Transmission substations connect two or more transmission lines. In the case where the lines have the same voltage, the substation may simply consist of switches that allow flow to be directed as desired plus monitoring and protective equipment such as circuit breakers. But in a substation with connected lines of different voltages, equipment includes transformers to convert between the two voltages, voltage regulators, switches, and monitoring and protective equipment. Voltage regulators include capacitors, reactors, and static var compensators (SVCs). Monitoring equipment may include phasor measurement units (PMUs) that allow measurement of current and voltage by amplitude and phase. PMU data combined with high-precision time synchronization via GPS allows transmission operators to compare measured values from different substations far apart to draw conclusions as to the state of the transmission grid on an ongoing basis in near real time. Almost all transmission substations now have SCADA-based communications and control so that conditions can be monitored by the system operator and lines can be remotely switched.
Transmission substations are most commonly owned by a transmission owner (TO) but may also be owned by a large industrial facility that has elected to purchase transmission level service from the transmission provider.