Illustrated Glossary » Substations

A substation is a high-voltage electric system facility. It is used to switch generators, equipment, and circuits or lines in and out of a system. It also is used to change AC voltages from one level to another, and/or change alternating current to direct current or direct current to alternating current. Some substations are small with little more than a transformer and associated switches. Others are very large with several transformers and dozens of switches and other equipment. There are three aspects to substations:

Figure 1. Typical substation
Figure 1. Typical 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.

A substation can have circuit breakers that are used to switch generation and transmission circuits in and out of service as needed or for emergencies requiring shut-down of power to a circuit or redirection of power.

The specific voltages leaving a step-up transmission substation are determined by the customer needs of the utility supplying power and to the requirements of any connections to regional grids. Typical voltages are:

  • High voltage (HV) ac:
  • 69 kV, 115 kV, 138 kV, 161 kV, 230 kV
  • Extra-high voltage (EHV) ac:
  • 345 kV, 500 kV, 765 kV
  • Ultra-high voltage (UHV) ac:
  • 1100 kV, 1500 kV
  • Direct-current high voltage (dc HV):
  • ±250 kV, ±400 kV, ±500 kV

Direct current voltage is either positive or negative polarity. A DC line has two conductors, so one would be positive and the other negative.

Figure 2. Step-up AC transmission substation

Figure 2. Step-up AC transmission substation

Figure 3. Step-up transmission substation to AC transmission lines

Figure 3. Step-up transmission substation to AC transmission lines

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. Oterwise, the power goes to a distribution substation.

Figure 4. Step-down transmission substation

Figure 4. Step-down transmission substation

Figure 5. Step-down power transformer

Figure 5. Step-down power transformer

Distribution substations are located near to the end-users. Distribution substation transformers change the transmission or 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.

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.

Figure 6. Distribution substation

Figure 6. Distribution substation

Figure 7. Distribution substation

Figure 7. Distribution substation

Figure 8. Distribution substation

Figure 8. Distribution substation

Figure 9. Distribution substation

Figure 9. Distribution substation

Underground distribution substations are also located near to the end-users. Distribution substation transformers 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:

Figure 10. Underground Distribution Substation

Figure 10. Underground Distribution Substation

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

Substations are designed to accomplish the following functions, although not all substations have all these functions:

  • Change voltage from one level to another
  • Regulate voltage to compensate for system voltage changes
  • Switch transmission and distribution circuits into and out of the grid system
  • Measure electric power qualities flowing in the circuits
  • Connect communication signals to the circuits
  • Eliminate lightning and other electrical surges from the system
  • Connect electric generation plants to the system
  • Make interconnections between the electric systems of more than one utility
  • Control reactive kilovolt-amperes supplied to and the flow of reactive kilovolt-amperes in the circuits

Additional information:

  • The Lineman's and Cableman's Handbook, Shoemaker, T. M., Mack, J. E., Tenth Edition 2002, McGraw-Hill.