Abstract:
In a thyristor control device which converts a first AC voltage to a DC voltage and converts the DC voltage to a second AC voltage to be supplied to a synchronous motor, a DC voltage detector is configured to detect the DC voltage, and is provided with an AC voltage detector configured to detect the second AC voltage and an arithmetic circuit configured to determine the DC voltage on the basis of the second AC voltage detected by the AC voltage detector. As a result, there is no need to separately provide a DV voltage detector, which makes it possible to make the device compact in size and cheap in price.

Description:
TECHNICAL FIELD 
     The present invention relates to a DC voltage detector and a power converter using the same, and in particular relates to a DC voltage detector configured to detect a DC voltage and a power converter using the same. 
     BACKGROUND ART 
     A thyristor starter includes a converter configured to convert a first three-phase AC power to a DC power, a DC reactor configured to smooth a direct current, and an inverter configured to convert the DC power supplied from the converter via the DC reactor to a second three-phase AC power which is supplied to a synchronous motor. The thyristor starter further includes an AC voltage detector configured to detect a first three-phase AC voltage input to the converter and a second three-phase AC voltage output from the inverter, and a control circuit configured to control the converter and the inverter on the basis of a detection result from the AC voltage detector. By controlling the second three-phase AC power, it is possible to start the synchronous motor from a stopped state and make it rotate at a predetermined rotating speed (for example, see Japanese Patent Laying-Open No. 2003-61380 (PTD 1)). 
     CITATION LIST 
     Patent Document 
     PTD 1: Japanese Patent Laying-Open No. 2003-61380 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the thyristor starter, in order to prevent a DC voltage supplied to the inverter from becoming excessively great to break down the inverter or the like, and/or to monitor the ripples of the DC voltage, it is preferable to provide a DC voltage detector configured to detect the DC voltage. However, if the DC voltage detector is provided separately, the thyristor starter will be made large in size and expensive in price. 
     Therefore, a primary object of the present invention is to provide a DC voltage detector compact in size and cheap in price and a power converter using the same. 
     Solution to Problem 
     A DC voltage detector according to the present invention is configured to detect, in a power converter which converts a first AC voltage to a DC voltage and converts the DC voltage to a second AC voltage to be supplied to a load, the DC voltage, and is provided with an AC voltage detector configured to detect the first AC voltage or the second AC voltage; and an arithmetic circuit configured to determine the DC voltage on the basis of the first AC voltage or the second AC voltage detected by the AC voltage detector. 
     Preferably, the load is a synchronous motor. The power converter is a thyristor starter for starting the synchronous motor, and includes a converter configured to convert a first AC power to a DC power, a DC reactor configured to smooth a direct current, and an inverter configured to convert the DC power supplied from the converter via the DC reactor to a second AC power which is supplied to the synchronous motor. Each of the converter and the inverter includes a plurality of thyristors. Each of the first AC voltage and the second AC voltage is a three-phase AC voltage. The first AC voltage is a voltage input to the converter. The second AC voltage is a voltage output from the inverter, and the frequency of the second AC voltage is variable. 
     Preferably, the arithmetic circuit, on the basis of the first or second AC voltage detected by the AC voltage detector and a plurality of control signals for controlling the plurality of thyristors included in the converter or the inverter, determines the DC voltage output from the converter or the DC voltage input to the inverter. 
     A power converter according to the present invention includes a converter configured to convert a first AC power to a DC power, a DC reactor configured to smooth a direct current, an inverter configured to convert the DC power supplied from the converter via the DC reactor to a second AC power which is supplied to a load, an AC voltage detector configured to detect a first AC voltage input to the converter or a second AC voltage output from the inverter, and a control circuit configured to control the converter and the inverter on the basis of a detection result from the AC voltage detector. The control circuit is configured to determine a DC voltage output from the converter or a DC voltage input to the inverter on the basis of the first or second AC voltage detected by the AC voltage detector. 
     Preferably, each of the first AC voltage and the second AC voltage is a three-phase AC voltage, and the frequency of the second AC voltage is variable. The load is a synchronous motor, the power converter is a thyristor starter for starting the synchronous motor, and each of the converter and the inverter includes a plurality of thyristors. 
     Preferably, the control circuit is configured to generate a plurality of control signals for controlling the plurality of thyristors included in the inverter on the basis of a detection result from the AC voltage detector, and determine the DC voltage input to the inverter on the basis of the plurality of control signals and the second AC voltage. 
