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SAFER, SMARTER, GREENER
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POWER TRANSFORMERS NEED REAL TESTS TO PROVE THEY CAN SURVIVE A SHORT CIRCUIT
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SAFER, SMARTER, GREENER
2 ENERGY Power transformers need real power to survive a short circuit
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CONTENTS 1. Executive summary ___________________________________________________________________ 05 2. Major power disruptions _______________________________________________________________ 06 2.1 Outages: a growing concern _________________________________________________________06 3. Short-circuit stresses in power transformers _______________________________________________08 4. Power transformer in-service failure______________________________________________________ 10 4.1 In-service failure statistics __________________________________________________________ 10 4.2 Consequences of transformer failures _______________________________________________ 11 5. Short-circuit withstand verification ______________________________________________________ 12 5.1 By design review __________________________________________________________________ 12 5.1.1 Comparison with a short-circuit tested reference transformer ___________________ 12 5.1.2 Checking against manufacturer’s design rules for short-circuit strength ___________ 12 5.2 By full-power testing ________________________________________________________________13
5.3 Limitations of the design review approach ____________________________________________ 14 5.3.1 Simulation tools are a simplification of the reality _______________________________ 14 5.3.2 The list of reviewed sub-components is not complete ___________________________ 15 5.3.3 The design review approach is static whereas the phenomena are dynamic ________ 15 5.3.4 Design review does not cover material and production deficiencies _______________15 5.3.5 Many failures occur in other than the ‘design reviewed’ sub-components _________ 15 5.3.6 There is no quality control on the performance of a design review _______________ 15 5.3.7 Conclusion __________________________________________________________________16 5.4 By testing of scale-models (‘mock-ups’) ________________________________________________16 6. Utility considerations _____________________________________________________________________ 17 7. Features of KEMA Laboratories ___________________________________________________________19 8. Laboratory capacity _______________________________________________________________________ 21 9. Test result statistics of power transformers __________________________________________________ 22 10. Failure modes ____________________________________________________________________________ 23 10.1 Observations from short-circuit tests _______________________________________________ 24 11. Discussion ____________________________________________________________________________ 25 12. Conclusions ______________________________________________________________________________ 25 13. References _______________________________________________________________________________ 26
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ABOUT THE AUTHORS
René Smeets Service Area Leader KEMA Laboratories René Peter Paul Smeets obtained a PdD degree for research work on switchgear. Until 1995, he was an assistant professor at Eindhoven University, the Netherlands. During 1991 he worked for Toshiba Co. in Japan in the development of vacuum interrupters. In 1995, he joined KEMA, the Netherlands. Currently, he is Service Area Leader with KEMA Laboratories, dealing with innovation and technology management. In 2001 he was appointed parttime professor at Eindhoven University, the Netherlands. In 2013 he became vice professor at Xi’an Jiaotong University, China. In 2008 he was elected Fellow of IEEE. He is convener, secretary and member of several CIGRE working groups, as well as convener of two IEC maintenance teams on high-voltage switchgear. Since 2008 he is the chairman of the ‘Current Zero Club’. In 2014, he published the book ‘Switching in Electrical Transmission and Distribution Systems’ with John Wiley UK. He got six international awards, authored more than 200 international papers on several aspects of power switching and testing technology, and presented many training courses all over the world.
Shankar Subramanay General Manager KEMA Laboratories Shankar Subramany is currently responsible for the High Power Laboratory of KEMA Laboratories in Arnhem, Netherlands, the largest short-circuit test laboratory in the world. He received his Master’s degree in High Voltage Engineering from the Anna University in Chennai, India. In 1986 he joined the Central Power Research Institute, Bangalore, India and worked in various roles including construction of the new high power laboratory and testing of MV and HV T&D components for short-circuit and switching performance. From 1998-2006, he worked at ABB, Sweden as Senior Test Engineer and Technical Manager of the High Power Laboratory and from 2006 as Manager of the High Power, High Current and Mechanical test laboratories at ABB. During those years he also held the position as Chairman of SATS Certification and was a member of TC17 of Swedish Electrotechnical Commission and STL Technical and Management Committees. Since 2013 he works at the KEMA Laboratories of DNV GL. In addition to participation in IEC standardisation teams he is currently also the Deputy Chairman of the STL Technical Committee.
René Bruil Head of Section KEMA Laboratories René Bruil is currently responsible for the planning of the High Power Laboratory and responsible for the transformer testing business of the KEMA Laboratories. He has more than 20 year of experience in the field of short-circuit tests on MV and HV equipment, 8 years as test engineer, 5 years in the certification department and the last years as planner of the High Power Laboratory and transformer business. He was member of the IEC working group MT45 for switches and is currently member of the STL Technical Committee.
Wilco Rorijs Principal Consultant Transformers Energy Advisory Wilco Rorijs is a principal consultant with the Energy Advisory unit at DNV GL, specializing in transformers. He joined DNV GL (formerly DNV KEMA) in September 2007. He is involved in many transformer and reactor related matters internationally. His current remit includes drafting technical specifications; performing factory audits; preparing tender evaluations; conducting design reviews; undertaking QA/QC; and witnessing of FAT and SAT of a wide range of transformers (distribution, GSU, auto, grid, phase-shift and HVDC) and reactors (shunt and series). He is also involved in power failure investigations (i.e. RCA) of transformers and reactors both at site as well as at the premises of manufacturers. Rorijs received his Bachelor degree of High Voltage Electro Technical Engineering at the HAN University, Arnhem, the Netherlands in 1992. After graduating he joined (Royal) Smit Transformers in the position of electrical design engineer. During his time at Smit, Rorijs became a senior electrical design engineer and designed many specialized power transformers which are installed in several countries worldwide. In 2007, he left Smit Transformers to join DNV GL as a transformers consultant. He is a member of the Dutch Cigré group A2 Transformers and a member of transformer related Cigré working groups.
Power transformers are the most expensive pieces of equipment in power systems. Interruption of service of transformers needs to be avoided at all time, given the enormous consequences. International studies have indicated that the failure rate of transformers is around 0.6%. Deeper study reveals that a major portion of these failures (up to 20%) is directly related to short circuits. During a short circuit, the large currents involved lead to severe mechanical forces and stresses in the transformer active parts, which may become deformed when the structural design of the transformer is not adequate. The verification of the ability of power transformers to survive short circuit is the subject of this paper. Two verification methods are practiced today. The first one is “design review”, in which third-party consultants check calculation results of forces and stresses and compare these with critical values based on tests or based on internal manufacturer’s rules. Design review is based on calculation results of idealized, homogeneous structures, it does not cover transient phenomena, it excludes a number of key subcomponents and it is not embedded in a strict quality surveillance system.
