Patent GB-1178560-A (GB)
Title: Improvements in or relating to Electrical Converters
IPC: H02M7/48
Filed: 1967-11-08
Abstract: 1,178,560. Converting. COMPAGNIE GENERALE D'ELECTRICITE. 8 Nov., 1967 [21 Nov., 1966], No. 50729/67. Heading H2F. In an A.C./D.C. or D.C./A.C. converter using solid-state controlled rectifiers, the latter conduct in pulses at a frequency considerably higher than the A.C. frequency, the time between pulses being controllable by a programming unit, and at least one rectifier has its firing instant controlled by a variable delay unit. The highfrequency pulses are obtained by an inductance and capacitance in series with the thyristor(s), the D.C. load or supply being connected across the capacitance through a second inductance of larger value. The first inductance may be provided by the leakage inductance of the transformer which connects the thyristor(s) to the A.C. supply or load. The invention is described as applied to a single-phase half-wave arrangement (Fig. 1, not shown), a single-phase arrangement with centre-tapped transformer and two thyristors (Fig. 3, not shown) giving a squarewave output, or a three-phase bridge arrangement (Fig. 5) giving a trapezoidal-wave output of reduced harmonic content. A greater number of phases may be used. The arrangements provide a unity power factor on the A.C. side. In Fig. 5 the thyristors T1-T6 are supplied with (or supply) three-phase A.C. through inductors 305-307 which resonate with capacitor 308 at a high frequency, this being converted into pulses by the rectifying action of the thyristors. The D.C. load (or supply) 310 is connected across the capacitor 308 through a large inductance 309, the arrangement being similar to that in Specification 1,109,779. A control unit 311 senses the voltages at the points x, y and z and supplies firing pulses for the thyristor gates at a-f subject to a variable input G which can modify the firing times in accordance with the desired output. The control unit (Fig. 10) comprises six identical comparators 601-606 which develop signals representing the difference between the instantaneous voltages at x, y and z, e.g. x-y in unit 601. This value if positive is passed through AND gates 612 and 614 to fire two of the thyristors, e.g. T1 and T4 from unit 601. The comparator 611 comprises three transformers (Figs. 11a and 11b, not shown) connected between the appropriate pairs of points and having a diode and resistor across each secondary. The difference signal is also passed through a third AND gate 613 to a mixer 615 which receives signals from all the comparators 601-606. The mixer output is compared at 616 with the voltage V c  across the capacitor 308 and passes on a signal if the capacitor voltage is less (when the arrangement is acting as a rectifier) or greater (when acting as an inverter). The signal is delayed in unit 617 by an amount dependent on the control signal G and applied to a bi-stable circuit 618 which energizes the AND gates 612 of comparators 604-606 in one half-cycle of the A.C. and the AND gates of 601-603 in the other halfcycle. The input to the bi-stable circuit is also fed to the AND gates 614 of all the comparators. The thyristors T1-T6 may comprise multiple units in series, parallel or series-parallel. The transformer 300 in Fig. 5 may have its secondaries in delta form. An L.C. filter is preferably included between each thyristor and the transformer to damp out transients; the L may be leakage inductance as may be the inductances 305-307.