This chapter focuses on the specifics of power electronic controllers as applied in substations for power transmission purposes. The dramatic development of power electronics with line and self-commutated converters in the past decades has led to significant progress in electric power transmission technology, resulting in advanced types of transmission systems, which require special kinds of substations. High-voltage power electronic substations consist essentially of the main power electronic equipment, that is, converter valves and flexible alternating current transmission systems controllers with their dedicated control and protection systems, including auxiliaries. High-voltage direct current offers frequency- and phase-independent short- or long-distance overhead or underground power transmission with fast controllability. The unified power flow controller uses a shunt-connected transformer and a transformer with series-connected line windings, both interconnected to a direct current capacitor via related voltage-source-converter circuitry within the control building. Valve losses in high-voltage power electronic substations are comparable in magnitude to those of the associated transformers.