Air has a dielectric strength of 33kV/cm. SF6 gas, on the other hand, has a dielectric strength of ~83kV/cm. Naturally, when the live parts are immersed in a pressurized gas chamber they can be placed a lot closer.
An inductor prevents sudden change in current magnitude. It is installed in substations to:
Capacitors consume active power and release reactive power. They also present a low impedance to harmonics in other words they attract harmonic frequencies. Thus, capacitor banks are installed in substations to:
Insulators make sure the energized parts do not touch the ground. All substation equipment in contact with an energized line or bus employs an insulator.
Relays need to know the current magnitude – either for metering or protection. A current transformer (CT) fulfills this role, stepping down thousands of amps to (typically) 5A which is then fed to a relay.
