Patent GB-1244038-A (GB)
Title: Apparatus for preventing overspeed in elastic fluid turbine driven generators
IPC: F01D21/02
Filed: 1968-11-13
Abstract: 1,244,038. Turbines. TOKYO SHIBAURA DENKI K.K. Nov.13, 1968 [Nov.14, 1967], No.53796/68. Heading F1T. [Also in Division G3] An elastic-fluid driven turbine-electric generator set having speed responsive valve means normally controlling the flow of working fluid to the turbine to maintain the speed of the set substantially constant has overspeed prevention apparatus comprising means to over-ride the normal speed control to rapidly close the valve means when the electrical load rapidly falls. The control system is applicable to both gas or steam turbine driven generator sets. In an embodiment of the latter, steam flow to the set 20, Fig.1, is controlled by an admission valve 32 actuated by a hydraulic servo-mechanism 31 normally responsive to a speed governor 30. An intercept valve is disposed between turbine stages. A watt meter 21 measures the generator output, i.e. the turbine load, and has a contact 211 which closes when the load exceeds 75% full load and a contact 212 which closes when the load is less than 25% full load. Closure of contact 211 energizes a relay 24 which instantly closes a contact 241. Relay 24 incorporates a time delay device so that contact 241 remains closed for a few seconds after opening of contact 211 resulting from any fall in load below 75%. Thus should the load fall rapidly from above 75% to below 25% there is an instant when both contacts 212 and 241 are closed so that a relay 25 is energized. The latter operates to provide a command signal to the servo-mechanism 31 of admission valve 32. The servomechanism includes, Fig.3a, a three-way valve 38 which normally permits pressure liquid to flow to below a land 33c to urge a rod 34 upwards against spring 37 in a cylinder 36 so that the top of the rod is held against the top of the cylinder. -In this position rod 34 is effectively rigid with cylinder 36 and thus follows the movement imparted to the latter by an arm 41 connected to the governor so as to control, by way of lands 33a, 33b running in fixed cylinder 35, the flow of fluid to servopiston 39 of admission valve 32. However when relay 25 transmits the command signal, valve 38 relieves pressure from below land 33c so that rod 34 moves rapidly downwards under the action of spring 37, causing pressure to be relieved from below servopiston 39. The latter thus moves downwards under the action of spring 40 to close valve 32 irrespective of the signal from the governor and thus prevent overspeeding of the set. A timer 27 connected in parallel with the relay 25 is arranged to deenergize the latter after a predetermined interval so as to restore normal governor control. The overspeed apparatus does not over-ride normal governor control if the load removal is gradual or if the load is rapidly reduced to a valve exceeding 25%. Under conditions of poor quality admission steam the opening of valve 32 may exceed 75% even though the load is less than 75%. To prevent the risk of overspeeding if the load is rapidly reduced to below 25% in these circumstances a contact 26 is provided which closes when the opening of valve 32 exceeds 75%. The overspeed apparatus can also over-ride governor control of the intercept valve. In a modification, the electrical circuit of Fig.1 may be replaced by a transistor circuit including a meter utilizing the Hall effect in place of the Watt meter 21. When applied to a gas turbine plant the control system operates on the valve controlling the flow of motive gas to the turbine, the fuel injection valve being linked to this valve.