Abstract:
A method and apparatus for turbine overspeed protection, useful for steam and gas turbines, is disclosed. The apparatus comprises a spring-loaded rod held by a plurality of energized solenoids in an operating position any time the turbine&#39;s shaft rotational speed is less than a trip rotational speed set-point. When the rotational speed reaches the trip rotational speed set-point, both solenoids are de-energized and the spring-loaded rod moves to provide turbine trip. Increased reliability of the solenoids is provided by compressing the spring during the resetting of the rod with an additional electromechanical actuator and by using a plurality of solenoids, each of which is able to provide the force required to hold the spring in its compressed state.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   Not applicable. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable. 
   REFERENCE TO MICROFICHE APPENDIX 
   Not applicable. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to overspeed protection. In particular, this invention relates to a method and apparatus for overspeed protection of a gas or steam turbine driving an electrical generator or other load from which the power consumed may rapidly drop. 
   2. Background Art 
   Generator breaker opening and other forms of rapid generator unloading can result in very high turbine shaft acceleration. Typically, a turbine will have a general speed control system, providing startup features and is made to maintain the turbine in continuous operation. Such a control system may or may not have an overspeed protection function. In addition, the turbine also typically has a dedicated overspeed protection system. When the speed control system does not operate properly, or when an upset occurs outside the ability of the speed control system to control, only the turbine overspeed protection system can prevent damage to the turbine and turbine shaft. 
   Traditionally, dedicated overspeed protection for gas and steam turbines was usually provided by a spring-loaded eccentric bolt (installed inside the turbine shaft) or a spring-loaded piston (installed outside the turbine shaft). Under high rotational speed conditions, either of these mechanisms was forced by centrifugal force to strike a lever providing a trip by closing the governor valves and trip valve(s), resulting in a turbine overspeed trip. Due to friction and wear, often an eccentric bolt does not work precisely and reliably. As a result, these bolts are now often replaced by an electronic overspeed trip device with electrical output acting on the lever or a spring-loaded rod or the valve itself. 
   The usual configuration for an electronic overspeed trip device comprises a solenoid valve which restrains the spring-loaded rod or valve when it is energized. Under normal turbine loading, this solenoid is energized. If the turbine experiences a high rotational speed, the solenoid is de-energized by the electronic overspeed trip device and the turbine trips and decelerates, perhaps shutting down entirely. Such an episode may occur immediately after an opening of the generator breaker or rapid generator unloading. A disadvantage of this solution is the high solenoid current required for spring compression for resetting the rod or valve decreases the reliability of the electronic overspeed trip device circuitry. 
   An unreliable solenoid power supply circuit may be the cause of false turbine trips due to insufficient current from the power supply. 
   BRIEF SUMMARY OF THE INVENTION 
   An object of this invention is the increased reliability of control of a solenoid restraining a spring-loaded rod or valve upon an overspeed event of a gas or steam turbine. This object is achieved by compressing a spring, usually compressed by the solenoid, during a reset in order to provide reduce the load the solenoid is under, thus reducing the solenoid current and eliminating the need for additional relays. The spring compression is provided by an electromechanical device which is not electrically connected with the overspeed protection circuit. 
   In particular, the electromechanical device
         compresses the spring, thereby unloading the solenoid before and during reset, and   decompresses the spring, reloading the solenoid after reset.       

   These steps, provided by an electromechanical actuator and associated lever, are not otherwise part of the turbine overspeed protection. In other words, the electromechanical device only comes to bear during a reset after an overspeed trip event. 
   With the additional electromechanical device carrying out the above steps, high current is not required for the solenoid to reset the spring-loaded rod or valve, yet the solenoid still provides the necessary high force to hold the spring-loaded rod or valve until an overspeed event occurs. 
