T1 Digital

Jean SANCHEZ, Mladen BANOVIC

Transformers MAGAZINE

VOL 1 ISSUE 1

Issues to Consider when Substituting Large Power Transformers in Generating Stations

Voltage Stresses on Solid-Liquid Insulation of Large Power Transformers

Classification of Transformers Family ISSN 1849-3319

1 | ISSUE 1, VOLUME 1 WWW.TRANSFORMERS-MAGAZINE.COM

POWER TRANSFORMER LIFE CARLOS GAMEZ

Basics of Power Transformers

Economical and Reliable Transformer Maintenance by Holistic Interpretation of Insulating Oil Condition A General Overview of Power Transformer Diagnosis

TRENDS IN POWER TRANSFORMER FAILURE ANALYSIS WALLACE BINDER MAGAZINE TRANSFORMERS ISSUE 1, VOLUME 1 | 1

Coiltech 2014 24-25 September Pordenone

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What visitors like about Coiltech

Producers of electrical motors, generators, transformers and other inductivities meet to discuss new projects and business development with market leaders from all major parts of the supply chain.

Exhibitors

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Leading international suppliers Innovative suppliers of all significant components and technologies Excellent networking opportunity Highly specialized technical presentations at the World Magnetic Forum Compact format, best use of time, excellent traffic connections

Sign up for your free e-ticket: www.visitcoiltech.com

marcogarofalo.net

It's not the size of the circuit that matters but the energy it transmits

With a creative, unique cost and time effective exhibition and conference format, Coiltech has successfully challenged old habits and grown in just four years into an established meeting point of the Coil Winding industry.

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Coiltech has a rebooking rate in excess of 90%. At other Coil Winding exhibitions it is about 75%

Number of unique Visitors* 1286 941

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397 2010

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For complete visitors profile, see www.quickfairs.net/coiltech

What exhibitors like about Coiltech Highly competent visitors with an increasingly international background The best visitor per exhibitor ratio of any Coil Winding Exhibition Turn-key stands for better use of time and resources Internet connection included Minimum effort for organisational issues thanks to the all-inclusive exhibition formula Transparent, all-inclusive cost with no surprises, three booth formats to choose from.

Choose your spot yourself: www.exhibitatcoiltech.com

*Coiltech is the only show in the industry with certified visitors data (ISFCERT).

Exhibitors sign-up dynamics*

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BASICS OF POWER TRANSFORMERS Jean SANCHEZ, Mladen BANOVIC What are the basic transformation principles and essential transformer parts? Power transformers are key elements of a high voltage electrical transmission grid, which adapt voltage levels to the different needs of electric power users at constant power (disregarding the losses).

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POWER TRANSFORMER LIFE Carlos GAMEZ Part 1: What does “transformer life” mean?

The concept of life for an electrical asset, such as a power transformer, is sometimes not properly understood. In this article we review what we mean when we refer to the “life” of a transformer and what constrains determine the design and operational parameters of these assets.

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TRENDS IN POWER TRANSFORMER FAILURE ANALYSIS Wallace BINDER This article will introduce the reader to the importance of failure investigations, discuss the need for guidelines and the standards organisations work that is underway to provide the guidelines. The concept of using the Scientific Method is introduced and the existing processes are described.

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CLASSIFICATION OF TRANSFORMERS FAMILY Mladen BANOVIC, Jean SANCHEZ Transformers are used in the electrical networks everywhere: in power plants, substations, industrial plants, buildings, data centres, railway vehicles, ships, wind turbines, in the electronic devices, the underground, and even undersea. It is very difficult to organise a structured overview of the transformer types. Here the attempt is to provide a relatively common point of view on most of those transformers types.

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CONTENT

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Influence of dielectric tests on main insulation design

The purpose of this paper is to indicate the most important aspects to be considered when checking the interchangeability of transformers, based on authors‘ experience and various standards requirements.

VOLTAGE STRESSES ON SOLID-LIQUID INSULATION OF LARGE POWER TRANSFORMERS Juliano MONTANHA Dielectric tests are mandatory to define the main insulation transformer design such as clearances between windings, windings to core and leads as well. Therefore, it is very important to understand how and where the voltages are distributed within the transformer during the dielectric tests.

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A GENERAL OVERVIEW OF POWER TRANSFORMER DIAGNOSIS Jean SANCHEZ, Mladen BANOVIC This paper discusses the main diagnostic methods that could be performed during service life of a transformer. It will also attempt to provide the purposes of the main diagnostic methods carried out by different power transformer stakeholders.

ISSUES TO CONSIDER WHEN SUBSTITUTING LARGE POWER TRANSFORMERS IN GENERATING STATIONS Relu ILIE, Isidor KERSZENBAUM

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ECONOMICAL AND RELIABLE TRANSFORMER MAINTENANCE BY HOLISTIC INTERPRETATION OF INSULATING OIL CONDITION Marius GRISARU

A transformer owner or a person responsible for its proper operation is faced with many diagnostic approaches and most of them are either inaccurate or unnecessary and irrelevant to individual case. It is important to carry out the tests that will not interfere with transformer operation and interpret the results holistically in the transformer exploitation context, manufacture, internal organisation politics and many other parameters.

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EVENTS

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Jean SANCHEZ, Mladen

Transformers BANOVIC

VOL 1 ISSUE 1

MAG AZI NE

Issues to Consider whe n Substituting Large Pow er Transformers in Generat ing Stations

Basics of Power Transformers

TRANSFORMERS MAGAZINE EDITORIAL BOARD Editor in Chief: Mladen Banovic, PhD; PUCARO Elektro-Isolierstoffe GmbH; Germany [email protected] EXECUTIVE EDITORS Michel Duval, PhD; Hydro Quebec; Canada Pierre Lorin; ABB; Switzerland Jean Sanchez, PhD; EDF; France Jin Sim; Jin Sim & Associates, Inc.; USA Juliano Montanha; SIEMENS; Brazil Craig Adams; TRAFOIX; Australia Arne Petersen; AP Consulting ;Australia

Voltage Stresses on Solid-Liquid Insulation of Large Power Transfor mers

Economical and Reliable Transformer Maintena nce by Holistic Interpretatio n of Insulating Oil Conditio n A General Overview of Power Transformer Diagnosis

Classification of Transformers Family ISSN 1849-3319

1 | 1, VOLUME 1 WWW.TISSUE RANSFO RMERS-MAGAZ

INE.COM

POWER TRANSFORMER LIFE CARLOS GAME Z

TRENDS IN POWER TRANSFORMER FAILU RE ANALYSIS WALL ACE TRANSBINDE

FORMERS R MAGAZINE ISSUE 1, VOLUM E1 | 1

Subscribe now! Subscribe to Transformers Magazine and keep track of the latest news and trends in the power transformer industry. Print Edition – 4 issues (1 year) for $ 76 Digital Edition – Free of charge subscription for registered users

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Art director: Momir Blazek Photo: Shutterctock.com Language Editor: Mirna Harwood ADVERTISING AND SUBSCRIPTION Marin Ante Dugandzic +44 20 373 474 69 [email protected] TRANSFORMERS MAGAZINE Transformers Magazine is published quarterly by Merit Media Int. d.o.o., Setaliste 150. brigade 10, 10 090 Zagreb, Croatia. Published articles don‘t represent official position of Merit Media Int. d.o.o. Merit Media Int. d.o.o. is not responsible for the content. The responsibility for articles rests upon the authors, and the responsibility for ads rests upon advertisers. Manuscripts, photos and other submitted documents are not returned. Subscription rate: $76 (1 year, 4 issues) Digital subscription: free for registered readers www.transformers-magazine.com REPRINT Libraries are permitted to photocopy for the private use of patrons. Abstracting is permited with credit to the source. A per-copy fee must be paid to the Publisher, contact Subscription. For other copying or republication permissions, contact Subscription. All rights reserved.

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Publisher: Merit Media Int. d.o.o. Setaliste 150. brigade 10, 10 090 Zagreb, Croatia Contact: +385 91 222 8820 Croatia Contact: +44 20 373 474 69 UK VAT number: HR09122628912 www.transformers-magazine.com Bank Name: Zagrebacka banka Bank identifier code: ZABAHR2X Bank IBAN: HR8023600001102375121 Director: Marin Ante Dugandzic

ADVERTISING Australasia: Vince Hantos [email protected] Tel: +61 40 768 03 31

India: Ashutosh Kumar Govil [email protected] Tel: +91 99 750 975 34

Northern Europe: Matti Stoor [email protected] Tel: +46 70 644 31 94

Spain & Portugal: Alfonso de Pablo Hermida [email protected] Tel: +34 91 715 77 92

Russian Federation: Alexander Drobyshevski [email protected] Tel: +79 03 618 33 42

ROW: Marin Ante Dugandzic [email protected] Tel: +44 20 373 474 69

EDITORIAL MESSAGE

Dear Readers,

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he idea about a transformers magazine was born from discussions on Linkedin Transformers forum, where people worldwide, from over 130 countries, take part in 24/7 discussions, share their experiences and learn about the latest issues related to transformers. There is a truly wide range of broad, comprehensive topics which include all aspects of transformers’ lifetime and its components such as: parameter specifications from the grid viewpoint, including smart grid, reliability and efficiency, design, manufacturing, testing, operation and maintenance, protection, monitoring, diagnosis, failure, research of the failure causes, standards, education etc. Some of the most accomplished experts have told me that through the forum they can still expand as well as refresh their knowledge, despite spending decades in the transformers field in the best possible environment. The content is specifically valuable to the somewhat less experienced, those with limited knowledge in this field. In order to prevent the content from being lost somewhere in virtual world in the electronic form, the idea of a magazine was born. A group of more active transformer community members made great effort in preparation of the magazine and, despite their numerous private and work commitments, the magazine has become a reality. The website has been active for some time now, and it regularly brings current global news related to transformers. The comments received so far have been very positive and encouraging, especially for the website maintenance team who endeavour not only to keep up the good work but to also strive for improvement in time. Print magazine brings technical articles regarding the most recent topics, with a particular focus on efficiency increasing solutions and solutions for smart grid.

efficient device with efficiency at rated load even greater than 99%. However, the efficiency on a system scale is not so high, due to a series of transformers on the electricity’s way from power plants to consumers, due to loading lower than rated, and due to lower efficiency of smaller transformers. It is estimated that about 10% of globally generated electricity is dissipated in grid losses, 40% of that being the losses in transformers. That means 4% of globally generated electricity is wasted in transformers. Significant power generation (and transmission) capacity and corresponding energy resources are needed just to supply transformer losses globally, which is a huge amount and a solid potential for reduction of the environmental impact. The additional problem is that transformer fleet ages globally. This significantly increases not only financial risks for utilities and industry, but also the risk of even larger environmental impact. Therefore, we encourage thinking and writing about solutions and strategies to lower environmental impact through further increase of efficiency, i.e. lower CO emission, failure prevention, extension of transformer lifetime, and decrease of other risks, particularly risks of endangering people’s lives and health. We would like to use this opportunity to invite you to participate in creation of the Transformers Magazine upcoming issues. The first issue covers some of the fundamental topics such as reviews of transformer types, insulation design, diagnosis, lifetime, failure research and transformer replacement. I wish you a pleasant reading. Mladen Banovic , Editor in Chief

Influence of transformers on the environment is often significantly underestimated, probably because it is a highly

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EDITORIAL BOARD

Editorial board of Transformers Magazine consists of experts

with diverse experiences and backgrounds, from different parts of the world, and of different age. This should ensure as broad view on the matter of the magazine’s scope as possible. Here we present the editors’ short thorough biographies. Michel Duval Dr Michel Duval obtained a B.Sc. and PhD. in chemical engineering in 1966 and 1970, and has worked for IREQ (Hydro-Quebec, Canada) since 1970. He has made significant contributions in 3 mains fields of R&D: dissolved gas-in-oil analysis (DGA), electrical insulating materials and lithium-polymer batteries. In the field of DGA, Dr Duval is well-known for his «Triangle method» of DGA interpretation, used worldwide. He has developed the use of gas-in-oil standards and participated in the development of the «Hydran» on-line monitor for hydrogen in oil. Dr Duval has established the levels of gas formation observed in various types of electrical equipment. He has been the Convenor of numerous IEC and CIGRE working groups and the principal author of several IEC international standards and CIGRE Technical Brochures on DGA. He is also very active in several IEEE working groups. Dr Duval holds 16 patents and is the author of more than 100 scientific papers and standards. He is a Fellow at the Chemical Institute of Canada, a Life Fellow of IEEE, and the recipient of IEC and CIGRE Awards and of the IEEE Herman Halperin Electric Transmission and Distribution Award for 2012.

Pierre Lorin Pierre Lorin holds M.Sc. Electrical & Mechanical. He graduated in Paris in 1992. From 1992 to 1996 he worked at the Swiss Federal Institute of Technology in Lausanne (EPFL) where he conducted researches for large utilities in Euro8

pe and North America on reliability and maintenance strategy for overhead lines. From 1996 till today he has been with ABB Power Products - Transformers Service. He has been active in Research & Development as well as Product Management, mainly involved in transformers reliability, diagnosis methods, on-line monitoring, maintenance, repair, and active noise control. He is now Head of Technology for Transformers Service activities globally and the author or co-author of several publications in this field. Pierre has represented Switzerland within the CIGRE Transformer Study Committee (A2) for 6 years. He is now leading the Advisory Group “Transformer Utilization” within CIGRE A2.

Jean Sanchez Jean Sanchez completed a Ph.D. degree on organising a general scheme for power transformers fault diagnosis in 2011 and worked in a french power transformer reparation factory. This work was based on formalising and systematising human expertise and experience on power transformer fault diagnosis of any kind, which was developed over the years in close collaboration between transformer users and the reparation factory on failures and active part reconstructions. He worked on many transformer designs (core as well as shell type, GSU, substation and industrial transformers), tests (electrical, oil analysis), fault expertise (onsite, and in reparation workshop), power ratings, and OLTC reparations (MR and old french designs). Today he is working on generator diagnosis (mechanical, electrical, ancillaries) and maintenance in a major French utility and is the executive editor of the Transformers Magazine. He also holds a Masters degree in Applied Physics. TRANSFORMERS MAGAZINE | Volume 1, Issue 1

Jin Sim Jin Sim holds a BSEE from Dankook University in Korea. He attended two graduate schools for Engineering and one graduate school for Business Administration. He has been in the transformer industry for over 37 years in design, development, manufacturing, testing and management. In 2013, Jin retired as a VP and Chief Technology Officer at SPX Transformer Solutions (formerly known as Waukesha Electric Systems) and founded his own consulting company; Jin Sim & Associates, Inc. He has been active in the Electric Power Industry as a leader of several working groups and subcommittees. Recently, Jin was the chairman of the IEEE Transformers Committee for 2002–2003. He was a member of the U.S. Technical Advisory Group for IEC Technical Committee 14, Power Transformers and an individual member of the CIGRE. He has also been the NEMA and IEEE delegate to the ASC C57 Committee.

Juliano Montanha Juliano Montanha holds a degree in Electrical Engineering from University of São Paulo – Brazil from 1998 when he started working at Siemens Power Transformer factory in Jundiai - Brazil. Juliano has been working with insulation technology regarding main insulation design up to 800 kV, including leads design and winding assembly. He was responsible for winding assembly standardisation for local market in 2000. Juliano is a high voltage expert at Siemens Transformer Group and has participated on a worldwide R&D research with the Siemens transformers factories. Between 2011 and 2013, he was responsible for insulation design group at Jundiai Siemens factory. He was also responsible for implementation of 500 kV electrode in Jundiai factory as well as transient studies for Power Transformers design like VFT studies and failure investigations. Juliano technically supported the manufacturing process of HVDC units at Siemens factory in Jundiai and Siemens factory in México in 2010. He has been a member of IEEE since 2014.

Craig Adams Craig Adams started his career in 1992 as an apprentice electrical engineering tradesperson with Capricornia Electricity Board before commencing further engineering studies. He joined GEC Alstom in 1997 as a Cadet Test Engineer at the Rocklea Works, and from 2001 to 2013 held positions of Test Manager and Product Quality Manager. In 2013 he founded TRAFOIX Pty Ltd and is currently its Director and Principal Consultant. He has extensive experience in all aspects of transformer testing, routine, type & special tests, and investigative diagnostic techniques including failure investigations and forensic strip downs. He has considerable experience with routine & type testing of HV & MV w w w . t ra n sfo r m e r s - m a g a z i n e . co m

apparatus including switchgear, capacitor banks, high voltage motors and generators, and in the design and upgrading of test facilities and equipment.

Arne Petersen Arne Petersen received degrees in Electrical Engineering from Odense Teknikum, Denmark, a Bsc.Eng degree from RMIT, Australia and an MBA degree from University of Queensland, Australia. He is also a chartered engineer and a fellow member of Institution of Engineers Australia. Arne has five years experience as a transformer designs engineer with a transformer manufacturer and three years experience in power station design. He also worked as a transformer specialist and manager of HV Plant for Powerlink, Qld, a major Australian transmission utility, for twenty-seven years. He is currently working as a self-employed consulting engineer specialising in transformer technology. Arne has conducted many investigations into transformer failures and fires in Australia and Southeast Asia. He is an active member of Cigre, both in Australia and internationally, he was active in several Cigre working groups and is currently contributing to Cigre Study committee for transformers (SC A2).

Mladen Banovic Mladen Banovic obtained his PhD degree from the University of Zagreb in 2012. He leads PUCARO‘s research and development in transformer insulation, the editorial board of Transformers Magazine, and the Transformers forum. He managed the development of insulation systems up to 1200 kV and has been involved in defining ABB‘s smart grid strategy covering transformer insulation and components. Prior to joining PUCARO, he led basic research of transformer insulation and transformer monitoring business. He worked on developing and deploying systems for automated testing of transformers in factories and onsite, software packages for automation of transformer design, 2D and 3D field calculation and analysis, etc. Mladen is active member of a few IEC, IEEE, and Cigre working groups. He also holds a degree in Electrical Engineering and postgraduate Master of Science degree from the University of Zagreb.

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PRODUCTS NEWS

IUS Technologies presents new smart transformer monitors USA, Texas: IUS Technologies unveiled groundbreaking distribution transformer monitors - the TM1000 and TM2000 at DistribuTECH in San Antonio.

Oil Filtration Systems announces two new series of transformer oil purification equipment USA, Texas: Oil Filtration Systems (ClarkReliance Company) has announced two new series of oil purification equipment to remove impurities from dielectric insulating oils.

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he new generation of monitors uses carbon nanotube technology and combines total combustible gas (TGS), temperature and load monitoring readings to provide real-time notification for single and three phase distribution transformers. The monitors are the newest additions to the Born Smart™ line of smart grid sensing devices by IUS Technologies. They have an accuracy rating of 0.2% to re-

GE‘s Digital Energy announces the latest version of Perception™

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he newly-announced purification systems are HVTOPS and DOPS: High Vacuum Transformer Oil Purification Systems (HVTOPS) are single pass systems intended for quick turnarounds. Dielectric Oil Purifier Systems (DOPS) are designed to remove all impurities in multi-pass. Unlike HVTOPS equipment, they do not pull a deep vacuum on empty transformers for dry out. Otherwise, they function the same as HVTOPS. To find out more, visit: www.oilfiltrationsystems.com

USA, Georgia, Atlanta: GE’s Digital Energy has launched the latest version of Perception (TM) software used by the utilities for management of their power transformer fleet.

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he new Perception (TM) is the latest version of the General Electric‘s Digital Energy risk management software, a package used to diagnose the condition of every transformer in the fleet.

MR announces new TAPCON® voltage regulator Germany, Regensburg: The market leader, Maschinenfabrik Reinhausen GmbH (MR) has released TAPCON®, the new voltage regulator.

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n addition to regulation tasks, the TAPCON(R) also supports special applications such as threewinding transformers, transformer banks, phase shifters or shunt reactors. The modular system makes it possible to match the regulator‘s power spectrum to the respective requirements, claims MR. TAPCON® is suitable for integration into redundant network systems with the RSTP and PRP protocols. Source: Maschinenfabrik Reinhausen GmbH

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mote transformer monitoring and provide a complete view of equipment health apart from regular manual inspection periods. TM1000 and TM2000 immediately detect normal, abnormal, caution or dangerous conditions and send alerts either when predetermined conditions are met or supply a constant stream of information. Source: Business Wire

Transformer diagnosis is performed by the asset owners in order to maintain a transformer fleet and reduce operating expenses. Source: Thomasnet News

Endesa installs thermographic camera to monitor transformers in Maragall substation Spain, Barcelona: The Maragall substation has been fitted with a tiny thermographic surveillance camera which records everything that happens in power transformers within a 24 hour period.

