Testing Of Power Transformers: Nagaraja.m.c

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TESTING OF POWER TRANSFORMERS NAGARAJA.M.C Executive Engineer (E), R.T South Division, KPTCL, Rajajinagar, K.P.T.C.L, Bangalore :560010

The History of the Transformer

Ottó Bláthy

Lucien Gaulard

Sebstian Ferranti

It all started in 1883 with one of the first implementations to the transmission line a single-phase 15 kVA, 1500/300 V transformer, by K. Zipernowski, M. Deri, O. Blathy (Ganz, Budapest) and others. Ottó Bláthy, Miksa Déri, Károly Zipernowsky of the Austro-Hungarian Empire First designed and used the transformer in

both experimental, and commercial systems. Later on Lucien Gaulard, Sebstian Ferranti, and William Stanley perfected the design.

Transformers "the heart of the alternating current system“ William Stanley Jr.

William Stanley's First Transformer built in 1885

Stanley's first transformer which was used in the electrification of Great Barrington, Massachusetts in 1886

When was the transformer invented? The property of induction was discovered in the 1830's but it wasn't until 1886 that William Stanley, working for Westinghouse built the first refined, commercially used transformer. His work was built upon some rudimentary designs by the Ganz Company in Hungary (ZBD Transformer 1878), and Lucien Gaulard and John Dixon Gibbs in England. Nikola Tesla did not invent the transformer as some dubious sources have claimed. The Europeans mentioned above did the first work in the field, George Westinghouse and Stanley made the transformer cheap to produce, and easy to adjust for final use.

Where were the first transformers used? The first AC power system that used the modern transformer was in Great Barrington, Massachusetts in 1886. Earlier forms of the transformer were used in Austro-Hungary 1878-1880s and 1882 onward in England. Lucien Gaulard (Frenchman) used his AC system for the revolutionary Lanzo to Turin electrical exposition in 1884 (Northern Italy). In 1891 mastermind Mikhail Dobrovsky designed and demonstrated his 3 phase transformers in the Electro-Technical Exposition at Frankfurt, Germany.

Transformer development timeline: 1830s - Joseph Henry and Michael Faraday work with electromagnets and discover the property of induction independently on separate continents. 1836 - Rev. Nicholas Callan of Maynooth College, Ireland invents the induction coil 1876 -

Pavel Yablochkov uses induction coils in his lighting system

1878 -1883 - The Ganz Company (Budapest, Hungary) uses induction coils in their lighting systems with AC incandescent systems. This is the first appearance and use of the toroidal shaped transformer. 1881 - Charles F. Brush of the Brush Electric Company in Cleveland, Ohio develops his own design of transformer (source: Brush Transformers Inc.)

1880-1882 - Sebastian Ziani de Ferranti (English born with an Italian parent) designs one of the earliest AC power systems with William Thomson (Lord Kelvin). He creates an early transformer. Gaulard and Gibbs later design a similar transformer and loose the patent suit in English court to Ferranti. 1882 Lucien Gaulard and John Dixon Gibbs first built a "secondary generator" or in today's terminology a step down transformer which they designed with open iron core, the invention was not very efficient to produce. It had a linear shape which did not work efficiently. It was first used in a public exhibition in Italy in 1884 where the transformer brought down high voltage for use to light incandescent and arc lights. Later they designed a step up transformer. Gaulard (French) was the engineer and Gibbs (English) was the businessman behind the initiative. They sold the patents to Westinghouse. Later they lost rights to the patent when Ferranti (also from England) took them to court.

1883 - In Hungary Ottó Bláthy had suggested the use of closed-cores, Károly Zipernowsky the use of shunt connections, and Miksa Déri had performed the experiments. They found the major flaw of the GaulardGibbs system were successful in making a high voltage circuit work using transformers in parallel. There design was a toroidal shape which made it expensive to make. Wires could not be easily wrapped around it by machine during the manufacturing process. 1884 - Use of Lucien Gaulard's transformer system (a series system) in the first large exposition of AC power in Turin, Italy... This event caught the eye of George Westinghouse who bought Gaulard and Gibbs Transformer design. The 25 mile long transmission line illuminated arc lights, incandescent lights, and powered a railway. Gaulard won an award from the Italian government of 10,000 francs. 1885 - George Westinghouse orders a Siemens alternator (AC generator) and a Gaulard and Gibbs transformer. Stanley begin experimenting with this system.

1885 makes the transformer more practical due to some design changes: "Stanley's first patented design was for induction coils with single cores of soft iron and adjustable gaps to regulate the EMF present in the secondary winding. (See drawing at left.) This design was first used commercially in the USA in 1886". William Stanley explains to Franklin L. Pope (advisor to Westinghouse and patent lawyer.) that is design was salable and a great improvement. Pope disagrees but Westinghouse decides to trust Stanley anyway. George Westinghouse and William Stanley create a transformer that is practical to produce (easy to machine and wind in a square shape, making a core of E shaped plates) and comes in both step up and step down variations. George Westinghouse understood that to make AC power systems successful the Gaulard design had to be changed. The toroidal transformer used by the Ganz Company in Hungary and Gibbs in England were very expensive to produce (there was no easy way to wind wire around an iron ring without hand labor).

1886 - William Stanley uses his transformers in the electrification of downtown Great Barrington, MA.This was the first demonstration of a full AC power distribution system using step and step down transformers. Later 1880s - Later on Albert Schmid improved Stanley's design, extending the E shaped plates to meet a central projection.

