Module 4 · Anatomy, land and a knowledge check

Take a GIS bay apart, then test yourself

Pick any numbered component to see what it does, its typical data at the selected voltage and what to watch for in service. Explode the bay to see where the gas-tight joints are, compare the land a GIS saves against an air-insulated yard, and finish with ten questions.

Line bay elevation · 220 kV · single-phase enclosure

3

SF6 circuit breaker

The only device in the bay that can interrupt fault current. Its interrupter sits in its own compartment at the highest gas density, about 6 bar, with an adsorber to trap decomposition products.

Breaking current: 50 kA for 1 s
Break time: 60 ms (3 cycles)
Interrupter: Puffer or self-blast, one break per pole

Watch for: Count operations and cumulative interrupted current (ΣI²t): contact and nozzle wear, not age, decides the overhaul.

The land a GIS gives back

A nine-bay double-busbar switchyard at 220 kV, drawn to scale. The illustrative areas include the GIS building and its gas-handling aisle.

AIS yard: 32,000 m²
GIS: 3,200 m²
GIS share: 10 %
Land saved: ≈ 4.0 football pitches

GIS against AIS, point by point

Aspect	Gas-insulated (GIS)	Air-insulated (AIS)
Insulation	SF6 at 4–6 bar inside earthed enclosures	Atmospheric air across open clearances
Land	About 10–15 % of an equivalent AIS yard	The baseline
Coastal salt and desert dust	Live parts sealed and unaffected	Insulator washing, flashover risk
Maintenance	Inspection roughly every 8–10 years, overhaul at 20–25	Frequent: isolator contacts, insulator cleaning
Safety	Every live part enclosed and earthed	Exposed live parts behind safety clearances
Erection	Factory-tested modules, weeks on site	Long civil and erection work
Cost	Higher equipment cost, far lower land cost	Lower equipment cost, high land cost in cities
Repair outage	Longer: gas handling and module transport	Shorter: parts are accessible
Seismic (Kutch is zone V)	Low and compact, good performance	Tall porcelain is more vulnerable
Environment	SF6 inventory to manage — GWP about 23 500	No gas insulation to manage

Knowledge check

Ten questions drawn from the four modules. Each answer comes with the reason.

Compared with air at the same pressure, the dielectric strength of SF6 is roughly…
- About the same
- About 2.5 to 3 times higher
- About 10 times higher
- Lower, but it cools arcs better
Why is each compartment supervised by a density monitor rather than a plain pressure switch?
- Density monitors are cheaper
- Pressure varies with temperature, but insulation strength depends on the gas density
- Pressure switches cannot be used with SF6
- Density monitors also measure moisture
In a puffer interrupter, where does the gas blast come from?
- A separate high-pressure reservoir
- The arc heating the gas only
- The moving cylinder compressing gas against a fixed piston as the breaker opens
- A compressor in the drive cabinet
Why is 89A refused while 52 is closed and carrying load (no bus transfer in progress)?
- 89A is too slow
- A disconnector has no arc-quenching means and cannot break load current
- It would overload the CT
- The VT would lose voltage
What makes the line-side fast-acting earth switch different from a maintenance earth switch?
- It is manually operated
- It has short-circuit making capability and can switch induced currents
- It is only used during commissioning
- It earths the busbar
At what instant does an AC breaker actually interrupt the current?
- The moment the main contacts part
- The moment the trip coil is energised
- At a natural current zero
- When the puffer pressure peaks
IEC 62271-203 limits the relative SF6 leakage rate of a GIS compartment to…
- 0.5 % per year
- 5 % per year
- 0.5 % per month
- 1 % per day
A UHF PD pattern with pulses at every phase angle, erratic amplitude and gaps in time most likely indicates…
- A void in a spacer
- A floating electrode
- A free metallic particle
- Corona from a protrusion
What is the job of a barrier (gas-tight) insulator?
- To measure gas density
- To divide the switchgear into separate gas compartments while supporting the conductor
- To vent gas during an internal arc
- To connect the enclosure to earth
Disconnector operations in GIS are known for producing…
- Very fast transient overvoltages with nanosecond rise times
- Harmonics at the 5th order
- Sustained power-frequency overvoltage
- DC offset in the CT

Question 1 of 10

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Compared with air at the same pressure, the dielectric strength of SF6 is roughly…