Evaluation Voltage Excitation Tests

Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramco’semployees. Any material contained in this document which is notalready in the public domain may not be copied, reproduced, sold, given,or disclosed to third parties, or otherwise used in whole, or in part,without the written permission of the Vice President, EngineeringServices, Saudi Aramco.

Chapter : Electrical For additional information on this subject, contactFile Reference: Evaluating Voltage Excitation Tests W. A. Roussel on 874-1320

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Evaluating Voltage Excitation Tests

Engineering Encyclopedia Electrical


Saudi Aramco DeskTop Standards

CONTENTS PAGES

EVALUATING TURNS RATIO TESTS ......................................................................................1

Turns Ratio Test Sets: Construction and Operational Principles ....................................1

Construction .......................................................................................................1

Operational Principles ........................................................................................4

Turns Ratio Tests: Purposes and Basic Techniques ........................................................5

Purposes .............................................................................................................5

Basic Technique .................................................................................................5

Identifications of Faults ...................................................................................................7

Shorted Turns .....................................................................................................7

Open Circuits .....................................................................................................7

Incorrect Number of Turns .................................................................................7

Tap-Changer Faults ............................................................................................7

Incorrect Winding Polarity .................................................................................8

Magnetic Core Damage......................................................................................8

EVALUATING INSTRUMENT TRANSFORMER RATIO AND EXCITATIONCURRENT TESTS........................................................................................................................9

Ratio and Excitation Current Testing of Instrument Transformers: Principles andTechniques.......................................................................................................................9

Principles............................................................................................................9

Technique for Performing CT Tests .................................................................12

Technique for Performing VT Tests .................................................................17

Evaluation Factors .........................................................................................................19

Accuracy Class.................................................................................................19

Ratio Error........................................................................................................20

Phase Angle Error ............................................................................................20

Magnetization Current......................................................................................22



Saudi Aramco DeskTop Standards

WORK AID 1: RESOURCES USED TO EVALUATE A TURNS RATIO TEST ....................23

Work Aid 1A: Non-Mandatory Test Report P-025, Oil-Filled Transformers(Handout 13)..................................................................................................................23

Work Aid 1B: ANSI/IEEE Standard C57 .....................................................................23

Work Aid 1C: Applicable Procedural Steps..................................................................24

WORK AID 2: RESOURCES USED TO EVALUATE INSTRUMENT TRANSFORMERRATIO AND EXCITATION CURRENT TESTS .......................................................................26

Work Aid 2A: Non-Mandatory Test Report P-014, Instrument Transformers(Handout 17)..................................................................................................................26

Work Aid 2B: ANSI/IEEE Standard C57 .....................................................................26

Work Aid 2C: Manufacturer’s Literature ......................................................................27

Work Aid 2D: Applicable Procedural Steps .................................................................28

GLOSSARY ................................................................................................................................31



Saudi Aramco DeskTop Standards 1

EVALUATING TURNS RATIO TESTS

Turns Ratio Test Sets: Construction and Operational Principles

ConstructionFigure 1 is an illustration of the front panel and the four leads of a transformer turns ratio (TTR) test set. Themajor components of the front panel and their functions are:• The excitation current meter. This meter indicates the magnitude of current that excites the transformer

under test.

• The voltmeter. The voltmeter indicates the terminal voltage of the test set’s internal hand-crankedgenerator.

• The null detector. This detector is a galvanometer that indicates the condition of balance in the test set’sinternal bridge circuit.

• The three decade switches. These switches are adjusted to achieve a null balance in the test set’s internalbridge circuit.

• The potentiometer. This potentiometer is used to make the fine adjustment of the null balance of the testset.

• The indicator windows. These windows display numerals that indicate the turns ratio of the transformerunder test.

• The grounding stud. This stud is used to connect the frame of the TTR test set to a grounding system forthe purpose of personal safety.

Figure 2 is a simplified schematic diagram of the electrical circuits of a transformer turns ratio test set. Thefollowing four subject headings explain the major functional components of this circuit.Hand-Crank Generator −− The hand-cranked generator produces a sine-wave alternating voltage ofapproximately 8 volts magnitude and 60 Hz frequency when it is cranked at 120 rpm.




