Patent GB-1230974-A (GB)
IPC: H02M7/515
Filed: 1967-07-06
Abstract: 1,230,974. A.C. line commutated inverters. P. CHADWICK, and N. G. HINGORANI. 20 Sept., 1968 [6 July, 1967], No. 31128/67. Heading H2F. In an A.C. line commutated invertor there is provided a control circuit which ensures that the extinction angle 8 is maintained at a greater value than the minimum permissable value # c  irrespective of transient variations in the commutation voltage e.g. during A.C. system faults and switching operations. In Specification 1,124,263 a constant extinction angle (C.E.A.) control signal is produced by integrating the commutating voltage from the instant -# + # c  and adding to the integrator output a signal significant of the required angle of advance #, the net (C.E.A.) signal being fed to a level detector to produce a firing pulse when zero. This arrangement is modified in the present Specification by adding to the C.E.A. waveform a predetermined fraction (K) of the commutation voltage waveform to give a resultant voltage wavform R = ##2E(Ksin wtcos wt-cos # c ) as shown Fig. 8. The phase angle at which the resultant waveform passes through zero going positive is compared with a fixed phase angle 2#/3 and a signal is derived, from which the angle of advance # is controlled such that the extinction angle is maintained greater than the minimum permissible value # c  during transient changes in the commutating voltage waveform. Control system.-Each value of the invertor has a control system as shown Fig. 7 also Fig. 9 (not shown). Level detectors A and B (Schmidt triggers) are connected via a centre tapped scaling transformer (not shown) to the main convertor transformer (not shown), detector A receives the negative of the commutating voltage signal received by B. Detector C receives the resultant waveform R which is the sum of the C.E.A. control output and the constant fraction of the commutating voltage which is added by means of resistors R and V R . A monostable circuit has an operating period determined by the desired period of transient detection analysis of the commutating voltage waveform, i.e. from wt = -# to wt = 2#/3 (graph b, Fig. 8), and is initially triggered by detector A. A bi-stable circuit is triggered to one state by detector B and will remain in this state until reset by detector C when summation waveform R goes positive through zero. Under steady state conditions, or under transient conditions which would tend to increase the extinction angle 8, such as when the voltage integral of the commutating voltage waveform in the half cycle beginning wt = 0 is greater than that in the first half cycle (Fig. 2, not shown) then the bi-stable circuit will be reset before the end of the monostable period (graphs b, c, Fig. 8) and there will be no output from the AND gate and the C.E.A. control will provide the firing angle. Under transients which would result in a reduction in 8, and failure of commutation, i.e. when the integral of the first half cycle from wt = -# is greater than that of the second from wt = 0, then the bi-stable and monostable circuits will produce negative outputs simultaneously (graphs b, and d) and the AND gate will produce a negative output pulse which is amplified and inverted (graph e) and fed to the firing circuit to fire the valve concerned, which will occur immediately after the R waveform goes positive. A secondary reset pulse is then fed to the bi-stable circuit to ensure that it is always reset before the next analysis.