Gas-insulated switchgear (GIS)

Gas-insulated switchgear (GIS) encloses the HV buses, breakers, isolators, earth switches and current transformers in earthed metal enclosures filled with SF6 gas. The design places the GIS in an indoor hall, where an air-insulated switchyard (AIS) needs an open yard of bays at the level's column width.

Read first: Bus schemes · Circuit breakers and isolators

1AIS or GIS per HV yardCopy link to AIS or GIS per HV yard

The insulation is chosen per HV yard on the Define page, and AIS is the default. A switching substation's lower yard has a choice of its own, so a 400 kV GIS can sit beside a 220 kV AIS. The MV switchgear is chosen separately, indoor switchgear or an outdoor MV AIS yard (Circuit breakers and isolators, MV switchgear).

The bus schemes of AIS are available in GIS (Bus schemes). The single-line diagram (SLD) draws the same circuits inside a dashed GIS outline. With a one-and-a-half breaker scheme the exits differ in height, so the SLD marks the exits and its note names the GIS. The SLD marks the exit of each circuit as an SF6/air bushing, a gas-insulated bus duct (GIB) or a cable end.

2The GIS hallCopy link to The GIS hall

The GIS stands in an indoor hall with an electric overhead travelling (EOT) crane, SF6 gas detection and exhaust ventilation at the bottom level. An erection and handling bay lies at the west end of the hall (Figure 1).

The hall's sizes are typical values per level, and the user can enter the GIS supplier's pitch, hall width and hall height. The bay pitch is the width of one bay module along the hall.

The ANSI / IEEE set's levels take the hall of the nearest IEC level: 69 kV from 66 kV, 115 kV to 161 kV from 132 kV, 230 kV from 220 kV and 345 kV from 400 kV. The SLD of the ANSI / IEEE set adds IEC 62271-203 to its notes, which its GIS outline names.

The hall's length is the erection bay, the bay positions at the pitch, and an end clearance of 1.5 m at each end wall. A future bay keeps its position in the hall.

3Bay modulesCopy link to Bay modules

Each circuit is one bay module along the hall, and the coupler or the bus section is one more. A one-and-a-half breaker scheme has three breaker positions per diameter, and a position whose breaker is not built yet is kept as a future bay. A bay module is three-phase enclosed and holds the circuit breaker, a three-position isolator and earth switch, a fast-acting earth switch, the current transformer and the gas density monitors.

The main buses run along the hall, three-phase enclosed, at the level's bus current. A double main bus has two. A single sectionalised GIS has one main bus in two sections, so the bill of materials counts one bus the length of the hall. The bus voltage transformers are inductive units inside the GIS, one per main bus or bus section, with three cores including a 0.2 metering core (Instrument transformers, Voltage transformers).

4Exits and linksCopy link to Exits and links

The lines and the bus reactor leave the hall to the north by short GIB runs to outdoor SF6/air bushings. Beyond each bushing stand an outdoor CVT, a surge arrester and the line gantry, at the AIS heights and column width of the level. The clearances of the line exits are therefore the AIS clearances.

The ICTs are fed to the south. At 220 kV and above the link is a GIB. Below it the link is an XLPE cable from a cable sealing end on the GIS to an outdoor termination at the transformer. A surge arrester stands at each ICT.

5SF6Copy link to SF6

The design counts the SF6 of the first fill per bay, including its share of the bus, and per metre of GIB. It adds 10 % as spare gas. The bill of materials adds a gas handling cart for recovery, filtering, vacuum and refill, and a portable leak detector.

6What changes in the layoutCopy link to What changes in the layout

With GIS, the hall takes the place of the HV bus and the bay columns. The aluminium tube of the HV yard is only the interconnection at the exits, 45 m on the default project against 916 m in AIS. The outdoor breakers and isolators of AIS (4 breakers and 13 isolators on the default project) are inside the bay modules.

The fenced area falls with the HV yard. The default 220 kV project's fence is 152.0 × 130.4 m in AIS and 152.0 × 94.4 m in GIS, 72 % of the AIS area. The hall itself is a civil item of the bill of materials: a reinforced concrete frame with cladding, of the hall's floor area.

7Earthing and lightning protectionCopy link to Earthing and lightning protection

The earth mat covers the whole site as for AIS, and the IEEE Std 80-2013 calculation of the touch and step voltages stays on that mat. Under the GIS hall the design adds a denser mesh at 0.5 of the mat's spacing, 1.72 m against 3.44 m on the default project. Each bay has 2 enclosure bonding conductors to the mat. The calculation does not credit the denser mesh. Leaving it out is conservative for the touch and step voltages.

The roof of the GIS hall is a protected level of its own for the lightning protection, like the control building's. It takes the IEC 62305-3 rolling sphere of lightning protection level I, of 20 m, at the hall's height of 10.0 m. A GIS yard has no outdoor HV bus, so the HV bus level has no points to protect.

8What the GIS supplier coversCopy link to What the GIS supplier covers

The design sets the GIS hall, the bays, the links, the gas and the earthing to typical values of the level. The IEC + CEA / CBIP checklist carries the items that depend on the supplier's GIS:

• the GIS type tested to IEC 62271-203: gas compartments, density monitoring and the internal-arc rating
• SF6 handling, recovery and leakage per gas compartment within the specified limit
• the transient overvoltages in the GIS of the fastest-front class of IEC 60071-1:2019 Table 1, assessed by the supplier
• the transient enclosure voltage, which depends on the GIS and is answered from the supplier's study

The ANSI / IEEE checklist carries the transient enclosure voltage item. These items are Not applicable when no HV yard is gas-insulated. The supplier's general arrangement replaces the typical pitch, hall width and hall height on the Define page.