Abstract:
A power conversion apparatus includes an inverter circuit including a switching element, a system voltage measurement unit measuring a system voltage of a power system, a voltage drop detector detecting a voltage drop of the power system, a carrier wave generator generating a carrier wave, a carrier wave frequency modulator increasing a frequency of the carrier wave, when the voltage drop is detected, a signal wave generator generating a signal wave to control the inverter circuit, a gate signal generator comparing the carrier wave with the signal wave, and generating a gate signal, and a power conversion controller controlling the inverter circuit, based on the gate signal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Continuation Application of PCT Application No. PCT/JP2011/053956, filed Feb. 23, 2011, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a power conversion apparatus to be applied to a generation system which interconnects with an alternating current power system. 
         [0004]    2. Description of the Related Art 
         [0005]    In general, a power conversion apparatus is used in a generation system which interconnects with an alternating current (AC) power system. The power conversion apparatus converts a direct current (DC) power into an AC power which synchronizes with the AC power system, to supply the power to the AC power system. Moreover, on an AC output side of the power conversion apparatus, an overcurrent relay is disposed to protect the power conversion apparatus. 
         [0006]    However, the overcurrent relay for use in this way performs the following false operation sometimes. When a system voltage drops owing to a fault or the like of the AC power system, an amplitude of a ripple of an alternating current output from the power conversion apparatus increases. In consequence, even when an instantaneous value of a current of a fundamental component is not in excess of a setting value at which the overcurrent relay operates, the instantaneous value due to the amplitude of the ripple of the current exceeds the setting value, so that the overcurrent relay operates sometimes. In this case, the overcurrent relay is to bring about a false operation. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         Patent Literature 1: U.S. Pat. No. 6,921,985 
       
     
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    An object of the invention is to provide a power conversion apparatus to be applied to a generation system which interconnects with an AC power system, so that a false operation of an overcurrent relay disposed on an AC output side can be prevented. 
         [0009]    In accordance with an aspect of the invention, there is provided a power conversion apparatus to be applied to a generation system that interconnects with an alternating current power system. The power conversion apparatus includes an inverter circuit including a switching element configured to convert a direct current power into an alternating current power; a system voltage measurement unit configured to measure a system voltage of the alternating current power system; a voltage drop detector configured to detect a voltage drop of the alternating current power system, based on the system voltage measured by the system voltage measurement unit; a carrier wave generator configured to generate a carrier wave; a carrier wave frequency modulator configured to increase a frequency of the carrier wave generated by the carrier wave generator, when the voltage drop is detected by the voltage drop detector; a signal wave generator configured to generate a signal wave to control the alternating current power output from the inverter circuit; a gate signal generator configured to compare the carrier wave generated by the carrier wave generator with the signal wave generated by the signal wave generator, and to generate a gate signal to drive the switching element; and a power conversion controller configured to perform power conversion control of the inverter circuit by pulse width modulation, based on the gate signal generated by the gate signal generator. 
         [0010]    Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0011]    The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
           [0012]      FIG. 1  is a block diagram showing a constitution of a dispersed generation system to which a control apparatus of an inverter according to a first embodiment of the invention is applied; 
           [0013]      FIG. 2  is a block diagram showing a constitution of a dispersed generation system to which a control apparatus of an inverter according to a second embodiment of the invention is applied; 
           [0014]      FIG. 3  is a block diagram showing a constitution of a dispersed generation system to which a power conditioner of a wind power generation system according to a third embodiment of the invention is applied; and 
           [0015]      FIG. 4  is a block diagram showing a constitution of a dispersed generation system to which a power conditioner of a wind power generation system according to a fourth embodiment of the invention is applied. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    Hereinafter, embodiments of the invention will be described with reference to the drawings. 
       First Embodiment 
       [0017]      FIG. 1  is a block diagram showing a constitution of a dispersed generation system  10  to which a control apparatus  2  of an inverter  1  according to a first embodiment of the invention is applied. It is to be noted that the same parts as in the drawings are denoted with like reference signs to omit detailed description of the parts, and different parts will mainly be described. Also in the subsequent embodiments, repeated descriptions are similarly omitted. 
