Partial discharge (PD) is high frequency electrical discharge that temporarily connects a small portion of insulation between two electrodes. This is due to the electric field strength overpowering the breakdown strength of that portion of insulation. PD occurs in insulation gaps caused by manufacturing or installation errors, aging or overstressing, physical damage etc. It leads to progressive deterioration and eventual failure of insulation.
A gap in insulation can be modeled as a capacitor. The impedance that a capacitor contributes to an electric circuit can be calculated using the following equation.
It can be observed from the above equation that as the frequency of the electric signal going through the capacitor increases, the lower the impedance becomes.
Partial discharge produces electromagnetic emissions (radio waves), light and heat, sound in the audio and ultrasonic frequency ranges as well as ozone and nitrous oxide gases. These parameters are detected by sensors and used as indicators of PD activity.
A high voltage power supply is required to test insulation. This supply must have low background noise and introduce a very low amount of PD in order to prevent interference with partial discharge sensors. An example of a PD test system is shown in figure 1. The elements include a PD free linear power supply, a coupling capacitor, a unit under test (UUT), an impedance matching network, a PD detector, and connections free of partial discharge.
Amp-Line Corp.’s (ALC) linear power sources are rated from 100 W – 1500 W with low noise interference (about 0.2-0.3 pC of PD). They can be used to drive external high voltage output transformers to achieve clean and regulated high voltage AC signals. They are perfect solutions for partial discharge test systems that require high voltage and have low tolerance for noise and PD interference.
