Power converter

A power converter can include an inverter including, connected in series, switching elements, the inverter circuit being connected to both ends of a DC power source series circuit resulting from connecting in series a DC power source and a DC power source. Also included can be an AC output terminal that is connected to a connection point of the switching element and the switching element, an AC output terminal that is connected to a connection point of the DC power source and the DC power source, a bidirectional switch element one end of which is connected to the AC output terminal and the other end of which is connected to a terminal of an AC power source and a bidirectional switch element one end of which is connected to the AC output terminal and the other end of which is connected to the AC power source.

BACKGROUND

1. Field of the Invention

The present invention relates to a power converter that generates predefined AC voltage using voltage of an AC power source and a DC power source. More particularly, the present invention relates to instantaneous voltage-drop compensation devices and an uninterruptible power source device that can supply stabilized voltage to a load, even upon occurrence of fluctuation of voltage of an AC power source and power failure in the AC power source.

2. Related Art

FIG. 15is a diagram for explaining a power converter of a continuous-inverter power feeding scheme disclosed in Japanese Patent Application Publication No. H7-337036 (also referred to herein as “Patent document 1”). The power converter converts temporarily the voltage of an AC power source to DC voltage, converts the DC voltage again to AC voltage, and supplies the voltage to a load. In the figure, the reference symbol1is a single-phase AC power source,2is a capacitor,3is a converter circuit,4is an inverter circuit,5is a filter circuit, and6is a load.

One end of the AC power source1is connected to an AC input terminal of the converter circuit3. The AC input terminal of the converter circuit3is connected to one end of the AC power source1. In the converter circuit3, one end of a reactor L is connected to the AC input terminal. The other end of the reactor L is connected to a connection midpoint of a switching element series circuit, in which switching elements Qp, Qn are connected in series. A capacitor series circuit in which capacitors Cp, Cn are connected in series, is connected to both ends of the switching element series circuit. The connection midpoint of the capacitor series circuit is connected to the other end of the AC power source1. The converter circuit3turns the switching elements Qp, Qn on and off, to rectify the voltage of the AC power source1, and charge the capacitors Cp, Cn to a predefined voltage. The capacitors Cp, Cn thus charged to a predefined voltage form a DC power source.

The capacitor2is connected between the AC input terminal of the converter circuit3and the connection midpoint of the capacitor series circuit. The inverter circuit4comprises series-connected switching elements Q1, Q2. The inverter circuit4is connected to a DC output terminal of the converter circuit3. The inverter circuit4turns on and off the switching elements Q1, Q2, to convert, to AC voltage, the voltage of the DC power source that comprises the capacitors Cp, Cn.

The filter circuit5is configured through connection in series of a reactor Lf1and a capacitor Cf1. One end of the filter circuit5is connected to the connection midpoint of the switching elements Q1, Q2. The other end of the filter circuit5is connected to the connection midpoint of the capacitor series circuit. The filter circuit5removes a high-frequency component from the AC voltage that is outputted from the inverter circuit4.

One end of the load6is connected to the connection point of the reactor Lf1and the capacitor Cf1. The other end of the load6is connected to the other end of the AC power source1. The AC voltage that is supplied from the inverter circuit4is outputted, via the filter circuit5, to the load6.

FIG. 16is a diagram for explaining a power converter of a continuous commercial-power feeding scheme disclosed in Japanese Patent Application Publication No. H11-178216 (also referred to herein as “Patent document 2”). In the figure, a switch7and the secondary winding of a transformer8are connected in series between an AC power source1and a load. The respective connection relationships between a converter circuit3, an inverter circuit4, a filter circuit5and a capacitor2are identical to those of the embodiment inFIG. 15. An AC input terminal of the converter circuit3is connected to one end of the primary winding of the transformer8. The connection midpoint of the capacitor series circuit is connected to the other end of the AC power source1and is connected to the other end of the primary winding of the transformer8. The connection point of the reactor Lf1and the capacitor Cf1is connected to one end of the load6.

The power converter supplies ordinarily voltage of the AC power source to the load. When the voltage of the AC power source1drops, the converter circuit3turns on and off the switching elements Qp, Qn, to generate thereby compensation voltage for compensating the voltage drop from the DC voltage at which the capacitor series circuit is charged.

The compensation voltage is superimposed on the voltage of the AC power source1, via the transformer8. The voltage resulting from superimposing the compensation voltage on the voltage of the AC power source1is supplied to the load6. Charging of the capacitor series circuit is carried out in this case by the inverter circuit4.

The switch7is opened when the AC power source1fails. The inverter circuit4turns on and off the switching elements Q1, Q2, to convert the DC voltage of the capacitor series circuit to AC voltage, and supply the voltage to the load6.

In the power converter illustrated inFIG. 15, however, AC-DC voltage conversion by the converter circuit3and DC-AC voltage conversion by the inverter circuit4are required until AC voltage is supplied from the AC power source1to the load6. The current that flows through the power converter passes at least once through each switching element of the converter circuit3and the inverter circuit4. That is, the current flowing in the power converter passes through switching elements at least two or more times. Accordingly, respective conduction losses derived from passage of current through the switching elements occur at both the converter circuit3and of the inverter circuit4.

The on and off operations of the switching elements Q1to Q4in the converter circuit3and the inverter circuit4are performed on the basis of the voltage of the DC power source that comprises the capacitors Cp and Cn. Accordingly, switching loss occurs when each element is turned on or turned off.

The power loss, including conduction loss and switching loss, in the switching elements is therefore substantial in the technology disclosed in Patent document 1. A problem arises herein in that the conversion efficiency of the power converter drops when power loss in the switching elements is large.

