Partial discharge is the process of dielectric breakdown of a small portion of a solid or a liquid electrical insulation system which is under high voltage stress. Partial discharges within an insulation system, unlike a corona discharge, may or may not reveal any visible discharges as the discharge events tend to have a more sporadic character. The effects of partial discharge within cables and other high voltage equipment should not be treated lightly as it can even lead to complete failure. Find out how to take the necessary precautions and also get information about Bushing monitoring at elec-di.com.

The partial discharge phenomenon (PD) usually commences within voids, cracks, in bubbles within liquid dielectrics or inclusions within solid dielectrics. It can also appear along the boundaries between different insulating materials. Because discharges are narrowed to only a part of the insulation they can only bridge the distance between electrodes in part. PD within insulating materials begins in the gas-filled voids of the dielectric. This is a consequence of increased voltage stress across the void and above the corona inception voltage. Once the process is triggered, the insulating materials start to deteriorate progressively. This deterioration process eventually leads to an electrical breakdown.

Partial discharge can be prevented by means of thorough design and material selection. When operating with high voltage equipment the use of PD detection equipment is mandatory. It is recommended that this equipment is used during the manufacturing stage as well. It is also important to have periodical check ups of the electrical equipment in order to avoid any unpleasant surprises. Reliable and long-term operation of high voltage equipment can only be properly assured by PD prevention and detection. Bushing monitoring is also very important where large power transformers are concerned. Problems in bushing can occur unexpectedly and easily pass undetected so thorough Bushing monitoring is the only reliable solution.

Various partial detection methods have been developed since the importance of preventing complete system failure has been fully acknowledged early in the last century. PD currents are usually of very short duration and discharges look like random pulses on an oscilloscope. PD magnitude is commonly measured in picocolumbs. One way of detecting PD is by means of calibrators. The voltage of the pulses is calibrated against the voltages obtained from a calibration unit which is discharged into the measuring instrument. The method of wideband PD detection is also widespread. With this type of detection, the coupling impedance comprises a low Q parallel-resonant RLC circuit which attenuates the existing voltage and amplifies the discharge generated voltage. Other methods include laboratory testing, narrow band detection circuits, differential discharge bridge methods, acoustic and ultrasonic methods and field testing.

Not all partial discharges are of the same gravity. Some discharges can in fact be extremely dangerous and cause a great deal of damage to the health of the insulation system. For example, the discharges within polymeric cables and accessories are particularly hazardous. However, other types of partial discharges are relatively harmless. Such an example would be the corona into the air from outdoor cable setting ends. Whenever testing for partial discharges is necessary that a professional engineer determines the exact type of discharge and its origin so that he becomes aware of the potential risks involved.

Partial discharge is an electrical discharge that can potentially cause serious problems amongst high voltage equipment. PD detecting as well as Bushing monitoring are very important for avoiding dangerous situations. At elec-di.com you will find all the information you need about PD instruments.