Causes and prevention methods of corrugated corona
There is a gap between the outer surface of the high-voltage stator winding groove and the groove wall, and the field strength EaM of the insulating surface at the gap can be calculated as a double-layer dielectric plate capacitor when it is not treated with anti-halation treatment.
Even at the operating voltage, the air gap between the winding insulation surface and the groove wall will produce corona. At the winding corners, vents and slots, these parts will corrode at lower operating voltages due to distortion of the electric field.
Practice has proved that when the rated voltage of the motor does not exceed 3kV, there is no corona phenomenon in the slot; when the rated voltage is 6 kV, corona may be generated in the slot. When the rated voltage is greater than 10 kV, virtually all the gaps are Corona will appear.
In order to eliminate the corona inside the insulation, it is necessary to standardize the insulation wrapping, impregnation and pressing of the coil so that the insulating layer is tight without air gap. For the coil above 6 kV, the internal quality of the insulation must be determined by measuring tgδ. . The operating experience proves that the Δtgδ is controlled within the allowable range, and it can be considered that the corona problem in the coil has been solved. Therefore, the anti-corona treatment only considers the corona on the surface of the coil.
To eliminate corona between the phase wall and the insulating surface, the most common method is to add a low-resistance anti-halation layer to the insulating surface. On the one hand, the electric field distribution of the ventilation slot is relatively uniform, and the axial magnetic field is reduced; on the other hand, the low-resistance anti-halation layer is short-circuited with the groove wall. If the resistance of the anti-halation layer is very low, the corona layer can be short-circuited by the gap between the insulating surface and the groove wall as long as the corona layer is stably grounded, and no corona is generated. However, in order to reduce the loss of the antihalation layer, the resistance of the antihalation layer should not be too low, so that the antihalation layer far from the contact point is at a potential determined by the voltage drop generated by the capacitive current on the antihalation layer. The smaller the surface resistivity ρs of the antihalation layer, or the shorter the distance between the contact points, that is, the better the grounding of the antihalation layer, the smaller the voltage drop generated by the capacitor current, and the less likely the corona is generated. Practice has proved that the contact point of the thermoplastic insulated coil and the groove wall is more, when ρs=104~105 ohms, the corona generation can be basically prevented.
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