1889 - Russian-born engineer developed the first three-phase transformer in Germany at AEG. He had developed the first three phase generator one year before. Dobrovolsky used his transformer in the first powerful complete AC system (Alternator + Transformer + Transmission + Transformer + Electric Motors and Lamps) in 1891.

1891 Early three phase transformer (circular core type) Siemens and Halske company 5.7 kVA 1000/100 V This transformer was created at the beginning of the modern electrical grid.

Main parts of Power Transformer • • • • • • • • • • • • •

CORE WINDING AND INSULATION SYSTEM TANK BUSHINGS RADIATORS TAP CHANGER- OLTC PRV CONSERVATOR BREATHER MARSHALLING BOX BUCHHOLTZ RELAY OSR MOG

CORE • It is made up of thin laminations of CRGO of thickness 0.28mm to 0.35mm. • Laminations are insulated from each other by oxide coating. • Magnetostriction in the core produces a vibration that causes audible noise called as Humming. (Due to contract and expansion of core material) • It is the backbone of the transformer from magnetic and mechanical aspect. • It provides a path for Magnetic flux. • It supports windings. • Low losses, better heat dissipation, low vibrations, Higher working flux, specific core losses W/Kg at peak flux density 1.7 Tesla at 50 Hz, ageing withstand capability.

ASSEMBLY OF TRANSFORMER CORE

WINDING AND INSULATION SYSTEM • Winding is made up of Copper material. • Wound concentrically around the core limbs. • LV winding with thick cross sectional area is placed near the core on the press board cylinder. • HV winding is placed above LV winding and is separated by press board cylinder and by oil ducts, hence HV winding is away from the core. • Superior Craft Paper Insulation is wrapped over conductor. • Most sensitive part subjected to Thermal, Mechanical and dielectric Stresses. • Major Insulations used in Windings are Thermally upgraded Craft Paper Cylinders, Synthetic Resins bonded press board cylinder and press board sheets. • Minor Insulations used in windings are Press Board spacers, Cellulose tape and synthetic enamel wire.

WINDING OF COILS USING VARIABLE SPEED MOTORS

WINDINGS OF POWER TRANSFORMERS WITH INSULATION

TRANSFORMER CORE AND WINDING ASSEMBLY UNDER OVEN FOR DRYING

TANK • It is made from Steel material. • Fabricated welded construction to obtain a box type construction. • Supports and encloses various parts. • Should be strong sturdy, free from vibrations. • Should withstand mechanical load, during handling, lifting and transport. • Should withstand Vacuum test and Pressure test • Tank cover supports, tap changers and other fitments. • It’s accessories includes lifting legs, manhole for inspection, valves for filling and draining of oils, earthing terminals.

TRANSFORMER TANK

BUSHINGS • Oil impregnated paper wound condenser bushing (OIP). • Provides insulated support to conductor connecting the external terminal to the Transformer winding. • Central Copper or Aluminum rod with surrounding condenser body • The Bushings are made of porcelain, the leads of HV and LV windings are brought out and connected to the terminals of the Bushings. • Oil filling between condenser and porcelain. • Bushing mounted vertically or inclined on tank over on turrets.

TRANSFORMER BUSHINGS

RADIATOR • It consists of a bank of tubes through which heat of oil is dissipated. • Oil is circulated inside the Radiator tube and air is circulated around Radiator tubes. • Both oil and air flow will be forced circulation. • It could be fitted on the tank. • For circulating oil, oil pump is provided. • For circulation of air, fans are provided.

TAP CHANGER- OLTC • On Load Tap Changer is mounted vertically within the Transformer Tank. • It consists of diverter switch and Tap selector and separate Driving Mechanism Box. • Driving Mechanism Box is fitted on the side of the Transformer Tank.

