Power transformers

When classified depending on the voltage levels in both sides of a transformer:

• step-up type (used in voltage increase from alternator-> transmission line)
• step-down type (used in voltage decrease from transmission line->distribution)

Classified depending on the insulation medium:

• mineral oil-filled type
• dry-type

Circuit Breakers :
These are normally classified according to the arc-quenching medium around the contacts:

• Air circuit breakers; Air Blast circuit breakers (ACB)
• Vacuum circuit breakers (VCB)
• Oil circuit breakers (OCB)
• Gas circuit breakers (eg: SF6 breakers)

Isolators:
These are mechanical devices used to open an electrical path. This is particularly vital as a visual indication of isolating high voltage components, which is not provided by a CB.

Current transformers:
A type of transformer used to reduce the magnitude of the flowing current in a conductor, so that current can be handled safely for measurement & instrumentation.

Primary side is the current measured and secondary side will have the reduced current. Reduction in magnitude will be determined by the turns ratio (e.g.: 400/5, 1000/5, 2000/5 etc. ).

110 kV High-voltage Current transformer in a grid substation

It should be noted that the secondary side of a CT is NEVER open-circuited. This is to avoid the dangerous high voltage present in the secondary side of the CT.
for e.g. :
If a 400A/5A CT is used in a transmission line rated at 132kV & 400A, secondary voltage will become 132*400/5 kV (= 10,560 kV) – if it is left open circuited.

Potential transformers

A type of transformer used to reduce the magnitude of the voltage in a conductor, so that voltage can be handled safely for measurement & instrumentation.

Like Ct's, reduction in voltage will be determined by the turns ratio of a PT.

Bus bars:
These are normally made of hollow Copper/Aluminium rods. The reason is to account for the high current flow so that normal cables would be unable to withstand the electrical stress produced.

Busbars as seen in a GSS

Surge restorers/ Lightning arrest or:
These are devices made for the protection of a power system arising from dangerous surges. These surges (high voltage impulses of shorter duration) are either from lightning or load switching.

These SA’s work allow the normal power frequency waves (50 / 60 Hz) but yield a grounding to surges (have very high frequency in the order of 10000 Hz).

Gas Insulated Sub-Stations (GIS):
A modern development is to make grid substation indoors. introduction of SF6 gas as insulation medium with very desirable characteristics. 

A gas-insulated indoor substation by ABB Inc

These type of substations occupy little space compared to the conventional outdoor stations, making them useful in densely populated cities or harshly-polluted areas.