Figure 1. Front Panel and Leads of a TTR Test Set




Figure 2. Simplified Schematic Diagram of a TTR Test Set




Balance Bridge Circuitry −− The circuit of a TTR test set (Figure 2) is a balance bridge type of circuit. Thehigh-voltage winding of the test set’s reference transformer and the high-voltage winding of the transformerunder test are the two arms of the balance bridge. When both of these high-voltage windings are generating thesame voltage, the null detector will have a center-of-scale deflection.Variable Ratio Reference Transformer −− The test set’s reference transformer has an adjustable turns ratio.The ratio is adjustable in the range of 0.001 to 130. The ratio of the reference transformer is adjusted bychanging the three decade switches and the potentiometer.Synchronous Detector −− The null detector is connected to the bridge circuit through a synchronous rectifier.These two components constitute a synchronous detector. This synchronous detector responds only to currentthat is the same frequency as the test set’s hand-cranked generator. The synchronous detector is not sensitive tonoise signals that might be induced in the test set’s leads by the strong magnetic and electric fields that exist inpower substations.

Operational PrinciplesThe operation of a TTR test set is based on the principle of voltage ratio being equal to turns ratio intransformers and the principles of a balance bridge measurement.Equivalence of Voltage Ratio and Turns Ratio −− The ratio of winding voltages of any two windings installedon the same segment of magnetic core is nearly identical to the ratio of the numbers of turns of the windings.This relationship is expressed mathematically as:

•VV

NN

1

2

1

2

=

• where V1 is the open-circuit voltage of the high-voltage winding, V2 is the open-circuit voltage of thelow-voltage winding, N1 is the number of turns in the high-voltage winding, and N2 is the number ofturns in the low-voltage winding.

For measurements of the turns ratio of power transformers and distribution transformers, this relationship istypically accurate to within 0.1% of the turns ratio value. A TTR test set actually measures voltage ratio. Thenumber that appears in the indicator windows is 1.0005 times the measured voltage ratio.




Balance Bridge −− By changing the decade switches and the potentiometer until the null detector indicates nullbalance, the test set’s reference transformer is made to have the same voltage ratio as the transformer under test.

Turns Ratio Tests: Purposes and Basic Techniques

PurposesThe purposes of turns ratio tests are to verify conformance of a power transformer to a purchase specificationand to troubleshoot faults in transformer windings.Verify Conformance to Purchase Specification −− Saudi Aramco purchase specifications for a powertransformer or a distribution transformer include a specification that the measured turns ratio of all windings beequal to the ratio of the winding voltages printed on the transformer’s nameplate within a tolerance of ±0.5%.Transformer turns ratio is measured as soon after delivery of the transformer as practical to verify conformanceto this purchase specification.Troubleshoot Faults −− If a transformer is suspected to have a winding fault, a turns ratio test is usuallyconducted to confirm that a fault exists and to distinguish whether the fault is a turn-to-turn fault or an open-circuit fault. Note: A transformer turns ratio test is not used alone to determine the suitability of a transformerfor continued service. Other tests are conducted such as a visual inspection, an oil dielectric test, an insulationpower-factor test, an insulation-resistance test, a combustible gas-in-oil test, an excitation current test, or aterminal-to-terminal resistance test.

Basic TechniqueThe basic technique of conducting a turns ratio test is to measure a ratio for every set of windings and for everywinding tap selection. The turns ratio test set leads are connected to the transformer winding terminals in aconfiguration that will excite the low-voltage winding of the transformer. Note: Although a tertiary windingmight have a lower voltage rating than a low-voltage winding, it is preferable to excite the low-voltage windingduring a turns ratio test.




Figure 3. A Nameplate Connection Diagram for a Power Transformer

Example A: How many measurements of turns ratio should be made on a three-phase transformer that hasthe nameplate connection diagram shown in Figure 3?

Answer: The connection diagram of Figure 3 shows three sets of windings. The de-energized tapswitch has five positions. For each set of windings, five measurements of ratio should bemade, one measurement for each tap-switch position. A total of fifteen measurements of turnsratio should be made.

Example B: To which winding terminals of the transformer represented in Figure 3 should the test set’sanvil clamps and alligator clamps be connected to measure the turns ratio of the phase-Awinding?

Answer: In order to excite the low-voltage winding, the anvil clamps should be connected to terminalsX0 and X1. The alligator clamps should be connected to terminals H3 and H1.




Identifications of FaultsThe nature of a transformer winding fault can be identified by interpreting the indications of a TTR test set.

