Partial discharge is mainly the discharge of the internal insulation of dry-type transformers, transformers and other high-voltage electrical equipment under the action of high voltage. This kind of discharge exists only in the local part of the insulation and does not immediately form the entire insulation penetration breakdown or flashover, so it is called partial discharge. Partial discharge is very weak, relying on the person’s intuition, such as eye hearing is imperceptible, only a highly sensitive partial discharge measurement instrument can detect it.
The internal insulation of the dry-type transformer is under the action of the working voltage for a long time in operation, especially with the increase of the voltage level, the electric field strength of the insulation is very high, and the partial discharge is easily generated at the weak insulation, and the cause of the partial discharge is : The electric field is too concentrated at a certain point, or if the electric field strength of a certain point is too large. For example, if the solid medium has bubbles, the impurities are not removed. The oil contains water, gas, and suspended particles. In the different media combinations, there is a serious problem at the interface. Electric field distortion. Traces of partial discharge often leave only a small spot on the solid insulation, or a dendritic burn mark. In oil, some decomposed small bubbles appear.
Although the partial discharge time is short, the energy is also very small, but it is very harmful, and its long-term existence will have a greater destructive effect on the insulating material. First, the insulating material adjacent to the partial discharge is directly bombarded by the discharged material. The damage caused by local insulation, and the second is the chemical action of the reactive gases such as heat, ozone, and nitrogen oxide generated by the discharge, so that the local insulation is corroded and aged, the conductance increases, and eventually thermal breakdown occurs. Dry-type transformers in operation, the internal insulation of the aging and damage, mostly from the beginning of the partial discharge.
Transformer partial discharge detection methods generally have:
1, electrical measurement method. Use an oscilloscope or radio jammer to find the characteristic waveform of the discharge or radio interference level.
2, ultrasonic testing. The sound waves appearing in the discharge are detected, and the sound waves are converted into electric signals, which are recorded on a magnetic tape for analysis. The distance from the detection point to the discharge point can be obtained by using the difference in the transmission time of the electric signal and the sound signal.
3, chemical measurement method. Detection of various dissolved gases in the oil content and increase and decrease changes in the law. The test method can detect changes in the composition, proportion, and quantity in the oil to determine whether there is partial discharge (or local overheating).
In addition, in recent years, partial discharge on-line detectors have been developed that can automatically detect partial discharges during transformer operation. In order to prevent the occurrence of partial discharge, the manufacturing company should conduct a reasonable structural design of dry-type transformers; meticulous construction, improve the purity of materials, and strictly handle the quality of each link. Operation units shall strengthen the maintenance and monitoring of dry-type transformers to effectively prevent the occurrence of partial discharge of dry-type transformers.