partial discharge is mainly the discharge of internal insulation of dry-type transformer, transformer and other high-voltage electrical equipment under the action of high voltage. this kind of discharge only exists in the partial position of the insulation and will not immediately form the whole insulation through breakdown or flashover, so it is called partial discharge. partial discharge is very weak, which can't be detected by human intuition, such as eyes, ears and ears. only a very sensitive partial discharge measuring instrument can detect it.
the internal insulation of dry-type transformer is under the action of working voltage for a long time in operation, especially with the improvement of voltage level, the electric field strength value of insulation is very high, and partial discharge is easy to occur in the weak part of insulation. the reason for partial discharge is that the electric field is too concentrated at a certain point, or the electric field strength at a certain point is too large, such as the solid medium has bubbles, and impurities are not removed; there are water, gas and suspended particles in the oil, and there are serious electric field distortion at the interface in different media combinations. the traces of partial discharge often leave only a small spot on the solid insulation, or a branch shaped burn mark. in the oil, there are some small bubbles of decomposition.
although the time of partial discharge is short and the energy is very small, it is very harmful. its long-term existence will cause great damage to the insulating materials. first, the insulating materials adjacent to the partial discharge will be damaged by the direct bombardment of the discharge particles. second, the chemical action of the heat, ozone, nitrogen oxide and other active gases generated by the discharge will cause the partial insulation to be damaged to corrosion aging, conductivity increases, eventually leading to thermal breakdown. in the operation of dry-type transformer, the aging and damage of internal insulation mostly start from partial discharge.
the detection methods of partial discharge of transformer generally include:
1. electrometric method. use oscillograph or radio jammer to find the characteristic waveform or radio interference degree of discharge.
2. ultrasonic method. detect the sound wave in the discharge, transform the sound wave into electrical signal, record it on tape for analysis, and use the difference of transmission time between electrical signal and acoustic signal to obtain the distance from the detection point to the discharge point.
3. chemical method. detect the content of various dissolved gases in the oil and the law of change. the test method can find the change of composition, proportion and quantity of oil, so as to determine whether there is partial discharge (or partial overheating).
in addition, in recent years, an on-line local discharge detector has been developed, which can automatically detect partial discharge in the operation of transformer.
in order to prevent the occurrence of partial discharge, the manufacturer shall carry out reasonable structural design for the dry-type transformer, carefully construct, improve the purity of materials, and strictly deal with the quality of each link. the operation unit shall strengthen the maintenance and monitoring of dry-type transformer to effectively prevent the occurrence of partial discharge of dry-type transformer.