Shorted TurnsA turn-to-turn short-circuit fault in a transformer winding will cause a change in the measured turns ratio. Forthis kind of fault, the measured ratio might be either greater than or less than the ratio measured at the time ofcommissioning.In some cases, a turn-to-turn short-circuit fault can be detected by a more than usual current indicated on theTTR test set’s excitation current meter. This meter is not marked in a standard engineering quantity such asamperes. For this reason, the most accurate evaluation of more-than-normal excitation current is to compare theexcitation current indication of the transformer having a suspected fault with the indication from a similarmodel of transformer that is known to be in good condition. A turn-to-turn fault additionally might require anextra turning force on the hand-cranked generator.

Open CircuitsAn open-circuit winding can be distinguished by indications of a normal generator voltage, a normal level ofexcitation current, but no deflection of the null detector.

Incorrect Number of TurnsA turns ratio test can detect that a transformer winding has an incorrect number of turns. The indication of anincorrect number of turns is that the measured turns ratio is different than the nameplate ratio of voltages bymore than 0.5%. The turns ratio test set will indicate a normal generator voltage, normal level of excitation,and normal deflection of its null detector.

Tap-Changer FaultsTap changer faults include short-circuit faults, open-circuit faults, broken components in the drive mechanismof a tap switch and misconnected winding leads. In the case of a tap changer fault, measured values of turnsratio will not be correct for some of the tap switch selections, but they will be correct for other tap switchselections.




Incorrect Winding PolarityIf the transformer’s winding polarity is not correct due to an incorrect internal connection or an incorrectterminal marking, the null detector will deflect to the right when the decade switches are set to 0.000. Note:The test set’s red-colored alligator clamp must be connected to the high-voltage winding terminal that has thesame instantaneous voltage polarity as the low-voltage winding terminal to which the red-colored anvil clampis connected.Example C: To which winding terminal of the transformer represented in Figure 3 should the test set’s red-

colored alligator clamp be connected if the red-colored anvil clamp is connected to terminalX1?

Answer: Because the H1 terminal has the same instantaneous polarity as the X1 terminal, the red-colored alligator clamp should connected to the H1 terminal.

Magnetic Core DamageThe indications of magnetic core damage are a large magnitude of excitation current, and a measured ratio thatis different than the ratio measured during the transformer’s commissioning test by more than 0.5%. Theindications of magnetic core damage are often difficult to distinguish from the indications of an incorrectnumber of turns. If core damage is suspected, visual inspections of the magnetic core and an insulationresistance test of the core-to-ground insulation should be recommended.




EVALUATING INSTRUMENT TRANSFORMER RATIO AND EXCITATIONCURRENT TESTSBecause many instrument transformers have a large primary impedance voltage drop at 8 volts excitationvoltage, a transformer turns ratio test set cannot be used to make consistently accurate measurements of turnsratio of instrument transformers. The alternative methods of measuring the turns ratio of an instrumenttransformer are explained under this subject heading. Also explained are the excitation current tests that areperformed on instrument transformers.

Ratio and Excitation Current Testing of Instrument Transformers: Principles andTechniques

PrinciplesThe principles of ratio tests and excitation current tests can be explained using an equivalent circuit diagram, aphasor diagram depicting associated circuit currents and voltages, and a description of the ratio correctionfactor.Equivalent Circuit −− Figure 4 is an equivalent circuit diagram of an instrument transformer. One winding ofthe instrument transformer is excited by a sine-wave voltage, V1. The other winding is open circuit. Thetransformer symbol represents an ideal transformer that has no winding impedance, has a turns ratio that isexactly equal to N1/N2, and has a voltage ratio E1/V2 that is exactly equal to its turns ratio.

Figure 4. Equivalent Circuit of an Instrument Transformer




Excitation Currents −− In the equivalent circuit of Figure 4, a susceptance element B and a conductanceelement G account for the excitation current, I. This excitation current is delayed in phase from the excitationvoltage V1. The amount of this phase delay is a non-linear function of the magnitude of the excitation voltage.Additionally, the magnitude of the excitation current is a non-linear function of the excitation voltage as shownin Figure 5.

Figure 5. Excitation Voltage Versus Excitation Current Curves

The equivalent circuit has elements R and X that represent the resistance and inductance of the instrumenttransformer’s primary winding. The flow of excitation current through these R and X elements causes voltagedrops proportional to IR and IX. Figure 6 is an open phasor diagram that shows the voltages in the primary-circuit loop and the open-circuit secondary voltage. This phasor diagram demonstrates that the open-circuitsecondary voltage V2 will not be exactly equal to V1 times the turns ratio N1/N2, and it also implies that therelative phase delay between V2 and V1 will not be exactly 180 degrees.