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ndesa decided to extend the strategic measures to make the facilities safer and fitted the camera in the Maragall substation a few months ago. The camera is very small; in fact, if not informed, one would not even be aware of its existence. It is suspended from the ceiling corner in Room 4. This device allows thermal monitoring, i.e. analysing the heat of the most sensitive elements of power transformers and they are used to signal a warning in the event of above normal heat detection.

The tiny thermographic camera has been a major leap forward in safety and possible fire prevention. Endesa is pleased with the results and now wants to export the surveillance camera. Source: El Periodico

TRANSFORMERS MAGAZINE | Volume 1, Issue 1

BUSINESS NEWS

TransnetBW commissions first vegetable oil 420 kV transformer from Siemens

Falfurrias Capital Partners acquire Efacec ACS USA, Georgia: Falfurrias Capital Partners, North Carolina-based private equity firm, has acquired Norcross, Georgia-based Efacec ACS, Inc. from the Efacec Group for undisclosed terms. The company name will change to Advanced Control Systems, Inc. (ACS).

Germany: TransnetBW has commissioned Siemens’ first vegetable oil transformer with 420 kV voltage level at the BruchsalKändelweg substation near Karlsruhe.

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pparently, the first power transformer in the world, insulated and cooled using vegetable oil, will link the 380 kV ultra-high voltage level with the 110 kV grid, writes EBR. The insulating oil for the new transformer is made from renewable plant resource, is completely bio-degradable, and much less flammable than mineral oil. “The bio-degradability of the insulating oil means that additional collecting vessels and separation systems are no longer required at the installation location, resulting in cost savings for these items.“ said Siemens Energy Transformers business unit CEO, Beatrix Natter. The transformer can be installed and operated in water conservation areas or areas where environmental protection applies as it is the world‘s first transformer at the 420 kV extra-high voltage level for which no water hazard classification must be issued. Source: EBR

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alfurrias Capital Partners formed NATDG in 2013 to acquire and grow well-performing USbased companies with strong balance sheets that support mission-critical elements of the electric utility grid. The first acquisition made by NATDG was ITEC, a Charlotte-based manufacturer of instrument transformer products. ACS‘ product portfolio includes SCADA, distribution management, outage management and substation automation. ACS was one of the first companies in the

ENG April 2014 ad print.pdf 1 28/03/2014 14:00:04

industry to deliver a truly integrated ADMS platform, combining distribution and outage management functions using a common network model and user interface. “ACS has a tradition of innovation that has kept us at the forefront of the transformation to an intelligent grid.” said José Barbosa, CEO of ACS. “Together with the NATDG leadership and the backing of Falfurrias Capital Partners, we will continue developing new solutions that empower our customers to meet the challenges they face today and in the years to come.” Source: Efacec

MR invests € 24 million ($ 33 million) in new materials management centre C

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Germany, Regensburg: Maschinenfabrik Reinhausen GmbH (MR), the world leader for the regulation of power transformers has invested € 24 million ($ 33 million) in strengthening the highly automated materials management centre in Regensburg. CM

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he company intends to reorganise the existing site and employ extra 100 people to work in the new buildings. “With this investment, the entire economy of the Regensburg site will be further strengthened.“ said the Mayor Hans Schaidinger. The main objectives of the investment were to reduce logistics costs and lead times by at least 25 %. The four camps operating in the urban area as well as the need to renew the warehouse equipment and reorganise all the processes were the reasons for the construction of the site. The construction of the new buildings and reorganisation of the factory premises is scheduled for completion by the end of 2015. The new, highly automated materials management centre is scheduled to start operation in early 2016. Source: Maschinenfabrik Reinhausen Gmbh

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Products of M&I Materials Ltd.

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TECHNOLOGY BASICS

ABSTRACT Power transformers are key elements of a high voltage electrical transmission grid, which adapt voltage levels to the different needs of electric power users at constant power (disregarding the losses). This electrical machine is classically constructed out of copper, steel, paper and insulating oil. All those materials are made into different components. The main ones are the windings, tap changing system, core, tank and bushings. The components are assembled together to produce a power transformer. Power transformers are developed worldwide using a few basic designs established almost a century ago, that can be largely adapted for many special applications [1].

Keywords basics, components, power transformers 12 12 | JULY 2013

Basics of Power Transformers What are the basic transformation principles and essential transformer parts? Introduction

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minimising Joules effect losses using high voltage over long distances.

owadays, electric energy is available almost everywhere, and we do not even think how it was produced. Production of the electric energy is possible by application of the power transformers.

Transformers have been used worldwide for many years and their availability and reliability is a major concern for all electricity users and the assets owners.

Power transformer is a complex assembly of elements based on decades-long, worldwide proven technologies. Those electrical machines are essential for power grids to transmit electrical power by

The basic facts and the main parts and components of power transformers are presented in this article to understand which diagnoses can be applied accordingly.

Basics of power transformers

TRANSFORMERS MAGAZINE | Volume 1, Issue 1

JeanSANCHEZ, SANCHEZ,Mladen Mladen BANOVIC BANOVIC Jean Electromagnetic basis A single-phase transformer is basically made out of two separate windings that are inserted into each other into a closed loop of magnetic core. The voltage ratio (V1/V2, where V1>V2) of the transformer is equal to the ratio of the number of turns of the two windings (N1/N2, where N1>N2) in a first approximation. It should be noted that a classic power transformer requires alternative voltage. In a rough approximation, if the transformer losses are disregarded, the power (Voltage x Current) transmitted through the windings has a lower current on the high voltage (HV) side than on the low voltage (LV) side. Moreover the Joules heat effect is proportional to the square of the current transmitted into any ordinary conductor like transformer windings or transmission lines. Both these effects combined at constant power of elevating voltage reduce heat dissipation accordingly by the square of the current, and enable the transmission of power of alternating current and voltage over very long distances from the energy producer to the energy consumer while limiting the power losses in the grid. This is possible due to a key grid component the power transformer. Most of them are the three-phase transformers or the three single-phase transformers.

voltage (lightning) and overcurrent (short circuits). The reader interested in finding out more about the theory and the practice regarding power transformers could read [2], a book continuously updated for almost one century ago!

um coils insulated mainly with several layers of paper between the turns. The two main winding designs and technologies have been developed over time with many variations: the core type and the shell type windings. The electromagnetic basis remains the same in both cases but the mechanical construction Active part is different. In the core type design, the The active part of a transformer is made of winding is “enclosing” the magnetic the elements that are in contact with the core legs, while in the shell type the core voltage and the current, and are mainly is “enclosing” (and running through) the windings. Every composed of windings, core, tap chan- Nowadays, electric energy is transformer manuger bushings. The facturer has its own other components are available almost everywhe- experience with these ancillary components re, and we do not even think technologies, neither not mentioned here. which is automahow it was produced. Pro- of ted. Windings

duction of the electric ener-

The windings of a gy is possible by application The manufacturing power transformer of windings involves are its main element, of power transformers. a lot of human labour like the heart in a huand requires signifiman. The windings are handmade out cant experience as well as application of copper, or sometimes out of alumini- of the highest quality standards. This is

Three points could be noted from these electromagnetic principles. First, with the voltage increase of an electrical network, the Joule losses are reduced. The trend in large countries like Canada, Brazil, Russia, China, South Africa, South Korea, USA, and Venezuela is the development of the 800 kV electrical networks, or the 1000 kV to 1200 kV ones in China and India, respectively. Secondly, the two main con-

The theoretical and practical principles of power tra­ nsformers have remained the same since more than a century ago. straints of power transformers are high voltage and high current, depending on whether the HV or LV is observed. Those constraints are taken into account when studying the transformer limits, like overw w w . t ra n sfo r m e r s - m a g a z i n e . co m

Figure 1: Three phase power transformer 13

TECHNOLOGY TECHNOLOGYBASICS BASICS because winding conductors are covered by a type of insulation such as varnish or insulating paper with a limited mechanical and thermal stability. Nevertheless, this insulation type provides protection from high overvoltages, high overcurrents, short-term overheating, and high mechanical stresses in order to prevent reduction of the insulation paper durability. It must be taken into account that the winding insulation cannot be easily repaired or replaced during the service life of a transformer and rewinding has to be performed only in a specialised workshop. A review of winding types used in power transformers is provided in the article [3]. Core

Figure 2: Insulation in power transformer

The core is an important part of a transformer and generally the heaviest one. Produced from steel, it has high magnetic permeability and provides low magnetic resistance to the magnetic flux. It is made from thin steel sheets with the thickness of a few tenths of a millimetre in order to reduce losses and magnetising current. The main way to produce a core is to stack the sheets, cut to desired size, onto the automatic machines, and then manually stack them to build a core. Wound cores provide much better productivity for single-phase small distribution transformers. The main core parts are the legs (vertical parts), and yokes (horizontal parts). The legs are mainly situated in a same plain but the three-phase transformers can have so called triangular-spaced core legs. This

The manufacturing of core and windings - the heart of a transformer, involves a considerable amount of manual labour even to date.

Figure 3: High voltage bushings of power transformer 14

type of transformer is called hexaformer. Small-size transformers, like distribution transformers, are sometimes produced as hexaformers but their market penetration is very low. Even transformers of up to 10 MVA have been produced in this form but the concept was cancelled due to complexity. They produce lower losses but their productivity is lower compared to transformers with a ‘traditional’ core. TRANSFORMERS MAGAZINE | Volume 1, Issue 1

Tap Changer

Insulating materials

Most transformers have additional turns added to the HV windings and some of those turns are linked to a device called the “Tap Changer”. It enables a specific range of the voltage variation during the transformer service life. The electric circuit of the windings and the tap changer

The three most typical insulating materials for the power transformers are: mineral oil, paper and pressboard in different forms. The mineral insulating oil is weighted in tons within the tank and can be used to assess many essential points about the condition of a transformer and some critical incipient faults. The paper insulates the winding turns, while the pressboard strengthens the electrical insulation and provides dielectric distance at specific locations, for example in the main duct between the windings.

Mostly used solid insulating materials, paper and pressboard, are organic and subject to aging. They cannot be repaired or replaced easily, therefore they limit the lifetime of a transformer. have some movable contacts. The two main types of tap changers are the DEenergized Tap Changer (DETC) - mechanically quite simple type that changes the voltage while the transformer is not loaded; and the On Load Tap Changer (OLTC) - a more complex type [4] which operates when the transformer supplies the load. It should be noted that the tap changers, the OLTCs in particular, are contributing to an increasing transformer failure rate, mainly due to the movable contacts wearing over the years (hot spots, aging mechanisms) [5]. Bushings

The bushings are the components that link the windings to a network through the grounded tank. High voltage bushings can be technically complex and, in some cases, their failure can lead to a transformer explosion quite rapidly. This is because one of the highest voltage gradients is between the HV bushing central part at full potential, and the grounded tank at the distance of just a few centimetres. The insulating oil just below is very flammable and if the bushing is sparking, it could generate a lot of energy, open the tank slightly and then ignite the oil, which could lead to an explosion. For this reason, the HV bushing is manufactured to withstand very high voltages within a small space filled with paper and oil between the bushing and transformer tank. w w w . t ra n sfo r m e r s - m a g a z i n e . co m

Insulating materials, such as paper, pressboard and mineral oil are organic materials subject to aging. As the solid insulation cannot be repaired or replaced easily like other transformer parts and components, it limits the transformer service lifetime. Therefore, the solid insulation lifetime is the main driver of the lifetime of a transformer.

Conclusion The basic facts about transformers and the main transformer parts and components are briefly described abo-

ve. Power transformers can be seen as main components of any high voltage grid, which reduce the losses during the delivery of electrical energy to wide areas. More details on the topics above can be found in the literature cited, including [6] and [7].

References [1] Mladen Banovic, Jean Sanchez, Classification of Transformers Family, Transformers Magazine, Vol 1, No.1., 2014 [2] Martin J. Heathcote, J&P Transformer book, Newnes, 13th edition, 2007 [3] Jean Sanchez, Classic Power Transformers Windings, Transformers Magazine, Vol 1, No.2., 2014 in print [4] Dieter Dohnal, On-Load Tap-Changers for Power Transformers A Technical Digest, MR Publications, 2009, source : www.reinhausen.com/XparoDownload. ashx?raid=15497 [5] An international survey on failures in large power transformers in service, CIGRE Electra No. 88, pp 21-42, 1983 [6] IEC chapters 1 to 21 of the 60076 power transformers standards, www.iec.ch [7] Handbook for Transformers, 3rd edition, ABB, 2010

Authors Jean SANCHEZ completed a Ph.D. degree on power transformers fault diagnosis in 2011 and worked in a French power transformer reparation factory. His work involved many transformer designs, tests, fault expertise, power ratings, and OLTC repairs. Today he is working on generator diagnosis in a major French utility and is the executive editor of the Transformers Magazine. He also completed a Masters degree in Applied Physics. Mladen BANOVIC obtained his PhD degree from the University of Zagreb in 2012. He leads PUCARO‘s research and development in transformer insulation and the editorial board of Transformers Magazine. He has been involved in the development of insulation systems up to 1200 kV and defining ABB‘s smart grid strategy covering transformer insulation and components. Prior to joining PUCARO, he led basic research of transformer insulation and transformer monitoring business. He also holds a degree in Electrical Engineering and postgraduate Master of Science degree from the University of Zagreb. 15

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Power Transformer ABSTRACT The concept of life for an electrical asset, such as a power transformer, is sometimes not properly understood. In this article, the first from the series of three articles, we review what we mean when we refer to the “life” of a transformer and what constrains determine the design and operational parameters of these assets.

Keywords power transformer, life, asset management, condition assessment, life extension 16

Part 1: What does “transformer life” mean?

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n our industry there are often terms or buzzwords that many of us use with liberty but might not fully understand their meaning. Justifiably, we use a particular word often because the concept it conveys is of critical importance to the safe and reliable operation of any electrical asset. We use this word to convey the idea of a time period in which an asset performs its intended function so it is important for any person involved in specifying, purchasing, testing, commissioning, maintaining, operating or disposing of

these assets to have a clear understanding of what life means in the context of power transformers. In this series of articles, I am going to talk precisely about this concept. Throughout the articles in the series, we will try to establish what life is and what it means for a power transformer. We will also explore the factors that affect it and consider the options that an asset owner can utilise to extend and optimise the transformer life. In the first article, I will try to establish the common definition and understanding of

TRANSFORMERS MAGAZINE | Volume 1, Issue 1

Carlos GAMEZ

Wait, what? … Are transformers not supposed to last 35 to 40 years? The simple answer is: not necessarily, it depends.

WHAT DOES “TRANSFORMER LIFE” MEAN? When we talk about concept of the transformer life or any other electrical asset for that matter, we also hear related terms and phrases that almost invariably show up in the same conversation. We often hear terms like: “life management”, “life expectancy”, “life extension”, “life cycle”, etc.

Life what transformer life is. This will hopefully serve as the foundation for the articles to follow in this series. This topic is vast and there are as many opinions as there are experts in our industry. I have tried to remain objective and base my comments on evidence and facts but I anticipate that my personal experience has found its way into these articles in one way or another. I have also assumed that the reader is familiar with the basic power transformer concepts and has seen or worked around these assets at least once in their career.

w w w . t ra n sfo r m e r s - m a g a z i n e . co m

We will somehow touch on all those concepts but with some luck, we will mention them following a logical sequence of ideas. I will attempt to explain these concepts in a clear and understandable way; how they relate to day to day operation of a transformer fleet and most importantly, what the user/owner of these assets can do in order to minimise the operation and maintenance costs as well as mitigate the risks of unexpected failures. The common denominator in the terms mentioned above is the word “life” so it seems only fair that we start by setting some fundamental understanding of what “life” means from the perspective of a power transformer. There are many types, designs and ways of manufacturing power transformers but for the purpose of this discussion, please consider insulating liquid - filled power transformers while you are reading articles in the series. However, the concepts explained and reviewed here can also be applied to dry transformers, instrument transformers, gas insulated transformers and other classes of specialty transformers to some extent. Common sense tells us that the life of an asset can be regarded as the period of time

in which the asset will reliably perform its intended function. This is not a bad start, but we can do better. Life can also be defined in statistical terms which are particularly useful for insurance companies as the “Mean Time Between Failures” or MTBF. This basically means that out of a population of transformers the time between initiation of service and failure is measured and averaged, providing a good idea of the life expectancy for a transformer belonging to that population or of one with very similar characteristics. Another good reference is to look at what the different standard committees worldwide have to say on the matter. The Australian Standard, AS 2374.7 – Power Transformers – Part 7: Loading Guide for Oil-Immersed Power Transformers establishes in: “1.4 General limitations and effects of loading beyond nameplate rating … 1.4.1.1 Factors influencing life duration The actual life duration of a transformer depends to a high degree on extraordinary events, such as overvoltages, short-circuits in the system, and emergency overloading. … The normal life expectancy is a conventional reference basis for continuous duty under normal ambient temperature and rated operating conditions. The application of a load in excess of nameplate rating and/or an ambient temperature higher than

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It is also not a coincidence that none of the standards mentioned above define what the life expectancy of a particular transformer should be.

“3. Definitions … 3.5 Transformer Insulation Life: For a given temperature of the transformer insulation, the total time between the initial state for which the insulation is considered new and the final state for which dielectric stress, short circuit stress, or mechanical movement, which could occur in normal service, and would cause an electrical failure”

Further to this definition, IEEE C57.911995 provides calculation formulas for “loss of life” as a percentage of “per unit” life expectancy, providing a normalised life loss equation under various overloading and stress circumstances. This relationship is commonly represented in the form of a Life vs.Temperature curve, as shown below.

1000

Per Unit of Normal Life

100

10

1

0,1

“3 Terms and Definitions … 3.10 transformer insulation life total time between the initial state for which the insulation is considered new and the final state when due to thermal ageing, dielectric stress, short-circuit stress, or mechanical movement, which could occur in normal service and result in a high risk of electrical failure 3.11 per cent loss of life

0,01

0,001

And finally from IEC 60076-7 – Loading guide for oil-immersed power transformers we read the following:

50

70

90

110

130

150

Hottest Spot Temperature (ƟH) [°C]

Figure 1: Transformer insulation life

170

190

equivalent ageing in hours over a time period (usually 24 h) times 100 divided by the expected transformer insulation life. The equivalent ageing in hours is obtained by multiplying the relative ageing rate with the number of hours” The discerning readers would have noticed that there is a common thread across these standards. They relate to transformer loading guidelines, mention temperature as an important factor and talk about the insulation and the insulation life. This already provides a hint into where we have to look to understand transformer life. It is also not a coincidence that none of the standards mentioned above define what the life expectancy of a particular transformer should be. Wait, what? ... Are transformers not supposed to last 35 to 40 years? The simple answer is: not necessarily, it depends. As we will discuss in these articles, the various mechanisms affecting the life duration of a particular unit are difficult to

Figure 2: Internal construction of a typical power transformer  18

TRANSFORMERS MAGAZINE | Volume 1, Issue 1

Carlos GAMEZ predict. Trying to anticipate events during the operation of any given transformer and how they will ultimately affect its life is an extremely complex problem.

In short, the life of the transformer as a whole is directly linked to the life of its insulation system.

