What Is the Dielectric Strength of Insulating Oil? Why Regular Testing?

After years of maintenance in the power system, I have found that many people know that transformers need to undergo "oil testing", but they cannot explain exactly what "dielectric strength" is measured. This article thoroughly explains this concept.

The essence of dielectric strength

Dielectric strength, also known as breakdown voltage, refers to the voltage value in kV at which the insulating oil film breaks down when a gradually increasing AC voltage is applied between two standard electrodes. The higher this value, the stronger the insulation ability of the oil; If the value is low, it indicates that the oil has deteriorated or been contaminated.

For example, insulating oil is like the "bodyguard" of the internal insulation system of a transformer, and the dielectric strength is the combat effectiveness index of this bodyguard. Once the combat effectiveness is exhausted, the transformer winding cannot be protected, which can result in equipment failure or even power grid accidents.

The main factors affecting dielectric strength

Moisture is the number one enemy. Even with trace amounts of moisture in insulating oil, the breakdown voltage will significantly decrease - studies have shown that increasing the moisture content from 10ppm to 30ppm may result in a decrease in breakdown voltage from above 60kV to below 30kV. Secondly, there are particle impurities. Conductive particles such as fibers and carbon particles form conductive bridges between the electrodes, directly leading to premature breakdown. In addition, acidic substances and oxides generated by oil aging can also reduce insulation performance.

Why do we need to test regularly

According to the regulations of the national power industry, the insulation oil of transformers in operation must be sampled regularly to measure the breakdown voltage. Transformers of 110kV and above are usually inspected once a year, and the cycle needs to be shortened when important equipment or operating environments are harsh. New oil storage, equipment overhaul, and abnormal operation must also be inspected. The insulation oil with unqualified breakdown voltage cannot be used anymore and must be filtered or replaced.

What can HZJQ-X1 do

According to the three major standards of GB/T507-2002, DL429.9-91, and DL/T846.7-2004, HZJQ-X1 automatically completes 1-10 boost breakdown tests within the range of 0-80kV, calculates the average value, and prints the test report with one click. The operator only needs to take the oil sample and adjust the electrode spacing, and the instrument automatically completes the entire process.