         [0018]    The dispersed generation system  10  comprises the inverter  1 , the control apparatus  2 , a DC power source  3 , a smoothing capacitor  4 , an AC filter  5 , an interconnection transformer  6 , an AC current detector  71 , an overcurrent relay  72 , an AC voltage detector  73 , and a DC voltage detector  74 . The dispersed generation system  10  is a generation system which interconnects with an AC power system including a system bus  7  and an AC power source  8 . 
         [0019]    The DC power source  3  supplies a DC power to the inverter  1 . The DC power source  3  may be any DC power source, as long as the DC power can be supplied to the inverter  1 . The DC power source  3  is, for example, a secondary cell, a solar cell, or a fuel cell. It is to be noted that the DC power source  3  may be a converter which converts an AC power into the DC power to supply the power to the inverter  1 , or the like. 
         [0020]    The inverter  1  is an inverter subjected to pulse width modulation (PWM) control. The inverter  1  converts the DC power supplied from the DC power source  3  into the AC power which synchronizes with the AC power source  8 . The inverter  1  supplies the AC power to the system bus  7  connected to the AC power source  8 , via the interconnection transformer  6 . In the inverter  1 , a power conversion circuit (an inverter circuit) is constituted of a switching element. The switching element is driven by a gate signal Gt output from the control apparatus  2 . In consequence, the inverter  1  performs the power conversion. 
         [0021]    The smoothing capacitor  4  is disposed on a DC side of the inverter  1 . The smoothing capacitor  4  smoothes the DC power supplied from the DC power source  3  to the inverter  1 . 
         [0022]    The AC filter  5  comprises a reactor  51  and a capacitor  52 . The AC filter  5  removes a noise output from the inverter  1 . 
         [0023]    The AC current detector  71  is a detector to measure an output current Iiv of the inverter  1 . The AC current detector  71  outputs the detected output current Iiv as a detection signal to the control apparatus  2  and the overcurrent relay  72 . 
         [0024]    The overcurrent relay  72  performs a protecting operation, when an instantaneous value of the output current Iiv measured by the AC current detector  71  is in excess of a beforehand setting value. 
         [0025]    The AC voltage detector  73  is a detector to measure a system voltage Vr of the system bus  7 . The AC voltage detector  73  outputs the detected system voltage Vr as a detection signal to the control apparatus  2 . 
         [0026]    The DC voltage detector  74  is a detector to measure a DC voltage Vdc which is to be applied to the DC side of the inverter  1 . The DC voltage detector  74  outputs the detected DC voltage Vdc as a detection signal to the control apparatus  2 . 
         [0027]    A DC current detector  75  is a detector to measure a direct current Idc which is to be input into the DC side of the inverter  1 . The DC current detector  75  outputs a detected direct current Idc as a detection signal to the control apparatus  2 . 
         [0028]    The control apparatus  2  comprises a power command calculation unit  21 , a current control unit  22 , a gate signal generation unit  23 , a carrier wave generation unit  24 , and a voltage drop detection unit  25 . 
         [0029]    The power command calculation unit  21  calculates a power command value Pr on the basis of the DC voltage Vdc detected by the DC voltage detector  74 , and the direct current Idc detected by the DC current detector  75 . The power command value Pr is a command value to an output power of the inverter  1 . The power command calculation unit  21  outputs the calculated power command value Pr to the current control unit  22 . 
         [0030]    Into the current control unit  22 , there are input the power command value Pr calculated by the power command calculation unit  21 , the output current Iiv detected by the AC current detector  71  and the system voltage Vr detected by the AC voltage detector  73 . The current control unit  22  is a processing unit to control the output current Iiv so that an output power of the inverter  1  follows the power command value Pr. The current control unit  22  calculates a voltage command value Vivr on the basis of the power command value Pr, the output current Iiv, and the system voltage Vr. The voltage command value Vivr is a command value to the output voltage of the inverter  1 . The current control unit  22  outputs the calculated voltage command value Vivr as a signal wave to the gate signal generation unit  23 . The signal wave is a sinusoidal wave. 
