Patent GB-1052682-A (GB)
IPC: F01D21/02
Abstract: 1,052,682. Speed governors. ENGLISH ELECTRIC CO. Ltd. Feb.25, 1965 [Feb.28, 1964], No. 8330/64. Heading G3B. [Also in Division F1] The invention relates to a control system for a prime mover and is described with reference to a power plant comprising a steam turbine 10 driving an alternator 13, the steam supply being controlled by a valve 11 which is adjusted by a hydraulic actuator 14. The control system comprises a mechanical speed sensitive governor 16 which in normal operation controls the actuator 14 through a hydraulic relay 15. The loading of the governor may be varied by means of a speeder motor 17. The control system comprises also a load-sensitive device 21 in the form of a rectifier disposed in the three-phase output lines of the alternator which supplies a D.C. output signal dependent on the load on the alternator 13, to an amplifier 19, the amplifier being connected to a motor 18 which may act on the actuator 14 to close the control valve. The amplifier is supplied with electric power from an alternating current source 20. The amplifier is also connected to contacts 23a of a pressure sensitive relay 23 which is responsive to steam pressure at an intermediate stage of the turbine 10 to operate when the steam pressure is equal to or greater than an upper set point corresponding to a load on the alternator of a proportion (say 50%) of a predetermined nominal full load value. A time-delay relay 24 is also connected to the amplifier 19 and is arranged to be energized when the alternator load is below (say) 45% of full load value while the steam pressure on the relay 23 is above its upper set point value. A D.C. power source is connected across contacts 24a, the contacts being connected in the circuit of the governor speeder motor 17 and also in the circuit of a solenoid 25a which when energized opens a relief valve 27 so as to limit the pressure in the hydraulic system between the governor 16 and the actuator 14. An isolating switch 26 is arranged to break the circuits of the speeder motor 17 and solenoid 25a when the setting of the governor is reduced to a pre-determined reduced speed value. In normal operation the control valve is under the control of governor 16 and the amplifier 19 is inoperative. If the load on the alternator now becomes reduced to its lower set point value, the steam pressure on the relay 23 still being at or higher than its lower set point value, the amplifier 19 causes the motor 18 to operate the actuator 14 so closing the valve 11, and at the same time energizes relay 24. Closing of the valve 11 causes the pressure in the turbine to fall and so the relay 23 opens so reducing the output of amplifier 19 to zero and de-energizes the motor 18 so restoring control of the turbine to the governor 16. After a delay of four seconds the relay contacts 24a are closed thus energizing the speeder motor 17 and the solenoid 25a. The relief valve 27 is thus opened thus placing a limit on the load the turbine could carry if load were restored and the turbine speed returned to normal. The speeder motor 17 adjusts the governor 16 reducing its setting so that the speed of the turbine is reduced. When the setting of the governor 16 has been reduced by the speeder motor 17 to a value corresponding to the predetermined alternator load required to operate the auxiliary equipment, the governor trips the isolating switch 26 which breaks the circuits of the speeder motor 17 and of the solenoid 25a so isolating the relief valve 27 since it is now no longer necessary to limit the oil pressure.