The second verification method is “short-circuit testing”, in which the complete transformer is subjected to real short-circuit current and thus to the same stresses as would occur in service. Short-circuit testing is the only complete verification method of short-circuit withstand capability of power transformers. KEMA laboratories are now ready to test power transformers with rated voltage up to 800 kV and power up to 500-600 MVA single phase (1500-1800 MVA three phase). In spite of the wide application of advanced calculation methods, still around 20–30% of the transformers, submitted to a short-circuit test fail to pass the short-circuit test. Mostly, the reason to fail is a reactance increase beyond the limit set by the standard, which indicates an unacceptable internal deformation. In a number of cases, however, unexpected events are triggered by short-circuit current which are outside of the “usual” failure modes, like breaking of a bushing, oil spill, internal flashover etc. The highest degree of reliability with respect to short-circuit withstand verification is through full-scale short-circuit testing in accordance with the international standards.
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MAJOR POWER DISRUPTIONS
Electricity is the life blood of modern society. Any interruption to its supply can have a huge impact on individuals, businesses and communities. The transmission and distribution (T&D) sector and the energy industry as a whole therefore put a great deal of effort into trying to avoid disruptions to the power supply. But are we doing enough and in the correct way?
2.1 Outages: a growing concern
European experiences are similar: a German/Swiss/Austrian survey indicates that 59% of outages are due to faulty equipment/human error, while the costs of outages can soar to 22 M€/hour in the major metropolises. What can network operators do to mitigate the risk of major power disruptions? To start, we must identify the root causes of power outages. Repeated research shows that the three main causes of unscheduled power outages are: weather events, equipment failure and human error (figure 1).
Super grids increase both the risk and the potential impact of outages. Losing power in one region is bad, but imagine what would happen if the supply to half a continent will be interrupted. Such a threat is already starting to attract headlines, with articles in many countries speculating on when and where ‘the big one’ will hit. Liability issues are becoming a major concern as the general public, as well as the legislative, financial and regulating bodies, are increasingly aware that power disruptions are not inevitable ‘acts of God’, but are in fact avoidable to a great extent. Investors are increasingly taking into consideration the quality of their (future) assets using a careful process of quality assurance. Electrical power outages, surges and spikes are estimated to cost more than $150 billion in annual damages to the U.S. economy. The downtime costs for data centres in particular are skyrocketing, with the average cost per minute in 2015 reaching $8,851: up 58% from the $5,600 per minute tabulated in 2010. The average cost of a data centre outage rose from $505,502 in 2010 to $690,204 in 2013 to $740,357 in 2015, representing a 38% increase in the cost of downtime. Maximum downtime costs are rising faster than average, increasing 81% percent since 2010 to a 2015 high of $2,409,991.
Animal Faulty equiment / human error Planned Unknown Vehicle accident
For example, a 2015 US outage survey showed that weather is the prime cause of outages (30%), followed by faulty equipment/human error (27%).
Weather / trees Overdemand
Faulty equipment/human error
Figure 1 - Reported outages by cause in 2015
Both equipment failure and human error can be reduced through better quality of network components and better educated people. And while the weather cannot be controlled, its impact on our electricity supply can be reduced by ensuring that local problems do not escalate across the entire grid. So this again comes down to the quality of our components and our people. The need to reduce the impact of weather-based outages will be even more urgent as the current models of climate change predict that more extreme weather events are going to occur.
Number of outages
The electricity industry is going through a transformation right now. The need to create an affordable and sustainable energy supply that can meet the ever-increasing demand for electricity is driving growth in (often remotely located) renewable sources and international electricity trading. That in turn is leading to the rise of so-called super grids: extremely high voltage, interconnected networks that transport electricity over much longer distances between different regions and countries.
900 800 700 600 500 2008 2009 2010 2011 2012 2013 2014 2015
Furthermore, the number of outages caused by faulty equipment/human error is still increasing (see figure 2): in 2015, it was up by 47% compared to 2008.
Year Figure 2 - Trend in outages caused by faulty equipment/human error
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SHORT-CIRCUIT STRESSES IN POWER TRANSFORMERS
The power transformer is the most vital substation component since its unavailability creates a major problem, given the high costs and the long time involved in repair or replacement. Power transformers are especially sensitive to short-circuit events, as will be made clear in the following. The effects of short-circuit currents in transmission and distribution networks for electric energy are tremendous, both for the equipment and for the stability of the networks. Since short circuits are not rare events (as a rule of thumb one short circuit per 100 km overhead line per year), short-circuit withstand capability is regarded as belonging to the main characteristics of the equipment installed. The capability to withstand a short circuit is recognized as a major and an essential requirement of power transformers. Failure to withstand results in damage to the internal (and even external) parts, and can lead, on short or longer term, to loss of service.
Radial and axial forces may have the following effect on the winding: n
The radial forces act in the radial direction with a pulsating radial force and tend to compress the turns in the inner (normally the lower voltage) winding and to expand the turns in the outer winding (higher voltage). When the mechanical design of the supports is not adequate, radial stresses may lead to buckling of the inner winding, which is observed frequently (see figure 4).
Free buckling Inner winding
External bulge Axial supporting strips
Figure 4 - Radial forces may result in buckling (schematically left and after a short-circuit test - right)
The axial forces act in the axial direction with a pulsating compression force. Axial and radial forces have been observed to result in spiralling and/or tilting of conductors (see figure 5). Through the deformation of turns, the oil flow may become obstructed, leading to the formation of hot spots or may lead to future winding short circuit.
Fr = radial force
Fa = axial force
Figure 3 - Radial (left, Fr) and axial forces (right, Fa) resulting from the leakage flux (B) and current (I)
Figure 5 - Axial forces may result in tilting (schematically)
Short-circuit current leads to electro-dynamic forces on the windings that cause mechanical stresses in the radial as well as axial direction. They result from the interaction of the current with the leakage magnetic field between the windings, in radial and axial direction, see figure 3.
Apart from winding deformation, a variety of deformation of internal, but also external parts has been observed in testing. Permanent deformation of windings may lead to immediate damage or long-term issues because of insulation damage, obstruction of oil flow, material weakness or loose parts. Damage and event break of bushings due to the mechanical shock has been observed as well. Transformers, like series reactors, have the ability to limit the short-circuit currents to values pre-dominantly determined by the transformer’s impedance. In this way, the design of a power transformer with respect to the short-circuit current withstand capability is focused towards the limitation of short-circuit current. In addition, the control of the forces and stresses exerted by the same short-circuit currents inside the transformer must be an integral part of the design process and quality verification. With an increase of the short-circuit power of power systems during the years, the most severe short-circuit currents will appear when the transformer is aged. These short-circuit currents have to be withstood without impairing the transformer. Short-circuit withstand capability should also cover the ability to withstand several full asymmetrical short-circuit currents in each phase and in each representative tap position without impairing the transformer suitability for normal service.
