Evaluation Voltage Excitation Tests

Engineering Encyclopedia Saudi Aramco DeskTop Standards

Evaluating Voltage Excitation Tests

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

Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is is not already 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, Engineering Services, Saudi Aramco.

Chapter : Electrical File Reference: Evaluating Voltage Excitation Tests

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Engineering Encyclopedia

Electrical Evaluating Voltage Excitation Tests

CONTENTS

PAGES

EVALUATING TURNS RATIO TESTS .................... ............................... ....................... ...................... ...................... ....................... .....................1 ..........1 Turns Ratio Test Test Sets: Construction and Operational Operational Principles .................... .............................. ................1 ......1 Construction ..................... ............................... ...................... ....................... ....................... ...................... ..................... ....................... ................1 ....1 Operational Principles ..................... ............................... ...................... ....................... ...................... ...................... ..................... ............ .. 4 Turns Ratio Tests: Purposes and Basic Techniques ...................... ................................. ...................... ..................... ............ .. 5 Purposes ..................... ................................. ....................... ..................... ...................... ....................... ..................... ...................... ......................5 ..........5 Basic Technique .................... ............................... ....................... ...................... ...................... ....................... ..................... .....................5 ...........5 Identifications of Faults ..................... ............................... ...................... ....................... ...................... ...................... ..................... ...................... .............. 7 Shorted Turns ..................... ............................... ...................... ....................... ....................... ...................... ..................... ...................... .............. ... 7 Open Circuits ..................... ............................... ...................... ....................... ....................... ...................... ..................... ...................... .............. ... 7 Incorrect Number of Turns ..................... ............................... ...................... ....................... ....................... ...................... ...............7 .....7 Tap-Changer Faults ..................... ............................... ...................... ....................... ....................... ...................... ..................... ................7 .....7 Incorrect Winding Polarity ..................... ............................... ...................... ....................... ....................... ...................... ...............8 .....8 Magnetic Core Damage .................... ............................... ....................... ...................... ...................... ....................... .....................8 ..........8 EVALUATING INSTRUMENT TRANSFORMER RATIO AND EXCITATION CURRENT TESTS............................ TESTS........................................ ....................... ..................... ...................... ....................... ..................... ...................... ....................... .............. ... 9 Ratio and Excitation Current Testing Testing of Instrument Transformers: Principles and Techniques .................... ............................... ....................... ...................... ...................... ....................... ..................... ...................... ....................... .....................9 ..........9 Principles................... Principles............................. ...................... ....................... ..................... ...................... ....................... ..................... ...................... .............. 9 Technique for Performing CT Tests ...................... ................................. ...................... ..................... .....................12 ...........12 Technique for Performing VT Tests.............................. Tests.......................................... ....................... ..................... ............ .. 17 Evaluation Factors ..................... ................................. ....................... ..................... ...................... ....................... ..................... ...................... ..................19 ......19 Accuracy Class............................. Class......................................... ....................... ..................... ...................... ....................... ..................... ............ .. 19 Ratio Error.................... Error............................... ....................... ...................... ...................... ....................... ..................... ...................... ..................20 ......20 Phase Angle Error ..................... ................................. ...................... ..................... ....................... ...................... ..................... ................2 .....2 0 Magnetization Current...................... Current.................................. ....................... ..................... ...................... ....................... ...................22 ........22

Saudi Aramco DeskTop Standards



WORK AID 1: RESOURCES USED TO EVALUATE EVALUATE A TURNS RATIO TEST ....................23 ....................23 Work Aid 1A: Non-Mandatory Test Report P-025, Oil-Filled Transformers (Handout 13) ..................... ................................. ...................... ..................... ....................... ...................... ..................... ....................... ...................... ...............2 .....2 3 Work Aid 1B: ANSI/IEEE Standard Standard C57 .................... ............................... ...................... ...................... ....................... ................2 ....2 3 Work Aid 1 C: Applicable Procedural Steps.......................... Steps...................................... ...................... ..................... ..................24 .......24 WORK AID 2: RESOURCES USED TO EVALUATE INSTRUMENT TRANSFORMER RATIO AND EXCITATION CURRENT TESTS .................... ............................... ....................... ...................... ...................... ..................26 ......26 Work Aid 2A: Non-Mandatory Test Report P-014, Instrument Transformers (Handout 17) ..................... ................................. ...................... ..................... ....................... ...................... ..................... ....................... ...................... ...............2 .....2 6 Work Aid 2B: ANSI/IEEE Standard Standard C57 .................... ............................... ...................... ...................... ....................... ................2 ....2 6 Work Aid 2C: Manufacturer’s Literature.......................... Literature...................................... ....................... ..................... .....................27 ...........27 Work Aid 2D: Applicable Procedural Steps Steps ...................... ................................. ...................... ..................... .....................28 ...........28 GLOSSARY .................... ............................... ....................... ...................... ...................... ....................... ..................... ...................... ....................... ..................... ...................31 .........31




EVALUATING TURNS RATIO TESTS Turns Ratio Test Sets: Construction and Operational Principles Construction Figure 1 is an illustration illustration of the front panel and the four leads of a transformer turns turns ratio (TTR) test set. The major components of the front panel and their functions are: The excitation current meter. meter. This meter indicates the the magnitude of current that excites excites the transformer • under test. •

The voltmeter. generator.