OLTC WINDING

5

4

3

2

1 R

R

Functional Diagram of M type OLTC

Transition Sequence of Diverter Switch

1 3 5 7 9 11

Output

2 4 6 8 10 12

Transition Sequence of Diverter Switch

1 3 5 7 9 11

Output

2 4 6 8 10 12

Transition Sequence of Diverter Switch

1 3 5 7 9 11

Output

2 4 6 8 10 12

Transition Sequence of Diverter Switch

1 3 5 7 9 11

Output

2 4 6 8 10 12

Transition Sequence of Diverter Switch

1 3 5 7 9 11

Output

2 4 6 8 10 12

Transition Sequence of Diverter Switch

1 3 5 7 9 11

Output

2 4 6 8 10 12

Transition Sequence of Diverter Switch

1 3 5 7 9 11

Output

2 4 6 8 10 12

Transition Sequence of Diverter Switch

1 3 5 7 9 11

Output

2 4 6 8 10 12

Transition Sequence of Diverter Switch

1 3 5 7 9 11

Output

2 4 6 8 10 12

Transition Sequence of Diverter Switch

1 3 5 7 9 11

Output

2 4 6 8 10 12

Transition Sequence of Diverter Switch

1 3 5 7 9 11

Output

2 4 6 8 10 12

Transition Sequence of Diverter Switch

1 3 5 7 9 11

Output

2 4 6 8 10 12

Transition Sequence of Diverter Switch

1 3 5 7 9 11

Output

2 4 6 8 10 12

Transition Sequence of Diverter Switch

1 3 5 7 9 11

Output

2 4 6 8 10 12

Transition Sequence of Diverter Switch

1 3 5 7 9 11

Output

2 4 6 8 10 12

Transition Sequence of Diverter Switch

1 3 5 7 9 11

Output

2 4 6 8 10 12

Transition Sequence of Diverter Switch

1 3 5 7 9 11

Output

2 4 6 8 10 12

Transition Sequence of Diverter Switch

1 3 5 7 9 11

Output

2 4 6 8 10 12

Transition Sequence of Diverter Switch

1 3 5 7 9 11

Output

2 4 6 8 10 12

Transition Sequence of Diverter Switch

1 3 5 7 9 11

Output

2 4 6 8 10 12

Transition Sequence of Diverter Switch

1 3 5 7 9 11

Output

2 4 6 8 10 12

Transition Sequence of Diverter Switch

1 3 5 7 9 11

Output

2 4 6 8 10 12

Transition Sequence of Diverter Switch

1 3 5 7 9 11

Output

2 4 6 8 10 12

Transition Sequence of Diverter Switch

1 3 5 7 9 11

Output

2 4 6 8 10 12

Transition Sequence of Diverter Switch

1 3 5 7 9 11

Output

2 4 6 8 10 12

Transition Sequence of Diverter Switch

1 3 5 7 9 11

Output

2 4 6 8 10 12

Transition Sequence of Diverter Switch

1 3 5 7 9 11

Output

2 4 6 8 10 12

Transition Sequence of Diverter Switch

1 3 5 7 9 11

Output

2 4 6 8 10 12

Transition Sequence of Diverter Switch

1 3 5 7 9 11

Output

2 4 6 8 10 12

Measuring of Transition Resistance

Pressure Relief Valve • Pressure Relief Valve (PRV) is mounted on the tank to permit venting out of the gases produced by oil. • It protects Transformer Tank against explosion.

CONSERVATOR • • •

• • • •

It is a large cylinder connected by pipe to the Transformer. Transformer Oil is filled up to certain level in the conservator. Expansion and contraction of transformer oil is accommodated by air cushion in the conservator. Direct contact with external air is avoided. It is of two types: Air Cushion connected with external atmosphere through Breather. Air cushion contained in Rubber bags called as Air cell or Bellow installed within the Conservator. Rubber Bag is filled with air of +ve Pressure

BREATHER • It is connected to Conservator. • It is filled with Dry Silica Gel. • Colour of the good Silica Gel is dark blue or blue colour. • Silica Gel absorbs the moisture and only the Dry Air to the Conservator. • Wet Silica gel is in Pink or pale Pink colour.

BUCHOLTZ RELAY • It is fitted in the pipe between Tank and Conservator. • It is a Gas operated Relay. • It gives alarm for incipient faults in the Transformer from the gas. • Gas pressure operates alarm contacts. • It also operates due to the loss of oil in Bucholtz Relay and conservator tank.

BUCHOLTZ RELAY

OIL SURGE RELAY • It is connected in-between OLTC chamber and its conservator. • It is also a Gas operated Relay.

Magnetic Oil Gauge • Magnetic Oil Gauge is fitted on the side of the conservator tank. • It shows the level of the oil in the conservator tank. • The minimum oil level in the conservator should be 30˚ level.

MARSHALLING BOX • It is placed on the side of the tank, fixed on the Brackets. • All instruments for Measurement Monitoring, Control are placed in this cabinet. • All Control cables and Power cables between Transformer, Cooler, Control Room, Auxiliary supply , Switch gear etc connected via control cabinet.

TRANSPORTATION OF POWER TRANSFORMER BY HYDRALLIC TRAILER

ON ROAD

ON RAILWAY TRACK

A TRANSPARENT VIEW OF POWER TRANSFORMER

1. Manufacturing Process. 2. Classification of testing. 3. Field Tests of power Transformers.

Manufacturing Process Of Transformer

RAW MATERIAL

• Raw material basically used is CRGO(cold rolled grained oriented)silicon steel. • The purpose of using CRGO is to reduce the Hysteresis Losses.

CORE CUTTING

• It is in the form of thin sheets & cut to size as per design. • Generally three different shapes of core laminations are used in one assembly.

COIL WINDING

• Coil Winding is of two types:• R-S COIL • HELICAL COIL • SPIRAL COIL

CORE FITTING

ASSEMBLY

• The core assembly is vertically placed with the foot plate touching the ground. the top yoke of the core is removed. The limbs of the core are tightly wrapped with cotton tape and then varnished.

• The coils as specified in the design may be of following types: • L.V COIL • H.V COIL

TESTING

TANKING

• COIL WINDING • CONNECTION

• Tanking is a procedure of embracement of complete unit (active unit) in a mild steel tank, the unit being enclosed includes (core entrapped with windings).

• ROUTINE TEST • RATIO METER METHOD • DIELECTRIC TESTS TESTING AT • IMPULSE TEST LEVELS POWER LEVEL • TEMPERATURE RISE TEST

DISMANTLIN G& DISPATCH

• After testing is completed and before dispatch from the factory all necessary work, e.g. removal of bushings etc. shall be performed. • A shipping list is made concerning the General arrangement drawing which list out the external attachments of a power transformer to be send to customers .

RAW MATERIAL USED  MATERIAL- CRGO SILLICON STEEL

 DIFFERENT GRADES OF CRGO  CTC-Continuous transposed material  TYPICAL LOSS VALUE (W/Kg) AT 50 Hz & B (Tesla) GRADE

LOSS VALUE

CRGO

B (Telsa)

 27M-4

0.83

1.22

 MOH

0.72

0.99

 ZH100

0.70

0.96

 ZDKH90

0.60

0.83

CTC (Continuous Transposed Conductors)

CORE CUTTING

E shape

I shape

V shape

COIL WINDING

HELICAL WINDING

CONTINUOUS DISC WINDING

SPIRAL WINDING

INTERLEAVED DISC WINDING

CORE FITTING  CONSTRUCTION 1.THREE LEG (NORMAL) 2.FIVE LEG (TO REDUCE HEIGHT)  CLAMPING ARRANGEMENT

1.END FRAME & INSULATION 2.CLAMPING PLATES 3.YOKE BOLTS OR YOKE STEEL BAND (NO HOLE PUNCHING REQ )  PROPER TIGHTENING TO REDUCE NOISE LEVEL  VARNISH AT EXPOSED SURFACES

The steps are being continued..

