In high-voltage stator windings, localized ionizing discharges with blue fluorescence, accompanied by a hissing sound, are commonly observed; this phenomenon is called corona discharge. Corona discharge produces ozone (O3) and nitrogen oxides (NO, NO2), which combine with moisture to form harmful acidic substances that corrode the insulation. Corona discharge can also cause localized heating of the insulation, accelerating insulation aging and, in severe cases, rapidly destroying the insulation. Today, Ms. Can will briefly discuss the prevention of corona discharge in high-voltage motors , as well as corona discharge and electro-corrosion problems in the slots.
Regarding winding corona
The parts of the winding most prone to corona discharge are: (1) the air gap in the inner layer of the coil insulation; (2) the coil slot outlet and ventilation opening; (3) the gap between the coil insulation surface and the slot wall; and (4) the gap between adjacent coils at the winding ends and the lead wire and end clamp.
When the anti-corona layer on the thermosetting insulation surface has poor or unstable contact with the tank wall, the contact point becomes intermittent under the action of electromagnetic vibration, causing spark discharge. The energy of this spark discharge can be hundreds of times that of corona discharge, and the local temperature can reach hundreds of degrees, causing pits deeper than 1 mm on the insulation surface. The position of the pits often changes due to vibration, which is the so-called electro-corrosion phenomenon.
Causes and prevention methods of corona discharge in the tank
There is a gap between the outer surface of the slot and the slot wall of the high-voltage stator winding. The electric field strength EaM at the gap on the insulating surface can be calculated as a double-layer dielectric plate capacitor when no anti-corona treatment is performed.
In formula (1):
Uφ——Phase voltage (kV);
δa — Installation gap (cm);
δi—Insulation thickness on one side (cm); Slot gap of the TuGuo-36 coil
εa — the dielectric constant of air;
εi — the dielectric constant of the insulation.
If the value of EaM is greater than or equal to the air breakdown field strength EbM in a uniform electric field, then corona discharge will occur.
Even at operating voltage, corona discharge will occur in the air gap between the winding insulation surface and the slot walls. At winding corners, ventilation ducts, and slot openings, corona discharge will occur at lower operating voltages due to electric field distortion.
Practice has shown 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 6kV, corona may occur in the slot; and when the rated voltage is greater than 10kV, corona will actually occur in all gaps.
To eliminate corona discharge within the insulation, the insulation wrapping, impregnation, and pressing of the coil must be standardized to ensure a tight, air-free insulation layer. For coils above 6 kV, the internal insulation quality must be assessed by measuring Δtgδ. Operational experience has shown that if Δtgδ is controlled within the allowable range, the corona discharge problem within the coil can be considered resolved. Therefore, anti-corona treatment only considers the corona discharge on the coil surface.
To eliminate corona discharge between the phase wall and the insulating surface, the most common method is to add a low-resistance anti-corona layer to the insulating surface. This serves two purposes: firstly, it makes the electric field distribution at the ventilation slot opening more uniform, reducing the axial magnetic field; secondly, the low-resistance anti-corona layer short-circuits the gap when it contacts the slot wall. If the resistance of the anti-corona layer is very low, as long as there is a stable grounding point, the gap between the insulating surface and the slot wall can be completely short-circuited, preventing further corona discharge. However, to reduce losses in the anti-corona layer, its resistance should not be too low. This ensures that the anti-corona layer farther from the contact point is at a potential determined by the voltage drop generated by the capacitive current on the anti-corona layer. The smaller the surface resistivity ρs of the anti-corona layer, or the shorter the distance between the contact points (i.e., the better the grounding of the anti-corona layer), the smaller the voltage drop generated by the capacitive current, and the less likely corona discharge will occur. Practical experience shows that when using thermoplastic insulated coils with more contact points with the slot wall, and when ρs = 10⁴~10⁵ ohms, corona discharge can be essentially prevented.
Causes and prevention methods of electrolytic corrosion in tank sections
On the surface of the anti-corona layer at the center of the span between the two contact points between the anti-corona layer of the slot coil and the slot wall, in addition to the voltage drop generated by the capacitive current, there is also the electromotive force induced by the main magnetic flux and slot leakage flux on the anti-corona layer. In motors with low line loads, the electromotive force induced by the slot leakage flux is small and can be ignored. Therefore, the maximum voltage at the center of the span between the two contact points is equal to the vector sum of the voltage drop generated by the capacitive current and the electromotive force induced by the main magnetic flux. Experiments have shown that under electromagnetic vibration, when this combined voltage reaches 130~150 volts, a strong spark discharge will occur in the gap, corroding the insulation. To avoid electro-corrosion, measures must be taken in the process to minimize the distance between stable contact points and to select a reasonable surface resistivity of the anti-corona layer.
For thermosetting insulated coils, the spacing between stable contact points is generally required to be less than 50 cm. Therefore, certain fixing measures must be adopted to ensure sufficient stable contact points between the coil surface and the slot wall. Simultaneously, the surface resistivity of the anti-corona layer should not exceed 5 × 10⁴ ohms in large-capacity motors and not exceed 5 × 10⁵ ohms in medium-capacity motors. To prevent excessive increases in anti-corona layer losses, the surface resistivity of the anti-corona layer should not be lower than 5 × 10³ ohms in both large-capacity and medium-capacity motors.
