Patent GB-499310-A (GB)
Title: Improvements in or relating to electric protective arrangements
IPC: H02H1/04
Filed: 1937-07-16
Abstract: 499,310. Protective cut-out systems. REYROLLE & CO., Ltd., A., and LEYBURN, H. July 16, 1937, No. 19812. [Class 38 (v)] A protective arrangement of the distance type having directional and distance-measuring relay devices whose contacts are connected in series to control the tripping of the breaker is provided with means whereby, on the occurrence of a fault such that inadvertent operation might result from sudden change in the power-flow conditions following clearing or partial clearing of the fault, tripping of the breaker is prevented for a time-interval which is not less than that required for the relay devices to assume the operative positions appropriate to the changed power-flow conditions. The conditions envisaged by the invention are shown by way of example in Fig. 1, in which a ring main with generators a, a<1> is sectioned at b .. e and the section-ends provided with breakers denoted by crosses at b<1> .. e<2>. Each breaker has a directional relay operative for incoming earth-leakage current, and distance relays the contacts of which are in series with those of the directional relays and cperate instantaneously up to 80 per cent of the section-length and after a timeinterval for faults bevond the " instantaneous" zone, through a time-lag relay. Distance relays with a time-characteristic are also referred to. An earth fault at f in section c close to the generator a is cut out at once at c<1> but after a time-delay at c<2> during which interval the power will change direction, as shown by the single and double-headed arrows, at the breaker b<2>, the distance relay at which will also have operated. If the latter fails to reset itself, when c<1> trips before the directional relay responds to the reverse flow produced by the tripping of c<1> the breaker b<2> is tripped incorrectly. This condition may also arise in parallel-feeder protection when power-flow suddenly changes in one feeder due to tripping of a breaker in the other, and in a system in which, due to opening of a breaker in a line beyond one of two generators connected by a feeder, oscillations set up in the feeder might operate the breaker at the generator nearest the fault. In order to avoid this, lock-out contacts C<2> additional to trip contacts C<1> are provided on the distance relays C, these contacts C<2> being in series with normallyclosed contacts B<2> of the directional relay B, the trip contacts B<1> of which are in series with C<1> and the time-lag relay E. Thus, in the conditions in question, B<2> remain closed and C<2> close thereby energizing a lock-out relay G from the residual-voltage winding H2, which relay lifts and opens the trip circuit at G<2>, and maintains its energizing circuit at G<1>, so long as a residual voltage exists, that is, until the section is cut out at both ends. In Fig. 5, the lock-out relay G is residual-current responsive, and normally short - circuited through contacts G<3> by contacts C<3> of relays C (only one shown). When C operates, and provided B does not operate to close contacts B3, the short-circuit is removed and, owing to G<3> opening, cannot be restored until the residual current is removed by cutting out the .feeder at both ends. In a modification of Fig. 4, Fig. 6 (not shown), the lock-out relay lifts when C<2> close, provided B<2> remain closed, but instead of the holding contacts G<1> it has a time-delayed opening which allows time for the relays to adjust themselves, and is fed from the tripping source F, for example. In a further modification of Fig. 4, Fig. 7 (not shown), contacts B<2>, C<2> are dispensed with, and a further directional relay, which is energized by the same quantities as B but operates in the opposite sense to B, energizes, when operated, the lock-out relay, which makes its holding circuit and opens the trip circuit. The arrangement is also stated to be adaptable to the already-described systems of Fig. 5 and Fig. 6 (not shown), and is suitable for ring mains with two or more generators as in Fig. 1, but (unlike Figs. 4-6) is not suitable for a parallel-feeder system or for a ring main with one generator between the fault and relay station. In Fig. 8, each of two parallel feeders is protected in the manner shown for one feeder, although the arrangement is applicable also to a ring main. The lock-out relay is operated and locked in at R<1> by momentary closure of contacts Q<2> when breaker Q is tripped (and gives rise to change in power flow in the upper feeder). The arrangements shown apply to directional earth faults, but by suitable modification may be adapted for load-power flow, as shown by triple-headed arrows in Fig. 1, and for interphase-fault distance protective systems.