Abstract:
An object of the present invention is to provide a control system and a control method for servo valve which can stably continue to control a servo valve when a break occurs in servo coils of a servo valve having a plurality of servo coils.  
     In the present invention, voltages of a plurality of servo coils  3 A,  3 B,  3 C are detected by wire break detecting means individually provided to servo amplifiers  2 A,  2 B.  2 C. The servo amplifiers  2 A,  2 B.  2 C performs wire break detection and wire break compensation.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a control system for servo valve which controls opening degree of the servo valve having a plurality of servo coils used in the field of chemical plant or electric-power generating plant and a control method for the servo valve, and a combined-cycle electric-power plant and a gas turbine plant having the control system for servo valve.  
         DESCRIPTION OF THE RELATED ART  
         [0002]    In general, in the field of chemical plant or electric-power generating plant, a servo valve having a plurality of servo coils is used as a valve for controlling an important plant amount. Even if a break occurs in one of the servo coils, the servo valve having a plurality of servo coils can control the servo valve by compensating the break in the servo coil using the other sound servo coils, and this can prevent a shutdown or a power swing of the plant, which in turn can improves the reliability of the operation.  
           [0003]    In order to perform the compensation of servo coil break, it is necessary to detect occurrence of a break in the servo coils. In the past, detection of wire break in the servo coil has been detected by a control operation unit such as a microcomputer outputting a servo control voltage demand based on an opening degree difference between a target opening degree and a real opening degree of the servo valve. This technology is disclosed, for example, in Japanese Patent Application Laid-Open No. 9-126351.  
           [0004]    In addition, there is proposed a method in which a break detecting resistor is connected to a plurality of servo coils in common, and a break in the servo coil is detected by a voltage of the break detecting resistor.  
           [0005]    Since the technology disclosed in Japanese Patent Application Laid-Open No. 9-126351 detects a break in servo coils using the control operation unit such as a microcomputer, the detection is dependent on the operation cycle, and accordingly there is a problem in the high speed operability. Since the operation cycle (sampling cycle) is 50 ms to 100 ms, the technology can not cope with the requirement of 5 ms.  
           [0006]    On the other hand, the technology of using the common break detection resistor to the plurality of servo coils has a problem in that when a failure occurs in the common break detection line, the function of break detection is lost.  
         SUMMARY OF THE INVENTION  
         [0007]    An object of the present invention is to provide a control system and a control method for servo valve which can stably continue to control a servo valve when a break occurs in servo coils of a servo valve having a plurality of servo coils, and to provide a combined-cycle electric-power generating plant and a gas turbine plant having the control system for servo valve.  
           [0008]    The present invention is characterized by that a break in a servo coil is detected by a voltage in each of a plurality of servo coils.  
           [0009]    A preferred embodiment of the present invention is that detection of wire break in the servo coil and compensation of wire break are performed by servo amplifiers.  
           [0010]    That is, since a break in the servo coil is detected by a voltage in each of a plurality of servo coils, the break can be instantaneously detected. Further, since the compensation of wire break in the servo coil is performed by sharing the load current of the broken servo coil among the unbroken servo coils, the control of servo valve can be stably continued. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0011]    [0011]FIG. 1 is a block diagram showing an embodiment in accordance with the present invention.  
         [0012]    [0012]FIG. 2 is a detailed block diagram showing an example of the microcomputer of FIG. 1.  
         [0013]    [0013]FIG. 3 is a detailed block diagram showing an example of the servo amplifier of FIG. 1.  
         [0014]    [0014]FIG. 4 is a graph explaining operation of the present invention.  
         [0015]    [0015]FIG. 5 is a graph explaining operation of the present invention.  
         [0016]    [0016]FIG. 6 is a graph explaining operation of the present invention.  
         [0017]    [0017]FIG. 7 is a graph explaining operation of the present invention.  
         [0018]    [0018]FIG. 8 is a graph explaining operation of the present invention.  
         [0019]    [0019]FIG. 9 is a graph explaining operation of the present invention.  
         [0020]    [0020]FIG. 10 is a graph explaining operation of the present invention.  
         [0021]    [0021]FIG. 11 is a graph explaining operation of the present invention.  
         [0022]    [0022]FIG. 12 is a system diagram showing a combined-cycle electric-power generating plant to which the control system foe servo valve in accordance with the present invention is applied.  
         [0023]    [0023]FIG. 13 is a system diagram showing a gas turbine plant to which the control system foe servo valve in accordance with the present invention is applied. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    An embodiment of the present invention is shown in FIG. 1 to FIG. 3. FIG. 1 is a block diagram showing the embodiment in accordance with the present invention. FIG. 2 is a detailed block diagram showing an example of the microcomputer of FIG. 1. FIG. 3 is a detailed block diagram showing an example of the servo amplifier of FIG. 1. Therein, the detailed block diagrams of the examples shown be FIG. 2 and FIG. 3 are the same structures for the microcomputers  1 B,  1 C and the servo amplifiers  2 B,  2 C, respectively.  