ASSEMBLY(BEFORE TERMINAL GEAR STAGE)  TOP YOKE REMOVAL  LEVELING OF BOTTOM SUPPORT BLOCK  LEG PREPARATION VISUAL CHECKING OF COIL  LOWERING OF COIL IN PROPER ORIENTATION  TOP INSULATION & CLAMPING RINGS  MOUNTING OF ENDFRAMES  SHRINKAGE IN HEATER  TOP YOKE FITTING  BTG TEST FOR CHECKING CORRECTNESS

DESCRIPTION OF TESTS ON TRANSFORMERS Sl. No.

NAME OF TEST/ CHECK POINT

PURPOSE OF TEST/ CHECK

1

Core insulation tests To check the insulation between Core (CC&CL) and Ground

2

Operational Checks on cooler bank (pumps & Fans), Operational Checks Breathers (silica gel or drycol), MOG, temperature gauges on protection System (WTI/OTI), gas actuated relays (Buchholz, PRD, SPR etc.) and simulation test of protection system

3

4

Test reveals the condition of insulation (i.e. degree of dryness of paper insulation), presence of any foreign Insulation contaminants in oil and also any gross defect inside the Resistance (IR) transformer (e.g. Failure to remove the temporary measurement transportation bracket on the live portion of tap-changer part) Measurement of C1 & C2 Capacitance and Tanδ in UST Capacitance and Tan mode. Changes in the normal capacitance of an insulator δ measurement of indicate abnormal conditions such as the presence of bushings moisture layer, short -circuits or open circuits in the capacitance network.

DESCRIPTION OF TESTS ON TRANSFORMERS Sl. No.

5

6

Name of Test/ Check point

Capacitance & Tan Dissipation factor /Loss factor and capacitance δ measurement of measurement of winding is carried out to ascertain the windings general condition of the ground and inter-winding insulation Turns ratio (Voltage To determine the turns ratio of transformers to identify any ratio) measurement abnormality in tap changers/ shorted or open turns etc

Vector Group & 7 Polarity 8

Purpose of test/ check

Winding resistance measurement

Magnetic Balance 9 test

To determine the phase relationship and polarity of transformers To check for any abnormalities due to loose connections, broken strands and high contact resistance in tap changers This test is conducted only in three phase transformers to check the imbalance in the magnetic circuit

Floating Neutral This test is conducted to ascertain possibility of short 10 point measurement circuit in a winding.

DESCRIPTION OF TESTS ON TRANSFORMERS Sl. No.

Name of Test/ Check point

Purpose of test/ check

11

This test is used to detect winding movement that usually occurs Measurement of Short due to heavy fault current or mechanical damage during Circuit Impedance transportation or installation since dispatch from the factory.

12

To locate defect in magnetic core structure, shifting of windings, failures in turn to turn insulation or problems in tap changers. Exciting/ Magnetising These conditions change the effective reluctance of the magnetic current measurement circuit thus affecting the current required to establish flux in the core

To measure the vibrations of core /coil assembly in the tank of the reactor. Movement of the core-coil assembly and shielding Vibration measurement structure caused by the time–varying magnetic forces results in 13 of Oil immersed vibration of the tank and ancillary equipment. These vibrations Reactor have detrimental effects such as excessive stress on the core-coil assembly 14

Operational checks on To ensure smooth & trouble free operation of OLTC during operation. OLTCs

DESCRIPTION OF TESTS ON TRANSFORMERS Sl. No. 15

16

17

18

Name of Test/ Check point

Purpose of test/ check

This test is performed to check the proper operation of Stability of Differential & REF protection of Transformer & Reactor by Differential, REF of simulating actual conditions. Any problem in CT connection, Transformer/ Reactor wrong cabling, relay setting can be detected by this test. Tests/ Checks on To ascertain the healthiness of bushing current transformer at the Bushing Current time of erection Transformers (BCTs) To assess the mechanical integrity of the transformer. Transformers while experiencing severity of short circuit current Frequency Response losses its mechanical property by way of deformation of the Analysis (FRA) winding or core. During pre-commissioning this test is required to measurement ascertain that Transformer active part has not suffered any severe impact/ jerk during transportation. Oil sample for DGA to be drawn from transformer main tank before commissioning for having a base data and after 24 hrs. of Dissolved Gas charging subsequently to ensure no fault gas developed after first Analysis (DGA) of charging. DGA analysis helps the user to identify the reason for oil sample gas formation & materials involved and indicate urgency of corrective action to be taken

DESCRIPTION OF TESTS ON TRANSFORMERS Sl. Name of Test/ No. Check point

Purpose of test/ check

A thermo vision Camera determines the temperature distribution on the surface of the tank as well as in the vicinity of the Jumper connection to the Thermo vision bushing. The information obtained is useful in Infrared scanning predicting the temperature profile within the inner 19 (IR surface of tank and is likely to provide approximate thermography) details of heating mechanism. Thermo vision scanning of transformer to be done at least after 24 hrs of loading and repeated after one week.

