What is Partial Discharge test and why it’s needed?

Corona (not the beer) is the generic name for electrical discharge that occurs in electrical insulation as a result of high velocity ionization under the influence of an electric field that exceeds the dielectric strength of the insulation.  Corona and the term partial discharge are used interchangeably when referring to this phenomenon in transformers. The partial discharge terminology is preferred since it most accurately describes the occurrence. Much has been written concerning the cause of partial discharges. Some of the common conditions that can initiate partial discharges are:

  • Improper site and/or weather conditions with humid/dusty environments, poor or no maintenance procedures.
  • Improper processing or drying of the insulation.
  • Over-stressed insulation due to lack of proper recognition of the voltage limitation of the insulation.
  • High stress areas on conducting parts which can be caused by sharp edges on either the conducting parts or ground plane.


The effect of partial discharge in a transformer is twofold.

  • One effect is that the ion and electron bombardment can be damaging to the insulation and shortens the life of the transformer.
  • The other effect is the transient currents produced due to partial discharge may interfere with electrical communications.

Partial discharge is defined as an optional test per IEEE C57.12.01.2015. MGM, for its own research and continual improvement performs this test periodically on a first design and or on prototypes. We also offer customer witness testing when specified at the time of order. 
MGM Transformer Company has been aware of the consequences of partial discharge in transformers for many years and has developed drying and processing procedures as well as insulation systems that virtually eliminates the presence of damaging partial discharges within our coils. The general procedure for partial discharge testing is to raise the voltage to 1.8 times the rated voltage and hold for a minimum of 30 seconds and then reduce the voltage to a level equivalent to 1.3 times and maintain it for 3 minutes and measure the partial discharge levels. The ambient levels of the instrumentation shall be considered when determining the final value of partial discharge. The maximum acceptable levels of partial discharge for resin-encapsulated windings is 50 pC (picoCoulumbs).