Figure 6. Phasor Diagram of Transformer Open-Circuit Voltages




Ratio Correction Factors −− The ratio correction factor is the true ratio of an instrument transformer divided byits nameplate ratio.

• FNNR

T

N

=

• where FR is the ratio correction factor, NT is the true ratio, and NN is the nameplate ratio.

Manufacturers publish ratio correction factor curves and phase-angle error curves for each different model ofcurrent transformer or voltage transformer. Figure 7 is an example set of curves. The principle use of a ratiocorrection factor and a value of phase-angle error is to correct the readings of a kilowatt-hour meter to which thecurrent transformer and potential transformer are connected. Correcting a kilowatt-hour reading produces avalue called true watts. The procedure for calculating true watts is beyond the scope of this Module.Correction factors and phase-angle errors do, however, have a secondary use in the evaluation of instrumenttransformer tests. This use of correction factors is explained in the procedure in Work Aid 2.

Figure 7. Ratio Correction Factor and Phase-Angle Error Curves




Technique for Performing CT TestsThe commissioning tests and periodic maintenance tests of a current transformer (CT) include measurements ofwinding resistance, of turns ratio, and excitation current. These measurements are explained in the next foursubject headings.Note: Many of the tests conducted on current transformers produce high voltage. They should only beperformed by experienced personnel who are familiar with the particular hazards related to current-transformer testing.The test results of current transformer ratio and excitation tests are influenced by the existence of residualmagnetism in the magnetic core of the current transformer. If alternating current in a CT winding is interruptedabruptly for any reason, or if a CT winding is excited with direct current, then a magnetism will remain in theCT magnetic core. When the CT is subsequently tested, the residual magnetism will cause a larger than normalexcitation current. Additionally, an accurate measurement of turns ratio might not be realized. Residualmagnetism is eliminated before conducting tests by applying, using a variable-voltage source, a sine-wavealternating voltage to the secondary terminals of a CT. The initial voltage is made large enough to saturate theCT’s magnetic core. Voltage is then gradually reduced to zero.Winding Resistance −− The winding resistance (terminal-to-terminal resistance) of the secondary winding of acurrent transformer is measured using a Kelvin Bridge or a digital low-resistance ohmmeter. The ohmic valueof terminal-to-terminal resistance and the estimated winding temperature are recorded during commissioning toestablish base data. Temperature-corrected values of winding resistance are measured during periodicmaintenance and are subsequently compared to the original values of resistance.Ratio-by-Voltage −− The turns ratio of a current transformer can be determined by measuring its voltage ratio.Figure 8 is a schematic diagram of a test circuit for measuring the voltage ratio of a CT. A variableautotransformer is used to excite the secondary winding with an adjustable alternating voltage. The magnitudeof this applied voltage should be small enough to avoid saturation of the CT’s magnetic core. High-impedancevoltmeters are connected in the circuit for measuring secondary and primary voltages. An ammeter isconnected in the secondary circuit to detect excessive excitation current (no more than 0.5 ampere).




Figure 8. Circuit for a Ratio-by-Voltage Test of a Current Transformer

The turns ratio is calculated using the following formula:

• NVVT

P

S

=

• Where NT is the turns ratio, VP is the measured primary voltage, and VS is the measured secondaryvoltage.

The evaluation of this calculated turns ratio is explained in the procedure of Work Aid 2.Note: If the CT to be tested is a window-type CT, a short conductor can be placed within the geometric centerof the window to serve the same function as the primary winding. The voltage VP can be measured from oneend of the conductor to its other end. The exact length of the conductor and the exact connection points of thevoltmeter’s leads are not critical factors for making an accurate measurement.Ratio-by-Current −− The turns ratio of a current transformer can be measured by connecting its primarywinding in series with the primary winding of another CT of known ratio (called a reference CT) and injectingcurrent into this series circuit with a primary-current injection test set. As shown in Figure 9, one ammeter isconnected into the secondary windings circuit of the reference CT, and another ammeter is connected into thesecondary winding circuit of the CT under test. The magnitude of current that is injected should be as close tothe rated primary amperes as the capacity of the test set will allow.




Figure 9. Circuit for a Ratio-by-Current Test of a Current Transformer

The ratio of the CT under test is calculated using the formula:

• N NIIT RR

T

=

• where NT is the ratio of the CT under test, NR is the ratio of the reference CT, IR is the magnitude ofcurrent measured in the secondary circuit of the reference CT, and IT is the magnitude of currentmeasured in the secondary circuit of the CT under test.

