Transformer Tech Procedures DPC 34-1032

8/10/2019 Transformer Tech Procedures DPC 34-1032

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ESKOM COPYRIGHT PROTECTED

B Morrison / August 2011 / Rev 0

PROCEDURE

Document Classification: Controlled Disclosur e Title: Distribution Standards Part 15:

DISTRIBUTION TEST

PROCEDURE FOR POWERTRANSFORMERS

Unique Identifier: 34-1032Document Type: DPC

Revision: 0 Published date: AUGUST 2011 Total pages: 73Review date: AUGUST 2016

COMPILED BY APPROVED BY FUNCTIONAL RESP AUTHORISED BY

_ _ _ _ _ _

H SITHOLE

_ _ _ _ _ _ _ _

A M A CRAIB

_ _ _ _ _ _ _ _

PR GROENEWALD

_ _ _ _ _ _ _ _

P MOYO Snr. Engineer Protection

Protection SC chair for TESCOD CMDT for DE (Dx)

DATE:30/6/2011 DATE: 30/6/2011 DATE: 30 June 2011 DATE:

Content

Page Foreword ........................................................................................................................................................ 3 Introduction .................................................................................................................................................... 4 1 Scope .................................................................................................................................................. 5 2 Normative references .......................................................................................................................... 5 3 Definitions and abbreviations .............................................................................................................. 6

3.1 Definitions .................................................................................................................................... 6 3.2 Abbreviations ............................................................................................................................... 6

4 Requirements ...................................................................................................................................... 6 4.1 Applicability of tests ..................................................................................................................... 6 4.2 Safety and preparation ................................................................................................................ 7

5 Power Transformer Routine tests .................................................................................................... 7 5.1 Voltage ratio test .......................................................................................................................... 7 5.2 Positive sequence impedance test .............................................................................................. 8 5.3 Winding resistance test ............................................................................................................. 10 5.4 Winding insulation test ............................................................................................................... 10 5.5 Core to earth insulation resistance test ..................................................................................... 11 5.6 Valve position checks ................................................................................................................ 12

6 Power Transformer Commissioning tests ...................................................................................... 12 6.1 Vector group test ....................................................................................................................... 12 6.2 Magnetisation test ...................................................................................................................... 16 6.3 Zero sequence impedance test ................................................................................................. 17 6.4 Built-in current transformer tests ............................................................................................... 25 6.5 Wiring Checks ............................................................................................................................ 25 6.6 Control cabling and contact insulation tests .............................................................................. 25 6.7 Ancillary devices: Oil and winding temperature instrument tests .............................................. 26 6.8 Ancillary devices: Buchholz relay tests ...................................................................................... 28 6.9 Ancillary devices: OLTC Surge relay test .................................................................................. 29

6.10 Ancillary devices: Pressure relief valve test .............................................................................. 29

6.11 Ancillary devices: Oil level gauge .............................................................................................. 30 6.12 Ancillary devices: Fans and Pumps ........................................................................................... 30


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DOCUMENT CLASSIFICATION:CONTROLLED DISCLOSURE DISTRIBUTION TEST PROCEDURE FOR POWERTRANSFORMERS

Unique Identifier: 34-1032Type: DPCRevision: 0Page: 2 of 73


When downloaded from the IARC WEB, this document is uncontrolled and the responsibility rests with the userto ensure it is in line with the authorised version on the WEB.

6.13 Verification of neutral earthing requirements and tank earthing ................................................ 30 7 Power Transformer Special tests ................................................................................................... 31

7.1 Polarisation Index (PI) test ........................................................................................................ 31

7.2 Bushing test (Tan Delta test) ..................................................................................................... 32 7.3 Sweep Frequency Response Analysis (SFRA) tests ................................................................ 32 8 Tap Change tests .............................................................................................................................. 33 9 Neutral Electromagnetic Coupler tests .............................................................................................. 34

Annex A - List of test sheets ........................................................................................................................ 37 Annex B - Impact assessment ..................................................................................................................... 70


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Foreword

This document together with the technical drawings and other reference material serves as a proceduredetailing the minimum on site tests required for new and refurbished transformers. The test procedure isalso applicable following a Master trip of transformers in service.

Revision history

This revision cancels and replaces Revision No. 0 of specification No. SCSPVADJ2.

Date Rev. CompiledBy

Clause Remarks

Aug 2011 0 H Sithole - Original document reformatted on latest document

template. Reference number changed to 34-1032.Document heading structure re-arranged under newheadings 4 9.Tests arranged in general format: Purpose, Procedureand Expected Results.

2 References updated.4 New section added indicating applicability of tests and

safety notifications.5.2.2 Note added on % impedance conventions.6.1.2 Vector group test diagrams redrawn to scale.6.4 Detailed CT test procedures deleted in favour of

reference to DPC 34-1035.6.8 Preferred Buchholz test method amended to be via

test push button, not gas injection.7.2 Tan D test method: reference made to DPC 34-734.7.3 Section added.

9.1.4 Added reference to NEC Oil temperature alarm andtrip settings.

Aug 2001 0 PA Gerber - First issue of SCSPVADJ2

Authorisation

This document has been seen and accepted by:

Name Designation

P Moyo Corporate Manager (Divisional Technology IARC)

PR Groenewald Technology Development Manager - Control Technologies

AMA Craib Protection Study Committee Chairperson

SJ van Zyl Protection Discipline Specialist

This procedure shall apply throughout the Distribution Division of Eskom Holdings Limited.


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Development team

The following working group members contributed towards the creation of the original document:Riaan Basson Western Region

Craig Clark Distribution Technology

Andrew Craib Southern Region

Andre De Witt North West Region

Mervin Delsing North West Region

Paul Gerber Central Region

Keith Harley Northern Region

Marius Jansen Van Vuuren Central Region

Lesley Meyer Western Region

Hilton Mills Eastern Region

Izak Van Der Merwe Western Region

Phillip Wadsworth Northern Region

The latest revision was compiled by Haggai Sithole with input from Paul Gerber, Stuart van Zyl, ArchiebaldMabula and Andrew Craib.

Introduction

This procedure details all tests that shall be carried out to a new or refurbished transformer on site beforethe unit may be switched in.

Transformers are tested in the following eventualities:

a) Newly constructed transformers located in the factory prior to being shipped;

b) On site after delivery by the supplier (the presence of Eskom EDFS personnel witnessing thetests and co-signing the test document is imperative).

c) Before moving a transformer from one site to another site. This is very important as anytransport-related damage, which can prove to be very expensive will not be accepted by anycontractor unless the contractor is aware that tests were completed beforehand and witnessedand/or signed off before commencing with the transport operation. Before allowing the contractorto proceed with the transportation, it is advisable to reach an agreement with the contractor onwhat proof he requires of the condition of the unit prior to despatch and confirmation of the unitbeing in the same condition upon delivery to the new site. Thus each transformer shall be testedtwice; before transportation commences and after off-loading at the destination site.

d) When a transformer has experienced a unit protection trip (i.e. a Master trip that is not solely dueto a sustained fault timeout); and

e) Before and after reconditioning of a unit.

It is recommended that transformers should be tested by the supplier or repair contractor on site whilebeing observed by an Eskom EDFS staff member. EDFS shall counter-sign the test documents onconclusion of the tests to verify that the tests have been completed and that the unit is fit for the intendedpurpose.