   In addition, the reliability of the overspeed protection system is further improved by the use of two solenoids, each of which providing sufficient force to hold the rod or valve in its operating position. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  is a schematic of a turbine overspeed protection electromechanical subsystem of an automatic turbine control system; 
       FIG. 2  is a schematic of a steam turbine and steam turbine control system; 
       FIG. 3  is a schematic of a gas turbine and gas turbine control system; and 
       FIG. 4  is a force-displacement plot for a solenoid. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The turbine overspeed protection electro-mechanic subsystem of a turbine automatic control system is shown on  FIG. 1 . The overspeed system shown in  FIG. 1  is shown in schematic form. Therefore, the orientation, that is, up and down and left and right, of the components in  FIG. 1  is not necessarily representative of an actual installation. However, it will be useful to refer to the orientation of  FIG. 1  in this specification. 
   Here a trip pilot valve  105  loaded by a spring  110  is connected with a trip lever  115  restrained (while the turbine  120  is loaded normally) by a hook on a protection lever  125 . Hydraulic connections of the trip pilot valve  105  with a hydraulic resetting device and with stop and governor valve actuators are not shown. The protection lever  125  is loaded by a protection lever spring  130 . 
   Engaging an end of the protection lever  125  opposite the protection lever spring  130 , is a spring-loaded rod  135  within a solenoid trip assembly  100 . A trip spring  140  applies force to the spring-loaded rod  135  in a downward direction according to the orientation of  FIG. 1 . Plates  145 ,  150  are fastened to the rod  135  and function to anchor two solenoids  155 ,  160 . The present invention is not limited to a specific number of solenoids  155 ,  160 . A plurality of solenoids  155 ,  160  provide greater reliability than a single solenoid since each solenoid  155 ,  160  can provide adequate force to hold the trip spring  140  in compression. A sliding plate  165  engaged by the trip spring  140  can be forced upward (in the orientation of  FIG. 1 ), by an auxiliary lever  170 . The auxiliary lever  170  is actuated by an electromechanical actuator  175  which is equipped with limit switches  180 ,  181 . 
   The solenoids  155 ,  160  and the electromechanical actuator  175  are under the governance of a controller  185 . The controller  185  utilizes a signal from at least one (typically three) speed sensor such as a Magnetic Pickup Unit (MPU)  190  activated by a gear  192  turning on a turbine shaft  195  on which the electric generator  198  is installed. 
   The turbine overspeed protection electromechanical subsystem operates as follows. 
   Before turbine startup, the electromechanical actuator  175  actuates the auxiliary lever  170 . The auxiliary lever  170  engages the sliding plate  165  and forces it against the spring to its high limit position. The achievement of the high limit position is sensed by the limit switch  181  and a signal to this effect is sent to the controller  185 . Thus, the force of the spring  140  is removed from the rod  135 . When the sliding plate  165  reaches its high limit position, the controller  185  energizes the solenoids  155 ,  160 , and they move the rod  135  to its upper position. As illustrated in  FIG. 4 , the force-displacement characteristics of the solenoids  155 ,  160  are such that, when the rod  135  is in its upper position, the force exerted by the solenoids  155 ,  160  to the rod  135  is significantly greater than when the rod  135  is in a lower position. 
   With the rod  135  in its upper position, the electromechanical actuator  175  relaxes, permitting the sliding plate  165  to return to its lowered position. Upon reaching this lowered position, the lower limit switch  180  sends a signal to the controller  185 . By returning the sliding plate  165  to its lowered position, spring force is returned to the rod  135  from the spring  140 . In this state, the spring-loaded rod  135  is in position to provide a turbine trip effected by de-energizing the solenoids  155 ,  160  and permitting the spring-loaded rod  135  to engage the protection lever  125 . 
   Once the solenoids  155 ,  160  are holding the spring  140  in compression, the trip pilot valve  105  is moved to its top limit via hydraulic pressure upon a hydraulic reset signal from the hydraulic reset device (not shown). The trip lever  115  is raised by the trip pilot valve  105  during this action. Once the trip lever  115  is engaged to the protection lever  125 , the hydraulic reset signal ceases. In this position, the stop and governor valves may be manipulated by their actuators. 
   The turbine  120  is now prepared for startup. Under normal turbine load, the controller  185  monitors the turbine&#39;s  120  rotational speed by the at least one speed MPU  190  activated by the gear  192 . The controller  185  controls the turbine&#39;s  120  speed and/or droop. 