Note: There are several disadvantages to measuring turns ratio by current. The primary-current test setneeded to perform this measurement is large and heavy. A primary-current injection test set will inject a non-sinusoidal current in some circumstances. There is a possible hazard of a back-feed voltage being induced inother CTs that are installed on the same bus. Additionally, special care must be taken to extend the test circuitconductors as far as possible along the axis of a CT to minimize the influence of stray magnetic flux.




Excitation Test −− A turn-to-turn short-circuit fault in the secondary winding of a CT or physical damage in theCT magnetic core is detected by conducting an excitation test. This test, sometimes called a magnetizationcurve test, is performed by measuring values of excitation current that flow in the secondary winding of a CT atvarious values of excitation voltage. Figure 10 is a diagram of a test circuit that can be used to perform anexcitation test. Depending on the ANSI/IEEE C57.13 standard voltage rating (10, 20, 50, 100, 200, 400, or800) of the CT, the magnitude of voltage needed to conduct the test might be as little as 10 volts or as great as800 volts. The evaluation of a CT excitation test is explained in the procedure of Work Aid 2.

Figure 10. Circuit for an Excitation Test of a Current Transformer

Polarity −− Figure 11 shows two circuit diagrams that represent two acceptable techniques for conducting apolarity test of a CT. In the circuit of Figure 11a, channel 1 of a dual trace oscilloscope displays the waveformof the alternating voltage that excites the secondary winding of the CT under test. Channel 2 of theoscilloscope displays the waveform of the voltage induced in the primary winding of the CT. Connected withthe polarities shown, both waves should appear on the display of the oscilloscope as being in phase. Note:Because all instrument transformers have a subtractive polarity, the instantaneous voltage polarity of their H1and X1 terminals should be the same.




In the circuit represented by Figure 11b, the reference CT has the same marked ratio as the CT under test and isknown to have a correct polarity. Ammeter A1 indicates the magnitude of current that flows in X1 terminal ofthis reference CT. The output of the primary-current injection test is adjusted until ammeter A1 indicates 5amperes, which is the rated secondary current of the reference CT. If the CT under test has the correct polarity,ammeter A2 will indicate zero.

Figure 11. Circuit Diagrams for Polarity Tests of CTs




Technique for Performing VT TestsThe commissioning tests and periodic maintenance tests of a voltage transformer VT include measurements ofvalues of winding resistance, a measurement of turns ratio, and an excitation current test. These measurementsand tests are explained in the next three subject headings.Note: Many of the tests conducted on voltage transformers produce high voltage. They should only beperformed by experienced personnel who are familiar with the particular hazards related to voltage-transformer testing.Winding Resistance −− The values of winding resistance (terminal-to-terminal resistance) of the primarywinding and the secondary winding of a voltage transformer are measured using a Kelvin Bridge, digital low-resistance ohmmeter, or a digital multimeter. These resistance values are corrected to a standard temperatureand are recorded during commissioning to establish base data. Temperature-corrected values of windingresistance are measured during periodic maintenance and are compared to the original values of resistance.Ratio-by-Voltage −− The turns ratio of a voltage transformer can be determined by measuring its ratio ofwinding voltages. Figure 12 is a schematic diagram of a test circuit for measuring the voltage ratio of a VT. Asingle-phase 120 volt source is used to excite the high-voltage winding. High-impedance voltmeters areconnected in the circuit for measuring the terminal-to-terminal voltages of the high-voltage winding and thelow-voltage winding. Note: The high-voltage winding is excited with low voltage (120 volts) for safetyreasons. Exciting the low-voltage winding with 120 volts would produce a voltage of hazardous magnitude formost models of voltage transformers.




Figure 12. Circuit Diagram for the Ratio Test of a Voltage Transformer

Excitation Current −− The excitation current of a voltage transformer is not usually measured duringcommissioning or during routine maintenance. Excitation current is measured if a fault is suspected to exist ina voltage transformer. Open-circuit excitation current should be no more than 2% of the rated current of thewinding that is excited with rated voltage.Polarity −− Figure 13 is a diagram that represents the circuit that is used to verify the correct polarity of a VT.In this polarity test, a jumper is temporarily connected between the two terminals of the VT that have polaritymarks. These terminals are also marked H1 and X1. A low-voltage AC source excites the high-voltage windingof the VT. The voltage, V1, that is measured between VT’s two high-voltage terminals should be less than thevoltage, V2, that is measured between the two terminals that do not have polarity marks.




