It is defined as the transmission and distribution network which receives electrical energy at a higher voltage and transfer to a consumer at low voltage.

Fig. Electrical Power Substation

The points that should be kept in mind while setting Substations

  1. Substation should be located at a proper site as far as possible.
  2. Substation should be located at the center of gravity of the load.
  3. Substation should provide a safe and reliable arrangement for safety consideration must be given to the maintenance of regulatory clearance.
  4. Substation should be easily operated at maintained it should involve minimum capital cost.

Functions of Electrical Power substation

  1. supply electric power to the consumers continuously.
  2. supply of electric power within specified voltage limits and frequency limits.
  3. shortest possible fault duration.
  4. optimum efficiency of plants and the network.
  5. supply of electrical energy to the consumers at lowest cost.

Classification of Substations

The electrical power substation can be classified as follows

  1. On the basis of type of supply
  2. On the basis of service rendered
  3. On the basis of substation design/installation of equipment or switch-gears
  4. On the basis of design configuration
  5. On the basis of importance
  6. On the basis of operating voltage

1. On the basis of type of supply

  1. AC Substation
  2. DC Substation

2. On the basis of service rendered

a). Transformer Substation

Transformers are installed on such substation to transform the power from one voltage level to other voltage levels.

b). Switching Substation

Switching Substations are meant for switching operation of power lines without transferring the voltages.

c). Converting Substation

AC to DC conversion is required. In HVDC transmission, converting substations are employed on both sides of HVDC link for converting AC to DC and again converting back from DC to AC. Converting power substations are also employed where a frequency is to be converted from higher to lower and vice-versa. This type of frequency conversion is required in connecting to the grid system.

3. On the basis of substation design/installation of equipment or switch-gears

a). Outdoor Substation

The various electrical equipments are installed in the switchyard below the sky. Electrical equipments are mounted on support structures to obtain sufficient ground clearance.

b). Indoor Substation

The apparatus is installed within the substation building. Such substations are usually for the rating of 66kV. Indoor substations are preferred in heavily polluted areas and power substation situated near the seas (saline atmosphere causes insulator failures results in flashover).

4. On the basis of design configuration

  1. Air insulated electrical power substation
  2. Gas insulated electrical power substation
  3. Hybrid electrical power substation

5. On the basis of importance

a). Grid substations

Grid substations are the stations from where bulk power is transmitted from one point to another point in the grid.

b). Town substations

Town substations stepped down the 33/11KV voltage and further distribute electrical power in the towns.Any fault or failure in such substation results in failure of supply for the whole of the town.

6. On the basis of operating voltage

a). High Voltage Substations

Voltage between 11 kV and 66 kV.

b). Extra high voltage Substations

Voltage between 132 kV and 400 kV

c). Ultra-high-voltage Substations

Voltage above 400 kV.


Q. State any four types of substations according to the service?

Ans. The substations, according to the service rendered are

Transformer substations, Switching substations, Converting substations and Frequency changing substations.

Q. State the relative merits of indoor and outdoor substations.

Ans. Merits of Indoor Substations

Less requirement of space, less maintenance and control cable length; protection from lightning; flexibility in installation, no dust and dirt.

Merits of Outdoor Substations

No building requirement, short erection time; Easy fault finding due to visibility of equipment; Easy repair work; Easier installation/extension; Availability of sufficient space between equipments.

Q. What arc the factors governing the selection of site for substations?

Ans. Factors governing the selection of site for a substation are:

1. Nearness to load centre
2. Availability of suitable and sufficient land
3. Availability of communication facilities
4. Drainage facilities
5. Availability of essential amenities to the staff.
6. Pollution free atmosphere
7. Facility for future extension
8. Distance from airport, military or police rifle training centre.

Q. List out the equipment used in a substation.

Ans. Equipment used in a substation are

Transformer(s); Bus-bars; Circuit breakers; CTs; PTs; Insulators; Isolators; Switchgear (fuses, air-break switches etc.), Protective relays; Surge arresters; Cables, Fire-fighting equipment; Batteries; Earthing arrangement, Wave traps, Reactors etc.

Q. What are the functions of panels and fire-fighting equipment in a substation?

Ans. Panels or cubicles in a substation are provided to support the various equipment like meters, relays etc. It saves the horizontal space and facilitates the supervisory staff to record readings easily. All the panels/cubicles are properly grounded.

The special type of fire extinguishers is installed in a substation to quench the electric fire in case of a fire accident because ordinary fire extinguishers or water cannot be used to quench electric fire as it is dangerous. They are provided in an accessible position and everyone in the substation must know its operation.

Q. What is a flexible bus?

Ans. The flexible bus consists of flexible ACSR or all aluminium alloy stranded conductors supported by strain insulators from each end.

Q. Name the interlockings provided with isolators.

Ans. The interlocking provided with isolators are:

1. Interlocking between three poles for simultaneous operation.
2. Interlocking with circuit breakers.

Q. In what way is an isolator different from an air-break switch?

Ans. Isolators are used to connect or disconnect the line only under no-load condition while air-break switches are used for making and breaking of the circuit under loaded condition. Air-break switches are provided with arc control devices.

Q. Why are isolators necessarily provided on the supply side of the circuit breakers?

Ans. Isolators are necessarily provided on the supply side of the circuit breaker in order to ensure isolation of the circuit breaker from the live parts for the purpose of maintenance.

Q. Give the sequence of operation during opening and closing of a circuit.

Ans. The sequence of operation of the circuit breaker, isolator and earthing switch during opening/closing of a circuit is given below.

While Opening: Open circuit breaker, open isolator and then close earthing switch if provided.
While Closing: Open earthing switch, close isolator and then close circuit breaker.

Electrical Power Substations

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