         [0031]    Into the voltage drop detection unit  25 , the system voltage Vr detected by the AC voltage detector  73  is input. The voltage drop detection unit  25  outputs a detection signal Sd to the carrier wave generation unit  24  on the basis of the system voltage Vr. The voltage drop detection unit  25  sets the detection signal Sd to “0”, when the system voltage Vr is not less than a predetermined reference voltage (usual time). The voltage drop detection unit  25  sets the detection signal Sd to “1”, when the system voltage Vr is below the predetermined reference voltage (at the drop of the system voltage Vr). 
         [0032]    Into the carrier wave generation unit  24 , the detection signal Sd from the voltage drop detection unit  25  is input. Two different frequencies are beforehand set to the carrier wave generation unit  24 . When the detection signal Sd is “0” (the usual time), the lower frequency is selected from the two frequencies. When the detection signal Sd is “1” (at the drop of the system voltage), the higher frequency is selected from the two frequencies. The carrier wave generation unit  24  generates a triangular wave as a carrier wave at the frequency selected in accordance with the detection signal Sd. The carrier wave generation unit  24  outputs the generated carrier wave to the gate signal generation unit  23 . 
         [0033]    Next, there will be described a method of determining the frequency of the carrier wave for use at the drop of the system voltage Vr. 
         [0034]    A current ripple to be superimposed on the output current Iiv of the inverter  1  is generated in accordance with the following equation. 
         [0000]        di/dt=ΔV/L   (1),
 
         [0000]    in which the left side is a change ratio of the output current Iiv of the inverter  1 . L is a reactor component between the inverter  1  and the system bus  7 . ΔV is a voltage drop of the system voltage Vr. 
         [0035]    The frequency of the carrier wave is set so that the current ripple predicted in accordance with the above equation is suppressed. 
         [0036]    Into the gate signal generation unit  23 , there are input the voltage command value Vivr calculated by the current control unit  22  and the carrier wave generated by the carrier wave generation unit  24 . The gate signal generation unit  23  compares the sinusoidal wave which is the signal wave with the triangular wave which is the carrier wave, to generate a pulse wave. The gate signal generation unit  23  outputs the generated pulse wave as the gate signal Gt to the inverter  1 . The gate signal Gt drives the switching element of the inverter  1 . In consequence, the output voltage of the inverter  1  is controlled. 
         [0037]    According to the present embodiment, when the system voltage of the interconnecting power system drops, the gate signal Gt is generated with the carrier wave of the high frequency. In consequence, an amplitude of the ripple of the current output from the inverter  1  can be decreased. 
         [0038]    Moreover, when the carrier wave is set to the high frequency only at the drop of the system voltage Vr, a failure ratio of the switching element of the inverter  1  can be decreased, as compared with a case where a carrier wave always having the high frequency is used. 
       Second Embodiment 
       [0039]      FIG. 2  is a block diagram showing a constitution of a dispersed generation system  10 A to which a control apparatus  2 A of the inverter  1  according to a second embodiment of the invention is applied. 
         [0040]    The dispersed generation system  10 A has a constitution where in the dispersed generation system  10  according to the first embodiment shown in  FIG. 1 , the control apparatus  2  is replaced with the control apparatus  2 A. The other respects are similar to those of the dispersed generation system  10  according to the first embodiment. 
         [0041]    The control apparatus  2 A has a constitution where in the control apparatus  2  according to the first embodiment, the carrier wave generation unit  24  is replaced with a carrier wave generation unit  24 A, and the voltage drop detection unit  25  is replaced with a voltage drop calculation unit  26  and a carrier frequency modulation unit  27 . The other respects are similar to those of the control apparatus  2  according to the first embodiment. 
