Electric power conversion device

The present invention relates to a power conversion device that converts DC power to DC power having different voltage, and in particular, relates to a power conversion device used for high-voltage DC power transmission.

In recent years, in offshore wind power generation, the amount of electric generation has been increasing with size increase in windmills, and the distance between a substation facility and windmills installed offshore has become further long. Powers generated by a plurality of electric generators are collected and transmitted to a substation facility. In order to cope with increase in the amount of electric generation by the electric generators and increase in the power transmission distance, high-voltage DC power transmission is considered in which the collected voltage is efficiently transmitted by direct current with increased voltage.

A DC step-up conversion unit which is a conventional power conversion device used for high-voltage DC power transmission has, in one mode, input terminals for receiving input DC power to be DC power. The DC step-up conversion unit has a converter section which has a plurality of insulation-type DC-DC converters and in which first terminals of the plurality of insulation-type DC-DC converters are connected in parallel to the input terminals. The DC step-up conversion unit has output terminals which are connected in series to second terminals of the plurality of insulation-type DC-DC converters and from which output DC power which is DC power having voltage stepped up by the converter section is outputted. The DC step-up conversion unit has, as a converter section controller for controlling the converter section so that voltage of the input DC power becomes predetermined voltage, a first converter section controller provided for each of the plurality of insulation-type DC-DC converters of the converter section, and a second converter section controller (see, for example, Patent Document 1).

In another example of conventional power conversion devices used for high-voltage DC power transmission, a plurality of DC/DC converters are connected in series on both the input side and the output side (see, for example, Patent Document 2).

Patent Document

Patent Document 1: Japanese Laid-Open Patent Publication No. 2015-6066

Patent Document 2: International Publication No. WO2013/013858

Problems to be Solved by the Invention

In the DC step-up conversion unit described in Patent Document 1, inputs of the plurality of insulation-type DC/DC converters are connected in parallel and outputs thereof are connected in series. Therefore, the usage is limited to the case where input voltage of the DC step-up conversion unit is equal to or lower than the withstand voltage of semiconductor switching elements in each insulation-type DC/DC converter. That is, it is difficult for the DC step-up conversion unit to use, as the input power, high-voltage DC power obtained by collecting generated powers from a plurality of electric generation devices.

The power conversion device described in Patent Document 2 is applicable to high-voltage input power, but the input voltage needs to be stable. Therefore, if power is inputted from a DC power source having varying voltage, it is difficult to reliably perform output control.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a power conversion device that, without increasing the element withstand voltages, allows input of high-voltage DC power from a power source having varying voltage and can reliably perform output control.

Solution to the Problems

A power conversion device according to the present invention includes: a power conversion unit including M number of DC/DC converters between first DC terminals and second DC terminals each pair of which is composed of both positive and negative terminals, where M is two or greater, the power conversion unit performing power transmission between the first DC terminals and the second DC terminals; and a controller for controlling the power conversion unit. The power conversion unit further includes one or more balancing circuits each connected between two of the DC/DC converters and balancing powers of the two DC/DC converters. In the M number of DC/DC converters, one of input and output thereof is defined as a first side and the other one is defined as a second side, first-side terminals of the DC/DC converters are connected so that current flows in common between both positive and negative terminals of the first DC terminals, and second-side terminals of the DC/DC converters are connected so that current flows in common between both positive and negative terminals of the second DC terminals. Each balancing circuit is connected between two pairs of the first-side terminals of the two DC/DC converters, and receives and passes power between the two pairs of first-side terminals.

Effect of the Invention

The power conversion device according to the present invention can achieve power conversion that allows input of high-voltage DC power without increasing element withstand voltage and enables reliable output control even when power is inputted from a power source having varying voltage.

FIG. 1 is a diagram showing the schematic configuration of a power transmission system to which a power conversion device according to embodiment 1 of the present invention is applied.

FIG. 2 is a diagram showing the configuration of the power conversion device according to embodiment 1 of the present invention.

FIG. 3 is a circuit diagram showing a DC/DC converter according to embodiment 1 of the present invention.

FIG. 4 is a block diagram showing the entire configuration of a controller of the power conversion device according to embodiment 1 of the present invention.

FIG. 5 is a control block diagram illustrating a second control unit for controlling the DC/DC converter according to embodiment 1 of the present invention.

FIG. 6 is a control block diagram illustrating a third control unit for controlling an auxiliary converter according to embodiment 1 of the present invention.

FIG. 7 is a diagram showing the configuration of a power conversion device according to embodiment 2 of the present invention.

FIG. 8 is a block diagram showing the entire configuration of a controller of the power conversion device according to embodiment 2 of the present invention.

FIG. 9 is a diagram showing the configuration of a DC/DC converter according to embodiment 3 of the present invention.

FIG. 10 is a circuit diagram of a converter cell according to embodiment 3 of the present invention.

FIG. 11 is a block diagram showing the entire configuration of a second control unit for controlling the DC/DC converter according to embodiment 3 of the present invention.