Figure 13. Circuit Diagram for the Polarity Test of a VT

Evaluation Factors

Accuracy ClassANSI/IEEE standard C57.13-1976 describes the classification system for instrument transformers that are usedin metering service. Accuracy classes are 0.3, 0.6, or 1.2. The accuracy class appears on the nameplate of theinstrument transformer and represents the maximum percentage difference between actual ratio and nameplateratio at rated voltage and rated current.Current transformers that are used in protective relaying service have an extra system of accuracy classification.Each relaying service CT has a letter C or T stamped on its nameplate followed by a number (10, 20, 50, 100,200, 400, or 800). The C classification is for CTs whose leakage flux does not have an appreciable effect onratio (window-type CTs or bar-type CTs). The T classification is for CTs whose leakage flux does have anappreciable effect on ratio (wound-primary CTs). The number represents the maximum voltage that will existat the secondary terminals of the CT while it is delivering 20 times its rated current and not exceeding a 10percent error in ratio.




Ratio ErrorFigure 14 is a table of values that represent the allowable limits of the ratio correction factor for an instrumenttransformer of accuracy class 0.3, 0.6, or 1.2.

Voltage Transformers Current Transformers

At 90 to 110 Percent RatedVoltage

At 100 Percent RatedCurrent


MeteringAccuracy

Class Minimum Maximum Minimum Maximum Minimum Maximum0.3 0.997 1.003 0.997 1.003 0.994 1.0060.6 0.994 1.006 0.994 1.006 0.988 1.0121.2 0.988 1.012 0.988 1.012 0.976 1.024

Figure 14. Table of Values of Ratio Correction Factors

Phase Angle ErrorThe allowable phase angle error of a metering service instrument transformer is related to its ratio correctionfactor. Figure 15 is a parallelogram that represents the limits of phase angle error for a metering service currenttransformer. Figure 16 is a parallelogram that represents the limits of phase angle error for a metering servicevoltage transformer. Phase angle error is not measured during commissioning tests or routine maintenancetests. Phase angle error is measured if a fault is suspected to exist in a metering service instrument transformer.




Figure 15. Limits of Accuracy for CTs in Metering Service

Figure 16. Limits of Accuracy for VTs in Metering Service




Magnetization CurrentAn excessively large magnetization current (excitation current) indicates a fault in the magnetic core of aninstrument transformer. Figure 17 is a typical plot of secondary excitation voltage versus magnetizationcurrent for a CT. The evaluation of a CT excitation test is explained in the procedure of Work Aid 2.

Figure 17. Excitation Voltage vs. Magnetization Current Curve for a CT




WORK AID 1: RESOURCES USED TO EVALUATE A TURNS RATIO TEST

Use the Work Aids and the procedure described below to evaluate the turns ratio test of a distributiontransformer or a power transformer.

Work Aid 1A: Non-Mandatory Test Report P-025, Oil-Filled Transformers (Handout13)

For the contents of Test Report Form P-025, refer to Handout 13. Note: Handout 13 was also used in WorkAid 3 of Module 3.

Work Aid 1B: ANSI/IEEE Standard C57Applicable excerpts from ANSI/IEEE Standard C57 are given below. Standard C57.12.01-1979 relates to dry-type transformers. Standard C57.12.00-1980 relates to liquid-immersed transformers.

9.1 Ratio. With rated voltage impressed onone winding of a transformer, all other ratedvoltages at no load shall be correct within0.5% of the nameplate markings. Rated tap voltages shall correspond to thevoltage of the nearest turn if the voltage perturn exceeds 0.5% of the desired voltage.

Figure 18. Excerpt from ANSI/IEEE Standard C57.12.01-1979

9.1 Tolerance for Ratio. With thetransformer at no load and with rated voltageimpressed on one winding of a transformer, allother rated voltages at no load shall be correctwithin 0.5% of the nameplate markings,except in cases where the rated tap voltagecorresponds to the voltage of the nearest turnbut still exceeds 0.5% of the desired voltagebecause the volts per turn of the unit exceedsthis tolerance.

