Current Transformer Testing

Outline Current Transformer CT Test Setup Tap, Ratio and Polarity Test Megger Test Excitation Test Testing CT’s in a Circuit Breaker Testing CT’s in a Three Phase Delta-Wye Transformer Digital Current-transformer Testers (EZCT-10)

Current Transformer Transformer is a transformer, no matter whether it is a voltage or current transformer. A Current Transformer circuit must be grounded; however, it should only be grounded in one location. A Current Transformer circuit secondary must never be opened when load is flowing in the Primary. The main difference between a potential transformer and a current transformer is the size of the core, size of the wire, number of turns and the impedance of the transformer.

CT TEST SETUP The size of the wire going to the primary bushings can be small since it will be carrying no current and less than one volt. A voltmeter is used to check tap voltage to verify that taps are brought out correctly to the CT terminal block. The ammeter is used to monitor current as the test-input voltage is increased from 0 to the desired test voltage volts making sure there are no shorted turns in the CT. The current should not exceed 5 amps. The dual voltmeter is used to verify the primary to secondary CT ratio by reading the two voltages at the same time. This is a ratio of voltage that is inversely proportional to the CT current ratio. The phase angle meter is used to check the correct polarity of the CT. The phase angle meter should be connected so that a reading of zero degrees indicates correct CT polarity while a reading of 180 degrees indicates incorrect CT polarity. The exact connection may vary depending on the type of phase angle meter used. The Excitation test can use the same setup.

Tap, Ratio and Polarity Test This test is to verify the correct CT polarity, full CT ratio, and taps. It also verifies taps are correctly brought out to the CT terminal blocks. Voltage applied may be determined by the CT ratio per the next page chart. This test is to verify the correct CT polarity, full CT ratio, and taps. It also verifies taps are correctly brought out to the CT terminal blocks. Voltage applied may be determined by the CT ratio per the following chart. The voltage will be evenly distributed in the CT. Therefore, the voltage at each tap should be in proportion to the CT tap ratio. Higher test input voltages may be used if higher CT primary voltages are desired. Reduce the voltage to zero before switching Off the AC source to prevent magnetizing of the CT core.

Using the test connections of Figure 1, turn On and increase the input voltage applied to the secondary of the CT while monitoring the dual voltmeter and ammeter. Set the applied secondary input voltage and read the CT primary voltage. The phase angle meter should read approx 0 degrees. To check the phase angle meter operation roll either set of leads (A&B or C&D) to see a change to read approx. 180 degrees; this verifies the correct CT polarity. With voltage still applied to the CT secondary, take the voltage readings from X1 to X2, X2 to X3, X3 to X4 and X4 to X5 to verify the taps. Reduce the voltage to zero before turning Off the source.

Megger Test This test is to verify that adequate insulation exists between the CT winding and ground. With one Megger lead connected to ground, touch the other Megger lead to one CT tap. The CT should typically read 75 mega ohms or more. If the readings are not within this limit, then check the wires, terminal blocks, or CTs for moisture, condensation, or anything else that could contribute to the low resistance readings Note that the 75 mega ohms guideline is only applicable for testing individual CT’s, not parallel CT circuits or long cable runs connected to the CT.

Excitation Test This test is to verify that the CT does not have any shorted secondary turns and to compare the measured exciting current against the manufacturer’s CT test curves. The best test method is to determine what voltage is required to approach the saturation point (knee of the curve) of the CT for the full winding and apply a voltage slightly below this value and read the current. Measured currents greater than those shown on the CT curves would indicate trouble within the CT. If the CT curves are not available and the current increases quickly with small increases in voltage there is trouble within the CT. This exciting current is compared to the CT curves for a specific voltage applied and the number of turns to which this voltage is applied. The lowest voltage that we would want to use would be 0.50 volts per turn connected from X1 to X5. You may also use the minimum number of turns calculation as shown below to determine test values. Measured currents greater than those shown on the CT curves would indicate trouble within the CT. If the CT curves are not available and the current increases quickly with small increases in voltage there is trouble within the CT.

