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Partial Discharge


What is parcial discharge technology?

At Prax we see a trend in the market that each time the   working voltage is getting higher and higher. A good example of this are the battery packs in electrical vehicles that can go up to 800V. This high voltage allow to work at high power without the need to increase the wire section, contributing to better overall performance. The transformer is a key element to provide the insolation in an electrical device like a on board charger or an EV charger. It is therefore of most importance to ensure that the transformer can withstand such high voltages without degradation of the isolation.

The partial discharge test gives us good knowledge about the quality of isolation and verifies that it won’t be damaged during operation life.

Partial discharge definition

For example, the IEC 61558-1 defines a partial discharge test as mandatory for applications which conform to basic or reinforced insulation, if the working voltage is greater than 750V.

According to IEC 60270:2000+AMD1:2015 CSV “a partial discharge (PD) is a localized electrical discharge that only partially bridges the insulation between conductors”.

When does a partial discharge occur?

  • When an isolation barrier of a transformer has a defect, such as an internal void, localized ionization will occur.
  • This ionization combined with a sufficiently high electrical stress produces partial discharges.
  • The partial discharge is expressed in pC.

PD Test


The Partial Discharge Test verifies if the charge on the stray capacitances is less than 10pC during the measurement period.


Partial discharge test procedure

The following graph shows how the partial discharge test is normally performed.

First step

  • First, we must define the working voltage of the transformer. If for example the working voltage of the transformer is 600Vrms the voltage that is applied between the primary and secondary windings is linearly increased to the value of 1.875 times the working voltage. In this example, it would be 1125Vrms volts. Then it is held for a maximum time of 5 seconds.

Second step

  • The voltage is then linearly decreased to 1.5 times the working voltage. In this example, it would be 900Vrms. It is held for a maximum of 15 seconds at the extension voltage. During this period, the partial discharge is measured in pico-coulombs.

Partial Discharge

  • The device under test passes the partial discharge test when the charge on the stray capacitance during the testing period is less than 10 pico-coulombs.


Partial discharge test equipment

The test equipment that is used for the partial discharge test consists of a high voltage test chamber, a partial discharge tester and a computer with the measurement software for viewing the measurements.

PD test equipment is an important tool for maintaining the reliability and safety of transformers in industrial applications.

Partial discharge benefits

  • Market trend: Working Voltage increase → Partial Discharge test becomes more important.
  • Partial Discharge Test gives reliability information of the isolation over component lifetime.
  • High voltage results in partial discharge test ensure high quality of the insolation.
  • PRAX experienced R&D team can test Partial Discharge up to 15kV to assure the reliability of the transformer design.
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