GB866031A - Improvements in or relating to protection devices for electric networks - Google Patents

Abstract

866,031. Protective cut-out arrangements. CHARBONNAGES DE FRANCE. July 7, 1958 [July 9, 1957; March 25, 1958; June 2, 1958], No. 21761/58. Class 38 (5). In a protective arrangement the occurrence of faults which may be of high resistance is detected by one or more relays which respond to an increase in the highfrequency current injected into the protected system by an electronic high-frequency generator through one or more series inductance - capacitance circuits presenting a low impedance to the high-frequency current, the power source and the apparatus supplied by the source being provided with parallel capacitors so as to form parallel inductance-capacitance circuits presenting a high impedance to the high-frequency current under normal conditions. The high-frequency generator may comprise a transistor oscillator 6, Fig. 1, and a coupling transformer 10 associated with a capacitor 9, a variable resistor 7 and a potentiometer 8 arranged to provide two high-frequency signals of suitable amplitudes with a phase displacement of 90 degrees between them. These signals are amplified in channels each comprising a transformer 15, push-pull power transistors 17, 18, a transformer 21 and push-pull power transistors 23, 24, the amplified signals being applied to the primary windings of transformers 27, 28 which have Scottconnected secondary windings to provide a balanced three-phase high-frequency current for injection into the protected three-phase power circuit through series connected inductors and capacitors 29 . . . 34. Various over-voltage protective devices are indicated as diodes 35, 36. Zener diodes 39, 40 and 41, 42, or as varistors 43, 44 associated with tertiary windings of the transformers 27, 28. Relays X1, X2 cause a warning to be given or the tripping of a circuit-breaker should either amplification channel fail. Fig. 1 also shows the capacitors 45 . . . 47 connected in parallel with the power supply source S. Fig. 2 shows one of a number of different arrangements described in which the relay or relays are associated with the highfrequency generator G and protect the system as a whole. In Fig. 2 the voltages produced across the series inductors 51 . . . 53 are rectified at 54 . . . 56 and applied to the operating coil of a common relay 57 which effects the opening of a circuit-breaker, contactor or main relay. Similarly, Fig. 6 shows an arrangement in which current transformers 71 . . . 73 associated with an individual circuit energize relays 74 . . . 76 through filters 81 and rectifiers 77 . . . 79. In Fig. 14 (not shown), the neutral of the supply source is connected through a capacitor to earth to enable protection to be obtained against earth faults. The Specification also describes single phase arrangements and refers to the use of electron tubes in the high-frequency generator.