The voltmeter indicates indicates the terminal terminal voltage of the the test set’s set’s internal hand-cranked hand-cranked

•

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

•

The three decade switches. These switches are adjusted to achieve a null balance in the test set’s internal internal bridge circuit. c ircuit.

•

The potentiometer. This potentiometer is used to make the fine adjustment of the the null balance of the test set.

•

The indicator windows. These windows display numerals that indicate indicate the turns ratio ratio of the transformer under test.

•

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

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


1



Figure 1. Front Front Panel and Le Leadsof a TTR Te T est Set


2



Fig Figure ure2. Simplifi Simplifie ed Schematic Diag Diagram of a TTR Te T est Set


3



set (Figure 2) is a balance bridge bridge type of circuit. The Balance Bridge Circuitry − The circuit of a TTR test set high-voltage winding of the test set’s reference transformer and the high-voltage winding of the transformer under test are the two arms arms of the balance bridge. When both of these high-voltage windings are generating generating the same voltage, the null detector will h ave a center-of-scale deflection. Variable ariable Ratio ReferenceTransforme former − 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 adjusted by changing the three decade switches an d the potentiometer. Synchronous Synchronous Detector − The null detector is connected to the b ridge circuit through a synchronous rectifier. These two components constitute a synchronous detector. This synchronous detector responds only to current that is the same frequency as the test set’s hand-cranked generator. The synchronous detector is not sensitive to noise signals that might be induced in the test set’s leads by the strong magnetic and electric fields that exist in power substations. substa tions.

Operational Principles The operation of a TTR test set is based on the principle of voltage ratio being equal to turns ratio in transformers and the principles of a balance bridge measurement. Equivalenceof Voltag Voltage Ratio and Turns Turns Ratio Ratio − The ratio of winding voltages of any two windings installed on 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: V1 N1

•

•

V2

=

N2

where V1 is the open-circuit voltage of the high-voltage winding, V 2 is the open-circuit voltage of the low-voltage winding, N 1 is the number of turns in the high-voltage winding, and N 2 is the number of turns in the low-voltage winding.

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


4



Balance BalanceBridge Bridge− By changing the decad e switches and the potentiometer until the null detector indicates null balance, balance , the test s et’s reference refer ence transformer tran sformer is made to have the same sam e voltage voltag e ratio as the transformer trans former under u nder test. tes t.

Turns Ratio Tests: Purposes and Basic Techniques Purposes The purposes of turns ratio tests are to verify conformance of a power transformer to a purchase specification and to troubleshoot faults in transformer windings. Verify ify Conformance Conformanceto PurchaseSpecificati cification on − Saudi Aramco purchase specifications for a power transformer or a distribution transformer include a specification that the measured turns ratio of all windings be equal 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 conformance to this purchase specification. Troubleshoot Faults − If a transformer is suspected to have a winding fault, a turns ratio test is usually conducted to confirm that a fault exists and to distinguish whether the fault is a turn-to-turn fault or an op encircuit fault. Note: A tr ansformer ansformer tur ns r atio test test is not used used alone alone to determine determine the suitabi li ty of a tr ansfor ansfor mer mer for cont in ued ser ser vice. Other t ests ests ar e conducted such as a visual in specti specti on, an oi l di electri c test, test, an insulat io n power power -factor test, test, an i nsulati nsulati on-r esistance esistance tes test, a combustibl combustibl e gas-i gas-i n-oil test, test, an excitation curr ent ent test, test, or a terminal -to-terminal resistance resistance tes test.

Basic Technique The basic technique of conducting a turns ratio test is to measure a ratio for every set of windings and for every winding tap selection. The turns ratio test set leads are connected to the the transformer winding terminals in a configuration that will excite the low-voltage winding of the transformer. Note: Note: Although a terti terti ary winding might have a lower vol tage r ating than a low- voltage winding, it i s pr efer efer able to excite excite the low- voltage winding duri ng a turns r atio test. test.


5



Figure 3. A Nam Nameplate Connection Diag Diagram for a Power Transforme former Exampl Examplee A: Answer:

Exampl Examplee B:

Answer:

How many many measu measurem rement entss of turn turnss ratio ratio shoul should d be made made on a thre three-p e-phas hasee trans transfor former mer that that has has the nameplate connection diagram shown in Figure 3? The connection diagram of Figure 3 shows three sets of windings. The de-energized tap switch has five positions. For each set of windings, five five measurements of ratio should be made, one measurement for each tap-switch position. A total of fifteen measurements of turns ratio should be made. To which which windin winding g termin terminals als of of the the trans transfor former mer repr represe esente nted d in Figu Figure re 3 shoul should d the test test set’s set’s anvil clamps and alligator clamps be connected to measure the turns ratio of the ph ase-A winding? In order to excite the low-voltage winding, the anvil clamps should be connected to termina ls X0 and X 1. The alligator clamps should be connected to terminals H3 and H1.


