Partial Discharge

Partial discharge (PD) is often an area of concern in a hipot or surge voltage test. Both testing methods serve as a basis to measure partial discharges in electrical devices.

  • A hipot test, which in this case must be completed with alternating current, measures the dielectric strength and also the partial discharge between windings and/or a winding and the ground.
  • In surge voltage testing, one “looks” directly into the winding (from winding to winding) in order to find problem areas in insulation there. A partial discharge test is completed here as well.

What is partial discharge?

As the word "partial" in "partial discharge" describes, in a partial discharge a disruptive discharge takes place in part of the insulation. During a hipot test with alternating current, there is no "partial disruptive discharge," but rather a full disruptive discharge between windings and ground/earth. In practice, it is frequently possible to hear crackling noises during a hipot test. If you do hear such crackling noises, no full disruptive discharge is taking place. Instead, you are only perceiving partial discharge effects. These can be measured and evaluated with a partial discharge test.

Where do partial discharges occur, and what consequences do they have?

A partial discharge can occur between windings which touch one another, as well as between the winding and the body. There is a partial insulation problem in one part of the wound material where these partial discharges occur. Such partial discharges (also called partial disruptive discharges) can turn into full disruptive discharges during operation – for instance in a motor. With partial discharge measuring devices from SPS electronic, collected information is used to assess a transformer or a set of transformers, for instance. Defective transformers can then be sorted out in order to avoid failures in the field.

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