The assembly of apparatus used to change some characteristic (e.g. voltage, a.c. to d.c., frequency, p.f. etc.) of electric supply is called a Electrical substation.
Need / Functions of a Electrical substation
An electricity supply undertaking generally aims at the following:
Supply of required electrical power to all the consumers continuously at all times.
Maximum possible coverage of the supply network over the given geographical area.
Maximum security of supply.
Shortest possible fault duration.
Optimum efficiency of plants and the network.
Supply of electrical power within targeted frequency limits.
Supply of electrical power within specified voltage limits.
The tasks associated with a major substation in the transmission system include the following:
Controlling the exchange of energy.
Protection of transmission system.
Ensuring steady state and transient stability.
Load shedding and prevention of loss of synchronism.
Maintaining the system frequency within targeted limits.
Voltage control, reducing the reactive power flow by compensation of reactive power, tap-changing.
Securing the supply by providing adequate line capacity and facility for changing the transmission paths.
Data transmission via power line carrier for the purpose of network monitoring, control and protection.
Determining the energy transfer through transmission lines and tie-lines.
Fault analysis and pin-pointing the cause and subsequent improvements.
Securing supply by feeding the network at various points. All these tasks are performed by the team
work of load-control centre and control rooms of substations. The substations perform several important tasks and are integral part of the power system.
Factors governing the selection of the site of the substation
Sub-stations are important part of power system. The continuity of supply depends to a considerable extent upon the successful operation of sub-stations. It is, therefore, essential to exercise utmost care while designing and building a sub-station.
The following are the important points which must be kept in view while laying out a sub-station:
Land: choose areas that minimize the need for earth movement and soil disposal.
Water: avoid interference with the natural drainage network.
Vegetation: choose low productivity fanning areas or uncultivated land.
Protected areas: avoid any areas or spots listed as protected areas.
Community planning: avoid urban areas, development land, or land held in reserve for future development.
Community involvement: engage community in the approval process.
Topography: flat but not prone to flood or water stagnation.
Soil: suitable for construction of roads and foundations; low soil resistivity is desirable.
Access: easy access to and from the site for transportation of large equipment, operators, and maintenance
teams.
Line entries: establishment of line corridors (alternatives: multi-circuit pylons, UG lines).
Pollution: risk of equipment failure and maintenance costs increase with pollution level.
Classification of substation
The substations, according to operating voltage, may be categorised as
High Voltage Substations (HV Substations) – Involving voltages between 11 KV and 66 KV. Extra High Voltage Substations – Involving voltages between 132 kV and 400 KV. Ultra High Voltage – Operating voltage above 400 KV
Classification based on type of voltage
A.C. Substation : EHV, HV, MV, LV; HVDC Substation. DC Substation
Classification based on indoor/outdoor:
Outdoor substation is under open sky. Indoor substation is inside a building.
Classification of substation based on configuration:
- Conventional Air insulated outdoor substation or,
- SF6 gas insulated substation (GIS),
- Composite substation having combination of the two
Classification of Substations Based on Construction:
- Indoor Substation. 2. Outdoor Substation. 3. Gas insulated Substation. 4. Underground Substation. 5. Pole mounted substation. 6. Plinth mounted Substation. 7. Compact/prefabricated substation.
Classification of substation based on Application/Service requirements:
(i) Transformer sub-stations. (ii) Switching sub-stations. (iii) Power factor correction sub-stations. (iv) Frequency changer sub-stations. utilisation. (v) Converting sub-stations.