In the power converter illustrated inFIG. 16, the transformer8is required in order to compensate for the voltage drop of the AC power source1. The size of the transformer8is large, since the latter must function at a commercial frequency. In the power converter illustrated inFIG. 16, moreover, the operations of the converter circuit3and the inverter circuit4must be switched in order to supply predefined AC voltage to the load6when a power failure occurs in the AC power source1.

A problem arises therefore, in the technology disclosed in Patent document 2, in that a large commercial transformer is required, which translates into a power converter of large size. A further problem is the occurrence of disturbances in the AC output voltage as a result of switching over between the operations of the converter circuit3and the inverter circuit4.

SUMMARY OF THE INVENTION

A goal of the present invention is to solve problems such as those of conventional technologies. Specifically, it is an object of the present invention to provide a power converter that can output AC output voltage, without occurrence of disturbances, during a voltage drop of an AC power source or during power failure in the AC power source. It is a further object of the present invention to provide a power converter that allows reducing power loss. Yet a further object of the present invention is to provide a power converter that requires no commercial-frequency transformer to perform a voltage compensation operation.

In order to attain the above goal, a first embodiment in which the present invention is used is a power converter that outputs AC voltage on the basis of an AC voltage command, comprising: a single-phase AC power source having a first AC terminal and a second AC terminal; and a DC power source series circuit, resulting from connecting in series a first DC power source and a second DC power source such that a neutral terminal, which is a connection point of the first DC power source and the second DC power source, is connected to the second AC terminal. The power converter outputs voltage of any one level selected from among four levels of voltage of a positive-side voltage of the DC power source series circuit, a negative-side voltage of the DC power source series circuit, zero voltage being the potential of the neutral terminal, and a voltage of the AC power source.

In the power converter, further, the positive-side voltage of the DC power source series circuit and the negative-side voltage of the DC power source series circuit are larger than an amplitude value of the voltage of the AC power source.

In the power converter, further, at each control period that is continuous and has a pre-established time width, the power converter sets, as a first voltage, a voltage of one level selected from among the four levels of voltage, and sets, as a second voltage, a voltage of another level selected from among the four levels of voltage. The power converter outputs complementarily the first voltage and the second voltage over respective predefined time widths.

In the power converter, further, the first voltage and the second voltage are voltages selected on the basis of the AC output voltage command and the voltage of the AC power source.

In the power converter, further, at each control period that is continuous and has a pre-established time width, the power converter selects as a first voltage, from among the four levels of voltage, a voltage having an absolute value equal to or higher than an absolute value of the AC output voltage command and having a value that is closest to a value of the AC output voltage command, and selects as a second voltage, from among the four levels of voltage, a voltage having an absolute value smaller than the absolute value of the AC output voltage command, and having a value that is closest to the value of the AC output voltage command. The power converter sets a time ratio of an output time of the first voltage and an output time of the second voltage, within each control period, to a predefined value, and complementarily outputs the first voltage and the second voltage, to thereby output AC voltage corresponding to the AC output voltage command.

At each control period that is continuous and has a pre-established time width, the power converter sets the positive-side voltage of the DC power source series circuit as the first voltage and sets the zero voltage as the second voltage when the AC output voltage command is equal to or higher than the zero voltage and the voltage of the AC power source is lower than the zero voltage. The power converter sets the positive-side voltage of the DC power source series circuit as the first voltage and sets the voltage of the AC power source as the second voltage when the AC output voltage command is equal to or higher than the zero voltage, the voltage of the AC power source is equal to or higher than the zero voltage, and the AC output voltage command is higher than the voltage of the AC power source. The power converter sets the voltage of the AC power source as the first voltage and sets the zero voltage as the second voltage when the AC output voltage command is equal to or higher than the zero voltage, the voltage of the AC power source is equal to or higher than the zero voltage and the AC output voltage command is equal to or lower than the voltage of the AC power source. The power converter sets the voltage of the AC power source as the first voltage and sets the zero voltage as the second voltage when the AC output voltage command is lower than the zero voltage, the voltage of the AC power source is lower than the zero voltage and the AC output voltage command is equal to or higher than the voltage of the AC power source. The power converter sets the negative-side voltage of the DC power source series circuit as the first voltage and sets the voltage of the AC power source as the second voltage when the AC output voltage command is lower than the zero voltage, the voltage of the AC power source is lower than the zero voltage and the AC output voltage command is lower than the voltage of the AC power source. The power converter sets the negative-side voltage of the DC power source series circuit as the first voltage and sets the zero voltage as the second voltage when the AC output voltage command is lower than the zero voltage and the voltage of the AC power source is equal to or higher than the zero voltage. The power converter sets a time ratio of an output time of the first voltage and an output time of the second voltage, within each control period, to a predefined value, and complementarily outputs the first voltage and the second voltage, to thereby output AC voltage corresponding to the AC output voltage command.

In the power converter, the output time of the first voltage is established on the basis of the AC output voltage command, the first voltage and the second voltage. In the power converter, the output time of the second voltage is a time resulting from subtracting the output time of the first voltage from the time of each control period.

In the power converter, further, the output time of the first voltage is a time corresponding to a value obtained by dividing a difference voltage between the AC output voltage and the second voltage by a difference voltage between the first voltage and the second voltage.

In the power converter, further, an average value of the AC output voltage in each control period is equal to an average value of the AC output voltage command within that control period.

In the power converter, further, the AC output voltage command is synchronized with the voltage of the AC power source. In the power converter, further, the power converter outputs the voltage of the AC power source when a deviation between the voltage of the AC power source and the AC output voltage command lies within a pre-established range.

Further, the power conversion circuit that outputs single-phase AC voltage in a power converter according to a first embodiment is configured out of an inverter circuit and a bidirectional switch circuit. The inverter circuit is a switching element series circuit resulting from connecting in series a positive-side switching element that is connected to a positive-side terminal of the DC power source series circuit, and a negative-side switching element that is connected to a negative-side terminal of the DC power source series circuit. The bidirectional switch circuit comprises a first bidirectional switch element one end of which is connected to the first AC output terminal, which is connected to the positive-side switching element and the negative-side switching element, and the other end of the first bidirectional switch element being connected to one end of the AC power source; and a second bidirectional switch element one end of which is connected to the first AC output terminal, and the other end of which is connected to the neutral terminal of the DC power source series circuit.

