Patent GB-1236327-A (GB)
Title: Arrangement for controlling an on-load tap changing apparatus
IPC: H01H9/00
Filed: 1969-11-03
Abstract: 1,236,327. Transformers. COMPAGNIE GENERALE d'ELECTRICITE. 3 Nov., 1969 [5 Nov., 1968], No. 53880/69. Heading H1T. [Also in Division H2] On-load transformer tap changing apparatus for use with non-sinusoidal loads comprises a mechanical tapping selector switch I, II, III, IV, V (Fig. 1a) two thyristor switches A, B (Fig. 2a), each paralleled by a protective non- linear device E A , E B , a device 210 for detecting the polarity of the load current passing through zero and a number of AND gates controlling the ignition and extinction of the thyristor switches A and B. The protective devices E A , E B  may be Zener diodes. Tap changing is effected by rotation of the selector switch I-V by a motor, the selector carrying an apertured diaphragm (shown as V in Fig. 1a and D in Fig. 2a) through which a beam of light impinges on one of two photo-transistors Q A  Q B  (Fig. 2a) controlling the logic circuit L from which ignition or blocking pulse are provided via OR gates 39, 40, 25, 26; gates P LA , P XA , P LB , P XB , and trigger circuits A 1 , A 2 , B 1 , B 2 , to the thyristor switches A and B. The motor of the selector is itself under control of a blocking device 51 (Fig. 20, not shown). For limited periods, during which switches A and B are open, the load current is carried by the Zeners such as E A , switch A being reclosed if B is slow to close. The tap changing may be in accordance with a stored programme but is supervised step by step by logic circuits to guard against current wave forms likely to cause damage e.g. inter-tap short circuits. The supervisory and control arrangements include a sequence commencement order store MC 19, 20 (Fig. 6b) energized when a pulse from a photo transistor 1A or 1B and a commencement signal I 1  reach AND 17 or 18. Subject to the arrival of the extinction order I 2  an extinction pulse for the appropriate thyristor switch is taken off at XA or XB. Subject to routing signal I 3  reaching AND's 5 and 6 the ignition order routing store 7, 8 is energized and on the appearance of ignition signal I 4  an ignition pulse is sent to the appropriate thyristor gate for switch A or B. The signals I 1 , I 2 , I 3  are produced from the circuit shown in Fig. 5a when energized by a pulse S 1  from a diode shunt circuit (Fig. 3a, not shown), the pulse S 1  occurring when the load current increasing from zero reaches 60 mA. I 2  is produced with a delay ( millisecond) after I 1  at least equal to the time interval between the passage through zero of the minimum load current and the point in time after which conduction by the appropriate thyristor cannot be stopped by turning off its control current. I 3  is 1 millisecond after I 2 . I 4  is produced by a pulse S 2  from the diode shunt circuit referred to above via inverters, differentiators and monostable triggers producing a delay of 270 microseconds. The circuit of Fig. 5a continues to produce I 1 , I 2  and I 3  (being reset by pulses from trigger 85 at 1 millisecond after I 3  so long as there is a pulse S 1 . Various logic circuits are provided to control ignition and extinction of the thyristors in the event of abnormal load current wave forms to prevent damage due e.g. to inter-tap short circuits or unduly long intervals between the extinction of one thyristor switch and ignition of the take-over thyristor switch. Details of the circuit controlling operation of the motor driving the selector switch are given (see Division H2).