GB575070A - Improvements in means for protecting electrical direct-current installations againstreverse currents - Google Patents

Abstract

575,070. Protective cut-out arrangements ; induction relays. AKT.-GES. BROWN, BOVERI, & CIE. Feb. 24, 1944, No. 3473. Convention date, March 12, 1943. [Class 38 (v)] A reverse-current protective arrangement for a d.c. system employs a three-winding transformer the primary and secondary of which are normally uncoupled bv saturating' the core in the normal direction by the third winding energized by a constant d.c. voltage. When the main current reverses the core is desaturated and the secondary energizes a signal or trip relay. As applied to a mechanical converter in which supply leads 15 are connected to a load over contacts 13 driven by a motor 14, each lead 15 includes the primary 2 of a transformer 1 having a polarizing winding 3 connected to a d.c. source 10 so as to saturate the transformer-core in the direction of the main current in 2. The series-connected secondaries 4 supply a relay-coil 6 with a current-impulse when, on reversal of the main current, one of the transformer-cores is desaturated. The impulse induces in a closed coil 18 an e.m.f. which reacts with a polarizing flux set up by windings 7 so as to rotate the contact 19 into a position in which it shortcircuits all the leads and is latched at 20, 21. The windings 6, 7 are distributed each to a pair of poles of a four-pole magnet. In a three-phase arrangement of Graetz-connected valve-rectifiers, each supply lead is connected to the mid point of the corresponding primary so that the core is magnetized in the same direction by the go and return currents. In this form the protective relay has a holding winding for maintaining it closed after the trip-impulse has passed, this winding being independently energized and de-energized by a manual switch for resetting purposes. The polarizing winding circuits of the transformers and the relay holding circuit include chokes to prevent induced currents therein, Fig. 4 (not shown).