GB1242673A - Overvoltage protection circuit - Google Patents

Abstract

1,242,673. Overvoltage protection; protecting converters. ENGLISH ELECTRIC CO. Ltd. 14 Aug., 1968 [18 Aug., 1967], No. 38291/67. Headings H2F and H2H. An overvoltage protective arrangement comprises a main thyristor 3A and an impedance 4A connected in a series circuit across the protected equipment. a transformer 13 having a secondary winding connected to provide a gating signal to the thyristor 3A when its primary winding is energized by the turning on of a control thyristor 20 on the occurrence of an overvoltage, a capacitor 25 charged in response to the overvoltage and effective to turn off the control thyristor 20 when the voltage is reduced, and circuit means for turning off the main thyristor 3A on cessation of the gating signal. In the arrangement shown, rectifier equipment supplying power for railway locomotives is protected against overvoltage due to lightning strikes by series connected main thyristors 3A, 3B with non-linear resistors 4A, 4B and inductors 10A, 10B for limiting the rate of rise of the current through the thyristors. Transformers 13, 14 have their primary windings connected in series with the thyristor 20 which conducts when the breakdown voltage of a zener diode 22 is exceeded. A spark gap 9 and a non-linear resistor 8 are connected across the equipments. Fig. 6 (not shown), illustrates the protection of thyristor equipment in which the protective arrangement is duplicated so as to provide protection against both forward and reverse overvoltages. The circuit means for turning off the main thyristors may comprise a connection 32 to the A.C. supply for the protected rectifier equipment, or an oscillatory circuit may be provided, Figs. 4 and 5 (not shown).