Abstract:
A method for controlling a turbine-generator including: detecting a power-load unbalance between a turbine and a generator; measuring the duration of a power-load unbalance; measuring the rate of loss of an electrical load; and regulating steam flow through the turbine responsive to the rate of loss of an electrical load and the duration of the power-load unbalance; all of which results in more accurate and robust control of turbine-generator speed.

Description:
BACKGROUND  
       [0001]     This disclosure relates generally to control systems for turbines, and more specifically to measuring generator operating parameters to detect a loss of load that results in a large unbalance between power and load that would result in turbine-generator overspeed trip and provide turbine overspeed protection that will avoid a turbine trip due to the emergency overspeed trip device.  
         [0002]     In a typical reheat turbine speed-load control system, valves regulate steam flow to both the high-pressure and reheat stages of the turbine in order to maintain the turbine speed at a predetermined reference speed. A main control valve regulates steam flow into the high-pressure stages and an intercept control valve regulates steam flow into the reheat stages. On the occurrence of an overspeed condition while the generator is separated from the system, the main control valve and intercept control valve are totally or partially closed to decrease steam flow into their respective stages to effect a reduction of turbine speed to the reference speed.  
         [0003]     These reheat turbine speed-load control systems operate effectively to prevent an emergency overspeed trip on the occurrence of sudden total load loss where the load lost is small compared to the total generating capacity of the turbine-generator set. In this case a rapid complete closure of the intercept control valve causes the loss of a major portion of the driving power normally provided by the reheat stages and similarly steam flow to the high-pressure stages is reduced by closure of the main control valve. The control basis for these valve closures is the difference between the actual speed and the reference speed.  
         [0004]     On the occurrence of sudden severe total loss of load where the load lost is comparable to a significant portion of the generating capacity of the turbine generator set, the turbine speed-load control systems, as described above, can not prevent an emergency overspeed trip. Delaying rapid complete closure of the intercept valve and control valve until the actual speed increases above the reference speed in order to avoid an emergency overspeed trip is not effective due to the larger control and intercept valve opening and the larger power density at high load conditions. Initiation of control valve and intercept valve closure must be advanced in time sufficiently to begin cut-off of steam prior to actual speed increase. Total electrical load loss is characteristically a rapid event when compared to rate of change for turbine power production. This situation can be used to provide detection of the conditions leading to turbine generator overspeed as described in this disclosure and prior art. When the characteristic conditions of load loss and unbalance between power and load are detected as described in this disclosure, rapid closure of turbine steam control valve and intercept valve is initiated, removing a major portion of the driving power to avoid an emergency overspeed trip. Hence, once the steam supply to the turbine is cut-off, the speed will increase due to the entrained steam mass as it proceeds to exhaust, but the speed increase will not be as rapid and will peak at a lower value thus avoiding an emergency overspeed trip. After the entrained steam is exhausted, speed will decrease and the turbine intercept valve and control valve will open based on speed-load control.  
       BRIEF DESCRIPTION  
       [0005]     Disclosed herein is a method for controlling a turbine-generator including: detecting an unbalance between turbine mechanical power and generator electrical load; measuring a duration of the power-load unbalance; measuring a rate of loss for an electrical load; and regulating steam flow through said turbine responsive to the rate of loss for an electrical load and the duration of said power-load unbalance.  
         [0006]     Further disclosed herein is a turbine-generator control system including: a power-load unbalance module; a turbine speed-load control module; and one or more close override devices wherein the power-load unbalance module detects a power-load unbalance between a turbine and a generator and identifies a cause of the power-load unbalance.  
         [0007]     Yet further disclosed herein is a system for anticipating and limiting overspeed of a turbine including: means for improved detection of a power-load unbalance; means for determining the duration of the power-load unbalance; means for calculating a current rate of change; and means for regulating steam flow through the turbine being responsive to at least one of the means for detecting a power-load unbalance, the means for determining the duration of the power-load unbalance, and the means for calculating a current rate of change.  
         [0008]     Other systems, methods, and/or computer program products according to exemplary embodiments will be or become apparent to one with skill in the art upon review of the following drawings and detailed description. It is intended that all such additional systems, methods, and/or computer program products be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     These and other features, aspects, and advantages of the disclosure hereof will become better understood when the following detailed description is read with reference to the accompanying figures, wherein:  
         [0010]      FIG. 1  depicts a typical reheat turbine-generator and associated control system;  
         [0011]      FIG. 2  depicts a block diagram of an exemplary embodiment of a reheat turbine-generator control system;  
         [0012]      FIG. 3  depicts a block diagram of an exemplary embodiment of a change detect function; and  
         [0013]      FIG. 4  depicts a typical response of a generator rate filter. 
     