6



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

Shorted Turns A turn-to-turn short-circuit short-circuit fault in a transformer winding winding will cause a change in the measured turns ratio. For this kind of fault, the measured ratio might be either greater than o r less than the ratio measured at the time of commissioning. In some cases, a turn-to-turn short-circuit fault can be detected by a m ore than usual current indicated on the TTR test set’s excitation excitation current meter. This meter is not marked in a standard engineering quantity such as amperes. For this reason, the most accurate evaluation evaluation of more-than-normal excitation current is to compare the excitation current indication of the transformer having a suspected fault with the indication from a similar model of transformer that is known to be in good condition. A turn-to-turn fault additionally additionally might require an extra turning force on the hand-cranked generator.

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

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

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


7



Incorrect Winding Polarity If the transformer’s winding polarity is not correct due to an incorrect internal connection or an incorrect terminal marking, the null detector will deflect to the right when the decade switches are set to 0.000. Note: The tes test set’ set’ s red-color ed alli gator clamp must must be connecte connected d to the high- voltage windi ng terminal that has the same instantaneous instantaneous voltage voltage polar ity as the low-voltage windi ng terminal to which the r ed-color ed-color ed anvil anvil clamp i s connected. Exampl Examplee C: To which which windin winding g termina terminall of the the trans transfor former mer repr represe esente nted d in Figur Figuree 3 should should the the test test set’s set’s redredcolored alligator clamp be connected if the red-colored anvil clamp is co nnected to terminal X 1? Because the H 1 terminal has the same instantaneous po larity as the X 1 terminal, the redAnswer: colored alligator clamp should connected to the H 1 terminal.

Magnetic Core Damage The indications of magnetic core damage are a large magnitude of excitation current, and a measured ratio that is different than the the ratio measured during the transformer’s transformer’s commissioning test by more than than 0.5%. The indications of magnetic core damage are o ften difficult to distinguish from the indications of an incorrect number of turns. If core damage is suspected, visual inspections of the magnetic core and an insulation insulation resistance test of the core-to-ground insulation should be recommended.


8



EVALUATING INSTRUMENT TRANSFORMER RATIO AND EXCITATION CURRENT TESTS Because many instrument transformers have a large primary impedance voltage drop at 8 volts excitation voltage, a transformer turns ratio test set cannot be used to make consistently accurate measurements of turns ratio of instrument transformers. The alternative methods of measuring the turns ratio of an instrument transformer are explained under this subject heading. Also explained are the excitation current tests tests that are performed perform ed on instrument instrum ent transformers trans formers..

Ratio and Excitation Current Testing of Instrument Instrument Transformers: Principles and Techniques Principles The principles of ratio tests and excitation current tests can be explained using an equivalent circuit diagram, a phasor phaso r diagram diagra m depicting depic ting associated ass ociated circuit currents c urrents and voltages, vo ltages, and a nd a descriptio d escription n of the th e ratio co rrection factor. Equivalent Circuit − Figure 4 is an equivalent circuit diagram of an instrument transformer. One winding of the instrument transformer transformer is excited by a sine-wave sine-wave voltage, V1. The other winding is open circuit. circuit. The transformer symbol represents an ideal transformer that has no winding impedance, has a turns ratio that is exactly equal to N 1/N2, and has a v oltage ratio E 1/V2 that is exactly equal to its turns ratio.

Fig Figure 4. Equivalent Cir Circuit cuit of an Instrument Transfo Transformer


9



Excitation Currents − In the equivalent circuit of Figure 4, a susceptance element B and a conductance element G account for the excitation current, I. I. This excitation current is delayed in phase from the excitation excitation voltage V 1. The amount of this phase delay is a non-linear non-linear function of the magnitude of the excitation excitation voltage. Additionally, the magnitude of the excitation current is a non-linear function of the excitation voltage as shown in Figure 5.

Fig Figure 5. Excitation VoltageVersus Excitation Curre urrent Curve Curves The equivalent circuit has elements R and X that represent the resistance and inductance of the instrument transformer’s primary winding. The flow of excitation current through these R and X elements causes voltage drops proportional to IR and IX. Figure 6 is an open phasor diagram that shows the voltages in the primarycircuit loop and the open-circuit open-circuit secondary voltage. This phasor diagram demonstrates that the open-circuit secondary voltage V 2 will not be exactly equal to V 1 times the turns ratio N 1/N2, and it also implies that the relative phase delay between V 2 and V 1 will not be exactly 180 degrees.

