Circuit arrangement for switchover of load between different sources of

Abstract

Two sources of alternating current, such as taps of a supply transformer, e selectively switchable across a load by means of two circuit branches each provided with a main switch and a collateral network in parallel therewith, this network comprising a series combination of a protective resistor, an ancillary switch and a pair of antiparallel thyristors. The gates of the thyristors of each pair are interconnected by a firing resistor in series with a disconnect switch; each gate, furthermore, is connected to its own cathode through a reverse-poled diode which enables the ignition of the opposite thyristor upon closure of the disconnect switch when the proper polarity of the source voltage appears across the network in the closed state of the associated ancillary switch. Prior to a switchover from one source to the other, with only the main switch of one branch closed, the ancillary switch of that branch is closed; this is followed immediately by a closure of the network's disconnect switch whereupon the associated main switch is opened. Next, the ancillary switch and the disconnect switch of the other network are closed in quick succession; there follows the reopening of the disconnect switch and the ancillary switch of the first network, closure of the main switch of the other branch and reopening of the disconnect and ancillary switches associated therewith.