TRANSFORMER CONSTRUCTION As we might suspect, the “heart” of a transformer are the core and the coils (see Figure 2.). They provide the critical function of transforming the incoming power into different levels of voltage and current by means of electromagnetic induction. When one thinks about the materials this subassembly comprises, one thinks of copper or aluminium for the conductors; structural steel for the tank, clamps and radiators; magnetic steel for the core; mineral oil for cooling and dielectric insulation; wood for the lead holding structure; ceramics for the bushings; paper boards and sheets, and tapes for electrical insulation. Out of all these components, the ones that are most sensitive to temperature and age are the ones with a cellulose constitution. Cellulosic materials are those which are derived from natural vegetable fibres such as wood, pressboard, Kraft paper, etc. You might ask, why paper? The fact is that paper in combination with an insulating liquid provides an excellent and versatile dielectric medium that can be applied to complex geometries. Paper also provides a good economic balance between the cost of the material and the function that it performs. However, paper is also the first material amongst the constituents of the core and the coils to degrade under thermal stress. While it would take several hundred degrees Celsius of temperature to melt or cause significant damage to the components made of steel, copper or metal, it takes merely around 120°C to start causing significant degradation of cellulosic materials. In fact, the main limitation of what temperatures are allowed to develop in a particular design is set by the paper. The transformer designer will make sure that not a single part of the in-

w w w . t ra n sfo r m e r s - m a g a z i n e . co m

sulation system is exposed to these kinds of temperature. And if the paper degrades? Why would that affect the functionality of the transformer? We know that the primary energy conversion function is performed by the conductors and the core, right? Well, it turns out that the insulation system is responsible for ensuring that conductive elements subject to a voltage difference stay electrically insulated. Should the insulation break down between two high voltage elements, it would cease to perform this function, the currents would flow through paths they are not supposed to follow and the eventual consequence is a catastrophic failure. In short, the life of transformer as a whole is directly linked to the life of its insulation system. If any part of the insulation system breaks down, the whole transformer

stops working. Albeit failures can occur in other components, such as core, bushings, current transformers, etc. it is commonly a failure in the insulation system that leads to catastrophic outcomes. Unsurprisingly, most of the efforts in the transformer manufacturing, operation and maintenance industries are aimed at improving, monitoring and maintaining performance of the insulation system. I hope you now have a better understanding of what life means in the context of power transformers. In the next article, we will delve a bit more into the molecular structure of the paper, what it means for its electrical and mechanical properties and what various mechanisms by which it degrades or loses those properties are.

Author Carlos Gamez currently works as a Principal Consultant and Product Manager at TxMonitor and is a member of the MM Group Holdings where he focuses in developing innovative solutions for the electrical asset management industry using both his technical and business acumen. After graduating in Electrical and Mechanical Engineering in 1996, Carlos started working as a Transformer Design Engineer at PROLEC-GE, the biggest transformer factory for General Electric on the American continent. Over the course of the following years, he gained expertise working in various roles in product development, manufacturing improvements, technology and software development, field engineering and customer service. In 2007 Carlos was seconded by General Electric to move to Perth, WA to start up the Transformer Division in order to provide field and workshop maintenance and repair services to customers across Australia. Having fulfilled this mission, in the early 2011 Carlos accepted the position of Principal Consultant with Assetivity, a leading consultancy firm in Asset Management. Over this period, Carlos developed a holistic point of view by working on projects within the Asset Management frameworks which eventually shaped the ISO 55000 set of standards published in 2014.

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Trends in Power Transformer Failure Analysis ABSTRACT This article will introduce the reader to the importance of failure investigations, discuss the need for guidelines and the standards organisations work that is underway to provide the guidelines. The concept of using the Scientific Method is introduced and existing processes are described. Limitations of postmortem investiga­ tions are identified along with best pra­ctices for investigations. 20

INTRODUCTION

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ailure investigations are becoming increasingly important in these days when assets like power transformers cost in the millions of euros or dollars and consolidation in the utility industry has resulted in the operators wanting the maximum capability from their assets. Transformer failures have been investigated since the beginning of the electric utility industry. Each manufacturer can probably identify their own problem areas from factory failures, quality programme results and experience. However, the ope-

rators (utilities) may not possess sufficient quantities of a manufacturer or particular design to recognise the problem areas. Statistics such as those which, in the US and elsewhere, might result in a recall of an automobile model which may not exist in quantities sufficient to establish patterns of defects. Further, there is no regulatory body to require such a “recall.” The owner/ operator of the transformer is expected to be an informed consumer. The good news is that the power transformer is a highly engineered and tested product which has a significant life span. The informed consumer can make judgments about operation and maintenance knowing the root cause of failures on the system. TRANSFORMERS MAGAZINE

Wallace Binder NEED FOR GUIDELINES Failure reporting has taken place in the form of surveys published by organisations such as the International Council on Large Electric Systems (CIGRE WG A2.37 Transformer Reliability Survey), Edison Electric Institute (EEI Transmission and Distribution Committee), and the Institute of Electrical and Electronics Engineers – Industry Applications Society (IEEE/IAS). The EEI stopped publishing the results of their survey about the time that the utility deregulation movement got underway in the US. Some of the statistics reported by IEEE/IAS in the “Color Book Series” rely on data collected by the US Army Corp of Engineers in the 1970‘s. It is now left largely to the user to develop their own reliability statistics for transformers. It was recognised long ago that the development of failure reporting guidelines was necessary. What was a defect to one user might be a major failure to another. This situation was observed in the data collected by EEI. This discrepancy led to the development of the IEEE Guide for Reporting Failure Data for Power Transformers and Shunt Reactors on Electric Utility Power Systems. Unfortunately, EEI no longer reported failure statistics a short time after publication of the Failure Reporting Guide. As the effort unfolded to develop failure statistics, it became clear that failure analysis guidelines were also necessary. The analysis guidelines, if effective, will result in four important things:

dismounting. The main following activities will be covered by this WG: - State of the art of postmortem analysis (IEEE C57.125-1991 and any other relevant existing documents) - How to make an external and internal inspection of different components - Important information to collect: diagnostics, protection, operation and maintenance records, etc. - Availability and significance of design data, material used, etc. - Documentation during the dismounting, check lists - Additional checks, e. g. clamping pressure… - Paper sampling: precautions, which winding, axial/radial position, correlation with temperature, number of samples, conservation and storage of the samples, parameters to be investigated (Task Force to be leaded by SC D1)

- Collection of pictures of postmortem analysis with examples of common failures and the associated failure investigation - Best practices for failure report and scrapping report - Economic aspect of postmortem analysis (cost, value, constraints, etc.)” In the IEEE/PES Transformers Committee there is a Working Group on revision of C57.125 Guide for Failure Investigation, Documentation, Analysis and Reporting for Power Transformers and Shunt Reactors which has the task of revising and merging two existing documents. These IEEE Guides have been used since they were originally published in the 1980‘s and 1990‘s and reaffirmed as recently as 2005. The current revision will provide updates to technologies used in testing and evaluating transformer condition and include the work on reliability assessment contained in the previous guide. The Working

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- Establish a common set of steps to investigate failures, - Ideally, reach the same conclusion on root cause when presented with the same data, - Result in sharing of performance between manufacturers and operators, and - Produce meaningful statistics for transformer performance (failure rates, meantime-to-failure, and so forth).

STANDARDS ORGANISATIONS WORK UNDERWAY In CIGRE, there is a Working Group A2.4.5 on Transformer Failure Investigation and Postmortem Analysis which is underway. The Working Group scope states, “This WG will develop a structured procedure from the decision to take the transformer and shunt reactor out of service to careful WWW.TRANSFORMERS-MAGAZINE.COM

ISSUE 1, VOLUME 1 | 21

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Group is preparing a document whose scope “...recommends a procedure to be used to perform a failure analysis and the reporting and statistical analysis of reliability of power transformers and shunt reactors used on electric power systems.” The Guide includes: - Definitions - Steps to Determination and Investigation of a Failure Occurrence - Preparation Items - Data Collection Checklists - Analysis of common failure modes - Failure Reporting guidelines (and gu­id­ ance to develop a statistical database for reliability evaluation). The Working Group has produced several drafts and expects to ballot in the next year.

SCIENTIFIC METHOD Application of the Scientific Method is necessary when investigating a transformer failure or suspected failure. This requires analysis of the facts and data present, establishment of an hypothesis of failure, testing the hypothesis against the available data, collecting more data to confirm or refute the hypothesis, and 22

reporting the results. Testing the hypothesis might include modeling or developing experiments to confirm the results (these experiments may take the form of comparison with other non-failed units of similar design, testing against established norms or comparison of test results on adjacent phases). After testing the hypothesis, it might be necessary to modify the hypothesis. This iterative process will come as close as possible to determining the root cause of the failure.

up to and following the supposed failure must be available for analysis. This requires collection of fault recorder, sequenceof-events recorder, protective relay operation, protective device operation (fuses or circuit breakers), and alarm conditions prior to and subsequent to the outage, if one occurred. This data can include past alarm conditions that have been corrected, previous trip operations that have occurred, and system events which are similar to the current event.

LIMITATIONS OF POSTMORTEM INVESTIGATIONS

If this type of data is not collected immediately after the suspected failure or if such data collection is not part of the practices of the utility, the failure event cannot be accurately reproduced. Likewise, if routine diagnostic testing is not part of the maintenance routine, comparison of test results with previous trends will be difficult or impossible. A well execut-

Power transformer failure investigations must start with an understanding of the failure mechanisms possible and an understanding of the system in which the transformer is applied. Relevant (and some irrelevant) information leading

Application of the Scientific Method is necessary when investigating a transformer failure or suspected failure. TRANSFORMERS MAGAZINE | Volume 1, Issue 1

Wallace Binder ed maintenance programme is critical to investigations. It must also be recognised that, in some cases, the damage done after a failure by the energy available on the power system can mask or destroy evidence of the root cause of the failure. This is one reason that routine monitoring of the transformer is important: - to observe detrimental trends in performance of the insulating system which, if left uncorrected, could lead to failure resulting in an outage, and - identify events which may have done damage to the transformer (such as damage resulting from through-faults, damage which is the result of improper or inadequate maintenance and repairs or damage from abnormal system conditions like over-voltages, abnormal frequency excursions, etc). Of course, not all failures manifest themselves in an outage or protective device operation. Routine testing or on-line monitoring can detect abnormal conditions. These situations are sometimes confounding to the transformer operator – Are these results conclusive enough to remove the asset from service to attempt repairs and can repairs be successful? There are few transformer operators who “run-tofailure” with the additional risks that this practice entails. But when is the appropriate time to remove a transformer from service? Too soon and the system is operating in a higher risk mode or the cost of a replacement is necessary. Too late, and the result may be a catastrophic failure. Transformers which have a history of test results can be better evaluated than those that do not. Some transformers have operated for years with low levels of combustible gas being generated. Some have long history of higher than normal tan-delta (power factor). It is important to know when a test result is out of tolerance and unacceptable. Trending of results may be the best approach for the examples given but for other tests, the results can lead to “go” or “no-go” results. Experience is generally necessary to properly make this determination. The C57.125 guide points the reader to possible conclusions given various sets of test data or observed conditions. Final determination of root cause ultimately requires disassembly or dismounting of the transformer to investigate the internal conditions found following the failure. w w w . t ra n sfo r m e r s - m a g a z i n e . co m

Factors to take into account when making the decision to remove a transformer from service should include: - risk of outage - consequence of outage - value of the transformer and surrounding equipment - the ability of the system to operate without the transformer in-service.

BEST PRACTICES FOR INVESTIGATIONS The rapid collection of data and observations from both the transformer and the system conditions at the time the decision is made to remove the transformer from service, either by human intervention or by automatic trip, is important to the successful analysis of transformer failures. It is also important to operate and investigate in a safe manner. This means that all corporate, regulatory, and rule-governed work practices must be followed to assure the safety of personnel and the public. Check with your local authorities to determine what practices must be observed. Discussion with experts and the manufacturer may lead to conclusions as to the root cause, even if the damage makes that difficult. Knowing the values of voltage stress on the insulation, for example, is an important factor to consider if there is evidence of insulation breakdown. Equally important, however, is modeling the voltage stress on the transformer provided by

the system. Papers have been written by investigators who have discovered voltage phenomena caused by external system conditions, which result in voltage stresses beyond the capability of the transformer insulation. This example, known as partwinding resonance, has been shown to be the cause of dielectric failure. There is a solution to prevent the occurrence and it becomes the decision of the transformer operator (utility, in most cases) as to whether the solution is economically justified for the risk and consequences. The design of the transformer and the design of the power system to which it is connected are important aspects of the investigation. The transformer has been designed according to the standards specified in the original purchase agreement. We will discuss the evolution of standards separately. System studies such as the load flow and short circuit capability of the system may be important to know in the analysis of the failure. Less routine studies to determine transient voltages during system disturbances may be required in some cases. Expertise in these areas should be made a part of the investigation team. In future articles, we will delve more deeply into the procedures and practices necessary to successfully determine the root cause of a failure. In addition, we will talk about the symptoms of known failure mechanisms.

Author Wallace Binder has experience in scope development, planning, design, construction, start-up, operation, and maintenance of distribution, transmission and customer utilisation substations, back-up power generation, transmission and distribution lines, and systems. Wallace Binder has been an active member of the IEEE/ PES Transformers Committee for more than 30 years. He has served twice as Chair of the Working Group on Failure Analysis, a position he currently holds. He served as Chair of the Transformers Committee for two years in the late 90‘s and has contributed to numerous guides and standards developed by the Transformers Committee. Wallace Binder is currently an independent consultant with his office located in Western Pennsylvania. He has served a variety of clients - both manufacturers and users of substation apparatus. 23

TECHNOLOGY BASICS

ABSTRACT Transformers are used in the electrical networks everywhere: in power plants, substations, industrial plants, buildings, data centres, railway vehicles, ships, wind turbines, in the electronic devices, the underground, and even undersea. The focus of this article is on transformers applied in the transmission of energy, usually called power transformers. Due to very versatile requirements and restrictions in the numerous applications, ranging from a subsea transformer to a wind turbine transformer, a small distribution transformer to a large phase shifter transformer, it is very difficult to provide a structured overview of the transformer types. Also, different companies supply different markets and each have their own classification of the transformers, which makes the transformer family even more difficult to assort. This paper will attempt to provide a relatively common point of view on most of those transformers types.

Keywords classification, distribution transformers, power transformers, reactors, transformers 24

Classification of Transformers Family 1. Introduction

T

ransformers basically perform a very simple function: they increase or decrease voltage and current for electric energy transmission. It is precisely stated what a transformer is in the International Electrotechnical Vocabulary, Chapter 421: Power transformers and reactors [1]: “A static piece of apparatus with two or mo­re windings which, by electromagnetic ind­ uction, transforms a system of alterna­ting voltage and current into another system of voltage and current usually of different values and at the same frequency for the purpose of transmitting electrical power.“ The focus of this article is on the transformers which enable transmission of energy in the electrical grid, while all other types, such as the instrument transformers (i.e. voltage and current transformers) and the audio transformers, etc., are excluded. The aim is to provide an overview of different types

of transformers in as systematic way as possible, rather than elaborating on each type. The most important international organisations with focus on such transformers are IEC1 through E14, its technical committee for the world standards; IEEE2 through the Transformers Committee mainly for the American standards and CIGRE3 through the Study Committee A2 Transformers which mainly produces technical brochures and guidelines on many subjects. Main standards for the transformers in question are the IEC 60076 [2] and the IEEE C57 [3] series.

2. Classification of transformers family As mentioned above, transformers perform a very simple function and they can have many applications. Transformers are IEC - International Electrotechnical Commission IEEE - Institute of Electrical and Electronics Engineers 3 CIGRE - Council on Large Electric Systems

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TRANSFORMERS MAGAZINE | Volume 1, Issue 1

Mladen BANOVIC, Jean SANCHEZ

Transformers exist for more than a century and they can be manufactured and used very differently according to customers needs.

used in every power plant, all grid substations, buildings, in the industry, the underground installations, wind turbines, on platforms, marine vessels, under the sea, etc. Due to peculiarities of all these applications, many different types of transformers have been developed in the course of history. To simplify the overview of many transformer types, it is useful to have some kind of systematic classification. However, this is not easy to do because there are many ways of doing it. The transformer types could be classified according to their power rating, voltage, current, weight, type of cooling etc., but such approach would have a limited applicability. Probably the simplest and the clearest transformer classification is according to the number of phases in: - single-phase transformers - three-phase transformers In a three-phase system, the single-pha­se units are used in a bank of three transformers linked together. A single threephase transformer costs approximately 15% less and occupies less space than one unit of three single-phase transformers within the same tank. However, due to limitations during the manufacturing and mainly transportation, particularly of large units, the transformers w w w . t ra n sfo r m e r s - m a g a z i n e . co m

sometimes must be produced as singlephase transformers. Another reason for using a single-phase unit rather than a three-phase unit, is the possibility of having a fourth identical unit as a spare. Despite its simplicity and clarity, this type of classification does not overly help in classification of the whole transformers family.

Despite not being a perfect one, perhaps the most practical classification used by the industry is the one according to the transformer application. According to this approach, transformers can be roughly classified as: - power transformers - distribution transformers - reactors

Classification according to basic technology of a transformer design and manufacturing is also often used. There are two main technologies for designing and manufacturing the transformers: - core type - shell type

This classification could, however, raise some questions. There are no obvious technical reasons for classifying a transformer as a distribution transformer rather than a power transformer but it is widely used in practice, and it is helpful. The term “distribution transformer” is somewhat used in the IEC 60076, while it is commonly used in IEEE C57. Some companies define distribution transformers as the power transformers below 10 MVA. The 2.5 or 5 MVA limits are also used elsewhere instead of 10 MVA.

In a shell-formed transformer, the primary and secondary windings are quite “flat” and are positioned on one leg surrounded by the core. In a core-formed transformer, cylindrical windings are like “coils” and cover the core legs. However, this classification is also limited in the large portfolio of either of those two transformer types. Transformers can be classified according to the insulating/cooling fluid in: - liquid-filled transformers - gas-filled transformers (mainly with SF6) - dry-type transformers As the dry-type, and particularly gasfilled/insulated transformers have limited applications in a large power system, this classification is also not perfect.

The classification above is even more dubious with regards to reactors, because they are not transformers at all but are usually grouped with transformers because they share most of the technology with power transformers and they are designed and produced in the transformer factories. It took me some time at the beginning of my carrier to distinguish a reactor from a transformer and I believe that others had a similar experience. The classification by application will be used in this article.

Since transformers have been in use, many different types have been produced and consequently their classification is quite challenging!

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TECHNOLOGY TECHNOLOGYBASICS BASICS 3. Power Transformers Power transformers cover the population of the largest transformer units by means of power and voltage ratings. Manufactured units range up to 1500 MVA, and up to 1785 kV. Several large phase shifting transformers consisting of two linked units have been manufactured with a combined capacity of 2750 MVA.

Medium Power Transformers (MPT) This group includes transformers with a power range from 60 to 200 MVA (or 40 to 250 MVA), and a high voltage of up to around 275 kV.

3.1 Generator step-up transformers (GSU) are essential components of the power plants linking the plant generator to the transmission network. Built as three single-phase or three-phase units in the core or shell technology, they transform voltage from the generator voltage level up to the suitable transmission voltage level, which may go up to 800 kV nowadays.

There are several different classifications of power transformers according to their power and voltage ratings or size, and/or according to the application.

Classification according to size Classification according to size is a bit ambiguous because different companies use different power and voltage range for particular types. This discrepancy may be due to the fact that manufacturers divide their portfolio according to the market they supply so, in their specific case, other classifications are considered pointless. Utilities can also have different fleets; therefore a certain classification can be better for each utility. The ranges mentioned here do not render presumption of general validity. Large Power Transformers (LPT) This group covers the largest units in the power transformers population with power of normally above 200 MVA (limits used range between 100 and 250 MVA) and High Voltage (HV) of usually at least 220 kV. Within this group there is less competition in the market but technical problems such as insulation problems (high dielectrical stress), magnetic problems (high leakage flux), thermal problems (high heating due to operational losses), mechanical problems (high forces due to short-circuit currents), transportation (large dimensions and very heavy weight), etc. are extremely high.

- step-down transformers - phase-shifting transformers (PST) - HVDC converter transformers - transformers for industrial applications - traction transformers - mobile transformers - test transformers

Figure 2: MPT [4]

Small Power Transformers (SPT) Transformers from roughly 10 to 60 MVA and a maximum service voltage of 170 kV belong to this group. Other limits used are from 5 to 40 MVA and up to 145 kV.

GSU transformers usually have deltaconnected Low Voltage (LV) windings (energised by the generator), and star connected HV windings (connected to the transmission lines). The connection of such transformers is mainly YNd. They often operate continuously at full load facing variations in voltage due to changes of the load or the network requirement for reactive power. High rated currents, particularly in the larger units, require a good control of the magnetic field to avoid localised overheating. GSU transformers can be very heavy for high power rated units which need to deliver the entire power of the power plant to the grid.

Figure 3: SPT 12,5 MVA with On-Load Tap Changer [4]

Classification according to application According to the application, power transformers can be divided in several subgroups: - generator step-up transformers (GSU) - system intertie (interconnecting) transformers

Figure 4: GSU transformer 890 MVA [4]

Power transformers cover the highest power and voltage ratings within the transformers family, and can be classified into several categories. Figure 1: LPT 265 MVA, 525 kV [4]

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TRANSFORMERS MAGAZINE | Volume 1, Issue 1

Mladen BANOVIC, Jean SANCHEZ

Phase shifting transformers are among the most complex transformers. They are mainly used to balance the exchange between two networks over two parallel lines as precisely as possible.