The power converter in which the present invention is used allows outputting voltage of any one level selected from among four levels of voltages, namely the positive-side voltage of the DC power source series circuit, the negative-side voltage of the DC power source series circuit, a neutral voltage of the DC power source series circuit, and the voltage of the AC power source. Herein, current need only pass through one bidirectional switch element during output of the voltage of the AC power source.

The power converter in which the present invention is used allows reducing the voltage that is applied to switching elements and bidirectional switch elements. Therefore, the power converter in which the present invention is used allows reducing power loss generated in switching elements and bidirectional switch elements.

The power converter in which the present invention is used divides a period of AC output voltage command into a plurality of control periods, and in each control period, outputs, over a respective predefined time, a first voltage, from among the four levels of voltage, being a voltage having an absolute value equal to or higher than an absolute value of the AC output voltage command and having a value that is closest to the AC output voltage command, and a second voltage, from among the four levels of voltage, being a voltage having an absolute value smaller than the absolute value of the AC output voltage command and having a value that is closest to the AC output voltage command, so that a desired AC voltage can be generated as a result.

Therefore, the power converter in which the present invention is used allows outputting AC voltage in which fluctuations of a power source voltage are compensated, even when no commercial-frequency transformer is utilized.

DETAILED DESCRIPTION

Embodiments of the power converter of the present invention are explained in detail next with reference toFIG. 1toFIG. 14. The power converter illustrated inFIG. 1toFIG. 14can be used in devices for supplying stable voltage to a load, even upon occurrence of voltage fluctuation of an AC power source or power failure of the AC power source, for instance in instantaneous voltage-drop compensation devices, uninterruptible power source devices and the like.

FIG. 1is a diagram for explaining a first embodiment of the power converter of the present invention. The power converter according to this embodiment generates four levels of voltage using the voltage of an AC power source and the voltage of a DC power source series circuit resulting from connecting two DC power sources in series.

In the figure, the reference symbol1is an AC power source, the reference symbol2is a capacitor, the reference symbol30is a DC power source series circuit, the reference symbol4is an inverter circuit, the reference symbol5is a filter circuit, the reference symbol6is a load, the reference symbol100is a bidirectional switch circuit, and the reference symbol200is a control circuit.

The AC power source1is a single-phase AC power source having a terminal R and a terminal S. A capacitor2is connected between the terminal R and the terminal S of the AC power source1. The DC power source series circuit30is a DC power source resulting from connecting a DC power source Psp and a DC power source Psn in series. The DC power source Psp is a positive-side DC power source. One end of the DC power source Psp is a positive-side terminal P that outputs voltage of positive polarity. The DC power source Psn is a negative-side DC power source. One end of the DC power source Psn is a negative-side terminal N that outputs voltage of negative polarity. A series connection point of the DC power source Psp and the DC power source Psn is a neutral terminal O that outputs zero voltage. The neutral terminal O is connected to the terminal S of the AC power source1.

The inverter circuit4is connected between the positive-side terminal P and the negative-side terminal N of the DC power source series circuit30. The inverter circuit4is configured in the form of a switching element series circuit. The switching element series circuit is a circuit resulting from connecting in series a switching element Q1one end of which is connected to the positive-side terminal P of the DC power source series circuit30, and a switching element Q2one end of which is connected to the negative-side terminal N.

The series connection point of the switching element Q1and the switching element Q2is connected to an AC output terminal U (first AC output terminal) for outputting single-phase AC voltage from the inverter circuit4. The neutral terminal O of the DC power source series circuit30is connected to an AC output terminal V (second AC output terminal) for outputting single-phase AC voltage from the inverter circuit4.

The bidirectional switch circuit100comprises a bidirectional switch element S1(first bidirectional switch element) and a bidirectional switch element S2(second bidirectional switch element). One end of the bidirectional switch element S1is connected to the AC output terminal U, and the other end is connected to the terminal R of the AC power source1. One end of the bidirectional switch element S2is connected to the AC output terminal U, and the other end is connected to the neutral terminal O of the DC power source series circuit30.

The AC output terminals U, V are connected to the load6via the filter circuit5. The filter circuit5removes the harmonic component in the AC voltage that is outputted from the inverter circuit4.FIG. 2AtoFIG. 2Dillustrate configuration examples of the bidirectional switch elements S1, S2. The bidirectional switch element illustrated inFIG. 2Ais configured through anti-parallel connection of two reverse blocking-type IGBTs (Insulated Gate Bipolar Transistors). The bidirectional switch element illustrated inFIG. 2Bis configured through anti-parallel connection of two sets of switch elements in which diodes and IGBTs having no reverse-blocking breakdown voltage are connected in series. The bidirectional switch element illustrated inFIG. 2Cis configured through anti-series connection of two sets of switch elements in which diodes are anti-parallel connected to IGBTs having no reverse-blocking breakdown voltage. The bidirectional switch element illustrated inFIG. 2Dis configured by replacing the IGBTs in the bidirectional switch element illustrated inFIG. 2Cby MOSFETs (Metal Oxide Semiconductor Field Effect Transistors).

The inverter circuit4and the bidirectional switch circuit100make up a power conversion circuit for generating AC voltage that is supplied to the load6. The power conversion circuit operates in such a manner that any one element from among the switching elements Q1, Q2and the bidirectional switch elements S1, S2is selected and turned on, and the other three elements are turned off.

