By hvhipot 
According to the principle and position of partial discharge, transformer partial discharge can be generally divided into corona, suspension, bubble discharge, discharge in oil these fHV Hipot Electric Co., Ltd. types.
Partial discharge of the transformer occurs when the field strength generated by the applied voltage in the electrical equipment is enough to make part of the insulation structure inside the transformer but not complete breakdown. Generally speaking, partial discharge generally occurs in the parts where the insulation is weak or the electric field strength is too high, which is caused by the uneven dielectric and electric field strength in each area of the insulation due to the air gap or impurities contained in the insulation material. Air bubbles in the oil discharge mechanism to explain, under a strong electric field of electronic current metal heating medium oil, transformer oil is decomposed to produce gas, or metal electrode surface itself contains pores, or caused by corona discharge electrode surface is not smooth and gasified oil medium, under the effect of voltage, bubbles start ionization, the result of the ionization by raising the temperature of the air bubble, volume increase, The ionization is further strengthened, the decomposition gas in the oil increases, and the gas channel is expanded. Then the electric field in the oil forms a gas “small bridge”, and the breakdown may occur on this channel, resulting in the discharge of bubbles in the oil.