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POWER TRANSFORMER IN-SERVICE FAILURE
4.1 In-service failure statistics
A number of international studies, all conducted between 1974 - 2005 report a failure rate(1) in the range of 0.4 – 3%.
Failure rate (%)
design Design ageing Ageing
In 2015 a new extensive survey covering the period 1996 - 2010 of 964 major transformer failures(2) during 167.459 transformer years from 56 utilities in 21 countries was reported by CIGRE WG A2.37. This study reports an overall failure rate of 0.57% (“one major failure per 200 transformers in a year”). In the table below a breakdown is given regarding voltage class:
unknown Unknown others Others
4 3 2 1 0 <150
69 ≤ kV < 100
100 ≤ kV < 200
200 ≤ kV < 300
300 ≤ kV < 500
500 ≤ kV < 700
kV ≥ 700
More detailed analyses on failed subcomponent, failure mode and failure cause reveals the following: Ageing and external short circuit are the largest known failure causes (12.3%, 11.6% resp.), see figure 6. n Windings are the most common failure location (40%), followed by tapchanger (27%) and HV bushing (14%), see figure 7. n Mechanical failures account for over 20% of all failures, the second largest after dielectric failures (36%). n The failure rates of GSU units is in all voltage classes higher than of substation transformers. n
that their failure rate is significantly higher than that of conventional station transformers, between 1.6 – 5.4%, depending on their size, see figure 8, compared to the 0.6 % of AC transformers. The failure rate increases as the unit rating increases. Higher rating presents a challenge of keeping the dimensions as small as possibe for transportation reasons and at the same time meet all the electrical clearances and provide sufficient cooling of all the parts.
Figure 8 - Failure rate of converter transformers by MVA unit rating (LCC projects)
From a compilation of failure data of various key components of DC installations shows that the converter transformer is responsible for 52% of the downtime of DC stations, followed by the DC transmission line (28%), see figure 9.
Transmission Transmission line line 28% 28%
AC trafo AC trafo AC excexc tr tr AC Valve Valve Control&Protection Control & protection DC Equipment DC equipment Other Other Transmission line line Transmission
AC trafo AC trafo 52%
OtherOther % 1%
DC Equipment equipment 3%
Control&Protection Control & 3%
EPRI (USA) maintains the IDB database that began to be populated in 2006. Meanwhile, it contains more than 20.000 US power transformers. One result that clearly stands out is that “inadequate short-circuit strength” is by far the largest failure cause, with 20% of the total of 654 clearly identified failures. The high-voltage winding is reported to be by far the most probably failure location (at 45% of the 1112 identified failed components). Converter transformers supplying DC links are of a special design. In several publications, their reliability (in LCC based DC projects) is addressed[8, 9, 10]. It is striking to observe
Figure 6 - Failure cause analysis based on 964 major failures
Table 1 - Failure rates of transformers in the CIGRE 2015 study
winding Winding tapchanger Tap changer
Valve 6% 6%
AC exc tr
AC7% exc tr 7%
In a five year international survey, covering 94 cases with an average loss of 183 MVA of capacity per incident, a total claim of 287M US$ is reported. In figure 10 this is quantified in detail. By far the biggest claim come from loss of GSUs (71%) followed by industrial transformer losses (20%) and utility application (7%). The loss per MVA was capitalized at US$ 9000. Further narrowing this down, the major causes of failure (capitalized at 150M US$) are “insulation failures”, that include defective installation, short circuit and insulation deterioration.
bushings Bushings other Other
Figure 7 - Failure location analysis based on 675 major failures for U ≥ 100 kV
Figure 9 - Breakdown of components of HVDC stations, responsible for downtime
4.2 Consequences of transformer failures
In such studies, failure rate is usually defined as: (n1+n2+ … +ni) . 100%; (N1+N2+ ... +Ni ).T with ni = number of failures in the i-th year, Ni = number of transformers operating in the i-th year, T = reference period (normally one year) (2) Any situation which required removal from service longer than 7 days, requiring opening of transformer or tapchanger tank, or bushing exchange. (1)
external short circuitcircuit External short
From the CIGRE 2015 study cited above, it is reported that 7.2% of the major failures result in a fire, and 6% in explosion or burst, in total in 126 incidents. Major failures related to bushings most often lead to severe consequences. From the 115 bushing failures, 41% resulted in fire or explosion. A noticeable increase of fire probability is observed at higher rated voltages: from 0.02% per year at 120 kV to 0.29% at 735 kV, most prominently (in around 50%) by bushing failures. From a recent Brazilian study, 19% results in an explosion and 9% in fire. Apart from the direct risk, also from financial and operational side, the loss of a transformer is very costly and the lead time to move from order to complete installation is anywhere between 6 and 24 months. One underwriter quotes: “Power transformers have long been a major underwriting concern. Failure of a single unit can result in widespread loss of service with considerable lost revenue as well as replacement and other collateral costs. As an object class, transformers have for decades been ranked in the top five by equipment insurers in terms of claims. One insurance company recently reported 25 transformer claims during one fiscal year. The largest transformer loss on record occurred at a power plant, leading to business interruption costs of more than $86 million. Had this event, which took place in 2000, occurred today, costs would have been much higher”.
Power transformers are considered as critical equipment because of the large quantity of oil in contact with high-voltage/high-current elements. Experience has shown an increasing number of transformer explosions and fires in all types of power plants worldwide.
business interruption damage
100 80 60 40 20 0 1997
Year Figure 10 - Claimed losses per year in property damage and business interruption due to transformer damage
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SHORT-CIRCUIT WITHSTAND VERIFICATION
5.1 By design review One of the methods for purchasers to assess the short-circuit current withstand capability of transformers is to conduct a design review, based on calculation results only. CIGRE issued guidelines on this method that are implemented in the international standard IEC 60076-5, annex A. This annex is an informative document giving guidelines only, and it is not a standard by itself. A design review is based upon the following data: a) Electromagnetic design data sheets; b) drawings/sketches of the winding and insulation arrangement with indication of the types of material; c) calculation of the (asymmetrical and symmetrical) short-circuit current values, covering the possible types of fault and tap positions; d) calculation of the main forces, with a possibility to simplify complicated configurations. Radial and axial forces on the windings are required, as well as end thrust forces on windings, on exits leads and on clamping parts, in total 5 parameters; e) main tensile and compressive stresses on windings, conductors, spacers, paper insulation, stack structures, press rings and tie rods. In total 11 parameters have to be quantified (for core-type transformers); f) drawings of the support and clamping structures; g) instructions for QA/QC concerning materials and manufacturing activities; h) checks regarding external components, eg. bushings.