Figure 19. Excerpt from ANSI/IEEE Standard C57.12.00-1980




Work Aid 1C: Applicable Procedural Steps1. Select one recorded value of turns ratio that represents a single tap switch selection of a single set of

windings. Note: This evaluation procedure is repeated for each recorded value of turns ratio.2. Determine from the transformer’s nameplate data the rated terminal-to-terminal voltage of the low-

voltage winding circuit. Note: If there is an on-load tap changer installed on the transformer that wastested, the recorded value of turns ratio will correspond to one of 33 different values of rated terminal-to-terminal voltage. Consult the tap changer’s nameplate to determine the rated terminal-to-terminalvoltage that corresponds to the particular on-load tap selection.

3. Calculate V1, the rated voltage of a low-voltage winding.• If the transformer tested is a single-phase transformer, V1 is equal to the rated terminal-to-terminal

voltage determined in step 2.

• If the low-voltage winding is connected in a delta circuit, V1 is equal to the rated terminal-to-terminalvoltage determined in step 2.

• If the low-voltage winding is connected in a wye circuit, V1 is equal to the rated terminal-to-terminalvoltage determined in step 2 divided by 1.732.

4. Determine from the transformer’s nameplate data the rated terminal-to-terminal voltage of the otherwinding circuit whose turns ratio was measured with respect to the low-voltage winding (usually thehigh-voltage winding, but will sometimes be the tertiary winding, or the fourth winding). Note: Ifthere is a de-energized tap changer, a link board, or a tap jumper associated with this winding, therecorded value of turns ratio will correspond to one of the five different values of rated terminal-to-terminal voltage. Consult the transformer’s nameplate to determine the rated terminal-to-terminalvoltage that corresponds to the particular de-energized tap selection.




5. Calculate V2, the rated voltage of the other winding.• If the transformer tested is a single-phase transformer, V2 is equal to the rated terminal-to-terminal

voltage determined in step 4.

• If the other winding is connected in a delta circuit, V2 is equal to the rated terminal-to-terminal voltagedetermined in step 4.

• If the other winding is connected in a wye circuit, V2 is equal to the rated terminal-to-terminal voltagedetermined in step 4 divided by 1.732.

6. Calculate the ratio of rated winding voltages:

• NVVV = 2

1

• Where NV is the ratio of rated winding voltages, V1 is the rated voltage of the low-voltage winding, andV2 is the rated voltage of the other winding.

7. Determine the measured turns ratio NT from the test data .8. Calculate the percentage deviation of the measured turns ratio from the ratio of rated winding voltages:

• %devN N

NT V

V

=−

× 100%

• Where %dev is the percentage deviation, NT is the measured turns ratio, and NV is the ratio of ratedwinding voltages.

9. For a commissioning inspection, compare the percent deviation (%dev) to the ratio tolerance specifiedin ANSI/IEEE standard C57 (Work Aid 1B). If the percent deviation is greater than the tolerance, areport of non-conformance should be made to the office of the Saudi Aramco Chief Engineer.

10. For a maintenance test or for cases where winding damage is suspected, compare NT to the originalturns ratio measured at the time of commissioning. A change of more than 0.5% in the value of NTindicates the need for additional electrical tests and mechanical inspections.




WORK AID 2: RESOURCES USED TO EVALUATE INSTRUMENTTRANSFORMER RATIO AND EXCITATION CURRENTTESTS

Use the Work Aids and the procedure described below to evaluate the voltage ratio test of a CT or VT, or toevaluate the excitation current (magnetization current) test of a CT.

Work Aid 2A: Non-Mandatory Test Report P-014, Instrument Transformers (Handout17)

For the contents of Test Report Form P-014, refer to Handout 17.

Work Aid 2B: ANSI/IEEE Standard C57Applicable excerpts from ANSI/IEEE Standard C57 are given below.

Voltage Transformers Current Transformers

At 90 to 110 Percent RatedVoltage



MeteringAccuracy

Class Minimum Maximum Minimum Maximum Minimum Maximum0.3 0.997 1.003 0.997 1.003 0.994 1.0060.6 0.994 1.006 0.994 1.006 0.988 1.0121.2 0.988 1.012 0.988 1.012 0.976 1.024

Figure 20 Excerpt from ANSI/EEE C57.13-1978, Ratio Correction Factors




Work Aid 2C: Manufacturer’s LiteratureApplicable excerpts from manufacturer’s literature are given below.

Figure 21. Excerpt from Instrument Transformer Manufacturer’s Literature




Work Aid 2D: Applicable Procedural Steps1. If the test data represents a ratio-by-voltage test of a current transformer (CT), determine from the test

data the measured secondary excitation voltage V2 and the measured primary voltage V1. Using thefollowing formula, calculate the turns ratio with the following formula:

• NVVT = 2

1

• Where NT is the turns ratio, V1 is the measured primary voltage, and V2 is the measured secondaryexcitation voltage.

