GB910106A - Improvements in and relating to electric control systems for prime movers driving electric generators - Google Patents

Abstract

910,106. Fluid-pressure servomotor control systems. ENGLISH ELECTRIC CO. Ltd. Oct. 29, 1958 [Nov. 1, 1957], No. 34176/57. Class 135. [Also in Group XXVI] A control system for a prime mover driving an electric generator comprises a speed-sensitive governor for controlling the supply of working fluid to the prime mover in a manner tending to maintain the speed substantially constant, a secondary control means responsive to an electrio quantity dependent on the load on the generator for interrupting or substantially reducing, on sudden reduction of the load from a predetermined intermediate value to a predetermined substantially lower value, the supply of working fluid rapidly and independently of the speed-sensitive governor and time delay means for subsequently rendering the secondary control means ineffective to further maintain the supply of working fluid interrupted or substantially reduced after the expiration of a predetermined time interval following the occurrence of the reduction in generator load, the secondary control means having a response time which is substantially smaller than that of the speed-sensitive governor. A steam turbine 11, Fig. 1, drives an alternator 14 and is normally controlled by a speed-sensitive governor 18 acting through an hydraulic servomotor 17 on a steam control valve 12. The in-phase component of the alternator load current is detected by a device 21 and used to energize a winding 20b of a magnetic amplifier 20, the arrangement being such that the amplifier output is substantially zero when the alternator load exceeds 20% of the full load value but rises rapidly, when the load falls below 20%. The output energizes a reluctance-type torque motor 19 which acts on an auxiliary valve of the servomotor 17 to close the valve 12 rapidly and independently of the governor 18 if (for example, due to a fault on the load circuit) the alternator load falls rapidly from above 30% to below 20%. Consequently, the steam pressure in the turbine falls, and a pressure relay 13 is operated when the pressure in an intermediate stage of the turbine falls to a low value corresponding to 30% of the full load of the alternator, opening its contacts 13a, Fig. 2, to de-energize a relay 24, and closing its contacts 13b to energize a timing relay 25. After a time delay sufficient for the speed-sensitive governor 18 to respond to the rise of speed caused by the fall in the alternator load, contacts 25a of timing relay 25 close to energize a closing coil 26a of a latching- type relay 26 (not shown). Contacts 26c on relay 26 close to energize a resetting winding 20c of the magnetic amplifier 20, thereby reducing the output of the amplifier substantially to zero and de-energizing the torque motor 19. In addition, contacts 26b open to de-energize the closing coil 26a, and contacts 26e close. The servomotor 17, once again under the control of the speed-sensitive governor 18, re-opens the steam valve 12 in accordance with the new alternator load. If, due for example to the persistence of the fault condition, the alternator load fails to rise to 20% within about 4 seconds, a timing relay 29 trips a circuit breaker 16 to disconnect the alternator 14 from the load circuit. This is brought about in the following manner. When the alternator load drops below 20%, a relay 23 in series with the winding 20b drops off to close contacts 23a and open contacts 23b, thereby short-circuiting a relay 27 and causing it to drop off so that contacts 27a open to isolate the relay 27 and contacts 27b close to energize a relay 28. Contacts 28a thereupon close to keep the relay 28 energized independently of contacts 24b of relay 24, and contacts 28b close to energize the timing relay 29. If, however, the alternator load rises above 20% before the expiry of the 4-second interval, relay 23 pulls on, so that relay 27 is re-energized and relays 28, 29 are de-energized. Since the resetting winding 20c remains energized, the loadsensitive device 21 remains ineffective to control the servomotor 17. If, eventually, the alternator load rises above 30%, the pressure relay 13 operates to re-open its contacts 13b to de-energize the timing relay 25 and re-close its contacts 13a to re-energize the relay 24. The resultant closure of contacts 24a energizes the tripping coil 26d of the relay 26 to trip the latter, so that contacts 26b close, contacts 26e open to deenergize the tripping coil 26d, and contacts 26c open to de-energize the resetting winding 20c so as to return the magnetic amplifier 20 to normal working. Instead of being energized by contacts 13b, the timing relay 25 may be energized by further contacts of the relay 23. If it is not necessary to disconnect the load circuit from the alternator under persistent fault conditions, the timing relay 29 and its associated control apparatus are omitted.