The present-day electrical power system is a.c. i.e. electric power is generated, transmitted, and distributed in the form of alternating current. The electric power is produced at the power stations which are located in favorable places, generally quite away from the consumers. It is delivered to the consumers through a large network of transmission and distribution.
The three types of substations that we are going to discuss would be:
1. Transformer Sub-Stations
Most of the sub-stations in the electrical power system are concerned with the changing of the voltage level of the electric supply. These types of substations are known as transformer sub-stations. It is because for changing the voltage level the transformer is the main equipment used in the power system. The transformer sub-stations may be classified into:
- (i) Step-up sub-station
- (ii) Primary grid sub-station
- (iii) Secondary sub-station
- (iv) Distribution sub-station
(i) Step-up sub-station
The generation voltage (11 kV in this case) is increased or stepped up to high voltage (220 kV) in order to meet the economy in the transmission of electric power. The sub-stations which accomplish this job are called step-up sub-stations.
(ii) Primary grid sub-station
From the step-up substation, electric power at 220 kV is transmitted by an overhead transmission system to the outskirts of the city. So these 220kv electric power is received by the primary grid sub-station which reduces the voltage level to 66 kV for secondary transmission. The primary grid sub-station is generally of the outdoor type.
(iii) Secondary sub-station
From the primary grid sub-station, electric power is transmitted at 66 kV by 3-phase, 3-wire system to various secondary sub-stations located at the strategic points in the city.
In a secondary substation, the voltage is further stepped down to 11 kV. The 11 kV lines run along the important roadsides of the city. It may be noted that big consumers (having a demand of more than 50 kW) are generally supplied power at 11 kV for further handling with their own sub-stations. The secondary sub-stations are also generally of the outdoor type.
(iv) Distribution sub-station
The electric power from 11 kV lines is delivered to distribution sub-stations. These sub-stations are located near the consumer’s localities and step down the voltage to 400 V, 3-phase, 4-wire for supplying to the consumers. The voltage between any two phases is 400V and between any phase and neutral, it is 230 V. The single-phase residential lighting load is connected between any one phase and neutral whereas 3-phase, 400V motor load is connected across 3-phase lines directly.
2. Pole-Mounted Sub-Station
It is a distribution sub-station placed overhead on a pole. It is the cheapest form of sub-station as it does not involve any building work. Fig (i) above shows the layout of the pole-mounted sub-station whereas Fig. (ii) shows the schematic connections. The transformer and other equipment are mounted on an H-type pole (or 4-pole structure). The 11 kV line is connected to the transformer (11kV / 400 V) through a gang isolator and fuses.
The lightning arresters are installed on the H.T. side to protect the substation from lightning strokes. The transformer steps down the voltage to 400V, 3-phase, 4-wire supply. The voltage between any two lines is 400V whereas the voltage between any line and neutral is 230 V.
The oil circuit breaker (O.C.B.) installed on the L.T. side automatically isolates the transformer from the consumers in the event of any fault. The pole-mounted sub-stations are generally used for transformer capacity up to *200 kVA.
3. Underground Sub-Station
In thickly populated cities, there is a scarcity of land, as well as the prices of land, are very high. This has led to the development of an underground sub-station. In such sub-stations, the equipment is placed underground. Fig. above shows a typical underground sub-station. The design of the underground sub-station requires more careful consideration than other types of sub-stations.
While laying out an underground sub-station, the following points must be kept in view:
- (i) The size of the station should be as minimum as possible.
- (ii) There should be reasonable access for both equipment and personnel.
- (iii) There should be provision for emergency lighting and protection against fire.
- (iv) There should be good ventilation.
- (v) There should be provision for remote indication of excessive rise in temperature so that the H.V. supply can be disconnected.
- (vi) The transformers, switches, and fuses should be air-cooled to avoid bringing oil into the premises.
