Method and apparatus for improving steam turbine control

Abstract

Steam turbine control systems often incorporate electromechanical pilot-valve actuator assemblies to regulate hydraulically-driven actuator pistons that modulate steam valves, thereby controlling turbine speed. The general operation and quick-response requirements of these pilot-valve actuator assemblies can suffer from shortcomings (such as, excessive friction, contaminated oil, and defective electromagnetic components) that degrade the control system's overall performance. For these reasons, this disclosure relates to a method for overcoming insufficient, pilot-valve actuator response by using an integrated control system dedicated to these electromechanical assemblies. Pilot-valve actuators can be equipped with a single coil or multiple coils (usually a primary and a secondary). In a dual-coil configuration, the primary is energized according to an output of a PID controller, whereas the secondary coil is regulated by a separate control element. When the speed at which the steam-valve actuator reacts is adequate, the energizing signals to the individual coils may be equal, or the secondary coil may not be energized at all. But whenever the steam-valve actuator's speed is not adequate, the secondary coil is quickly energized to a higher level; consequently assuring prompt and reliable response of the integrated, turbomachinery speed-control system.

We claim:  
     
         1 . A method for controlling a steam turbine system comprising a steam turbine, a hydraulic steam-valve actuator, instrumentation providing a signal proportional to a steam-valve actuator's piston velocity, a pilot valve for manipulating a position of the steam-valve actuator's piston, and an electromechanical pilot-valve actuator containing at least one individually-energized coil, the method comprising: 
 (a) sensing the steam-valve actuator's piston-velocity signal; and    (b) sending a signal to at least one individually-energized coil within the pilot-valve actuator, a strength of the signal being based on an error between the sensed piston velocity and a set point for the piston velocity.    
     
     
         2 . The method of  claim 1 , wherein the set point for the steam-valve actuator's piston velocity is calculated, based on an error between a steam turbine controlled-variable and a set point.  
     
     
         3 . The method of  claim 2 , wherein the steam turbine controlled-variable is a steam turbine rotational-speed.  
     
     
         4 . The method of  claim 2 , wherein the steam turbine controlled-variable is a steam-valve actuator's piston position.  
     
     
         5 . The method of  claim 1 , wherein the steam turbine system also comprises a control system sensing at least one controllable variable, and controlling the variable to the variable's set point; and at least one coil is energized by a signal, based on an output from the system controlling at least one controllable variable.  
     
     
         6 . The method of  claim 5 , wherein the controllable variable is the velocity of the steam-valve actuator's piston.  
     
     
         7 . The method of  claim 5 , wherein the controllable variable's set point is proportional to an error between a steam turbine rotational-speed and a rotational-speed set point.  
     
     
         8 . The method of  claim 5 , wherein the controllable variable's set point is proportional to an error between a steam-valve actuator's required piston velocity and an actual piston velocity.  
     
     
         9 . The method of  claim 5 , wherein the controllable variable is an error between a pilot-valve's position and a position at a predetermined point in time.  
     
     
         10 . The method of  claim 9 , wherein the predetermined point in time is the point at which a magnitude of the steam-valve actuator's piston velocity becomes less than a magnitude of a required piston velocity.  
     
     
         11 . The method of  claim 5 , wherein at least one coil is not energized whenever a magnitude of the steam-valve actuator's piston velocity is greater than or equal to a magnitude of a required piston velocity.  
     
     
         12 . The method of  claim 5 , wherein at least one coil is energized to a level determined by the sum of: 
 (a) a value proportional to the error between the sensed piston velocity and a set point for the piston velocity; and    (b) a value proportional to an output from the system controlling at least one controllable variable.    
     
     
         13 . The method of  claim 12 , wherein all coils are energized by values proportional to the sum.  
     
     
         14 . The method of  claim 1 , wherein the steam-valve actuator's piston-velocity signal is generated by the steps of: 
 (a) sensing a steam-valve actuator's piston position;    (b) generating a signal, based on the piston position; and    (c) computing a first time-derivative of the piston-position signal.    
     
