GB656787A - Improvements in or relating to electrical power supply equipment including standby - Google Patents

Abstract

656,787. Electricity supply. STANDARD TELEPHONES & CABLES, Ltd. March 1, 1948, No. 6428. [Class 38 (iv)] Subsituting sources in service; synchronizing.-Standby plant for supplying an alternating current load ACL upon failure of the normal mains supply A.C., comprises a battery B, a dynamotor or like converter A and relays so controlled by means responsive to the mains voltage that under normal conditions the machine A runs from the mains as a synchronous motor in parallel with the load, but upon the mains voltage falling below a predetermined value the connection between the mains and the load is interrupted and the battery is connected to the D.C. side of machine A which then operates as a converter and supplies the load from its A.C. side. The drawing shows the conditions of the relay contacts when the system is completely deenergized. To energize the load the mains switch SW1 is closed and, if the voltage is above a predetermined value, marginal relay JV picks-up, followed by relief relay RR and contactor CO, thereby completing the load circuit at col and rr3. A regulator ACR provides for automatic control of the load voltage. Machine A is now run up from the battery by closing switches SW2, SW3, causing the pick-up of relays CTR and X with consequent energization of the motor circuit over resistor R5 and contacts ctr1, mc2 (rr4 and co2 now being open). A thermal delay relay TDR, energized upon closure of ctr1, operates after about one second to effect energization of a delay relay DR and to prepare a circuit for energizing a machine-connection contactor MC. Operation of contact dd1 of relay DR short-circuits resistor R5, causing the speed of machine A to rise above the synchronous speed, the voltage generated at the A.C. end of the machine then being sufficient to operate a relay AO, which at contacts aol completes the circuit of machine contactor MC. Operation of the last connects the machine to the mains supply at mc1 and disconnects the battery at mc2, so that the machine is pulled into synchronism and runs from the mains. Relay TDR remains operated by the e.m.f. generated by the D.C. end of machine A, but its energizing current is reduced to a holding value by a shunt circuit R4 completed by contacts mc3, so that TDR quickly releases to deenergize contactor MC if the machine fails to synchronize. Another attempt to synchronize automatically follows, but if the machine fails to reach the necessary speed within a given time the prolonged opening of contacts mc4 allows relay X, which is very slow to release, to drop out, thereby interrupting the holding circuit of relay CTR and shutting down the machine. If, when machine A is normally running from mains, the mains supply fails, relays JV, RR and contactor CO drop out in succession, isolating the mains at contacts co1 and rr3 and connecting the battery to the machine over contacts rr4 and co2. Upon restoration of mains supply (at a voltage higher than that which caused JV to drop out, due to the now open contacts rr2) rr3 and col re-close to connect the load to the mains, D.C. supply to the machine being interrupted by the opening of rr4 and co2. If the machine loses synchronism relay TDR automatically initiates an attempt to re-synchronize. Alternative connections for the shunt field of the machine A and for the relay RR are described.