What is the role of a Substation in the Electrical Power Systems

Sub-station plays a vital role in the electrical power system. The continuity of the electrical power supply depends on the successful operation of the sub-station. Sub-station is a central location where the electrical power is supplied from the powerhouse and this electrical power is supplied through a network of transmission & distribution lines. Therefore it is essential to take care of while designing and building sub-station.

“The assembly of apparatus used to change some characteristic (voltage, ac to dc, frequency, Pf) of an electric power supply is called sub-station.

There are few points which should be kept in consideration while laying out sub-station.

1. It should be located at a proper site, as far as possible; it should be located at the center of gravity of the load.
2. It should provide a safe and reliable supply to the consumers. Also for reliability purpose, it should be designed and constructed in a proper manner.
3. It should be operate and maintained easily.
4. It should be built on economic cost.

Classification of sub-stations

There are several ways of classifying sub-stations. But here we will discuss two main categories of sub-stations.

• Service requirement

Transformer sub-stations: Those sub-stations which change the voltage level of the incoming voltage and delivers at some other voltage level to the consumers.

• Switching sub-stations: These sub-stations do not change the voltage level of incoming and outgoing lines. Simply they perform the switching operation of the power lines.
• Power factor correction sub-station:  That sub-station which improves the power factor level of the system is called power factor correction sub-station. These sub-stations generally uses synchronous condenser as the power factor improvement equipment.
• Frequency changer sub-stations: Those sub-stations which change the supply frequency are known as frequency changer sub-station. Such frequencies may be required for industrial utilization.
• Converting sub-stations: Those sub-stations which convert the Ac quantity to Dc quantity are called converting sub-stations. These sub-stations receive power as an Ac quantity and then they convert it to Dc with the help of rectifiers.
• Industrial sub-stations: Those sub-stations which provide electrical power to the individual industries are called industrial sub-stations.

Constructional features

According to constructional features, sub-stations has many components like circuit breakers, switches, fuses, instruments etc. which must be arranged in a proper manner to ensure the reliable service. Construction wise they are classified as:

• Indoor sub-station: These kinds of sub-stations are installed for 11kv voltage due to economic consideration. This is built under the shades due to the contaminated atmosphere; these sub-stations can be erected for voltage up to 66kv.
• Outdoor sub-station:  when the voltage level goes beyond 66kv, the equipment are installed outdoor because of such high voltage. The equipment and conductors require free space.
• Underground sub-stations: In congested areas where free space for the installment of sub-stations is very difficult, then the sub-stations are built underground.
  

• Pole mounted sub-stations: This kind of sub-station is installed outdoor on 4 pole structure. It is the cheapest form of sub-station for a voltage not exceeding 11kv or 33kv in some case.

  

Transformer sub-station:

Most of the power system is concerned with the voltage level changing which can be known as transformer sub-station because the transformer is the main component to change the voltage level, transformer sub-station can be classified into,

1. a) Step up sub-station               b) Primary sub-station
2. c) Secondary sub-station          d) Distribution sub-station

In the following diagram a typical electrical supply system showing the position of above types of sub-stations.

• Step up sub-station: This kind of sub-station step up the generating voltage (11kv) to high voltage (220kv) to affect the economy of the electric power system. This is known to step up sub-station, which is generally located in the powerhouses.
• Primary sub-station: The 220 kV is transmitted through 3 phase 3 wire overhead system from step sub-station to the load center. The power is then received by the primary grid sub-station which reduces the voltage level to 66kv for secondary transmission.
• Secondary sub-station: The electric power of 66kv from primary grid station is transmitted at 3 phase 3 wire system to various secondary sub-stations located at different positions. At secondary sub-station, the voltage level is reduced to 11kv, which transmission lines run along the roadside.
• Distribution sub-station: These kinds of sub-stations receives 11kv power from the secondary sub-stations and located near the localities of consumers. Here the voltage is stepped down to 400v, 3 phase 3 wires for the consumers. The voltage b/w two phases are 400v while b/w single phase and neutral is 220v.