Fig Figure 6. Phasor Diag Diagramof Tr Transformer Open-Cir n-Circuit cuit Voltag Voltages


10



Ratio Corr Correction Factors Factors − The ratio correction factor is the true ratio of an instrument transformer divided by its nameplate ratio. NT FR = • NN •

where FR is the ratio correction factor, N T is the true ratio, and N N is the nameplate ratio.

Manufacturers publish ratio correction factor curves and phase-angle error curves for each different model of current transformer or voltage transformer. transformer. Figure 7 is an example set set of curves. The principle use of a ratio correction factor and a value of phase-angle error is to correct the readings of a kilowatt-hour meter to which the current transformer and potential transformer transformer are connected. Correcting a kilowatt-hour reading reading produces a value 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 instrument transformer tests. This use of correction factors is explained in the the procedure in Work Aid 2.

Fig Figure 7. Ratio Corre orrection Factor and Phase-Ang -Angle Err Error Curve Curves s


11



Technique for Performing CT Tests The commissioning tests and periodic maintenance tests of a current transformer (CT) include measurements of winding resistance, of turns ratio, and excitation current. These measurements are explained in the next four subject headings. Note: M any of the tests tests conducted on curr ent tr ansformer s pr oduce hig h voltage. They should only be perfor med med by expe experr ienced ienced personne personnell who ar e famil famil iar with t he par par ticul ar hazar hazar ds r elated elated to curr ent- ent- tr ansfor ansfor mer mer testing. testing. The test results of current transformer ratio and excitation tests are influenced by the existence of residual magnetism in the magnetic core of the current transformer. transformer. If alternating current in in a CT winding is interrupted abruptly for any reason, or if a CT winding is excited with direct current, then a magnetism will remain in the CT magnetic core. When the CT is subsequently tested, the residual magnetism will cause a larger than normal excitation current. Additionally, an accurate measurement of turns ratio might not be realized. Residual magnetism is eliminated before conducting tests by applying, using a variable-voltage source, a sine-wave alternating voltage to the secondary terminals of a CT. The initial voltage is made large enough to saturate the CT’s magnetic core. Voltage is then gradually reduced to zero. resistance) of the secondary winding of a Winding WindingRe Resistance − The winding resistance (terminal-to-terminal resistance) current transformer is measured using a Kelvin Bridge or a digital digital low-resistance ohmmeter. The ohmic value of terminal-to-terminal resistance resistance and the estimated winding temperature are recorded during commissioning to establish base data. Temperature-corrected values of winding winding resistance are measured during periodic maintenance and are subsequently co mpared 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 circuit for measuring the voltage ratio ratio of a CT. A variable autotransformer is used to excite excite the secondary winding with an adjustable adjustable alternating voltage. The magnitude of this applied voltage should be small enough to avoid saturation of the CT’s magnetic core. High-impedance voltmeters are connected in the circuit for measuring secondary and primary voltages. An ammeter is connected in the secondary circuit to detect excessive excitation current (no mo re than 0.5 ampere).


12



Fig Figure 8. Circuit ircuit for a Ratio-by-Voltag Ratio-by-VoltageTest of a Curr Curre ent Trans Transformer The turns ratio is calculated using the following formula: V NT = P • VS •

Where NT is the turns ratio, V P is the measured primary voltage, and V S is the measured secondary voltage.

The evaluation of this calculated turns ratio is explained in the procedure of Work Aid 2. Note: If t he CT to be tested tested is a wi ndow-t ype CT, a shor shor t conductor can be pl aced withi n the geometri geometri c center center of the window to se serr ve the same same functi on as the pri mar y windi ng. The vol vol tage V can be measured me asured fr om one P end of the conductor to i ts other other end. The exact exact length of the conductor and the exact exact connection poi nts of the voltmeter voltmeter ’ s leads leads are not cri tical factors for making making an accurate meas measureme urement. nt. Ratio-by-Current − The turns ratio of a current transformer can be measured by connecting its primary winding in series with the primary winding of another CT of known ratio (called a reference CT) and injecting current into this series circuit circuit with a primary-current injection injection test set. As shown in Figure 9, one ammeter is connected into the secondary windings circuit of the reference CT, and another ammeter is connected into the secondary winding circuit of the CT under test. The magnitude of current that is injected should be as close to the rated primary amperes as the capacity of the test set will allow.


13



Fig Figure 9. Circuit ircuit for for a Ratio-by-Curr Ratio-by-Curre ent Test of a Curre urrent Transformer The ratio of the CT under test is calculated using the formula: IR N T = NR • IT •

where NT is the ratio of the CT under test, N R is the ratio of the reference CT, I R is the magnitude of current measured in the secondary circuit of the reference CT, and I T is the magnitude of current measured in the secondary circuit of the CT under test.

Note: Ther Ther e are sev several eral di sadvantages advantages to meas measuri uri ng turns rati o by curr ent. ent. The pr imar y-curr ent tes test set set neede needed d to perfor m this meas measur ur emen ementt i s large and heavy. heavy. A pr imar y-curr ent ent i njection test test set set wi ll inj ect ect a non- sinusoidal cur r ent in some some cir cumstances cumstances.. There is a possib possib le hazard of a back-feed back-feed voltage being ind uced i n other CT s that ar e i nstall ed on the same same bus. Addi ti onal ly, special special car e must must be taken taken to extend extend the test test cir cuit conductor s as far as possib possib le alo ng the axis of a CT to mini mize the infl uence of stray magnetic fl ux.


