Power conversion apparatus

A power conversion apparatus includes an inverter circuit, a system voltage measurement unit measuring a system voltage, a voltage drop detector detecting a voltage drop of a power system, based on the system voltage, a direct current power measurement unit measuring a direct current power to be input into the inverter circuit, an alternating current command value calculator calculating an alternating current command value to control an alternating current output from the inverter circuit, based on the direct current power and the system voltage, and a current limiter that decrease a current limit value to limit the alternating current command value, when the voltage drop is detected.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a power conversion apparatus to be applied to a generation system which interconnects with an alternating current power system.

2. Description of the Related Art

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.

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 the false operation.

CITATION LIST

Patent Literature

BRIEF SUMMARY OF THE INVENTION

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.

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 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 direct current power measurement unit configured to measure a direct current power to be input into the inverter circuit; an alternating current command value calculator configured to calculate an alternating current command value to control an alternating current output from the inverter circuit, based on the direct current power measured by the direct current power measurement unit and the system voltage measured by the system voltage measurement unit; and a current limiter configured to decrease a current limit value to limit the alternating current command value calculated by the alternating current command value calculator, when the voltage drop is detected by the voltage drop detector.

DETAILED DESCRIPTION OF THE INVENTION

First Embodiment

FIG. 1is a block diagram showing a constitution of a dispersed generation system10to which a control apparatus2of an inverter1according 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.

The dispersed generation system10comprises the inverter1, the control apparatus2, a DC power source3, a smoothing capacitor4, an AC filter5, an interconnection transformer6, an AC current detector71, an overcurrent relay72, an AC voltage detector73, and a DC voltage detector74. The dispersed generation system10is a generation system which interconnects with an AC power system including a system bus7and an AC power source8.

The DC power source3supplies a DC power to the inverter1. The DC power source3is, for example, a secondary cell, a solar cell, or a fuel cell.

The inverter1is an inverter subjected to pulse width modulation (PWM) control. The inverter1converts the DC power supplied from the DC power source3into the AC power which synchronizes with the AC power source8. The inverter1supplies the AC power to the system bus7connected to the AC power source8, via the interconnection transformer6. In the inverter1, 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 apparatus2. In consequence, the inverter1performs the power conversion.

The smoothing capacitor4is disposed on a DC side of the inverter1. The smoothing capacitor4smoothes the DC power supplied from the DC power source3to the inverter1.

The AC filter5comprises a reactor51and a capacitor52. The AC filter5removes a noise output from the inverter1.

The AC current detector71is a detector to measure an output current Iiv of the inverter1. The AC current detector71outputs the detected output current Iiv as a detection signal to the control apparatus2and the overcurrent relay72.

The overcurrent relay72performs a protecting operation, when an instantaneous value of the output current Iiv measured by the AC current detector71is in excess of a beforehand setting value.

The AC voltage detector73is a detector to measure a system voltage Vr of the system bus7. The AC voltage detector73outputs the detected system voltage Vr as a detection signal to the control apparatus2.

The DC voltage detector74is a detector to measure a DC voltage Vdc which is to be applied to the DC side of the inverter1. The DC voltage detector74outputs the detected DC voltage Vdc as a detection signal to the control apparatus2.

A DC current detector75is a detector to measure a direct current Idc which is to be input into the DC side of the inverter1. The DC current detector75outputs a detected direct current Idc as a detection signal to the control apparatus2.

The control apparatus2comprises a power command calculation unit21, a current command value calculation unit22, a limiter23, a current control unit24, a PWM control unit25, and a voltage drop detection unit26.

The power command calculation unit21calculates a power command value Pr on the basis of the DC voltage Vdc detected by the DC voltage detector74, and the direct current Idc detected by the DC current detector75. The power command value Pr is a command value to an output power of the inverter1. The power command calculation unit21outputs the calculated power command value Pr to the current command calculation unit22.

Into the current command calculation unit22, there are input the power command value Pr calculated by the power command calculation unit21, the output current Iiv detected by the AC current detector71and the system voltage Vr detected by the AC voltage detector73. The current command value calculation unit22calculates a current command value Iivr0to control the output current Iiv so that the output power of the inverter1follows the power command value Pr. The current command calculation unit22outputs the calculated current command value Iivr0to the limiter23.

Into the voltage drop detection unit26, the system voltage Vr detected by the AC voltage detector73is input. The voltage drop detection unit26outputs a detection signal Sd to the limiter23on the basis of the system voltage Vr. The voltage drop detection unit26sets 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 unit26sets 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).

