Engineer's Notes: primary substations and GIS
Security is designed in
The busbar scheme decides what happens when something fails. A single busbar loses everything for a busbar fault; a double busbar loses circuits only for the minutes it takes to move them; breaker-and-a-half loses nothing. Pick the scheme from the consequence of an outage.
GIS where space and climate demand it
In GCC cities and dense Asian centres, GIS indoors is the norm: small, enclosed and protected from sand, salt and heat. Outside the cities, AIS stays common because land is cheap and repairs are quick.
SF6 needs stewardship
SF6 is an excellent insulating gas and the most potent greenhouse gas known. Density monitoring, careful handling and records keep leaks small, and SF6-free switchgear is now available for many applications.
Plan capacity ahead
Firm capacity, not installed capacity, is what customers can rely on. Transformer and switchgear lead times of two years or more mean the next stage must be planned well before demand reaches the limit.
Frequently asked questions
What is the difference between GIS and AIS?
Air-insulated switchgear (AIS) uses open air as insulation, so it needs large clearances and outdoor space. Gas-insulated switchgear (GIS) puts the conductors inside earthed metal enclosures filled with SF6 or an alternative gas, which typically cuts the switchgear footprint by 85 to 95 percent and protects the equipment from sand, salt and pollution.
Which busbar scheme should a 132 kV substation use?
It depends on how serious it is to lose the substation. Double busbar with a coupler is common for urban 132 kV GIS because circuits can move to the healthy bus in minutes after a busbar fault. Breaker-and-a-half and ring bus give more security at transmission level, while sectionalised single bus suits smaller zone substations.
What is firm capacity?
Firm capacity is the load a substation can supply with its largest transformer out of service, usually (n minus 1) times the transformer rating, sometimes increased by a short-time emergency overload factor. Utilities plan reinforcement before the maximum demand reaches it.
Why do SF6 density relays have alarm and lockout stages?
The first stage warns that gas is leaking and needs topping up. The second stage blocks breaker operation, because below that density the insulation and breaking capability are no longer guaranteed. The relays are temperature compensated so they respond to gas density, not to daily temperature swings.
Are there alternatives to SF6 in high voltage switchgear?
Yes. Vacuum interrupters combined with clean air insulation, and gas mixtures based on fluoronitrile or fluoroketone with much lower global warming potential, are now available from several manufacturers, increasingly up to 145 kV and above.
Topics: primary substation · zone substation · bulk supply point · busbar scheme · double busbar · breaker-and-a-half · ring bus · GIS · AIS · hybrid switchgear · SF6 density relay · leakage rate · GWP · SF6-free · firm capacity · N-1 · emergency loading · OLTC · AVC relay
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