    
     DETAILED DESCRIPTION  
       [0014]      FIG. 1  illustrates a typical turbine-generator  10  that is controlled by a turbine control system  30 . The typical turbine-generator  10  illustrated is a tandem-compound reheat design and is connected to a steam boiler  12  with a steam reheater  26 . The tandem-compound turbine-generator consists of a high-pressure turbine  14 , a reheat turbine  16 , a low-pressure turbine  18 , a common shaft  20 , an electric generator  22 , and one or more valves. In the typical turbine-generator  10 , steam from the steam boiler  12  passes through a high-pressure turbine  14 , then through the steam re-heater  26  and on through the reheat turbine  16  and through the low-pressure turbine  18 . The high-pressure turbine  14 , the reheat turbine  16 , and the low-pressure turbine  18  are mechanically connected to the common shaft  20 . Steam flow through the multiple turbines stages causes the common shaft  20  to rotate, which in turn drives the electric generator  22  which generates electricity. The electric generator  22  supplies electric power to a load  28 . Steam flow through the multiple turbine stages and thus electric power production is controlled by a main steam stop valve  50 , an inlet control valve  52 , a reheat intercept valve  54 , and a reheat stop valve  56 . The main steam stop valve  50 , the inlet control valve  52 , the reheat intercept valve  54 , and the reheat stop valve  56  are controlled by the turbine control system  30 .  
         [0015]     The turbine control system  30  includes a turbine speed-load control module  32 , a power-load unbalance module  34 , and one or more fast close override modules  36 . The turbine speed-load control module  32  controls the position of the main steam stop valve  50 , the inlet control valve  52 , the reheat intercept valve  54 , and the reheat stop valve  56 . The power-load unbalance module  34  detects a stage pressure of the turbine, illustrated in  FIG. 1  as the steam leaving the high-pressure turbine  14 , and the current generated by generator  22 . Additionally, the power-load unbalance module  34  utilizes algorithms, described in further detail herein, to detect a loss of load event and to issue a power-load unbalance event signal to the fast close override devices  36  associated with the inlet control valve  52  and the reheat intercept valve  54 . The fast close override devices  36  cause the inlet control valve  52  and the reheat intercept valve  54  to rapidly close and cut off steam to the steam turbine resulting in turbine power reduction. The reopening of the inlet control valve  52  and the reheat intercept valve  54  is coordinated by both the power-load unbalance module  34  and the speed-load control module  32  to prevent overdriving of the load and consequent emergency overspeed trip action. In an exemplary embodiment, the functions of the power-load unbalance module  34  and the turbine speed-load module  32  are combined into a single control module.  
         [0016]     Turning now to  FIG. 2 , a block diagram of an exemplary embodiment of a turbine control system  30  is shown. The turbine control system  30  includes one or more sensing devices  98 , such as current transformers, which are used to sense one or more generator phase currents. The generator phase currents are rectified and presented to a summing junction  100  that generates a total summed rectified current. The total rectified current is then processed by one or more multipliers  102  and  104  to yield a per unit current signal  107 . A turbine stage pressure signal, here reheat pressure signal  105 , which is a measurement of turbine mechanical power, is normalized by multiplier  106  to yield a per unit mechanical power signal  109 . The per unit current signal  107  is subtracted from the per unit mechanical power signal  109  by a summing junction  108  and then compared to a threshold value  111  by a comparator  112 . The comparator  112  generates a Boolean signal, true or false, which is presented to a time delay pick up  116 .  
         [0017]     The time delay pick up  116  is programmed to output a power-load unbalance out of limit signal  120  which is a logic signal of true when the input to the time delay pick up  116  is true and remains true for at least ten milliseconds. Otherwise, the power-load unbalance out of limits signal  120  remains at a logic level of false. The minimum sustained power unbalance duration is verified by the time delay pick up  116 . The input to the time delay pick up  116  must be held true for ten milliseconds before the output of the time delay pick up  116  is set to true. If the input to the time delay pick up  116  becomes false before the end of ten milliseconds then the ten millisecond timer of the time delay pick up  116  is reset to zero and must again be held true for a minimum of ten milliseconds. At any time, if the input to the time delay pick up  116  goes to a logic level of false, the power-load unbalance out of limits signal  120  will go to a logic level of false. It will be appreciated that the duration of time required for the time delay pick up  116  can be set to any suitable time and that the time of ten milliseconds as used, based on the transient electrical load characteristics detected by sensing devices  98 , herein is the amount of time used in an exemplary embodiment of the time delay pick up  116 .  
         [0018]     Continuing with  FIG. 2 , a rate of change detect function  110  computes the measured current rate of change. The details of the rate of change detect function  110  are illustrated in  FIG. 3 . The output of the rate of change detect function  110  is compared to a per unit current rate threshold signal  113  with a comparator  114 . In an exemplary embodiment, the per unit current rate threshold signal  113 , based on the transient electrical load characteristics, is set to a value of −37.5 PU/sec. If the per unit current rate threshold signal  113  is exceeded by the output of the rate of change detect function  110  and the input to the time delay pick up  116  has been sustained for ten milliseconds or more, then an edge-triggered pulse  118  will be generated as a power-load unbalance current rate out of limit signal  122 . The power-load unbalance current rate out of limit signal  122 , which will last for a configured duration of time, is necessary for the reheat intercept valve  54  to function in the desired manner; that is to actuate and then close after the configured duration. In an exemplary embodiment, the configured duration of time is at least twelve milliseconds.  
         [0019]     The turbine control system  30  also includes a AND gate  124 , an OR gate  126 , a master set-reset logic element  136 , a secondary set-reset logic element  128 , and one or more time delay pick up elements  130 ,  132 . It is necessary for both the power-load unbalance current rate out of limit signal  122  and the power-load unbalance out of limit signal  120  to be simultaneously true in order to initiate and latch a power-load unbalance event, this condition is determined by the AND gate  124 . When the power-load unbalance out of limit signal  120  becomes false it initiates a reset signal to the master set-reset latch element  136 , which clears the power-load unbalance event. In an exemplary embodiment of the reheat turbine-generator control system  30 , an optional programmable delay time may be added before a power-load unbalance event is initiated. The optional programmable delay time is controlled by the OR gate  126 , the secondary set-reset latch element  128 , and the time delay pick up elements  130 ,  132 . The power-load unbalance event will use the optional programmable delay time only if power-load unbalance delay enabled signal  134  has a logical value of true.  
         [0020]     Turning now to  FIG. 3 , a block diagram of an exemplary embodiment of the rate of change detect function  110  is shown. The rate of change detect function  110  has one or more inputs including: the per unit current  107 ; a power unbalance logic signal  200 ; and a nominal per unit current operating point of the turbine-generator system  202 . The output of the rate of change detect function  110 , a per unit current rate of change  226 , is characterized by the following equation:  
         dl   dt     =         PerUnit   ⁢           ⁢     Current   ⁡     (     t   2     )         -     PerUnit   ⁢           ⁢     Current   ⁡     (     t   1     )               t   2     -     t   1             
 
