Transformer oil test (routine test)
The routine tests for transformer oil include breakdown voltage measurement, dielectric loss factor measurement, water content determination, gas content determination and dissolved gas chromatographic analysis.
Breakdown voltage measurement
Test Objective
The breakdown voltage of transformer oil is an important indicator for assessing the degree of contamination by water and suspended impurities in the oil: the lower the breakdown voltage of the oil, the poorer the overall insulation performance of the transformer, which directly affects the safe operation of the transformer; therefore, it is necessary to conduct strict tests and control the breakdown voltage of transformer oil within different ranges.
Test method
The test method for measuring the breakdown voltage of transformer oil shall be carried out in accordance with the provisions of GB/T 507.
Experimental instruments
It is recommended to use a fully automatic breakdown voltage tester – an insulating oil dielectric strength tester. The entire set of instruments should meet the following conditions:
a) Oil cup: Made of transparent glass or plastic, with an effective volume between 300 mL and 500 mL, and equipped with a lid.
b) Electrodes: Made of brass or stainless steel and ground, they can be spherical or hemispherical in shape. The geometrical dimensions should comply with the provisions of GB/T507, and the distance between the electrodes is 2.5mm.
c) Voltage regulating device: An automatic voltage boosting system is adopted, and an autotransformer voltage regulator or a resistor voltage divider can be used.
d) Transformer: When the power supply on the low-voltage side of the step-up transformer is 50 Hz, the output voltage waveform on the high-voltage side should be approximately sinusoidal, with a crest factor within the range of √2 (1 ± 5%). The voltage rise rate should be controlled at 2 kV/s, and when the voltage exceeds 15 kV, the minimum short-circuit current of the transformer should be greater than 20 mA.
e) Protective devices: These include a current-limiting resistor connected in series with the test electrode and a circuit breaker connected to the primary circuit. The circuit breaker should operate within 0.02 seconds after the sample is broken down to protect the test equipment.
f) Other parts: The insulating oil dielectric strength tester should be equipped with an automatic stirrer and a safety dust cover. It is better to have the functions of automatically displaying and recording the breakdown voltage value, calculating the average value and the deviation from the specified value, and printing the test results.
Sample taking of test specimens
The test sample shall be taken in accordance with the method specified in GB7597.
Experimental process
The test process shall be carried out in accordance with the following requirements:
a) Be familiar with the user manual of the insulating oil dielectric strength tester, connect the test instruments as required, debug them to normal operation, and have them verified and qualified by the metrology department.
b) When using it for the first time, the electrode should be disassembled. Thoroughly clean the electrode and oil cup with a cleaning agent, then rinse several times with distilled water and dry them completely with a hair dryer. When installing the electrode, adjust the electrode gap to 25mm with a standard gauge. Under normal usage conditions, it is not necessary to disassemble and clean the electrode each time.
c) Pour the test oil that has been balanced to room temperature into the oil cup and wash it twice. Finally, the oil surface of the test oil poured into the oil cup should be 40mm above the electrode.
d) Cover the safety dust cover, select the standard value, and press the “Start” button. The instrument will automatically start the test. After each test, the instrument will print the breakdown voltage value. After six tests, the instrument will print the average value of the six breakdown voltages and the deviation from the standard value.
Precautions for the Experiment
Please note the following requirements:
First, check the AC power supply. Only when it meets the requirements of the instrument can the power be connected.
b) The instrument housing and grounding point should be reliably grounded.
c) After cleaning the oil cup and electrodes, it is strictly forbidden to touch them with hands or unclean objects.
d) If the oil cup has not been used for a long time or the measured breakdown voltage value is too low (below 10 kV), it should be re-cleaned in accordance with the requirements of 7.15.
e) The relative humidity of the environment shall not exceed 75%.
f) The validity period of the calibration of the measuring instrument should also be regularly determined, and its accuracy should be guaranteed within 25mm ± 0.1m.
g) When the deviation between the sample and the specified value is greater than 10%, attention should be paid. It is better to take samples again and conduct parallel tests to confirm whether it is caused by improper operation.
Post time: Apr-02-2026