14



Excitation Test − A turn-to-turn short-circuit fault in the secondary winding of a CT or physical damage in the CT magnetic core is detected by conducting an excitation test. This test, sometimes called a magnetization curve test, is performed by measuring values o f excitation current that flow in the secondary winding of a CT at various values of excitation voltage. Figure 10 is a diagram of a test circuit that can be used to perform an excitation test. Depending on the ANSI/IEEE C57.13 standard voltage rating (10, 20, 50, 100, 200, 400, or 800) of the CT, the magnitude of voltage needed to conduct the test might be as little as 10 volts or as great as 800 volts. The evaluation of a CT excitation excitation test is explained in in the procedure of Work Aid 2.

Fig Figure 10. Circuit ircuit for an Excitation Test of a Curre urrent Trans Transformer Polarity − Figure 11 shows two circuit diagrams that represent two acceptable techniques for conducting a polarity test of a CT. In the circuit c ircuit of Figure 11a, 1 1a, channel cha nnel 1 of a dual du al trace oscillosc o scilloscope ope displays dis plays the th e waveform wave form of the alternating voltage voltage that excites the secondary winding winding of the CT under test. Channel 2 of the oscilloscope displays the waveform of the voltage voltage induced in the primary winding of the CT. Connected with the polarities shown, both waves should appear on the d isplay of the oscilloscope as being in phase. Note: Becaus Because e all instrument transformers have a subtractive polar ity, the i nstantane nstantaneous ous voltage voltage polar ity of their H 1 and X ter minal s shoul shoul d be the same. same. 1


15



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

Fig Figure 11. Circuit ircuit Diagrams for Polarity Polarity Tests of CTs


16



Technique for Performing VT Tests The commissioning tests and periodic maintenance tests of a voltage transformer VT include measurements of values of winding resistance, a measurement of turns ratio, and an excitation current test. These measurements and tests are explained in the next three subject headings. Note: M any of the tests tests conducted on voltage transfor mer mer s pr oduce hig h voltage. They should only be perfor med med by expe experr ienced ienced personne personnell who ar e famil famil iar with t he par par ticul ar hazards related to voltage- tr ansfor ansfor mer mer testing. testing. (terminal-to-terminal resistance) of the primary Winding WindingRe Resistance − The values of winding resistance (terminal-to-terminal winding and the secondary winding of a voltage transformer are m easured using a Kelvin Bridge, digital lowresistance ohmmeter, or a digital multimeter. These resistance values are corrected to a standard temperature and are recorded during commissioning to establish establish base data. Temperature-corrected values of winding resistance 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 of winding voltages. Figure 12 is a schematic schematic diagram of a test circuit for measuring the voltage ratio of a VT. A single-phase 120 volt source is used to excite excite the high-voltage winding. High-impedance voltmeters are connected in the circuit for measuring the terminal-to-terminal voltages of the high-voltage winding and the low-voltage winding. Note: The high-vol tage winding i s excited excited with low vol tage (120 volts) volts) for safety r easons easons.. Exciti ng the low-voltage windi ng with 120 vol ts would would pr oduce a voltage of hazar hazar dous magnitude for most most models of vol tage tran sformers.


17



Figure 12. Circuit ircuit Diag Diagram for the Ratio Te Test of a VoltageTransforme former Excitation Curr Curre ent − The excitation current of a voltage transformer is not usually m easured during commissioning or during routine maintenance. Excitation current is measured if a fault is suspected to exist in in a voltage transformer. Open-circuit excitation current should be no more than 2% of the rated current of the winding 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 polarity marks. These terminals terminals are also marked H 1 and X 1. A low-voltage AC source source excites the high-voltage winding of the VT. The voltage, V 1, that is measured between VT’s two high-voltage terminals should be less than the voltage, V 2, that is measured between the two terminals that do not have polarity marks.


18



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

Evaluation Factors Accuracy Class ANSI/IEEE standard C57.13-1976 describes the classification classification system for instrument transformers that are used in metering service. Accuracy classes are 0.3, 0.6, or 1.2. The accuracy class appears on the the nameplate of the instrument transformer and represents the maximum percentage difference between actual ratio and nameplate ratio at rated voltage and rated current. Current transformers that are used in protective relaying service have an extra system of accuracy classification. 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 on ratio (window-type CTs or bar-type CTs). CTs). The T classification is for for CTs whose leakage flux does have an appreciable effect on ratio (wound-primary CTs). CTs). The number represents the maximum voltage that will exist at the secondary terminals of the CT while it is delivering 20 times its rated current and not exceeding a 10 percent error in ratio. ra tio.


