S08 Short Circuit Presentation

Marcel Fortin Eng, Charlotte, SC, March 17, 2008

Short-Circuit Strength and Short Circuit Testing of Power and Distribution Transformers

Charlotte, North Carolina, March 17, 2008

Presented by: Marcel Fortin (Consultant), Pierre Riffon (Hydro-Québec), Juergen Gerth (ABB) and Richard P. McLaughlin (Kema Powertest)


Introduction


Presented by: Marcel Fortin EngTest specialistConsultant


Contributors

Marcel Fortin: test specialist retiredfrom Hydro-Québec High Power LaboratoryJuergen Gerth: Technical Manager of ABB Inc. Power Transformer Division at Varennes, Quebec, Canada.Pierre Riffon: test specialist for the Hydro-Québec Quality Control DepartmentRichard P. McLaughlin: Test Engineer Supervisor, KEMA Powertest.


Presentations

Standard Status – M FortinDesigning Transformers to Withstand Short-Circuit Forces – J GerthShort-Circuit Withstand of Power Transformers, Utility Perspective – P RiffonShort-Circuit Testing of Liquid FilledTransformers – R McLaughlinSome Particular Cases – M Fortin


Standard Requirements and Status




Continuous revision of C57.12.00 and C57.12.90

C57.12.00 sets requirements for short circuit withstand of oil filled transformers, clause 7 and table 21C57.12.90 sets the short-circuit test procedure, including the pass criteria and diagnostic tools, clause 12.A TF chaired by myself is responsible for reviewing clause 12 of C57.12.90


Short Circuit withstand requirements

IEEE C57.12.00-2006, clause 7.1.1, states:"Liquid-filled transformers shall be designed and constructed to withstand the mechanical and thermal stresses produced by external short circuits under the conditions specified in 7.1.3, 7.1.4, and 7.1.5. The external short circuits shall include three-phase, single line-to-ground, double line-to-ground, and line-to-line faults on any one set of terminals at a time.“IEC 60076-5 has the same design requirements


Short Circuit testing requirements

IEEE C57.12.00, clause 8.2 and table 21, states that short circuit test is:

A design test for transformers rated 500 kVA and belowA test that may be specified by the purchaser (other) for transformers rated 501 kVA and higher (clause 8.2.3).

IEC 60076-5 has also the same design requirements


Status of changes recommended by the Task Force

No changes to C57.12.00Abandon of work on a short circuit test guide (PC57.133)

This guide project covered interpretation of LVI (low voltage impulse) diagnostic methodThe proposed change to C57.12.90 include removal of LVI as a diagnostic method


Status of changes recommended by the Task Force

Major changes proposed to C57.12.90 are:Withdrawal of LVIMake dissolved gas analysis (DGA) a compulsory diagnostic toolIntroduction of other diagnostic tools: FRA, FRSL, TOP Make the pre-set method the preferred one and keep the post-set method as an alternativeIntroduce requirements and guidance for testing special transformers as axial splitAdd informative notes and figures to clarify the requirementsAdd an informative annex giving test circuit diagrams for singlephase testing of 3 phase transformers

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Designing Transformers to withstand

Short-Circuit Forces

Jürgen Gerth Technical ManagerABB Inc.Varennes, Quebec Canada

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Designing Transformers to withstand Short-Circuit Forces

In case of external short-circuits the first peak of the fault current over the transformer will increase to a multiple of the rated current.

The fault current is governed by:

Open-circuit voltage

Source and transformer impedance

Instant of fault onset

Displacement of current

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2D Plot of the magnetic stray field lines of a two winding transformer.

Radial direction

Axial direction

The stray flux has components in axial and radial direction.

There are field compoments outside the windings.

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Electromagnetic forces tend toincrease the main insulation duct and to increase existing un-symmetries.