         [0042]    Into the voltage drop calculation unit  26 , the system voltage Vr detected by the AC voltage detector  73  is input. When the system voltage Vr is below a predetermined reference voltage (at the drop of the system voltage), the voltage drop calculation unit  26  calculates a voltage drop ΔV by subtracting the system voltage Vr from a rated voltage. The voltage drop calculation unit  26  outputs the calculated voltage drop ΔV to the carrier frequency modulation unit  27 . 
         [0043]    Into the carrier frequency modulation unit  27 , the voltage drop ΔV calculated by the voltage drop calculation unit  26  is input. The carrier frequency modulation unit  27  calculates a frequency command value fr on the basis of the voltage drop ΔV. The frequency command value fr is calculated so that the value increases as the voltage drop ΔV increases. The carrier frequency modulation unit  27  outputs the calculated frequency command value fr to the carrier wave generation unit  24 A. 
         [0044]    An equation with which the frequency command value fr is calculated on the basis of the voltage drop ΔV is derived similarly to the first embodiment. 
         [0045]    The carrier wave generation unit  24 A modulates the frequency of a carrier wave so that the frequency becomes the frequency command value fr calculated by the carrier frequency modulation unit  27 . The carrier wave generation unit  24 A generates the carrier wave at the frequency modulated to the frequency command value fr. The carrier wave generation unit  24 A outputs the generated carrier wave to the gate signal generation unit  23 . The other respects are similar to those of the carrier wave generation unit  24  according to the first embodiment. 
         [0046]    According to the present embodiment, the frequency of the carrier wave is modulated in accordance with the voltage drop ΔV, so that the carrier wave can be generated at such a minimum frequency that an overcurrent relay  72  is not operated. In consequence, the frequency is not increased more than necessary, and hence a failure ratio of a switching element of the inverter  1  can be decreased as compared with the first embodiment. 
       Third Embodiment 
       [0047]      FIG. 3  is a block diagram showing a constitution of a dispersed generation system  10 B to which a power conditioner  20  of a wind power generation system according to a third embodiment of the invention is applied. 
         [0048]    The dispersed generation system  10 B has a constitution where in the dispersed generation system  10  according to the first embodiment shown in  FIG. 1 , the control apparatus  2  is replaced with a control apparatus  2 B, the DC power source  3  is replaced with a wind power generator  11  and a converter  12 , and an AC current detector  76  is added. The power conditioner  20  has a constitution including the inverter  1 , the converter  12 , the control apparatus  2 B, the smoothing capacitor  4 , and the AC filter  5 . The other respects are similar to those of the dispersed generation system  10  according to the first embodiment. 
         [0049]    The wind power generator  11  is a generator to generate an AC power by use of a wind power. The wind power generator  11  supplies the generated AC power to the power conditioner  20 . 
         [0050]    The power conditioner  20  is a power conversion apparatus to convert the AC power supplied from the wind power generator  11  into the AC power which synchronizes with a system voltage Vr. The power conditioner  20  supplies the converted AC power to the system bus  7  via the interconnection transformer  6 . 
         [0051]    A DC side of the converter  12  is connected to a DC side of the inverter  1  via a DC link  13 . That is, the converter  12  and the inverter  1  constitute a back to back (BTB) converter. An AC side of the converter  12  is connected to the wind power generator  11 . The converter  12  converts the AC power generated by the wind power generator  11  into a DC power, to supply the power to the inverter  1 . 
         [0052]    The converter  12  is an inverter subjected to PWM control. In the converter  12 , a power conversion circuit is constituted of a switching element. The switching element is driven by a gate signal Gt output from a converter control unit  31  of the control apparatus  2 B. In consequence, the converter  12  performs the power conversion. 
         [0053]    The control apparatus  2 B has a constitution where in the control apparatus  2  according to the first embodiment, the converter control unit  31  is disposed in place of the power command calculation unit  21 . An inverter control unit  32  is constituted of the current control unit  22 , the gate signal generation unit  23 , the carrier wave generation unit  24 , and the voltage drop detection unit  25 . The other respects are similar to those of the control apparatus  2  according to the first embodiment. 