   However, should the rotational speed reach its trip set point, the controller  185  will de-energize the solenoids  155 ,  160 . With the solenoids  155 ,  160  de-energized, the spring-loaded rod  135  is forced downward by the spring  140  to a lower position where the spring-loaded rod  135  engages the protection lever  125 , forcing one end of the protection lever  125  downward in the orientation of  FIG. 1 . This action releases the trip lever  115  from its captive position hooked on the protection lever  125 . When the trip pilot valve  105  is released along with the trip lever  115 , the spring  110  forces the trip pilot valve  105  to its lower position, causing the closing of the stop and governor valves via their actuators controlled by the trip pilot valve  105 . Thus the turbine  120  no longer has energy input and is permitted to shut down. 
   Each solenoid  155 ,  160  is sized to provide sufficient force, alone, to maintain the spring  140  in its compressed state. Therefore, failure of either solenoid  155 ,  160 , singly, will not result in a false trip of the turbine  120 . 
     FIGS. 2 and 3  show how the present invention fits into a steam turbine control system and a gas turbine control system, respectively. 
   In  FIG. 2 , a steam turbine  210  is shown driving a load  220 . Examples of loads  220  driven by steam turbines  210  are generators  198 , compressors, and pumps. This invention is not limited to a particular load  220 . The load  220  may include a monitoring and/or control system for that load  220 . 
   A speed controller  230  may comprise one or more separate components. The speed controller&#39;s  230  functions may include any of the following:
         1. Startup sequencing.   2. Turbine rotational speed control.   3. Generator droop control.   4. Overspeed protection.   5. Emergency shutdown.       

   As input signals, the speed controller  230  receives information from at least one rotational speed sensor  240  such as an MPU. Preferably, a plurality of said rotational speed sensors  240  are utilized for additional reliability. In a typical installation, three such rotational speed sensors  240  are found. Additional input signals may include information about the load  220  such as a status of a generator breaker or an indication of surge in a compressor. Valve position signals may be fed back into the speed controller  230 , and other signals, typically found in turbine installations, may also be received by the speed controller  230 . With the information received as inputs, the speed controller  230  manipulates a trip and throttle valve  250  and a throttling valve or a steam rack  260  used for metering a steam flow rate through the steam turbine  210  for governing purposes. An overspeed function within the speed controller  210  system also controls the electromechanical actuator  175  for resetting the spring-loaded rod  135  and the solenoids  155 ,  160  within the solenoid assembly  100 . The solid arrows between the electromechanical actuator  175 , solenoid assembly  100  and the trip pilot valve  105  represent the mechanical interactions of the auxiliary lever  170 , protection lever  125 , and trip lever  115 . 
   Hydraulic fluid, shown as heavy, long dashed lines, passes through the trip pilot valve  105  before passing through individual pilot valves for the actuator manipulating the trip and throttle valve  250  and the throttling valve or steam rack  260 . In this way, if the trip pilot valve  105  is in its tripped position, the actuators for the trip and throttle valve  250  and the throttling valve or steam rack  260  will cause these valves to close, causing the steam turbine  210  to shut down. 
   A corresponding system for a gas turbine  310  is shown in  FIG. 3 . The load  220 , potentially with its control and/or monitoring system, is shown being driven off the turbine shaft  195 . 
   The fuel is metered into the gas turbine  310  through one or more fuel valves  350 ,  360 . The positions of these fuel valves  350 ,  360  are specified by the speed controller  230 . The actuators for the fuel valves  350 ,  360  are charged with hydraulic fluid that passes through the trip pilot valve  105 . Again, if the trip pilot valve  105  is in its tripped position, the actuators for the fuel valves  350 ,  360  will cause these valves to close, causing the gas turbine  310  to shut down. 
   The above embodiment is the preferred embodiment, but this invention is not limited thereto. It is, therefore, apparent that many modifications and variations of the present invention are possible in light of the above teachings. Hence, it is to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.