The direction of forces is always directed perpendicular to the magnetic field lines

Forces usually are split into the two components

• Radial forces

• Axial forces

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Buckling:Characteristic failure mode for inner winding

• Buckling of the inner winding• Diameter increase of the outer winding, rupture of cable

• Spiralling of end turns in helical winding

Spiralling:Characteristic failure mode for inner and outer winding

Radial forces failure modes:

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Conductor tilting Bending of cables

Collapse of end support

Axial forces failure modes:

Telescoping of windings Damage of conductor insulation

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Steps to design transformers to survive short-circuits

• Three phase fault on HV, LV and Tertiary side

• Line to line fault on HV, LV and Tertiary side

• Line to ground fault on HV and LV side

Consider the network impedance in line with the Standard or the spec.Consider different tap changer positions

Consider that impedances are subject of tolerances

Consider that current limiting reactors may show saturation effects

Step 1: Run fault current calculations, covering all external fault modes:

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• Radial forces and copper stresses in all windings• Spiralling forces on the exits

• Axial compression forces and:• Bending stresses in the cables

• Stresses on the paper of the winding conductors

• Critical tilting stresses of the winding conductors

• Axial forces on the end supports and stresses in the mechanical support structure

Step 2: Calculate the forces and the stressesStep 2: Calculate the forces and the stresses

The highest fault current will not always lead to the highest forces in a winding. Superimposed fields of other windings may create higherstresses

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• Design the windings radially self supporting• Select the cable dimensions, copper yield strength and epoxy

bonding to meet the stresses

• Focus on axial ampere-turn balancing optimization of all windings,

• Require close manufacturing tolerances,

• Provide solid clamping of the windings,

• Provide safe fastening of the winding exits and connection leads.

Step 3: Take the right design measures

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Which simplifications are applied during design?

• The forces alternate with the square of the current. During design only the peak force is calculated and considered as a static force. Adynamic factor may be used to consider the dynamic effects.

• Core and coils are a 3 D arrangement. The routine field calculations are ran with 2 D programs,

• Layer and helical windings have a pitch creating variable displacements. It depends on the manufacturer’s rules which displacement is considered,

• The calculation of the forces on the winding exits is based on asimplified model.

• The mechanical withstand limits of helical windings against spirallingforces are based on limited number of experiments.

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Which improvements have been achieved over the last 20 years?

• Computer programs run fault current and force calculations for all fault conditions and all winding connections automatically,

• 3 D magnetic tools have been applied for R&D work,

• Epoxy bonding of winding conductors has become available and the application became a standard practice,

• Hard and very hard drawn copper is applied more frequently,

• Pre-compressed pressboard is exclusively used,

• There is more focus on the short circuit strength during the design stage,

• More experience has been gained in the field and by testing.

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Why do transformers fail testing?

• Due to design weakness because: • The rules did not fully cover the case,

• There are effects which have not been considered,

• Due to shortcomings during manufacturing,

• Due to transportation issues or poor test preparations,

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Benefit and risk of testing

Test results are an important feedback and a valuable source forimprovements. Due to the high cost and the limited number of test facilities the number of tests will remain limited.

Submitting a transformer to a short-circuit test is always connected to high expenses for the purchaser. In case of a failure the expenses become very high for purchaser and manufacturer.

A short-circuit test therefore involves a risk for both parties.

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Personal observations after 40 years in business

• Designing for short-circuit strength was not high on the list 40 and 30 years ago,

• Radial buckling, axial winding collapse and broken press rings were a non-rare experience,

• In the meantime the design philosophy has changed,• More experience has been gained, mainly after performing tests,

• There are better design rules, the calculation tools were improved, and there are better materials to strengthen windings and winding supports,

• I did not see a winding with buckling, collapsing, tilting indication for more than 10 years in a transformer designed during that period.

Pierre Riffon, Eng., Charlotte, SC, March 17, 2008

Short Circuit Withstand of Power Transformers, Utility Perspective


Presented by: Pierre Riffon Eng.Test SpecialistHydro-Quebec


Type of faults to be considered

All type of external faults such as:

Line faults, 1φ, 2φ, 3φ (not so severe if far from transformer).Substation faults 1φ, 2φ, 3φ, Severe cases, highest fault current, not frequent.Worst case: 3 phases short-circuit caused by forgotten safety grounding devices after maintenance…


Fault occurrence – utility experience

Line faults statistics (IEC 62271-310), more frequent than substation faults, approximately 4 to 6 faults per year per line, typically due to lightning strokes.


