Transformer Substations in an Electric Power Supply System

The greater part of the substations in the power systems are focused on changing of the voltage level of the electric supply. These are termed to as transformer substations since the transformer is the key component used to change the voltage level.

Transformer substations may be classified further into the following substations depending on the purpose they serve:

• Step-up substation
• Primary grid substation
• Secondary substation
• Distribution substation

These substations are demonstrated in the block diagram of an electric supply system:

Each substation in the electric power system performs specific functions as discussed below:

Step-up Substation

The generation voltage (11 kV) is stepped up to high voltage (220 kV) to alter the cost of electric power transmission. This substation is of outdoor type.

Primary grid Substation

From the step-up substation, the electric power at 220 kV is transmitted by 3-phase, 3 wire overhead system to areas the border a city. Here, the electric power is received by the primary grid substation which reduces the voltage level to 66 kV for secondary transmission. The primary grid substation is generally of outdoor type.

Secondary Substation

The power from the primary grid substation, is transmitted at 66 kV by 3-phase, 3-wire system to various secondary substations that are located at strategic points in the city. At a secondary substation, the voltage is further dropped down to 11 kV. The 11 kV lines run along the major road sides of the city. Industrial consumers, with high demand, are generally supplied power at 11 kV for further handling with their own substations. Secondary substations are usually of outdoor type.

Also read: Key Factors to Consider In Substation Design

Distribution Substation

The electric power from 11 kV lines is delivered to distribution substations. These substations are typically located near the consumer 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 400 V and between any phase and neutral is 230 V. For example, the single phase residential lighting load is connected between any one phase and neutral while 3-phase, 400 V motor load is connected across 3-phase lines. These substations are commonly of pole-mounted type.

Related: Features and Classification of Substations