Abstract: A multilevel convertor having a set of series-connected capacitors and a set of associated series-connected switching elements employs a main control loop which controls some aspect of an AC system, e.g. busbar voltage, by controlling the switching times of the switching elements. The convertor also comprises one or more subsidiary control loops for establishing a desired relationship between mean DC voltage levels on the capacitors. The subsidiary control loops combine first signals proportional to DC voltage levels on the capacitors and second signals proportional to tap currents associated with the capacitor nodes to provide signals which modulate the timing of the switching of switching elements connected to those capacitors which are out of balance, while leaving alone the timing of the switching of switching elements associated with those capacitors which have the required voltage across them.

Abstract: A D.C. current transformer (used for measuring D.C.) comprising a common primary winding and a pair of saturable magnetic cores on which respective secondary windings are wound and connected in opposite series in series to a source of alternating e.m.f., is provided with phase-sensitive device for determining the sense of the unknown D.C. in the primary winding. The phase sensitive device periodically senses and integrates the voltage (whose waveform is the -45.degree. to +45.degree. portion of a sine wave) across at least one of the secondary windings. The sense of the integrated waveform depends on whether said voltage is initially positive or negative, and is determined by the sense of the unknown D.C. in the primary winding. Preferably said voltage is sensed at each half cycle of said alternating e.m.f.

Abstract: A multi-loop control system for convertors of the kind used in h.v.d.c. transmission and industrial applications. The invention consists in a control circuit which effects transfer between two or more control loops, e.g. a constant-current loop and a constant .alpha. loop, in a smooth and precise manner, without extra smoothing lags, even in the presence of ripple in the controlled quantities.

Abstract: A multi-phase inverter comprising a bridged array of thyristors periodically fired in sequence by a source of firing pulses has a circuit arrangement for controlling each extinction angle (.gamma.) of the thyristors individually. Said circuit arrangement comprises an adder for summing .gamma. signals and adder for differencing said sums to generate an error signal and, said error signal being applied to said source of firing pulses at appropriate phase angles to appropriate thyristors so as to tend to equalize said sums, the number of pairs of differenced sums being equal to the number of thyristors in the array. Preferably each error signal acts as a constant multiplier of a correction signal which is a harmonic of the A.C. output frequency of the inverter. The arrangement improves the efficiency by enabling each .gamma. to be minimized individually without risk of commutation failure.