Transformers failures caused by short circuit events – Hydro-QuébecHydro-Quebec transformer short-circuit failures have been compiled for the period of 1970 to 1998. The results are:

9,20

11,15

ComparisonHQ/CIGRE

1970 – 1998

0,092

0,015

Failure rate

1993 –1997

%

0,184

0,145

Failure rate

1970 -1998

%

3

1

Number of faults1993 -1997

6

10

Number of faults1970 -1998

4,603255765

1,156912145 -170

ComparisonHQ/CIGRE1993 - 1997

Number of transformer

- years

System voltage

A CIGRE TF with WG 12.19 made survey in 1997 (18 utilities in 11 countries). Time frame of the results: 1993 – 1997 (5 years)


Transformers failures caused by short circuit events – Hydro-Québec

Well spread among 7 different manufacturers (no specific manufacturer nor design).Hydro-Quebec short-circuit related failure rates significantly higher than the figures reported by CIGRE.Short-circuit failure rates improved in the last 5 years (1993- 1998).Average age of transformers when failing: 19 years (CIGRÉ), 17 years (HQ).


Transformer failure during short-circuit tests - CIGRÉ

CIGRE SC12 has conducted in 1998 a worldwide survey among high-power laboratories (12 labs). The results are:

22,9≤ 2,5

41,9> 100

12,1> 40 and ≤ 100

22,3> 2,5 and ≤ 40

Average failure rate during tests%

Transformer power rating (MVA)

Transformer impedances for units rated 25 to 200 MVA were in the range of 14,3% to 19,5%.


Demonstration of transformer ability to withstand short-circuit currentsManufacturer: Calculations at the design stage. Calculation methods should be based on experience gained on similar transformer testing (not always the case). IEC 60076-5 does accept calculations and does give calculations guidance and criteria.User: Shall review the manufacturer short-circuit calculation results and transformer design and decide if a test is needed.


Is a short-circuit test to be considered?Technical aspects to consider:

Mandatory for distribution transformers (≤ 500 kVA)Is the test possible? Not possible for large units ( >400 MVA). Only few laboratories (outside North America) are able to test units rated over 100 MVA.What is the application (normal substation, GSU, interconnections between two systems (phase-shifter or HVDC), etc..)


Is a short-circuit test to be considered?Technical aspects to consider:

Is a spare available?Is that a single and unique order or this is an order that can be related to several identical units (e.g. distribution transformers or standardisation of a specific design);Can the power be fed through a parallel path (generally possible in power substations but not in generating plants).


Is a short-circuit test to be considered?

Technical aspects to consider:

Short-circuit occurrence and amplitude;Utility service experience;Manufacturer performance in service;Manufacturer experience of short-circuit tests;Risky design, small safety margins, new design, etc…


Is a short-circuit test to be considered?Economical aspects to consider:

Testing costs (including transport costs) and associated risks (project delays in case of a failure);Evaluation of revenue losses, mainly for bulk power applications like interconnections, generating power stations, large industrial plants, etc.;Large number of units to be ordered e.g. distribution transformers



Economical aspects to consider:

Duration of an outage;For GSU applications, is it possible to store the energy e.g. in a water reservoir for hydro power;Time needed to replace the failed transformer;Costs for transformer replacement;Costs of energy replacement;Costs of power replacement.



A composite evaluation of all aspects (technical and economical) shall be done. Costs of performing a short-circuit test series shall then be compared with the costs, inconveniences and risk of having a failure in service.


Hydro-Quebec policy regarding the need of performing short-circuit tests

Short-circuit tests on critical power transformers since 1997. Short-circuit tests on small power transformers and distribution transformers are still performed. Can be easily tested in North American Laboratories without excessive transport costs.Nowadays, short-circuit tests on large power transformers can be hardly justified because of the extreme transport costs for sending transformers to laboratories outside North America.


