In the production process of high-voltage motor windings, in addition to the quality requirements of the insulation materials themselves, process control is particularly important, such as the consistency of the insulation stack, the tightness of the tape, the effect of dipping paint, etc., are all very important control elements. In addition to the insulation thickness requirements, the air holes in the windings are minimized and the windings are made into a solid unit.

Although the production process is designed to minimize internal porosity, it is generally inevitable that voids are present in the resin-impregnated mica tape insulation structure of a high-voltage rotating electrical machine. Since partial discharge is common in high-voltage motors, the mica in the insulation structure only ensures the operating life of the motor under specified aging conditions.

The inner layer of the main insulation of the motor winding, the separation of the conductor and the insulating material, the damage of the antihalation layer, the gap at the end of the winding and the surface all lead to the discharge phenomenon, which is also the quality control point of the winding manufacturing. In addition, foreign conductive materials can also cause partial discharge of the windings, and clean production is also necessary for high-voltage motor manufacturing.

Discharge caused by different links

1. Internal stratification of main insulation

The internal delamination of the main insulation may be due to improper resin impregnation during production or incomplete curing of the insulation structure or excessive mechanical or thermal stress during operation. Insulation aging can also cause delamination of the insulating structure, and delamination caused by aging is usually a long-term process. Therefore, the delamination of the old insulation structure is a clear sign of insulation aging. Large pores form a large surface that causes a relatively high energy discharge that can severely damage the insulation. In particular, delamination reduces the heat transfer of the insulation, resulting in accelerated aging. Therefore, when evaluating partial discharges, careful consideration of stratification factors is required.

2. Separation of conductor and insulation material

The air or the inflated elongated slot formed by the separation of the conductor and the insulating material creates a partial discharge that is embedded between the main insulation and the conductor stack. Overheating or extreme mechanical stress can result in separation between layers.

3. Slot discharge

In high-voltage motors, when a portion of the anti-halation layer is damaged, a slot discharge occurs. The trough discharge may be due to an increase in the local electric field strength due to the residual material of the anti-corona material, or due to the movement of the circle/bar in the groove or in the groove region, for example due to material deposition, erosion, wear, chemical attack or manufacturing. The defect causes the slot die pressure to be lost. When the coil/wire bar is loose, the electromagnetic force will cause it to vibrate within the gear, resulting in anti-corona and insulation wear on the groove. In the case where local damage has occurred in the anti-halation layer of the groove portion, there is a partial discharge inception voltage having a high pulse amplitude between the grounded metal electrode (slot iron) and the main insulating surface. This type of discharge mainly occurs where the bar or coil field strength of the stator winding is high. Partial discharge accelerates aging due to erosion of the main insulation. In severe cases, loose coils can also cause mechanical wear.

Slot discharge caused by high voltage can cause insulation erosion, which is more serious at the line/wire bar near the high pressure side. Therefore, when phase discharge occurs, off-line partial discharge measurements will provide different partial discharge intensities, sometimes at the outgoing and neutral points.

The vibration of the coil/bar in the slot during operation of the motor may be the initial state of damage to the anti-halation layer of the groove, for example due to the action of electromagnetic force causing the coil/bar to loosen. Under certain conditions, vibrational sparks may occur during motor operation. This is caused by the intermittent interrupt current caused by the voltage generated by the electromagnetic induction. Although it is not a partial discharge phenomenon, it may occur during measurement.

When the groove anti-halation layer is deteriorated by the vibration spark, the groove discharge can be measured after the motor is stopped. The vibration spark is caused by the magnetic field. This phenomenon mainly occurs on the wire rod or the line of the outlet end of the groove to the neutral point. The phenomenon can be measured on both sides of the winding. The phenomenon of vibration sparks can be observed when performing comparative measurements (including image analysis).

4. Winding end gap and surface discharge

In the end regions of the windings, partial discharges may occur at locations where the electric field strength is high, such discharges often occurring at the interface between different portions of the ends of the stator windings. If the poor design interface, pollution, porosity, thermal effects, etc. make the end anti-halation layer of the winding become ineffective, the reliable field strength gradient is no longer guaranteed, surface discharge will occur, and the insulating material is gradually eroded. Although partial discharges produce relatively rapid changes due to surface effects, this is usually a slow failure mechanism. Partial discharges can change relatively quickly due to surface effects.

In addition, since the interface is not sufficiently clean, such as components of the end support structure, partial discharges may occur between the phases, or a relative discharge may occur at the winding end surfaces.

5. Foreign conductive discharge

Windings mixed with conductive impurities may cause partial discharge intensity concentration. This can result in partial damage to the insulation. Larger metal parts are more destructive, such as damaged bolts or screws and forgotten tools. In addition to localized concentrated discharge and its potentially damaging effects, it can also cause secondary impact on the equipment, such as mechanical wear caused by metal fragments.

Reprinted from the network