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Keywords

Voltage ratio, vector group, zero sequence impedance, Tan Delta, NEC, NEC/R, transformer, temperature.

Bibliography

J&P Transformer book, 10th edition, by S Austen Stignant and A.C. Franklin

TTS 004, NEC/NER/Auxiliary Transformer Test Sheet .

SCSASAAQ1, Distribution Standard Part 7: Substations Section 4, Quality Control Process for the checkingof Distribution substation construction before handing over for commercial operation ,

AB KIHLSTRMS MANUAL TD50/TD76

1 Scope

This procedure covers in detail, the common tests that need to be performed on power transformers andearthing compensators and associated ancillary equipment (temperature gauges etc) and providescomprehensive test sheets for reference purposes by Eskom technical staff or contractors.

2 Normative references

Parties using this procedure shall apply the most recent edition of the documents listed below:

DISSCAAD3,Specification for large power transformers up to 132 kV, in the rating range of 2,5 MVA to 80MVA,

EPC 32-846, Operating Regulations for High-Voltage systems

DMN 34-101, Work with/on extension/single ladders

DPC 34-227, Pre-task planning and feedback process

DPC 34-734, Procedure for tan-delta testing

DPC 34-908, Procedure for Barricading

DST 34-1710, Provision and use of Personal Protective Equipment

DPC 34-2149, Policy for neutral earthing of electrical networks

DST 34-2151, Standard list for Protection Technician tools and test equipment

SCSPVACL6, Procedure for using Fall Arrest System

DPC 34-1035, Current Transformer Test Procedure.


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3 Defini tions and abbreviations

3.1 Definitions

None.

3.2 Abbreviations

CT: Current transformer

EDFS: Electricity Delivery Field Services

EDM: Electricity Delivery Management FS: Field Services

HV: High voltage, usually the primary winding

LV: Low voltage, usually the lowest voltage winding (i.e. tertiary winding)

MCB: Miniature circuit breaker

MIB: Marshalling interface box

MV: Medium voltage, usually the secondary winding

Note: In the context of three winding transformers, it is most convenient to speak of windings rather than voltagelevels: primary, secondary and tertiary. This helps to avoid confusion in cases where the primary andsecondary windings are rated for HV voltages.

NEC: Neutral Electromagnetic Coupler

NCS: National Calibration Services

SFT: Sustained Fault Timer

TCD: Tap change drive

TMK: Transformer marshalling kiosk VT: Voltage transformer

4 Requirements

4.1 Applicabilit y of tests

The test procedure for transformers is presented in three sections: Routine Tests, Commissioning Testsand Special Tests.

Routine tests are those that shall be conducted following a Master trip (excluding SFT operation), duringmajor maintenance and during commissioning.


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Commissioning tests are those tests that in addition to routine tests are applicable at the time ofcommissioning of the transformer.

Special Tests are those that are only required for specific purposes. These tests are performed only ondemand.

The procedure also describes on-load tap changer tests and NEC and NEC/RT tests. These tests shalltypically be conducted during commissioning and major maintenance.

4.2 Safety and preparation

The following minimum safety procedures and preparations shall be adhered to:

a) Field personnel must be equipped with all relevant tools as per DST 34-2151.

b) Ensure that ladders are used as per DMN 34-101. Where work is to be carried out above 2mfrom the ground make use of a Fall Arrest System as per SCSPVACL6.

c) All personal protective equipment shall be acquired and worn in accordance with DST 34-1710.

d) Perform pre-task planning and risk assessment in accordance with DPC 34-227 before thecommencement of work and continuously during the task execution. Also refer to an approvedcritical task analysis where applicable.

e) Ensure that the transformer under test is isolated and earthed from the network in accordancewith the ORHVS (see EPC 32-846).

f) Ensure that the necessary barricading is in place as per DPC 34-908. Ensure that the barricadingis not attached to live structures as to prevent climbing on a transformer while any part thereof isenergised.

g) Ensure that the correct apparatus is isolated, earthed (grounded) and barricaded.

h) Before making contact with the transformer, check that the structure is earthed.

i) No work shall commence before permission is given by the authorized person and the workpermit is signed by all applicable persons.

j) Ensure that work is carried out within the restraints of the permit.

5 Power Transformer Routine tests

5.1 Voltage ratio test

5.1.1 Purpose

The ratio test is done to compare the actual transformer voltage ratios to those stated on the nameplate.

5.1.2 Procedure

In the case of a two winding transformer, the ratio test shall be done from the HV to the MV winding on alltaps.


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The ratio test on three winding transformers shall be done as follows:

a) HV to MV winding all taps.

b) HV to LV (tertiary winding) Minimum, Nominal and Maximum taps only (Taps 1, 5 and 17).

c) MV to LV winding Nominal tap only (Tap 5).

The ratio tests shall be performed using a calibrated ratio meter (NCS accredited calibration).

An alternative test method that may be used is to apply a 400 Volt three phase test voltage to the HVwinding and measuring the corresponding MV and LV voltages. It is important from a safety aspect toensure that the test voltage is always connected to the highest voltage winding.

The preferred test voltage to be used in performing the above tests is 400 Volts, three phases. In the eventthat a three phase supply is unavailable, then a single phase supply can be used as indicated below inFigure1.

When performing this test remove all neutrals from the station earth. Disconnect the surgearrestors/arrestor earths. This is a standard safety test procedure to ensure only the neutral on the test setis effectively connected to earth.

Figure 1. Ratio test performed using a 230Vac sing le phase source

5.1.3 Expected test resul ts

The test results from the ratio test shall not differ from the nameplate values by more than 0.5%.Deviations are indicative of either a faulty tap change selector or inter-turn winding faults.

5.2 Positive sequence impedance test

5.2.1 Purpose

This test is usually performed to ensure the stated nameplate impedance and transformer actualimpedances match. A significant deviation provides indication of impending transformer failure. This canbe due to short-circuited windings, damaged tap change mechanism, damaged core etc.

~

N

c

b

a

B

C

A

V V


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5.2.2 Procedure - two winding transformer

The test can be performed automatically by means of a ratio meter. An alternative method is possible, byshort-circuiting the MV winding and supplying the HV winding with a test voltage, usually from a stationthree phase 400 Volt supply or three phase alternator. Figure 2 below details a typical test set up.

Figure 2. Positive sequence impedance test performed on 2 windi ng transformer using a400VAC three phase source

The test is to be performed on the lowest tap, the nominal tap and the highest tap. Once the test has beenperformed, and the results recorded on the test sheet, Equation 1 is used to calculate the actual percentageimpedance.

100Voltage)(Rated

73.1 2%

MVA Rated I

V z

Measured

Measured

Note: there is some variability between transformer suppliers in the calculation of the % impedances of off-nominal tappositions. Most suppliers use the voltage rating and MVA rating of the specific tap under test as Rated Voltage and RatedMVA in Eq.1. This result is frequently termed the % impedance. Suppliers may also calculate the % ohmic impedance,using the nominal voltage rating and nominal MVA rating in all calculations. The impedance results of most moderntransformers are quoted using the tap voltage and tap MVA (equivalent on all taps). ACTOM transformers are one exceptionto this, using the % ohmic calculation method.