         [0025]    In FIG. 1, the microcomputer  1 A, the servo amplifier  2 A and the servo coil  3 A are connected so as to form one set, and the microcomputer  1 B, the servo amplifier  2 B and the servo coil  3 B are connected so as to form one set, and the microcomputer  1 C, the servo amplifier  2 C and the servo coil  3 C are connected so as to form one set. Although FIG. 1 shows the example of three servo coils  3 A to  3 C, the present invention is not limited to three servo coils, and can be applied to a case of two servo coils or a case of four or more servo coils.  
         [0026]    The embodiment will be explained by taking the set of the microcomputer  1 A, the servo amplifier  2 A and the servo coil  3 A.  
         [0027]    Real opening degrees AI 1 , BI 1  of the servo valve  4  are input to the microcomputer  1 A shown in FIG. 2 from the opening meters  5 A,  5 B, and a practical opening value is selected by a high-value selector  7 , and a control command based on a difference between a preset target value and an opening degree signal output from the high-value selector  7  is generated by a difference calculator  8 , and then the control command is transmitted to the servo amplifier  2 A by outputting it to an operational amplifier  25  shown in FIG. 3.  
         [0028]    Therein, in a switch (T 1 )  10  shown in FIG. 2, when a state D 1  for starting servo valve control is input from a control start relay  6  to bring the switch (T 1 )  10  in a starting state (the relay contact is ON), the output from the difference calculator  8  is selected to bring the system to the control mode state. When the control start relay  6  is OFF state (the relay contact is OFF), the switch  10  selects an output of a constant value generator  9  to keep the command to the servo valve  3 A in an initial state, that is, a full-open or full-close state.  
         [0029]    The command to the operational amplifier  25  is operated by a multiplier  11  to be output as a servo control command. In the servo amplifier  2 A, the operational amplifier  25  receiving the servo control command transmits the output to a voltage/current converter  26  to convert the voltage to a current to be output.  
         [0030]    A contact signal of the control start relay  6  is input to a switch  27 , and the current is output to the servo coil  3 A if the control start relay  6  is in ON-state to turn the switch  27  ON.  
         [0031]    Similarly, in the B system of the microcomputer  1 B, the servo amplifier  2 B and the servo coil  3 B, a current is output to the servo coil  3 B by the microcomputer  1 B and the servo amplifier card  2 B. In the C system of the microcomputer  1 C, the servo amplifier  2 C and the servo coil  3 C, a current is also output to the servo coil  3 C by the microcomputer  1 C and the servo amplifier card  2 C.  
         [0032]    Thus, the servo valve  4  is driven by the sum of the current signals individually output to the servo coil  3 A, the servo coil  3 B and the servo coil  3 C.  
         [0033]    Therein, the servo amplifiers  2 A to  2 C may be mounted on a single card.  
         [0034]    [0034]FIG. 4 is shows an example of the characteristic of an operation result executed by the microcomputer  1 . The difference (mA) between the target value (mA) and a current value corresponding to the detected opening degree (%) value output from the high-value selector  7  is calculated by the difference calculator  8 , and the magnitude of the calculation result of difference is output to the servo amplifier  2 A as the servo control command.  
         [0035]    [0035]FIG. 5 shows an example of changes in current flowing in the servo coils  3 A to  3 C and changes in total current flowing in the servo coils. The differences obtained from calculations in the microcomputers  1 A to  1 C are input to the servo amplifiers  2 A to  2 C, and currents are out put to the servo coils  3 A to  3 C, respectively. The servo coils  3 A to  3 C receive the output currents having a current error of ±8 mA and a voltage of 1V to 5V from the servo amplifiers  2 A to  2 C to drive the servo valve  4 . That is, when the real opening value is equal to the target value, the current outputs of the servo amplifiers  2 A to  2 C are 0, and the servo valve  4  is in balance by 3V (assuming that the resistance of each coil is 375Ω, a value of current flowing in it is 8 mA) per one servo coil  3  (the total voltage value is 9V by the three coils  3 A,  3 B,  3 C, and the total current value is 24 mA).  
         [0036]    The detection of wire break and the compensation of wire break in a case of a break in one servo coil will be described below.  
         [0037]    Referring to FIG. 3, if a wire break occurs in the servo coil  3 A, the wire break is detected by a wire break detection voltage monitoring part  28 A which monitors voltage of a current loop formed up to the servo coil  3 A by the current output from the voltage/current converter  26 . That is, the wire break is detected by a voltage drop of the wire break detection voltage monitoring part  28 A when the wire break occurs in the servo coil  3 A. The operational principle of the wire break detection by the voltage monitoring of the current loop is obvious from the Ohm&#39;s law.  
