Fall 2002 1 P otential transformers (PTs) are necessary to a power system for metering and protective re-laying to convert higher system volt ages to lower control voltages that are practical from an equip- ment, operation, and safety perspective. There are four basic styles. Potential Transformers (PTs) or Voltage Transformers (V Ts) PTs or VTs are the most common devices used. These devices are con- ventional transformers with two or three windings (one primary with one or two secondary). They have an iron core and magnetically couple the primary and secondary. The high side winding is constructed with more copper turns than the secondary(ies), and any voltage impressed on the primary winding is reflected on the second ary windings in direct proportion to the turns ratio or PT ratio. The secondary windings often have two taps with a nominal voltage of 115/67 V, 120/69 V, or 120/208 V . This style o f PT is the most accurate style in use. A typical PT is shown in Figure 1. Control Potential Transformers (CPTs) CPT s are conventional t ransformers with a higher V A rating than VT’s and are used to supply control power for circuit breakers, motor starters, and other control equipment. These transformers are not designed for accuracy and should not be used in revenue metering or primary protec- tion systems. Special attention should be paid to following: • The equipment ratings con- nected to the secondary should be verified to ensure the combi- nation of devices does not ex- ceed the VA rating of the trans- former. • The l ocati on of th e pri mary con- nection is also important as a CPT connected to the load side of the circuit breaker or motor starter will prevent the devices connected to the secondary from operating until after the breaker or starter has been closed. • All CPTs that suppl y trip power should be supplemented with an external device such as a ca- pacitive trip unit to provide at least one trip signal after a power system loss of voltage. Some typica l CPT s are shown in Figure 2. A typical applic ation is shown in Figure 3. Figure 1 by Les WarnerValence Engineering Technologies Ltd. Testing o fPotential Transformers
P otential transformers (PTs) are necessary to a power system formetering and protective re-laying to convert higher system voltages to lower control voltages that are practical from an equip-
ment, operation, and safety perspective. There are four basic styles.Potential Transformers (PTs) or Voltage Transformers (VTs)
PTs or VTs are the most common devices used. These devices are con-ventional transformers with two or three windings (one primary withone or two secondary). They have an iron core and magnetically couplethe primary and secondary. The high side winding is constructed withmore copper turns than the secondary(ies), and any voltage impressedon the primary winding is reflected on the secondary windings in directproportion to the turns ratio or PT ratio. The secondary windings oftenhave two taps with a nominal voltage of 115/67 V, 120/69 V, or 120/208V. This style of PT is the most accurate style in use. A typical PT is shownin Figure 1.
Control Potential Transformers (CPTs)CPTs are conventional transformers with a higher VA rating than VT’s
and are used to supply control power for circuit breakers, motor starters,and other control equipment. These transformers are not designed foraccuracy and should not be used in revenue metering or primary protec-tion systems. Special attention should be paid to following:
• The equipment ratings con-nected to the secondary should be verified to ensure the combi-nation of devices does not ex-ceed the VA rating of the trans-former.
• The location of the primary con-nection is also important as aCPT connected to the load sideof the circuit breaker or motorstarter will prevent the devicesconnected to the secondary fromoperating until after the breakeror starter has been closed.
• All CPTs that supply trip powershould be supplemented withan external device such as a ca-pacitive trip unit to provide atleast one trip signal after apower system loss of voltage.Some typical CPTs are shown inFigure 2. A typical application isshown in Figure 3.
Figure 1
by Les Warner Valence EngineeringTechnologies Ltd.
T estingof Potential Transformers
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Figure 2
Capacitive Voltage Transformers (CVTs)CVTs are often used in high voltage (115 kV and
higher) applications and use a series of capacitors asa voltage divider. The capacitors are connected be-tween the system voltage and the polarity terminal of a PT. As the PT secondary is normally groundedthrough an internal jumper, an external switch is of-ten used to isolate the PT from ground during test-ing. The correct switch position must be determined before energizing the CVT to ensure correct opera-tion. An internal schematic diagram of a CVT is shownin Figure 4. CVTs are less expensive and are often usedin place of high voltage PTs to reduce capital costs.
Current/Voltage TransformersIn order to reduce costs and save valuable real es-
tate inside a substation, combined instrument trans-
formers are being installed more and more frequently.These devices combine a current transformer andpotential transformer in one device. The combined in-strument transformers should be tested as two sepa-rate components.
Basic Guide for PT TestingFollowing is a basic guideline for PT testing using
the most basic of equipment, arranged in accordancewith NETA standards for easy reference.
