A repair shop undertook the maintenance of a 10kV high-voltage motor . After the test was completed, an alarm occurred during the withstand voltage process. Subsequent insulation resistance testing of the motor revealed no abnormalities. Further inspection revealed that the motor fault was a typical flashover. Ms. Can has compiled some actual photos to share and analyzed the causes of the flashover.
Fault Case Description
The motor has a compact structure and a relatively small cavity, but considering the overall distribution of the stator windings, this problem shouldn't have occurred. Further inspection revealed blackened marks along a section of the winding end surface. Below the winding area were some very fine iron particles. After cleaning these iron particles and repairing the burned areas, the problem was resolved.
Based on the symptoms of the fault and some actual anomalies, it can be basically attributed to a typical flashover. Further investigation revealed that the cause was that during the machining of the motor base, the blind hole of the fixed end cover was opened, and fine iron powder generated during tapping and assembly entered the motor cavity. During the operation of the motor, the iron powder in the end holes above and on the side of the base was shaken off and fell to the position below the base, "perfectly creating" the conditions for flashover to occur.
The consequences of this case serve as a reminder that while holes in motor components may seem simple, it is crucial to distinguish between through holes and blind holes. Strict adherence to the drawings is essential, especially regarding blind holes, as they always serve a specific purpose and rationale.
What is flashover ?
Flashover refers to the breakdown between two electrodes that occurs within a gas cavity along the surface of a solid insulator under high voltage. The voltage between the two electrodes just before a flashover occurs is called the flashover voltage. Due to the influence of factors such as the surface condition and shape of the solid insulation, the flashover voltage is always lower than (at most equal to) the spark discharge voltage of a gas gap with the same electrode structure and distance.
Insulators or bushings of high-voltage power transmission and transformation equipment contaminated with dirt (industrial filth, salt, etc.) may experience a significant decrease in flashover voltage when exposed to moisture (especially fog, dew, frost, or light snow), even flashing at the operating voltage of the electrical equipment, causing serious accidents. This condition is called pollution flashover and can be prevented by improving insulation design and regular cleaning.
A flashover is a destructive discharge phenomenon that occurs along the surface of a solid insulation in a gaseous or liquid dielectric. Flashovers commonly occur at the interface between the gas and the solid dielectric. For example, destructive discharges along the surface of an insulator string or bushing are called flashovers. Therefore, the term flashover is only used to describe discharge phenomena under specific conditions.
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