         [0038]    After the wire break detection voltage monitoring part  28 A detects the wire break, an OR circuit  31  transmits a wire break detection signal expressing the servo coil  3 A being broken from the servo amplifier  2 A to the microcomputer  1 A. In the microcomputer  1 A, by the wire break detection signal of the OR circuit  31 , a pickup timer  17  and a weight timer  18  shown in FIG. 2 execute operation to memorize the first wire break in a flip-flop  22 .  
         [0039]    Although the case of occurrence of a wire break in the servo coil  3 A is explained above, in a case of occurrence of a wire break in the servo coil  3 B or  3 C the similar operation is performed by the microcomputer  1 B or  1 C and the servo amplifier  2 B or  2 C.  
         [0040]    In the case of occurrence of the wire break in the servo coil  3 A, the switches  14  of the microcomputers  1 B,  1 C select 1.5 as the constant from the constant value generators  12  by the output of the OR circuits  29  to multiply the servo control command by 1.5 using the multipliers  11 . That is, in a case where a wire break occurs in one of the servo coils  3 A to  3 C (in this case,  3 A), the control command is increased to 1.5 times of the normal value in the microcomputers  1 .  
         [0041]    In the servo amplifier  2 A, the output of the weight timer  45  of the first wire break is memorized in a counter  46  (the first wire break is 1) to hold a monitor relay  47 . The operation of the compensation of wire break is performed by selecting the gain 1.5 of the constant value generator  40  using the switch  43  and by multiplying the output of the operational amplifier  25  by 1.5 using the multiplier  49 .  
         [0042]    Therein, FIG. 6 shows the control characteristics in the case where the wire break occurs in the servo coil  3 A. FIG. 7 shows the characteristics of the operation of voltage signal in the servo amplifier  2 A and the output to the servo valve  4  at compensating the wire break.  
         [0043]    It is clear from FIG. 7 that the output of the servo coil  3 A is decreased because the wire break occurs in the servo coil  3 A, but the outputs of the servo coil  3 B and the servo coil  3 C are increased by the gain of 1.5, and accordingly the output of the servo valve  4  is not changed and kept in a state similar to the state of three coils alive. That is, as shown in FIG. 6, by increasing values of the respective control commands for the servo coils  3 B and  3 C to 12 mA, the opening degree (%) of the servo valve  4  can be kept constant.  
         [0044]    Next, it is assumed that a wire break occurs in the servo coil  3 B under a state where control is being performed using the servo coil  3 B and the servo coil  3 C. In this case, the wire break is detected by the wire break detection voltage monitoring part  28 B of the servo amplifier  2 B, similarly to the wire break of the servo coil  3 A. After the wire break detection voltage monitoring part  28 B detects the wire break, the OR circuit  31  shown in FIG. 3 transmits a wire break detection signal expressing occurrence of the second wire break of the three coils from the servo amplifier  2 B to the microcomputer  1 B.  
         [0045]    In the microcomputer  1 B, by the wire break detection signal of the OR circuit  31  of the servo amplifier  2 B, the flip-flop  33  memorizing the first wire break and a time pickup  23  holds an AND circuit  30 . The switch  16  selects 3.0 from the constant value generator  15  to multiply the servo control command by 3.0 using the multiplier  11 .  
         [0046]    [0046]FIG. 8 and FIG. 9 show the control characteristics in the case where the wire break further occurs in the servo coil  3 B. At occurring of the wire break in the servo coil  3 B, the control command is increased to 3.0 times of the normal value in the microcomputers  1 , and at the same time the second wire break is memorized in the counter  46  (the second wire break is  2 ) in the servo amplifier  2 B. Further, the switch  44  selects the gain 3.0 of the constant value generator  41  by holding of a monitor relay  48 . The servo control command of the servo amplifier  25  is multiplied by 3.0 by the multiplier  49  to perform operation of the compensation of the wire break. It can be understood from FIG. 9 that the output of the servo coil  3 B is decreased because of occurrence of the wire break, but the output of the servo coil  3 C is increased by the gain of 3.0, and accordingly the total output of the servo valve  4  is not changed. That is, as shown in FIG. 8, by increasing the value of the control command for the servo coil  3 B to 24 mA, the opening degree (%) of the servo valve  4  can be kept constant.  
         [0047]    In order to perform a rapid closing (or rapid opening) operation of the servo valve  4 , the output of the servo coil  3 C is changed to 0. That is, the control start relay  6  in the microcomputer  1 C is switched OFF. Since the switch (T 1 )  10  selects 0 of the output of the constant value generator  9 , the servo control command of the microcomputer  1  rapidly falls to 0, as shown in FIG. 10. Therefore, the servo amplifier  2 C generates a fully-closing output, and the servo valve  4  is rapidly closed, as shown in FIG. 11.  