CLASSIFICATION OF TESTS • ROUTINE TESTS

• TYPE TESTS • SPECIAL TESTS

(A)

ROUTINE TESTS:

1. Measurement of winding resistance for each tap position (As per IS 2026 PI Clause 16.2) 2. Measurement of voltage ratio, verification of polarity and vector group (As per IS 2026 PI Clause 16.3)

3. Measurement of No load losses & Magnetizing current (As per IS 2026 PI Clause 16.5) 4. Measurement of impedance Voltage/Short Circuit impedance and full load loss at principal tap (As per IS 2026 PI Clause 16.4) 5. Measurement of insulation resistance (As per IS 2026 PI( Clause 16.6)

6.

DIELECTRIC TESTS a) b)

Power frequency/separate source AC voltage withstand (As per IS 2026 PIII Clause 16.7) Induced over voltage test (As per IS 2026 PI Clause 16.7)

7.

Testing of transformer oil (as per IS 335: 1992)

8.

Tests on ON LOAD tap changer (As per IS 2026 PI Clause 16.9)

9.

Pressure test for oil leakage an a completely assembled transformers (as per Cl.No. 17.32 (b) of CBIP specifications)

(B) TYPE TESTS: Following type tests shall be conducted on transformer: 1. 2.

3. 4. 5.

6. 7. 8.

Measurement of winding resistance (Cl.No. 16.2 of IS 2026 Part I - 1977) Measurement of voltage ratio and check of voltage vector relationship (Cl.No. 16.3 of IS 2026 Part I - 1977) Measurement of impedance voltage/short-circuit impedance (principal tapping) and load loss

(Cl.No. 16.4 of IS 2026 Part I - 1977) Measurement of no load loss and current (Cl.No. 16.5 of IS 2026 Part I - 1977) Measurement of insulation resistance (Cl.No. 16.6 of IS 2026 Part I - 1977) Dielectric tests (Cl.No. 16.7 of IS 2026 Part I - 1977) Temperature – rise (Cl.No. 16.8 of IS 2026 Part I - 1977) and Tests on on-load tap-changers, where appropriate (Cl.No. 16.9 of IS 2026 Part I - 1977)

(C) SPECIAL TESTS: Following special tests shall be conducted on Transformer: 1.

Measurement of acoustic Noise level (at no load, at normal voltage and at normal frequency)

2.

Measurement of Zero phase sequence impedance.

3.

Magnetic Balance Test

4.

Measurement of auxiliary losses like losses of cooling fans.

5.

Measurement of Tan delta & capacitance of winding.

6.

Measurement of harmonics at No load current.

7.

Jacking test

NOTE:-The above stated tests has to be agreed between the purchase and supplier. Hence purchaser should clearly specify conducting above tests in the P.O.

FIELD TESTS 1. I.IR Measurement measurementsof Impedance voltage/short circuit 2. Ratio test current Testsgroup on OLTC 3. II. Vector test and Cooler controls

Alarm and trip teststest 4. III. Magnetization current Core Isolation 5. IV. Magnetic balance test test ( for 3 phase unit ) of oil for DGA & of other parameters 6. V. C &Testing Tan delta measurement bushings and windings. Oil filling , oil circulation and air venting. 7. VI. Winding resistance measurement

(1)

MEASUREMENT OF INSULATION RESISTANCE:

Reference Standard Equipment required

: IS 2026-Cl.No. 16.6 : 5 KV / 10kV Insulation Tester.

TEST METHOD/PROCEDURE:

The mean temperature of transformer oil and air temperature (ambient) will be measured & will be recorded immediately prior to the test. The insulation resistance of each winding, in turn to all the other windings, core & frame or tank together & to earth shall be measured & recorded as follows.

a) b) c)

High voltage winding to low voltage winding. High voltage winding to low voltage windings, tank & earth Low voltage winding to high voltage winding, tank & earth However, insulation resistance is highly temperature dependent and thus the results should be corrected to a standard temperature, usually 40°C. A good thumb rule is that for every 10°C increase in temperature, the current doubles while resistance halves. Most modern insulation testers offer displays that provide the operator with both a digital readout of the result and some form of analog readout. When an insulation tester is “hooked up” to the item to be tested, and when the test is started several things occur.

The three different currents ---Capacitive charging Dielectric absorption Conduction/leakage are following.

`The sum of these three currents will cause instrument display to vary with the reading increasing, initially quickly and then more slowly with time. With an analogue display, the movement of the pointer may provide information to an experienced operator. The movement of pointertraveling smoothly or stuttering?, rising steadily or intermittently dropping back. When doing insulation testing, more the operator knows about the results (during and after test) it is better on how to correct the problem. Condition of Test results Fairly higher values and well maintained

What to do No cause for concern

Low but well maintained

Condition is probably all right may simply be the type of insulation is poor.

Fairly high values but showing a constant tendency towards lower values

Locate and remedy the cause and check the down award trend

INSULATION RESISTANCE OF 3-WINDING TRANSFORMER IS MEASURED AS FOLLOWS: [TEST VOLTAGE TO BE APPLIED: 5 KV]

TERMINALS

INSULATION RESISTANCE IN MEGA OHMS

1

2

3

4

LINE [-]

LINE[+]

At 60 seconds

At 600 seconds

HV

LV+TV+Tank

Earthed LV

HV+TV+Tank Earthed

TV

HV+LV+Tank Earthed

HV

LV

LV

TV

TV

HV

Polarization index = 4/3 5

PURPOSE: The insulation resistance test is one that can be easily performed in the field. Many manufacturers require that this test be made prior to energizing a transformer, to preclude start up failure caused by the entry of moisture into the transformer during shipment or storage. The insulation resistance test can detect other ground circuit that may exist in the transformer caused by shipping damage. This test checks the complete circuit, bushing, leads and coils. Acceptance Criteria For megger results -At 30° C of temperature.

