ABSTRACT
This chapter examines the application of power semiconductors to high-voltage direct current (DC) transmission. It also examines the use of power semiconductors for voltage regulation. AC transmission dominates electricity supply by virtue of ease of generation, motor characteristics and the ability to change voltage magnitudes easily using transformers. High-voltage DC (HVDC) transmission uses a lighter, and hence cheaper, construction than an equivalently rated alternating current (AC) system with two conductors instead of three, and operation with cable is simplified because of the reduction of capacitive effects. HVDC systems do require complex and expensive terminal arrangements involving fully controlled converters. The term 'thyristor valve' is used in relation to HVDC systems to refer to the series/parallel grouping of individual thyristors used. Typical control schemes for HVDC transmission systems include constant DC current, constant DC voltage, equidistant firing angle operation and, in the case of the inverting station, operation with constant extinction angle.