3.2 System intertie (interconnecting) transformers connect AC systems of different voltage levels so that active as well as reactive power can be exchanged between them. They can have fully separate windings or electrically connected windings, in which case they are called autotransformers. Transformers with separate windings provide a galvanic insulation between the two voltage systems. Autotransformer, compared to a transformer with separate windings of the equivalent rating, is a more compact and economical solution. Typical voltage ratio between HV and LV is between 1 and 2 for autotransformers. However, as they have connected windings, there is no galvanic insulation between the two interconnected systems.

Figure 5: Autotransformer 250 MVA [4]

3.3 Step-down transformers can be classified as a variety of the system intertie transformers. Their purpose is a voltage transformation from the transmission voltage level down to an appropriate distribution level. However, with penetration of distributed generation energy direction can change, and when it happens a stepdown transformer becomes a step-up transformer. 3.4 Phase-shifting transformers (PST) are among the most complex transformers. Their purpose is to control power flow between the parallel power lines or cables; w w w . t ra n sfo r m e r s - m a g a z i n e . co m

or between the two independent power systems. To achieve this, they are designed not to significantly change the voltage magnitude but mainly its phase angle (hence their name). Voltage magnitude and angle are controlled by superimposing induced secondary voltage and some other voltage with necessary phase displacement to the main line voltage. When transformer has one active part, the additional de-phased voltage can be taken from the winding onto another core limb but two active parts are necessary for higher ratings so the other voltage is taken from the second active part. In such arrangement, the first unit is called the booster or series transformer, and the second magnetising, regulating, or shunt transformer. All those interconnections needed for the production of the desired voltage and phase displacement make the transformer more complex in all stages of manufacturing, from design to production and testing. In addition to these complications, the terminology is also not simple. Besides PST, other names such as phase angle regulating (PAR) transformer or quadrature booster are also used. The reference standard with dual IEC-IEEE logo is IEC 62032 Guide for the application, specification, and testing of phase-shifting transformers [5]. PSTs are sometimes considered a variety of the system intertie transformers as they are often installed in the substa­tions but from the application point of view, they can be classified as a par­ticular kind of power transformer.

Figure 6: PST 600 MVA, 230 kV [4]

3.5 HVDC converter transformers are in fact AC transformers. The name HVDC comes from the application in the HVDC converter station, which converts AC current and voltage to DC, and vice versa. Hence, a HVDC transformer is the essential component in the HVDC transmission system. Reasons for using the HVDC systems are loss reduction in some transmission lines, connecting AC systems with different frequencies, co­n­ necting non-synchronous systems or using underground or undersea long transmission lines.

Figure 7: HVDC converter transformer [4]

Due to operation in a converter station close to power electronic converters, the transformer is subject to DC electric stress and high current harmonics. Therefore, it has to be designed and manufactured with special consideration for DC insulation and harmonics and as a result, it contains much more solid insulation compared to a classic power transformer. The reference IEC standard is IEC 61378-2. Convertor transformers - Part 2 Transformers for HVDC applications [6]. 3.6 Transformers for industrial applications are used in the industrial plants for supplying high energy demanding objects like furnaces or converters. Those transformer power ratings are approximately 10 MVA but can be technically very complex due to specific needs and/or very high operational constraints. Furnace power transformers have to provide very high currents, close to shortcircuit currents, at relatively low voltages in the steel melting and the metallurgical industry. Capacity can range up to several hundred MVA with high LV currents, even more than 150 kA; and wide secondary voltage range. Due to extreme LV currents, the On-load tap-changer 27

TECHNOLOGY TECHNOLOGYBASICS BASICS (OLTC) is systematically placed on the HV side. The secondary load can be AC or DC.

capacity. Such compact design uses less material, which increases the losses and temperatures above the limits for the materials normally used like cellulose and mineral oil. Therefore, aramid, a polyamide in combination with high temperature resistant insulating fluids is used.

Figure 9: Traction transformer for a high speed train [4] Figure 8: Furnace power transformer 95 MVA [4]

Converter transformers face higher load current harmonics due to the distorted waveform caused by the semiconductor converters connected to the transformer. Typical applications are: rectifiers for large drives, electrolysis, scrap melting furnaces and inverters for variable speed drives. Other applications can be chemical electrolysis, DC arc furnaces, graphitising furnaces, traction substations, copper refining etc.

3.8 Mobile transformers are used when power needs to be supplied temporarily to a particular place, like in cases of a system failure, system maintenance, natural disasters, terrorist attacks, civil construction, etc. After system restoration, or completion of the construction work, the requirement for power supply will be significantly changed. Therefore, in such cases, a permanent substation is not an economical solution.

The main drivers for a design of a mobile transformer are weight and size to facilitate road transportation. Mobile transformers for mobile substations are manufactured up to a 100 MVA rating, and up to 245 kV. Those transformers can have many different voltages levels. 3.9 Test transformers are specific transformers designed for a given testing application in the high voltage industry. They can be technically very complex with special requirements regarding very high operating voltage, short-circuit withstand capability or ability to provide many different voltage levels on one unit. They are mostly used in test facilities of the high voltage equipment manufacturers, for example of power transformers or circuitbreakers.

4. Distribution Transformers Distribution transformers are those used in distribution of electricity close to consumers. Apart from substations, their us­ ual environments are buildings, shopping ce­ntres, data centres, industry plants, shi­ ps, the underground, under the water etc. We can see different classifications used by the industry. According to the coolant

The reference IEC standard is IEC 613781 Converter transformers – Part 1: Transformers for industrial applications [7]. 3.7 Traction transformers are used to supply traction (railway) system and vehicles (locomotives). Nowadays, there are different types of traction systems worldwide such as: - DC system (0.6 - 3 kV) - AC systems: - 12 kV 50 Hz - 15 kV 16 2/3 Hz (this system tends to decrease worldwide over the years) - 25 kV 50 or 60 Hz Traction transformers are supplied by stationary transformers (line feeder transformers, mainly single-phase substation transformer). Locomotives with DC drives require a three-phase rectifier transformer, also called an on-board transformer. These transformers can travel around as fast as 570 km/h, and it is not surprising that critical parameters for such transformers are weight, size and appropriate cooling 28

Figure 10: Mobile transformer [4]

Distribution transformers are those used in distribution of electricity close to consumers. TRANSFORMERS MAGAZINE | Volume 1, Issue 1

Mladen BANOVIC, Jean SANCHEZ

Apart from substations, usual environments for distribution transformers are buildings, shopping centres, data centres, industry plants, ships, the underground, under the water etc.

used, they can be classified as: - liquid-filled distribution transformers - dry-type transformers

4.3 Padmount transformers or padmounted transformers are ground mounted distribution transformers placed in a locked steel cabinet on a concrete pad. All energised parts are securely enclosed in a grounded metal housing so that the transformer can be installed in places that do not have room for a fenced enclosure.

This is not an exhaustive list of distribution transformer types but it is deemed enough for the purpose of this article.

Figure 15: Padmount transformer [4]

Figure 11: Liquid-filled hermetically sealed distribution transformer [4]

Figure 13: Substation transformer 5 MVA [4]

4.1 Substation transformers are transformers used in distribution substations which transfer power from the transmission system to the area distribution system. Transmission voltage level is up to 110 kV, while the distribution level is usually up to 36 kV.

4.4 Polemount transformers or polemounted transformers, as the name says, are mounted on utility poles. These transformers typically service rural and urban residential and commercial areas. In rural areas they typically supply households or farms, while in urban areas, they are used for industrial and commercial lighting applications. Due to weight restrictions, the polemount transformers are built for voltages up to 36 kV.

4.2 Unit substation transformers are used in commercial and industrial applications to convert distribution voltage to the utilisation voltage designed for an easy interconnection with primary and secondary switchgear, and for an indoor or outdoor placement. Figure 12: Dry-type transformer [4]

Liquid transformers can have a conservator or they can be hermetically sealed. According to the application, they can be classified as: - substation transformers - unit substation transformers - padmount transformers - polemount transformers - drives transformers - wind turbine transformers - underground transformers - subsea transformers, etc. w w w . t ra n sfo r m e r s - m a g a z i n e . co m

Figure 16: Polemount transformer [4]

Figure 14 : Unit substation transformer [4]

4.5 Drives transformers or Variable Speed Drive (VSD) transformers are used in speed regulation systems of the electric motors in many industrial applications, such as pumps, ventilators, compressors, rolling mills, paper machines and an innumerable amount of different machines used in manufacturing and other industries. They can be built as liquid-filled or dry-type, with the capacity of up to approximately 5 MVA. 29

TECHNOLOGY TECHNOLOGYBASICS BASICS underwater installations mainly in oil and gas industry. As they can be placed at depths down to 3000 m, this requires double barriers to compensate temperature and pressure changes associated with seawater. They are oil-filled with voltage rating of up to 72 kV and variable power rating. Due to the requirement for a maintenance-free service, natural heat of sea water convection is used.

Figure 17: VSD transformer for high speed compressor 6 MVA [4]

4.6 Wind turbine transformers steps up turbine generator output voltage from a few hundred volts to the collector system‘s medium voltage level. Besides restrictions and requirements for size, weight, and fire behaviour, they are exposed to the severe service conditions such as variable loading, harmonics, switching surges and transient over-voltages. Consequently, there are problems in the operation which lead to the use of insulating materials like aramid in combination with high temperature insulating fluids. They have characteristic slim form and are hermetically sealed. Liquid-filled transformers have capacity up to 4 MVA but dry-type transformers are also used up to 2 MVA.

circuit breakers, meters, etc. are housed. The capacity can range from small distribution units, to medium sized power transformers, up to 50 MVA. Larger units are used in the underground substations in cities and megacities with an extreme scarcity of space where high costs of land can justify the construction costs of an underground substation twice as high.

4.8 Subsea transformers are used in distribution systems to supply subsea equipment like pumps, compressors and other electrical components used in the

4.7 Underground transformers are placed completely below the ground level. They are designed for installation in an underground vault – a structure or a room where power transformers, network protectors, voltage regulators, 30

5. Reactors As mentioned above, reactors are not transformers but are included here because they share most of technology with transformers and are very briefly described. They have different applications, but here only two types are mentioned: - shunt reactors - series reactors Basically a reactor can be considered as a “one winding transformer” and can be either a single or three phase. Reactors are covered by the IEC standard IEC 60076-6 Part 6: Reactors [9]. They are rated in reactive power: MVAR.

Figure 19: Underground submersible transformer [4]

Figure 18: Wind turbine transformer [8]

Figure 20: Subsea transformer 1.6 MVA [4]

5.1 Shunt reactors are used in a power system to moderate the effect of voltage increase along the power line when the line is energised but is either not loaded or slightly loaded. This effect is called

Reactors are not transformers but are included here because they share most of technology with transformers.

TRANSFORMERS MAGAZINE | Volume 1, Issue 1

Mladen BANOVIC, Jean SANCHEZ

Shunt reactors are used in a power system to moderate the effect of voltage increase along the power line when the line is energised but is either not loaded or slightly loaded.

of transformer manufacturers [4], and other sources on the Internet [8], as well as IEC and IEEE standards. The authors would like to thank Mrs Kristina Holmstrom-Matses for the support and approval to use ABB photos in this article.

Bibliography:

Figure 21: Shunt reactor 30 MVAr [4]

Ferranti effect. This is because the line capacitance, which draws capacitive current, can cause voltage increase. One way to compensate that effect is by using the shunt inductance (reactor). That way, the energy efficiency of the system is improved. To improve the compensation of reactive power, a shunt reactor can have an OLTC with typical regulating range from 50 % to 100 % power. Shunt reactors can be connected directly to the power line (mainly several hundred kV) or to a tertiary winding of a transformer (mainly around 20 kV). 5.2 Series reactors or current limiting reactors are used in a power system to reduce short circuit currents with the aim to use circuit breakers with lower short circuit breaking capacity. Theoretically they could also be used to “adapt” the short-circuit impedance of a newly replaced transformer within an existing installation, mainly to adapt the shortcircuit capability of the circuit breaker, and to somehow achieve the same voltage drops as with the previous unit. They can be also used to limit inrush current of large motor drives. w w w . t ra n sfo r m e r s - m a g a z i n e . co m

Acknowledgement During the preparation of this article authors consulted books [10] and web sites

[1] International Electrotechnical Vocabulary, Chapter 421 : Power transformers and reactors [2] IEC chapters 1 to 21 of the 60076 Power Transformers Standards, IEC [3] IEEE C57.12.xx IEEE Standard for General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers, IEE Power & Energy Society, 2010 [4] ABB Group, www.abb.com, Courtesy of ABB [5] IEC 62032 ed2.0 Guide for the application, specification, and testing of phase-shifting transformers, IEC, 2012 [6] IEC 61378-2 ed1. Convertor transformers Part 2 Transformers for HVDC applications, IEC, 2001 [7] IEC 61378-1 Converter transformers – Part 1: Transformers for industrial applications, IEC, 2011 [8] Paul Anderson, Turbine Base and Transformer of Tower No 24, http://s0.geograph.org. uk/photos/79/81/798125_4ff07f9e.jpg, current 18.03.2014. [9] IEC standard IEC 60076-6 Part 6: Reactors [10] Handbook for Transformers, 3rd edition, ABB, 2010

Authors: Mladen BANOVIC obtained his PhD degree from the University of Zagreb in 2012. He leads PUCARO‘s research and development in transformer insulation and the editorial board of Transformers Magazine. He has been involved in the development of insulation systems up to 1200 kV and defining ABB‘s smart grid strategy covering transformer insulation and components. Prior to joining PUCARO, he led basic research of transformer insulation and transformer monitoring business. He also holds a degree in Electrical Engineering and postgraduate Master of Science degree from the University of Zagreb. Jean SANCHEZ completed a Ph.D. degree on power transformers fault diagnosis in 2011 and worked in a French power transformer reparation factory. His work involved many transformers designs, tests, fault expertise, power ratings, and OLTC repairs. Today he is working on generator diagnosis in a major French utility and is the executive editor of the Transformers Magazine. He also completed a Masters degree in Applied Physics. 31

TECHNOLOGY

Voltage Stresses on Solid-Liquid Insulation of Large Power Transformers Influence of dielectric tests on main insulation design ABSTRACT Due to the time cost replacement of a failed large power transformer, reliability and good performance in service are fundamentals for long-term operation. Insulation coordination studies are the base to evaluate the occurrence of the most important transients like overvoltages, system disturbances, lightning discharges and switching operations which reach the transformer terminals connected to the system. A set of dielectric tests are required to be performed at the manufacturer facilities as a function of the transformer insulation level according to the standards. Such dielectric tests are mandatory to define the main insula32

tion transformer design such clearances between windings, windings to core and leads as well. Therefore it is very important to understand how and where the voltages are distributed within the transformer during the dielectric tests. This article presents an overview about the voltage distribution of the main dielectric tests within the winding assembly of a three phase 40 MVA 138/13,8kV wye-delta regulating transformer.

Keywords insulation design, dielectric test, main gap, fieldplot TRANSFORMERS MAGAZINE | Volume 1, Issue 1

Juliano MONTANHA INTRODUCTION

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ower transformers are required to be designed for long term operation with good performance and reliability as they are one of the most important pieces of equipment in a power system. Therefore they should be designed to withstand all types of transients during their life in service. Most transients are caused by natural phenomena like lightning atmospheric discharge or caused by the system switching operations or disturbances. It is much too complicated and complex to represent such stresses by tests as the transients are basically dependent on system characteristics and the geographic conditions where the transformer is connected and installed. International committees have established workgroups consisting of Manufactures, Utilities, Generators and Academic Societies. These workgroups have published standards e.g. IEEE and IEC to provide a standardised testing methodology. These documents describe the controlled conditions to be simulated at manufacturer’s test field. They specify which tests must be performed on the transformer and the conditions for acceptance. They are known as dielectric tests. The type and level of the test is a function of the insulation level of the transformer specified by a customer. The dielectric tests and the application of appropriate protection devices defined by insulation coordination study have the aim to assure a reliable operation of the transformer without failure during its life in service. Therefore the transformer’s insulation design like core and coil assembly, and leads are submitted to Final Acceptance Tests at manufactures facilities to prove its ability to withstand different kinds of dielectric stresses. Once approved, the transformer is expected to operate in service with reliability for the long-term withstanding possible transients from the system.

Different types of dielectric tests are required to be carried out on transformers as per standards at manufacturers facilities to check the transformer design.

secondary windings are grounded and tank as well. Then the test is performed on secondary windings with the voltage source connected and the primary windings grounded. During this test an AC source is applied with approximately 2 times the rated voltage of on the transformer under test. Primary and secondary transformer voltage circuit is tested separately. When tested, all bushings of the same circuit (primary or secondary) are connected together, including neutral in a wye connection. The bushings from the other circuit are directly grounded. Figure 1. shows the equipotential lines during an applied voltage test of 230kV on the primary windings (high voltage) of a three phase wye-delta regulating transformer as an example.

This article presents an overview about the dielectric stresses on the main insulation of medium power transformers of 40 MVA designed with solid-liquid insulation. The insulation levels are given in Table 1. Table 1: insulation level of the transformer with full insulated neutral in kV

1a

Primary Secondary Wye connection Delta connection Phase Neutral Phase Rated Voltage 138 - 13,8 Applied Voltage 230 230 34 Induced Voltage 230  Lightning Impulse 550 550 110

Separate AC source withstand voltage test (Applied Voltage Test) The main proposal of this test is to check the major insulation between windings and each winding to ground. Each circuit is tested separately. For a two winding transformer, two tests are required. When the voltage source is connected to primary windings, the w w w . t ra n sfo r m e r s - m a g a z i n e . co m

1b Figure 1: Applied voltage test on high voltage, wye connection 33

TECHNOLOGY

Applied voltage test is an AC voltage test either on primary or secondary of the transformer. Higher voltage gradients appear typically between primary and secondary circuits.

Figure 2. shows the equipotential lines distribution during a lightning voltage test on high voltage terminal of a three phase wye-delta regulating transformer, same example of the Figure 1. As the neutral bushing is grounded and connected directly on regulating winding, the regulating winding assumes the potential zero. Figure 2b shows the voltage distribution at the peak of the voltage. Observe that the equipotential lines have different distribution in comparison with the applied voltage test. Now the stress between the HV and R windings is higher.

Lightning Impulse Test Impulse test system is necessary to produce the wave shape 1.2/50 µs as required by the standards. The generator is a very large piece of equipment composed of an RLC circuit in modular stages. The impulse generator is set up in various series and parallel configurations to achieve the wave form 1.2/50 µs and the required impulse voltage level. The test is applied individually to all terminals of the transformers. When one terminal is tested, the other bushings are solid grounded, grounded via resistances or grounded via a shunt for fault detection purposes. This is done for each terminal with the specified lightning impulse level.

During the lightning impulse test, transient voltages appear within the windings but this is not addressed in this article.

Lightning impulse test is a unipolar wave shape applied individually on each terminal of the transformer. Voltage distribution is different from applied voltage test.

2a

3a

2b Figure 2: Lightning Impulse test in a wye-delta transformer

3b Figure 3: Induced voltage test in a wye-delta transformer

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TRANSFORMERS MAGAZINE | Volume 1, Issue 1

Juliano MONTANHA Induced voltage test During this test an AC voltage source is connected on the transformer under test. Depending on the highest voltage of equipment under test, Um, a single or three phase test is required by the standards. Figure 3. shows as example a single phase induced voltage test in a three phase wye-delta regulating transformer, same example of Figure 1.

For AC dielectric stress in a solid-li­ quid structure of a large power tra­n­ sformer it is very important to know the voltage distribution in the wi­ ndings. The electrical field in the oil gaps is higher than in solid insulation because of permittivity ratio.

During the single phase induced voltage test, the neutral terminal is enhanced to 1/3 of the required voltage on line terminal to ground. This means that the voltage distribution is different from the applied and lightning impulse tests. The neutral has 33% of the potential of the voltage applied.

AC Design To determine the AC dielectric stress in a solid-liquid structure of a large power transformer it is very important to know the voltage distribution within the windings. This depends on the way the windings are connected into the test circuit. It is also important to define the clearances between the windings and windings and core.

State-of-the-art for Large Power Transformers design for AC system is composed of solid-liquid insulation. For transformers filled with mineral oil, the permittivity ratio between mineral oil and the pressboard is in the range of (ξóil: ξpsp = 1:2). This results in a higher electrical field in the oil gaps for AC stresses. For tests like switching and lightning impulse tests, the evaluation of the equipotential lines distribution is performed considering those wave shapes as an AC equivalent stress by using conversion factors.