19



Ratio Error Figure 14 is a table of values that represent the allowable limits of the ratio correction factor for an instrument transformer of accuracy class 0.3, 0.6, or 1.2. Voltage Transformers At 90 to 110 Percent Rated Voltage Metering Accuracy C la s s 0 .3 0 .6 1 .2

M inim u m 0 .9 9 7 0 .9 9 4 0 .9 8 8

Maximum 1 .00 3 1 .00 6 1 .01 2

Current Transformers At 100 Percent Rated Current

Minimum 0 .9 9 7 0 .9 9 4 0 .9 8 8

M a xim um 1 .0 03 1 .0 06 1 .0 12

At 10 Percent Rated Current

M inim um 0 .9 9 4 0 .9 8 8 0 .9 7 6

M aximum 1 .0 0 6 1 .0 1 2 1 .0 2 4

Fig Figure 14. Tableof Value Values of Ratio Ratio Corr Corre ection Factors

Phase Angle Error The allowable phase angle error of a metering service instrument transformer is related to its ratio correction factor. Figure 15 is a parallelogram that represents the limits of phase angle error for a metering service current transformer. Figure 16 is a parallelogram that represents represents the limits of phase angle error error for a metering service voltage transformer. Phase angle error is not measured during commissioning tests or routine routine maintenance tests. Phase angle error is measured if a fault is suspected suspected to exist in a metering service instrument instrument transformer.


20



Fig Figure 15. L imits of Accuracy for for CT C Ts in Me Metering Service

Fig Figure 16. L imits of Accuracy for for VT VTs in Me Metering Service


21



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

Fig igure1 e17. ExcitationVoltagevs.MagnetizationCurre rentCurvef eforaCT


22



WORK AID 1:

RESOURCES USED TO EVALUATE A TURNS RATIO TEST

Use the Work Ai ds and and the procedure descri descri bed below below to evaluate the tur ns rati o test test of a distri bution transformer transformer or a power power transforme transformer. r.

Work Aid 1A:

Non-Mandatory Test Report P-025, Oil-Filled Transformers (Handout 13)

For the contents of Test Report Form P-025, refer to Handout 13. Note: Handout 13 was also used used in Wor k Aid 3 of Modul e 3.

Work Aid 1B:

ANSI/IEEE Standard C57

Applicable excerpts from ANSI/IEEE Standard Standard C57 are given below. Standard C57.12.01-1979 relates to drytype transformers. Standard C57.12.00-1980 relates to liquid-immersed liquid-immersed transformers. 9.1 Ratio. With rated voltage impressed on one winding of a transformer, all other rated voltages at no load shall be correct within 0.5% of the nameplate markings. Rated tap voltages shall correspond to the voltage of the nearest turn if the voltage per turn exceeds 0.5% of the desired voltage.

Fig Figure 18. Excerpt from fromANSI/I NSI/IE EEE Standard Standard C57.12.01-1979

9.1Tolerance for Ratio. With the transformer at no load and with rated voltage impressed on one winding of a transformer, all other rated voltages at no load shall be co rrect within 0.5% of the nameplate markings, except in cases where the rated tap voltage corresponds to the voltage of the nearest turn but still exceeds exceed s 0.5% of the desired d esired voltage because becaus e the volts v olts per turn of the unit un it exceeds excee ds this tolerance.

Fig Figure 19. Excerpt from fromANSI/I NSI/IE EEE Standard Standard C57.12.00-1980


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Work Aid 1C: 1. 2.

3. •

Applicable Procedural Steps

Select Select one record recorded ed valu valuee of turns turns rati ratio o that that repres represent entss a sing single le tap switch switch select selection ion of a single single set of windings. Note: This evaluati evaluati on procedure is r epeate epeated d for each recor recor ded value of turns r atio. Determ Determine ine from from the the tran transfo sforme rmer’s r’s namepl nameplate ate data data the the rate rated d termi terminal nal-to -to-t -term ermina inall volt voltage age of the the lowlowvoltage winding circuit. Note: If there is an on-load tap changer changer install ed on the tr ansfor ansfor mer mer t hat was tested, tested, the r ecorded ecorded value of tur ns ratio wil l corr espond espond to one of 33 different values of r ated ter ter minal- to-terminal voltage. Consult Consult the tap changer changer ’ s name nameplate plate to determine determine the r ated ter ter minal- to-terminal voltage that corr esponds esponds to the parti cular on-load t ap selec selection. tion. Calculate V 1, the rated voltage of a low-voltage winding. If the transformer tested is a single-phase transformer, V 1 is equal to the rated terminal-to-terminal voltage determined in step 2.

•

If the low-voltage winding is connected in a delta circuit, V 1 is equal to the rated terminal-to-terminal voltage determined in step 2.

•

If the low-voltage winding is connected in a wye circuit, V 1 is equal to the rated terminal-to-terminal voltage determined in step 2 divided by 1.732.

4.