When the switching element Q1is on, positive voltage of the DC power source Psp is outputted to the AC output terminal U. When the switching element Q2is turned on, negative voltage of the DC power source Psn is outputted to the AC output terminal U. When the bidirectional switch element S1is turned on, R terminal voltage of the AC power source1is outputted to the AC output terminal U. When the bidirectional switch element S2is turned on, zero voltage is outputted to the AC output terminal U. That is, the power conversion circuit can output, to the AC output terminal U, voltage of any one level from among four levels of voltage, namely the positive voltage of the DC power source Psp, the negative voltage of the DC power source Psn, the R terminal voltage of the AC power source1and the zero voltage, through selection and turning-on of any one element from among the switching elements Q1, Q2and the bidirectional switch elements S1, S2.

The filter circuit5is configured through connection in series of a reactor Lf1and a capacitor Cf1. The filter circuit5is connected between the AC output terminal U and the AC output terminal V (hereafter, between the AC output terminals U and V). The load6is connected to both ends of the capacitor Cf1. The filter circuit5removes a harmonic component from the AC output voltage Vus that is outputted between the AC output terminals U and V. The voltage outputted from the filter circuit5is supplied to the load6.

A control circuit200divides the period of a below-described AC output voltage command into a plurality of control periods. At each control period, the control circuit200generates control signals G1, G2for turning on and off the switching elements Q1, Q2, and control signals Gs1, Gs2for turning on and off the bidirectional switch elements S1, S2. Each control period is a continuous period having a pre-established time width.

The length of the control period for controlling the turning on and off of the switching elements Q1, Q2and the bidirectional switch elements S1, S2will be referred to hereafter as a switching period T. The switching frequency determined by the switching period T is preferably a sufficiently high frequency with respect to the frequency of the AC output voltage command. For instance, the switching frequency is preferably 1 kHz or higher in a case where the frequency of the AC output voltage command is a commercial frequency. The switching period T need not necessarily be synchronized with one period of the AC output voltage command, and may be asynchronous therewith.

FIG. 3is a block diagram for explaining the operation whereby the control circuit200generates control signals. The control circuit200receives a voltage Vrs of the AC power source1as detected by a voltage detector301, a voltage Vps of the DC power source Psp as detected by a voltage detector302, and a voltage Vns of the DC power source Psn as detected by a voltage detector303. On the basis of relationships between these three voltages, the control circuit200generates the control signals G1, G2for controlling the turning on and off of the switching elements Q1, Q2, and the control signals Gs1, Gs2for controlling the turning on and off of the bidirectional switch elements S1, S2.

Specifically, the control circuit200generates the control signals G1, G2and the control signals Gs1, Gs2as described below. An AC output voltage command generation circuit201generates an AC output voltage command Vus* on the basis of the voltage Vrs of the AC power source1. For instance, the AC output voltage command Vus* is synchronized with the voltage Vrs of the AC power source1, and has an amplitude identical to that of the voltage rating of the AC power source1.

The AC output voltage command Vus* can be set to an AC output voltage command that is asynchronous with the voltage Vrs of the AC power source1. The AC output voltage command Vus* can be set to an AC output voltage command having an amplitude different from that of the voltage rating of the AC power source1.

The voltage Vrs of the AC power source1and the AC output voltage command Vus* are inputted to a voltage determination circuit202. The voltage determination circuit202outputs a domain signal δ to which the corresponding switching period T belongs, using the voltage Vrs of the AC power source1and the AC output voltage command Vus*. The domain signal δ is classified into domains 1 to 6.

FIG. 4is a diagram for explaining the relationship between domain determination performed by the control circuit200, a pulse width command α, and element selection. When the relationship between the AC output voltage command Vus* and the voltage Vrs of the AC power source1is Vus*≧0 and Vrs<0, the voltage determination circuit202determines the switching period T to be domain 1.

When the relationship between the AC output voltage command Vus* and the voltage Vrs of the AC power source1is Vus*≧0 and Vrs≧0 and Vrs<Vus*, the voltage determination circuit202determines the switching period T to be domain 2.

When the relationship between the AC output voltage command Vus* and the voltage Vrs of the AC power source1is Vus*≧0 and Vrs≧0 and Vrs≧Vus*, the voltage determination circuit202determines the switching period T to be domain 3.

When the relationship between the AC output voltage command Vus* and the voltage Vrs of the AC power source1is Vus*<0 and Vrs<0 and Vrs≦Vus*, the voltage determination circuit202determines the switching period T to be domain 4.

When the relationship between the AC output voltage command Vus* and the voltage Vrs of the AC power source1is Vus*<0 and Vrs<0 and Vrs>Vus*, the voltage determination circuit202determines the switching period T to be domain 5.

When the relationship between the AC output voltage command Vus* and the voltage Vrs of the AC power source1is Vus*<0 and Vrs≧0, the voltage determination circuit202determines the switching period T to be domain 6. In each domain, one element from among the four elements is selected as an H-arm element and another element is selected as an L-arm element. The remaining two elements not having been selected as an H-arm element or an L-arm element constitute off-state arm elements.

The H-arm element is an element that by being turned on can output, to the AC output terminal U, a voltage (first voltage), from among the four levels of voltage, that is equal to or higher than the AC output voltage command Vus* and that is closest to the AC output voltage command Vus*. The H-arm element is on during a time (H-arm on-state time) corresponding to the below-described pulse width command α.

The L-arm element is an element that by being turned on can output, to the AC output terminal U, a voltage (second voltage), from among the four levels of voltage, that is smaller than the AC output voltage command Vus* and that is closest to the AC output voltage command Vus*. The L-arm element is on during a time (L-arm on-state time) resulting from subtracting the H-arm on-state time from the switching period T.

