GB257699A - Electric protective arrangement - Google Patents

Abstract

257,699. British Thomson-Houston Co., Ltd., Fetch, H. S., and Wheatcroft, E. L. E. June, 26, 1925. Cut-out arrangements. - In actuating or direct current circuit-sections or apparatus, in order to protect against shortcircuits within the section or apparatus, an impedance or resistance is connected across a transformer or shunt in the main circuit and is designed to h a v e electrical properties similar in all respects to those of the section or apparatus to be protected. The protection is effected by the interaction, through a relay of the wattmeter type, of two voltages (i) proportional to the voltage at the end of which the relay is arranged and (ii) similarly proportional to the " apparent " voltage at the far end and served from the vectorial difference between the voltage drop across the impedance or resistance and that at the near end. Voltage (ii) is proportional to that at the far end when there is no fault, and, in fault conditions, proportional to the voltage that would obtain at the far end if there were no fault although the section were called upon to carry the fault current. The apparent voltage at the far end, measured as indicated, changes sign only if a fault occurs within the section; the relay operates in response to this change. As shown in Fig. 3 applied to a single-phase circuit, the secondary 23 of a voltage transformer 16 supplies the winding 25 of the relay 24 and the current transformer 22 supplies the impedance 19 which reproduces the characteristics of the feeder 11. The other winding 26 of the relay is energized in accordance with the vectorial difference between the voltages across the secondary and across the impedance by the arrangement of connections shown, and when this energization changes in sign the relay operates. In the application to three-phase circuits or apparatus the voltages which interact in the relay are proportional to apparent voltages both at the far end of the protected section &c. As shown in Fig. 4, impedances 19 as before (one for each phase) are connected across current transformers 22 in each phase and potential transformers 17 with one side earthed are connected across the phases. The potential transformer secondaries and the impedances are interconnected and connected to the two coils 25, 26 of relays 24, one only of which is shown, so that the winding 25 is energized in accordance with the apparent voltage between a phase and earth at the far end and the other winding 26 in accordance with the apparent voltage, at the far end between the two remaining phases. In the event of a short-circuit or earth within the section one of these voltages reverses and the relay operates. An additional impedance 33 may be arranged at the star point of the impedances 19 in order that the reduction of apparent voltage due to an earth fault may be greater than the corresponding reduction due to a phase fault of similar magnitude. The impedances are overcompounded so that the relay for a section will operate. when a fault occurs just beyond its section, but in order to obtain selectivity time-lag devices or biassing transformers as described in Specifications 193,434 and 216,254 are. used. In the preferred form, over-compounded impedances divided each into two parts are employed, one part (corresponding to say 70 per cent of the impedance of the section) being connected through an instantaneously-acting relay and the other part (corresponding to say 60 per cent) being connected through a time-lag relay. One protective relay only per phase may be used in this arrangement with a time-lag relay to shift the relay connection after a certain time from one tapping point of the relay to another. An arrangement for protecting a direct-current circuit is shown in Fig. 7 by means of a polarized reverse-current relay having a two-part polarizing directly across a shunt 19 in the main circuit winding 25 connected across the line and a twopart core 26 one part 39 of which is connected and the other part 40 across the line. Coil 26, considered as a whole, is in effect energized by the apparent voltage at the far end and, when this voltage reverses, the relay operates.