Determ Determine ine from from the the tran transfo sforme rmer’s r’s namepla nameplate te data data the the rated rated termin terminalal-toto-ter termin minal al voltag voltagee of the other other winding circuit whose turns ratio was measured with respect to the low-voltage winding (usually the high-voltage winding, but will sometimes be the tertiary winding, or the fourth winding). Note: If there is a de-ene de-enerr gized gized tap changer changer , a l ink boar d, or a tap j umper umper ass associated with thi s winding, the r ecorded ecorded value of turns rati o wil l cor r espond espond to one of the five differ ent ent values of r ated termi termi nal-t o- terminal voltage. Consult Consult the transformer’ s nameplate nameplate to determine determine the r ated terminal -to-termi nal voltage that cor r esponds esponds to the parti cular de-energize de-energized d tap selection. selection.


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5.

Calculate V 2, the rated voltage of the o ther winding. If the transformer tested is a single-phase transformer, V 2 is equal to the rated terminal-to-terminal

•

voltage determined in step 4. If the other winding is connected in a delta circuit, V 2 is equal to the rated terminal-to-terminal voltage

•

determined in step 4. If the other winding is connected in a wye circuit, V 2 is equal to the rated terminal-to-terminal voltage

•

determined in step 4 divided by 1.732. 6.

Calc Calcul ulat atee the the rat ratio of of rat rated ed wind windiing volt voltag ages es:: V2 NV = V1

•

Where NV is the ratio of rated winding voltages, V 1 is the rated voltage of the low-voltage winding, and

•

V2 is the rated voltage of the other winding. 7. 8. •

•

D et etermin e the m ea easured turns ratio N T from the test data . Calcul Calculate ate the percen percentag tagee devia deviatio tion n of the measure measured d turns turns ratio ratio from from the ratio ratio of of rated rated windin winding g volt voltage ages: s: NT − N V × 100% %dev = NV Where %dev is the percentage deviation, N T is the measured turns ratio, and N V is the ratio of rated winding voltages.

9.

10.

For a commi commissi ssioni oning ng insp inspect ection ion,, compar comparee the the perce percent nt devi deviati ation on (%de (%dev) v) to to the the rati ratio o toler toleranc ancee speci specifie fied d in ANSI/IEEE standard C57 (Work Aid 1B). If the percent deviation is greater greater than the tolerance, a report of non-conformance should be made to the office of the Saudi Aramco Chief Engineer. For a maint maintena enance nce test test or or for for cases cases wher wheree windi winding ng damage damage is suspec suspected ted,, compa compare re N T to the original turns ratio measured at the time time of commissioning. A change of more than 0.5% in the value of N T indicates the need for additional electrical tests and mechanical inspections.


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WORK AID 2: RESOURCES USED TO EVALUATE INSTRUMENT TRANSFORMER RATIO AND EXCITATION CURRENT TESTS Use the Work Ai ds and and the procedure des descri bed below below to evaluate the voltage rati o test test of a CT or VT, or to evaluate evaluate the excitati excitati on cur r ent ent (magnetization (magnetization curr ent) test test of a CT.

Work Aid 2A:

Non-Mandatory Test Report P-014, Instrument Transformers (Handout 17)

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

Work Aid 2B:

ANSI/IEEE Standard C57

Applicable excerpts from ANSI/IEEE Standard C57 are given below. Voltage Transformers At 90 to 110 Percent Rated Voltage Metering Accuracy C la s s 0 .3 0 .6 1 .2

M inim u m 0 .9 9 7 0 .9 9 4 0 .9 8 8

Maximum 1 .00 3 1 .00 6 1 .01 2

Current Transformers At 100 Percent Rated Current

Minimum 0 .9 9 7 0 .9 9 4 0 .9 8 8

M a xim um 1 .0 03 1 .0 06 1 .0 12

At 10 Percent Rated Current

M inim um 0 .9 9 4 0 .9 8 8 0 .9 7 6

M aximum 1 .0 0 6 1 .0 1 2 1 .0 2 4

Fig Figure 20 Exce E xcerpt from fromANSI/E NSI/EE EE C57 C 57.13-1978, Ratio Corre orrection Factors


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Work Aid 2C:

Manufacturer’s Literature

Applicable excerpts from manufacturer’s literature are given below.

Fig Figure 21. Excerpt from Instrument Transfo Transformer Manufacturer’s Lite Literature


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Work Aid 2D: 1.

•

•

Applicable Procedural Steps

If the the test test data data repre represen sents ts a ratioratio-byby-vol voltag tagee test test of of a curr current ent transf transform ormer er (CT) (CT),, deter determin minee from from the the test test data the measured secondary ex citation voltage V 2 and the measured primary voltage V 1. Using the following formula, calculate the turns ratio w ith the following formula: V2 NT = V1 Where NT is the turns ratio, V 1 is the measured primary voltage, and V 2 is the measured secondary excitation voltage.

2.