5.2 By full-power testing n
it is manufactured according to the same QA/QC practices; n the margins for short-circuit strength of both designs overlap. For the evaluation, the parameters in 5.1d, e are tabulated together with the reference ones. The design under review is considered capable to withstand the dynamic effects of the short circuit on condition that none of the force and stress parameters exceed the corresponding ones calculated for the tested reference transformer by 20% (and in case of three parameter values by 10%). 5.1.2 Checking against manufacturer’s design rules for short-circuit strength In this method, evaluation is by checking the force and stress parameters against the manufacturer’s design rules for short-circuit strength. These rules shall be based on a “solid experimental basis”, a number of short-circuit withstand tests of actual transformers or the outcome of tests performed on representative transformer models combined to any indirect supportive evidence based on long duration, trouble free operation of a number of transformers in the field. In order to substantiate this, the manufacturer has to submit the following data: a list of transformers built that were subjected to short-circuit tests, including its nameplate data; n results of these tests and possible impacts on design rules; n the technical standards of short-circuit strength, used in regular design as well as production activity; n service records and in-field failure rates regarding short-circuit performance and units produced;
In short-circuit testing, the transformer is subjected to the actual short-circuit current. Tests are performed in every phase, each one to be subjected to the full asymmetrical current, one time at minimum tap position, one at nominal and one at maximum, during 0.25 s of current. In total, nine tests in a three-phase transformer. After every test, the short-circuit reactance is measured. Increase of short-circuit reactance beyond a certain value(3) , laid down in IEC 60076-5, is an indication of unacceptable winding deformation and leads to a negative test result. As part of the certification procedure, a detailed out-of-tank active part inspection must be carried out after having performed the test, followed by a repetition of the dielectric tests at 100% of the specified voltage level. After passing all the requirement, a type test certificate is issued, see figure 11.
5.1.1 Comparison with a short-circuit tested reference transformer This method describes comparison with a reference transformer that passed short-circuit tests successfully on the condition that: The design under review can be considered similar to the reference transformer. In IEC 60076-5, Annex B, “similarity” is defined by having an absorbed power in a range between 30% and 130%, and exceeding forces and stresses by not more than 120%; n it is designed using the same calculation methods and withstand criteria; n
For the evaluation, the forces and stress parameter values are to be compared with the critical values that the manufacturer has adopted in his design practice. These data are to be collected in a table that allows direct comparison between “actual”, “allowable” and “critical” values. The transformer is considered able to withstand the dynamic stresses on condition that none of the force and stress figures exceed the allowable force of stress and remains below 80% of the critical stresses.
The great step forward that calculation methods have made in last decades, together with the increased emphasis on cost reduction, can lead to a practice of designing close to the margin, whereas in the past – due to a greater uncertainty and lower cost pressure – a larger safety margin was built in. This suggests that advanced design methods not automatically guarantee a more reliable product. The highest degree of reliability with respect to short-circuit withstand capability is through full-scale short-circuit withstand testing in accordance with the international standards.
In the design evaluation of the transformer, two alternative methods can be adopted: comparison with a reference, short-circuit tested transformer or checking against manufacturer’s own design rules for short-circuit strength.
Test experience shows that, in spite of the ubiquitous use of simulation programs in the design of transformers, still around 25% of the transformers do not pass the standardized short-circuit test, see section 9. The large costs and the possible delay of projects that results at a failure to pass makes it clear that manufacturers do everything they can to ensure a positive test. The fact that nevertheless 25% fails to pass illustrates that the result of the test cannot be predicted beforehand by calculation.
Figure 11 - Example of a KEMA Type Test certificate of short-circuit performance
Short-circuit testing is considered a better means of ascertaining the real performance of equipment at short-circuit, since such a test demonstrates that both construction and design are adequate. This is recognized in an IEC 60076-5 Annex B that states (for transformers > 2.5 MVA): “for the purpose of evaluation the unit under consideration may be simultaneously compared with a limited number of transformers that have passed the short-circuit test successfully and match most - but not all the characteristics considered in Annex A”.
Short-circuit tests do not reduce the life-time of a welldesigned transformer. Reputable manufactures agree on the fact that a properly designed transformer with enough margin to handle the electro-dynamic stresses, the effect of the short-circuit stress will be that the windings undergo a certain settling. The effect of the settling is that the stiffness of the windings increases and this is visible in a small variation (if any) in the reactance values measured between the first tests, but becoming smaller or nihil at the last tests, see section 10.1.1. Such a transformer is even stronger after the short-circuit tests than before, and can be put safely in service. Large utilities in for example Italy, Canada, Turkey have not seen any service problems on the units which have been short-circuit tested. Chinese experience shows that 40 transformers (110 - 220 kV) put into service after passing short-circuit tests, function without problem during the monitoring period from a few months to more than 5 years.
For transformers of category III (above 100 MVA) this value is 1%; for smaller transformer this number depends on the winding geometry.
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5.3 Limitations of the design review approach CIGRE studies show that calculation methods are an indispensable tool in the design phase of electrical equipment and modern, multi-physics finite-element 3D simulation tools can predict internal stresses. However, they cannot be applied for the verification of equipment performance verification. This also applies to power transformers. For a number of reasons, listed below, the value of design review is limited, and generally insufficient as replacement of short-circuit withstand verification by laboratory testing. 5.3.1 Simulation tools are a simplification of the reality By means of simplifications it is possible to calculate approximately the highest stresses that occur. In order to evaluate these stresses, it is necessary to compare these with critical values (see section 5.1.2) of forces and stresses. This implies that also data of failed short-circuit tests shall be available, which can only be the case when a database of solid testing experience is at hand. More generally: It is very important that the manufacturer can demonstrate that the calculations and considerations of the design characteristics and manufacturing practices used are based upon and validated by short-circuit tests. The most obvious simplifications are: disregard of the influence of the other phases on the magnetic fields in a certain winding, calculation of the forces and stresses for the current peaks and RMS values only, consideration of the windings as rigid (i.e. without any flexibility or settling effects), consideration of the windings as rotational symmetrical, etc. The most onerous circumstances can be selected by such calculations and simulations. An exclusive focus on the copper yield strength is not sufficient. Therefore, at the design stage extra, difficult to quantify, margins need to be implemented to cover such effects. Another simplification is the assumption of rotational symmetry of the windings. IEC 60076-5 does not take into account deviations from the ideal cylindrical shape, not from discontinuities both from design and manufacturing point of view.