     
         15 . The method of  claim 3 , wherein the set point for the steam-valve actuator's piston velocity is calculated by the steps of: 
 (a) calculating a speed controller output, based on the steam turbine rotational-speed and set point;    (b) sensing a steam-valve actuator piston position;    (c) calculating a difference between the speed controller output and the steam-valve actuator position; and    (d) calculating the set point for the steam-valve actuator's piston velocity as being proportional to the difference between the speed controller output and the steam-valve actuator position.    
     
     
         16 . An apparatus for controlling a steam turbine system comprising a steam turbine, a hydraulic steam-valve actuator, instrumentation providing a signal proportional to a steam-valve actuator's piston velocity, a pilot valve for manipulating a position of the steam-valve actuator's piston, and an electromechanical pilot-valve actuator containing at least one individually-energized coil, the apparatus comprising: 
 (a) means for sensing the steam-valve actuator's piston-velocity signal; and    (b) means for sending a signal to at least one individually-energized coil within the pilot-valve actuator, a strength of the signal being based on an error between the sensed piston velocity and a set point for the piston velocity.    
     
     
         17 . The apparatus of  claim 16  also comprising means for calculating the set point for the steam-valve actuator's piston velocity, based on an error between a steam turbine controlled-variable and a set point.  
     
     
         18 . The apparatus of  claim 17 , wherein the steam turbine controlled-variable is a steam turbine rotational-speed.  
     
     
         19 . The apparatus of  claim 17 , wherein the steam turbine controlled-variable is a steam-valve actuator's piston position.  
     
     
         20 . The apparatus of  claim 16 , wherein the steam turbine system also comprises a control system with means for sensing at least one controllable variable, and means for controlling the variable to the variable's set point; and at least one coil is energized by a signal, based on an output from the system controlling at least one controllable variable.  
     
     
         21 . The apparatus of  claim 20 , wherein the controllable variable is the velocity of the steam-valve actuator's piston.  
     
     
         22 . The apparatus of  claim 20  also comprising means for calculating the controllable variable's set point as proportional to an error between a steam turbine rotational-speed and a rotational-speed set point.  
     
     
         23 . The apparatus of  claim 20  also comprising means for calculating the controllable variable's set point as proportional to an error between a steam-valve actuator's required piston velocity and an actual piston velocity.  
     
     
         24 . The apparatus of  claim 20  also comprising means for calculating the controllable variable as an error between a pilot-valve's position and a position at a predetermined point in time.  
     
     
         25 . The apparatus of  claim 24  comprising means for determining the predetermined point in time as the point at which a magnitude of the steam-valve actuator's piston velocity becomes less than a magnitude of a required piston velocity.  
     
     
         26 . The apparatus of  claim 20 , wherein at least one coil is not energized whenever a magnitude of the steam-valve actuator's piston velocity is greater than or equal to a magnitude of a required piston velocity.  
     
     
         27 . The apparatus of  claim 20  including means for energizing at least one coil to a level determined by the sum of: 
 (a) a value proportional to the error between the sensed piston velocity and a set point for the piston velocity; and  
 (b) a value proportional to an output from the system controlling at least one controllable variable.  
 
     
     
         28 . The apparatus of  claim 27  including means for energizing all coils by values proportional to the sum.  
     
     
         29 . The apparatus of  claim 16 , wherein the steam-valve actuator's piston-velocity signal is generated, the apparatus comprising: 
 (a) means for sensing a steam-valve actuator's piston position;    (b) means for generating a signal, based on the piston position; and    (c) means for computing a first time-derivative of the piston-position signal.    
     
     
         30 . The apparatus of  claim 18 , wherein the set point for the steam-valve actuator's piston velocity is calculated, the apparatus comprising: 
 (a) a speed controller calculating its output, based on the steam turbine rotational-speed and set point;    (b) a steam-valve actuator piston-position sensor;    (c) a difference calculator for calculating a difference between the speed controller output and the steam-valve actuator position; and    (d) a set point calculator for calculating the set point for the steam-valve actuator's piston velocity as being proportional to the difference between the speed controller output and the steam-valve actuator position.