•

If the the test test data data repre represen sents ts a ratioratio-byby-cur curren rentt test test of of a CT, CT, dete determi rmine ne from from test test data data the measu measured red primar primary y current I1, and the measured secondary current I 2. Calculate the turns ratio using the following formula: I2 NT = I1

•

Where NT is the turns ratio, I 1 is the measured primary current, and I 2 is the measured secondary current.

3.

If the the tes testt data data rep repre rese sent ntss a rati ratio o test test of of a CT, CT, calc calcul ulat atee the the name namepl plat atee turn turnss rati ratio. o. IS NN = IP

•

•

Where N N is the nameplate turns ratio, I P is the rated primary current and I S is the rated secondary current.

4.

If the test test data data repres represent entss a ratio ratio test test of of a CT, calcul calculate ate the rat ratio io correc correcti tion on fact factor: or: NT

•

FR =

•

Where FR is the ratio correction factor, N T is the true ratio, and N N is the nameplate ratio.

NN


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5.

6. 7.

8.

If the CT test ested is a met meter eriing-c ng-cllass ass CT, CT, comp compar aree F R with the maximum and minimum allowable ratio correction factors factors from Work Aid Aid 2B. If F R is outside of the minimum or maximum limit for the CT’s accuracy class, the CT is not suitable for continued service. Note: Wheneve Wheneverr F i s calcul calcul ated using using R the result result of a r atio-by-vol tage tes test, evaluate evaluate the r atio test test by t he minimum and maximum li mits that appear appear i n the “ 100% rated curr ent” ent” column of the table in Work Aid 2B. If the CT is a re relayi laying ng-c -cllass ass CT CT, the the maxi maximu mum m F R is 1.012 and the minimum F R is 0.988. If FR is outside of the minimum or maximum limit, the CT is not suitable for continued service. If the the test test data data repr represe esents nts an an excita excitati tion on curre current nt test test,, plot plot the the excit excitati ation on curre current nt versu versuss excita excitati tion on volta voltage ge data points on a cop y of the manufacturer’s excitation curve. Note: Work Aid 2C i s an examp example le of a manufacturer manufacturer ’ s excitati excitati on curve. Eval Evalua uate te all all plo plott tted ed data data poin points ts acco accord rdin ing g to to the the foll follow owin ing g cri crite teri ria: a:

•

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 be less than 95% of the v alue of the manufacturer’s curve.

9.

•

•

If more than one data plot fails to meet these criteria, the CT is not suitable for contin ued service. Note: Work Aid 2C i s an exam example ple of a manufacturer’ manufacturer’ s excitati excitati on curr ent ent curve. If the the test test data data repre represen sentt a ratio ratio test test of a voltag voltagee trans transfor former mer (VT), (VT), determ determine ine from from test test data data the the measured secondary excitation voltage V 2 and the measured primary voltage V 1. Using the the following following formula, calculate the turns ratio with the following formula: V1 NT = V2 Where NT is the turns ratio, V 1 is the measured primary voltage, and V 2 is the measured secondary excitation voltage.


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10. •

•

If the the test test data data repre represent sentss a rati ratio o test test of of a VT, VT, calc calcula ulate te the the namep nameplat latee turns turns ratio. ratio. V NN = P VS Where N N is the nameplate turns ratio, V P is the rated primary voltage, and V S is the rated secondary voltage.

11.

If the the test test data data repre represen sents ts a rati ratio o test test of a VT, calcul calculate ate the rati ratio o correc correcti tion on facto factor: r: NT

•

FR =

•

Where FR is the ratio correction factor, N T is the true ratio, and N N is the nameplate ratio.

12.

NN

Compare the F R of the VT with the maximum and minimum allowable ratio correction factors of Work Aid 2B. If F R is outside of the minimum or maximum limit for the VT’s accuracy class, the VT is not suitable for continued service.


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GLOSSARY High-Voltage Winding

The winding of a transformer that has the largest rated terminal-toterminal voltage.

L ow-Voltag -VoltageWinding Winding

The winding of a transformer that has the second largest rated terminalto-terminal voltage.

Phase Angle Er Error

For a voltage transformer, phase angle error is the relative displacement of voltage between the transformer’s high-voltage winding and its lowvoltage winding. For a current transformer, phase angle error is is the relative displacement of phase between the the transformer’s transformer’s primary current and its secondary current.

Primary Winding

The winding of a transformer transformer that normally receives power from from the distribution system.

Ratio Corr Correction Factor Factor

The true ratio of an instrument transformer divided by its nameplate ratio.

Secondary Winding Winding

The winding of a transformer that normally delivers power into the distribution system.

Tertiary Winding

The winding of a transformer that has the third largest rated terminalto-terminal voltage.

TTR Test Set Turns Ratio

A transformer turns ratio test set. 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 lower voltage that is installed on the same seg ment of magnetic core.

VoltageRatio

The ratio of the voltage in a full winding of higher voltage with respect to the voltage of a full winding of lower voltage that is installed on the same segment of magnetic core.


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Evaluation Voltage Excitation Tests

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