From the design point of view: n All kind of magnetic field deviations from rotational symmetry such as yoke, unwound leg, other phase and tank wall; n all kind of mechanical discontinuities in the cylindrical shape, such as transpositions inside the winding, crossovers in disk windings, pitch of a winding and insulation supports inside the winding; n type of winding with parallel conductors and current division; n friction between turns and impact on insulation; n the interaction between phases, both magnetic and mechanical; n dynamic aspects, such as mechanical resonance in axial direction and the influence of the oil. From the manufacturing point of view: Quality of cured epoxy (bonding strength as a function of operating temperature) and the quality of the epoxy bonding process; n roundness of winding; n dimensional tolerances; n balancing windings on axial symmetry; n paper and insulation material shrinkage after the drying process; n final pressing and tightening procedure before assembly in the tank. n
5.3.2 The list of reviewed sub-components is not complete The design review should check all critical force and stress values, but it is impossible to ensure that all necessary aspects are considered. One omission is that calculations related to lead supports and connections to bushings are not requested. Some aspects that are not (or insufficiently) considered and/or deviate from (not validated) assumptions during the design stage related to the short-circuit withstand capability are: n cross overs of turns (inside the winding) and transpositions of parallel conductors (inside the winding); n exit leads of the windings, fixation to prevent movement and friction (wear of insulation) of exit lead; n support of cleats and leads and connections to tapchanger; n support of leads to bushings; n stability of the radial support of windings, for example spacers used during the winding of the coil, untreated, dried, dried and oil impregnated;
Stability of the axial support of windings. The axial pressure applied on the individual windings of the entire winding block is sensitive for the shrinkage of the insulation material, because the height of the winding is partly made up by copper and partly by insulation material. The shrinkage of copper is fixed and negligible compared to insulation material. Difference in shrinkage of the insulation material blocks in the overall winding block assembly can lead to different heights of the individual windings in the commonly pressed winding block and therefore different pressure on the individual windings. During short circuit it can occur that windings will be over-pressed resulting in damage of conductors (crossovers or transpositions) or that windings are under-pressed and insulation blocks will come loose and may come out destroying the mechanical stability of the winding (block) and resulting in an electrical and/or mechanical collapse of the winding. A large scale study showed that aged transformers are more prone to short-circuit winding deformation because of diminishing winding fastening strength due to insulation shrinkage and pieces of insulating material disappear or become displaced.
5.3.3 The design review approach is static whereas the phenomena are dynamic The electrodynamic stresses are varying in time and are varying spatially. Complicated, three-dimensional time varying fields and forces stressing various non-homogeneous and non-linear structures and materials cannot be covered adequately by simulation. The transient mechanical behaviour of the windings (natural frequencies, damping, non-linear effects) and the production tolerances (tolerances in materials, processing, assembling, etc.) make it very complicated to exactly simulate the winding’s behaviour. As the dynamic behaviour of the windings during a short circuit is not mentioned in design review no aspects of inter-winding (dynamic) oil flow or tank stress calculations are considered. Moreover, design review does not assess the fact that successive short circuits lead to a sequence of events. Typically, after the first application of a short circuit in a test, a slight reactance increase is observed (see figure 25). Successive movement of the winding, core, clamping arrangements and leads due to accumulated stresses during a series of short-circuit tests, leading to a failure after a certain number shots cannot be simulated/calculated.
5.3.4 Design review does not cover material and production deficiencies Natural variations in properties of material, quality assurance, workmen’s skills etc., especially in the less experienced companies, cannot be taken into account, nor can any design review reveal deficiencies resulting from this. From CIGRE studies, it became clear that models can predict stresses but the prediction of yield to stresses is another matter, since inhomogeneity of material determines where and when a disruptive event (breakdown, rupture, burst etc.) occurs. 5.3.5 Many failures occur in other than the “design reviewed” sub-components Complicated ‘secondary’ physical phenomena like shock waves in oil, shocks and vibrations (leading to untimely falling-off of buchholz relays or damage to ancillary equipment - bushings, tap-changers etc.) are normally not considered at all in calculation methods and design review. There is no quality control on the performance for the design calculations. The theoretical evaluation of the ability to withstand the dynamic effects of short-circuit will in general be performed by non-specialized purchasers and/or (independent) skilled and/or unskilled third party inspectors or consultants. No defined procedure is available how to judge the theoretical evaluation and how it can be assessed. Statement letters provided are in general unique to the inspecting body. The theoretical evaluation procedure leaves opportunities for “shopping” to get “somewhere” a positive statement letter. The definition of a “similar” design (in IEC 60076-5 annex B) is too ambiguous and may lead to confusion. This is a difficult point since purchasers generally do not have the specialist’s knowledge to decide that an available short-circuit tested transformer can be considered as “similar” to the transformer under evaluation. On the other hand, a short-circuit tested transformer is certified by an accredited test laboratory which covers everything. The certificates, see figure 11, provided by accredited test laboratories are uniform in style and acknowledged everywhere without any restriction.
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16 ENERGY Power transformers need real power to survive a short circuit
6 5.3.6 Conclusion The short-circuit withstand capability of a transformer cannot be presented in a set of equations and summarized in a spreadsheet table as in the IEC 60076-5 Annex A. Verification based on design review is an oversimplification of the existing knowledge and it neglects the vast design and manufacturing experiences of the different manufacturers. Theoretical evaluation seemed to be introduced because of short-circuit testing limitations in the past, as an alternative in case short-circuit testing was not possible. In due time, test laboratories have increased their short-circuit power and can cover the vast majority of power transformer ratings, see section 8. According to IEC 60076-5 it might seem that design review unrestrictedly can be chosen as an equivalent for shortcircuit testing, but this was not the original purpose of the theoretical evaluations that are behind a design review.
As such, testing of models plays an important role in linking theory and practice and they assist in material selection, choice of geometric arrangements and manufacturing procedures. The contribution that model testing can make is as follows: n Evaluation of the dynamic characteristics of materials; n definition of critical stress levels which can lead to failure; n validation or calibration of analytical and numerical modelling methods; n investigation of the effects of material properties and manufacturing process variables; n examination of the mechanical effects of repeated short circuits. Experiments on models are insufficient to warrant shortcircuit withstand capability of a complete transformer. Calculations and tests on models alone are not sufficient to renounce the short-circuit capability of a transformer.
There is very limited information on direct evidence of failures in service after short-circuit withstand evaluation. From a survey conducted by CIGRE SC12 (covering 121460 transformer years in the period 1993-1997) 15 failures attributed to short-circuit were identified. In 5 cases (33%) design reviews had been performed, whereas none of the failed units (or similar designs) was short-circuit tested.
