Substation Main Functions and Classification

Electrical substation

An electrical substation is an assemblage of electrical components including busbars, switchgear, power transformers, auxiliaries etc. These components are connected in a definite sequence such that a ckt. can be switched off during normal operation by manual command and also automatically during abnormal conditions such as short-circuit.

Basically an electrical substation consists of number of incoming ckt. and outgoing ckt. connected to a common Bus-bar 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.
3. Ensure steady State & Transient stability.
4. Load shedding and prevention of loss of synchronism. Maintaining the system frequency within targeted limits.
5. Voltage Control; reducing the reactive power flow by compensation of reactive power, tap-changing.
6. Securing the supply by proving adequate line capacity.
7. Data transmission via power line carrier for the purpose of network monitoring; control and protection.
8. Fault analysis and pin-pointing the cause and subsequent improvement in that area of field.
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

Classification

The substations can be classified in several ways including the following:

1 Classification based on voltage levels

e.g. : A.C. Substation : EHV, HV, MV, LV; HVDC Substation.

2 Classification based on Outdoor or Indoor

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

3 Classification based on configuration

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

4 Classification 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 consumer.
• Mining Substation – Needs special design consideration because of extra precaution for safety needed in the operation of electric supply.
• Mobile Substation – Temporary requirement.

Important Notes:

Primary Substations receive power from EHV lines at 400KV, 220KV, 132KV and transform the voltage to 66KV, 33KV or 22KV (22KV is uncommon) to suit the local requirements in respect of both load and distance of ultimate consumers. These are also referred to ‘EHV’ Substations.

Secondary Substations receive power at 66/33KV which is stepped down usually to 11KV.

Distribution Substations receive power at 11KV, 6.6 KV and step down to a volt suitable for LV distribution 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
• Busbar
• Transformers
• Bus post insulator & string insulators
• Substation Equipment such as circuit-beakers, isolators, earthing switches, surge arresters, CTs, VTs, neutral grounding equipment.
• Station Earthing system comprising ground mat, risers, auxiliary mat, earthing strips, earthing spikes & earth electrodes.
• Overhead earthwire shielding against lightening strokes.
• Galvanised steel structures for towers, gantries, equipment supports.
• PLCC equipment including line trap, tuning unit, coupling capacitor, etc.
• Power cables
• Control cables for protection and control
• Roads, Railway track, cable trenches
• Station illumination system

2 Main Office 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 fighting system
• Diesel Generator (D.G.) Set
• Oil purification system

Secondary Distribution

Distribution Substation