GB573748A - Improvements in or relating to protective systems for locking the protection relays of electrical lines in unstable conditions - Google Patents

Abstract

573,748. Protective cut-out systems. COMPANIA PARA LA FABRICACION DE CONTADORES Y MATERIAL INDUSTRIAL SOC. ANON., and VITEAU, P. March 31, 1944, No. 6015. [Class 38 (v)] A system for distance-protection against faults is prevented from responding to asynchronous fluctuations by blocking relays remaining continuously locked whilst the frequency of the fluctuations exceeds a given value. The tripping of a circuit-breaker 3 controlling a line 2 by the closure of contact 4<1> of a relay 4 of a protective device 1, is controlled by a blocking device comprising two impedance relays 5, 6 associated respectively with auxiliary relays 7, 8 energised by a voltage source 9. A fault causes simultaneous closure of contacts 5<1>, 6<1> of relays 5, 6, but as relay 7 operates faster than relay 8 the former operates to open contacts 7<11> and prevent operation of relay 8, and to close contact 7<1> giving control of the circuit breaker to relay 4. With asynchronous line fluctuations of less than criticial frequency, when impedance decreases below Z1, relay 6 operates first to close contact 6<1> thereby energising a condenser 10, and, througb contacts 7<11>, the relay 8, the contacts 8<1> closing and contacts 8<11> opening to break the circuit-breaker trip circuit. When the impedance < Z2 < Z1 relay 5 operates to energize relay 7 and open contacts 7<11> but relay 8 remains energised through closed contact 8<1>. As impedance rises above Z2<SP>1> > Z2 contacts 5<1> open to de-energize relay 7, and as it rises above Z1<SP>1> > Z1 contacts 6<1> open but relay S remains energized during a time td by discharge through it of the condenser 10. If the frequency of fluctuation is greater than the critical value the relay 8 is still in the energized position when relay 6 again closes so that the protection remains locked when time between the opening and closing of contacts 6<1> is less than td. A modification is described in which the auxiliary relay associated with relay 6 short-circuits the coil of the auxiliary relay associated with relay 5 to afford the locking, and the delay in unlocking is provided by the former auxiliary relay having a short-circuited ring, Fig. 2, (not shown).