What is Partial Discharge

Focus Partial Discharge

Partial Discharge (PD) , as defined by IEC 60270 ,is an electrical discharge that does not completely bridge the space between two conducting electrodes. The discharge may be in a gas filled void in a solid insulating material, in a gas bubble in a liquid insulator, or around an electrode in a gas. When partial discharge occurs in a gas, it is usually known as corona. Partial discharge is generally accepted as the predominant cause of long term degradation and eventual failure of electrical insulation. As a result, its measurement is standard procedure in the factory testing of many types of high voltage equipment. In addition, partial discharge activity can be tested for or monitored on in-service equipment to warn against pending insulation failure.




Measuring partial discharge in GIS

As customers increasingly push to adopt condition-based maintenance for Gas-Insulated Substations (GIS), new opportunities are arising for periodic or permanent measurement of partial discharge. Traditionally, high voltage substations are air insulated, but the clearances required between phases were huge. This results in rather large installations, making them difficult to house in urban environments where space is at a premium. To overcome this constraint, a parallel technology was developed, the Gas-Insulated Substation (GIS), using a gas, for example sulphur hexafluoride (SF6), at high pressure. SF6 has excellent dielectric properties and is used as the insulating medium between the phases and earth.

As a consequence, a GIS is much more compact. In fact, gas-insulated substations can be down to one-tenth the size of their air-insulated cousins, depending on the voltage level. The use of gas insulation in the power system network has developed rapidly thanks to its compact nature, low maintenance requirement and reliable operation. But the reliability of the GIS equipment can be undermined by the presence of free particles that originate mainly from the mechanical vibrations, from moving parts in the system such as breakers or disconnectors, or even from the manufacturing process.

They are rare, but can locally generate high electric fields exceeding the structure’s design limits and initiate partial discharges (PD) forming free electrons and ions in the insulation. Repeated partial discharges are capable of triggering a progressive carbonisation of spacers that can slowly build up over the years until they produce a flashover, or failure of the switchgear insulation structure resulting in the entire installation, or parts of it, being shut down.

Measuring partial discharges

When partial discharges occur (resulting in voltage drops of less than a nanosecond), they generate electromagnetic waves that propagate through the switchgear. These waves can be measured by means of different technologies operating in a variety of frequency ranges. Detecting partial discharges in lower frequency ranges can be carried out by taking measurements with acoustic sensors. In the medium frequency range, between a few kHz and a few MHz, measurements are usually made by means of a coupling capacitor. The disadvantage of using this device is that it is large and not suitable for online monitoring. However, partial discharges in pressurised gas can be measured in the Ultra High Frequency (UHF) range between 100 MHz and 2 GHz. The added advantage here is that this allows the whole substation to be permanently monitored and the location of PD activity can also be pinpointed. Demand for this level of monitoring is particularly high in the Middle East, though less pronounced in Europe, where utilities are more hesitant to make the additional outlay required.

Benefits of On-Line Partial Discharge Measurements

It is truly a predictive test, indicating insulation degradation in advance of the failure. It is a unobstructed test, requiring no interruption of service and is performed under normal operating voltage, load and environmental conditions. it does not test to failure or adversely affect the equipment under test in any way. It does not use any overvoltage, thereby not exposing the tested equipment to higher voltage stresses than those encountered under normal operating conditions.

Trending can be accomplished by storing baseline measurement results to allow comparison with future tests. In many instances the site of the partial discharge occurrence can be located within the cable/plant under test, so the localised problem can be repaired. compared with off-line testing, allowing annual surveys to be performed economically at most facilities.

[Reference] IEC 60270:2000/BS EN 60270:2001 “High-Voltage Test Techniques – Partial Discharge Measurements” IEC 61934:2006 “Electrical insulating materials and systems – Electrical measurement of PD under short rise time and repetitive voltage impulses” IEC 60664-4:2007 “Insulation coordination for equipment within low-voltage systems – Part 4: Consideration of high-frequency voltage stress” IEC 60034-27:2007 “Rotating electrical machines – Off-line partial discharge measurements on the stator winding insulation of rotating electrical machin es” IEEE Std 436™-1991 (R2007) “IEEE Guide for Making Corona (Partial Discharge) Measurements on Electronics Transformers” IEEE 1434–2000 “IEEE Trial-Use Guide to the Measurement of Partial Discharges in Rotating Machinery” IEEE 400-2001 “IEEE Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable Systems”