Grid Sub-Stations (GSS)

This is an arrangement of electrical conductors, towers, protective equipment, transformers etc for the operation and maintenance of a power transmission network.

Flow of equipments in a Typical Grid SubStation in USA
Image from: www.osha.gov

Most common equipments found in a GSS with few abbreviations are:

1. Power transformers
2. Circuit Breakers (CB)
3. Isolators
4. Current transformers (CT)
5. Potential transformers (PT)/ Capacitive voltage transformers (CVT)
6. Bus bars (BB)
7. Surge arrestors (SA)
8. Line trap (for PLC communication)
9. Earthing transformers
10. Auto-Recloser
11. Overhead earth wire
12. Underground earthing system

Power transformers:

When classified depending on the voltage levels in both sides of a transformer:
1. step-up type (used in voltage increase from alternator-> transmission line)
2. step-down type (used in voltage decrease from transmission line->distribution)

Hermetically sealed Mineral oil-filled transformer 
Image from: www.toppowertransformer.com

Classified depending on the insulation medium:
1. mineral oil-filled type
2. dry-type

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

SF6-filled High Voltage Gas Circuit Breaker
Image from: www.made-in-china.com

1. Air circuit breakers; Air Blast circuit breakers (ACB)
2. Vacuum circuit breakers (VCB)
3. Oil circuit breakers (OCB)
4. 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.

Air-break Isolator 
Image from: www.yashexports.tradeindia.com

These can be operated only in Off-Load condition.

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 tranformer 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 CTs, reduction in voltage will be determined by the turns ratio of a PT.

Busbars:
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 arrestors/ Lightning arrestor:
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.

Surge/lightning arrestor

Image from: www.electrical-res.com

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 charctersitics.

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.

For a deatiled study visit wikipedia.