Within the switching period T, the off-state arm elements are constantly off. Returning toFIG. 3, the pulse width command selection circuit203receives the voltage Vrs of the AC power source1, the voltage Vps of the DC power source Psp, the voltage Vns of the DC power source Psn, the AC output voltage command Vus* and the domain signal δ. On the basis of these input signals, the pulse width command selection circuit203computes the pulse width command α (ratio of on-state time with respect to the switching period T) for the H-arm element.

The pulse width command α in domains 1 to 6 is worked out based on the following expressions.
Pulse width command α in domain 1α=Vus*/Vps(1)
Pulse width command α in domain 2 α=(Vus*−Vrs)/(Vps−Vrs)  (2)
Pulse width command α in domain 3α=Vus*/Vrs(3)
Pulse width command α in domain 4α=Vus*/Vrs(4)
Pulse width command α in domain 5 α=(Vus*−Vrs)/(Vns−Vrs)  (5)
Pulse width command α in domain 6α=Vus*/Vns(6)

The comparator204receives the pulse width command α and a carrier signal Sc generated by a carrier signal generation circuit206. The comparator204compares the pulse width command α and the carrier signal Sc, and generates a signal Hon for turning on the H-arm element. When the H-arm on-state signal Hon is at a high level, the H-arm element is on for the H-arm on-state time, within the switching period T.

A logic inverter207inverts the high level or low level of the H-arm on-state signal Hon to the low level or the high level, and generates a signal Lon for turning on the L-arm element. When the L-arm on-state signal Lon is at a high level, the L-arm element is on for the L-arm on-state time, within the switching period T.

A pulse distribution circuit205receives the H-arm on-state signal Hon, the L-arm on-state signal Lon, and the domain signal δ. The pulse distribution circuit205distributes the H-arm on-state signal Hon to the control signal of the H-arm element that is selected in accordance with the domain signal δ. The pulse distribution circuit205distributes the L-arm on-state signal Lon to the control signal of the L-arm element that is selected in accordance with the domain signal δ. The pulse distribution circuit205generates then control signals for turning off the off-state arm elements for the duration of the switching period T.

As described above, the H-arm element is an element that, by being turned on, can output, between the AC output terminals U and V, a voltage (first voltage), from among the four levels of voltage, that is equal to or higher than the AC output voltage command Vus* and that is closest to the AC output voltage command Vus*. The L-arm element is an element that, by being turned on, can output, between the AC output terminals U and V, a voltage (second voltage), from among the four levels of voltage, that is smaller than the AC output voltage command Vus* and that is closest to the AC output voltage command Vus*.

In domain 1, with reference toFIG. 4, the H-arm element is the switching element Q1, the L-arm element is the bidirectional switch element S2, and the off-state arm elements are the switching element Q2and the bidirectional switch element S1. In domain 2, the H-arm element is the switching element Q1, the L-arm element is the bidirectional switch element S1, and the off-state arm elements are the switching element Q2and the bidirectional switch element S2. In domain 3, the H-arm element is the bidirectional switch element S1, the L-arm element is the bidirectional switch element S2and the off-state arm elements are the switching elements Q1and Q2. In domain 4, the H-arm element is the bidirectional switch element S1, the L-arm element is the bidirectional switch element S2and the off-state arm elements are the switching elements Q1and Q2. In domain 5, the H-arm element is the switching element Q2, the L-arm element is the bidirectional switch element S1, and the off-state arm elements are the switching element Q1and the bidirectional switch element S2. In domain 6, the H-arm element is the switching element Q2, the L-arm element is the bidirectional switch element S2, and the off-state arm elements are the switching element Q1and the bidirectional switch element S1.

An explanation follows next, with reference toFIG. 5AtoFIG. 7E, on the relationship between the AC output voltage Vus and the turning on and off operations of the four elements within the switching period T in a case where the AC output voltage command Vus* is positive (domains 1 to 3).

FIG. 5Ais a diagram for explaining a positive AC output voltage Vus outputted between the AC output terminals U and V in domain 1.FIG. 5BtoFIG. 5Eare diagrams for explaining the operations of the switching elements Q1, Q2and the bidirectional switch elements S1, S2in this domain.

In this domain, the H-arm element is the switching element Q1. The L-arm element is the bidirectional switch element S2. The off-state arm elements are the switching element Q2and the bidirectional switch element S1. Accordingly, the switching element Q1is on for the on-time TH1(FIG. 5B). Therefore, the bidirectional switch element S2is on for the on-time TL1(FIG. 5E). Herein, the switching element Q2and the bidirectional switch element S1are off (FIG. 5C),FIG. 5D).

The on-time TH1is a time calculated for the switching period T, on the basis of the pulse width command α worked out based on Expression (1). The on-time TL, is a time resulting from subtracting the on-time TH1from the switching period T.

When the switching element Q1is on, the voltage Vps of the DC power source Psp, being the first voltage, is outputted between the AC output terminals U and V. When the bidirectional switch element S2is on, the zero voltage, being the second voltage, is outputted between the AC output terminals U and V (FIG. 5A). The average value of the voltage that is outputted between the AC output terminals U and V is equal to the AC output voltage command Vus*.

The voltages that are outputted within the switching period T may be the second voltage and the first voltage, in this order. The same applies to the explanation hereafter.FIG. 6Ais a diagram for explaining a positive AC output voltage Vus outputted between the AC output terminals U and V in domain 2.FIG. 6BtoFIG. 6Eare diagrams for explaining the operations of the switching elements Q1, Q2and the bidirectional switch elements S1, S2in this domain.

In this domain, the H-arm element is the switching element Q1. The L-arm element is the bidirectional switch element S1. The off-state arm elements are the switching element Q2and the bidirectional switch element S2. Accordingly, the switching element Q1is on for the on-time TH2(FIG. 6B). Thereafter, the bidirectional switch element S1is on for the on-time TL2(FIG. 6D). The switching element Q2and the bidirectional switch element S2are off (FIG. 6C),FIG. 6E).

