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.
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.