In the past, utilities mainly relied on the selection of trusted manufacturers to secure the short-circuit capability of their transformers. However, in recent times the situation has changed, with more and more users asking for tests on critical units or prototypes of series of identical units. This may be because a long term confidence between purchaser and manufacturer is less and less achievable because of increasing deregulation. The high failure rate in service due to poor short-circuit performance before the year 2000 led to the adoption of short-circuit testing as a method for quality improvement. As an example: short-circuit failure rate was 0.4% in France, 0.35% in Italy, 1.2% in Turkey. In China 84% of all internal failures was due to short circuit  and in India over 80% of the failures were caused by winding displacement. The experience achieved through tests, and sometimes even if the test result is negative, turns out to be a precious source of information and knowledge. Critical spots can be detected and simple measures be set up, often with negligible impact on costs, which result in being highly beneficial also with respect to long term reliability. A significant and positive influence of short-circuit testing on the reduction of the rate of faults in a large overall system during many decades is reported[18, 31, 32]. In the 1960s it was reported that in the US from 50% - 85% of the transformer failures were due to short-circuit withstand deficiency, whereas in 1979 this percentage was estimated to have dropped to 20% - and remains to this rather high level, see section 4.1.
5.4 By testing of scale-models (“mock-ups”) Sometimes testing of downscaled models (or “mock-up”) of actual transformers is practiced, mainly because the actual transformer’s capacity is too big for any laboratory. Examples are large GSU’s for nuclear power plants and phase-shifting transformers. The results of such tests are useful to obtain input data for models, to validate certain calculation methods, or to study relevant phenomena because such “research oriented” models allow additional observations, such as strain gauge measurements or visual observation of moving parts through observation ports. In figure 12 a test on a mock-up is shown.
Figure 12 - Testing of a mock-up of a 570 MVA transformer
One might state a parallel with the dielectric withstand capability: “Although the design of the insulation structure is based on the calculation of voltage distributions along the windings, also supplemented by thousands of tests and measurements on simple insulating structures or on more complex models, nobody would accept to abstain from performing dielectric tests on a transformer”.
A major utility’s experience indicates that premature O&M failures are occurring due to accelerated ageing and/or weakening of short-circuit withstand capability of transformer due to repeated short circuits in the underlying system. The number of through faults seen by their transformers is high and has a cumulative effect on the mechanical weakening of the winding supports and insulation, and it increases the probability of premature failure of the transformer. To check that manufacturers ensure short-circuit withstand capability as per network requirement/specification of their transformers during design/manufacturing, this utility introduces short-circuit withstand test for at least one transformer for each manufacturer which will be type tested and benchmarked for future projects.
Other major utilities[19, 29,34, 35] require suppliers to pass a learning path towards a successful design through full-power short-circuit testing thereby using it as an essential and successful tool for quality improvement. The considerations above have led to a rapid increase in testing of large power transformers in the world’s test facilities. In figure 13, the number of short-circuit tests of transformers ≥ 25 MVA is plotted vs. time, performed at the KEMA Laboratories. In this figure, a distinction is made between the test results showing no problem at short-circuit tests and tests that lead to a problem, see chapter 7.
initially not OK
25 20 15 10 5 0 '96 '97 '98 '99 '00 '01 '02 '03 '04 '05 '06 '07 '08 '09 '10 '11 '12 '13 '14 '15
Figure 13 - Number of power transformer (≥ 25 MVA) short-circuit tests performed 1996-2015 at KEMA Laboratories. Indicated is the fraction of initially “not OK” and “OK”.
Of course it is impractical and even impossible to verify short-circuit withstand of all transformers by testing. In the following cases, purchasers have good reasons to specify short-circuit withstand tests: n In case of utilities, whose transformer fleet mainly consists of standard units. If several identical units are going to be purchased at one time, or if it deals with units to be manufactured on the basis of new or revised designs, it may be good practice to submit the first unit of that series to a short-circuit test, and then, only after the successful completion of that test, release the following units for production: the “learning path” approach;
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18 ENERGY Power transformers need real power to survive a short circuit
in case of transformers performing a key function in the network, for which the reliability issue is of prime importance. This applies, for example, to key GSU transformers (already having a relatively high failure rate, see section 4.1) and unit auxiliary power supply transformers installed in nuclear power stations. strategic interconnection (auto)transformers located in strategic stations or huge consumption centers (server parks); transformers requiring a special design, for example entailing a primary winding and two equally-sized secondary windings, each with half power rating compared to the primary (axially split winding type); transformers to be installed in networks with high fault incidence, expected to face a heavy duty operation consisting, for example, of tens of short circuits per year. Any forecast of a sharp increase of the short-circuit power level in the network should also receive due attention with respect to this issue; track feeding transformers; transformers having a low short-circuit impedance and/or installed in solidly earthed systems. This is often the case in USA, where transformers often have lower short-circuit impedance than in Europe.
Short-circuit test laboratories for power transformers draw their short-circuit power either directly from the power grid (grid-supplied lab, see figure 14, upper) or from generators (generator supplied lab, see figure 14, lower).
Grid supplied lab
FEATURES OF KEMA LABORATORIES
Making switches (MS in figure 14) that switch in the short-circuit current. Also these switches can be much more accurately controlled at medium voltage level. When high-voltage breakers are used (HV CB right side in figure 14 upper), the statistical nature of the pre-strike upon closing makes it difficult to close at the exact voltage phase that will result in a full asymmetrical current as required by the standard. This may lead to unnecessary trial testing with insufficient asymmetry levels.
At present, KEMA Laboratories uses six generators as its power source. These operate at a voltage 15 – 17 kV. A number of circuit topologies have been designed for optimum conformity with the service situation:
Three-phase tests. Three-phase transformers should preferably be tested three-phase. In case the voltage range is not sufficient or the short-circuit power is not enough, the testing authorities may use single-phase testing instead of three-phase testing. As in three-phase testing, at each test one phase is subjected to the specified (peak) current value, see figure 15 where the asymmetrical peak is applied to the upper phase. During later tests the other phases are subjected to the required current.
Generator supplied lab
Figure 14 - Lay-out of grid- and generator supplied test laboratories. T: laboratory transformer, TO: transformer under test; HV CB: high-voltage circuit breaker, MB: master breaker, MS: making switch, G: generator, M: motor, L: reactor
Grid supplied labs have the advantage that very high power can be drawn from the grid - when the grid operator gives permission - but regarding security for the transformer under test, generator supplied labs offer lower risk because switching occurs at the output voltage level of the generator, usually around 15 kV: Master breakers (MB in figure 14), that need to interrupt current, can be designed to act very fast at this voltage level. Very fast interruption of short-circuit current, in combination with a special protection system prevents major damage to the transformer under test in case of an initial failure, like an internal arc, or fire because of oil spill. In our case, the master breakers can de-energize the circuit within 6 ms. In grid supplied laboratories, master breakers have to operate at high-voltage, which implies that breaking times of several tens of ms are unavoidable.