2. If the test data represents a ratio-by-current test of a CT, determine from test data the measured primarycurrent I1, and the measured secondary current I2. Calculate the turns ratio using the following formula:

• NIIT = 2

1

• Where NT is the turns ratio, I1 is the measured primary current, and I2 is the measured secondary current.

3. If the test data represents a ratio test of a CT, calculate the nameplate turns ratio.

• NIINS

P

=

• Where NN is the nameplate turns ratio, IP is the rated primary current and IS is the rated secondarycurrent.

4. If the test data represents a ratio test of a CT, calculate the ratio correction factor:

• FNNR

T

N

=

• Where FR is the ratio correction factor, NT is the true ratio, and NN is the nameplate ratio.




5. If the CT tested is a metering-class CT, compare FR with the maximum and minimum allowable ratiocorrection factors from Work Aid 2B. If FR is outside of the minimum or maximum limit for the CT’saccuracy class, the CT is not suitable for continued service. Note: Whenever FR is calculated usingthe result of a ratio-by-voltage test, evaluate the ratio test by the minimum and maximum limits thatappear in the “100% rated current” column of the table in Work Aid 2B.

6. If the CT is a relaying-class CT, the maximum FR is 1.012 and the minimum FR is 0.988. If FR isoutside of the minimum or maximum limit, the CT is not suitable for continued service.

7. If the test data represents an excitation current test, plot the excitation current versus excitation voltagedata points on a copy of the manufacturer’s excitation curve. Note: Work Aid 2C is an example of amanufacturer’s excitation curve.

8. Evaluate all plotted data points according to the following criteria:• Any value of excitation current plotted below the broken line of the manufacturer’s curve should not

exceed the value of the manufacturer’s curve by more than 25%.

• Any value of excitation voltage plotted above the broken line of the manufacturer’s curve should not beless than 95% of the value of the manufacturer’s curve.

If more than one data plot fails to meet these criteria, the CT is not suitable for continued service.Note: Work Aid 2C is an example of a manufacturer’s excitation current curve.

9. If the test data represent a ratio test of a voltage transformer (VT), determine from test data themeasured secondary excitation voltage V2 and the measured primary voltage V1. Using the followingformula, calculate the turns ratio with the following formula:

• NVVT = 1

2

• Where NT is the turns ratio, V1 is the measured primary voltage, and V2 is the measured secondaryexcitation voltage.




10. If the test data represents a ratio test of a VT, calculate the nameplate turns ratio.

• NVVN

P

S

=

• Where NN is the nameplate turns ratio, VP is the rated primary voltage, and VS is the rated secondaryvoltage.

11. If the test data represents a ratio test of a VT, calculate the ratio correction factor:

• FNNR

T

N

=

• Where FR is the ratio correction factor, NT is the true ratio, and NN is the nameplate ratio.

12. Compare the FR of the VT with the maximum and minimum allowable ratio correction factors of WorkAid 2B. If FR is outside of the minimum or maximum limit for the VT’s accuracy class, the VT is notsuitable for continued service.




GLOSSARY

High-Voltage Winding The winding of a transformer that has the largest rated terminal-to-terminal voltage.

Low-Voltage Winding The winding of a transformer that has the second largest rated terminal-to-terminal voltage.

Phase Angle Error For a voltage transformer, phase angle error is the relative displacementof voltage between the transformer’s high-voltage winding and its low-voltage winding. For a current transformer, phase angle error is therelative displacement of phase between the transformer’s primarycurrent and its secondary current.

Primary Winding The winding of a transformer that normally receives power from thedistribution system.

Ratio Correction Factor The true ratio of an instrument transformer divided by its nameplateratio.

Secondary Winding The winding of a transformer that normally delivers power into thedistribution system.

Tertiary Winding The winding of a transformer that has the third largest rated terminal-to-terminal voltage.

TTR Test Set A transformer turns ratio test set.Turns Ratio The ratio of the number of turns in a winding of higher voltage of a

transformer with respect to the number of turns in a winding of lowervoltage that is installed on the same segment of magnetic core.

Voltage Ratio The ratio of the voltage in a full winding of higher voltage with respectto the voltage of a full winding of lower voltage that is installed on thesame segment of magnetic core.

Documents

Evaluation Voltage Excitation Tests