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Substation Main Functions and Classication


30 March 2012

Primary distribution - Alstom 145kV Gas-insulated switchgear - GIS (F35) at the Shuqaiq HV substation in Saudi Arabia

An electrical substation is an assemblage of electrical components including busbars, switchgear, power

transformers, auxiliaries etc. These components are connected in a denite sequence such that a ckt. can be
switched o during normal operation by manual command and also automatically during abnormal conditions such
as short-ckt.
Basically an electrical substation consists of number of incoming ckt. and outgoing ckt. connected to a common Busbar systems. A substation receives electrical power from generating station via incoming transmission lines and
delivers elect. power via the outgoing transmission lines.
Substation is integral part of a power system and form important links between the generating station,
transmission systems, distribution systems and the load points.

Main tasks of major sub-stations in the T&D

Main tasks associated with major sub-stations in the transmission and ditribution system include following:
1. Protection of transmission system.
2. Controlling the Exchange of Energy.


2. Controlling the Exchange of Energy.

3. Ensure steady State & Transient stability.
5. Voltage Control; reducing the reactive power ow by compensation of reactive power, tap-changing.
6. Securing the supply by proving adequate line capacity.
8. Fault analysis and pin-pointing the cause and subsequent improvement in that area of eld.
9. Determining the energy transfer through transmission lines.
10. Reliable supply by feeding the network at various points.
11. Establishment of economic load distribution and several associated functions.

Types of substation
The substations can be classied in several ways including the following:

1 Classication based on voltage levels


2 Classication based on Outdoor or Indoor

Outdoor substation is under open skv. Indoor substation is inside a building.

3 Classication based on conguration

Conventional Air insulated outdoor substation or
SF6 Gas Insulated Substation (GIS)
Composite substations having combination of the above two

4 Classication based on application

Step Up Substation Associated with generating station as the generating voltage is low.
Primary Grid Substation Created at suitable load centre along primary transmission lines.
Secondary Substation Along secondary transmission line.
Distribution Substation Created where the transmission line voltage is step down to supply voltage.
Bulk supply and industrial substation Similar to distribution sub-station but created separately for each
Mining Substation Needs special design consideration because of extra precaution for safety needed in the2/5

operation of electric supply.
Mobile Substation Temporary requirement.

Important Notes:
consumers. These are also referred to EHV Substations.
purposes, normally at 415 volts.

Substation parts and equipment

Each sub-station has the following parts and equipment:

1 Outdoor Switchyard
Incoming Lines
Outgoing Lines
Bus post insulator & string insulators
Substation Equipment such as circuit-beakers, isolators, earthing switches, surge arresters, CTs, VTs,
neutral grounding equipment.
Overhead earthwire shielding against lightening strokes.
Galvanised steel structures for towers, gantries, equipment supports.
Power cables
Control cables for protection and control
Roads, Railway track, cable trenches
Station illumination system

2 Main Oce Building

Administrative building
Conference room etc.

3 Switchgear and Control Panel Building


Low voltage a.c. Switchgear

Control Panels, Protection Panels

4 Battery Room and D.C. Distribution System

D.C. Battery system and charging equipment
D.C. distribution system

5 Mechanical, Electrical and Other Auxiliaries

Fire ghting system
Diesel Generator (D.G.) Set
Oil purication system

Secondary Distribution

Secondary substation - Vilalonga VLG 66/20 kV

Distribution Substation