     Preferably, the control circuit is configured to generate a plurality of control signals for controlling the plurality of thyristors included in the converter on the basis of a detection result from the AC voltage detector, and determine the DC voltage output from the converter on the basis of the plurality of control signals and the first AC voltage. 
     Preferably, the thyristor starter starts a synchronous generator in a power plant as the synchronous motor. 
     Advantageous Effects of Invention 
     In the DC voltage detector according to the present invention, the DC voltage is determined on the basis of the first or second AC voltage detected by the AC voltage detector already existing in the power converter. Thereby, it is possible to make the power converter compact in size and cheap in price. 
     Moreover, in the power converter according to the present invention, the existing control circuit determines the DC voltage of one terminal or the other terminal of the DC reactor on the basis of the first AC voltage or the second AC voltage detected by the existing AC voltage detector. Thereby, it is not necessary to separately provide a DC voltage detector, which makes it possible to make the power converter compact in size and cheap in price. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a circuit block diagram illustrating a configuration of a thyristor starter according to an embodiment of the present invention, 
         FIG. 2  is a circuit diagram illustrating a main part of a synchronous motor illustrated in  FIG. 1 , 
         FIG. 3  is a circuit diagram illustrating a configuration of an arithmetic circuit included in a control circuit illustrated in  FIG. 3 , 
         FIG. 4  is a time chart illustrating operations of the arithmetic circuit illustrated in  FIG. 3 , and 
         FIG. 5  is another time chart illustrating operations of the arithmetic circuit illustrated in  FIG. 3 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     As illustrated in  FIG. 1 , a thyristor starter according to an embodiment of the present invention is configured to start a synchronous motor  11  in response to a three-phase AC power received from a commercial AC power supply (power system)  1 , and is provided with AC voltage detectors  2  and  7 , alternating current detectors  3  and  8 , a converter  4 , a DC reactor  5 , an inverter  6 , a rotor position detector  9 , and a control circuit  10 . The thyristor starter is further provided with a U-phase line UL, a V-phase line VL and a W-phase line WL which are all connected between commercial AC power supply  1  and converter  4 , and a R-phase line RL, an S-phase line SL and a T-phase line TL which are all connected between inverter  6  and synchronous motor  11 . 
     The three-phase AC voltage of a commercial frequency generated by commercial AC power supply  1  is supplied to converter  4  through U-phase line UL, V-phase line VL and W-phase line WL. AC voltage detector  2  detects a line voltage Vuv between U-phase line UL and V-phase line VL and a line voltage Vvw between V-phase line VL and W-phase line WL, and provides a signal indicating the detected values to control circuit  10 . Alternating current detector  3  detects a current Iu flowing through U-phase line UL, a current Iv flowing through V-phase line VL and a current Iw flowing through W-phase line WL, and provides a signal indicating the detected values to control circuit  10 . 
     Converter  4  converts a three-phase AC power from commercial AC power supply  1  to a DC power. Specifically, converter  4  includes thyristors  21  to  26 . The anodes of thyristors  21  to  23  are connected to U-phase line UL, V-phase line VL and W-phase line WL, respectively, and the cathodes thereof are all connected to a high-voltage output terminal  4   a . The cathodes of thyristors  24  to  26  are connected to U-phase line UL, V-phase line VL and W-phase line WL, respectively, and the anodes thereof are all connected to a low-voltage output terminal  4   b.    
     The gates of thyristors  21  to  26  receive control signals φ 21  to φ 26  from control circuit  10 , respectively. As control signals φ 21  to φ 26  are set at an “H” level, thyristors  21  to  26  are turned on accordingly. Turning on thyristors  21  to  26  at a predetermined timing makes it possible to convert the three-phase AC power to the DC power. 
     DC reactor  5  is connected between high-voltage output terminal  4   a  of converter  4  and a high-voltage input terminal  6   a  of inverter  6  and is configured to smooth the direct current. Low-voltage output terminal  4   b  of converter  4  is directly connected to a low-voltage input terminal  6   b  of inverter  6 . 