The on-time TH2is a time calculated for the switching period T, on the basis of the pulse width command α worked out based on Expression (2). The on-time TL2is a time resulting from subtracting the on-time TH2from the switching period T.

When the switching element Q1is on, the voltage Vps of the DC power source Psp, being the first voltage, is outputted between the AC output terminals U and V. When the bidirectional switch element S1is on, the voltage Vrs of the AC power source1, being the second voltage, is outputted between the AC output terminals U and V (FIG. 6A). The average value of the voltage that is outputted between the AC output terminals U and V is equal to the AC output voltage command Vus*.

FIG. 7Ais a diagram for explaining a positive AC output voltage Vus outputted between the AC output terminals U and V in domain 3.FIG. 7BtoFIG. 7Eare diagrams for explaining the operations of the switching elements Q1, Q2and the bidirectional switch elements S1, S2in this domain.

In this domain, the H-arm element is the bidirectional switch element S1. The L-arm element is the bidirectional switch element S2. The off-state arm elements are the switching element Q1and the switching element Q2. Accordingly, the bidirectional switch element S1is on for the on-time TH3(FIG. 7B). Thereafter, the bidirectional switch element S2is on for the on-time TL3(FIG. 7D). The switching element Q1and the switching element Q2are off (FIG. 7C),FIG. 7E).

The on time TH3is a time calculated for the switching period T, on the basis of the pulse width command α worked out based on Expression (3). The on-time TL3is a time resulting from subtracting the on-time TH3from the switching period T.

When the bidirectional switch element S1is on, the voltage Vrs of the AC power source1, being the first voltage, is outputted between the AC output terminals U and V. When the bidirectional switch element S2is on, the zero voltage, being the second voltage, is outputted between the AC output terminals U and V (FIG. 7A). The average value of the voltage that is outputted between the AC output terminals U and V is equal to the AC output voltage command Vus*.

FIG. 8AtoFIG. 10Eare diagrams for explaining the relationship between the AC output voltage Vus and the turning on and off of the four elements within the switching period T in a case where the AC output voltage command Vus* is negative (domains 4 to 6).

FIGS. 8A-8Eare diagrams for explaining the AC output voltage in domain 4. Domain 4 is a domain wherein, by virtue of circuit symmetry, the switching elements Q1, Q2and the bidirectional switch elements S1, S2operate substantially in the same way as in the case of domain 3. In this domain, a negative voltage of which average voltage is equal to the AC output voltage command Vus* is outputted between the AC output terminals U and V.

FIGS. 9A-9Eare diagrams for explaining the AC output voltage in domain 5. Domain 5 is a domain wherein, by virtue of circuit symmetry, the operations of the switching element Q1and the switching element Q2are reversed, and operations substantially identical to those in the case of domain 2 are performed. In this domain, a negative voltage of which average voltage is equal to the AC output voltage command Vus* is outputted between the AC output terminals U and V.

FIGS. 10A-10Eare diagrams for explaining the AC output voltage in domain 6. Domain 6 is a domain wherein, by virtue of circuit symmetry, the operations of the switching element Q1and the switching element Q2are reversed, and operations substantially identical to those in the case of domain 1 are performed. In this domain, a negative voltage of which average voltage is equal to the AC output voltage command Vus* is outputted between the AC output terminals U and V.

As described above, the power conversion circuit according to the present embodiment selects the H-arm element and L-arm element that are necessary in order to generate the AC output voltage Vus that is equal to the AC output voltage command Vus*, at each switching period T. The power conversion circuit according to the present embodiment turns on, for a respective predefined time, the H-arm element and the L-arm element, within the switching period T; thereby, voltage such that the average voltage thereof is equal to the AC output voltage command Vus* can be generated between the AC output terminals U and V.

For instance, when the voltage Vrs of the AC power source1is lower than the AC output voltage command Vus*, the power conversion circuit according to the present embodiment superimposes, for a predefined time, the voltage Vps and Vns of the DC power source series circuit30on the voltage Vrs of the AC power source1, through operation in domain 2 and domain 5, so that the AC output voltage Vus can be generated thereby.

When the voltage Vrs of the AC power source1is higher than the AC output voltage command Vus*, the power conversion circuit according to the present embodiment steps down the voltage of the AC power source1, through operation in domain 3 and domain 4, so that the AC output voltage Vus can be generated thereby.

The power conversion circuit according to the present embodiment can further generate AC output voltage Vus of opposite polarity to that of the voltage Vrs of the AC power source1, through operation in domain 1 and domain 6. The phase of the AC output voltage Vus that is generated herein deviates significantly from that of the voltage Vrs of the AC power source1.

That is, the power converter according to the present embodiment can maintain the AC output voltage Vus that is supplied to the load6at the AC output voltage command Vus*, by using the voltage Vrs of the AC power source1and the voltages Vps, Vns of the DC power source series circuit30.

The power converter according to the present embodiment cannot output voltage that is higher than the voltage Vps of the DC power source Psp or voltage that is lower than the voltage Vns of the DC power source Psn. Accordingly, it is appropriate to perform a protective operation, which involves for instance turning off all elements, when the AC output voltage command Vus* is higher than the voltage Vps of the DC power source Psp, or when the AC output voltage command Vus* is lower than the voltage Vns of the DC power source Psn.

The switching element Q1may be maintained constantly in an on-state when the AC output voltage command Vus* is higher than the voltage Vps of the DC power source Psp. The switching element Q2may be maintained constantly in an on-state when the AC output voltage command Vus* is lower than the voltage Vns of the DC power source Psn.

