Patent Application
20060042265
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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.
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
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.
The turbine overspeed protection electro-mechanic subsystem of a turbine automatic control system is shown on
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
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
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's 120 rotational speed by the at least one speed MPU 190 activated by the gear 192. The controller 185 controls the turbine'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
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.
In
A speed controller 230 may comprise one or more separate components. The speed controller's 230 functions may include any of the following:
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
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.