     Inverter  6  converts the DC power supplied from converter  4  through DC reactor  5  to a three-phase AC power of a desired frequency. Specifically, inverter  6  includes thyristors  31  to  36 . The anodes of thyristors  31  to  33  are all connected to high-voltage input terminal  6   a , and the cathodes thereof are connected to R-phase line RL, S-phase line SL and T-phase line TL, respectively. The anodes of thyristors  34  to  36  are connected to R-phase line RL, S-phase line SL and T-phase line TL, respectively, the cathodes thereof are all connected to low-voltage input terminal  6   b.    
     The gates of thyristors  31  to  36  receive control signals φ 31  to φ 36  from control circuit  10 , respectively. As control signals φ 31  to φ 36  are set at the “H” level, thyristors  31  to  36  are turned on accordingly. Turning on thyristors  31  to  36  at a predetermined timing makes it possible to convert the DC power to the three-phase AC power of a desired frequency. 
     The three-phase AC power generated by inverter  6  is supplied to synchronous motor  11  via R-phase line RL, S-phase line SL and T-phase line TL. As illustrated in  FIG. 2 , synchronous motor  11  includes a R-phase coil Cr, an S-phase coil Cs and a T-phase coil Ct. One terminal of each of coils Cr, Cs and Ct is connected to each of R-phase line RL, S-phase line SL and T-phase line TL, respectively. The other terminal of each of coils Cr, Cs and Ct is connected to a neutral point together. As coils Cr, Cs and Ct are supplied with the three-phase AC power, a rotating magnetic field is generated thereby to rotate a rotor (not shown). 
     Referring back to  FIG. 1 , AC voltage detector  7  detects a line voltage Vst between R-phase line RL and S-phase line SL and a line voltage Vst between S phase line SL and T-phase line TL, and provides a signal indicating the detected values to control circuit  10 . Alternating current detector  8  detects a current Ir flowing through R-phase line RL, a current Is flowing through S-phase line SL and a current It flowing through T-phase line TL, and provides a signal indicating the detected values to control circuit  10 . Rotor position detector  9  detects a rotor position of synchronous motor  11 , and provides a signal indicating the detected value to control circuit  10 . 
     Control circuit  10  controls converter  4  and inverter  6  on the basis of the signals from detectors  2 ,  3  and  7  to  9 . For example, control circuit  10 , on the basis of the detection results from AC voltage detector  2 , alternating current detector  3  and rotor position detector  9 , generates control signals φ 21  to φ 26  for thyristors  21  to  26  included in converter  4 . Further, control circuit  10 , on the basis of the detection results from alternating current detectors  3  and  8 , AC voltage detector  7  and rotor position detector  9 , generates control signals φ 31  to φ 36  for thyristors  31  to  36  included in inverter  6 . 
     In starting synchronous motor  11  from a stopped state, control circuit  10  gradually increases the output from inverter  6  and increases the frequency of the three-phase AC electric power in synchronization with the rotation of synchronous motor  11  at the same time so as to increase the rotating speed of synchronous motor  11  from 0 to a predetermined value. 
     Further, control circuit  10 , on the basis of control signals φ 31  to φ 36  and the detection results from AC voltage detector  7 , determines a DC voltage VDC between input terminals  6   a  and  6   b  of inverter  6 . The determined DC voltage VDC, for example, is used to prevent inverter  6  from being broken down as DC voltage VDC becomes excessive great (i.e., provide an overvoltage protection) and/or to monitor the ripples of DC voltage VDC. 
     Such kind of thyristor starter is used in a power plant, for example, to start a synchronous generator from a stopped state as a synchronous motor. When the synchronous generator is being driven to rotate at the predetermined rotating speed as the synchronous motor, while detaching the thyristor starter from the synchronous generator, the synchronous generator is driven to rotate by a gas turbine, for example, to generate AC power. 
       FIG. 3  is a circuit diagram illustrating a configuration of an arithmetic circuit  40  included in control circuit  10 . In  FIG. 3 , arithmetic circuit  40  is provided with AND gates  41  to  46 , switches  51  to  56 , subtractors  61 ,  63  and  66 , and adders  62 ,  64  and  65 . 
     AND gate  41  outputs a logical product signal of control signals φ 31  and φ 35 . AND gate  42  outputs a logical product signal of control signals φ 32  and φ 34 . AND gate  43  outputs a logical product signal of control signals φ 32  and φ 35 . AND gate  44  outputs a logical product signal of control signals φ 33  and φ 35 . AND gate  45  outputs a logical product signal of control signals φ 33  and φ 34 . AND gate  46  outputs a logical product signal of control signals φ 31  and φ 36 . 