Into the limiter23, the current command value Iivr0calculated by the current command calculation unit22is input. The limiter23limits the current command value Iivr0by a limit value. The limiter23outputs the limited current command value Iivr0to the current control unit24.

In the limiter23, two limit values are set. The limiter23switches the limit value in accordance with the detection signal Sd input from the voltage drop detection unit26. At the usual time (when the detection signal Sd is “0”), the limiter23limits the current command value Iivr0by use of a maximum current value in an allowable range of the output current of the inverter1as the limit value. At the drop of the system voltage (when the detection signal Sd is “1”), the limiter23limits the current command value Iivr0by the limit value smaller than that at the usual time.

Next, a way to obtain the limit value for use at the drop of the system voltage Vr will be described.

A current ripple to be superimposed on the output current Iiv of the inverter1is generated in accordance with the following equation.
di/dt=ΔV/L(1)
in which the left side is a change ratio of the output current Iiv of the inverter1. L is a reactor component between the inverter1and the system bus7. ΔV is a voltage drop of the system voltage Vr.

The limit value is set so that the current ripple predicted in accordance with the above equation is suppressed.

Into the current control unit24, there are input the output current Iiv detected by the AC current detector71and a current command value Iivr limited by the limiter23. The current control unit24calculates a voltage command value Vivr to control an output voltage so that the output current Iiv of the inverter1follows the current command value Iivr. The current control unit24outputs the calculated voltage command value Vivr to the PWM control unit25.

Into the PWM control unit25, the voltage command value Vivr calculated by the current control unit24is input. The PWM control unit25generates a gate signal Gt so that the output voltage of the inverter1is controlled to the voltage command value Vivr. The gate signal Gt drives the switching element of the inverter1. In consequence, the inverter1is subjected to PWM control.

According to the present embodiment, when the voltage drop is detected by the voltage drop detection unit26and the limit value to limit the current command value Iivr0is set to be smaller than that at the usual time, the output current Iiv of the inverter1can be decreased. In consequence, the ripple of the output current Iiv of the inverter1can be prevented from being in excess of the setting value of the overcurrent relay72. In consequence, the false operation of the overcurrent relay72can be prevented.

The control apparatus2controls the inverter1to output the power command value Pr calculated by the power command calculation unit21. Therefore, when the limit value to limit the current command value Iivr0is decreased, the output voltage of the inverter1increases. The control to increase the output voltage of the inverter1at the drop of the voltage of the system bus7due to a system fault or the like is control opposite to usually performed control to decrease the output voltage of the inverter1. However, the control apparatus2decreases the output current of the inverter1only at the voltage drop of the system voltage Vr, so that it is possible to prevent the false operation of the overcurrent relay72due to the ripple of the output current Iiv of the inverter1.

Second Embodiment

FIG. 2is a block diagram showing a constitution of a dispersed generation system10A to which a control apparatus2A of an inverter1according to a second embodiment of the invention is applied.

The dispersed generation system10A has a constitution where in the dispersed generation system10according to the first embodiment shown inFIG. 1, the control apparatus2is replaced with the control apparatus2A. The other respects are similar to those of the dispersed generation system10according to the first embodiment.

The control apparatus2A has a constitution where in the control apparatus2according to the first embodiment, the limiter23is replaced with a limiter23A and the voltage drop detection unit26is replaced with a voltage drop calculation unit27and a limit value calculation unit28. The other aspects are similar to those of the control apparatus2according to the first embodiment.

Into the voltage drop calculation unit27, a system voltage Vr detected by an AC voltage detector73is input. When the system voltage Vr is below a predetermined reference voltage (at the drop of the system voltage), the voltage drop calculation unit27calculates a voltage drop ΔV by subtracting the system voltage Vr from a rated voltage. The voltage drop calculation unit27outputs the calculated voltage drop ΔV to the limit value calculation unit28.

Into the limit value calculation unit28, the voltage drop ΔV calculated by the voltage drop calculation unit27is input. The limit value calculation unit28calculates a limit value Lr on the basis of the voltage drop ΔV. The more voltage drop ΔV, the less limit value Lr is calculated. The limit value calculation unit28outputs the calculated limit value Lr to the limiter23A.

The limiter23A limits the current command value Iivr0by the limit value Lr calculated by the limit value calculation unit28. The other aspects are similar to those of the limiter23according to the first embodiment.

According to the present embodiment, the limit value Lr to limit the current command value Iivr0is changed in accordance with the voltage drop ΔV, so that the output current Iiv can be limited by the minimum limit value Lr at which the overcurrent relay72is not operated. In consequence, as compared with the first embodiment, an output voltage of the inverter1can be prevented from being unnecessarily increased.

