High voltage earthing switch general description:
Earthing switches shall be designed and rated in accordance with the operation requirements under normal and abnormal operation conditions, the environmental conditions and the performance requirements. In general, the DC disconnector and earthing switches shall be similar to the AC disconnector and earthing switches having the equivalent continuous current rating and basic insulation level.
This earthing switch designed for no-load switching and are able to connect or disconnect low currents in HVDC systems as HVAC systems. Every earthing switch must be capable of conducting its rated short-time withstand current without damage.
Earthing switches can be combined with the adjacent disconnecting switches to form one unit or can also be installed separately.
Filter Earthing Switch(FES) IN HVDC grids:
The main function of the Filter Earthing Switch(FES) IN HVDC grids is to earth the filters on the DC side, e.g. for maintenance, when DC filters are present.
The position of Filter Earthing Switches show in figure No:1 in red circles:
Filter Earthing Switch operation sequence in HVDC grids:
In normal operation the earthing switches are open. Operation of the Filter Earthing Switch (HV) (Earthing switch on the HV pole side) requires an open disconnecting switch FD (HV) (Filter disconnecting switch on HV pole side) and a closed disconnecting switch FD (NB) (Filter disconnecting switch on NB side). The earthing switch Filter Earthing Switch (NB) (Earthing switch on NB side) can then be closed after both disconnecting switches FD (HV) and FD (NB) have been opened. Interlocking needs to be provided to ensure that the high-voltage filter capacitor is discharged before closing of the Filter Earthing Switches.
Filter Earthing Switch specifications:
➧There are no requirements of the Filter Earthing Switch for breaking capacity.
➧The Filter Earthing Switches must be capable of earthing a partly discharged filter connected through the disconnecting switch FD (NB) to the neutral bus.
➧There are no requirements of the earthing switch FES (NB) in respect of making capacity. Conventional earthing switches can be used in most cases.
➧Maintenance of FES can be performed after disconnection of filter.
➧The DC withstand voltage across open contacts and to earth should be the same as for other equipment connected to the pole line and the neutral bus.
Also Filter Earthing Switchs (FES) have no normal current in normal condition except very low earth leakage current, they have making capacity as filter trap charge current amount in HVDC grid.
The most of Filter Earthing Switch class are E0 according to IEC standard.
We can see a kind of HVDC earthing switch in figure No:2 as below made by Coelme-Egic company:
Figure No:2 (The distance to ground potential (“G”) can reach values of 9000 mm and even higher through specific design).