Patent GB-667687-A (GB)
Title: Improvements in or relating to electric protective arrangements
IPC: H02H3/30
Filed: 1949-05-25
Abstract: 667,687. Protective cut-out arrangements. REYROLLE & CO., Ltd., A., and EDGLEY, R. K. May 24, 1950. [May 2, 1949] No. 13987/49. Class 38 (v). To prevent incorrect operation of a Merz- Price current balance protective arrangement due to transient current surges, the protective relay of the arrangement is energized by means of a transductor having transient bias and smoothing windings arranged so that the D.C. component of the transient current surge due to an external fault condition or switching operation has the effect of biassing the relay against operation. When applied to a three phase circuit, Fig. 1; a transductor D<SP>1</SP>, D<SP>2</SP>, D<SP>3</SP> may be associated with each phase. In this Figure, the protected circuit A<SP>1,</SP> A<SP>2</SP>, A<SP>3</SP> is provided at each end with current transformers B<SP>1</SP>, B<SP>2</SP>, B<SP>3</SP>, the secondaries of the current transformers being interconnected by pilot wires C<SP>1</SP>, C<SP>2</SP>, C<SP>3</SP> and connected so that current circulates in the pilot circuits. Each transductor is provided with an A.C. operating winding H, H<SP>1</SP> and the transient bias winding F connected in series with each other and between normally equipotential points in the pilot circuit. A shortcircuited smoothing winding G is so arranged on the centre limb E of the transductor that it is inductively linked with the transient bias winding F but not with the A.C. operating winding H, H<SP>1</SP> or with the secondary winding J, J<SP>1</SP> which may energize the relay K through a rectifier K<SP>1</SP>. The smoothing winding G has the effect of short-circuiting magnetically the winding F when the current in winding F is a symmetrical alternating current. Thus in the event of a fault within the protected section an out-of-balance current flows through the windings, F, H, H<SP>1</SP>, and as the D.C. component of the fault current is small or of very short duration, the effect of the winding F is nullified by the winding G so that the winding H, H<SP>1</SP> induces current into the secondary winding J, J<SP>1</SP> which energizes the relay K to effect tripping of the circuit. Should a transient current surge originating externally occur, any out-of-balance current which flows through the windings F, H, H<SP>1</SP> will have a substantial D.C. component so that the winding F becomes effective to partially saturate the transductor core and reduce the effective transformation ratio between the operating winding H, H<SP>1</SP> and the secondary winding J, J<SP>1</SP> so preventing the operation of the relay K. The A.C. operating winding of each transductor may be energized from the secondary of an auxiliary transformer, whose primary is connected between the equipotential points and the bias winding F may be connected in series with either the primary or the secondary of this transformer. In a modification, Fig. 3 (not shown), each transductor is provided with two cores, the windings H, H<SP>1</SP>; J, J<SP>1</SP>; F; G. being shared equally between the two cores, but arranged so that there is no resultant inductive coupling between the windings H, H<SP>1</SP>, J, J<SP>1</SP> and the windings F, G. In both arrangements the operating and secondary windings H, H<SP>1</SP>, J, J<SP>1</SP> may be combined into an autotransformer winding or alternatively the operating and bias windings H, H<SP>1</SP>, F, may be combined into a single winding having unequal portions disposed on the two outer limbs E<SP>1</SP>, E<SP>2</SP> of the core, Fig. 2 (not shown) or on the two cores. Various other arrangements and dispositions of the transductor windings are described or referred to. If a single transductor is used in a three phase circuit the current transformers B<SP>1</SP>, B<SP>2</SP>, B<SP>3</SP> at each end are connected to a summation transformer. It is stated that the invention is especially suitable for earth fault protection, bus-bar zone protection and for the protection of a power transformer to prevent incorrect tripping due to charging current when the. transformer is switched in.