     One terminal of switch  51  receives AC voltage Vrs. Switch  51  is turned on as the output signal of AND gate  41  is at the “H” level, and is turned off as the output signal of AND gate  41  is at an “L” level. One terminal of switch  52  receives AC voltage Vrs. Switch  52  is turned on as the output signal of AND gate  42  is at the “H” level, and is turned off as the output signal of AND gate  42  is at the “L” level. Subtractor  61  subtracts a voltage V 52  of the other terminal of switch  52  from a voltage V 51  of the other terminal of switch  51 . 
     One terminal of switch  53  receives AC voltage Vst. Switch  53  is turned on as the output signal of AND gate  43  is at the “H” level, and is turned off as the output signal of AND gate  43  is at the “L” level. One terminal of switch  54  receives AC voltage Vst. Switch  54  is turned on as the output signal of AND gate  44  is at the “H” level, and is turned off as the output signal of AND gate  44  is at the “L” level. Subtractor  63  subtracts a voltage of the other terminal of switch  54  from a voltage of the other terminal of switch  53 . 
     Adder  65  adds an AC voltage obtained by delaying the phase of AC voltage Vrs by 180° and an AC voltage obtained by delaying the phase of AC voltage Vst by 180° to generate an AC voltage Vtr (Vtr=−Vrs−Vst). One terminal of switch  55  receives AC voltage Vtr. Switch  55  is turned on as the output signal of AND gate  45  is at the “H” level, and is turned off as the output signal of AND gate  45  is at the “L” level. One terminal of switch  56  receives AC voltage Vtr. Switch  56  is turned on as the output signal of AND gate  46  is at the “H” level, and is turned off as the output signal of AND gate  46  is at the “L” level. Subtractor  66  subtracts the voltage of the other terminal of switch  56  from the voltage of the other terminal of switch  55 . Adders  62  and  64  are configured to generate a DC voltage V 62  by adding voltages V 61 , V 63  and V 64  output respectively from subtractors  61 ,  63  and  66 . 
     Diagrams (a) to (i) in  FIG. 4  and diagrams (a) to (e) in  FIG. 5  each is a time chart illustrating the operation of arithmetic circuit  40  illustrated in  FIG. 3 . As illustrated in the diagrams (a) to (f) of  FIG. 4 , each of control signals φ 31  to φ 36  is a clock signal having a predetermined cycle. If one cycle is expressed by 0 to 2π, control signal φ 31  becomes the “H” level from 0 to 2π/3 in one period. Control signal φ 32  becomes the “H” level from 2π/3 to 4π/3 in one cycle. Control signal φ 33  becomes the “H” level from 4π/3 to 2π in one period. 
     Each of control signals φ 34  to φ 36  is a signal obtained by delaying each control signals φ 31  to φ 32  by π, respectively. In other words, control signal φ 34  becomes the “H” level π from it to 5π/3 in one period. Control signal φ 35  becomes the “H” level from 0 to n/3 and from 5π/3 to 2π in one period. Control signal φ 36  becomes the “H” level from π/3 to π in one period. 
     Further, as illustrated in the diagram (g) of  FIG. 4 , AC voltage Vrs fluctuates sinusoidally, and it becomes a positive voltage from 0 to π and becomes a negative voltage from π to 2π. AC voltage Vst is a voltage obtained by delaying AC voltage Vrs by 2π/3. AC voltage Vtr is a voltage obtained by delaying AC voltage Vst by 2π/3. 
     During the interval of 0 to 7π/3, control signals φ 31  and φ 35  both become the “H” level, thyristors  31  and  35  in  FIG. 1  are turned on, and DC voltage VDC is applied to coils Cr and Cs in  FIG. 2 . The output signal of AND gate  41  in  FIG. 3  becomes the “H” level, and switch  51  is turned on. Thereby, as illustrated in the diagrams (a), (e), (g) and (h) in  FIG. 4 , Vrs=V 51 =VDC during the interval of 0 to 7π/3. 
     Further, during the interval of π to 4π/3, control signals φ 32  and φ 34  both become the “H” level, thyristors  32  and  34  in  FIG. 1  are turned on, and DC voltage VDC is applied to coils Cs and Cr in  FIG. 2 . The output signal of AND gate  42  of  FIG. 3  becomes the “H” level, and switch  52  is turned on. Thereby, as illustrated in the diagrams (b), (d), (g) and (i) in  FIG. 4 , Vrs=V 52 =−VDC during the interval of π to 4π/3. 
