Patent GB-289470-A (GB)
Title: Improvements in and relating to automatic electric switching systems
IPC: H02H3/06
Filed: 1928-04-25
Abstract: 289,470. British Thomson-Houston Co., Ltd., (Assignees of Anderson, A. E.). April 28, 1927, [Convention date]. Switching systems.-An automatic control system for alternative a.c. supply sources adapted to be connected to a local circuit comprises means responsive to the phase relation between the two sources to allow reconnection of the preferred source when its voltage has become restored after a failure (the emergency source being at such time connected to the load) only when synchronism exists, but should synchronism not occur within a predetermined time the emergency source is disconnected and the preferred source switched in. Either source may be selected as the preferred source. Excessive overload effects a lockout of both sources until the lock-out relay has been reset. With the switch 15 to the left and both circuit breakers 4, 5 open, simultaneous energization of sources 1, 2 causes voltage relay 10, energized by the preferred source 2, and relay 11, energized by the emergency source, to lift. Relay 10 energizes time-delay relay 12 which completes the circuit of hesitating relay 20 through interlocks on both circuit breakers and on the lock-out relay 25. The circuit-breaker 5 is thus closed and source 2 connected to load circuit 3. When the source 2 fails relay 10 drops, de-energizes relay 12 and energizes time-delay relay 13. The trip coil 9 is thus energized through an interlock on c.b. 5 and source 2 is disconnected. Hesitating relay 19 is energized by a contact on c.b. 5 when it drops, through interlocks on the lock-out relay and c.b. 4. Circuit breaker 4 is thus closed and source 1 connected. When the source 2 again becomes normal relay 10 lifts, energizing time-delay relay 12 and de-energizing time-delay relay 13. The former cannot energize relay 20 since contacts 47 on c.b. 4 are open, but when contacts 17 of the synchronizing relay close relay 20 is energized through the same interlocks as before. Source 2 is thus reconnected in parallel with source 1, but the trip coil 8 is energized by contact 79 on c.b. 5 through contacts of relay 12 and interlocks on c.b. 4 and source 1 disconnected. Should synchronism be delayed, time delay relay 18, which is energized through an interlock on c.b. 5 when relay 12 lifts, will energize the trip coil 8 and contacts 47 on c.b. 4 will energize relay 20 through an interlock on lock-out relay so connecting in source 2 after disconnecting source 1. Should c.b. 4 be closed when source 1 is de-energized and source 2 energized the lifting relay 12 will energize the trip coil 8 through an interlock on c.b. 4 and the lower contacts of relay 11. The contacts 47 of c.b. 4 will then energize relay 20 as before and cause source 2 to be connected. An overload relay 23 energizes both trip coils through the lock-out coil 25 on excessive overload and the latter prevents reclosure of either circuit breaker. Throwing switch 15 to the right makes source 1 the preferred source and source 2 the emergency source. The synchronizing relay may be of any suitable kind responsive to phase relation only or to frequency relation also. Relay 11 prepares a circuit for relay 13 which is held open by relay 10. The coils 40, 41 of a synchronizing device 16 are also energized from the respective sources, but the time delay of this device is preferably longer than that of relay 12.