5.2.3 Procedure - three winding transformer

The procedure here is the same as for the two winding transformer except that the impedance shall bemeasured between the HV and MV windings, the HV and LV (Tertiary) and the MV and LV. Again the testvoltage is applied from the higher voltage winding and the lower voltage winding is short-circuited. Figure 3shows a typical set up for the HV to LV test.

Figure 3. Posit ive sequence test performed on 3 winding transf ormer using a 400VAC threephase source

Equation 1 is then used to calculate the impedances per winding.

N

c

b

a

B

C

A

~V

AV

A

A

V

(Eq.1)


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The nameplate and measured values should not differ by more than 10% in comparison. Where variancesoccur, confirm that this is not due to the voltage and MVA bases used in the % calculation (see noteabove).

5.3 Winding resistance test

5.3.1 Purpose

Employed as a type test, this test provides the necessary data to allow for the separation of I2R and eddy-current losses in the windings. As a routine test, it acts more as a simple comparison between phases tocheck for obvious discontinuities in the windings.

5.3.2 Procedure

The winding resistance, measured phase-to-phase for delta windings, phase-to-neutral for star windings, isto be measured with an NCS accredited resistance meter and recorded on the appropriate test sheet.


This is principally a comparative test, where the results from different phases should be similar. Themanufacturers acceptance test results can be used as a comparison of the readings to ensure that they arewithin the specified limits.

5.4 Winding insulation test

5.4.1 Purpose

The purpose of this test is to check that the transformer is not put into service with compromised insulation.The insulation of a transformer can be compromised by the following causes: water contaminated paper oroil, heat damaged insulation due to severe overloading or a transformer fault and mechanical damage dueto poor transport preparation or a fault.

5.4.2 Procedure

All terminations to the primary connections must be disconnected including the neutral(s) and the earthstrap to the core (if it is brought out).

The insulation resistance is measured winding to winding and all windings to ground. The following criteriaare used to determine which test voltage to use:

Transformers > 2 MVA and 10 MVA, use test voltage up to 2 kV DC

Transformers > 10 MVA, use test voltage of up to 5 kV DC

Using an electronic megger, apply DC voltage for one minute.


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5.4.3 Expected resul ts

Minimum insulation values may be determined using the following formula:

R

kVA

E C

Where R = Minimum insulation resistance (M )

C = 1.5 for oil filled transformers at 20 C, assuming the insulating oil is dry, acid and sludge free.= 30.0 for untanked oil-impregnated transformers.

E = voltage rating (Volts). Phase-to-phase for delta-connected and phase-to-neutral for star-

connected.kVA = rated capacity of the winding under test. If the winding under test is 3 phase and the three

individual windings are being tested as one, the rated capacity of the complete 3 phasewinding is used.

[Source: IEEE C57.125-1991. Guide for Failure Investigation, Documentation, and Analysis for PowerTransformers and Shunt Reactors]

NOTE. If initial test results are not satisfactory, the guard terminal on the megger instrument should be used to enhance theaccuracy of the measurements. A wire wrapped around the bushings and connected to the guard terminal on the meggerserves the purpose. The line and earth terminals on the megger are connected as before between the transformer phases andearth.

5.5 Core to earth insu lation resist ance test

5.5.1 Purpose

Transformer core types are of the core form or shell form. In both forms, the core is insulated from the tankand earth. The core is connected to earth in order to prevent a voltage rise from occurring on the coreduring operation (via a single point).

The purpose of this test is to measure the insulation resistance between the core and earth.

5.5.2 Procedure

The core to earth resistance is measured using a 500V megger to ensure that the core is connected at asingle point. The single intentional core earth is disconnected prior to conducting the test.


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5.5.3 Expected resul ts

Typical insulation resistance ranges for various conditions of core are indicated below:

Condition of equipment Core insulation resistance Condition of resistance

New > 1000 MOhms

Service aged > 100 MOhms Normal

10 100 MOhms Indicative of insulation deterioration

< 10MOhms Sufficient to cause circulating currents

[Source: Eskom Enterprises. Operating Standard No. OPS-007. Rev.4]

5.6 Valve posi tion checks

Before tests on the transformer are commenced, ensure that the valves are open. Ensure that youunderstand the markings on the valve. The valves are usually located on the transformer tank between thecooling fins and the tank on the feeder pipe. This is for the top and bottom. Valves are usually situated oneither side of the oil pump where fitted. If the valve markings are not logical to you or if you are unsure forany reason, consult the transformer manual or the manufacturer. There may also be valves between theconservator tank and transformer main tank. Check these as well.

Prior to switching in the transformer, check the position of all valves. Once the transformer is in service andis on load a check of the temperature by hand at the bottom and the top of the fins should verify that thevalves are open. The bottom part should be cooler than the top. Complete this check without contraveningthe ORHVS regulations and only if it is safe to do so. You do not have to leave ground level. Stand at asafe distance from the fans as they may switch in at any moment and cause injury.

If a valve on the outlet side of an oil pump is left closed, the pump may be damaged. Hence even whendoing tests on the control circuits and checking pump motor direction etc. the position of the valves arecritical.

6 Power Transformer Commissioning tests

6.1 Vector group test

6.1.1 Purpose

The vector group of the transformer must be tested and checked to match the vector group on thenameplate. If the unit has been incorrectly connected internally and it is put into service without aconfirmation test this may have serious consequences: causing serious damage to the transformer andother transformers paralleled with it.

6.1.2 Procedure

The vector group test may be done using a ratio meter. As there are many test units on the market, it isadvisable to follow the manufacturers instructions. Modern ratio meters commonly perform the vectorgroup test automatically. Should such a test set not be available or the test set fails, the test may also beperformed in the following way:


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a) Strap the HV and MV White phase bushings together and apply a three phase 400 Volt powersupply to the HV bushings.

b) Measure the voltage magnitudes (and angles if possible) between: HV Red phase to MV Redphase (VaA), HV Red phase to MV Blue phase (VcA), HV Blue phase to MV Red Phase (VaC)and HV Blue phase to MV Blue phase (VcC).

Scale phasor diagrams, and expected measurement results are presented for Yd1, Yd11, Yd5 and Yd7transformers in Figures 4a to 4d respectively (assuming a 132/44kV transformer).

Figure 4a. Vector group test: vector diagram for a 132/44kV Yd1 transformer


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Figure 4b. Vector group test: vector diagram for a 132/44kV Yd11 transf ormer

Figure 4c. Vector group t est: vector diagram for a 132/44kV Yd5 transformer

Figure 4d. Vector group test: vector diagram for a 132/44kV Yd7 transformer


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It is clear from Figures 4a 4d that the measurement results differ markedly for transformers of differentvector groups.

NOTE: it is also possible to calculate the expected voltage magnitudes that will be present using complex algebra (rememberto divide 400V by 3).

6.1.3 Vector group test - quick reference table

Figures 2a 2d indicate the expected measurement results for transformers of voltage ratio 3:1 (e.g.132/44kV), and differing vector groups. Expected voltage magnitude results for transformers of differentvoltage ratios are presented in Table 1 below. It is assumed in all cases that the HV test supply is rated at400V. The results of Table 1 may be scaled for lower/higher test voltages.