Excitation Test If the voltage source available for testing is not high enough to approach the saturation point, the test can be run with a lower voltage and or fewer secondary turns. The lowest voltage that we would want to use would be 0.50 volts per turn connected from X1 to X5. You may also use the minimum number of turns calculation as shown below to determine test values. Following is an equation that can be used to calculate the minimum number of turns that may be used and a table showing several typical CT ratios and the minimum number of turns for them as derived from the formula. This is for the minimum number of turns only, not the suggested number of turns to use for testing. Use the same test setup in Figure 1 except connect the Variac to the required secondary tap position. Apply voltage and record the current. Do not exceed 5 amps. Record the value of current, voltage and turns used for future reference.

Excitation Test CAUTION: If voltage is applied to less than the full secondary winding of a CT, the CT can act as an auto transformer and build high voltage across the full winding. Damage can occur if greater than 1000 volts is created across the full winding. This can happen whenever voltage is applied to a low number of turns. Care should be exercised not to use so small a number of turns which results in damage to the CT.

Testing CT’s in a Circuit Breaker

Testing CT’s in a Single Phase Transformer In testing CTs inside power transformers, it has been found that the low side bushings of the single or three phase transformers have to be shorted together with any type of jumpers in order to test the high side CTs. Shorting the high side bushings, that are not being tested, however; does not affect testing of the low side CTs. In any case, it is recommended that when testing CTs inside transformers, all bushings not being tested be shorted together and tied to non-polarity.

Testing CT’s in a Three Phase Delta-Wye Transformer With all bushings, not being tested, shorted together and connected to non-polarity

Digital Current-transformer Testers (EZCT-10) The EZCT is a microprocessor-based, current-transformer test set. This rugged and portable test set can perform the current transformer (CT) excitation, CT current-ratio, and winding polarity tests. Current transformers can be tested in their field-mounted configuration, eliminating the need to remove bushings or current transformers from the host equipment. The EZCT is capable of outputting 50 Vac at 2A and 200 Vac at 2A. The EZCT-10 has all the features of the EZCT, but uses a heavy-duty transformer to perform the CT excitation test. It is capable of outputting 50 Vac at 10A and 200 Vac at 10A. The CT excitation test is performed using the ANSI/IEEE C57.13.1, IEC 60044-1 test method. The EZCT applies an AC variable test voltage (up to 1,200 Vac) to the CT’s secondary windings. The EZCT records and displays the test voltage and excitation current applied to the current transformer during the excitation test. Once tests are completed, up to 10 excitation curves and knee-point voltages of the tests can be plotted on the built-in thermal printer. Ratio and polarity test: A test voltage is applied on the CT’s X terminals and the induced voltage is measured across the CT’s H1 and H2 terminals. The current-ratio is displayed on the screen and stored in memory. The current-ratio measuring range is from 0.8 to 5,000. Winding polarity is displayed as a “+” sign (in-phase) or “-” sign (out-of-phase) and is annotated with the phase angle in degrees. The test voltage range (50V at 2A, 200V at 2A,1,200V at 1.5A) is selected with a switch on the control panel. A voltage control knob is used to control the variable test voltage output. This test set can perform the current transformer (CT) excitation, CT current-ratio, and winding polarity tests.

EZCT Cable Connections Always connect the EZCT to the substation ground before connecting any test cables. The EZCT is supplied with a 20-foot X test cable and a 35-foot H cable. Only the X cable connection is required to run the transformer excitation test. The H and X cable connection is required to run the transformer ratio test. A typical connection is shown in Figure 2.

Connection to CT’s on Y Transformer

Connection to CT’s on Delta Transformer

Typical EZCT Connection to Multiple CT’s When performing an excitation test, the EZCT applies an AC voltage to the CT secondary winding. The user raises this AC voltage until the CT windings reach the saturation point. The user then slowly lowers the test voltage back to zero volts. The voltage and current readings will be recorded to plot the excitation curve. A typical excitation test contains a maximum of 32 data points. There may be less than 32 data points depending on how quickly the voltage is lowered to zero or due to the CT excitation curve characteristics. The EZCT also calculates the knee point voltages (using IEEE 45, IEEE 30, ANSI 10/50, IEC 60044-1 10/50 standards) and prints them on the tabulated report. The CT turns ratio is measured using the voltage method. An AC voltage is applied to the CT secondary winding. The induced voltage on the CT primary winding is then measured. The CT turns ratio is the voltage ratio measured between the secondary and the primary windings. Do not change the voltage selector switch during a test. Failure to observe this warning may damage the EZCT. A CT will typically reach saturation when the excitation current reaches about 0.8A.