Corona in Generators

Corona in a generator refers to the glow discharge caused by ionization of the surrounding air due to uneven electric field distribution and excessively high local field strength at certain parts of the insulation surface of the generator's stator high-voltage windings. It occurs on the surface of the insulation and differs from the corona we are familiar with near conductors under general outdoor high-voltage electric fields. Compared with other forms of partial discharge, the discharge intensity of generator corona itself is not very high, but its presence significantly reduces the performance of the insulation material.

Factors Contributing to Generator Corona

1.Related to altitude: The higher the altitude, the thinner the air, and the lower the corona inception voltage.

2.Related to humidity: Increased humidity reduces surface resistivity, leading to a decrease in corona inception voltage.

3.Related to the temperature of the end high-resistance anti-corona layer: If the resistance of the high-resistance anti-corona layer is high at room temperature, the corona inception voltage increases with temperature rise; otherwise, the opposite occurs.

4.Related to the potential and electric field distribution at the location of the winding bar: The higher the potential, the more likely corona is to occur; the more uneven the electric field distribution, the more likely corona is to occur.

Areas Prone to Corona in Generators

1.Winding bar exit from the slot: The exit of the winding from the slot is a typical bushing-type structure, where the electric field is highly concentrated, making it the most prone area for corona.

2.Ventilation duct in the iron core section: The ventilation duct steel has sharp edges, which easily cause local uneven electric fields.

3.Areas where the winding bar surface has poor contact with the iron core slot or has air gaps.

4.Between different phase winding bars at the end: The electric field distribution at the winding end is complex, especially at the contact points and edges of the coil with end hoops, binding ropes, and shim. Due to technological reasons, it is often difficult to completely eliminate air gaps, and corona is likely to occur in these gaps.

Measures for Generator Anti-Corona

1.Select corona-resistant materials for the generator's insulation as much as possible;

2.Use corona-resistant paint during generator impregnation;

3.Consider the harsh working conditions in motor design and reduce electromagnetic loads;

4.Adopting anti-corona semiconductor materials, anti-corona structures, and anti-corona processes can all increase the corona inception voltage of the generator, thereby effectively preventing the occurrence of corona.

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