Table 1. Vector group test qu ick reference guide of expected voltage measurements per vectorgroup and transformation ratio

Using 400V injected on HV( White phases s trapped)Yd1 Transformation Ratio

2 3 4 6 12a-A 246 291 316 342 370c-A 246 291 316 342 370a-C 446 419 411 404 400c-C 246 291 316 342 370

Yd11 Transformation Ratio2 3 4 6 12

a-A 246 291 316 342 370c-A 446 419 410 404 400a-C 246 291 316 342 370c-C 246 291 316 342 370


a-A 580 518 488 456 427c-A 446 420 410 404 400a-C 580 518 488 456 427c-C 580 518 488 456 427


a-A 580 518 488 456 427c-A 580 518 488 456 427a-C 446 420 410 404 400c-C 580 518 488 456 427

Applic ation example : When testing a 132/66kV Yd1 transformer (transformation ratio of 2), Table 1indicates that VaA = VcA = VcC = 246V. VaC = 446V.


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The calculated/measured vector group shall match that stated on the nameplate, and shall be compatiblewith the design philosophy of the application (i.e. match the vector groups of parallel transformers).

6.2 Magnetisation test

6.2.1 Purpose

This test is done to provide an indication of the no-load current drawn by the transformer; hence themagnetisation current set up due to losses in the electric circuit and losses in the magnetic circuit due toe.g. shorted core laminations (due to damaged insulating vanish between the laminations. The insulatingvarnish could be damaged by sustained high current faults resulting in a heated core. Loss of insulationbetween laminations also results in increased eddy currents, and hence increased magnetising. Acomparison of the values obtained during commissioning and after a fault could provide an indication of adamaged core.

6.2.2 Procedure

The magnetisation test must be done using three phase 400V supply test equipment. If not available, arudimentary single phase supply test can be done as shown in Figure 3 below. For a two windingtransformer, the magnetisation test must be done from the secondary winding. In the case of a threewinding transformer, the magnetisation test must be done from the tertiary side. Where the MV or LVwinding is not accessible this test shall be performed on the HV side.

Figure 5. Magnetisation test performed using a 230Vac sing le phase source


The results obtained are not that easy to quantify: essentially one must do the test with the transformer new(manufacturers test result and site acceptance test result). The magnetisation test result can then becompared to the original result (when using the same test methodology and applied sources). Alternatively,compare results with readings obtained from an identical unit at the substation. Any large deviation shallthen be cause for further investigation.

N

c

b

a

B

C

A

~V A


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6.3 Zero sequence impedance test

6.3.1 Purpose

The calculation of earth fault currents on the power system requires the zero-sequence impedance oftransformers to be known. It is however not possible to directly measure the zero-sequence impedance ofa transformer. With the aid of the following tests, we can calculate the equivalent zero-sequenceimpedance(s) of the transformer which then be used in fault calculations.

The flow of zero-sequence current through a transformer occurs under earth fault conditions and only if thetransformer has a star winding with the neutral point grounded and has a secondary/tertiary winding whichis able to produce balancing zero-sequence currents. If there is no secondary or tertiary winding whichproduces the balancing zero-sequence currents, the zero-sequence impedance of the transformer would belimited by the tank-delta effect.

The following transformer types may be tested:

Two-winding transformers -

Star-delta (YNd*)

Delta-star (Dyn*)

Star-star (YNyn*)

Three-winding transformers -

Auto-transformers with delta tertiary (YNa0d*)

Star-star-delta (YNyn*d*)Where: * represents the vector shift with respect to the primary vector

N and n are the neutral points of star connected windings brought out through bushings on thetransformer

Notes : 1) All tests should be carried out at nominal tap.

2) Perform all tests with a test current of 10 amps (where possible)

3) All impedances are referred to the high voltage winding

Where available, it is preferred that the manufacturers Zo values are used as the low magnitude testcurrents on site do not necessarily result in realistic Zo values.

6.3.2 Procedure

6.3.2.1 Notation

The following notation is in this section:

H High voltage winding

M Medium voltage winding, (Common winding on auto-transformers)

L Low voltage winding, (Tertiary winding on three-winding transformer and the secondary of a two-winding transformer)


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I Measured test current

V Measured test voltage

n turns ratio (n= Nprimary/Nsecondary )

Zo zero sequence impedance in Ohms

Z H-M Leakage impedance between HV winding and MV winding

Z H-L Leakage impedance between HV winding and LV winding

Z M-L Leakage impedance between MV winding and LV winding

Z H-ML Leakage impedance between HV winding and combined MV and LV windings

Z M-HL Leakage impedance between MV winding and combined HV and LV windings

Zo H Equivalent zero-sequence impedance of HV winding

Zo M Equivalent zero-sequence impedance of MV winding

Zo L Equivalent zero-sequence impedance of LV winding

6.3.2.2 Injection tests

This section describes the test procedure to be performed per transformer vector group. All tests are to beconducted on nominal tap.

TYPE: YNd*

Connect the transformer as follows:

1) Apply a three phase short to the HV terminals

2) Leave the LV terminals open circuit

3) Connect a single phase supply between the HV terminals and the neutral as shown in the diagrambelow:

Figure 6. Zero sequence test procedure using 230VAC source

Record the test voltage and current readings from the volt- and ammeters indicated.

A

V

AC

a

b

c

A

B

C

N

230 V

Isolatingtransformer

Variac

HV


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TYPE: Dyn*


1) Apply a three phase short to the LV terminals

2) Leave the HV terminals open circuit

3) Connect a single phase supply between the LV terminals and the neutral as shown below.

Figure 7. Zero sequence test using 230Vac source: 2 winding Dyn transformer


TYPE: YNyn*

TEST 1 Impedance measured Z H-L


1) Leave the LV terminals open circuit

2) Apply a three phase short to the HV terminals

3) Connect a single phase supply between the HV terminals and the neutral as shown in the diagrambelow

Figure 8a. Zero sequence Z H-L test for YNyn transformer.


TEST 2 Impedance measured Z M-L


1. Apply a three phase short to the LV terminals

2. Leave the HV terminals open circuit

3. Connect a single phase supply between the HV terminals and the neutral as shown in the diagrambelow

A

V

A C2 3 0 V

I s o l a t i n gt ra n s f o r m e r V a r i a c

A

B

C

a

b

c

n

H V L V

A

V

A C

HV LV

Isolatingtransformer Variac

230 V

A

B

CN

a

b

cn


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Figure 8b. Zero sequence Z M-L test for YNyn transformer


TEST 3 Impedance measured Z H-ML



2. Apply a three phase short to the HV terminals

3. Connect a single phase supply between the HV terminals and the neutral as shown in the diagram:

Figure 8c. Zero sequence Z H-ML test for YNyn transformer


TEST 4 Impedance measured Z M-HL





Figure 8d. Zero sequenced Z M-HL test for YNyn transformerRecord the test voltage and current readings from the volt- and ammeters indicated.

HV

A

V

A C

LV


A

B

C

a

b

c

nN

230 V

HV

A

V

A C

LV


A

B

C

a

b

c

nN

A

V

A C

HV LV


230 V

A

B

CN

a

b

cn


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TYPE: Auto-transfor mer with delta terti ary (YNa0d*)

NOTE: The tertiary winding is to remain open circuit for all the tests.