Tests performed on Hydro-Quebec's power transformers since 1997

N/A

1st test series:Impedance variation: YesVisual inspection: Yes, axial failure

2nd test series:Impedance variation: No, less than 2%Dissolved gas-in-oil analysis: Yes, presence of acetylene.Visual inspection: Yes, axial failure

Failure has been observed by

NoYesB47 MVA, 170 kV / 25 kV

YesNoA2,5 MVA,Special

application

Modifications made on the initial design

Succeed at the

first trial

ManufacturerDescription


Tests performed on Hydro-Quebec's power transformers since 1997

Impedance variation: No less than 2% but more than 1%Dissolved gas-in-oil analysis: Yes, presence of acetylene.Visual inspection: Yes, spiralling failure of the LV winding

Arcing fault to the tankImpedance variation: YesDissolved gas-in-oil analysis: Yes, presence of acetylene.Visual inspection: Yes, axial failure

Failure has been observed by

YesNoC775 MVA, 230 kV / 24 kV

GSU

YesNoC102 MVA,GSU

Axial-split230 kV /11 kV /

13,8 kV

Modifications made on the initial design

Succeed at the

first trial

ManufacturerDescription


Benefits of performing short-circuit tests

Feedback for improving calculations, design evaluation and manufacturing methods;Changes and improvements made on a design which has initially failed a short-circuit test are normally implemented as a common manufacturing practice.Increase manufacturer and utility knowledge.Long-term improved reliability for all similar transformers.


Does a short-circuit test damage the transformer?

Hydro-Quebec have not seen any service problems on the units which have been short-circuit tested.EDF (France) and ENEL (Italy) who did perform extensive short-circuit test campaigns did not report any particular problems with these units.


How to be sure that a transformer subjected to a short-circuit test has no

internal damagesBy doing additional diagnostic tests such as FRA or FRSL;By adding a dissolved gas-in-oil analysis after tests;By doing a mandatory and detailed out-of-tank active part inspection;


How to be sure that a transformer subjected to a short-circuit test has no

internal damagesBy requesting a temperature rise test at maximum rating after the active part inspection;By requesting dielectric type tests to be performed after the active part inspection at 100% of the rated test levels;By requesting all other type and routine test at 100% of the various ratings.


Conclusions

Short-circuit failures represent only a few percentage of total transformer failures but do generally result in catastrophic failures.Test is the only method to guaranty the ability of a transformer to withstand a short-circuit.Tests are providing valuable inputs to designers and utilities for improving actual and future designs and service continuity.


Conclusions

Short-circuit tests are expensive and an extensive technical, economical and risk evaluation should be conducted before requiring a test.Adequate diagnostic methods, active part inspection and final type and routine tests at 100% of the rated levels are needed to insure that a short-circuit test did not damage the transformer.

Richard P. McLaughlin, Charlotte, SC, March 17, 2008

SHORT-CIRCUIT TESTING OF LIQUID-FILLED

TRANSFORMERSCharlotte, North Carolina, March 17, 2008

Presented by: Richard P. McLaughlin Supervising Test EngineerKema Powertest, LLC


CONSIDERATIONS BEFORE TESTING

SPECIFY WHICH TESTS TO PERFORMDictated by the client as to which

tests are to be witnessed and/or performed by the test lab.


SCHEDULING OF SUFFICIENT TEST TIME

Depending on the above specified tests to be performed the test lab will submit an estimated time quotation to perform all tests for approval.

CONSIDERATIONS BEFORE TESTING


INFORMATION NECESSARY FOR SHORT-CIRCUIT TEST

TRANSFORMER NAMEPLATE INFORMATIONRequire the MVA, voltage ratings,

and total percent impedance to calculate the necessary short-circuit test levels. Each test transformer is calculated on an individual basis.


POST-SET VS. PRE-SETTypically perform most short-circuit tests

with the pre-set method due to a generator fed source (2250MVA short-circuit generator). Post-set testing can be performed on a

strict individual basis depending on the test transformer voltage, short-circuit current ratings, and no-load losses.

