How to Test insulating Oil Breakdown Voltage by Oil BDV Tester

Why should the insulating oil breakdown voltage(BDV) test be performed on the insulating oil of the transformer?

Insulating oil is widely used in high-voltage electrical equipment such as transformers, oil circuit breakers, oil-filled cables, power capacitors and bushings. Its functions are as follows:
1. Insulation. Impregnates and protects solid insulation such as transformers, cables, capacitors, etc., fills the air bubbles in the insulation, prevents the intrusion of air or moisture, and ensures reliable insulation.
2. Cooling effect. It has a good cooling effect on electrical equipment such as transformers.
3. Arc extinguishing effect. The insulating oil in the oil circuit breaker not only has the function of insulation, but also has the function of extinguishing the arc, so that the circuit breaker can cut off the arc quickly and reliably.
During the operation of the transformer, the insulating oil will gradually change its performance due to the action of high oxygen humidity, high temperature, sunlight, strong electric field and impurities, so that it cannot fully exert its insulating effect. Therefore, it is necessary to regularly conduct relevant tests on insulating oil to identify whether its performance has deteriorated. Among them, the oil breakdown voltage (BDV) test is a very important test item.

The principle of insulating oil BDV test:

The test principle of insulating oil BDV testing is to place the insulating oil sample into the oil cup, add a set rate and continuously increase the test voltage until the insulating oil is broken down, or the boost voltage reaches the Maximum test voltage value, thus get the dielectric strength of the insulating oil. Since the distribution of various impurities present in the insulating oil is affected by uneven motion (Brownian motion of impurities in the oil), etc., the test result of the breakdown voltage has dispersion, so several testing are required

Since the oil breakdown voltage test conditions are strict, the withstand voltage test of the insulating oil is carried out in a special breakdown voltage tester.

Insulating Oil BDV Testing Procedure

1. Clean and dry the oil test cup, standard gauge, glass rod for stirring and glass plate for covering the oil cup.
2. Use a clean and dry oil sample bottle to take oil samples at the sampling port of the transformer (the sampling port should be cleaned, dried and processed), insulating oil should be cooled down to 27°C temperature before testing.
3. Rinse the oil cup three times with the sample oil. Use a gauge to adjust the distance between the electrodes of the oil cup (2.5 mm), then slowly inject the test oil into the oil cup along the stirring glass rod until it is ≥10 mm above the electrode, then cover the glass cover and let it stand still for 15 minutes to make the air bubbles in the oil overflow.
4. Then turn on the power supply, and increase the voltage evenly at a speed of 3~5kV/s until the oil cup electrode breaks down, and (KOF overcurrent trip) disconnect the power supply.
5. Use a stirring glass rod (about 2 mm in diameter) to gently move between the electrodes to remove carbon particles on the surface of the electrodes due to breakdown.
6. After stirring, stand still for 5 minutes and pressurize again.
7. The breakdown should be repeated six times in total, the first time is not calculated, and the average value of the five breakdowns is taken as the final oil pressure resistance value.

8. If insulating oil BDV value is above 30 KV, transformer oil is good.

Transformer insulating oil BDV test result:

Sample test: Six independent breakdown voltage tests were conducted on Oil Sample

1. The breakdown voltage values (BDV) of each test were:

Test 1: 52 KV
Test 2: 42 KV
Test 3: 47 KV
Test 4: 40 KV
Test 5: 43 KV
Test 6: 40 KV

2.Calculate the average BDV value: average the breakdown voltage values of 6 tests. The calculation method is as follows:
Average BDV=
52+42+47+40+43+40)/ 6 = 44 KV

Result recording: The calculated average breakdown voltage value of 44 KV is used as the final breakdown voltage value (Oil BDV Value) of the insulating oil sample.

JY6611 transformer insulating oil breakdown voltage tester:

1. The working power of the transformer insulating oil withstand voltage tester is AC220V. The ground terminal of the enclosure must be connected to a standard ground wire (done before power is applied).
2. Carefully install and check the oil cup. Note the axis of the poles. Adjust the small plate electrode spacing to 2.5 mm.
3. Turn on the "power switch". The power indicator is on. At this time, the voltmeter displays "00.0". Indicates that the meter is normal.
4. The pressure must be reduced before boosting, and then the boosting test must be carried out. Otherwise, the instrument will refuse to work.
5. Pour the transformer oil into the oil cup and clean it twice. Then pour the prepared oil sample into the oil cup, put the oil cup on the electrode holder, close the oil cup cover, and press the interlock switch (do not press the interlock switch to refuse the operation).
6. Let stand for 10 to 15 minutes. Remove air bubbles.
7. During the test, for example, although the digital display of the transformer insulating oil withstand voltage tester cannot reach 60KV, the turntable of the voltage regulator has reached the limit switch. This phenomenon is caused by insufficient external voltage (not related to the instrument).
8. During the test, if the transformer insulating oil breakdown voltage tester is abnormal, press the "stop switch" to force the power off (during the retest process, the "stop open" switch resets)
9. Press the "Boost switch" to automatically boost the pressure; when the oil sample is broken down by high voltage, the overcurrent protection system device will cut off the high voltage within 0.02 seconds. The numerical value shown here is the breakdown voltage value of the oil.
10. Press the "step-down switch", the voltage stabilizer will automatically return to zero, and the second test can be completed after 5 minutes.

Please Check More Transformer/Substation Testers from Kingrun:

What are the Acceptance Testing Items after Transformer Installation, Commissioning and Repair?

What is the Difference Between Flash Point and Ignition Point of Transformer Oil?

Key Points to Note in Transformer Oil Breakdown Voltage (BDV) testing

High Precision Insulating Oil Tester--BDV & Tan Delta

How to Judge Transformer Fault Through Transformer Oil