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.
[4] Traditionally, high voltage substations are air insulated. But the clearances required between phases and between phases and earth are 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 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.
According to David Gautschi, Alstom Grid electrical engineer, “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 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.” Repairs – often involving the manufacture of specific parts – can take several weeks to complete.
