Transmission substations are located where two transmission lines meet. They may tie together multiple lines owned by a single transmission owner, or they may form an interconnect between lines owned by different entities. Often the substations connect lines of different voltages and must transform the voltage so that it is the same for both lines. Transmission substations may also connect a DC transmission line to AC transmission lines in which case they must convert DC to AC power.
The simplest transmission substation simply connects two lines of the same voltage. In this case the substation includes switches to allow the two lines to be connected or separated, and to allow lines to be isolated for fault clearance or maintenance. The substation would also include circuit breakers to protect against transmittal of faults, other protective equipment, and monitoring equipment.
More complex transmission substations interconnect lines of different voltages or interconnect DC lines to AC lines. This requires large transformers to transform the voltages, and in the case of DC to AC, also requires inverters to convert the power. Switching and protective equipment is also necessary. Other components often located in a transmission substation include:
- Voltage support or power factor correction equipment such as capacitors, reactors, static VAR compensators (SVCs), or static compensators (STATCOM)
- Monitoring and control components of the SCADA system
- Phasor measurement units (PMUs)
- Phase angle regulators (PARs)
In recent years transmission-level battery storage has begun to be included in some substations as well.
A small transmission substation might also be owned by an industrial facility that has elected to purchase transmission level service from the transmission provider. An example might be a steel mill that requires power at 138 kV. Since distribution level service will not be adequate to run the equipment in the mill, transmission level service is needed. In this case the customer may take service at a higher transmission voltage (typically at a lower rate) and maintain its own substation to deliver the needed voltage to its facility.