Moment of maximum stress
Full asymm. current in one phase Current in three-phase test
Current in 1.5 phase test Current in three-phase test
Current in 1.5 phase test Figure 15 - 1.5 Phase test method showing that at the moment of maximum stress, the current in all phases is identical to a full three-phase condition
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20 ENERGY Power transformers need real power to survive a short circuit
8 Make switch
Transformer under test l
1,5Uphase 1/2l 1/2l
Figure 16 - Test-circuit for 1.5 phase short-circuit tests
Single phase tests. In case the test station’s power is not enough for using the 1.5 phase method, a real single- phase method could be applied, like the method used for single-phase transformers. In that case the terminals of the other phases are open.
In order to be prepared for testing of very large transformers up to the 800 kV class, investments are made for increasing the short-circuit capacity of the author’s laboratory by 50%. This is achieved by installing two additional generators and four short-circuit transformers, making available a direct test-power of approx. 15000 MVA. Figure 17 shows one of the new generators.
In this figure, tests of the past years are plotted using the rated voltage and rated power as coordinates, distinguished by the test method. In addition, several new possibilities are entered as an indication. The actual feasibility for testing, choice of test method and test circuit will ultimately depend on parameters such as short-circuit reactance of transformer to be tested, the specific grid short-circuit power etc. Key assets of any laboratory are the short-circuit transformers that need to adapt the supply voltage to the required voltage of the transformer under test. For the laboratory expansion project, four new short-circuit transformers have been designed, tested and installed. They will be used in addition to the six existing laboratory transformers (2x14 kV/2x36 kV, 250 kV isolation to earth). The ten transformers in series realize a direct testing voltage of 550 kV. This implies that having the laboratory extension available, single phase transformers in the 500 – 600 MVA class can now be short-circuit tested (up to 1500-1800 MVA three phase). Voltage wise, transformers with rated (single phase) voltages up to 800 kV can be tested as well, which has indeed be demonstrated, see figure 19, showing an 800 kV transformer after a short-circuit test.
Figure 17 - New generator, 2500 MVA 16.7 - 60 Hz
Technically, the impact of the extension is plotted in figure 18 showing the technical feasibility for testing of the various voltage and power ratings of transformers before and after the extension of the laboratory.
1.5 phase tests. With the single-phase method, known as 1.5 phase method, the phase under test is connected in series with the other two phases, which are connected in parallel, figure 16. The RMS currents in the two parallel connected phases are 50% of the specified three-phase value. Figure 15 illustrates this condition. The evolving stresses in the two parallel connected phases are in this case lower than in the case of three-phase tests, but at the most critical moment, at the asymmetrical current peak in the fully stressed phase, all currents are momentarily identical to the situation of a three-phase test. In this topology only half the power is required from the test station compared to three-phase testing. Special attention has to be given to the neutral of the Y-connected windings, which in many other test stations is at a voltage level of 0.5 pu. In our practice this remains at ground potential.
Figure 19 - An 800 kV transformer under test
100 0 0
Figure 18 - Past and newly added test potential for power transformers vs. rated power, voltage and test method
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22 ENERGY Power transformers need real power to survive a short circuit
TEST RESULT STATISTICS OF POWER TRANSFORMERS
During the past 20 years, in total 297 times a test access for a transformer larger than 25 MVA (258 transformers from which 39 are re-tested) has been counted: In 230 cases, the transformer showed no problem at the test-site. These transformers initially passed the short-circuit test. The final test-result is not always known because there was in a limited number of cases inspector’s involvement in the subsequent routine tests and the visual inspection. n
In 67 cases a transformers showed a problem due to short-circuit stresses that became immediately apparent at the test site. Mostly, this problem was an unacceptable increase of short-circuit reactance due to the short-circuit stress, but a range of other, immediately evident problems also occurred; 39 transformers from the latter group had been re-tested after modification in the factory and most did not show a problem at the test site at the re-test; In 7 cases, transformers after having experienced no problem at the test site, did not pass the routine tests and/or visual inspection after the tests.
From these results, an initial failure rate is defined as the ratio of tests that resulted in failure to pass the test at first access (67 times) and the total tests (297). Thus, the initial failure rate is 23%. In figures 20 and 21 results are shown, differentiated in both power- and voltage class. The results suggest a tendency of the highest initial failure rates for the highest ratings: the failure rate of the largest transformers (> 300 kV or > 200 MVA), around 100 tested, is in the range of 30%.
Number of tests
Evidence of damage, as suggested by measured reactance variation is usually confirmed by visual inspection. In addition, other, most clearly recognizable defect are evident directly at the test site or upon inspection of the internal parts.
40 20 0 25-50
Figure 20 - Initial power transformer failure rate for various ranges of MVA rating
initially not OK initially OK
80 60 40 20 0
Figure 21 - Initial power transformer failure rate for various ranges of kV (primary) rating
The failure rate at testing, observed by KEMA Laboratories is in the same order as the experience reported by another major test laboratory that reports a failure rate of 20 – 25% out of 20 units > 100 MVA. Other sources state an overall failure rate of 23% for a total of 3934 tests and 21.4% failure (2.5 – 100 MVA), 41.9% failure (> 100 MVA). Test failure rates of 15 test-laboratories worldwide are summarized in figure 22.
Axial clamping system: looseness of force in axial clamping, of axial compression force, of axial supporting spacers and of top and bottom insulating blocks; n windings: axial shift of windings, buckling, tilting of conductors; n cable leads: mechanical movement, for instance from tap changer to regulating windings; deformed or broken leads, outward displacement and deformation of exit leads from inner windings; broken exit leads; n insulation: crushed and damaged conductor insulation; displacement of vertical oil-duct spacers; dielectric flashover across HV-winding or to the tank; displacement of pressboard insulation; tank current due to damaged conductor insulation; n bushings: broken or cracked bushings (see figure 23), leading to oil spraying; n enclosure: spraying of oil (see figure 24), exhaust of hot gases, evaporated oil, measurement of current to enclosure. n
On the other hand, in the cases (the vast majority) that the reactance change is within the tolerances set by the standards, it is the author’s observation that (visual) inspection sometimes still leads to rejection of a certificate. Visual inspection is necessary, because deformations and displacements in supporting structures, clamping systems, insulating materials, winding exit leads, external connections from the coils to the tap changer and within the on-load tap changer cannot be detected by the reactance measurements only. In addition, defects to the regulation winding, often not detectable by impedance change can only be confirmed by visual inspection. The authors conclude that the reactance variation is a very good tool to assess short-circuit withstand capability right after the short-circuit test. It can not be used as a ‘passed’ indicator, only as a ‘not-passed’. Our experience with the short-circuit reactance measurements is that for power transformers a variation of more than 1.0% indicates a large deformation in one or more windings. Also a gradually increasing variation during the short-circuit tests, although in total not more than 0.5% to 1.0%, indicates a progressive movement of conductors. Variations of the reactance values between the short-circuit tests in an unusual way are an indication of instability of the windings.
