Patent GB-1016171-A (GB)
Title: Improvements in and relating to static convertors
IPC: H02M7/515
Filed: 1961-04-05
Abstract: 1,016,171. Converting. ENGLISH ELECTRIC CO. Ltd. April 4, 1962 [April 5, 1961], No. 12183/61. Heading H2F. [Also in Division H3] A static converter comprises a plurality of controllable rectifiers controlled by pulses from a switching circuit comprising a plurality of thyristors, rendered conductive sequentially. Fig. 2 shows a circuit for converting shortduration, three-phase pulses into long-duration (up to 120 degrees) pulses for controlling a mercury-arc rectifier. Silicon controlled rectifiers (thyristors) 30, 31, 32 are connected across a D.C. supply provided by rectifying at 52 the output of a three-phase transformer 51. Assuming thyristor 30 has been triggered by a short-duration, positive pulse at the gate electrode, the cathode attains a positive potential, energizing grid No. 1 of the mercury-arc rectifier (not shown). This condition is maintained for 120 degrees when the next thyristor 31 is triggered. Capacitor 46 is initially discharged hence the anode potential of 31 will fall while capacitor 46 is charging, this fall being passed via capacitor 62 to turn off thyristor 30. Similarly, thyristor 31 will be turned off when thyristor 32 is triggered. Fig. 13 shows an alternative circuit in which a common anode resistor 36<SP>1</SP> provides the turn-off coupling between the thyristors 30, 31, 32. A six-phase, mercury-arc converter having a double star connection with inter-phase reactor can be controlled by two of the circuits shown in Figs. 2 or 13. Protecting converters.-Diodes 42, 43, 44 bypass positive surges at the grids of the mercuryarc rectifiers to the reservoir capacitors 55, 56. A protection circuit 48 disconnects the supply from the thyristors if three thyristors conduct simultaneously. Fig. 12 shows a circuit for use in block 48. If three thyristors should conduct simultaneously, the voltage developed across resistor 82 will be sufficient to turn on the unijunction transistor 72 which turns on thyristor 71 by discharging capacitor 83 into its gate electrode. Resistors 78, 79 have the same value so that the potential at their junction will fall to zero, this fall being passed via capacitor 80 to turn off thyristor 70 and hence remove the supplies to the thyristors in Figs. 2 or 13. The supply is restored when capacitors 80, 74 have recharged sufficiently to re-trigger thyristor 70. Controlling thyristors.-Transistor 7 (Fig. 3) becomes conductive during each cycle only when the combined value of the direct control voltage fed in via diode 12 and the alternating voltage fed via transformer 29, is negative. A positive-going pulse is thus derived at the collector of transistor 7 which is fed to the base of transistor 13 (Fig. 4) so that transistor 13 becomes conductive at the trailing edge of the pulse from 7, the change of conducting states of these transistors being accentuated by positive feedback via resistor 16 and capacitor 17 to the base of transistor 7. Capacitor 18 discharges via transistor 13 and transformer 15 giving a short-duration positive pulse in the secondary winding starting at a time depending on the control voltage fed via diode 12 and is used to trigger one of the thyristors 30 ... 32 (Fig. 2). The range of control exhibited by the voltage fed via diode 12 is improved by adding a triangular wave to the peaks of the sine wave fed via transformer 29 (Fig. 3). This waveform may be generated from a sine wave by the circuit shown in Fig. 5 comprising Zener diodes 23, 24.