A: Winding and bushing insulation resistance, absorption ratio and polarization index
1. Test procedure
Measure and record the ambient temperature and humidity, and record the average value of the top oil temperature of the transformer as the winding insulation temperature.
Before measuring, short-circuit the tested winding to ground and fully discharge all windings.
The non-tested windings are short-circuited to ground, and the terminals of the tested windings are short-circuited. The insulation resistance values of 16, 60, and 600s are measured and recorded.
Turn off the megohmmeter and discharge the winding circuit under test to ground.
Measure other windings
2. Matters needing attention
a) When measuring the absorption ratio, attention should be paid to the error caused by time.
b) During the test, note that the L end and E end of the megger cannot be reversed.
c) Try to eliminate the influence of surface leakage current during the test.
d) Accurately record the top oil temperature, because the insulation resistance of the transformer changes significantly with temperature changes.
B. DC leakage current of winding and bushing
1. Test procedure
a) Disconnect each winding lead of the transformer, connect the test high voltage lead to the tested winding, and short-circuit the other non-tested windings to ground.
b) Prepare the test according to the wiring diagram, and ensure that all test equipment and instruments are connected correctly and the instructions are correct.
c) Record the top oil temperature and ambient temperature and humidity.
d) Start the test after confirming that everything is normal. First, pressurize to the test voltage in sections with no load to check whether the test equipment is well insulated and whether the wiring is correct.
e) Apply the DC power supply output to the winding of the tested transformer, pressurize it to 0.5 times the test voltage during measurement, and read the leakage current value after 1 min. Then pressurize to the test voltage, and read the leakage current value after 1 min.
f) After the test of the tested winding is completed, reduce the voltage to zero, cut off the power supply, and perform other operations after fully discharging.
2. Matters needing attention
a) The test voltage of the graded insulating transformer should be in accordance with the voltage grade standard of the tested winding, but it should not exceed the withstand voltage level of the neutral point insulation.
b) High-voltage leads should use shielded wires to avoid the influence of lead leakage current on the results. High-voltage leads should produce corona if small.
c) The microammeter should be measured on the high-voltage side.
d) The assessment of insulation under negative DC voltage is more stringent, and negative polarity should be adopted.
e) Since the factory test generally does not measure the DC leakage current, the value of the DC leakage current should comply with the relevant standards, which is the basis for future pre-test comparison and judgment.
f) If the leakage current is abnormal, it can be eliminated by drying or shielding.
C. tgδ of winding and bushing
1. Test procedure
a) Measure and record the top oil temperature and ambient temperature and humidity.
b) Connect the test circuit according to the wiring diagram of the instrument, and pay attention to the insulation problem of the test high-voltage line to the ground.
c) Carry out the test in accordance with the operating instructions of each dielectric loss tester.
2. Matters needing attention
a) Dielectric loss measurement can find that the entire transformer is damp, the insulating oil is deteriorated, and serious local defects, but for the local defects of large transformers. Its sensitivity is low.
b) In the test, the voltage of the high-voltage test line is 10 kV, and attention should be paid to the problem of ground insulation.
EP Hipot Electric Co., Ltd.
