This article describes the function and the hardware of a typical scheme for high-voltage direct current power transmission. Its basis is a 2 x 500MW back-to-back scheme to interconnect the Western and Southern power regions of India. This technology permits sophisticated and rapid control of power system quantities - power, reactive power, frequency and voltage - by adjusting the timing of thyristor valve firing in relation to AC system voltage. The latest high-voltage thyristor and convector control equipment is described.