Patent GB-547861-A (GB)
Title: Improvements in and relating to distance protective arrangements for electric systems
IPC: H02H1/00
Filed: 1941-06-13
Abstract: 547,861. Protective cut-out systems. BRITISH THOMSON-HOUSTON CO., Ltd. June 13, 1941, No. 7507. Convention date, June 14, 1940. [Class 38 (v)] An impedance relay comprising a polyphase directional element acting as the operating element, and a polyphase voltage-responsive restraining element preventing operation until predetermined impedance conditions obtain in the circuit, has the restraint varied so as to provide the same distance-response with symmetrical three-phase faults as with faults involving only two phases. In the arrangement shown, the directional element 15 and the restarting element 30 are structurally of the character described in Specification 508,005 and the latter is restrained in accordance with the area of the voltage-triangle, as set out in Specification 361,193. Element 15, which is responsive to outgoing power from station 12, has salient poles on a stator 16, the adjacent pairs of poles being energized by currents and voltages in quadrature with one another. Thus the poles 17 ... 24 are respectively energized by the quantities IB, EAC, IA, -EAB, IC, EAB, IA, and -EAC in the line IIA, IIB, IIC. The rotors 27, 42 are fixed to a shaft 43 carrying the trip arm 44 which is biased "open" by a spring 46. The poles 33, 35, 37, 39 of element 30 are energized by voltages EAB, -EAB, EAB, and EAB, whereas poles 34, 38 are both energized by EBC, and, in order to double the restraint, poles 32, 36 may be both energized by the voltage -EBC, this last condition being subject to contacts 47, 61 of a further relay 48 being closed. Relay 48 is a negative phase-sequence element and therefore responds to faults involving only two phases, to open contacts 61, 47 against spring-action 62 and halve the restraint. For this purpose the element, which is structurally similar to the others, has its poles 50 ... 57 energized from line current transformers in the manner shown. The mathematical considerations underlying this arrangement are set out in the Specification. In a modified arrangement of element 30, Fig. 2 (not shown), a resonance condenser is inserted in series with the windings on poles 33, 35, 37, 39, and the windings on the remaining poles are in series with a resonance condenser and resistance, an adjustable part of which can be short-circuited by closure of contacts 47. In a further modification, Fig. 3 (not shown), the phase-currents are compared with one another in pairs in three current-balance relays, unbalance in any one relay due to an unsymmetrical fault causing opening of contacts in series with contacts 47. In Fig. 4, contacts 47<SP>1</SP> are in series with two parallel-connected sets of contacts 80, 81, of impedance relays 75 ... 77 having current operating coils 78 and voltage restraining coils 79. On balanced conditions contacts 80 or 81 are all closed, these being of the make-beforebreak type, whereas on unbalanced faults one or two of the relays will operate to open the circuit of contacts 47<SP>1</SP> in series with contacts 47. Separate relays are provided to deal with earth faults.