| 摘要 |
The circuit (7) to be protected is controlled by a circuit-breaker (1) whose mechanism (2) opens in response to the movement of a trip-coil (3) plunger (9). The coil is connected in series with the output terminals (51,52) of a protective relay (4) across two conductors of the protected circuit. Conditions in the latter are monitored eg. by a core-balance CT(5), whose output is processed (14) by the relay's processor (14), powered from the primary circuit via a voltage reduction and rectifying unit (15). On detecting a differential fault current, the processor triggers a thyristor (16) to complete the trip-coil circuit and trip the circuit-breaker. A varistor (8) is connected between the trip-coil center tapping (11) and the opposite circuit conductor. During transient disturbances in the protected circuit, varistor current is limited by the impedance of the series-connected first half-winding (10), while the varistor characteristic restricts the voltage applied across the relay (51,52), with which it is effectively paralleled. Also, the transient current in the first half-winding is insufficient to actuate the plunger. This half is further divided, with a central insulating screen, to deter inter-turn flashovers. Two variants, providing enhanced security, are also described. |