Third Embodiment

FIG. 3is a block diagram showing a constitution of a dispersed generation system10B to which a power conditioner20of a wind power generation system according to a third embodiment of the invention is applied.

The dispersed generation system10B has a constitution where in the dispersed generation system10according to the first embodiment shown inFIG. 1, the control apparatus2is replaced with a control apparatus2B, the DC power source3is replaced with a wind power generator11and a converter12, and an AC current detector76is added. The power conditioner20comprises the inverter1, the converter12, the control apparatus2B, the smoothing capacitor4, and the AC filter5. The other aspects are similar to those of the dispersed generation system10according to the first embodiment.

The wind power generator11is a generator to generate an AC power by use of wind power. The wind power generator11supplies the generated AC power to the power conditioner20.

The power conditioner20is a power conversion apparatus to convert the AC power supplied from the wind power generator11into the AC power which synchronizes with the system voltage Vr. The power conditioner20supplies the converted AC power to the system bus7via the interconnection transformer6.

A DC side of the converter12is connected to a DC side of the inverter1via a DC link13. That is, the converter12and the inverter1constitute a back to back (BTB) converter. An AC side of the converter12is connected to the wind power generator11. The converter12converts the AC power generated by the wind power generator11into a DC power, to supply the power to the inverter1.

The converter12is an inverter subjected to PWM control. In the converter12, a power conversion circuit is constituted of a switching element. The switching element is driven by a gate signal Gtc output from a converter control unit31of the control apparatus2B. In consequence, the converter12performs the power conversion.

The control apparatus23has a constitution where in the control apparatus2according to the first embodiment, the converter control unit31is disposed in place of the power command value generation unit21. An inverter control unit32is constituted of the current command value calculation unit22, the limiter23, the current control unit24, the PWM control unit25, and the voltage drop detection unit26. The other aspects are similar to those of the control apparatus2according to the first embodiment.

The AC current detector76is a detector to measure an alternating current Ig which is to be input from the wind power generator11into the converter12. The AC current detector76outputs the detected alternating current Ig as a detection signal to the converter control unit31.

Into the converter control unit31, there are input the alternating current Ig detected by the AC current detector76, the DC voltage Vdc detected by the DC voltage detector74, and the direct current Idc detected by the DC current detector75.

The converter control unit31generates a gate signal Gtc to control the converter12, on the basis of the alternating current Ig detected by the AC current detector76, the DC voltage Vdc detected by the DC voltage detector74, and the direct current Idc detected by the DC current detector75. The converter control unit31outputs the generated gate signal Gtc, to drive the switching element of the converter12.

The converter control unit31calculates the power command value Pr to control the inverter1. The converter control unit31outputs the calculated power command value Pr to the current control unit22.

According to the present embodiment, in the power conditioner20of the wind power generation system, a function and an effect similar to those of the first embodiment can be obtained.

Fourth Embodiment

FIG. 4is a block diagram, showing a constitution of a dispersed generation system10C to which a power conditioner20C of a wind power generation system according to a fourth embodiment of the invention is applied.

The dispersed generation system10C has a constitution where in the dispersed generation system10B according to the third embodiment shown inFIG. 3, the power conditioner20is replaced with the power conditioner20C. The other aspects are similar to those of the dispersed generation system10B according to the third embodiment.

The power conditioner20C has a constitution where in the control apparatus2B of the power conditioner20according to the third embodiment, the limiter23is replaced with the limiter23A according to the second embodiment, and the voltage drop detection unit26is replaced with the voltage drop calculation unit27according to the second embodiment and the limit value calculation unit28according to the second embodiment. The other aspects are similar to those of the power conditioner20according to the third embodiment.

According to the present embodiment, in the power conditioner20C of the wind power generation system, a function and an effect similar to those of the second embodiment can be obtained.

It is to be noted that in the second embodiment and the fourth embodiment, the limit value Lr is calculated on the basis of the voltage drop ΔV, but the limit value may be selected from previously set limit values. When the limit value corresponding to the voltage drop ΔV is selected, a function and an effect similar to those of the respective embodiments can be obtained.

Moreover, in the third embodiment and the fourth embodiment, the constitution using the wind power generator11has 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 a form of energy other than wind power, as long as the generator generates the AC power.

Furthermore, in the respective embodiments, the limit value and an equation to obtain this limit value may not be based on the above equation (1). For example, the limit value may be obtained by empirical rule or know-how.

Moreover, in the respective embodiments, the interconnection transformer6interposed between the dispersed generation system10and the AC power system may not be disposed. In this case, the voltage detected by the AC voltage detector73is an electricity at the same measuring position as that of the current detected by the AC current detector71.

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.