GB267930A - Improvements in and relating to protective arrangements for electric systems - Google Patents

Abstract

267,930. British Thomson-Houston Co., Ltd., (Assignees of Traver, O. C.). March 18, 1926, [Convention date]. Cut-out arrangements. - In the protection of parallel feeders, in addition to the usual differential selective protection by means of current transformers 22, 23 connected in a circulating current system, the selective tripping being effected by the closure of both an over-current relay 11 in the equi-potential connection 20, 21 and a directional relay 13 with its current coil 14 in the same equi-potential connection, summation overload protection is provided operative when the vectorial sum of the currents in the feeders exceeds a predetermined value. This additional protection is afforded by an overcurrent relay 25 with its coil 26 connected across an impedance 28 in the circulating-current circuit, the equi-potential point 20 being a point on this impedance. By these means the relay 25 is always energized by a current proportional to the vectorial sum of the line currents whether one or two lines are in service and, therefore, either line when in service alone may be operated at a current equal to that of the normal current of the two lines together, without changing the relay adjustment. In modifications, the relay winding is connected across the middle half of the impedance or across a secondary winding (the impedance acting as the primary) and having half the number of turns of the impedance.