As per CBIP specifications the acceptable values are 2 Mega ohms per KV but minimum values which are considered at present in KPTCL are as follows

As per CBIP specifications the acceptable values are 2 Mega ohms per KV but minimum values which are considered at present in KPTCL are as follows : a)

(b)

i) Minimum 200 M ohms, for below 6.6 KV class ii) Minimum 400 M ohms between 6.6KV &11KV class Parameters iii) Minimum 500 M ohms upto 33KV class iv) Minimum 600 M ohms upto 132KV class v) Minimum 650 M ohms upto 220KV class Polarization Index test

The measurement of insulation resistance will be done and expressed in mega-ohms at the end of 60 seconds, & 600 seconds. The ratio of second reading to the first reading is called “POLARISATION INDEX”.

The simplest implementation of the time resistance test for solid insulation is represented by the popular polarization index (PI) test, which requires only two readings followed by a simple division: the one minute reading is divided into the ten-minute reading to provide a ratio. The result is a pure number and can normally be considered independent of temperature since the thermal mass of the equipment being tested is usually so great that the overall cooling which takes place during the 10 minutes of the test is negligible. In general, a low ratio indicates little change, hence poor insulation, while a high ratio indicates the opposite. References to typical PI values are common in the literature, which make this test very easy and readily employed.

A test like the Polarization Index is particularly useful because it can be performed on even the largest equipment, and yields a self-contained evaluation based on relative readings rather than absolute values. But no P.I. can be calculated with a tester of limited range, because “infinity” is not a number ! Advanced testers reach the teraohm range, and therefore, do not run off the graph. The largest and newest capital equipment can be readily tested to yield repeatable data for recording and subsequent trend evaluation. The following charge highlights selected PI values and what they mean to the operator. Polorization Index

Insulation Condition.

<1

Poor

1-2

Questionable

2-4

Okay

>4

Good

Values above 4 indicate excellent equipment for which no action is likely to be necessary within the immediate maintenance schedule. The operator may be called upon to make critical judgments, however. Some high values of PI above 5.0 could indicate brittle or cracked insulation: this should be fairly obvious.

(2)

MEASUREMENT OF VOLTAGE RATIO AND POLARITY:

Reference Standard Equipment required

: IS 2026-Part 1 : 1977 Cl.No. 16.3/IEC-76 : Transformer turns ratio meter

TEST METHOD/PROCEDURE: This test will be carried out with the help of direct reading turns-ratio meter. This test will be done on all taps & all phases. The ratio meter uses the principle of comparison of voltage fed to H.V. winding and voltage induced in the LV winding. The ratio meter is used in a bridge circuit where the voltages of windings of the transformer under test are balanced against the voltages developed across the fixed and variable resistance of the ratio meter, By selecting correct ratio on ratio meter, null will be obtained on the null detector usually galvanometer is incorporated along with ratio meter. This method also confirms the polarity of the windings since a zero reading would notbe obtained if one of the windings connections are reversed. Ratio readings on each tap will be recorded. This procedure will be repeated for all 3 phases in turn. In the case of double ratio transformers, the tests should be carried out for both ratios.

PURPOSE: The purpose of this test is to find out ratio of HV to LV which is Generally 6:1 for Power transformers at normal Tap.

per phase voltage on HV side Turns ratio= per phase voltage on LV side No. of HV turns

Turns ratio= No. of LV turns For example, per phase HV voltage = 66000 V, per phase LV Voltage = 11000 V 66000 Turns ratio= 11000 Therefore Turns ratio= 6.

Further, this test is to ensure that all windings have the correct number of turns according to design and should be connected properly. If a transformer is equipped with either off load tap changer or on load tap changer then the ratio tests are also carried out at the various positions of the tap changer. The object of ratio tests at different tap positions is to ensure that all the winding taps are made at the correct turns and that the tap connections are properly made to the tap changer.

Acceptance Critera/ Tolerance on test Parameter

: Tolerance permissible is +/-0.5% of declared ratio on all the taps.

(3)

VERIFICATION OF VECTOR GROUP:

TEST METHOD/PROCEDURE: Reference Equipment required

: IS 2026-Cl.No. 16.3 : Refer figure No. 3

The LV vector will be superimposed on HV by connecting 1U and 2U terminals 2u 1U

2n 2v 2w

1W

1V

The three phase voltage will be applied to the line terminals of HV winding & voltage measurement will be done across. 1U-1V

1V-2V

1V-2W

1V-1W

1W-2V

1W-2W

1W-1U

1U-2N

1V-2N

Checks :

voltage measured across, must satisfy following conditions

1V-2V = 1V-2W 1W-2V > 1W-2W 1U-2N + 1V-2N = 1U - 1V

PURPOSE:

The purpose of this test is to find out whether the winding connection is as per customer’s specifications. That is, this test is to ensure that all the windings are assembled in correct physical orientation and they are connected properly to provide the desired phase relationship.

Acceptance Criteria

: Should confirm the vector group required

(4)

MEASUREMENT OF MAGNETISING CURRENT

Reference Standard

: IS 2026-Cl.No. 16.5

• The test is conducted by injecting 3-phase or two-phase voltages on L.V or H.V windings keeping the other side open.

