Why does partial discharge occur in motors?

When the surface of a solid medium is contaminated and becomes damp, the soluble salts, acids, and alkalis contained in the contamination layer dissolve in the water film, forming ionic conductivity, causing a large leakage current to pass through the contaminated surface; the surface flashover voltage is significantly reduced, and in severe cases it can be reduced to less than 10% of the flashover voltage of a clean surface. In fog, dew, drizzle, and melting snow, insulators may even flash over at the working voltage, seriously affecting the safe operation of the power system. Therefore, pollution flashover has become the main basis for selecting the external insulation level of high-voltage and ultra-high-voltage electrical equipment.

Partial discharges are common in the insulation structure of high-voltage motors, but their size, quantity and location depend on the motor design, materials, manufacturing process, quality, operating environment and aging. For a specific motor design, the characteristics of the insulation materials used, the manufacturing method, the operating conditions, etc. can greatly affect the number, location, characteristics, development trend and significance of partial discharges. For a given motor, in most cases, the characteristics of partial discharges can be used to identify and distinguish different partial discharge sources, which can also be verified by additional diagnostic tests and visual inspections.

Usually, partial discharge occurs at locations where the dielectric properties of the insulating material are uneven. At these locations, the local electric field strength will increase, and due to the excessive local electric field strength, it will cause local breakdown. This local breakdown will not lead to the breakdown of the entire insulating structure. Usually, partial discharge requires a certain amount of gas space to develop, such as gas pores filled inside the insulator, near the conductor or the insulator interface.

When an inhomogeneous local field strength exceeds its breakdown field strength, partial discharge occurs, resulting in several partial discharge pulses in one cycle of applied voltage. In rotating electrical machines using mica insulation, there are inevitably many defects, such as pores in the insulation material and aging parts at the delamination of winding insulation. Therefore, partial discharge sources of different strengths need to be superimposed for measurement.

The stator winding insulation structure will have partial discharge during operation. Inorganic mica materials are inherently resistant to partial discharge. However, the presence of significant partial discharge in the motor is usually a sign of insulation defects, such as manufacturing quality problems or deterioration during operation, and it is not the direct cause of its failure. In addition, it depends on the partial discharge source and its amplitude under specific conditions, which may turn into an important factor in local insulation aging. The failure time is independent of the level of partial discharge, but is closely related to other factors, such as operating temperature, slot wedge condition, pollution level, etc.

Specific partial discharge measurements and analysis can be used for quality control of new windings and winding components as well as for early detection of insulation defects caused by thermal, electrical, environmental and mechanical stresses in operation, which can lead to insulation failure.