Write for us Are you looking for a fresh challenge and want to get involved? Transformers Magazine is the primary source for the latest power transformers industry news, expert views, technology, and trends, with particular focus on green solutions and solutions for smart grid. We publish scientific and professional articles on topics covering all aspects of transformer R&D, design, manufacturing, testing, operation, maintenance, monitoring, diagnosis, fault investigation, decomissioning and more. More info: www.transformers-magazine.com/write-for-us.html

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TECHNOLOGY

Streamlines

PHOTO: Johnny Chan LinkedIn

Equipotential lines Figure 4: AC field plot in a core window section

Figure 4. shows one simulation for AC distribution between low and high voltage windings in a core window section. The equipotential lines are the lines where the charges are under the same potential. The electrical field intensity is proportional to the potential gradient. The streamlines are perpendicular to the potential lines and show the path and direction of the forces of the electrical field over the charges.

Streamlines are perpendicular to the potential lines. They indicate the path and direction of the forces of the electrical field over the charges.

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Author Juliano MONTANHA holds a degree in Electrical Engineering from University of São Paulo – Brazil from 1998 when he started working at Siemens Power Transformer factory in Jundiai - Brazil. Juliano has been working with insulation technology regarding main insulation design up to 800 kV, including leads design and winding assembly. He was responsible for winding assembly standardisation for local market in 2000. Juliano is a high voltage expert at Siemens Transformer Group and has participated on a worldwide R&D research with the Siemens transformers factories. Between 2011 and 2013, he was responsible for insulation design group at Jundiai Siemens factory. He was also responsible for implementation of 500 kV electrode in Jundiai factory as well as transient studies for Power Transformers design like VFT studies and failure investigations. Juliano technically supported the manufacturing process of HVDC units at Siemens factory in Jundiai and Siemens factory in México in 2010. He has been a member of IEEE since 2014. TRANSFORMERS MAGAZINE | Volume 1, Issue 1

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TRENDS TRENDS

ABSTRACT Basically, the life expectancy of a power transformer is quite long, known to be around forty years of service before it needs replacing. Nevertheless, during service life of a power transformer, it is necessary to constantly maintain it, mainly in order to make the transformer operate safely as long as possible. This paper will discuss the main diagnostic methods that could be performed during service life of a transformer. It will also attempt to provide the purposes of the main diagnostic methods carried out by different power transformer stakeholders.

A General Overview of Power Transformer Diagnosis When diagnosis is used and where to find raw data? Considering ‘open databases’. Introduction

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Keywords

resuming they operate normally, average life expectancy of power transformers is around forty years. The manufacturer’s transformer warranty is valid for one to a few years according the initial contract.

power transformers, diagnosis, failures, maintenance

After commissioning and expiration of the initial warranty, the transformer

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asset is under the full responsibility of its owner. To make transformers operate safely for as long as possible, it is sometimes necessary to assess its condition in order to prevent a major failure or any significant fault. Also, from the construction of a transformer, or a fleet of transformers, up to the end of its service life, many diagnoses may be performed and analysed.

TRANSFORMERS MAGAZINE | Volume 1, Issue 1

JeanSANCHEZ, SANCHEZ,Mladen Mladen BANOVIC BANOVIC Jean The last worldwide study on transformer failures was conducted in 1983 by CIGRE Working Group [1] and it produced results based on a precise population of transformers over period of ten years. The average failure rate was around 2 %. Some main points in the report indicated that On Load Tap Changers (OLTC) seemed to increase the risk of transformer failures and that more transformer failure data had to be gathered worldwide and assessed. A significant amount of transformer failure data analysis would statistically lead to more precise studies and appropriate recommendations about transformer failure prevention, maintenance and diagnosis to improve their service life expectancy. The worst kind of damage that could happen to a transformer is an explosion so we will discuss different ways of preventing such a disastrous event and other possible hazards.

Figure 1: Healthy substation three-phase power transformer [4]

Classic tests and information used for power transforElectrical tests mer diagnosis Failure and condition diagnosis of power transformers can be regarded as a way of testing an ability to withstand voltage and current over time by assessing all possible parts and components related to it. Two main test types are carried out on power transformers - the factory tests and the field tests. There are two types of factory tests: Quality Assurance (QA) Tests for the manufacturer use during the construction process, and the Factory Acceptance Tests (FAT) which are contractual between the manufacturer and the client. Those tests are often carried out according to the IEC standard 60076 [2] and are conducted in the client’s presence. Afterwards, most of the tests can be carried out onsite [3] but hardly any standard provides precise criteria for the result assessment. At best, they provide trends or slight indications, apart from the oil analysis that has been extensively studied. Finally, field test expertise is required in majority of cases in order to understand and diagnose potential or effective problems, especially if the previous test results indicated an incipient fault, a failure or any out of the ordinary condition of the transformer. w w w . t ra n sfo r m e r s - m a g a z i n e . co m

Electrical tests are divided in two groups: Low Voltage (LV) and High Voltage (HV) tests where the limit is set at 1 kV. Most HV tests, such as induced voltage tests, partial discharge (PD) test or lightning impulse test, require the usage of heavy or highly specialised test devices and the appropriate expertise to interpret the results. Those tests are mostly carried out in the factory and rarely onsite. On the other hand, LV tests, like winding resistance, insulation resistance, tangent delta and voltage ratio, are relatively easy to perform in the factory as well as onsite. Some recent tests with fingerprints over a frequency band (range) are becoming frequently used as they are easy to carry out, although the results are still not so

easy to interpret. These types of tests are the Frequency Response Analysis (FRA), from a few Hz to several MHz, mostly used for identification of winding mechanical deformation caused by transportation shock or short circuits, or dielectric spectroscopy, which is a tangent delta test from low (around 100 Hz) to very low (a few mHz) frequency, used to assess the moisture enclosed in the solid insulation of the transformer.

Chemical tests Two types of most frequently carried out chemical tests are: insulating oil tests and solid insulation tests when paper is accessible (very seldom except at the end of a transformer service life). Solid insulation tests estimate the “remaining life” by measuring the insulation Degree of Polymerisation (DP). There are many tests based on insulating oil (especially mine

Power transformers last for decades. Worldwide analysis of major failures could help towards enabling such long service life but it has hardly been updated in the last 30 years. ISSUE 1, VOLUME 1 | 39

TRENDS TRENDS

ral oil) studies but the best-known one is the Dissolved Gas Analysis (DGA). It is quite cheap compared to the transformer cost and has been proven to lead to a reliable diagnosis of the transformer‘s active part. The well-accepted standard for interpretation of the DGA is IEC 60599 [5], including the Duval triangle although further studies have shown some limitations.

Operational information An often forgotten but very useful source of information for any diagnosis is all the operational information such as: recent voltage levels, load history, overvoltages, short circuits, temperatures, protection activations if any, etc.

Different stages of power transformer service life and its diagnosis requirements The life of a transformer could be roughly divided into three main parts that would have different diagnostic requirements: the design up to the manufacturing stage, the onsite commissioning, and finally the service life up to the removal from service due to a devastating failure or failure prevention. Basically, the insurance companies have a choice to take action or offer advice during the service life stages although the insurance companies are mainly concerned with transformer transportation or reduction of service life risks.

Transformer diagnosis is practiced from the design stage to the end of service. Most of it is focused on how to manage the transformer service life safely for as long as possible. 40

From customer requirement to the Final Acceptance Test When a transformer customer specification has been submitted to a manufacturer, a design will be adopted. It may be of interest to the client to have design reviews with the manufacturer to check if all fixed parameters for the final product fit its operational needs. Then, the workshop performs some internal QA tests during the manufacturing to be sure the transformer assembly is going well, and diagnoses and resolves any existing problem. Finally, the FAT test gives an opportunity to detect any major or minor defects of the transformer under high constraints (i.e. voltages, temperature rises) at the transformer’s final construction stage; for example, problems in insulation, design, welding or bolting could appear and be fixed at the last stage.

From factory to commissioning A critical stage for transformer is the transportation to the site after the FAT tests and its commissioning. A form of monitoring can be arranged during the transportation in order to detect shocks. Some LV and chemical tests are carried

TRANSFORMERS MAGAZINE | Volume 1, Issue 1

JeanSANCHEZ, SANCHEZ,Mladen Mladen BANOVIC BANOVIC Jean out again for commissioning purpose prior to the first onsite energisation. These tests check if the transportation and the commissioning processes have caused any potential hazard following the FAT tests at the factory. Up to this point, all the diagnostic tests would have been carried out in order to find and fix any faults during the early stage of the transformer service life.

Transformer diagnosis periodicity is linked to its maintenance policy. All this combined could give more or less options to manage the asset easily over time.

From site to the end of service Basically, the transformer service life should be quite long, measured in decades, before removal from the service due to a failure or preventive action. Typically, the failure rate of power transformers follows a “bath curve” [6], as in French Transmission System Operator grid fleet.

gnosis carried out to locate the precise failure and suggest different ways of fixing it. This kind of maintenance and diagnosis policy can be a problem because the interventions to fix faults are usually quite expensive and some repair work, such as untanking or rewinding, can be very time consuming as most active parts cannot be fixed without untanking or opening the tank.

At this point, two main kinds of diagnostic tests can be carried out over time: condition diagnosis (off-line and/or online based) and failure diagnosis. The condition diagnosis is carried out periodically and preventively, and assesses the trend of different parameters (like oil analysis) to test a “steady” condition of the transformer. This kind of diagnosis leads to relatively minor and precise actions to prevent degradation and major failure from happening. On the other hand, some transformer owners carry out transformer diagnostic tests only when a failure has already developed to the stage impossible to ignore. This type of failure diagnosis is a “one shot” dia-

Every power transformer owner has a choice to follow the manufacturer maintenance recommendations or to set their own power transformer maintenance policy, like the main French electric utilities, which maintain large transformers fleets. For example, any transformer owner who wishes to set their own transformer maintenance policy could find many points to adapt in the Cigre Technical Brochure [7] guidelines. Almost every maintenance policy includes some tests (chemical oil analysis, sometimes electrical (LV), and visual inspections) but often disregards the operation parameter history that could potentially help to resolve many

problems. LV tests detect major electrical variations that should not normally happen during a transformer service life unless a major internal failure happens within the active part. Comprehensive oil analysis gives a large amount of useful information about the condition and age of the transformer through liquid insulation diagnosis and solid insulation by deduction. The formation and progress of slow occurring internal incipient faults like hot spots or paper degradation (compared to fast transients phenomenon) could be detected by DGA [8] and tracked over time. The quality of the mineral insulating oil, such as acidity or water content, can also be assessed [9] to diagnose complete condition of the transformer. Condition diagnostic test mainly serves the purpose of preventive and periodic maintenance. It analyses trends (i.e. monitors devices) and diagnoses various progressive faults of the transformers. On the other hand, a failure diagnosis is used to detect and locate a major problem such as a problem caused by the lack

Figure 3: Example of transformer fleet failure rate in function of its age [5] w w w . t ra n sfo r m e r s - m a g a z i n e . co m

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In the future some kind of technical transformer open database would allow any party to conduct focused studies on transformers life management and perhaps, steer this field in new directions.

of maintenance. In most cases, it implies significant actions to fix internal problems that could have been prevented or minimised through periodic analysis.

From one transformer to a large number of transformers, a fleet management Most power transformer owner companies have a whole fleet of transformers. In order to apply or adapt maintenance policies to the whole fleet, two main points must be considered: transformer condition and failure analysis of the existing fleet throughout its service life [10], and Health Index that compares all the transformers through the analysis of many tests and information which result in a unique score by apparatus [11]. Both methods enable the owner company to prioritise actions needed for the specific transformers in the fleet.

Free power transformer data Many methods of power transformer diagnosis are studied worldwide but the feedback on failure data is extremely difficult to gather and study. The last known large study was conducted 30 years ago [1], and new attempts to update it were abandoned due to lack of data provided by the utilities and transformer users worldwide. Any detailed information, even anonymous, regarding failure and operation of transformers can be firstly organised internally and then submitted externally to any groups (e.g. CIGRE) working on transformer failures and reliability. There will always be room for more precise studies in this field and perhaps in the future, some kind of technical transformer open database would allow any interested party to conduct those kinds of studies, and then steer the transformer life management field into new directions. 42

Conclusion Power transformer diagnosis is the combination and interpretation of many different kinds of data (electrical, chemical, operational, and offline information) from every stage of the transformer’s life. From its correct assembly, to its service condition diagnosis and prevention maintenance to make it operate for as long as possible. At the final stage, the purpose of failure diagnosis is to locate a fault, mostly within the active part of a transformer, and provide often expensive options to repair it or the advice about safe usage up to the end of its service life. Just as the world electrical consumption is still increasing, so does the mostly aging transformer fleet. All this will allow the transformer diagnosis to be a promising and useful field in the years coming! Sharing and improving open knowledge about transformers globally and across many industries has become possible

in the last few years with the professional open forum on the Internet [12].

References [1] An international survey on failures in large power transformers in service, CIGRE Electra N°88, pp 21-42, 1983 [2] IEC chapters 1 to 21 of the 60076 power transformers standards, www.iec.ch [3] Service Handbook for Transformers, ABB, 2007 [4] Dr. Maik Koch, Neuer Leistungstransfor­ mator, http://commons.wikimedia.org/wiki/File:­ Leistungstransformator_neu.jpg, current 21.03.2014. [5] IEC 60599, Mineral oil-impregnated electrical equipment in service – Guide to the interpretation of dissolved and free gases analysis, 2nd edition, 2007 [6] Transformer Refurbishment Policy at RTE Conditioned by the Residual Lifetime Assessment, CIGRE A2-204, R. Blanc et al., 2008 [7] Guide for Transformer Maintenance, CIGRE Technical Brochure 445, 2011 [8] Zhenyuan Wang, Artificial Intelligence Applications in the Diagnosis of Power Transformer Incipient Faults, Ph.D. at Virginia Polytechnic Institute and State University, 2000 [9] IEC 60422, Mineral insulating oils in electrical equipment – Supervision and maintenance guidance, 3rd edition, 2005 [10] William H. Bartley P.E., Analysis of Transformer Failures, International Association of Engineering Insurers 36th Annual Conference, 2003 [11] Ali Naderian Jahromi et al., An Approach to Power Transformer Asset Management Using Health Index, IEEE Electrical Insulation Magazine, 2009 [12] Transformers Group on LinkedIn since 2008, www.linkedin.com/groups?gid=772397

Authors Jean SANCHEZ completed a Ph.D. degree on power transformers fault diagnosis in 2011 and worked in a French power transformer reparation factory. His work involved many transformer designs, tests, fault expertise, power ratings, and OLTC repairs. Today he is working on generator diagnosis in a major French utility and is the executive editor of the Transformers Magazine. He also completed a Masters degree in Applied Physics. Mladen BANOVIC obtained his PhD degree from the University of Zagreb in 2012. He leads PUCARO‘s research and development in transformer insulation and the editorial board of Transformers Magazine. He has been involved in the development of insulation systems up to 1200 kV and defining ABB‘s smart grid strategy covering transformer insulation and components. Prior to joining PUCARO, he led basic research of transformer insulation and transformer monitoring business. He also holds a degree in Electrical Engineering and postgraduate Master of Science degree from the University of Zagreb. TRANSFORMERS MAGAZINE | Volume 1, Issue 1

TRANSFORMER IN GRID

ABSTRACT For availability reasons, many generating utilities keep in storage custom designed spare transformers, readily available and identical to their critical large power transformers. However, in case an exact replacement is not available, the only option the generating utility has is to search for a substitute transformer that, as a minimum, is able to offer a temporary solution. The purpose of this paper is to indicate the most important aspects to be considered when checking the interchangeability of such substitute transformer, based on authors‘ experience and on various standards requirements.

Keywords power plants, power transformers, interchangeability. 44 44 | JULY 2013

TRANSFORMERS MAGAZINE

Relu ILIE, Isidor KERSZENBAUM

Issues to Consider when Substituting Large Power Transformers in Generating Stations INTRODUCTION

T

he power transformer is a reliable device, yet not failurefree. It has no rotating parts, consequently neither the typical faults of rotating machines. On the other hand, the large transformers are oil-immersed and suffer from other faults, mainly chemically or electrically related. A transformer internal fault may be very difficult to locate and to repair. In many cases, the owner may decide that it is faster and cheaper to buy a new transformer than to repair an old damaged one. Even when the repair is worthwhile, it may last for many months. Almost all large transformers used in power plants are customdesigned. Keeping in storage suitable spare transformers is a common practice, for the purpose of avoiding long unplanned outages and high economic losses. In case an exact replacement is not available, the only option the generating utility has is to search for a substitute transformer that, as a minimum, is able to offer a tem-

If a critical power plant transformer fails and an identical spare is not available, the generating utility will search for a substitute transformer as a minimum temporary solution.

When designing a new plant, the selection among these alternatives is generally based on consideration of some form of strategic reserve, as well as available space. Two half-size transformers may be selected in place of a single full-size transformer in order to reduce the cost of the spare. For similar reasons, a generator transformer may be made as a bank of single-phase units. Normally the cost, mass, and loss of such solutions are larger than for a single three-phase transformer; however, they may be preferable if transport size or weight limits apply. The three single-phase transformers provide independent magnetic circuits (see dedicated section below), representing high magnetizing impedance for zero-sequence voltage components. Therefore, a delta equalizer winding is normally provided, implemented by external connection between phase units. Some layouts may further complicate the spare transformer availability, like UAT with three-winding design.

DIMENSIONS AND WEIGHT Any physical size and weight limitations should be checked, for example for installation on an existing foundation. Special installation space restrictions may influence the insulation clearances and terminal locations on the transformer.

porary solution that may introduce some operational constraints. The goal of this paper is to mention the most important aspects to be considered when checking the interchangeability of such substitute transformer. The discussion is based on the various requirements included in relevant American and European standards. The most common generating station arrangements are shown in Fig. 1: a unit generator-transformer block configuration, and a unit generator-transformer with generator breaker. The vital large transformers in a power plant are the unit step-up/main/generator transformer (UT), the auxiliary transformer (UAT) and the station service/reserve transformer (SST). A failure of any one of these transformers may lead to unit shutdown or start-up unavailability.

GENERAL LAYOUT The UT may consist of a single three-phase unit, two half-size three-phase units, or three single-phase units. This is the most evident aspect to consider when a substitute transformer is needed. WWW.TRANSFORMERS-MAGAZINE.COM

a) Unit generator-transformer block configuration

b) Unit generator-transformer with generator breaker

Fig. 1. Common generating station arrangements.

UT and UAT are connected to the generator through isolated phase bus ducts. The high/extra high voltage terminals of the UT may be connected to gas insulated switchgear. The medium voltage connections to plant auxiliaries are also normally done via the rigid, non-segregated phase bus bars or cables. All these aspects must be checked and solved when looking for a transformer replacement. ISSUE 1, VOLUME 1 | 45

TRANSFORMER IN GRID RATED FREQUENCY In the unlikely situation a transformer designed for a different frequency is considered, the following applies. The rated frequency radically affects the transformer design and operation. The general formula of voltage e induced by a variable flux φ in a coil with N turns is e = -N dφ/dt. Assuming a sinusoidal flux φ = Φm cos ωt, the induced voltage becomes e = ω N Φm sin ωt. Its rms value will be, in terms of core cross section A and flux density Bm [1]: E = 2π/√2 f N Φm = 4.44 f N A Bm.



(1)

According to (1), operating at 50 Hz a transformer designed for 60 Hz means that the same voltage can be achieved only by a substantial increase in the flux density. The core iron will become heavily saturated, the excitation current will rise and also the hysteresis losses (proportional to the area of the hysteresis loop), which could severely overheat and damage the laminations. When operating at 60 Hz a transformer designed for 50 Hz, there is too much iron in the core. The hysteresis losses will be higher than the 50 Hz designed value (because iron volume and increased frequency), thus decreasing the efficiency. More importantly, the eddy currents losses will heat up the laminations, because they tend to increase as the square of the frequency. Operation of a 50 Hz rated transformer at 60 Hz may be sometime possible, but it may have to be derated from its nameplate MVA rating [2], depending on the new versus rated voltage.

MVA RATING The substitute transformer rated MVA is obviously a first parameter to consider. When a new plant is designed, the UT rating is chosen to ensure that it will not represent a bottleneck of unit capability, under any possible operating condition. However, a substitute UT poses a different perspective: using a smaller MVA rating than the original one means generating unit derating, nevertheless it is normally preferable to a complete shut-down, taking into consideration the long time needed to obtain an exact replacement.