         [0021]     In order to make the above calculation the per unit current  107  at time t 1  and the per unit current  107  at time t 2  must be determined. In an exemplary embodiment, the per unit current  107  is sampled at 500 Hz and stored in a circular list  208 , which is used in calculating the per unit current rate of change  226 .  
         [0022]     Continuing with  FIG. 3 , a rate discriminator filter  206  is characterized by the following transfer function:  
         H   ⁡     (   s   )       =     S     S   +   50           
 
 The rate discriminator filter  206  has the desired characteristic for detecting an actionable current rate of change when used with rectified current inputs. When the output of the rate discriminator filter  206  drops below a set threshold, a large current drop is indicated. After detecting a large current drop, an initial time tag (t 1 )  214  is then stored and the sampled output of the discriminator rate filter  206  is stored in a buffer  222 . The set threshold level of the discriminator rate filter  206  is based on the nominal per unit current operating point of the turbine-generator system  202 . 
 
         [0023]     Additionally, when the power unbalance logic signal  200  is a logical true the output of the discriminator rate filter  206 , which is stored in the buffer  222 , is scanned to find a minimum and the corresponding time tag (t 2 )  216 . If a minimum is not achieved in the ten millisecond time frame then the current measured at ten milliseconds is used as the minimum and time tag (t 2 )  216  is thereby established. With the time tag (t 1 )  214  and time tag (t 2 )  216  established, the per unit current stored in the circular list  208  can be then accessed for the computation of the current rate of change  226 . As depicted in  FIG. 2 , the output of the rate of change detect function  110 , the per unit current rate of change  226 , is sent to per unit current rate of change comparator block  114  for signal processing of the logic signal to generate a twelve millisecond pulse  118  indicating that a power-load unbalance current rate out of limit has been detected.  
         [0024]     A power-load unbalance event is triggered when both the power-load unbalance current rate out of limit signal  122  and the power-load unbalance out of limit signal  120  are true. Once a power-load unbalance event is triggered the reheat turbine-generator control system  30  actuates at least one of the main steam stop valve  50 , the inlet control valve  52 , the reheat intercept valve  54 , and the reheat stop valve  56 . The fast close override devices  36  may be used to cause the inlet control valve  52  and the reheat intercept valve  54  to rapidly close and cut off steam to the steam turbine resulting in turbine power reduction. Additionally, the reopening of the inlet control valve  52  and the reheat intercept valve  54  is coordinated by both the power-load unbalance module  34  and the speed load control module  32 .  
         [0025]      FIG. 4  illustrates a typical response of the discriminator rate filter  206  to a full load loss, in this example a 100% load loss is experienced in 0.006 seconds as established by 60-Hz line characteristics. The characteristics of the discriminator rate filter  206  show an output dynamically changing when there is a rapid change in input. Furthermore, the discriminator rate filter  206  output relaxes back to a “zero” output when the input is no longer changing. The output minimum of the discriminator rate filter then establishes the point in time at which the input stopped falling. This characteristic is very useful for establishing the time tag t 2   216  and further to more accurately determine per unit current rate of change.  
         [0026]     While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.