Corona characteristics of flaws in a solid encapsulated high voltage inverter transformer

Thermal stress cracks have been observed in high voltage solid encapsulated transformer cells resulting from stress concentrations of the order of 10 at the interface between a copper plated shield and the resin. These cracks result in a degradation of CIV by a factor of 6. They are characterized by corona pulse spectra having very high amplitudes. Pulses of 50 to 500 pC have been observed at an overvoltage of only 10 to 15 percent.