test instructions for potential transformer

POTENTIAL TRANSFORMER

TEST INSTRUCTION CORE ENGG ELECTRICAL

Document Number : REL-COENG-NEE-A14-U-00001 (TI102) TEST INSTRUCTION - TI102 Sheet 1/4

1.1 OBJECTIVE ......................................................................................................................... 1 1.2 SCOPE ............................................................................................................................... 1 1.3 PREPARATION ..................................................................................................................... 1 1.4 INSULATION RESISTANCE (IR) MEASUREMENT OF PRIMARY ..................................................... 1 1.5 INSULATION RESISTANCE (IR) MEASUREMENT OF SECONDARY ............................................... 2 1.6 POLARITY CHECKS .............................................................................................................. 2 1.7 ROUGH RATIO MEASUREMENT .............................................................................................. 3 1.8 PT OIL TESTS...................................................................................................................... 3 1.9 CAPACITANCE & TAN DELTA MEASUREMENT ........................................................................... 4

1.1 Objective

The tests are carried out to verify the PT’s healthiness after transportation and erection, and also periodically during service. They also verify that the PT of correct specification is installed.

1.2 Scope

a. These instructions apply to PTs installed in switchgear at all voltage levels. They also apply to PTs installed in Generator PT cubicles and in Switchyard.

b. All the listed tests shall be conducted during commissioning. The same instructions may be followed during routine testing every 3 years. The routine tests to be carried out are IR test and Capacitance & tan delta test. Oil may be tested less frequently, once in 6 years.

1.3 Preparation

a. Confirm that all primary circuits are de-energised, isolated, earthed and locked out. Take a valid PTW.

b. Disconnect all wires on the PT secondary terminals. However, in case of PTs already in service, do not disturb any secondary circuits.

c. In most cases the primary is isolated by racking out. For high voltage PTs, ensure that the Test tap (for Tan delta) is earthed. Confirm that the PT body is earthed, if a terminal is provided.

d. PT Testing will comprise of the following work, in the same sequence. Record the results in the format TF102.

1.4 Insulation Resistance (IR) of primary

a. Disconnect the earth connections on the primary and secondary neutrals. Reconnect after the test.

b. Use an insulation tester of appropriate voltage. A 1kV tester is used for LV PTs and for 3.3kV PTs. A 5kV tester is good for 6.6kV and above. It is sufficient to test all three phases together.

c. Test the Primary-to-Earth IR, as per Fig. 1. Ensure that the PT body or truck is earthed. d. Test the Primary-to-Secondary IR, as per Fig. 2 e. Note the IR readings, test voltage and the Serial number of the tester




f. The minimum acceptable value will be rated kV + 1 MΩ. (8MΩ for 6.6kV PT)

1.5 Insulation Resistance (IR) of Secondary

a. In case of routine testing, disconnect the earth link in the PT secondary circuit. Test all phases together, along with the connected cables, relays and meters.

b. Use an insulation tester of 500V. c. Test the Secondary-to-Earth IR, as per Fig. 3 d. Test the Secondary-to-Secondary IR, as per Fig. 4. Repeat for each pair of secondaries. e. Note the IR readings, test voltage and the Serial number of the tester f. The minimum acceptable value will be 1 MΩ.

1.6 Polarity Checks

a. Disconnect the PT secondary circuit connection. b. Connect the test setup as per Fig. 5 c. Use a fresh cell of 1.5V / 3.0V. Use any Analogue meter in DC mA range or DC mV range

(indicated in the figure as AVO) d. Touch the lead of ‘R’ Phase momentarily to +ve of the cell. The meter shall deflect in

“positive” (clockwise) direction, and then fall back to zero e. The meter will deflect in the reverse direction when the lead is disconnected f. If the meter deflection is incorrect, correct the polarity marks on the secondary.




g. Precautions: Do not connect the battery cell to the PT primary for more than 3 seconds. h. Repeat the test for all phases and all windings.

1.7 Rough Ratio measurement

a. The purpose is to simply verify the connected ratio of the PT, and is not to test its accuracy.

b. Isolate the external circuits from PT secondary terminals. c. Disconnect the earth connection on the neutral of the primary. d. Connect the test setup as per Fig. 6. Increase the variac voltage gradually to 200V or to

some other convenient value. e. Note the primary and secondary voltages and calculate the ratio. Verify the ratio against

the ratio mentioned in the scheme drawing. f. The maximum acceptable deviation of the measured ratio from the calculated ratio shall

be ±10%.

g. Repeat the test for all phases and all windings.

1.8 PT oil tests

a. The oil in oil-filled PTs needs to be tested at the time of commissioning and periodically thereafter.

b. However, if the PT is hermetically sealed, the oil shall not be disturbed for testing, except when a leak is observed and oil contamination is suspected. The oil sampling, replacement or topping-up activity shall be carried out in consultation with the




manufacturer. c. Refer to TI201 Oil sampling and Test instruction for details.

1.9 Capacitance & Tan delta measurement

a. Oil-filled PTs are additionally tested for Capacitance & Tan delta. b. The test is carried out after the Insulation resistance measurement. c. Refer to TI203 Capacitance & Tan delta Test instruction for details.

test instructions for potential transformer

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