A substation is a high-voltage electric facility. It is used to switch generators,

equipment and circuits or lines in and out of system.

A Substation is also used to change AC voltages from one level to another, or

change alternating current to direct current or direct current to alternating

current. There are small substations and large one. The small substations are

little more than a transformer and relate switches, while the large one has

several transformers, dozens of switches and equipment.

Substation Transformer Types 

• Step-Up Transmission Substations


• Step-Down Transmission Substation


• Distribution Substation


• Underground Distribution Substation

Step-Up Transmission Substation:

A step-up transmission substation receives electric power from a nearby

generating facility and uses a large power transformer to increase the voltage

for transmission to distant locations. A transmission bus is used to distribute

electric power to one or more transmission lines. There can also be a tap on

the incoming power feed from the generation plant to provide electric power to

operate equipment in the generation plant.

The typical voltage are:

• High voltage (HV) ac:                           69kV, 115kV, 138kV, 161kV, 230kv


• Extra-high voltage (EHV) ac:                345kV, 500 kV, 765 kV


• Ulta-high voltage (UHV) ac:                  1100 kV, 1500 kV


• Direct-current high voltage (dc HV):      +250kV, +400kV, +500 kV

Step-Down Transmission Substation:

Step-down transmission substations are located at switching points in an

electrical grid. They connect different parts of a grid and are a source for

subtransmission lines or distribution lines. The step-down substation can

change the transmission voltage to a subtransmission voltage, usually 69 kV.

The subtransmission voltage lines can then serve as a source to distribution

substations. Sometimes, power is tapped from the subtransmission line for use

in an industrial facility along the way. Otherwise, the power goes to a

distribution substation.

Distribution Substations:

Distribution substations are located near to the end users. Distribution

substation transformers changes voltage to lower levels for use by the end

users. Typical distribution voltages vary from 34,500Y/19,920 volts to

4,160Y/2400 volts.

34,500Y/19,920 volts is interpreted as a three-phase circuit with a grounded

neutral source. This would have three high-voltage conductors or wires and

one grounded neutral conductor, a total of four wires. The voltage between

the three phase conductors or wires would be 34,500 volts and the voltage

between one phase conductor and the neutral ground would be 19,920 volts.

From here the power is distributed to industrial commercial, and residential

customers.

Underground Distribution Substation:

Underground distribution substations are also located near to the end-users.

Distribution substation change the subtransmission voltage to lower levels for

use by end-users. Typical distribution voltages vary from 34,500Y/19,920 volts

to 4,160Y/2400 volts.

An underground system may consist of these parts:

• Conduits


• Duct Runs


• Manholes


• High-Voltage Underground Cables


• Transformer Vault


• Riser


• Transformers