Patent GB-1279856-A (GB)
Title: Improvements in starting control scheme for rectifier-inverter systems
IPC: G05F1/10
Filed: 1969-08-14
Abstract: 1279856 Static converter systems GENERAL ELECTRIC CO 14 Aug 1969 [23 Aug 1968] 40677/69 Heading H2F [Also in Division H3] Relates to a starting arrangement for static converters, particularly converters interconnected by an H.V. D.C. transmission link, the converters having a control system whereby in normal operation firing pulses from a pulse generator are applied to each valve in the respective converter in sequence, e.g. with 60 degrees interval between pulses, each valve conducting for 120 degrees so that at least two valves are always conducting simultaneously. If when starting a converter, the valves are fired in normal sequence, the first one fired could not conduct as there would be an incomplete path for current through the converter bridge. Thus the present starting arrangement provides a means whereby initially the firing pulses although generated are prevented from being applied to the valves and then in response to a start signal causes firing pulses to be applied simultaneously (as shown in Fig. 2F) to two valves in the bridge, i.e. to a selected valve and the one preceding it in the normal steady state firing sequence. In the case of a rectifier bridge and inverter bridge interconnected by a D.C. transmission link it is arranged for firing pulses to be applied to two valves in each bridge simultaneously whereby a path is provided for current flow between the bridges, as illustrated, Fig. 1 (not shown). Control system, Fig. 3 illustrates a typical system for controlling one converter bridge. Signal representing actual and desired converter power conditions are fed to a summing point 13 and a resulting error signal is amplified and passed through a starting bias circuit 15 to a firing time computer 19 which also receives signals representing A.C. system voltages. The computer generates a train of discrete pulses which are converted by a distributer 20 into a family of six control signals (Fig. 2B) at points 21 to 26, at 60 degree intervals and each subsisting for 120 degrees. The delay angle α relating the leading edge of the control signals to the positive-going zero crossing of the anode voltages of the various valves, is determined by the magnitude of the error signal. The control signals are fed via a logic gate circuit 28 to valve firing system units 31 to 36 which are operative when activated to transmit relatively short firing pulses to the valves 1 to 6 in turn. Starting. The gate 28 is operative initially to prevent activation of units 31-36 and is subsequently disabled only when a positive voltage exists across both valves to be fired and when the delay angle is suitable. The delay angle may be about 50 degrees at starting in order to limit the initial build-up of D.C. The instant for disabling the gate 28 after a "Start" switch 40 is actuated and thus permitting firing pulses to the two valves, is established by a pulse mixing means 42 (detailed, Fig. 4, not shown, see Division H3) which derives spike pulses C1 (Fig. 2C) from the leading edges of the control signals from the distributer 20. The spikes are stretched to give pulses C2 and if the converter is to be linked to a remote converter the pulses C2 are fed to a coincidence detector (Nand gate) 44 (detailed, Fig. 5, not shown) together with similar pulses received at a terminal 121 from the remote converter. Upon simultaneous receipt of pulses the detector 44 produces an output to a switch driver 45. Upon coincidence of the input from the detector 44 with a starting command signal 41 when start switch 40 is actuated; the switch driver 45 disables the gate 28 and unblocks the firing signals. Since two control signals still subsist at this instant the logic means 27 will pass energizing pulses to two consecutively numbered components, e.g. 31 and 32 of the firing system, which transmit short turn ON pulses (Fig. 2F) simultaneously to two valves, e.g. 1 and 2, which are normally fired in succession. The bias means 15 (detailed in Fig. 8, not shown, see Division H3) preconditions the firing angle computer 19 to override other inputs to establish an initial firing angle that ensures positive anode voltage at the two valves and avoids premature extinction of these valves before current is commutated to the next valve in the normal sequence.