• Precautions This test should be done before DC measurements of winding resistance to reduce the effect of residual magnetism. The transformer may be demagnetized before commencement of magnetizing current test.

(5) MAGNETIC BALANCE & MEASUREMENT OF MAGNETISING CURRENT TEST: Reference Standard : IS 2026 Equipment : Multi meter TEST METHOD / PROCEDURE: This test is carried out preferable from LV side of the transformer provided the voltage class of the LV winding is more than 6.6 KV. The voltage available at mains shall be applied across the winding terminals pertaining to ‘U’ phase and the corresponding transferred voltages across ‘V’ & ‘W’ phases shall be measured. Then similarly the mains voltage shall be applied to ‘V’ & ‘W’ phase in turn and measured across each of the remaining two phases. In all the three cases the voltage distribution shall be checked. TABLE OF MAGNETIC BALANCE TEST TYPICAL VALUES OF A 10MVA POWER TRANSFORMER

The magnetizing current shall be measured at the time of application of voltage across individual phases.

1U-1V

1V-1W

1W-1U

2N-2U

2N-2U

2N-2V

401

321

80

402

309

93

202

403

201

201

401

200

70

333

403

99

302

401

Acceptance Criteria/

: The values shall be recorded for reference Tolerance on Test Parameters

(5) TAN DELTA AND CAPACITANCE TEST: Reference Standard Equipment

: IS 2026 : 12KV Capacitance and tan delta bridge

TEST METHOD / PROCEDURE: Measurement will be done with a direct reading of capacitance in micro/pico farads and tan delta in percentage under UST and GST mode to measure the capacitance between the windings and capacitance with respect to ground as follows.

CAPACITANCE & TAN DELTA VALUES OF BUSHING. Terminals

Sr.No. of Bushing

Test voltage 5KV

Capacitance in pico farads

Tan Delta in %

The capacitance values 10KVare taken for future 1U 5KV reference only. 10KV 1V The value of Tan delta shall be less than 0.5%. 5KV 1W

10KV 5KV

2U

10KV 5KV

2V

10KV 5KV

2W

10KV

(7)

MEASUREMENT OF WINDING RESISTANCE:

Reference Standard : IS 2026-P I/1997 (Clouse No. 16.2) Equipment required : Kelvin double Bridge/wheat stone Bridge. High accuracy digital resistance meters & Thermometers. TEST METHOD/PROCEDURE:

The resistance of each windings of Principal, maximum and minimum tap or each taps shall be measured depending on the value of the resistance using Kelvin Bridge or high accuracy digital resistance meters.

PURPOSE: This is a measure of the resistance of the copper conductor in the transformer windings.

The resistance measurements taken at known temperatures have two important functions. 1. Calculation of the temperature of the windings during the temperature rise-test 2. Calculation of I2 R components of the winding losses and stray losses. Resistance measurement across the transformer terminals provide an assessment of the quality of internal connections made to the transformer windings. Loose or defective connections are indicated by usually high or unstable resistance windings. Acceptance criteria Tolerance on test parameters

: +/-3% of designed value of resistance

(8)

MEASUREMENT OF IMPEDANCE VOLTAGE/SHORT CIRCUIT CURRENT.

Reference standard Equipment required

: IS 2026-Cl.No. 16.4 : Voltmeters, Ammeters, milli-ammeter.

TEST METHOD/PROCEDURE: The impedance voltage / short circuit impedance ( on principal tapping only) and load loss shall be measured at rated frequency by applying an approximately sinusoidal supply of rated current to HV winding with the other LV winding short circuited, with the windings connected on the relevant tapping. On Field- 3-phase currents is passed to the High voltage winding keeping the other winding shorted (Two Winding). The Phase currents and neutral currents are recorded. IMPEDANCE: The impedance is normally expressed in terms of percent of rated voltage. The impedance voltage is that voltage required circulating the rated current through the primary winding with secondary shorted. The impedance is composed of two components, the percent reactance voltage (% X) and the percent resistance voltage (% R). The equation expressing the percent impedance is as follows: % Z = √ (% X)2 + (% R)2

The percent resistance voltage is the ratio of the winding losses over the rated KVA and can be determined as follows: % R = winding losses (watts)/10 x KVA The percent reactance voltage can be determined by substituting for % R and %Z. The %R and %X components are important in calculating the regulation of the transformer.

PURPOSE : The purpose of this test is to measure I2R loss in winding and stray losses in the tank & find out impedance level during short circuit test. The impedance test is made to verify the design impedance. The impedance is also important when paralleling two or more transformers. A transformer dissipates load loss that depends upon the transformer load current . Load loss is a cost to the user during the life time of transformer. Maximum values of the load loss of transformers at rated current are specified and often guaranteed by the manufacturer. Load loss measurements are made to verify that the load loss does not exceed the specified or guaranteed value. Acceptance Criteria/ : The losses & impedance shall be within Tolerance on test guaranteed values mentioned in the purchase Parameters order or + 10% tolerance on total losses and +/-10% tolerance on declared impedance

(9) TESTING OF TRANSFORMER OIL (as per IS 335) The transformer oil sample of power transformer should have following characteristics as per IS 335:

Conducting DGA of Transformer oil

Portable DGA testing kit

Schedule of characteristics Sl.No.