1. Visual and Mechanical InspectionsCompare equipment nameplate data with drawingsand specifications.• Every PT test form should include the serial num-
ber, model number, ratios and accuracy class.• The serial number is important for PT identifica-
tion and comparisons between your results and themanufacturer’s.
• The model number is important for comparison of test results to the manufacturer’s specifications andfor ordering replacement or spare PTs or parts.
• The PT ratio is the most important piece of infor-mation and must be recorded from the nameplateor the design criteria. The ratio determines the PToperating characteristics. If the PT has multiple taps(different possible ratio combinations), all tapsshould be recorded for future reference in case anew PT ratio is required for the application.
• The accuracy class indicates the PT’s performancecharacteristics. A PT’s accuracy is dependant on thenumber of devices connected to the secondary ter-minals. PT load is also called the burden and is usu-ally defined in volt-amperes and power factor at120 secondary volts. Standard burden designationsare shown in Table 1. Classifications for accuracyhave also been designated as shown in Table 2. It ispossible that one PT can be rated for different ac-curacies at different burdens. For example a PT can be rated 0.3W, 0.6Y and 1.2ZZ. However, if the ac-tual burden or power factor falls outside the guide-lines in Tables 1 and 2, the PT’s accuracy is not guar-anteed.
and that the ground connection can not be easilyremoved while the PT is in service (e.g., groundingpoint made on the line side of test switches).
• Ensure, if possible, that multiple grounds do notoccur, particularly when synchronizing systems areenabled.
Verify correct operation of transformer withdrawalmechanism and grounding operation.• When the PT is withdrawn, ensure that there is no
longer any connection with the primary electricalsystem. It is a good idea to remove all fuses as asafety precaution to prevent back-feeding danger-ous voltages into the system during testing. Thefuses should not be replaced until all PT and con-trol system testing is completed and the PTenergization is imminent.
• Use an ohmmeter or contact resistance test set tomeasure the resistance between the PT primary andground when in the fully withdrawn position toensure the PT grounding device is operational.
Verify correct primary and secondary fuse sizes.• Inspect the fuses and ensure they are sized correctly
for the particular application and meet the designrequirements. Typically, “E” rated current limitingfuses are used to protect PTs.
2. Electrical TestsPerform insulation resistance tests winding-to-windingand each winding-to-ground.To test the insulation integrity you will need amegohmmeter or high-potential test set rated to pro-duce the specified maximum test voltage. The testvoltage should never exceed 1.6 times the PT ratingunless authorized by the PT manufacturer. Use thefollowing test procedure for PT insulation tests:a. Isolate all windings from ground. b. Install a jumper across each full winding.
Based on Tables 1 and 2, our example PT (0.3W, 0.6Yand 1.2ZZ) will have the following operating charac-teristics:• 0.3W indicates the PT will operate with an accu-
racy between 99.7-100.3 percent if:1. The operating system power factor is greater than
0.6.
2. The secondary connected devices do not exceed12.5 VA and operate at a power factor greater than0.7.
• 0.6Y indicates the PT will operate with an accuracy between 99.4-100.6 percent if:1. The operating power factor is greater than 0.6.2. The connected devices do not exceed 25 VA and
operate at a power factor greater than 0.85.• 1.2ZZ means the PT will operate with an accuracy
between 98.8-101.2 percent if:1. The operating power factor is greater than 0.6.2. The connected devices do not exceed 25 VA and
operate at a power factor greater than 0.85.
Inspect physical and mechanical condition.The PT should be checked for any cracks or other ob-vious damage that may have occurred during ship-ping or installation.
Verify that all grounding connectionsprovide contact.• Use an ohmmeter or contact resistance test set to
measure the resistance between the PT primarygrounding connection and a known ground to en-sure an electrical connection exists. Visually inspectthe connection to ensure it is tight and protectedfrom obvious physical damage.
