Patent GB-718816-A (GB)
Title: Improvements in electrical protective equipment
Inventor: RYDER CLIFFORD
IPC: H02H7/045
Filed: 1951-05-01
Abstract: 718,816. Protective cut-out systems. METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. April 30, 1952 [May 1, 1951], No, 10226/51. Class 38(5) In a current or voltage balance protective arrangement for a polyphase system including a star-connected winding or circuit, relay means are rendered sensitive to earth faults, as well as to phase faults, by means which apply a current or voltage derived from the neutral or earth current of the winding or circuit to each of the phases of the polyphase balance circuits to balance the components dependent on the zero component of the main current in the event of a through earth fault and to produce unbalance in the event of an internal earth fault. In Fig. 2, delta-connected current transformers CA1, CB1, CC1 associated with 'the delta-connected winding T1 of a power transformer and star-connected current transformers CA2, CB2, CC2 associated with star-connected winding T2 of the power transformer are interconnected by conductors a, b, c and n and the operating coils OC1, OC2, OC3 of a three-phase relay or of three single-phase relays, the resulting current-balance circuits being unbalanced by currents dependent on the zero component of the main currents in the event of a through earth fault. To remove this unbalance under such conditions, current transformers CNA, CNB, CNC associated with the neutral or earth conductor E are star-connected between the conductors a, b, c and n and supply currents dependent on the earth current to the balance circuits to keep them balanced. However, in the event of an internal earth fault, the currents produced in the balance circuits by the transformers CNA, CNB, CNC are not balanced by those due to the other current transformers and the relay coils OC1, OC2, OC3 are energized. The relay or relays may be restrained under through fault conditions by windings RCA1, RCA2, RCB1, RCB2, RCC1. RCC2 connected in series in the conductors a, b, c. Additional restraint coils RCNA, RCNB, RCNC may be connected in the secondary circuits of the current transformers CNA, CNB and CNC. Fig. 3 (not shown) illustrates the applications of the above arrangement to a three winding transformer. Fig. 4 shows a voltage balance arrangement for the protection of a transformer feeder, the relay operating coils OC1, OC2, OC3 at each end of the protected section being connected in series with pilot conductors P1, P2, P3, and the restraining coils RC1, RC2 and RC3 being connected between the pilot conductors and a neutral pilot conductor PN. These voltage-balance circuits are supplied from the main current transformers through auxiliary current transformers AT1, AT2 and from the transformers CNA, CNB, CNC by auxiliary transformers CT1, CT2, CT3 connected in series with the pilot conductors. Under through fault conditions the voltages applied to each pilot wire P1, P2 and P3 sum to zero, but under internal fault conditions the components derived from the transformers CNA, CNB, CNC are no longer balanced by components derived from the other transformers, so that the relay operating coils are energized. A voltage-balance arrangement applied to a tee'd transformer feeder is illustrated in Fig. 5 (not shown). In each embodiment, a moving coil D.C. relay such as that of Specification 718,815 may be used, in which case the operating coils would be connected in or to the balance circuits through full-wave dry-plate rectifiers. The various restraining windings may be wound on a summation transformer which supplies a single restraining winding on the relay through a rectifier. Reference is also made to the use of induction relays. Harmonic restraining arrangements as described in Specificaton 718,815 may also be used.