GB1246703A - Apparatus for stabilising the voltage of a supply main - Google Patents

Abstract

1,246,703. Automatic voltage stabilization with varying inductive loads. LICENTIA PATENTVERWALTUNGS - G. m. b. H. March 24, 1969 [March 23, 1968(2)], No. 15412/69. Heading G3R. The voltage of an A. C. supply main which feeds a rapidly-varying inductive load is stabilized by maintaining the reactive current substantially constant, using inductors which are in series or in parallel with the load and which are controlled by electronic switches so that the inductance value can be altered with a rapidity of the order of one half to a cycle of the mains supply. The operation of an arrangement comprising a variable inductor in series with an inductive load having a variable resistance, Fig. 1 (not shown) is explained with the aid of a circle diagram Fig. 6 (not shown), from which are derived the various values of the inductor required to maintain the reactive current constant over a range of load resistances. In the extreme case of low load resistance the inductor acts as a current limiter. The variable inductor may comprise three series-connected inductors stepped in the ratio 1 : 2 : 4 and each in parallel with anti-parallel thyristors, Fig.3 (not shown). The latter should be fired at the peak value of the inductor voltage to prevent the occurrence of aperiodic components, Fig. 4 (not shown). If the variable inductor is in parallel with the load, Fig. 2 (not shown) its value is varied so that the reactive current is maintained at a value corresponding to the minimum value of the load resistance. Such an arrangement is shown in Fig. 7 wherein the load is an arc furnace R and a series connected inductor L, and the variable inductor comprises inductors L 12 , L 23 , L 31 controlled by anti-parallel valves. The load currents are sampled in each cycle or half cycle by a high speed electronic computer R 12 , R 23 , R 31 , each of which controls a respective valve-control circuit S 12 , S 23 , S 31 so that the inductors L 12 , L 23 , L 31 are switched to supplement the fluctuating load current with a delay of about one half to one cycle. The reactive current is thereby stabilized at a maximum value which may be compensated by capacitors C. The use of separate computers and control circuits enables the inductors to be switched to convert an asymmetrical loading situation (if such exists) to a symmetrical one, in accordance with the Steinmetz formula. Typical sampling instants and supplementing effects are described, Fig. 8 (not shown).