     Therefore, as illustrated in the diagram (a) of  FIG. 5 , output voltage V 61  of subtractor  61  is equal to DC voltage VDC from 0 to π/3 and from π to 4π/3 in one cycle. Similarly, as illustrated in the diagram (b) of  FIG. 5 , output voltage V 63  of subtractor  63  is equal to DC voltage VDC from 2π/3 to π and from 5π/3 to 2π in one cycle. Further, as illustrated in the diagram (c) of  FIG. 5 , output voltage V 66  of subtractor  66  is equal to DC voltage VDC from 7π/3 to 2π/3 and from 4π/3 to 5π/3. Thereby, as illustrated in the diagrams (d) and (e) of  FIG. 5 , output voltage V 62  of adder  62  is equal to DC voltage VDC from 0 to 2π. DC voltage VDC in the diagram (e) of  FIG. 5  is actually measured by a DC voltage detector connected between input terminals  6   a  and  6   b  of inverter  6  in  FIG. 1 . 
     As described above, in the present embodiment, arithmetic circuit  40  included in control circuit  10  determines input voltage VDC for inverter  6  on the basis of AC voltages Vrs and Vst detected by AC voltage detector  7  which is an already existing component. Therefore, it is not necessary to separately provide a DC voltage detector, which makes it possible to make the power converter compact in size and cheap in price. 
     Although in the present embodiment, AC voltage Vtr is determined by adder  65  from AC voltages Vrs and Vst, it is also acceptable to detect a line voltage between T-phase line TL and R-phase line RL by the AC voltage detector as AC voltage Vtr. In this case, adder  65  is removed and AC voltage Vtr detected by the AC voltage detector may be applied to one terminals of switches  55  and  56 . 
     Although in the present embodiment, DC voltage VDC between input terminals  6   a  and  6   b  of inverter  6  is determined on the basis of control signals φ 31  to φ 36  from thyristors  31  to  36  and the detection values from AC voltage detector  7 , it is needless to say that the DC voltage between output terminals  4   a  and  4   b  of converter  4  may be determined on the basis of control signals φ 21  to φ 26  from thyristors  21  to  26  in the same manner. 
     Although in the present embodiment, inverter  6  is described as a 6-phase converter including six thyristors  31  to  36 , it is not limited thereto, and it is needless to say that inverter  6  may be configured to include any number of thyristors other than six. For example, in the case where inverter  6  is a 12-phase converter including 12 thyristors and/or even in the case where inverter  6  is a 24-phase converter including 24 thyristors, it can be used to detect DC voltage VDC in the same manner as that in the present embodiment. 
     Similarly, although in the present embodiment, converter  4  is described as a 6-phase converter and six thyristors  21 - 26 , it is not limited thereto, and it is needless to say that converter  4  may be configured to include any number of thyristors other than six. For example, in the case where converter  4  is a 12-phase converter including 12 thyristors and/or even in the case where converter  4  is a 24-phase converter including 24 thyristors, it can be used to detect the DC voltage in the same manner as that in the present embodiment. 
     It should be understood that the embodiments disclosed herein have been presented for the purpose of illustration and description but not limited in all aspects. It is intended that the scope of the present invention is not limited to the description above but defined by the scope of the claims and encompasses all modifications equivalent in meaning and scope to the claims. 
     REFERENCE SIGNS LIST 
       1 : commercial AC power supply;  2 ,  7 : AC voltage detector;  3 ,  8 : alternating current detector;  4 : converter;  4   a : high-voltage output terminal;  4   b : low-voltage output terminal;  5 : DC reactor;  6 : inverter;  6   a : high-voltage input terminal;  6   b : low-voltage input terminal;  9 : rotor position detector;  10 : control circuit;  11 : synchronous motor;  21 - 26 ,  31 - 36 : thyristor;  40 : arithmetic circuit;  41 - 46 : AND gate;  51 - 56 : switch;  61 ,  63 ,  66 : subtractor;  62 ,  64 ,  65 : adder; Cr: R-phase coil; Cs: S-phase coil; Ct: T-phase coil; RL: R-phase line; SL: S-phase line; TL: T-phase line; UL: U-phase line; VL: V-phase line; WL: W-phase line