Improvements in electricity distribution systems and in load-shedding devices therefor

摘要

703,663. Load shedding arrangements. VENNER, Ltd. [Formerly VENNER TIME SWITCHES, Ltd.]. Oct. 31, 1952 [Aug. 2, 1951], No. 18344/51. Drawings to Specification. Class 38 (4) The voltage reduction in the supply system consequent upon overloading is utilized to cause the disconnection of the whole or part of any consumption circuit which at any time during the period of reduced voltage draws a current in excess of standard value allocated to that circuit. The voltage reduction may also effect a reduction in the magnitude of the standard value of current for the circuit as the magnitude of the voltage reduction is increased. In one arrangement, a relay for controlling the disconnection of the circuit has two opposed windings carrying, respectively, currents proportional to the system voltage and the circuit current, the iron armature of the relay, which operates contacts in the consumption circuit, being urged in a direction to open the contacts when difference in the field strengths produced by the two windings exceeds a predetermined amount. In another i arrangement a modified wattmetric type induction disc relay is employed, the relay having the usual voltage and current coils and, in addition, a further constantly but adjustably excited "current" coil which opposes the current coil proper and which may derive its excitation from a winding coupled to the voltage coil. Under normal voltage conditions and provided the current in the consumption circuit does not exceed a predetermined value, the torque produced on the induction disc by the three coils is sufficient to overcome the action of spring tending to return the disc to a position at which it closes a pair of contacts controlling a relay for disconnecting the consumption circuit from the supply. Upon reduction of supply voltage, the reduced torque on the induction disc causes a corresponding reduction in the maximum value of current which can be taken from the circuit before disconnection occurs. The relay controlled by the contacts of the induction disc is arranged to latch-out when operated and may be manually reset after steps have been taken to reduce the load on the circuit to a value below the maximum then permissible.