• Use an ohmmeter or contact resistance test set tomeasure the resistance between the PT secondarygrounding connection and ground to ensure anelectrical connection is installed. Special attentionis required to ensure that the PT secondaries of anyconnected group are grounded at one point only
Table 1 — Standard Burdens for PotentialTransformers
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c. Connect the megohmmeter toone terminal of the H side of thePT and the negative to one sideof the X side and to ground.
d. Increase the voltage slowly tothe test voltage and let stand forone minute. Monitor the insula-tion resistance during the test toensure that the reading is not lin-ear and does not increase expo-nentially as the test voltage in-creases. Either condition couldindicate insulation failure, poorinsulation, and/or improper testconnections.
e. Record the insulation resistance,temperature, humidity, andequipment designation.
f. Use Table 3, Insulation Resis-tance Compensation Factors, todetermine the equivalent insu-lation resistance at 20 ° C.
g. Check NETA Acceptance TestingSpecificationsTable 10.9 to ensurethe 20 ° C equivalent resistance isnot lower than the specifiedvalue or compare results to pre-vious test results. Ensure previ-ous results have been convertedto equivalent 20 ° C resistances.
h. Repeat steps a through g, ex-changing primary and second-ary connections to include allthe following tests for two- andthree-winding PTs. Use the dia-grams in Figure 5 to help visu-alize the connections.
Perform a polarity test on each transformer to verify the polarity marks or H1-X1 relationship
as applicable.We describe the two polarity test methods availableusing the most basic equipment.
Test polarity using dc voltageTo test polarity using dc voltage, a lantern battery anda voltmeter with an analog scale is required. Use thefollowing steps for dc polarity testing as shown in Fig-ure 6:• Connect the positive lead of the voltmeter to the
marked terminal (H1) of the high-voltage side of the PT and the negative lead to the nonmarked.
• Calculate the expected voltage using the batteryvoltage and the PT ratio (battery voltage x PT ra-tio). If the expected voltage exceeds the meter rat-ing, switch the battery to the primary side of thePT and voltmeter to the secondary side. Recalcu-late the expected voltage and set the voltmeter scaleaccordingly (battery voltage / PT ratio).
• Connect the negative terminal of the battery to thenonpolarity of the PT winding under test. Momen-tarily touch the battery positive terminal to the po-larity terminal of the PT winding under test.
• Closely watch the scale of the voltmeter. It shouldmove in the positive direction. This happens in afraction of a second and the meter must be moni-tored very closely. If the voltmeter kicks in the posi-tive direction the polarity marks are correct, and if it kicks in the negative direction then the polaritymarks are incorrect.
Test polarity using ac voltageTo test polarity using ac voltage, a variable transformerand voltmeter are required. Use the following stepsto test for PT polarity using the ac method as shownin Figure 7:• Connect a variable transformer across the primary
winding of the PT.• Connect a voltmeter (VM1) across the primary PT
winding and variable transformer.• Connect a voltmeter (VM2) from the polarity mark
of the H side to the nonpolarity mark of the X side.• Connect the nonpolarity mark of the H side wind-
ing to the polarity mark of the X side winding.• Increase the voltage to a known value. Calculate
the expected value ([VM1/PT ratio]+[VM1]). If VM2 displays the expected result, the PT polaritymarkings are correct. If VM2 is less than the ex-pected result, the test connection or the PT polaritymarkings are incorrect. (Note: VM1 and VM2 can be one voltmeter switching between positions if thetest voltage remains stable.)
Figure 6Figure 7
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Figure 8
Perform a turns ratio test on all tap positions,if possible.Testing the ratio of a PT is a simple test and only re-quires a variable transformer and a voltmeter. Use thefollowing procedure for PT ratio testing:
• Connect the variable transformer across the primarywinding.
• Increase the voltage to the test voltage (typicallyan easy multiple of the PT ratio, e.g., 35:1 V PT ra-tio = 35 V). Calculate the expected secondary volt-
age (test voltage / PT Ratio).• Measure the secondary voltage and compare to the
expected result.• After the ratio tests have been completed, ensure
that the connection is left as specified.
Note: Never energize the secondary winding andmeasure the primary winding as dangerous voltagescould be created.
All of the above-mentioned tests use the same prin-ciples as the fancy test equipment available today, soshould they ever fail (because that never happens) youcan get your trusty voltmeter and variable transformerand keep on testing.
products?pnlid=5&id=sp-pwrWestinghouse Technical Data Sheet 45-910; Instrument
Transformers Technical Data, Accuracy standards In-dex; December 1945
Northern Alberta Institute of Technology; ElectricalEngineering Technology Program; Principle of Operation of Potential Transformers.
Les Warner graduated from the Electrical Engineering Tech-
nologies Program at the Northern Alberta Institute of Technol-ogy in 2001. Les has been a valuable employee of Valence Engi-neering Technologies Ltd. for the last year in which he has de-voted majority of his expertise to maintenance testing and com-missioning in the cogeneration, oil & gas and production plantenvironments.