INFORMATION NECESSARY FOR SHORT-CIRCUIT TEST


PERFORMANCE OF SHORT-CIRCUIT TESTING

SET-UPDepending on the calculated

impedance and short-circuit test levels will dictate how extensive the set-up needs to be. Typically is a minimum of a ½ to ¾ of a work-shift.



INITIAL IMPEDANCE MEASUREMENTSMeasure on all necessary taps

that the short-circuit tests are to be apllied to. Typically only takes a several minutes for each tap position.



SHORT-CIRCUIT CURRENT CALCULATIONSCalculate the required short-

circuit current values based on the limitations and equations in IEEE C57.12.00 and C57.12.90.



SHORT-CIRCUIT TEST PROCEDURETypical procedure for each winding

of a three-phase unit:Impedance - 2 full-level short-

circuit current tests - Impedance.



SHORT-CIRCUIT TEST PROCEDUREAlternate procedure for each

winding of a three-phase unit:Impedance - 6 full-level short-

circuit current tests - Impedance.



SHORT-CIRCUIT TEST PROCEDURETypically it takes a minimum of 1

work-shift to perform the typical short-circuit test procedure. This is depending on the performance of the test transformer and the test station.



POST IMPEDANCE MEASUREMENTSMeasure on all necessary taps

that the short-circuit tests were apllied to. Typically only takes a several minutes for each tap position.


CONCLUSIONS

The impedance variation larger than that specified in the test standard is the most common after the short-circuit tests.


CONCLUSIONS

KEMA (Arnhem) shows that in 11 years of testing (of 77) large transformers (25 - 440 MVA, 20 -500 kV), 31% fails initially to pass the short-circuit test, mostly because an unacceptable increase in reactance. 1

1 KEMA'S TEST EXPERIENCES WITH SHORT-CIRCUIT WITHSTAND CAPABILITY OF LARGE POWER TRANSFORMERS : R. P. P. Smeets, L. H. te Paske, P.P. Leufkens - KEMA T&D Testing Services, Arnhem, the Netherlands


CONCLUSIONS

Every tested transformer provides many levels of concern, all must be accounted for with every short-circuit test.


CONCLUSIONS

Each test facility shall make accommodations for the areas covered, and not covered, in this tutorial depending on their circumstances presented to them at the time of the test.


Some Particular Cases




Content

Low X/R transformersMulti winding autotransformersNon-circular coilsAxial split transformers


Axial splitAxial-split transformers

comprise transformers having 3 or more windings with 2 or more windings physically superimposed axially one the same magnetic leg. Each of the axial-split windings can be connected to different sources and/or loads by different sets of bushings.

H1

source

load 1

load 2


Axial split

Fault shall be appliedto only 1 axial split windingTo lower the magnetizing inrushthe post-set method ispreferredClause 12.2.3.1 of the draft and CIGRE paper 12-104

H1

source

load 1

load 2


Non circular coilsMany lower size and distribution transformers are builtusing non-circular coils (square or rectangular)On the first asymmetry, somethime on the calibration, these windings will bulge radially and show a highimpedance variationUsually, after the first maximum peak test the windingshave bulged to their limit and most of the time no impedance variation, or very light, is observed on following testsHeat run test and visual inspection is of prime importance to ascertain that the transformer have passed the test.


Other multiwinding transformers

Single phase 120/240 V transformersSingle phase multiple connectiontransformers/ autotransformers

HV: 14400 VLV: 3 x 2400 V windings

Transformer connection: 14400/2400 or 7200 VAutotransformer connection: 14400/7200 V


Other multiwinding transformers

During testing, each winding shall be subjected to its maximum calculated fault current on at least one test. In general, a given fault type and location will not produce the maximum fault current in more than one winding, so it will be necessary to make tests with several different connections to fully evaluate the capability of all windings. (clause 12.2.2.1 of C57.12.90 – 2006)

S08 Short Circuit Presentation

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