<=2.5 MVA 2.5-40 MVA 41-100 MVA >100 MVA
40 30 20 10
Figure 23 - Breaking of a bushing and oil spill Figure 24 - Oil spill upon short circuit application (4) The data in this figure are compiled based on 4961 tested transformers, of which 1140 (23%) failed to pass. 400 are transformers > 40 MVA, of which 83 failed to pass. This figure has indicative value only, since various time periods, testing practices and testing purposes are considered.
Another item to be mentioned is the behaviour of buchholzrelays, as quite often buchholz-relays operate unnecessary due to the vibrations that occur during short-circuit conditions. Therefore, the behaviour of such relays is carefully monitored during testing and the observations are reported.
A wide variety of defects are revealed such as:
Commonly, the reason of not passing short-circuit tests is because the winding reactance change (usually an increase) is larger than specified in the standards.
initially not OK
Number of tests
Having 15000 MVA of installed short-circuit capacity (world’s largest), the author’s laboratory has tested transformers up to very high MVA and kV ratings. An evaluation has been made of short-circuit tests performed in the 20 year period 1996-2015. The tests were performed in accordance with IEC standard or IEEE standards on transformers with rated power up to 440 MVA and primary voltage up to 800 kV. The population includes single-phase and three-phase transformers, auto-transformers, step-up-, converter-, railway-, auxiliary- and three-winding transformers, 16.7, 50 and 60 Hz transformers, YD-, DY-transformers and YY autotransformers. The largest transformers tested are 334 MVA single-phase and 440 MVA three-phase. In detail, the test-experience is as follows:
Test failure rate (%)
Figure 22 - Test failure rates of test-laboratories worldwide (data used from )4
Although the standards do not require (S)FRA measurement, FRA measurements are regularly performed before and after short-circuit application. In some cases, a significant difference can be observed. Although a FRA pattern contains in principle more information than a single reactance measurement at a given frequency, it appeared difficult to correlate the observed shifts in the FRA patterns to visible internal deformations. More research is needed to establish knowledge rules.
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24 ENERGY Power transformers need real power to survive a short circuit
11 10.1.1 Observations from short-circuit tests In the case that a continuous increase of winding reactance is observed during all tests, it is reasonable to assume (by extrapolation) that the transformer undergoes accelerated ageing due to the short-circuit tests. In figure 25, such an example is given. Herein, the change in reactance after successive short-circuit tests is plotted. The left graph indicates a common failure in all three phases (after visual inspection this turns out to be deformation of the helical windings), whereas the right graph suggests a failure of the W phase only (which turned out to be a defect of the axial clamping system). The other phases (U, V) will not deteriorate due to high current stress.
Short-circuit reactance (0.5%/div)
TAP 5 Isc=82%
TAP 3 Isc=100%
TAP 5 Isc=90%
TAP 1 Isc=118%
TAP 3 Isc=100%
TAP 1 Isc=110%
phase U 0
phase V 0
Phase U under test
Phase V under test
Phase W under test
6 test number
Phase U under test
Phase V under test
Careful observation of the reactance evolution, measured after each test, can reveal short-circuit current withstand deficiency of the regulation winding. According to IEC 60076-5, 9 tests have to be performed in total (on a three-phase transformer) with 3 tests in minimum tap position, 3 tests in nominal and 3 tests in maximum tap position. A (measured) example of this procedure is shown in figure 26. As a result of this test, the observed short-circuit reactance change is very close to 1% (in phase 3). What is striking, however, is that the total reactance change (after completion of all tests) in the minimum tap position (0.1%) is much smaller than the value in the maximum after-test tap position (1%). Apparently, inclusion of the regulation winding brings the total reactance change from 0.1% to 1%. Given the fact that the contribution of the regulation winding to the total externally measurable short-circuit reactance is only a fraction of the total reactance, it is reasonable to expect that the reactance change of the regulation winding alone is far larger than 1%. The effect of this was confirmed in several case in recent years, where visual inspection of the regulation winding revealed unacceptable damage to this winding.
Phase W under test
Figure 25 - Evolution of short-circuit reactance increase during two test series Left: transformer with common failure in all phases Right: transformers with a failure in phase W only
Failures of transformers in service are well-known, but the reason for failure is often not clear[6, 39, 40]. One could wonder on what causes the high failure rate, observed by short-circuit test laboratories worldwide compared to the lower failure in service, found to be below 1%[6, 41], mainly due to defects in windings. The main reason of this discrepancy must be the severity of the tests, compared to actual service conditions. From an enquiry of CIGRE WG 13.08 it can be concluded that on a statistical basis, large power transformers have to face several full and many small short-circuits during their life, more precisely: the 90 percentile was estimated to be 4 full short-circuits in 25 years.
0,6 phase 3 0,4
0,2 test number
5 rated tap
10 tap11 min. after tests
Figure 26 - Evolution of reactance change in short-circuit test that reveal large impedance change of regulation winding
Thus, it must be assumed that this actual (full) short-circuit current in service is normally smaller than the rated shortcircuit current for which the transformer is designed. Because of the expected future increase in system power, especially in rapidly developing countries, this situation may change and the fact must be faced that during the life of the ageing transformer, its withstand against short circuits will be brought to the limit.
Short-circuit current leads to extreme mechanical forces in transformers which need to be managed in the design by adequate clamping and support of all relevant subcomponents, not only the winding. n Short circuits are a major contributor to damage to power transformers in service (up to 20% of the major failures). n Short-circuit testing is the only complete verification method of short-circuit withstand capability of power transformers. KEMA Laboratories are now ready to short-circuit test power transformers with rated voltage up to 800 kV and power up to 1000 MVA (based on three-phase transformer banks). The actual limit depends on many variables. n Design review is based on calculation results of idealized, homogeneous structures, it does not cover transient phenomena, it excludes a number of key subcomponents and it is not embedded in a strict quality surveillance system. It is not a good representation of the reality and it cannot serve as a complete and therefore as a reliable verification tool. n Failure of passing a short-circuit test is in the 20 – 30% range as confirmed by major test laboratories worldwide. n
26 ENERGY Power transformers need real power to survive a short circuit
Power transformers need real power to survive a short circuit ENERGY 27
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