Patent GB-1365817-A (GB)
Title: Margin angle control arrangements for electric power converters
IPC: H02M7/757
Filed: 1972-01-31
Abstract: 1365817 Converting GENERAL ELECTRIC CO 31 Jan 1972 [29 Jan 1971] 4396/72 Heading H2F In or for a polyphase converter 11, a margin angle control system comprises means 13 ... 24 for deriving signals representative of the conductive period of the respective converting elements 1 ... 6, means 25 for monitoring the A.C. voltages across the respective elements 1 ... 6, means 26 for deriving margin angle signals representative of the interval between cessation of current conduction through an element 1 ... 6 and the passage through zero potential across that element, a selector 27 responsive to said margin angle signal for selecting the signal representative of the smallest margin angle. As described, the converter bridge 11 may comprise thyratrons, ignitrons, mercury arc rectifiers or series/parallel arrangements of thyristors and may be employed in H.V.D.C. power transmission or in a reversible rolling mill drive. Two series-connected bridges 11 may alternatively be fed from star/delta transformers. The conducting period of the elements 1 ... 6 are monitored by current transformers 13 ... 15 the secondaries of which are bridged by anti-parallel diodes (Fig. 2, not shown) so as to provide rectangular pulses of opposite polarity, alternate pulses representing the conductive period of respective elements, e.g. 1, 4 in an arm of the bridge 11. The pulses are lengthened to overcome transformer transients and separated by means of a transistor/Zener diode circuit 21 ... 23. Square waves representing the voltage across the individual elements 1 ... 6 are derived at 25 from a three-phase transformer supplied by a tertiary winding of transformer 12. Six bi-stable flip-flops F1 ... F6 are each set on receipt of the trailing edge of the appropriate current pulse and reset by the square wave from the voltage monitor 25. Pulses from the respective flip-flops F1 ... F6 thus represent the margin angles of the respective elements 1 ... 6 and are fed to respective circuits, one of which is shown in Fig. 4, in which the width-modulated margin angle signals are converted into amplitude-modulated signals. The negative-going edge of the margin angle signal is differentiated at 84 and used to turn on transistor 87 which discharges capacitor 89. Transistors 82 and 83 are also turned on so as to provide a constant-current charging path for capacitor 89, the charging being terminated at the end of the margin angle signal by the turnoff of transistor 83. The D.C. level across capacitor 89 is retained until the next margin angle signal so that transistor 91 provides a sawtoothnotched D.C. level. This signal is passed to an OR gate comprising diodes 931 ... 936 which selects the smallest margin angle signal and passes it via transistor 95 and circuits 96, 101 (Fig. 5) to a storage capacitor 103, charged via a FET 104 during each margin angle period as selected by diodes 1141 ... 1146. Transistors 108, 111, 112 thus provide a D.C. signal representing the smallest margin angle. Automatic control of margin angle.-The minimum margin angle signal from Fig. 5 is compared at 28 (Fig. 1) with a first reference and the error is passed via resistor 29 to a regulator 32 comprising a variable frequency oscillator which controls the valve firing system 34 and thus corrects the margin angle. The loop gain is increased by diode 31 if the margin angle is too small. If the error is excessive compared at 35 with a further reference, a by-pass circuit 30, 36 is operated giving a rapid advance in margin angle to a value corresponding to the first reference.