GB118437A - A System for the Protection of Three-phase Alternating Current Systems. - Google Patents

Abstract

118,437. Jensen, R. J. Aug. 24, 1917. Protective arrangements; electro - magnetic switches.-For the protection of three-phase maina against faults which develop unsymmetrical loading in a normally symmetrically-loaded system, relays are inserted in the mains to release the main switch, the relays having a fixed field system passed by two only of the phase currents producing two magnetic fields which have a resultant causing the relay to operate on unsymmetrical loading, but have no effect on the relay on symmetrical loading. In the form shown in Fig. 6, the equal coils a, b each in one of the phases produce a resultant field which normally has a phase displacement of 90 degrees in relation to the field produced by the moving coil c which is passed by the third phase current and, consequently, the relay is not operated. In the event of fault however, the displacement differs from 90 degrees and, when the fault reaches a certain value,'the relay is operated. The relay may be provided with time-element. The coils a, b may be wound upon a horse-shoe core and the coil c arranged between the poles. For complete protection two relays as above described are necessary, the fixed field coils of the first carrying say the currents of phases 2 and 3, respectively, and the moving coil carrying the current of phase 1, while the corresponding coils of the second relay carry the currents of phases 1, 3 and 2. To deal with a leak when the phases are unloaded, a maximum relay is inserted in the neutral wire of the current transformers for the field coils and, to prevent the unnecessary operation of this relay by higher harmonies, an inductive resistance is connected in series with the relay and the relay and inductive resistance are shunted by an ohmic resistance. Symmetrical overload is dealt with by a single overload relay in one of the phases. In the induction-type relay shown in Fig. 13, the coil c carrying the current of the third phase is replaced by an induction disk s. The field coil n carries one of the phase currents directly, but the field coil p carries a current so displaced in relation to another of the phase currents that its phase is directly opposite to that of the current in the field coil n. This displacement is produced by the ohmic resistance q and the inductive resistance r. On the occurrence of a fault between any two of the mains, the currents in the windings n, p no longer nullify their effects in the relay, but produce a resultant field which operates the disk. The disk s may be dispensed with by employing a closed magnet having a secondary winding in which current is induced to release the main switch when a fault occurs.