         [0048]    The servo valve is controlled as described above. Since a break in the servo coil is detected by a voltage in each of a plurality of servo coils, the break can be instantaneously detected. Further, since the compensation of wire break in the servo coil is performed by sharing the load current of the broken servo coil among the unbroken servo coils, the control of servo valve can be stably continued.  
         [0049]    Further, in the above embodiment, when operation of rapid closing or rapid opening of the servo valve is required, the operation time of the servo valve can be shortened by directly inputting the condition to the servo amplifier. Therein, at that time when the servo valve is fully closed or fully opened, the load current applied to the servo coils does not continue to be supplied to the servo coils, but is interrupted. Therefore, the life time of the coils can be extended.  
         [0050]    As described above, since a break in the servo coil is detected by a voltage in each of a plurality of servo coils, the break can be instantaneously detected. Further, since the compensation of wire break in the servo coil is performed by sharing the load current of the broken servo coil among the unbroken servo coils, the control of servo valve can be stably continued.  
         [0051]    Description will be made below on a case where the control system for servo vale described above is applied to a combined-cycle electric-power plant.  
         [0052]    [0052]FIG. 12 is a system diagram showing an embodiment of a combined-cycle electric-power generating plant. Referring to FIG. 12, the combined-cycle electric-power generating plant comprises a gas turbine unit composed of a compressor  50  for compressing air, a combustor  51  for burning a gasified fuel such as LNG or the like, and a turbine  52  which is driven by burned gas obtained by mixing the air compressed by the compressor  50  and the burned gas burned by the combustor  51 ; a generator  53  linked to the gas turbine; an exhaust heat recovery boiler  54  for generating steam using high temperature exhaust gas exhausted from the gas turbine  52 ; and a steam turbine  55  driven by the generated steam from the exhaust heat recovery boiler  54 , and further comprises a condenser for condensing steam from the steam turbine  55  and a pump  57  for feeding condensate water to the exhaust heat recovery boiler  54 .  
         [0053]    In the combined-cycle electric-power plant shown by FIG. 12, the gas turbine system and the steam turbine  55  are coaxially linked to the generator  53  in order to make the plant compact. However, there are some cases where the gas turbine system and the steam turbine  55  are separately linked to individual generators.  
         [0054]    In the combined-cycle electric-power plant, a servo valve  4 A is placed in a pipe for conveying the gasified fuel such as LNG or the like to the combustor  51 . The servo valve  4 A is controlled by a gas turbine control unit  58 , but the gas turbine control unit  58  is constructed so as to include the control system for servo valve described in FIG. 1 to FIG. 3.  
         [0055]    Similarly, servo valves  4 B,  4 C,  4 D are individually placed in pipes conveying generated steam from the exhaust heat recovery boiler  54  to the steam turbine unit  55 . The servo valves  4 B,  4 C,  4 D are controlled by a steam turbine control unit  59 , but the steam turbine control unit  59  is constructed so as to include the control system for servo valve described in FIG. 1 to FIG. 3.  
         [0056]    By applying the servo valve control system as described above, since a break in the plurality of servo coils exciting to operate closing and opening each of the servo valves  4 A to  4 D is detected by a voltage in each of a plurality of servo coils, the break can be instantaneously detected. Further, since the compensation of wire break in the servo coil is performed by sharing the load current of the broken servo coil among the unbroken servo coils, the control of servo valve can be stably continued. Therefore, the combined-cycle electric-power generating plant can be stably operated.  
         [0057]    Description will be made below on a case where the control system for servo vale described above is applied to a gas turbine plant.  
         [0058]    [0058]FIG. 13 is a system diagram showing an embodiment of a gas turbine plant. Referring to FIG. 13, the gas turbine plant comprises a compressor  59  for compressing air; a combustor  60  for burning a gasified fuel such as LNG or the like; a turbine  61  which is driven by burned gas obtained by mixing the air compressed by the compressor  59  and the burned gas burned by the combustor  60 ; and a generator  62  linked to the gas turbine  61 .  
         [0059]    In the gas turbine plant, a servo valve  4 E is placed in a pipe for conveying the gasified fuel such as LNG or the like to the combustor  60 . The servo valve  4 E is controlled by a gas turbine control unit  63 , but the gas turbine control unit  63  is constructed so as to include the control system for servo valve described in FIG. 1 to FIG. 3.  
         [0060]    By applying the servo valve control system as described above, since a break in the plurality of servo coils exciting to operate closing and opening the servo valve  4 E is detected by a voltage in each of a plurality of servo coils, the break can be instantaneously detected. Further, since the compensation of wire break in the servo coil is performed by sharing the load current of the broken servo coil among the unbroken servo coils, the control of servo valve can be stably continued. Therefore, the gas turbine plant can be stably operated.