Patent GB-522783-A (GB)
Title: Improvements in or relating to protective devices for periodically actuated electriccontact arrangements
IPC: H02H7/10
Filed: 1938-12-09
Abstract: 522,783. Arc-rupturing in switches. PATENTVERWERTUNGS GES. HERMES. Dec. 9, 1938, No. 36012. Convention date, Dec. 9, 1937. [Class 38 (v)] [Also in Group XXXV] A protective device for moving-contact current - converters and other periodicallyactuated contact-devices, comprises a locking device controlled by the current so as, on fault, to prevent the opening of current circuits which at the time are closed, thus, in the case of converters, preventing the flow of equalizing currents which would damage the contacts by arcing. The locking may be effected by mechanical action on parts of the driving device, which method is suitable for reciprocating contacts as described in Specification 493,436, or by short-circuiting the main contacts by parallel auxiliary contacts which are closed by relays before the driving device again opens the corresponding contacts, which method is suitable for rotating contact devices. Preferably all the contacts are locked in closed condition, those which are open at the moment of fault being allowed to close, but the opened contacts may, alternatively, be locked open. Quick-acting switches or fuses may be arranged in series with the contacts in order to clear the fault, the switches being provided with means for automatically switching-in again, the latter process being effected, for example, in the way set out in Specification 504,531, [Group XXXV]. The protective device may be set in operation by discharge valves, the control grids of which are influenced by the current to be interrupted. When the contacts are in series with chokes arranged to produce a flattened curve in the neighbourhood of the current-zero as in Specification 492,369, the chokes are constructed so that short-circuit current is limited to two to four times the nominal current, in which case, with a temporary disturbance, a complete switching-off is unnecessary, whereas with longer disturbances, a less rapid-acting switch can be employed for switching out. Where two sets of three pairs of contacts are employed for full-wave rectification, after the manner of the Graetz connection for rectifying valves, two switching arrangements are used for bridging all the contact-pair, each arrangement comprising three fixed contacts and a movable contact bridging these three fixed contacts. A direct short-circuit of the D.C. mains is avoided by separating into two groups. Release of the contacts may be effected magnetically by a grid-controlled, gas-discharge . valve, the ignition impulse of which is obtained from six chokes in the anode-leads, polarized so as normally to be saturated, but arranged to be unsaturated by flow of a reverse current and thus ignite the valve by grid-control. Alternatively, the grid may be controlled by a voltage supplied by the armature of small synchronous motor energized by the three-phase supply, the changes in the armature-current in this motor, when the supply voltage is upset, igniting the valve. The rotor of the motor can be excited by be direct voltage so that sudden variation therein ignites the valve. Instead of bridging relays, mercury vapour valves may short-circuit the contacts being protected, liquid mercury being used as both electrodes so that the valves conduct in both directions: For a six - phase rectifier in Graetzconnection, two valves, each with three anodes, are necessary. The ignition can be produced by a grid-controlled discharge valve and condenser, and when ignited the valve has a voltage-drop of about 20 volts which is small enough to protect the main contacts from arcing.