TEST 1 - Impedance measured, Z H-L



2. Leave the MV terminals open circuit


Figure 9a. Zero sequence Z H-L test for autotransformer (YNa0d)


TEST 2 - Impedance measured, Z M-L

Connect the transformer as follows:1. Apply a three phase short to the MV terminals

2. Leave the HV terminals open circuit

3. Connect a single phase supply between the MV terminals and the Neutral, as shown in the diagram:

Figure 9b. Zero sequence Z M-L test for autotransformer (YNa0d)


TEST 3 - Impedance measured, Z H-ML



2. Apply a three phase short to the MV terminals

A

V

A C 230 V

LVHVMV

A

B

C

N

a

b

c

3A

3B

3C


A

V

A C 230 V

VariacIsolating

transformer

A

B

C

N

a

b

c

3A

3B

3C

HV MV LV


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Figure 9c. Zero sequence Z H-ML test for autotransformer (YNa0d)


TEST 4 - Impedance measured, Z M-HL

Connect the transformer as follows:1. Apply a three phase short to the HV terminals


3. Connect a single phase supply between the MV terminals and the neutral as shown in the diagram:

Figure 9d. Zero sequence Z M-HL test for autotransformer (YNa0d)


TYPE: YNyn*d*

NOTE: The tertiary winding is to remain open circuited for all the tests.

TEST 1 - Impedance measured, Z H-L


1. Apply a three phase short to the HV terminals2. Leave the MV terminals open circuit


Figure 10a. Zero sequence Z H-L test for YNynd transformer

A

V

A C 2 3 0 V

L VH VM V

A

B

C

N

a

b

c

3 A

3 B

3 C

Isola t ingt r a n s f o r m e r Va r i a c

A

V

A C 230 V

VariacIsolating

transformer

C

B

A

N

a

b

c

3A

3B

3C

HV MV LV

A

V

A C

HV MV LV


230 V

A

B

C

N

a

b

c

n

3A

3B

3C


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TEST 2 - Impedance measured, Z M-L


1) Apply a three phase short to the MV terminals

2) Leave the HV terminals open circuit

3) Connect a single phase supply between the MV terminals and the Neutral, as shown in the diagram:

Figure 10b. Zero sequence Z M-L test for YNynd transformer


TEST 3 - Impedance measured, Z H-ML



2. Apply a three phase short to the MV terminals3. Connect a single phase supply between the HV terminals and the neutral as shown in the diagram:

Figure 10c. Zero sequence Z H-ML test for YNynd transformer


TEST 4 - Impedance measured, Z M-HL




A

V

A C

HV MV LV

VariacIsolating

transformer

230 V

A

B

C

N

a

b

c

n

3A

3B

3C

A

V

A C

H V M V L V

Va r i a cI s o l a t i n g

t r a n s f o r m e r

2 3 0 V

A

B

C

N

a

b

c

n

3 A

3 B

3 C


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3. Connect a single phase supply between the MV terminals and the neutral as shown in the diagram:

Figure 10d. Zero sequence Z M-HL test for YNynd transformer


6.3.2.3 CalculationsUse the appropriate test sheet to gather the required test values. Once the test data has been acquired,the formulae below can be used to determine the equivalent star zero-sequence impedance of thetransformer.

YNd*: Zo = IV3 /ph See Figure 6

Dyn*: Zo =I

Vn 32 /ph See Figure 7

YNyn*, auto-transformers (YNa0d*) and YNyn*d*

Test 1: Z H-L = IV3 See Figures 8a, 9a, 10a

Test 2: Z M-L =I

Vn 32 See Figures 8b, 9b, 10b

Test 3: Z H-ML = IV3 See Figures 8c, 9c, 10c

Test 4: Z M-HL = IVn 32 See Figures 8c, 9c, 10c

It can be shown that

MLHLHLML ZZZZ 0 or HLMLMLHL ZZZZ 0

Also, Zo H = Z H-L Zo L and Zo M = Z M-L Zo L

The zero-sequence impedance of a YNyn* transformer (Zo) equates to: Zo = Zo H + Zo M= Z H-M

If all four tests have been done then the relationship: Z H-L Z M-HL = Z M-L Z H-ML should be satisfied. If not,

the test results should be verified.

A

V

A C

HV MV LV

VariacIsolating

transformer

230 V

A

B

C

N

a

b

c

n

3A

3B

3C


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The calculated values should not differ from the nameplate values (where provided) by more than 10% .

NOTE: To check that this calculated zero-sequence impedance of the transformer is correct, it can be compared to thepositive sequence impedance of the transformer since these two values are very close in magnitude. For three phase twowinding core-type transformers typical values for Zo are between 0.85 and 0.9 Z1.

6.4 Built-in current transformer tests

6.4.1 Purpose

To ensure that all built-in current transformers comply with the specification on the nameplate in terms ofratio, accuracy class and polarity.

6.4.2 Procedure

All current transformers shall be tested in accordance with DPC 34-1035 Current Transformer TestProcedure .

6.5 Wiring Checks

6.5.1 Purpose

To ensure that all connections correspond with the manufacturers drawings, all terminations are suitableand tight and that all voltage dependant devices have the correct voltage rating.

6.5.2 Procedure

Check the MIB, TCD and all other ancillary device termination boxes and ensure the correct wiring,tightness of terminals, continuity of circuits, correct voltages and correct installation. In particular:

a) The wiring and ferruling should be checked. The ferruling must correspond to the wiring numbers onthe diagram.

b) All wiring to be correctly lugged and crimped with the correct size lug as per the wiring.

c) Check control circuits as per manufacturers diagrams.

d) Check that all termination rails and terminals are correctly marked as per manufacturers diagrams.

6.6 Control cabling and contact insulation tests

6.6.1 Purpose

To ensure that the control cables and the associated wiring of ancillary devices (e.g. Buchholz relays, levelgauges etc.) have the appropriate insulation integrity so that all the devices on the transformer convey the

correct signals to the transformer and tap change control scheme.


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6.6.2 Procedure

Conduct an insulation resistance test from all unique wiring circuits to earth with a 500V DC insulationresistance tester.

6.6.3 Expected test resul t

The insulation resistance between any wire or contact and earth should be greater than or equal to 20Mega-Ohms.

6.7 Ancill ary devices: Oil and winding temperature instrument tests

6.7.1 General

The oil and winding temperature gauges provide protection against internal transformer faults andoverloading. Overheating the winding of a transformer can dramatically reduce the life of transformerssince the winding insulation deteriorates rapidly under these conditions. It is therefore not advisable tooperate a transformer outside its specified limits. Protection against overload is therefore based on windingtemperature, which is usually measured by a thermal imaging technique. Since the temperature of thewinding is proportional to the square of current passing through it, the winding temperature circuit makesuse of a current transformer wired in series with one phase of the transformer winding on the load side(usually the red phase). The output of the current transformer feeds a heating element that is located in thepocket for the winding temperature gauges probe.