Characteristics

Required

Method of Tests

Remarks

-1

-2

-3

-4

-5

The coil shall be clear and transparent and free from suspended matter or sediments

A representative sample of the oil shall be examined in a 100 mm thick layer at 0 27 C

-

i)

Appearance

ii)

Density at 29.5 C Max

iii)

Kinematic viscosity. Max, at

0

0.89 g/cm

3

IS 1448 [P:16] 1977

See NOTE 1

IS 1448 [P:25] 1976

-

IS 6104 : 1971

-

0

a)

27 C

b)

40 C

0

27 Copy Submitted to:

Under consideration 0

iv)

Interfacial tension at 27 C, Min

0.04 N/m

v)

Flash point pensky-Marten (closed), Min

140 C

0

IS 1448 [P:21] 1970

-

vi)

Pour point, Max

-6 C

IS 1448 [P:10] 1970

-

vii)

Neutralization value a) Total acidity,

0

IS 1448 [P:2] 1967 0.03 mg KOH/G

Max

Alcoholic potassium hyudroxide solution of 0.02 N should be used in place of 0.1 N indicated in test method

b) Inorganic acidity/alkalinity

Nil

do

viii)

Corrosive sulphur

Non-corrosive

Annex B

-

ix)

Electric strength

IS 6792 : 1972

-

(breakdown voltage) a) New unfiltered oil, Min

30KV (rms)

b) After filtration, Min If the above value is not attained, the oil shall be filtered 60KV (rms) x)

Dielectric dissipation factor 0 (tan q) at 90 C, Max

0.002

See Note 2

IS 6262 : 1971

See note

xi)

Specific resistance (resistivity)

IS 6103 : 1971

See note 2

0

a) At 90 C, MIn 0

b) At 27 C, Min

35 x 10

12

1500x 10 xii)

ohm-cm 12

ohm-cm

Oxidation stability a) Neutralization value after oxidation, Max

Annex C

-

IS 12177 : 1987

-

0.4 mg KOH/g

b) Total sludge, after oxidation Max 0.1 % by weight xiii)

Ageing characteristics after accelerated ageing (open beaker method with copper catalyst)

-

a) Specific resistance (resistivity)

Method A

0

1) at 27 C, Min 0

2) at 90 C, Min b) Dielectric dissipation 0 factor (Tan q) at 90 C, Max c) Total acidity, Max d) Total sludge, Max

IS 6103 : 1971 2.5 x 10

12

ohm-cm

0.2 x 10

12

ohm-cm

0.2

IS 6262 : 1971

0.05 mg KOH/G

IS 1448 [P : 2] : 1967

0.05% by weight

Annex A of IS 12177

xiv)

Presence of oxidation inhibitor

The oil shall contain antioxidant additives

IS 13631 : 1992

See Note 3

xv)

Water content, Max

50 ppm

IS 13567 : 1992

-

xvi)

SK value

Under consideration

Annex D

-

a)

BDV of Transformer oil – Testing procedure

Reference standard : IS 6792 : 1992 Equipment : Transformer oil BDV tester Circuit diagram : Refer Figure No. 9 The oil sample shall be taken out from the bottom sampling valve of the transformer in an oil cup. The oil cup shall be rinsed with the oil twice or thrice before filling it with the oil sample. The gap between the two electrodes shall be set to 2.5mm with Go-No Go Guage. The gap between them shall be set to an accuracy of +/- 0.01 mm by means of thickness gauge. The axis of electrodes is immersed to a depth of approx 40 mm. Test for breakdown voltage shall be made at an atmosphere of less than 50% relative humidity. The oil shall be allowed to settle down for the period of 10 minutes. The voltage shall be gradually raised form zero till the breakdown occurs. Six such readings shall be taken at the interval of 1 minute & the average of six readings shall be taken as the breakdown voltage of the oil sample under test. Acceptance Criteria Tolerance on test Parameters

: Minimum B.D.V. is 50 for cold oil as per KPTCL p.o. specifications

(10) TESTS ON ON - LOAD TAP CHANGER (AT RATED VOLTAGE & RATED CURRENT) Test will be carried as per IS 2026 part I – 1997 (Cl. No. 16.9)

Figure: Assembling of EMR OLTCs for 100MVA Transformer at M/s. EMCO Ltd, Thane.

1) Functional test:

a) b) c)

d)

After tap changer and transformer have been fitted together the following tests are carried out at 100 percent of the rated auxiliary supply voltage. Following performed without failure. Eight complete operating cycles with the transformer not energised. One complete operating cycle with the transformer not energized, with 85 percent of the rated auxiliary supply voltage . One complete operating cycle with the transformer energized at rated voltage and frequency at no load; and Ten tap-change operations with + 8 steps on either side of the principal tapping with one winding short-circuited.

PURPOSE: The correct functioning of all control devices, including limit switches and mechanical end stops will be demonstrated.

(11) Inspection of Accessories of Power Transformer i.

Oil conservator and air cell

Oil conservator shall be checked for physical dimensions. Air cell shall be tested by applying certain pressure for definite time to see air leakage.

(11) Inspection of Accessories of Power Transformer ii. Pressure relief device

Pressure relief device shall be tested for designed pressure

(11) Inspection of Accessories of Power Transformer iii. Magnetic oil gauge Magnetic oil gauge shall be tested for its operation as per the manual

(11) Inspection of Accessories of Power Transformer iv. Marshalling Box/Cooler cabinet for oil pumps and fans Marshalling box wiring shall be tested to withstand 2KV for one minute. All the wiring connections shall be tested in accordance with approved Drawing. All the operations of cooling fans and oil pumps shall be tested for its operation as per Technical specifications.

SOME OF THE FAILURES

Failure of 400/220kv/33kv, 315MVA transformer (BHEL make) at bamnauli substation of delhi transco on 11-02-2008 Courtesy : CEA

THANK YOU

9448365077

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