In the power converter illustrated inFIG. 13, the turning on and off operations of the switching elements of the inverter circuit are performed between the positive-side voltage and the negative-side voltage of the DC power source series circuit. In the power converter according to the present embodiment, however, the turning on and off operations of the switching elements and bidirectional switch elements are performed between the first voltage and the second voltage. As described above, the first voltage is a voltage equal to or higher than the AC output voltage command Vus* and closest to the AC output voltage command Vus*. The second voltage is a voltage that is lower than the AC output voltage command Vus* and closest to the AC output voltage command Vus*. AsFIG. 5AtoFIG. 10Emake clear, the voltage difference between the first voltage and the second voltage is significantly smaller than the magnitudes of the voltages Vps, Vns of the DC power source.

Therefore, the switching losses that occur upon turning on and turning off of the switching elements of the inverter circuit4according to the present embodiment are smaller than the switching losses of the switching elements of the power converter illustrated inFIG. 13. Similarly, the switching losses that occur upon turning on and turning off of the bidirectional switch elements of the bidirectional switch circuit100according to the present embodiment are smaller than the switching losses of the switching elements of the power converter illustrated inFIG. 13.

That is, the switching loss in the power converter according to the present embodiment can be made smaller than that of the inverter circuit4illustrated inFIG. 13when the switching frequency of the power converter according to the present embodiment is set to the same switching frequency as that of the inverter circuit4illustrated inFIG. 13.

Preferably, in particular, the AC output voltage Vus is synchronized with the voltage Vrs of the AC power source1. Synchronizing the AC output voltage Vus to the voltage Vrs of the AC power source1makes it possible to further reduce the voltage that is applied to the switching elements Q1, Q2and the bidirectional switch elements S1, S2. The switching loss incurred by these elements can be further reduced as a result.

The AC output voltage Vus of the power converter according to the present embodiment varies between the first voltage and the second voltage. The voltage applied to the reactor Lf1is accordingly smaller. Ripple current flowing in the reactor Lf1is proportional to the voltage time product (voltage variation range×voltage pulse width), and inversely proportional to the inductance value. For a same inductance value, the voltage time product is smaller in the power converter of the present embodiment, and the ripple current flowing in the reactor Lf1is accordingly smaller. A smaller ripple current entails a smaller loss (mainly iron loss) in the reactor Lf1, and it becomes therefore possible to reduce loss in the reactor Lf1.

The inductance value of the reactor Lf1can be reduced when ripple currents are set to be identical. The size of the reactor Lf1can be reduced in such a case. Even upon occurrence of power failure in the AC power source1, the H-arm element and the L-arm element in the power converter of the present invention can be selected at each switching period T, in accordance with the same logical process as when the AC power source1is in a normal condition. The AC output voltage Vus can be maintained at the AC output voltage command Vus* through turning on and off of the selected H-arm element and L-arm element in the same way as when the AC power source1is in a normal condition.

In the power converter according to this embodiment, control for maintaining the AC output voltage Vus at the AC output voltage command Vus* requires therefore no means for detecting a power failure in the AC power source1.

FIG. 11is a diagram for explaining another relationship between domain determination performed by the control circuit200, the pulse width command α, and element selection.FIGS. 12A-12Eis a diagram for explaining the AC output voltage Vus in domain 7, and the operations of the switching elements Q1, Q2and the bidirectional switch elements S1, S2.

The configuration of the control circuit200is identical to that of the control circuit illustrated inFIG. 3. Herein, however, the voltage determination circuit202determines also domain 7 in addition domains 1 to 6 illustrated inFIG. 4. Domain 7 is a domain for outputting voltage of the AC power source1between the AC output terminals U and V.

The explanation thereafter with reference toFIG. 3,FIG. 11andFIGS. 12A-12Ewill focus on the operation of the control circuit200for domain 7, and portions shared with domains 1 to 6 explained above will be omitted as appropriate.

The AC output voltage command Vus* and the voltage Vrs of the AC power source1are inputted, at each switching period T, to the voltage determination circuit202. The voltage determination circuit202determines the switching period T to be domain 7 when the relationship between the two voltages satisfies the condition |Vus*−Vrs|<ΔVus*. The voltage determination circuit202outputs then a domain signal δ denoting domain 7.

Herein, ΔVus* is a reference quantity for determining that the value of the voltage Vrs of the AC power source1lies within a predefined range with respect to the value of the AC output voltage command Vus*. In a case where the load6allows for a fluctuation of the input voltage within a range of AC output voltage command Vus*±10%, the reference quantity ΔVus* is equivalent to 10% of the AC output voltage command Vus*. The reference quantity ΔVus* may be established in accordance with some other condition.

Upon input of a domain signal δ denoting domain 7, the pulse width command selection circuit203fixes the pulse width command α to 1.0. When the pulse width command α is 1.0, the comparator204generates the signal Hon for turning on the H-arm element throughout the switching period T, regardless of the magnitude of the carrier signal Sc. In the switching period T, thus, the H-arm on-state signal Hon is constantly at a high level and the L-arm on-state signal Lon is constantly at a low level.

Upon input of the domain signal δ denoting domain 7, the pulse distribution circuit205sets the bidirectional switch element S1as the H-arm element. The pulse distribution circuit205sets the switching elements Q1, Q2and the bidirectional switch element S2as the off-state arm elements. Accordingly, the pulse distribution circuit205outputs a control signal Gs1of the bidirectional switch element S1that is at a high level during the switching period T, and control signals G1, G2and Gs2of the switching elements Q1, Q2and the bidirectional switch element S2that are at a low level during the switching period T.

Therefore, the bidirectional switch element S1is on and the switching elements Q1, Q2and the bidirectional switch element S2are off during the switching period T determined to be domain 7. The voltage Vrs of the AC power source1is outputted thus between the AC output terminals U and V through the operation of the four elements.

