Patent GB-1480832-A (GB)
Title: Static power conversion arrangement and method
IPC: G05F1/70
Filed: 1974-07-18
Abstract: 1480832 Reactive current converters; frequency converters WESTINGHOUSE ELECTRIC CORP 18 July 1974 [18 July 1973] 28147/74 Heading H2F [Also in Division H3] In a static circuit for providing controllable reactive VA to at least one A.C. supply line, and second parallel-connected cycloconverters are used to couple a high frequency link generator (e.g. a resonant circuit or a synchronous capacitor) to an A.C. supply line, the cycloconverters being controlled to reduce the variation of the quadrature component of their combined input current so as to reduce variation in the reactive current output provided by the high frequency link generator, whereby the physical size of the high frequency link generator can be limited. As shown in Fig. 1, a circuit for supplying reactive VA to an A.C. system comprises a forced-commutated cycloconverter FCC13, fed by a reactive power source in the form of a synchronous capacitor or a tuned circuit 15 and which is coupled to a naturallycommutated cycloconverter NCC11. Each cycloconverter 11 or 13 may take the form of a three-phase bridge (Fig. 5, not shown) using back-to-back thyristors in the naturally-commutated circuit and bridges of four diodes and one transistor in the forced-commutated circuit. NCC11 and FCC13 are operated at equal and opposite phase angles on the high frequency side so that no reactive power at the fundamental high frequency is taken from the tuned circuit 15. Much smaller tuned circuit components may thus be employed. The low frequency sides of NCC 11 and FCC13 are coupled via interphase transformers to the three-phase system V S1  ... V S3 . In Figs. 12 and 14 (not shown) two or four NCCs are employed and are controlled so as to minimize variation in the reactive power taken from tuned circuit 15 and thus prevent changes in the tuning of the circuit. Control of reactive current.-The current in the low frequency side of the cycloconverters 11, 13 is either leading or lagging depending on the relative magnitude of the cycloconverter output voltage and A.C. system voltage. The reactive current can thus be controlled by control of cycloconverter output voltage. In Figs. 6 and 7 (not shown) pulses are derived from the crossing points of the high frequency side voltages with a phase-adjustable low frequency reference voltage and are distributed by ring counters and gates to the main converting elements. The reactive current into and out of the converters is thus adjustable by control of the phase of the reference voltage. In Fig. 9, a D.C. voltage dependent on the voltage of the high frequency supplied is produced by transformers 139 and rectifiers 141 which, after comparison with a reference 149, is fed to LEDs 161, controlling a phase-shifting bridge of cadmium sulphide photo-cells 165 and capacitors 169. A multiplier 157 adjusts the amplitude of the low frequency voltage V S1  and feeds it to the phaseshifting bridge where its phase is changed according to the amplitude of the high frequency side and modified by adjustment of potentiometer 149 in response to the power factor on the low frequency side (Figs. 10, 11, not shown). Starting.-High frequency currents in tuned circuits 15 are initiated by a synchronous generator 137 or by action of the FCC13.