If the substitute transformer rated power is lower than the original transformer one, its suitability shall be checked according to standards and operating restrictions may apply. In the case of a transformer (especially UT) substitution, it is essential to pay attention to the differences among the various standards about the definition of rated power. The IEC 60076-1 [3] definition implies that rated MVA is the apparent input power, received when rated voltage is applied to the primary winding and rated current flows through the terminals of the secondary winding; the output power is, in principle, the rated power minus the power consumption in the transformer (active and reactive losses). Differently, by the North America conventions (IEEE C57.12.80 [4]), the rated MVA is the output power that can be delivered at rated secondary voltage. 1

If the transformer nameplate mentions a certain rated power and if it was designed in conformity with the American standards, the significance is that its primary (connected to the generator) would be able to receive a higher than rated power. If for instance, the UT impedance is 15%, it should be able to accept a primary MVA higher up to about 15% than the rated one, including load and magnetizing losses (in the worst conditions of lagging power factors). Exact values can be obtained by using the equivalent circuit of the transformer. Contrarily, such intrinsic capability will not be available in transformer exhibiting the same rated MVA, but designed to IEC requirements. The situation inverts under leading power factors (Mvar absorbed from the system) but this is a less likely regime, especially when a substitute transformer is involved. A transformer may be able under some limitations to carry loads in excess of nameplate rating. In the case of a substitute transformer, the expected regime is a long-time emergency loading that may persist for months. Both IEEE and IEC have standards specially dedicated to such loading aspects (IEC 60076-7 [5], IEEE C57.91 [6]). The application of a load in excess of nameplate rating involves accelerated ageing and risk of premature failure, for instance: deterioration at high temperatures of conductor insulation, other insulation parts and oil, overheating of metallic parts, increased gassing in the oil, high stresses in bushings, tap-changers, connections and current transformers, brittleness of gasket materials. In general, the larger the transformer the more vulnerable it is. Both above mentioned standards give similar maximum temperature limits that should not be exceeded in case of long–time loading beyond the nameplate rating: current 1.3 per-unit, winding hot-spot temperature 140 ºC (instead of 120 ºC at normal loading), top-oil temperature 110-115 ºC (instead of 105 ºC at normal loading). The increased ageing rate due to a hot-spot temperature of 140 ºC is 17.2 times higher than normal1 for upgraded paper insulation and 128 times higher than normal1 for non-upgraded paper insulation [5].

RATED VOLTAGES Finding a substitute transformer with suitable primary and secondary rated voltages is a challenging task, difficult to be selected intuitively. Additionally, a transformer design for a certain voltage determines the size of the core and has a significant impact on the overall transformer size and cost. The system and transformer medium/high/extra high voltage ratings are well standardized and correlated in ANSI C84.1 [7] and IEC 60038 [8]. However, the generators standards do not specify standard series, neither preferable values for the rated stator voltage. The generator stator voltage rating is normally fixed by agreement, and in many cases it is simply the generator manufacturer decision, according to its available design. Therefore, it is quite difficult to match between UT/UAT primary rated voltage and generator rated voltage. The next section deals in details with transformers having rated voltages lower than expected operational voltages. Contrarily, it may be possible to use transformers with rated voltages higher

“Normal” means here a relative ageing rate of 1.0, corresponding to 98 ºC for non-upgraded paper and to 110 ºC for upgraded paper [5]

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than the operational ones, providing that the rated current will not be exceeded. Practically in such case the substitute transformer might be oversized and then unsuitable, because of larger core size and longer insulation distances. Ideally, it is desirable for a generator to be able to absorb Mvar to its limit when the system voltage is at its highest expected level and to produce Mvar to its limit, when the system voltage is at its lowest expected level. This is seldom possible for a fixed tap setting in the UT, and thus a compromise may have to be made by selecting the appropriate tap rating to meet the most likely operating condition. It is important to note that the reactive power consumed by the UT will absorb a significant part of the generator Mvar output under most conditions.

Finding a substitute transformer with suitable primary and secondary rated voltages is a challenging task; various boundaries can be analyzed using graphical representations.

References [13] and [14] propose other graphs, more complete but also more complex, which take in consideration additional restrictions, such as: generator maximum excitation limit, generator under-excited reactive ampere limit, UT limits (at lower than rated tap voltage), turbine MW limit, and auxiliary bus-bars (motors) voltage limits. Such graph shows the reactive power transferred to/from the grid (at UT high voltage side) as a function of system voltage, and forecast the area of allowable operation. This graph can be also obtained in a spreadsheet using suitable equations (Fig. 3).

OVERVOLTAGE LIMITS The purpose of this section is to show how low the substitutetransformer rated voltages may be in order to withstand the highest expected voltages on its terminals. The standards define the maximum winding voltage based on the insulation withstand capability (in IEC 60076-3 [15] it is called the highest voltage for equipment, while in ANSI C84.1 [7] it is the maximum system voltage)

IEEE C57.116 [9] recommends selecting the main parameters of UT (rated voltages, rated MVA, impedance, and over-excitation) by portraying graphically their effect under various operation conditions. While this method is mainly dedicated to a new plant project, it may be also used in case of a substitute transformer. A typical graph (Fig. 2) shows the change in generator voltage with generator reactive load, for various constant transmission system voltages. The graph allows predicting the Mvar capability (both lead and lag) at any given system voltage, keeping the ±5% generator voltage limits, as required by IEEE C50.12 [10], IEEE C50.13 [11] and IEC 60034-1 [12]. The example in Fig. 2 was built on a spreadsheet using equations based on transformer phasors diagram [9]. For a potential substitute transformer, such graphs should be drawn for any available tap/ratio at anticipated MW, and analyzed in order to choose the most suitable tap voltages (according to the forecasted system voltage profile) that will pose minimum restrictions to unit operation: range of available Mvar, synchronization ability, etc. Of course, a substitute transformer may not allow a full range of loading; normally some limitations (e.g. in reactive capabilities) may be acceptable. Fig.3. Reactive power transferred to/from system as a function of system voltage (operation area limit).

Fig. 2. Change in generator voltage with generator reactive load for various system voltages. w w w . t ra n sfo r m e r s - m a g a z i n e . co m

According to (1), the ratio between the voltage and frequency (V/ Hz) of the system to which the transformer is connected determines the nominal flux density at which the transformer operates (assuming the number of turns at a particular tap will remain constant). Normally, the transformer is economically designed to operate at as high as possible flux density, while avoiding saturation of the core. System frequency is normally controlled within close limits; thus, the system voltage is the main factor responsible for over-fluxing (over-excitation). When the V/Hz ratios are exceeded, saturation of the magnetic core of the transformers may occur, 47

TRANSFORMER IN GRID and significant stray flux may be induced in non-laminated components that are not designed to carry flux. This can cause severe localized overheating in the transformer and eventual breakdown of the core assembly and/or winding insulation. As mentioned before, the IEC 60076-1 [3] definitions imply that rated voltage is applied to primary winding, and the voltage across the secondary terminals differs from rated voltage (defined in noload condition) by the voltage drop/rise in the transformer. Contrarily, by IEEE C57.12.00 [16] the rated output is delivered at rated secondary voltage; according to IEEE definition, allowance for voltage drop has to be made in the design so that the necessary primary voltage can be applied to the transformer (at a secondary load lagging power factor of 0.80 or higher). For instance, a UT with a impedance of 15% (typical range: 13–17%) designed according to IEEE shall withstand continuously primary voltages as high as 110% of the rated value when fully loaded at a power factor of 0.80 (value calculable using the transformer equivalent circuit). Such hidden capability will not be available in the case of a transformer which follows the IEC requirements. According to IEC 60076-1 [3], a transformer shall be capable of continuous operation up to 105% voltage or V/Hz (the standard meaning is per each winding). IEC 60076-8 [17] adds that this is not meant to be systematically utilized in normal service, but should be reserved for relatively rare cases of emergency service under limited periods of time. IEEE C57.12.00 [16] defines the over-flux capability in a different way: secondary voltage and V/Hz up to 105% of rated values when load power factor is 0.80 or higher (lagging). Taking into account the previous considerations, this additional requirement may result in 10-15% primary overvoltage (and over-excitation) for a fully loaded generator step-up transformer (UT) built under IEEE. Fortunately, generators are only capable of continuous operation until 5% above or 5% below their rated voltage, by [10]-[12] (i.e. V/Hz until 105% at generator base) so the above overvoltage capabilities are rarely exploited. For UAT and SST the impedance is usually smaller than for UT, they often work not fully loaded and the primary voltage will normally not rise by more than a few percentage points for a secondary increase of 5%.

dedicated V/Hz protection exists). Both IEC 60076-1 [3] and IEEE C57.12.00 [16] allow continuous operation at no load at a voltage or V/Hz up to 110% of the rated values. For the particular case of transformers connected directly to generators in such a way that they may be subjected to load rejection (i.e. configurations without generator breaker), [3] has an additional requirement: to be able to withstand 1.4 times rated primary voltage for 5 seconds.

CONNECTION ARRANGEMENT AND PHASE DISPLACEMENT Normally the high voltage system is grounded, leading the UT’s high voltage windings to be star connected, with the neutral often solidly grounded. The reason for this is mainly related to equipment insulation level and system protection requirements. The significant consequence as regards the transformer high voltage windings is the possibility to use non-uniform, cheaper insulation systems (i.e. the neutral terminal insulation is designed with a lower insulation level than assigned for the line terminals). On the other hand, it is convenient to have the low voltage windings delta connected (the delta circuit provides a path for third-order harmonics of the magnetizing currents, thus reducing the voltage waveform distortion; it also stabilizes the neutral point potential in the case it is left ungrounded). The most common connection / angular (phase) displacement used for large UTs is YNd1 [9], [14], however YNd11 is also encountered (Fig. 4).

a) YNd1 b) YNd11 Fig. 4. Most common connection/phase displacement used for UTs.

Overvoltage limits as defined by various standards have to be carefully understood and examined against the operation conditions to prevent any incompatibility. Another aspect related to an overvoltage condition is whether the UT and UAT will be subjected to load rejection. Sudden loss of load can subject these transformers to substantial overvoltage. If saturation occurs, substantial exciting current will flow, which may overheat the core and damage the transformer. A sudden unit unloading during a fault may be caused by the clearing of a system fault and, hence, the machine may be at the ceiling of its excitation system; the unit transformer may be excited with voltages exceeding 130% of normal [18], [19]. With the excitation control in service, the over-excitation will generally be reduced to safe limits in a few seconds; with the excitation control out of service, the over-excitation may be sustained and damage can occur (unless 48

a) Dyn11 b) Dyn1 Fig. 5. Most common connection/phase displacement used for UATs.

For similar reasons, commonly the UAT primary windings (connected to the generator side) are delta connected, while the secondary ones are star connected through a current-limiting resistor. The most common connections used for UAT are Dyn11 [14] or Dyn1 [9] (Fig. 5). According to IEEE C57.12.00 [16], in Yd or Dy transformers the low voltage shall lag the high voltage by 30°; the connection Yd1 for step-up transformer matches this standard, while Dy11 for step-down transformers does not; howTRANSFORMERS MAGAZINE | Volume 1, Issue 1

ever, a standard Dy1 UAT with phase sequence externally reversed on both sides, as explained below, is equal to Dy11. The IEC standards do not have such restrictions.

The connection configurations, phasor group (angular displacement) and layout terminal marking (sequence) differ according to American or European standards. If the UT is YNd1 and the UAT is Dyn11, the medium voltage auxiliary system has zero-phase shift compared with the high/extra high voltage system (Fig. 1a). During unit start-up or shut-down, the medium voltage busses fed from the UAT and SST secondary windings are briefly paralleled (by the fast transfer scheme), so both must be in phase. Additionally, the high and extra high voltage systems are always in phase so the SST must produce zerophase displacement and therefore, usually it is a star-star as YNyn0 transformer. In same cases it will have a delta-connected tertiary winding, for the reasons mentioned above. For modern transformers, it is matter of the grid configuration and/or protection requirements whether the SST is provided with a delta tertiary or not. The connection configurations and phasor groups are drawn in Fig. 4 and 5 according to IEC 60076-1 conventions [3]. Since that standard mentions that terminal marking on the transformers follows national practice, Fig. 4 and 5 use the American practice IEEE C57.12.70 [20]. If the original transformer should be replaced by a non-identical substitute, matching the three-phase connections and phase-angle relations may be a complicated or even an impossible task. Normally it is not possible to substitute a UT or UAT having the vector group number 11 with a vector group number 1 transformer (or the opposite); the reason was explained above: these two transformers link between two rigid phasor systems. Only in the case of a unit equipped with generator breaker (Fig. 1b), it may be possible to use a YNd11 UT instead of a YNd1 one (or contrary) assuming no rapid transfer is performed to other auxiliary bus-bars. However, such substitution will affect the secondary circuits (at least the differential protection) and changes will be required in relays settings and/or matching by intermediate transformers. It is recommended to check also any potential influence on synchronizing circuits; it is a good practice to use supervision sync-check relay with two or three phase-sensing circuits. Theoretically, it is possible to keep the original vector group while using a different group transformer. For example, an UAT with vector group 11 may be used in place of an original device with vector group 1 (or inverse), by reversing the phase sequence on both sides of the transformer. Such change is shown in Fig. 6a: as viewed from the external line connections, the Dy11 transformer became a Dy1 one. Unfortunately, the rigid isolated phase bus bars on the generator side and non-segregated bars on medium voltage side will not allow such cross-connections in most cases. The substitution may be complicated by the transformer layout terminal marking and sequence. According to IEEE C57.12.70 [20], w w w . t ra n sfo r m e r s - m a g a z i n e . co m

the terminals are marked as in Fig. 7a, i.e. the H1 lead is brought out as the right-hand terminal as seen when facing the high voltage side. Other countries may use different standards; for instance, the English practice is to locate the high voltage terminals from left to right when facing that side [21]; the German DIN 42402 rule is the terminals are arranged from right to left as viewed from the low voltage side [22] (Fig. 7b). For instance, if an UAT designed according to the German standard with a vector group Dy11 is installed in place of an original UAT designed according to an American one, without any change in the external connections, it will externally appear as a Dy1 transformer (Fig. 6 b). Taking in account both vector group and terminal sequence aspects, an Yd1 American transformer is interchangeable with a European Yd11, without any external modifications.

TAPS AND TAP-CHANGER The original transformer may be equipped with on-load tapchanger or de-energized tap-changer. Normally a substitution for a limited period of time with a fixed turns-ratio transformer will be possible in an emergency. When choosing the most suitable tap of the substitute transformer, it is indispensable to check whether it is a full-power tapping (e.g. suitable for a current equal to the rated power divided by the tap voltage). By IEEE C57.12.00 [16], whenever a transformer is provided with de-energized taps, they shall be full capacity taps. Transformers with on-load tap-changer shall be capable of delivering rated MVA at the rated voltage and on all taps above rated voltage. However, for taps below the rated voltage, they shall be capable of just delivering the rated current related to the rated voltage (i.e. these taps may be of reduced MVA, unless specified otherwise). By IEC 60076-1 [3] all taps shall be full-power taps, except when specified otherwise.

a) Dy11→Dy1 b) Dy11→Dy1 By reversing the phase sequence By using a substitute transformer on both sides of the transformer with different terminals layout Fig. 6. Modifications of transformer phase displacement.

Fig. 7. Transformer layout terminal marking.

Almost all transformers used in power plants are at least equipped with de-energized tap-changers. Sometimes, especially in 49

TRANSFORMER IN GRID case of units equipped with generator breakers, it is difficult to ensure the unit auxiliaries suitable voltage under any operation regime. To overcome this problem, UAT with on-load tap-changer may be required. In some cases, UT or SST are equipped with on-load in place of de-energized taps to allow for large variations in transmission system voltage.

IMPEDANCE When designing a new plant, the selection of transformer shortcircuit impedance (in fact the reactance, the resistance being negligible for large transformers) is subject to conflicting demands: low enough to limit the voltage drop and to meet stability requirements, but also suitable high to set the system short circuit levels according to economic limitations of the switchgear and other connected plant. If a generator breaker is used, regulation of the UAT with the generator offline should also be considered. These aspects should be also considered for a replacement transformer.

Different impedance of replacement transformer can impact on switchgear capability, voltage drop, and parallel operation.

A slightly different turns ratio and/or different impedance (resulting in different regulation) between the substituted transformer and the other two will result in voltage unbalance and thus, negative sequence voltages and currents. This condition will not lead to an increase in generator neutral current, although it introduces other problems discussed elsewhere in this paper. However, different winding capacitances to ground between the substituted and the other two transformers will result in an increase in neutral generator currents, with possible adverse impact on the protective scheme of the generator neutral. Fig. 8 shows the typical arrangement of a large generator grounded at the neutral via a single phase grounding transformer with a resistive load connected to the secondary winding, sized to reduce any fault current to about 15 to 25 Amps. The sizing of the neutral resistor is a simple calculation that can be found in any good book on generator protection or in the standard IEEE C62.92.2. It is mainly dependent on the value of the capacitance to ground of the generator as well as all other equipment connected to its stator leads. Fig. 8 shows the capacitances to ground of all windings and the isolated phase bus for each of the three phases. In the figure, the capacity to ground of the UT and UAT(s) are lumped in a single delta-connected component.

That means that large MVA at low impedances may require significant bulky transformers, and permissible transport limits of dimensions and weight may be reached. It is at this stage that the use of single-phase units may need to be considered.

Under normal conditions, all the capacitances to ground are balanced among the phases and the neutral current is very close to zero. However, if a substitute one-phase transformer is installed with different capacitance, the circuit becomes unbalanced, and the neutral current will grow. Finding the value of the unbalanced current requires solving the unbalanced circuit by any of several available methods. One such method requires the delta connection of capacitors to be replaced by its wye (star) equivalent. All series resistances and reactance are neglected. Then, Fig. 8 can be simplified to the circuit shown in Fig. 9. In the circuit, all the variables are vector quantities, with exception of Rr which is the grounding resistance referred to the primary side. Voltages are assumed balanced and generator / isolated phase bus capacitances to ground equal in each phase. Given that the capacitance reactance to ground is much higher than the series impedance of the windings/buses, these last are disregarded. Following, and using superposition, the total neutral current IN can be calculated by solving for each phase a circuit as shown in Fig. 10 (example for phase A) and then summing together.

UNBALANCE WHEN USING SINGLE PHASE UT

Following the calculation of the neutral unbalanced current, proper setting of the neutral protection can be done.

Since reactance is a result of leakage flux, low reactance is obtained by minimizing leakage flux and doing this requires a large core and an expensive transformer. Conversely, if high reactance can be tolerated, a smaller core can be provided and so a less expensive transformer. It should be noted that since the rated MVA (S) appears in the numerator of the expression for percentage impedance (z%) calculated from its ohmic value (Zohm), the value of percentage impedance tends to increase as the transformer rating increases:

z% = Zohm x S / U2. (2)

In addition to all the issues discussed in this paper that must be considered when selecting a substitute transformer, single-phase transformer replacement from a three-phase step-up unit introduces an additional concern: generator neutral current unbalance. Neutral current of the main generator is monitored in many power plants, in particular those with large generating units. Neutral current can be the result of a ground fault on the generator stator windings, on the circuit (isolated phase bus or cables) between the generator and UAT(s) and UT, or on the delta-connected windings of the UT or UATs. Upon exceeding certain amplitude, the generator neutral current alarms and might also trip the unit, by an overcurrent relay in the neutral circuit, or, by an overvoltage relay connected across the neutral resistor. 50

Fig. 8. Equivalent Circuit of the generator, isolated phase bus, and the delta-connected windings of the UT and UATs. TRANSFORMERS MAGAZINE | Volume 1, Issue 1

nally separate V/Hz protection. The curves that define generator and transformer V/Hz limits should be coordinated to properly protect both. When the transformer rated voltage is equal to the generator rated voltage, the same V/Hz relay that is protecting the

Protection circuits may be required to be modified and protections settings shall be eventually adapted when using a substitute transformer.

Fig. 9. Simplified circuit for calculating the unbalanced neutral current.

OVERCURRENT CONSIDERATIONS The mechanical force and the thermal short-circuit requirements described in transformer standards are normally satisfactory for UTs. However, the UAT must be designed to mechanically and thermally withstand the environment in which it operates. The standard requirements for network applications may be not be adequate for certain types of three-phase through-faults on the secondary of the UAT, because of the following: slower dc component decrement, longer short-circuit duration, possibility of higher primary voltage subsequent to breaker trip (load rejection) in block schemes [9]. Another eventuality is the fast transfer of load from UAT to SST (or vice-versa), which may lead, under certain conditions, to high-circulating currents flowing through the two transformers exceeding their mechanical design capability [9]. In general, examination of the aforementioned requires consulting with the manufacturer.