Also in the case where the AC output voltage command Vus* has negative polarity, the control circuit200performs the same operations as in the case where the AC output voltage command Vus* has positive polarity. In the switching period T determined to be domain 7, only the bidirectional switch element S1is on, while the switching elements Q1, Q2and the bidirectional switch element S2are off. Accordingly, only the bidirectional switch element S1incurs conduction loss on account of current application. No conduction loss occurs in the switching elements Q1, Q2and the bidirectional switch element S2, since no current flows through these. Moreover, there occurs no switching loss, since none of the elements undergoes an on or off operation.

Therefore, power loss can be further reduced by providing domain 7 in the operation of the power conversion circuit. Next,FIG. 13is a diagram for explaining a second embodiment of the power converter of the present invention. The power converter according to the present embodiment is configured by using herein a half-bridge converter circuit31as the DC power source series circuit30of the embodiment illustrated inFIG. 1.

The converter circuit31has, as main constituent elements, a series circuit of a positive-side switching element Qp and a negative-side switching element Qn, a series circuit of a positive-side capacitor Cp and a negative-side capacitor Cn, and a reactor L. One end of the reactor L is connected to one end of the AC power source1, and the other end is connected to a connection midpoint of the switching elements Qp, Qn. The series circuit of the capacitors Cp, Cn is connected in parallel to the series circuit of the switching elements Qp, Qn. The connection midpoint of the capacitors Cp, Cn is connected to the other end of the AC power source1, and is also connected to the AC output terminal V.

When the voltage of the AC power source1has positive polarity with respect to the AC output terminal V, firstly, the switching element Qn is turned on, and the switching element Qp is turned off. Through turning-on of the switching element Qn, voltage resulting from adding the voltage of the capacitor Cn and the voltage of the AC power source1is applied to the reactor L, and energy accumulates in the reactor L. Next, the switching element Qn is turned off and the switching element Qp is turned on. When the switching element Qn is turned off, the energy accumulated in the reactor L is charged to the capacitor Cp.

When the voltage of the AC power source1has negative polarity with respect to the AC output terminal V, firstly, the switching element Qp is turned on, and the switching element Qn is turned off. Through turning-on of the switching element Qp, a voltage resulting from adding the voltage of the capacitor Cp and the voltage of the AC power source1is applied to the reactor L, and energy accumulates in the reactor L. Next, the switching element Qp is turned off, and the switching element Qn is turned on. When the switching element Qp is turned off, the energy accumulated in the reactor L is charged to the capacitor Cn.

The on-off operations of the switching elements Qp, Qn are performed at a frequency that is sufficiently higher than the frequency of the AC power source1. Through the on-off operations of the switching elements Qp, Qn, the voltage of the capacitor Cp and the capacitor Cn is maintained at a predefined voltage that is higher than the voltage of the AC power source1.

The DC power source series circuit30of the power converter of the present invention can be configured thus in the form of a half-bridge converter31. The capacitor Cp of the half-bridge converter31corresponds to the positive-side DC power source Psp of the DC power source series circuit30. The capacitor Cn of the half-bridge converter31corresponds to the negative-side DC power source Psn of the DC power source series circuit30.

In the present embodiment, the action and effect of circuits other than the half-bridge converter31are identical to the action and effect of the power converter according to the first embodiment explained with reference toFIG. 1throughFIG. 12E.

Next,FIG. 14is a diagram for explaining a third embodiment of the power converter of the present invention. In the power converter according to this embodiment, the DC power source series circuit30of the embodiment illustrated inFIG. 1is configured in the form of a three-level rectifier32.

The three-level rectifier32has, as main constituent elements, a series circuit of a positive-side diode Dp and a negative-side diode Dn, the series circuit of the positive-side capacitor Cp and the negative-side capacitor Cn, a bidirectional switch element S3, and the reactor L. One end of the reactor L is connected to one end of the AC power source1, and the other end of the reactor L is connected to a connection midpoint of the diodes Dp, Dn. The series circuit of the capacitors Cp, Cn is connected in parallel to the series circuit of the diodes Dp, Dn. The connection midpoint of the capacitors Cp, Cn is connected to the other end of the AC power source1, and is also connected to the AC output terminal V. The bidirectional switch element S3is connected between the connection midpoint of the diodes Dp, Dn and the connection midpoint of the capacitors Cp, Cn.

When the voltage of the AC power source1has positive polarity with respect to the AC output terminal V, firstly the bidirectional switch element S3is turned on. Upon turning-on of the bidirectional switch element S3, the voltage of the AC power source1is applied to the reactor L, and energy accumulates in the reactor L. Next, the bidirectional switch element S3is turned off. Upon turning off of the bidirectional switch element S3, the energy accumulated in the reactor L is charged to the capacitor Cp.

On the other hand, when the voltage of the AC power source1has negative polarity with respect to the AC output terminal V, firstly the bidirectional switch element S3is turned on. Upon turning-on of the bidirectional switch element S3, the voltage of the AC power source1is applied to the reactor L, and energy accumulates in the reactor L. Next, the bidirectional switch element S3is turned off. Upon turning off of the bidirectional switch element S3, the energy accumulated in the reactor L is charged to the capacitor Cn.

The on-off operations of the bidirectional switch element S3are performed at a frequency that is sufficiently higher than the frequency of the AC power source1. Through the on-off operations of bidirectional switch element S3, the voltage of the capacitor Cp and the capacitor Cn is maintained at a predefined voltage that is higher than the voltage of the AC power source1.

The DC power source series circuit30of the power converter of the present invention can be configured thus in the form of the three-level rectifier32. The capacitor Cp of the three-level rectifier32corresponds to the positive-side DC power source Psp of the DC power source series circuit30. The capacitor Cn of the three-level rectifier32corresponds to the negative-side DC power source Psn of the DC power source series circuit30.

In the present embodiment, the action and effect of circuits other than the three-level rectifier32are identical to the action and effect of the power converter according to the first embodiment explained with reference toFIG. 1throughFIG. 12E.