The temperature gauges are normally mounted vertically next to the transformer tank, or for newertransformers inside the MIB. Two small oil pockets are normally located at or near the top of thetransformer tank, one for the winding temperature and one for the oil temperature. This is in the case oftwo-winding transformers. With three-winding transformers a winding temperature gauge is normallyprovided for each winding and one for the oil temperature, thus there should be four pockets for three-winding transformers. In order to ensure good heat transfer, each pocket has to be filled with oil.

A traditional mechanical temperature gauge, which consists of a measuring system via gears, levers andsignalling contacts, has a capillary tube running from it, and has a probe at the end of the capillary tubewhich fits into the pocket on the transformer. The probe is normally provided with a flange and fitted to thepocket by means of screws and sealed with a gasket. The thermometer dial has two pointers, one for theactual temperature and the other for indicating the maximum temperature reached by the unit. The latter ismoved by the former and remains at the maximum position reached, it can be reset by turning the screw onthe cover in an anti-clockwise direction until it touches the other pointer.

More modern transformers feature a digital temperature instrument with a mechanical gauge acting asback-up. Temperature measurement for digital instruments is achieved using a PT100 sensor that is fittedinto an oil pocket and wired to the instrument. The principle of operation is to measure the resistance of aplatinum element. A PT100 sensor has a resistance of 100 Ohms at 0 C and 138.4 Ohms at 100 C. Acurrent transformer input is provided to the instrument for use in an algorithm to determine the windingtemperature based on the oil temperature and set constants. Suitable setting constants are determinedduring heat run tests during prototyping of each power transformer design.


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6.7.2 Testing procedure

6.7.2.1 CalibrationWhen checking the calibration of the temperature gauges, the probe for each temperature gauge should beplaced in a bath of oil together with a control thermometer. A heating element should be immersed in thebath of oil and its temperature increased until the maximum reading on the temperature gauges arereached. This temperature should be maintained for a minimum of 10-15 minutes to ensure that the probeswill have, with certainty, attained the temperature of the oil. When carrying out this procedure, the liquidmust be kept in motion continuously. The heating element should then be switched off and readings takenat 5 C intervals of the control thermometer and each temperature gauge. This shall be repeated as the oilcools down until the minimum temperature (ambient) is reached. The error should not exceed 2% in therange above 50 C. When inserting the probes in the pocket, ensure that the oil in the pocket is at thecorrect level. Ordinary transformer oil should be used for this purpose. Adjust the oil level to about 20mmbelow the top edge of the pocket of the transformer when the probe is inserted.

6.7.2.2 Alarm and trip cont acts

Two signalling contacts are normally provided for each temperature gauge where no additional cooling ofthe transformer is required (i.e. fans or pumps). For mechanical gauges, each contact holder is providedwith a temperature scale. The required operation value should be set on each scale. For digitalinstruments, this is achieved using programmable settings. The values should be:

Function Alarm Trip Fan1 Fan2

Oil temperature: 95 C 105 C 60 C 80 CWinding temperature: 110 C 120 C 75 C 85 C

Where possible, operate the contacts and check that they close at the set values. This is done on

mechanical gauges by using the lever on the rotating shaft. This lever should be pushed down slowly,which in turn causes the temperature dial pointer to move, until the set value is reached.NOTE 1: When operating the output contacts in this manner, the lever on the shaft must never be forced upwards, as this maydamage the instrument. Never attempt to move the dial pointer for this test, rather use the lever on the shaft. Also rememberto reset the maximum demand pointer after replacing the covers to the gauges.

NOTE 2: Older instruments used mercury-wetted contacts. When using the mercury operated contact type temperaturegauges, it should be ensured that the gauges are in a perfectly upright (vertical) position since this can influence the operatinglevel of the gauge. These contacts can operate due to earth tremors or bumps to the transformer (caused by through faults),and gauges using such should generally be replaced with ones using microswitch contacts.

6.7.3 Winding temperature gauge temperature rise test

A typical winding temperature circuit is shown in the Figure 11 below.

Figure 11: Typical w inding temperature gauge circuit

connectedin series with

red phase winding

Winding temperaturecurrent transformer

TMK

TMK

heater element

correctionresistor

pump motor

contactor


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The heating element is adjusted so that the indicator will show the temperature of the hottest part of thewinding (hot spot temperature). This adjustment is based on a heat run test. The correction resistor isadjusted to achieve the correct temperature rise for the transformer. The test is carried out by, injecting

rated current into the heater element circuit. Readings are then taken of the initial temperature and the finalwinding temperature after a period of 15 minutes. The overall temperature change from start to finish isconsidered to be the winding temperature rise above the top oil temperature under the simulated load. Thisvalue varies from one transformer to the next, according to the design of the transformer. Compare thereadings obtained with the values given by manufacturer. Any deviation from the expected results must becorrected by altering the resistance setting of the correction resistor.

To set the winding temp resistance on HV, MV or LV instruments, the following information is required.

Whenever a transformer, whether new or old is to be tested, the manufacturers test report which containstest results of the winding temperature at 100% load, should be included.

For example: HV = 22,1 C, MV = 26,7 C, LV = 23,4 C

These values are called the test gradient values.

For the example above, a 132/66/22 kV Yn0d1 40 MVA & 10 MVA for the tertiary transformer is used.

CT ratios HV 200/2 testing @ full load HV I = 175 Amps

MV 200/2 testing @ full load MV I = 349 Amps

LV 190/2 testing @ full load LV I = 262 Amps Line = 151 Amps phase

Following procedure of Kihlstrom manual (TD50/TD76) the resistance values are calculated as follows.

HV 22, 1 C = 1, 1 In Therefore I2/I1 = 1, 1/1, 75 = 0, 63 Thus reading value from table 1, 35

MV 26, 7 C = 1, 1 In Therefore I2/I1 = 1, 26/1, 75 = 0, 72 Thus reading value from table 1, 55

LV 23, 4 C = 1, 1 In Therefore prim/sec = 151/190/2 = 1, 59 I2/I1 = 1, 26/1, 59 = 0, 79

Thus reading value from table 1, 8

(See section under AKM Supplementary Information in Annex A)

6.8 Anci llary devices: Buchholz relay tests

The Buchholz relay is a gas-actuated relay mounted in the pipe connecting the transformer and theconservator. It contains an alarm and a tripping device, each consisting of a float hinged to a frame. Thealarm float is situated in the upper part and the tripping float in the lower part of the housing. Each deviceconsists of a switch that is operated when the float drops down. This switch can have one or two contactsdepending on the type ordered. Under normal conditions the relay will be filled with oil and both floatssuspended in the oil from their respective hinges. The Buchholz relay normally has two tubes running downto the side of the transformer tank below the relay. They are labelled TEST and SAMPLE, the TEST cockbeing used when testing the relay and the SAMPLE cock being used when taking oil samples for testingpurposes.

When a serious internal fault occurs, the generation of gas is so rapid that an oil surge is set up through therelay. This oil flow will impinge on the flap fitted to the lower float, causing it to operate. In the event of oil

loss to the transformer, both alarm and trip will operate in turn.There are many different relay designs and thus small changes in operation can occur.


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6.8.1 Procedure

The alarm and tripping functions of a Buchholz relay are most readily tested using a mechanical push-button provided on the relay. By pressing the push-button slowly, the alarm and trip outputs should operatein turn. This push-button (if provided) is normally protected by a screw-on cap on top of the relay.

