GB718815A - Improvements in and relating to electrical protective equipment - Google Patents

Abstract

718,815. Protective cut-out arrangements; electromagnetic relays. METROPOLITAN VICKERS ELECTRICAL CO., Ltd. April 22, 1952 [May 1, 1951], No. 10224/51. Class 38(5) In a protective arrangement of the kind having relay means differentially responsive to the currents flowing into and out of a protected machine, transformer, busbar, feeder or other section of a system so as to discriminate between internal and external fault conditions, the operating and restraining coils of the relay means are connected with a common series resonant circuit for energization of the operating coil mainly in dependence on the fundamental frequency of the system and for energization of the restraining coil mainly or largely in dependence on a harmonic or harmonics of the system current. In Fig. 1, current transformers CT1, CT2 on each side of a protected transformer T supply their difference current to an auxiliary transformer ACT1 which supplies a resistor R through a series capacitance XC and inductance XL, the operating coil OC of a relay being connected across the resistance R through a rectifier SRI and the restraint coil HRC of the relay being connected across the series capacitance and inductance through a rectifier SR2 and rheostat R1. The series resonant circuit XC, XL is tuned to the fundamental frequency of the system so that it passes current of the fundamental frequency to the coil OC. During through fault and transient conditions, the harmonics in the differential current produce a voltage across the series circuit XC. XL so that the coil HRC is energized to restrain operation of the relay. The relay may be provided with an additional restraint coil TRC energized through a rectifier SR3 from an auxiliary transformer ACT2 in accordance with the circulating current of the transformers CT1, CT2 to produce further relay restraint during through fault conditions. Figs. 2 and 2A (not shown) illustrate the invention applied to the protection of three-winding transformers. As applied to the balanced-voltage protection of a feeder, Fig. 4, the resonant series circuit XL, XC is connected across a summation transformer ST, the harmonic restraining coil HRC is connected across the inductance XL and the operating coil OC is connected in series in a pilot circuit P1, P2, which is connected across the capacitance XC at each end of the protected feeder. Under through fault conditions, the harmonics produce a greater voltage across the inductances than.across the capacitances so that the relays are restrained from operation, but under internal fault conditions, the operating coils are energized by the out-ofbalance current in the capacitances XC, the restraint being insufficient to prevent operation of the relays due to the small harmonic content of the feeder current. In the feeder protective arrangement of Fig. 4A (not shown), the restraint coils HRC are connected across the resonant series circuits and load restraint coils are connected across the capacities in addition to the pilot circuit including the operating, coils. Two relays may be arranged at each end of a protected section, Fig. 5 (not shown). Fig. 3 (not shown) illustrates a busbar protective arrangement. Electromagnetic relays. A relay, Fig. 6, for use in the above arrangements comprises a potmagnet assembly 20 having a central pole 21 and a permanent magnet 22 secured to the pot by a bolt 23. A moving coil assembly 24 is axially movable in an air gap between the pole 21 and an annular pole 25, the coil assembly being mounted on a rod 26 supported by leaf springs 27, 28. A back stop 32 co-operates with the rod 26 and a contact 29 carried by the coil assembly co-operates with a fixed contact 30 carried on a spring blade 31. It is stated that the relay provides a linear response with low operating burdens. Specification 523,603 is referred to.