PT testing
List of test to be carried out for testing during commissioning of the PT
1. Physical inspection and name plate observation.
2. Insulation resistance of the PT.
3. High voltage dry power frequency test for one minute.
4. Insulation resistance test repetition after HV test.
5. Measurement of Resistance of primary and secondary winding.
6. Rough ratio test.
7. Secondary induced over voltage test and secondary winding burden measurement.
8. Secondary circuit burden measurement
9. Polarity test.
1. Insulation Resistance Test:
There are certain conditions which causes insulation failure are, Electrical, Mechanical, and
Thermal stress, Chemical attack, environmental conditions. No insulation is perfect, which
causes small amount of leakage current flows w.r.t. ground. Leakage current measurement is
the basic which in turn gives the insulation resistance by ohms law.
Testing Frequency:
Once in Two year.
Connections and measurement modes:
Equipment’s required:
1. 5 KV IR tester.
2. 230 V AC supply and supply board.
Reading of IR taken in following way:
a. HV-all Secondary wdg + Gnd:
This mode gives the reading of the resistance of HV to secondary winding and ground. Reading
should take for 1min. Test voltage is to be selected 5 KV.
b. Sec wdg-all other secondary wdg + Gnd
This mode gives the insulation resistance of secondary winding with respect to other
winding and ground. Repeat the test for all winding individually for 1min each. Do the
connection as per the below diagram for achieving above described mode. Test voltage to
be selected is 500V.
c. Tap-Gnd:
This mode gives the value of insulation resistance of tap to Ground, which indicates the
paper insulation condition. Do the test connection as per below diagram and perform the
test for 1 min.
Acceptance Criteria of IR value:
As per thumb rule insulation value should be greater than (KV+1) MΩ where KV is the
voltage level of test equipment under test.
2. High Voltage Test on Voltage Transformer:
This is high voltage insulation power frequency withstand test for 1 min. This is destructive
test.
Test value to be selected is: 80% of (2X HSV+1).
For every repetition of the test, high voltage test value to be reduced by 80% of value
calculated by above formula. While doing the test necessary safety precaution should be taken
(Place of the object including test instrument should be cordoned off, adequate clearances
maintained between Hi pot kit and grounded structures). Note down the leakage current.
For old PT which is going to be used for commissioning, the value of the High voltage to be
applied is (1.5 times the HSV)
Equipments required:
a. HV test kit with control unit.
b. Tong tester, Multimeter.
c. 1 phase AC supply with extension board.
d. Shorting wires with crocodile clips.
Acceptance criteria
Monitor the leakage current for 1min. Test kit should not trip during the test. If it is
tripping besides the all physical inspection done, same should be conveyed to HOD.
3. Repeat IR test after hipot:
As per point no.2 repeat the IR test after hi-pot and ensure the readings are ok as compared
to previous values observed during test 2.
4. Rough Ratio Test:
Rough ratio test is carried for checking the PT ratio as per the name plate. See below the test
connection.
Equipments required:
a. Single phase Variac.
b. Single phase AC voltage source.
c. Multimeter
d. Probes with Banana and crocodile clips
Acceptance Criteria:
Deviation with expected value :   3 %
5. Secondary Induced over voltage with Secondary impendence Measurement Test:
This test is basically carried out for the secondary impedance measurement. See the test
connection in below diagram. Burden of secondary winding calculated from the applied voltage
and input current. Voltage needs to be monitored in this test, so that for applied voltage &
current, burden of winding should not exceed the Burden mentioned on R&D plate for the
particular winding.
Equipments required:
a. 1 Phase Variac.
b. Multimeter.
c. Tong tester.
d. Probes.
Acceptance Criteria:
Observed current should not exceed the calculated current for 63.5 Volts secondary.
7. Polarity Test:
This test is carried out to confirm the polarity of the winding. See the test connection below.
Equipments required:
a. 9 V battery.
b. One way switch.
c. DC analogue meter.
d. Probes.
Acceptance Criteria:
Positive deflection should be shown when the correct connection as per polarity as shown in
Diagram. If backward deflection comes for positive polarity the PT needs to be rejected after
discussion with Head QAIT.