Where a test button is not provided on a specific relay, the relay should be tested by injecting nitrogen gasinto the TEST cock. The pressure shall be between 2 and 4 kPa until both the alarm and trip operates inturn. After connecting the hose of the gas bottle to the TEST cock and adjusting the gas pressure to therequired level, open the TEST cock and force the gas into relay. Both the alarm and trip output contactsshould be measured to verify correct operation, making sure that the alarm contact operates before the tripcontact. Once the relay has been tested in this way, the gas should be removed to restore the relay tonormal. To release the gas from the relay, the TEST cock should be opened until the relay has filled upwith oil once again and all excess gas bubbles have been removed. A bottle or can should be used tocatch the oil that will be forced out at the cock once the gas is released.

NOTE: exercise caution when testing a Buchholz relay using gas injection. There have been incidents in which excessive gaspressure was used, sufficient to operate the transformers pressure relief valve.

6.9 Anci llary devices: OLTC Surge relay test

The tap-changer tank is supplied with a pressure relay that operates a signalling contact in the event of anunusual pressure rise due to an internal fault in the tap-changer tank. Usually the smaller tanks have adirect connection to the atmosphere via a breather and the larger tanks are connected to the conservator.The pointer on the pressure valve has two positions, TEST and SERVICE. When the transformer is put intoservice the pressure valve should be locked with a padlock in the SERVICE position.

6.9.1 Procedure

The pressure valve can be tested by using a pressure source (a nitrogen gas bottle should be adequate)and connecting it to the nipple of the valve. The pointer on the pressure valve should be set to TEST. Theoperating level of the pressure valve of the tap-changer for the transformer should be obtained from themanufacturers documentation issued with the transformer. For smaller tanks this value is typically between30 and 50 kPa, and for the larger tanks they are typically between 50 and 100 kPa. The signalling contactshould be measured and correct operation verified when testing the pressure valve.

After completion of the test, the unit should be bled to remove all gas bubbles and the pointer should bereturned to the SERVICE position and the padlock secured on the unit.

6.10 Anci llary devices: Pressure relief valve test

A flashover or short-circuit occurring in an oil filled transformer is usually accompanied by an overpressurein the tank, due to the gas being formed by the decomposition and evaporation of the oil. By providing thetransformer with an explosion vent or pressure relief valve the overpressure can be limited, therebyminimising damage to the transformer.

6.10.1 Procedure

The pressure relief valve is provided with a signalling switch that is mounted on the side of the cover. Thehousing of the switch is a sealed, weatherproof type with single-pole, double-throw switch that is activatedby the upward movement of the outer valve ring. Once the switch has operated, it maintains this positionand gives a continuous signal until it is manually reset.


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To test its operation, the switch has to be manually activated, refer to the transformer manual on how thisshould be done. Once the switch has operated, the contact should be measured to verify operation. Theswitch should then be reset using the reset lever.

6.11 Anci llary devices: Oil level gauge

6.11.1 Purpose

To verify that the oil level gauge shall correctly alarm a low oil level.

5.15.2 Procedure

Essentially this should be checked with the oil out of the conservator tank, thus before filling. Check afterfilling that it is clear of the alarm. If there is a mechanical means of checking the contact status this may bedone. This procedure is recommended for existing in commission transformers as it is an expensiveexercise to change the oil level of a full transformer.

6.12 Anci llary devices: Fans and Pumps

6.12.1 Purpose

To ensure that all ancillary devices are functioning correctly, so that the transformer can continuouslyperform at the rated capacity.

6.12.2 Procedure

a) Check direction of rotation on all fans. This is marked on the blade or cowling. Failing this, checkwith the transformer supplier. The draught is usually upwards or into the fins.

b) Check direction of rotation on all pumps, often flow meters are present to indicate correct oil flowdirection. It is usually a simple matter to determine the direction of the cooling pumps, the hot oil isless dense and thus rises to the top of the transformer tank and thus also the cooling radiators orfins. The natural convection cycle is then to circulate the oil from the top of the transformer tank tothe cooling radiators and back into the transformer tank at the base of the transformer.

c) Check overcurrent protection for the fan and pump motors at either the contactor overload or MCBsupplying the fans or pumps. Usually this is a three phase circuit but may be a single phase circuit.Check for obvious signs of damage to the MCBs, and that the MCB may be opened to isolate the fansupply, and closed without trouble.

6.13 Verif ication of neutral earth ing requirements and tank earth ing

6.13.1 Purpose

This test is performed so as to ensure that the transformer is adequately earthed before energisation ispermitted. This is for new and existing units. Depending on the voltage level of the transformer used byEskom, the insulation level may be specified as follows:

a) Fully insulated Neutral need not be earthed (grounded).


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b) Partially graded Neutral to be protected via a surge arrestor, or solidly earthed (grounded).

c) Fully graded Neutral shall be solidly earthed (grounded).

6.13.2 Procedure

The decision as to whether or not to earth the neutral of a given transformer shall be based on therequirements of DPC 34-2149, Policy for neutral earthing of electrical networks. Partially graded HVwindings require that suitable surge protection be fitted to an unearthed neutral as outlined in DISSCAAD3and DPC 34-2149.

The transformer tank shall be earthed with 4 earths to the station earth. The earthing conductor shall beselected to be suitably rated for the maximum neutral current (see DISSCAAD3).

7 Power Transformer Special tests

7.1 Polarisation Index (PI) test

7.1.1 Purpose

Knowing the polarisation index of a transformer can be useful in appraising the fitness of the unit forservice.

The polarisation index is useful in evaluating windings for:

Build up of dirt or moisture;

Gradual deterioration of the insulation (by comparing results of tests made earlier on the same unit);

Fitness for overvoltage tests; and

Suitability for operation.

7.1.2 Procedure

The index is calculated from measurements of the winding insulation resistance.

Before measuring the insulation resistance, remove all external connections to the transformer andcompletely discharge the windings to earth

Proceed by applying 2500 Volts DC between the winding and earth using a direct-indicating, power-drivenMegohmmeter. The voltage is applied for 10 minutes and kept constant for the duration of the test.

The Polarisation Index is calculated as the ratio of the 10-minute to the 1-minute value of the insulationresistance, measured consecutively.

The insulation is considered good if the polarisation index is between 2 and 4.


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7.2 Bushing test (Tan Delta test)

7.2.1 Purpose

The bushings of a transformer are tested before fitting and/or transport by means of a tan delta test set.This is to ensure that this critical component does not have an insulation problem. If the bushing isincorrectly assembled, loses its insulating oil or has contaminated oil or insulation, the bushing may fail onenergisation and possibly cause damage to the complete transformer.

7.2.2 Procedure and Expected Resul ts

A detailed test procedure is found in DPC 34-734 Procedure for Tan Delta Testing .