         [0054]    The AC current detector  76  is a detector to measure an alternating current Ig which is to be input from the wind power generator  11  into the converter  12 . The AC current detector  76  outputs the detected alternating current Ig as a detection signal to the converter control unit  31 . 
         [0055]    Into the converter control unit  31 , there are input the alternating current Ig detected by the AC current detector  76 , the DC voltage Vdc detected by the DC voltage detector  74 , and the direct current Idc detected by the DC current detector  75 . 
         [0056]    The converter control unit  31  generates a gate signal Gtc to control the converter  12 , on the basis of the alternating current Ig detected by the AC current detector  76 , the DC voltage Vdc detected by the DC voltage detector  74 , and the direct current Idc detected by the DC current detector  75 . The converter control unit  31  outputs the generated gate signal Gtc, to drive the switching element of the converter  12 . In consequence, an output power of the converter  12  is controlled. 
         [0057]    The converter control unit  31  calculates a power command value Pr to control the inverter  1 . The converter control unit  31  outputs the calculated power command value Pr to the current control unit  22 . 
         [0058]    According to the present embodiment, in the power conditioner  20  of the wind power generation system, a function and an effect similar to those of the first embodiment can be obtained. 
       Fourth Embodiment 
       [0059]      FIG. 4  is a block diagram showing a constitution of a dispersed generation system  10 C to which a power conditioner  20 C of a wind power generation system according to a fourth embodiment of the invention is applied. 
         [0060]    The dispersed generation system  10 C has a constitution where in the dispersed generation system  10 B according to the third embodiment shown in  FIG. 3 , the power conditioner  20  is replaced with the power conditioner  20 C. The other respects are similar to those of the dispersed generation system  10 B according to the third embodiment. 
         [0061]    The power conditioner  20 C has a constitution where in the control apparatus  2 B of the power conditioner  20  according to the third embodiment, the carrier wave generation unit  24  is replaced with the carrier wave generation unit  24 A according to the second embodiment, and the voltage drop detection unit  25  is replaced with the voltage drop calculation unit  26  according to the second embodiment and the carrier frequency modulation unit  27  according to the second embodiment. The other respects are similar to those of the power conditioner  20  according to the third embodiment. 
         [0062]    According to the present embodiment, in the power conditioner  20 C of the wind power generation system, a function and an effect similar to those of the second embodiment can be obtained. 
         [0063]    It is to be noted that in the second embodiment and the fourth embodiment, the frequency of the carrier wave is modulated to the frequency command value fr calculated on the basis of the voltage drop ΔV, but the frequency may be selected from beforehand set frequencies. When the frequency corresponding to the voltage drop ΔV is selected, a function and an effect similar to those of the respective embodiments can be obtained. 
         [0064]    Moreover, in the third embodiment and the fourth embodiment, the constitution using the wind power generator  11  has been described, but the invention is not limited to this constitution. The generator may be a generator (for example, a hydroelectric power generator) which uses energy other than the wind power, as long as the generator generates the AC power. 
         [0065]    Furthermore, in the respective embodiments, the frequency of the carrier wave and an equation to obtain this frequency may not be based on the above equation (1). For example, the frequency of the carrier wave may be obtained by empirical rule or know-how. 
         [0066]    Moreover, in the respective embodiments, the interconnection transformer  6  interposed between the dispersed generation system  10  and the AC power system may not be disposed. In this case, the voltage detected by the AC voltage detector  73  is an electricity at the same measuring position as that of the current detected by the AC current detector  71 . 
         [0067]    It is to be noted that the present invention is not restricted to the foregoing embodiments, and constituent elements can be modified and changed into shapes without departing from the scope of the invention at an embodying stage. Additionally, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the foregoing embodiments. For example, several constituent elements may be eliminated from all constituent elements disclosed in the embodiments. Furthermore, constituent elements in the different embodiments may be appropriately combined.