SECONDARY CIRCUITS When using a substitute transformer, the protection circuits may be required to be modified because the transformer’s rated power changed, and/or bushing current transformers have different turns ratio, and/or differing secondary current or burden capabilities, etc. Substitute-transformer’s bushing current transformers, with different rated secondary current, ratio or burden than those of the original one, may lead to saturation and wrong operation of the differential protection. If the turns ratio or vector group of the alternative transformer is different from the original unit, it should be taken into account regarding current transformers ratio and connections. Some differential relays (mainly numerical) can internally accommodate the phase shift of the transformer, or differences in these ratios. Other relays (mainly electromechanical) do not have this versatility, and pose difficulties or need external auxiliary current transformers. In case of transformer replacement it is important to verify adequacy of the transformer over-excitation protection. Often, protection and output limiting functions are provided in the generator excitation equipment, but it is a good practice to apply additiow w w . t ra n sfo r m e r s - m a g a z i n e . co m

generator may be set in such a way that also protects the transformer. In some cases, however, the rated transformer voltage is lower than the rated generator voltage and common protection may not be applicable; in such a case, it is desirable to provide supplementary protection for the transformer..

Fig. 10. One of the three circuits to be solved for calculating the neutral unbalance current.

ADDITIONAL CONSIDERATIONS Additional aspects that should be checked when deciding for a transformer substitution are: - Details of type and arrangement of terminals, for example connections to overhead line, isolated phase bus, or cable box or gas insulated bus bar. - Isolated phase bus ducts with accompanying strong magnetic fields that may cause high circulating currents in transformer tanks and covers resulting in excessive temperatures (when corrective measures are not included in the design). - Transformers operation in parallel (e.g. half-sized UTs), needing careful matching of phase-angle, ratio and impedance in order to avoid circulating current risks [17]. - Unusual voltage conditions including transient over-voltages, resonance, switching surges, etc. which may require special consideration in insulation design. - Derating due to high harmonic load current. - Unusual environmental conditions (altitude, special cooling air temperature, explosive atmosphere, etc.). - Details of auxiliary supply voltage (for fans and pumps, tapchanger, alarms, etc.). - Sound-level restrictions. - Losses level (usually not relevant in case of an emergency situation transformer substitution).

CONCLUSIONS The large transformers used in power plants have particular characteristics and specific custom designs. Keeping in stock spare transformers identical to the critical ones in operation is an expensive strategy; however it ensures the lowest risk. The advantageousness of this policy increases in the case of multiple 51

TRANSFORMER IN GRID standardized generating units (one example of interchangeable generator transformer is detailed in [23]). Checking the suitability of a different substitute transformer is a complex task. As a first feasibility check, confirm the transformer rated frequency fits your grid, preliminarily look for an available transformer ratio close to the ratio of secondary system voltage to generator rated voltage (about ±5%), and roughly appreciate the transformer largeness, heaviness and available power. Following is a partial list of the main parameters that must be checked: - UT secondary tap voltage should not be lower than 95% of the grid’s maximum expected voltage (this value typically equals the rated system voltage). - UT primary (low) voltage should not be lower than 95% of generator rated voltage for transformers designed according to IEEE C57.12.00, and not lower than generator rated voltage for transformers designed according to IEC 60076-1. - UAT primary voltage should normally match the rated generator voltage; a lower voltage (up to 95% of generator rating) may be possible only after investigation of the risk for over-excitation. - Check connection and phase displacement suitability. - Analyze MVA rating suitability. If taps below the rated voltage shall be used, check their MVA capability. - Transformer rated voltages higher than the expected operating ones mean mandatory reducing the MVA to avoid exceeding the rated current. - Check dimensions and weight suitability. - Analyze the unbalance that may be introduced when replacing a single-phase transformer from an UT made of three sing-phase transformers. - Estimate transformer short circuit impedance influences. - Confirm through fault and fast load transfer capability in case of UAT/SST replacement. - Verify the implications on secondary circuits (mainly protection and synchronizing). - Thoroughly analyze the synchronizing and loading capabilities for any available tap, under various operation conditions, using graphs as in Fig.2 or Fig.3. - Check the transformer suitability for parallel operation (if pertinent to the particular layout). - In the eventuality of two units connected to the same bus, one of them having a substitute UT (with different MVA or ratio or impedance), the possible effect on generators different behavior should also be considered.

REFERENCES [1] M. G. Say, Alternating Current Machines, 4th edition, Wiley, 1976, pp.14, 76. [2] B. Lawrie, “How does frequency affect transformer operation“, EC&M Magazine, Dec. 1992. [3] Power Transformers – Part 1: General, IEC Standard 60076-1, Ed. 3.0, April 2011. [4] IEEE Standard Terminology for Power and Distribution Transformers, IEEE Standard C57.12.80-2010, Dec. 2010. [5] Power Transformers – Part 7: Loading Guide for Oil-Immersed Power Transformers, IEC Standard 60076-7, First Ed., Dec. 2005. [6] IEEE Guide for Loading Mineral-Oil-Immersed Transformers and Step-Voltage Regulators, IEEE Standard C57.91-2011, March 2012. [7] American National Standard for Electric Power Systems and Equipment - Voltage Ratings (60 Hertz), ANSI Standard C84.1-2006, Dec. 2006. [8] IEC Standard Voltages, IEC Standard 60038, Ed. 7.0, June 2009. [9] IEEE Guide for Transformers Directly Connected to Generators, IEEE Standard C57.116-1989, Sep. 1994. 52

[10] IEEE Standard for Salient-Pole 50 Hz and 60 Hz Synchronous Generators and Generator/Motors for Hydraulic Turbine Applications Rated 5 MVA and Above, IEEE Standard C50.12-2005, Feb. 2006. [11] IEEE Standard for Cylindrical-Rotor 50 Hz and 60 Hz Synchronous Generators Rated 10 MVA and Above, IEEE Standard C50.13-2005, Feb. 2006. [12] Rotating electrical machines – Part 1: Rating and performance, IEC Standard 60034-1, Ed. 12.0, Feb. 2010. [13] A. W. Goldman, “Selection of Generator Step-up Transformer Ratings“, IEEE Trans. Power Apparatus and Systems, vol. PAS-100, No. 7, pp. 3425-3431, July 1981. [14] A. W. Goldman, C. G. Pebler, Power Plant Electrical Reference Series, Vol. 2, Power Transformers, EL-5036-V2, EPRI, 1987. [15] Power transformers – Part 3: Insulation levels, dielectric tests and external clearances in air, IEC Standard 60076-3, Ed. 3.0., July 2013. [16] IEEE Standard for General Requirements for Liquid-immersed Distribution, Power, and Regulating Transformers, IEEE Standard C57.12.00-2010, Sep. 2010. [17] Power Transformers – Application Guide, IEC Standard 60076-8, First Ed., Oct. 1997. [18] IEEE Guide for Protecting Power Transformers, IEEE Standard C37.91- 2008, May 2008. [19] IEEE Guide for AC Generator Protection, IEEE Standard C37.102-2006, Feb. 2007. [20] IEEE Standard for Standard Terminal Markings and Connections for Distribution and Power Transformers, IEEE Standard C57.12.70-2011, Feb. 2012. [21] M. J. Heathcote, The J&P Transformer Book, 13th edition, Elsevier, 2007, p. 457. [22] ABB Switchgear Manual, 11th edition, Cornelsen, 2006, p. 559. [23] Modern Power Station Practice, Vol. D Electrical Systems and Equipment, British Electricity International, Pergamon Press, 1992, pp.253-257.

Authors Relu Ilie received the MS degree in electrical engineering from the Technical University of Iaşi, Romania. He is in charge of Power Plant electrical systems and equipment for the Israel Electric Corporation. Relu Ilie has 30 years of experience in generation and transmission field, mostly dedicated to electrical machines issues: design and failure analysis, maintenance and inspections, monitoring and testing, technical specifications, life extension, condition assessment, etc. (email: [email protected]). Isidor “Izzy” Kerszenbaum received the BSc EE from the Technion – Israel Institute of Technology, and the MS and PhD (EE) from the University of the Witwatersrand. His employment experience includes working as large electric machines design engineer for GEC in South Africa, and manager of R&D for a power distribution transformer manufacturer (ITC) in California. He also worked as protection engineer and large apparatus consultant with three different utilities. He is a past chairman of the PES Electric Machines Committee. Currently Dr. Kerszenbaum is chairman of the IEEE Working Group 10 on Online Monitoring of Large Synchronous Generators.. He is a Fellow of the IEEE. Dr. Kerszenbaum spent 2002 in the US House of Representatives as a IEEE-AAAS Science and Technology Fellow. TRANSFORMERS MAGAZINE | Volume 1, Issue 1

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JULY 2013 | 53

DIAGNOSIS

A fault is not a failure

Economical and reliable transformer maintenance by holistic interpretation of insulating oil condition ABSTRACT The importance and cost of transformers is described and stressed in all the literature. Transformers are one of the most important and vital links in the electricity supply chain. Despite the very high cost related to transformer failures, the tests and diagnoses of the equipment itself, the transformer owner or a person responsible for its proper operation is faced with many diagnostic approaches and most of them are either inaccurate or unnecessary and irrelevant to individual case. Of course the lack of specialists in transformer diagnoses causes engineers in charge of transformer maintenance to rely on test and service provider companies. Naturally, the main target of those companies is to promote their business. It is very important to only carry out the tests that will not interfere with transformer operation and interpret the results holistically in the transformer exploitation context, manufacture, internal organisation politics and many other parameters. Missing one or several parts of the testing can lead to completely wrong overall diagnosis.

Keywords

transformer oil analysis, dissolved gas analysis, risk management, corrosive sulfur. 54 54 |

ISSUE 1, VOLUME 1

Introduction A few misconceptions about the transformer industry: - „If my old transformers are still in good condition, it is not necessary to worry about replacing them.“ - „Transformer technology is the same as it was 50 years ago; same steel, same oil and same cellulose.“ - „We intend to keep an old transformer in operation without maintenance until it fails, than replace it with a new one.“ - „The testing guidelines published in some of the standard guides also apply to my transformers.“ All those misconceptions can lead to very costly outcomes, e.g. replacement of a good quality old transformer by a less robust new transformer, or a dramatic failure scenario. Transformer maintenance is commonly compared to medicine, and as such, it has to be based on reliable tests and diagnoses which must be carried out by a specialist who is familiar with the latest technologies and issues regarding transformers and who is able to recognise specific conditions of the specific piece of equipment. Like human beings, transformers differ one from the other, even the ones made by the same manufacturer and in the same housing, and the twins. TRANSFORMERS MAGAZINE

Marius GRISARU mandatory, e.g. the former IEC60422 allowed 0.5, acidity. Back then, almost all oil types were suitable for any transformer. 2. With such variety of insulating oils today, it is very difficult to establish the differences and advantages of so many oil types and liquids. Of course, the oxidation stability coupled with potential of sulfur corrosion and additives are the main concerns for all industries due to too many recurring incidents related to these problems. The transformer purchaser chooses the liquids manly according to the initial price which can lead to very costly outcomes. However, the price is not a good criterion as some of the most expensive insulating liquids can also damage transformers. The only solution is to have the correct unbiased knowledge. 3. Solid insulating materials are also challenging to implement. Although there are very durable insulation materials, this is not always the best choice pricewise. The designer and the transformer owner have to know and understand the needs and problems of all materials and their compatibility. Transformer maintenance is more of a subjective art then an accurate science. Since 1885, transformers have been responsible for transportation of the electrical energy from the power station to the end user. Since then, the principle of transformer operation has not changed significantly. However, many researchers still continue the efforts to make them more efficient, economical, and environmentally friendly. These features are often results of conflicting requirements for high reliability and improving operation conditions. In the new competitive environment of post-privatisation age, transformers should transmit more MVA with less insulation for longer periods of time. Compared to dry transformers, oil filled transformers have the advantage of greater possibility of oil tests. All those parameters impose a real dilemma for the transformer owners and contribute to discrepancies in proper maintenance, reliable electricity supply and shareholders’ profits. The transformer technology and demands change dramatically these days and the transformers are no more „low tech“.

V.V. Sokolov et al. [1] stated that more than 70% of transformer condition insight can be gathered from oil analysis. Most transformer owners worldwide rely mainly on oil tests to diagnose abnormal conditions in transformer operation. Oil tests may even be the only planning method for transformer operation to some utilities and users.

Today a much more compact transformer has to transform more energy at higher voltages and in significantly less space. These requirements impose a continuous search for new materials, new designs and new maintenance strategies to enable longer transformer operation.

Major roles of oil inside the transformers are: - Oil is a part of insulating system along with cellulose. - Oil must have acceptable dielectric properties. - Oil has to flow through the internal parts of the transformer.

This paper will focus on insulating materials inside the transformers. Those materials are responsible for the majority of failures and catastrophic events but, nevertheless, can be monitored during transformer operation:

Like human beings, transformers differ one from the other, even the ones made by the same manufacturer and in the same housing, and the twins.

1. Choosing the right oil type for each transformer type. The variety of insulating oils increase and change constantly. 50 years ago, users had access to a very limited assortment of oil types, such as the well known PCB. Inhibited mineral oil with non-strict demands, such as low oxidation stability and lower breakdown voltage, was also available. At this time, the oil test limits were not WWW.TRANSFORMERS-MAGAZINE.COM

ISSUE 1, VOLUME 1 | 55

DIAGNOSIS - Insulating oil has to efficiently transfer transformer heat. - Oil has to safely store important data about transformer health. - Transformer oil should remain in an acceptable condition for the most of transformer life, at least 25 years. - Transformer oil should not harm the environment. - Insulating oil should not ignite easily. - Transformer oil has to dissolve impurities and sludge at maximum length. - Mineral insulating oil should not have lubricant properties. Routine oil tests are used to obtain maximum information with minimum investment. In addition, there are supplementary tests and special tests that can indicate a specific condition of the oil or the transformer. Oil testing process is divided into three separate parts: - sampling - analysing - diagnostics or conclusions based on the results and condition of the transformer All those steps should be performed according to only one standardisation body, e.g.: IEC, ASTM or other regional or national standards. IEC transformer oil sampling and test standards are precise and the user who abides by these standards can be assured that they will end up with reliable diagnosis. Other standardisation bodies also provide valuable sampling and testing procedures. The users should always make sure they follow all the recommendation and instructions for sampling via testing and up to the final diagnosis. Although most of the steps are clearly indicated, the quality of the diagnosis depends on the experience of the laboratory staff and the person who determines the final diagnosis. In most cases, the electrical and physical tests are performed only after receiving a warning sign from the oils tests.

DGA Sulfur Oil tests Furan DP

SFRA, FDS Dissipation factor

Megger Ratio IR thermo tests Acustics VB monitoring FB PD

=

DC

Figure 1: Correct interlacement of all the tests can avoid unpleasant situations

Transformer oil specification and limit values were changed dramatically during the last few years. Those changes reflect the need for an improved final product that will fulfil demands from transformers. In the past, the insulating oil contained more natural antioxidants that prolonged their life but affected the electrical properties. The new generations of transformer oil meet more stringent electrical property requirements and are refined accordingly. Now more users swap non-inhibited products to oils that include antioxidant inhibitors. Those oils have better electrical properties and provide better oxidation stability. However, they 56

depend more on the antioxidant content. The abundance of aromatic molecules and non-carbon atoms in modern oil is reduced to improve the electrical properties.

2. Transformer oil tests are categorised in two major groups: Oil tests for evaluation of transformer condition: Dissolved gas analysis, Furan compounds in oil, dissolved metals, and oil tests for evaluating oil conditions: 2.1 Dissolved gas analysis (DGA) is the single most popular and most indicative test for evaluation of condition of the transformer in service [2]. Along with the evident advantages, a DGA technique has a few important drawbacks. Here is a brief description: The main goals of DGA-based diagnosis methods are: - to suggest the severity of the failure and to plan maintenance - to establish a prognosis, which consists of inferring the apparatus diagnosis and possible evaluation of the malfunctions later on. Since these methods are prognostic, the results will have subjective aspects. Transformer data with significant impact on DGA evaluation are: - tap changer type - manufacture and construction technology - oil type - oil preservation system type - loading condition - maintenance Dissolved gases can be present in transformer oil due to following reasons: - natural aging of the materials - oil oxidation - gases generated due to transformer overload - gases generated due to a fault - other specific causes Major DGA advantages compared to other diagnostic methods are: - can be used to check almost all transformer internal components. Like the blood in living beings, the oil reaches all internal transformer parts. - sensitive method; capability to detect incipient transformer malfunctions using highly sensitive detectors. - data gathering; the test reveals problems than occurred during the sampling intervals. - economical sample intervals; due to gas production mechanism, it is possible to test the health of the unit once per year. Major DGA disadvantages are: - The technique is sometimes too sensitive. - According to Transformer Cigre WG [3], 30% of data regarding transformer health cannot be revealed by the DGA. - Some rapidly occurring failures damage the equipment before they can be detected by the DGA, even with the newest online monitors. DGA has to be performed by skilled personnel from the sampTRANSFORMERS MAGAZINE | Volume 1, Issue 1

y = 0,081x + 648,647 R² = 0,830

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y = 8,074x R² = 0,978

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10 11 12 13 14 15 16 17 18 19

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Figure 2: Comparison of repeatability of 2 DGA measurements, Head space according to IEC60567, 7.5 and portable device.

2000

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Portable Device

Area

8000

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7

number of tests

Ethane

y = 15,211x R² = 0,992

7 8 9 10 11 12 13 14 15 16 17 18 19 number of tests

8

y = 0,069x + 254,910 R² = 0,721

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7 8 9 10 11 12 13 14 15 16 17 18 19 number of tests

CO Repeatability CO Repeatability

12000

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The only way to overcome these difficulties is to use gas in oil mixtures with known concentrations. Although the existing standards describe methods for preparation of such mixtures, most of the users avoid preparing them due to the procedure complexity. Commercial gases in oil mixture are very limited and expensive, and do not provide reliable drawing calibration curves. Nitrogen

H2 Repeatability

100

Concentration [ppm]

ling phase up to the diagnosis. All test stages are susceptible to error. Some important aspects: - sampling: Glass Syringe is preferable oil container. The oil has to be representative, i.e. enough oil has to be allowed to flow before the sample is taken because in cases of active failure, the sampling can assist in locating the source of gases. - analysing: There are many methods of testing dissolved gases in the oil. Each method can measure gas in the oil differently as each method involves different extraction or uses different measurements. The basic assumption that gases in oil obeyed to Ostwald coefficients in all measure techniques was proved to be incorrect by a different researcher [4], [5]. Apart from the methods described in the standards, some portable and online devices use nonstandardised techniques. Detailed review of some of those techniques can be found on the CIGRE report [6]. Figure 2. shows a comparison of repeatability of a portable device vs. the standard head space method.

0

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Acetylene Ethylene Methane Hydrogen

150 50 0,5 0,5 0,5 0,5 0,5 0,5 1

Detection limit

4585 14276 139,5 109,5 109,5 104 106 111,5 109,5

Commercial gases

30211 9040 178 113 129 118 111 111 107

Concentration measured by IE

Figure 3: Calibration curves at Israel Electric and comparison to gas in oil commercial mixtures

w w w . t ra n sfo r m e r s - m a g a z i n e . co m

57

DIAGNOSIS Table 1: Comparison of DGA test methods and current IEC and ASTM

DGA method

Calculation

IEC 60567

ASTM D3612

Compatibility & Remarks

Vacuum extraction by partial degassing

Ostwald coefficient gas in oil and gas peaks calibrated by gas in gas

7.3

A

Yes

Stripping extraction method

Efficiency coefficients and gas peaks calibrated by gas in gas

7.4

Multi-cycle vacuum extraction using Toepler pump apparatus

Absolute volume of gases and gas in gas calibration

7.2

No

Best and absolute method

Headspace method

IEC calibrated by gas in oil standards

7.5

C

No







ASTM by Ostwald coefficients and gas peaks calibrated by gas in gas

The repeatability and accuracy of method is variant due to difference in injection and calculation methods.