7.3 Sweep Frequency Response Analysi s (SFRA) tests

7.3.1 Purpose

Sweep Frequency Response Analysis tests are special tests, sometimes performed as part of thecommissioning tests, during preventative maintenance and after fault trips. SFRA assesses the mechanicalcondition of transformers, and is one of several methods used to detect mechanical failures. In light ofEskoms sourcing of transformers on a global basis under the Sisonke project, it is now deemed wise toperform SFRA tests as mechanical failure might result from transport damage. The tests are to be done onthe transformer prior to dispatch from the factory and similar tests done when the transformer is finallyinstalled and commissioned on site. A comparison is then made of the reference thumb prints obtained priorto the transformer leaving the factory and the thumb print obtained during commissioning. It must be notedthough that the SFRA is one of several methods that is used to detect mechanical damage; other methodssuch as Dissolved Gas Analysis (DGA) leakage reactance etc. should also be used.

7.3.2 Procedure

The following test procedure is only cursory. Detailed information can be obtained from transformersuppliers as well as suppliers of suitable test equipment such as Doble and Omicron.

Multiple low voltage measurements at varying high frequencies are taken of the transformer parameters ofresistance, inductance and capacitance between the windings and between windings and core, and thumbprints produced. It is thus obvious that highly specialised equipment such as the Doble M5100 test set orequivalent is essential and manufacturer instructions must be followed if meaningful results are to beobtained. Close liaison with the test set equipment supplier is also essential in interpreting the resultsobtained. From some equipment manufacturers experience, certain frequency bands indicate differentproblem conditions e.g. 2 kHz indicates core deformation, an open circuit or shorted turns; 400 kHz to2MHz indicates movement of main and tap winding leads; 2 kHz to 20 kHz indicates bulk windingmovement relative to each other, clamping structure; 20 kHz to 400 kHz indicates deformation between themain and tap winding. It is thus important to keep a record of the initial test results as they are used forcomparison purposes and also for trending purposes during subsequent tests. Specific SFRA fingerprintsfor the Sisonke transformers can be downloaded from the Sisonke webpage. It is stressed again that adetailed test procedure and correct interpretation of the results may be obtained from the equipmentsuppliers.


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8 Tap Change tests

8.1.1 Local electri cal operation

8.1.1.1 Purpose

To verify that local Raise/Lower control is operational (where provided)

8.1.1.2 Procedure

Raise and lower operation of the tap changer is possible from the TCD when the tap change scheme isselected to Manual. Some drives also include a Local/Remote switch which must be selected to Local foroperation via the TCD. Check operation by making use of the Lower/Raise control switch.

8.1.2 Inter tap time and transitional tap time

8.1.2.1 Purpose

To verify correct operation of tap change drive mechanism and diverter contacts.

8.1.2.2 Procedure

Checking of the transition time is not necessary at every overhaul. However, tests should be conducted atmajor overhauls, contact changes etc. This is a specialised task usually conducted by tap change drivemaintenance staff.

The Inter tap time is measured using a stop watch when tapping through the change over tap i.e. tap 8 9or 9 10. The result is needed for the setting of some voltage regulating relays.

8.1.3 Emergency Stop

8.1.3.1 Purpose

To verify the emergency stop circuit (if fitted) should it be required to operate under an emergency.

8.1.3.2 Procedure

Check emergency stop by giving raise or lower pulse and after a second press emergency stop. Operation

should be interrupted. Reset the emergency stop, operation should now be completed.

8.1.4 Limit switches upper and lower

8.1.4.1 Purpose

To verify correct operation of electrical and mechanical TCD interlocks so that the TCD and diverter contactmechanism are not damaged or by over driving the mechanism at the minimum and maximum tap eitherelectrically or mechanically.

8.1.4.2 Procedure

Check the limit stops by operating the tap changer at each of the end positions. When trying to operate itelectrically beyond the end position, the motor should not start. Check the mechanical end stop by trying to


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hand crank it beyond the end position. After a couple of turns on the hand crank it should be mechanicallystopped.

Operate the tap changer electrically to the other end position and repeat the test procedure above. Thisprocedure shall be done with utmost care as serious mechanism damage can occur should the electricalinterlocking be faulty.

8.1.5 Manual crank interlock

8.1.5.1 Purpose

To ensure that the TCD cannot be operated electrically whilst the manual crank handle is in use and thusinjure personnel.

8.1.5.2 Procedure

As soon as the hand crank is fitted for manual operation, the operating circuit of the motor contactor isopened, thus whilst the crank handle is fitted to its shaft the TCD cannot be operated electrically. Thisshould be verified.

8.1.6 Analogue/Binary tap posit ion indicator

8.1.6.1 Purpose

To ensure that the tap position indicator on the transformer is reading the correct tap number and that theencoder used is the correct unit for the tap change scheme.

8.1.6.2 Procedure

The correct operation of the encoder shall be evident from the voltage ratio tests and the readout of theTCD tap position indicator. The tap position indicator on the tap change scheme will either be an analogue(pointer) or a binary (digital) type. Ensure that the encoder used is the correct unit. The analogue unit isusually easy to spot as it has a multitude of resistors on the transmitter plate. The binary encoder is usuallydriven by auxiliary contacts from the transmitter plate and is a separate unit. In some cases a bank ofrelays may emulate the typical binary encoder function. The manufacturers manual should detail themethods used and the terminals where the functions are located.

NOTE: Tap change control scheme using REG-D and REG-DA voltage regulating relays have options where by it is possibleto interface an analogue (resistor chain) encoder with the digital relay.

9 Neutral Electromagnetic Coupler tests

The NEC tests are done in a similar way to the power transformer tests and the corresponding testprocedure shall be used as a basis.

9.1.1 Vector group test

This test is carried out as per Section 6.1. A scale phasor diagram including expected measurement resultsare presented for a Dyn11 transformers in Figures 12 (assuming an 11kV/400V transformer).


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Figure 12. Vector group test: vector diagram for an 11kV/400V Dyn11 transformer

It is evident from Figure 12 that with the B and b phases strapped and Vac being 400V, the measurementsbetween phases are very similar, and care must be taken to identify the higher voltage.

9.1.2 Auxil iary transformer posit ive sequence impedance test

This test is conducted in the same manner as is described in Section 5.2.

9.1.3 Zero sequence impedance test

9.1.3.1 Purpose

To measure the zero sequence impedance of the NEC.

9.1.3.2 Procedure

The primary bushings of the NEC under test are strapped, and a test voltage, usually 230 or 400V AC, isapplied from the phase to the neutral bushings. The test current is measured at the neutral bushing. Thezero sequence impedance Z 0 is calculated as:

measured

measuredo I

VZ 3 /phase

Next, measure the DC resistance between two phases of the NEC under test. The resistance per winding,r, is equal to half of the measured resistance.

Next, measure the resistance (Rt) of one phase to neutral ie (A-ZN). The measured resistance will includethe current limiting resistor fitted inside the tank. One can determine the resistance of the resistor, R, bysubtracting the winding resistance, r:

R = Rt - r

Where R is the resistance of the current limiting resistor

r is the per phase resistance of the winding


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The relationship between the Z 0 impedance derived earlier, and the component resistances and reactanceis given as: Z0 = (3 x R) + r + jX0. This relation can be used together with the previouslymeasured/calculated values to derive X0. The values r, R and X 0 can then be checked against the

nameplate values.

9.1.4 Oil temperature inst rument tests

The oil temperature instrument shall be tested in accordance with Section 6.7. By consensus from existingNEC/RT manufacturers, the recommended settings are:

Alarm = 80 C

Trip = 100 C


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Transformer Tech Procedures DPC 34-1032