Suitable for factory test

Yes

B

Not suitable for factory test

Not suitable for factory test





The laboratory or transformer expert should issue a qualitative diagnoses based on reliable results from representative oil from a specific transformer. Here are some options:

According to this concept, we can reveal most transformer faults and avoid devastating failures. Here are some examples:

6000 5000 TCG [ppm]

- to use any of the existing mathematic models or software to prepare the transformer diagnosis. - to use internal prepared models for diagnosis. - to take advantage of personal experience based on as many successful failure diagnoses as possible.

TCG concentration vs. date 7000

4000 3000 2000 1000 0 24.5.02

10.12.02

28.6.03

14.1.04

1.8.04

Date

Table 2: Diagnosis methods and related problems

Method Principle Key gas

Gas identification and proportion

Rogers

Ratios of 3 combustible gases

Donnenberg

Ratios of 4 combustible gases

Figure 4. Case 1 Transmission transformer, DGA + findings

IEC 599

Other ratios of same 4 gases

Duval Triangle

Graphic Monograph – Triangle

Figure 4. shows a minor failure that was discovered by an accurate DGA test so serious consequences were avoided. This case emphasises the importance of taking accurate measurements.

IEEE C57-106

Absolute concentration and increase rate

Japan method

Graphic Monograph – Rectangular

PowerGen Method

Score for each gas, ratio and other oil tests

Artificial intelligence Different calculations approach still research CIGRE TF11

Calculations of 90% values for transformer



„families“ – with similar design and operati



on conditions.

58

Most transformer owners worldwide rely mainly on oil tests to diagnose abnormal conditions in transformer operation TRANSFORMERS MAGAZINE | Volume 1, Issue 1

Date

16.12.2001 12.6.2003 11.2.2004 26.3.2004 30.3.2004 13.4.2004 10.5.2004 5.8.2004 18.10.2004 17.5.2005 15.5.2005 29.8.2005

TG 6 5 7 8 6 6 7 8 7 9 4 8

H2

26 44 43 375 350 346 332 259 199 112 60 728

CH4 49 66 77 110 104 103 113 114 117 121 60 155

C2H2 C2H4 C2H6 0 0 0 139 123 129 115 31 12 1 1 187

34 39 44 70 67 67 71 69 67 71 37 91

24 29 36 34 32 32 32 30 31 42 22 43

CO 423 825 796 863 797 788 907 876 880 882 429 884

CO2 1482 4125 3973 4438 4294 4160 4254 4869 5120 5115 2781 8400

Figure 5: Case 2 double winding failure. The failure was revealed twice.

Figure 5. shows an interesting case which confirms that any sign of acetylene indicates a failure. If the transformer still operates after Buchholz relay there is a chance it could suffer a devastating failure.

issued by the contracted laboratory. One of the conventional practices in the industry is the cross section test, i.e. sending the same oil to several laboratories and comparing the results.

2.2 Oil quality tests described by main standardisation bodies

The quality manager of each laboratory should make effort to estimate the results of each oil test in the laboratory if they are also performed by any other quality accredited laboratory.

Table 3. displays some major differences between IEC [7] and ASTM [8]. The user has to know the exact method in order to test the oil. Each method can generate different values accordingly. 2.3 Proficiency tests for insulating oil tests Proficiency test or Round Robin comparison is the single most important tool for laboratory performance evaluation. The transformer owner has to be totally confident in the result

Proficiency tests are carried out by 19 worldwide laboratories, some of them well known and accredited and others without accreditation and poor quality control. Figures 10. and 11. show laboratories that take the test quality into account and achieve results much closer to the average. It is not surprising that only dissipation factor shows similarly scattered results within both laboratories groups. Three other analyses are more sensitive to the quality of the procedures and the operator skills.

Table 3: Comparison of major test methods for insulating oil of the 2 major standardisation bodies.

Parameter

Test type

IEC

ASTM

Compatibility

Break down voltage

Routine

IEC 30156

D1816 in service

No, different condition test

D877 new oil



Water in oil

Routine

IEC 60184

D1533

Yes, partially dependent on top oil temperature?

Acidity

Routine

IEC62021-1

D664

Potentiometric

Potentiometric

IEC62021-2 Colorimetric

D974 Colorimetric

Partially, different condition tests but results on the same scale if the tests are properly carried out No, absolute method for acidity

Dissipation factor

Routine

IEC60247

D924

No, different condition test

Antioxidant

Routine



IEC60666

D2668

content

Partially, IEC more sensitive and accurate

Interfacial tension

Complementary IEC

D971, to be changed

D971

Yes, accurate with automatic equipment





Routine

w w w . t ra n sfo r m e r s - m a g a z i n e . co m

59

DIAGNOSIS

100% 90%

80%

15,6

0,0856

44,78

0,2692

8,2

0,010468

70% 60%

50% 40% 30%

15,14

20%

0,013738

laboratories as possible. - Proficiency test is the only available option to control transformer oil measurements. - Accredited laboratories achieve less scattered results than the non-accredited ones. - Scattered results can also occur in accredited laboratories. - Round Robin points to inaccuracies and restores the quality of oil measurements.

10% 0% BDV

Acid

water

tan delta

range accredited range non accredited

Figure 6: Graph showing scattered results from the accredited laboratories and the non-accredited ones with poor quality test programme.

Scattered values can end in erroneous diagnosis regardless of how sophisticated the software that makes those recommendations is. Some laboratory results can cause incorrect costly maintenance or leave the transformer untreated. The figures represent the limits according to IEC 60422 standard: fair and poor values. We can make the following conclusions from this study: - For measurements without obtainable standard materials, the only “true value” is the average obtained from as many qualitative

0,25

2.4 Furan compounds, product of cellulose decomposition in oil Measurements: according to IEC 61198 and qualitative laboratory performance with controlled output. The laboratory has to perform comparison (proficiency test or PT) with other laboratories at least once a year. Frequency: - transformer up to 30 years of age, once every 2 years - transformer over 30 years of age, once a year - new transformer should be tested a month after energising - a month after each oil treatment Although furan compounds measurement has been performed for more than 15 years, no reliable diagnosis has been developed by any standardisation body regarding the DGA and oil analysis. In furan diagnosis, the experience and knowledge of the expert who performs the diagnosis is much more important.

Dissipation factor for oil C 0,35

0,2

0,2

Acidity for oil B

0,3 0,25

0,15

0,1

0,1

0,15

0,1

0,1

0,05

0,05 0

0

30

0,2

0,2

Water content of oil H

120 100

25 20

20 15

Breakdown voltage for oil E

15

80 60

10

40

5

20

0

0

60

50

Figure 7: Comparison of some oil tests within 19 laboratories.

60

TRANSFORMERS MAGAZINE | Volume 1, Issue 1

3. Correlation between oil parameters Based on reliable oil tests at Israel Electric laboratories, we correlate different parameters such as the acidity and dissipation factor with the concentration of the anti oxidant inhibitor for 400 transformers. Not surprisingly, we found that if the inhibitor content is above 40% of the initial level, then both parameters will be in good condition without an exception. If the inhibitor has been consumed, then the acidity and dissipation factor can be abnormal. No correlation between water content and inhibitor content was found. We found that furan compounds appear only in cases of high moisture and high acidity. Tg Delta vs. Inhibitor content of the transformer oil 0,45

Tg Delta values are displayed up to 0.4 % inhibitor

Tg Delta inhibitor content [%]

0,4

Tg delta

0,35 0,3 0,25 0,2

- The transformer life can be improved by: - preventing impurities inside the transformer - keeping humidity and oxygen away by proper sealing - maintaining the inhibitor level - efficient cooling

4. Potential sulfur corrosion Sulfur corrosion is one of the most researched subjects in the transformer industry since the beginning of the millennium. Several major organisations such as Cigre [9], Doble [10], Sea Marconi [11], Terna [11], and others have already published extensive reports on this topic. According to the reports, the DBDS substance causes sulfur corrosion. The oils at Israel Electric were tested for DBDS in 2005 and no evidence of this substance was found. In 2009, we discovered that some of our important GSU transformers suffer the consequences of sulfur corrosion despite no presence of DBDS in the oil. Figures 9. to 11. show several internal copper parts covered by the product of sulfur corrosion phenomena.

0,15 0,1 0,05 0

1

31

61

91

121 151 181 211 241 271 301 331 361

The data is shown from the low to high Inhibitor value

Values of acidity vs. %of Inhibitor content of the oil 0,40

% inhibitor

Acidity [mg KOH/gr] Inhibitor content [%]

0,35

mg KOH/gr

0,30 0,25

Figure 9: LV side (10.5 KV) flexible connection from a GSU transformer affected by sulfur corrosion

0,20

0,15 0,10 0,05 0,00

1

31

61

91

121 151 181 211 241 271 301 331 361

Data arranged from low to high values of inhibitor content

Figure 8: Inhibitor Survey in Israel Electric

Correlation between oil quality and DGA and Furan: - Humidity, elevated temperature, electrical and mechanical disturbance, and impurities in oil affect the transformer isolation (oil and paper). Gases and furans are generated according to the fault severity. - During the rapid oil ageing, the inhibitor disappears and consequently the acid rises. The process continues with the production of sludge, the sludge being deposited on the hot area inside the transformer and the whole process is intensified by the positive feedback. The sludge prevents heat dissipation through the oil. - The acid and sludge are one of the factors that affect the dielectric integrity of the oil/paper isolation. Destruction of the cellulose produces water that amplifies the aging process. w w w . t ra n sfo r m e r s - m a g a z i n e . co m

Figure 10: Typical contaminated bolt from a GSU transformer affected by sulfur corrosion

We discovered the phenomena before the failure occurred due to intensive electrical and chemical testing. The affected parts were examined by Electronic microscope. Figures 9. and 10. show some of the findings on the affected wires. We conclude that the mechanism of sulfur corrosion in our oil type is different from the mechanism suggested by CIGRE WG A2-32 [11]. The explanation of the sulfur corrosion mechanism in oil without DBDS is still in progress. Also the mitigation of this phenomena and monitoring of the severity will be discussed in the new CIGRE WG A2-40, to be published in its 2014 report. 61

DIAGNOSIS

Wire racture 1Date:16/04/2009 08:06:03Image size:1024 x 768Mag:5000xHV:10.0kV

Scan Date:16/04/2009 08:07:25 th.:0.39kcps

HV:10.0kV

Puls

Figure 11: Metallurgic test. Copper wire on SEM

Conclusion: How to improve chemical service? [2] IEC standard 60569, 1999 Our formula for optimal chemical service in the electrical industry: - comparison to standard materials and other laboratories abroad - International Standard Societies membership and community support from fellow labs - maintaining quality database for each transformer - good communication with transformer owners and clients - dedicated and reliable service The best DGA method for special tests such as heat run test, is a multi-cycle vacuum extraction using Toepler pump apparatus but IEC head space, 7.5., is the most sophisticated and time consuming for DGA routine. Out of the routine oil tests, the most important one is the antioxidant content test if the oil contains it, followed by the moisture, BDV, dissipation, IFT, and acidity test. For sulfur corrosion CCD (covered conductor deposition) test by IEC or DOBLE must be in line with DIN and ASTM. The disadvantage of those methods is that they are qualitative only. SEA Marconi published a quantitative method recently that is at the end of the standardisation process. The main and most important issue is to understand the exact need of each transformer based on the condition, remaining service life and even the organisation politics. Conducting tests with single quality laboratory can greatly assist in reliable diagnosis and help in making the right decision for high investments. Knowledge exchange of users and experts worldwide is also very important as well as to be up to date with the latest developments and issues that arise constantly in this industry.

Bibliography: [1] V. Sokolov et al: Transformer Fluid: A Powerful Tool for the Life Management of an Ageing Transformer Population, EuroTechCon 2001, p. 211. 62

[3] CIGRE A2.34, No. 445, Guide for Transformer Maintenance, Working Group, A2.34 [4] CIGRE D1.01/A2.11, No. 296, Recent developments in DGA interpretation, 2006 [5] IEC standard 60567, 2005 [6] CIGRE D1.01 (TF 15), No. 409, Report on Gas Monitors for Oil-Filled Electrical Equipment [7] IEC standard 60422, 2005 [8] ASTM D 3487 Specification for Mineral Insulating Oil Used in Electrical Apparatus [9] F. Scatiggio, V. Tumiatti, R. Maina, M. Tumiatti, M. Pompili, and R. Bartnikas, Corrosive Sulfur in Insulating Oils: Its Detection and Correlated Power Apparatus Failures, IEEE Transactions on Power Delivery, Vol. 23, No. 1, January 2008 [10] Griffin, P. J., and Lewand, L. R., Understanding Corrosive Sulfur Problems in Electric Apparatus, Proceedings of the Seventy-forth Annual Conference of Doble Clients, Boston, MA, Insulating Materials Session 1, 2007. [11] CIGRE WG A2-32, No. 378, Copper sulphide in transformer insulation

Author Marius GRISARU was a sub engineer in electricity from 1984. In 1991, he obtained his M.Sc in Electrochemistry and Polarography at Technion. Since then, he has been with Chief Chemist at Israeli Electric, Generation Division, specialising in Analytical Chemistry, focused on lube oil and particularly in insulating oil tests and estimation of transformer health. He is leading the insulating oil tests and treatments in Israel for Israel Electric and the local users. He is also an Assistant Instructor at Technion for undergraduate students. Marius is a participant member of ASTM, IEC, Cigre and other international working groups as well as the author and co-author of several papers on insulation oil methodology. TRANSFORMERS MAGAZINE | Volume 1, Issue 1

Jean SANCHEZ, Mladen BANOVIC

Need to Improve Performance? We Support You! Management Consulting, Transformer Processes and Technologies Interim Management. Transformer Operations & Sales Liaison and Representative Services

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EVENTS

Transform your business at CWIEME Berlin If you only visit one trade show this year, make it CWIEME Berlin – the world’s largest and most comprehensive event in the transformer manufacturing industry. This summer over 7,000 engineers, designers, buyers and academics from around the globe will gather at the Messe Berlin in Germany for CWIEME Berlin 2014 – the world’s largest annual meeting place for the electrical manufacturing, insulation and coil winding community. From the 24th to the 26th of June, some 750 suppliers from over 40 countries are expected to attend, including top names in the transformer manufacturing industry Waasner elektrotechnical, Trench France, Nynas, Thyssenkrupp and AEM cores. Over the three-day event, held in Germany’s buzzing capital, these companies and more will be exhibiting the latest electromagnetic insulation materials, coils, electric motors, transformers and repairs solutions. Under one roof you can find everything from insulation, magnets – everything related to your products, so we are always interested and fascinated to be here. – Muhammad Salman, Siemens AG

64

Free to attend, CWIEME Berlin is not just an opportunity to source new products and suppliers. CWIEME Berlin is valued throughout the industry as an occasion to network, learn about the latest trends and technical advances – and keep an eye on the competition. This year the themes of increasing electrification – both in emerging markets and new applications – and scouting young engineering talent will be brought sharply into focus.

Valuable industry insight As well as a chance to speak one-on-one with the industry’s top experts, visitors to CWIEME Berlin will benefit from a rich seminar programme. CWIEME Central will incorporate bitesized technical presentations on a variety of industry developments and panel discussions. This year’s event will also allow the best and brightest students to present their latest research pro-

TRANSFORMERS MAGAZINE | Volume 1, Issue 1

jects in the field of electrical engineering and design as part of the CWIEME Challenge. Not only is this a fantastic opportunity for students to gain valuable industry experience and receive feedback on their work, but for companies to hear about current university-led research projects, find academic partners for new business ventures – and spot talented new recruits. Meanwhile, CWIEME Berlin’s Machinery Trail will provide regular live product presentations and machinery demonstrations across a wide variety of exhibitor stands, allowing visitors to see the latest technology in action.

CWIEME Berlin

My favourite thing about CWIEME Berlin is seeing lots of old faces, brushing up my network, seeing new products and staying in touch with the industry. – Malte Jaensch, Porsche Engineering

For further information, please visit: www.coilwindingexpo.com/berlin

Now in its 18th year, CWIEME Berlin has grown to become the definitive event for the global coil winding, insulation and electrical manufacturing industries and the 2014 edition is set to have a greater and more engaging offering than ever before. Visit www.coilwindingexpo.com/berlin to register for your free visitor’s entry pass today.

Dates: 24th-26th June 2014 Venue: Messe Berlin, Messedamm 22, 14055 Berlin, Germany Opening times: Tuesday and Wednesday 09:00-18:00, Thursday 09:00-16:00 Admission: Free

Or contact: Simon Matthews / i2i Events Group Tel: +44 (0) 203 033 2133 Email: [email protected] Hannah Kitchener / SE10 Tel: +44 (0)207 923 5863 Email: [email protected]

CWIEME Berlin is part of i2i Events Group. i2i Events Group enables more than 200,000 buyers and producers to meet and trade at some of the greatest exhibitions, large scale events and festivals in the world.

w w w . t ra n sfo r m e r s - m a g a z i n e . co m

65

EVENTS

International Conference of Doble Clients

Transform Americas

6th April - 11th April 2014.

Exhibitions and technical presentations about transformer testing and diagnostics 26th May - 28th May 2014.

Westin Hotel Copley Place, Boston, Massachusetts, USA The conference covers topics from asset management and replacement strategies, storm preparedness, operating concerns and diagnostic approaches, giving an opportunity to learn and discuss ways to optimise operations and maximise performance – all to ensure the reliable flow of power.

Hannover Messe 2014 The main stop along the road to the Factory of the Future 7th April - 11th April 2014. Hannover, Germany Hannover Messe 2014 illuminates how the energy system transformation can be made possible: a quarter of all exhibitors are involved with energy production, distribution or storage. International companies and research institutes showcase the wide range of technologies for implementing “green manufacturing,” and how they can even ensure long-term cost advantages.

Hilton Hotel, Cartagena, Colombia TRANSFORM is a leading event for the industry in innovation and aims to encourage companies to share information about power transformers. It seeks to reach out and connect specialists worldwide, bringing its mission to Latin America, a region of high economic growth and home to valuable allies of our business.

IEEE Electrical Insulation Conference (EIC) The 32nd edition of the Electrical Insulation Conference 8th June - 11th June 2014. Sheraton Philadelphia Downtown Hotel, Philadelphia, USA 2014 the Dielectrics and Electrical Insulation Society (DEIS) offers presentations and short courses on the practical applications of electrical insulation systems and materials for all types of electrical and electronic equipment. Exhibits are also planned where manufacturers of various types of insulating materials and test equipment will display their products.

2014 IEEE/PES T&D Conference & Exposition 14th April - 17th April 2014. Chicago, Illinois, USA 2014 IEEE/PES T&D Conference & Exposition will showcase the technologies, products, companies and minds that will lead our industry through the next fifty years – and beyond.

Life of a Transformer Seminar The World’s Premier Training Experience on the Most Critical Aspects of LPTs 19th May - 23rd May 2014.

Cwieme Berlin The world’s largest international coil winding, insulation & electrical manufacturing exhibition 24th June - 26th June 2014. Berlin, Germany

CWIEME Berlin has the largest collection of coil winding, insulation and electrical suppliers, and brings together over 750 suppliers showcasing their products, services and technologies. It attracts 7,000 buyers and design engineers, who will be looking to forge new partnerships, network with old colleagues, and to discuss and explore potential new projects.

Padua, Italy

You work with transformers every day, but do you know everything you need to know about transformers? Learn practical information from industry experts for immediate, measurable impact at any point in the lifecycle of your transformers.

WEIDMANN Transformer & Technology Seminar 2014 2nd July - 4th July 2014.

2014 IEEE Innovative Smart Grid Technologies Conference (ISGT) Asia 20th May - 23rd May 2014. Kuala Lumpur, Malaysia The conference promotes the advancement of smart grid technology and research and serves as a platform where researchers, academics, policy makers and industries can share and exchange new ideas, learn from each other, discuss and debate on issues related to Smart Grid Implementation and its related matters.

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ISSUE 1, VOLUME 1

Hotel Seedamm Plaza Pfäffikon SZ, Switzerland This newly offered 3-day transformer seminar includes an array of industry experts and companies presenting and discussing topics such as materials and components used in transformers, design of distribution and power transformers, factory testing, operation of transformers and on-site testing. This unique course also covers topics like manufacturer‘s qualification, design reviews, transformer-grid interaction, transformer noise, aging of materials, electrical test methods and monitoring.

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provides information about the transformer industry related fa­ irs and conferences; provides a forum for information exch­ ange, technology advanceme­ nts discussions or troublesho­ o­ting; and offers access to a global audience of prospective buyers to our advertisers. Our readers are professionals in the transformer industry who have an opportunity to continuously improve and grow.

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