FIG. 12 is a block diagram showing the entire configuration of a controller of the power conversion device according to embodiment 3 of the present invention.

FIG. 13 is a control block diagram illustrating a first control unit for controlling the power conversion unit according to embodiment 3 of the present invention.

FIG. 14 is a control block diagram illustrating control for the entire DC/DC converter, in the second control unit according to embodiment 3 of the present invention.

FIG. 15 is a control block diagram illustrating a cell control unit for controlling the converter cell, in the second control unit according to embodiment 3 of the present invention.

FIG. 16 is a control block diagram illustrating a third control unit for controlling an auxiliary converter according to embodiment 3 of the present invention.

FIG. 17 is a circuit diagram showing a converter cell according to embodiment 4 of the present invention.

Hereinafter, embodiment 1 of the present invention will be described.

FIG. 1 is a schematic configuration diagram showing an example of a power transmission system 1 including a power conversion device 100 according to embodiment 1 of the present invention. As shown in FIG. 1 , the power transmission system 1 includes: a wind power generation system 200 and a power conversion device 100 provided offshore; an electrical substation 300 provided onshore; and a DC power transmission cable 400 as a DC power transmission line.

The wind power generation system 200 includes: electric generators 210 a , 210 b as a plurality of electric generation devices; power conditioners 220 a , 220 b which receive respective generated powers and perform conversion from AC power to DC power; and step-up DC/DC conversion units 230 a , 230 b which step up voltages of DC powers outputted from the power conditioners 220 a , 220 b . In the wind power generation system 200 , DC powers outputted from the step-up DC/DC conversion units 230 a , 230 b are collected together and outputted.

The DC power outputted from the wind power generation system 200 is inputted to the power conversion device 100 . DC power outputted from the power conversion device 100 is transmitted to the electrical substation 300 via the DC power transmission cable 400 . The electrical substation 300 includes: a DC/AC conversion unit 310 which receives DC power outputted from the power conversion device 100 and converts the DC power to AC power; and a transformer 320 which steps up voltage of the AC power from the DC/AC conversion unit 310 .

In the example shown in FIG. 1 , the power source is wind power generation, but is not limited thereto. Another power source in which the amount of power generation varies, e.g., a power supply using natural energy such as a photovoltaic generation device, may be used.

In this case, the wind power generation system 200 includes two electric generators 210 a , 210 b , two power conditioners 220 a , 220 b , and two step-up DC/DC conversion units 230 a , 230 b . However, without limitation thereto, the numbers of these components are optional. Further, although the wind power generation system 200 and the power conversion device 100 are provided offshore and the electrical substation 300 is provided onshore, the arrangement is not limited thereto.

Although the case of applying the power conversion device 100 to the power transmission system 1 is shown, the power conversion device 100 may be applied to DC distribution or the like, instead of DC power transmission.

FIG. 2 is a diagram showing the configuration of the power conversion device 100 .

As shown in FIG. 2 , the power conversion device 100 has first DC terminals 100 A and second DC terminals 100 B each pair of which is composed of both positive and negative terminals, and includes, between the first DC terminals 100 A and the second DC terminals 100 B, a plurality (M number) of DC/DC converters 10 ( 10 a to 10 h ), and auxiliary converters 20 ( 20 a to 20 g ) as balancing circuits. Each auxiliary converter 20 is connected between two DC/DC converters 10 .

In this case, the auxiliary converters 20 each connected between adjacent two DC/DC converters 10 are used as the balancing circuits, but without limitation thereto, anything that is connected between two DC/DC converters 10 and balances powers of the two DC/DC converters 10 may be used.

In addition, the power conversion device 100 includes a controller for performing output control of the DC/DC converters 10 and the auxiliary converters 20 of the main circuit (power conversion unit). The controller includes: a first control unit 110 for controlling the entire main circuit; second control units 120 ( 120 a to 120 h ) for controlling the respective DC/DC converters 10 ; and third control units 130 ( 130 a to 130 g ) for controlling the respective auxiliary converters 20 .

Input voltage Vin of the power conversion device 100 is controlled to be target voltage Vin*, e.g., 25 kV, by the power conversion device 100 . Output voltage Vout of the power conversion device 100 is controlled to be predetermined voltage, e.g., 350 kV, by the DC/AC conversion unit 310 of the electrical substation 300 .

In the present embodiment 1, DC power from the wind power generation system 200 is inputted to the first DC terminals 100 A. That is, the first DC terminals 100 A serve as the input terminals of the main circuit (power conversion unit) of the power conversion device 100 , and the input voltage Vin is voltage of the first DC terminals 100 A. In addition, the second DC terminals 100 B serve as the output terminals of the main circuit of the power conversion device 100 , and the output voltage Vout is voltage of the second DC terminals 100 B. Output power from the power conversion device 100 is outputted to the electrical substation 300 via the DC power transmission cable 400 (not shown).

Each DC/DC converter 10 includes: DC terminals 5 A (hereinafter, first-side terminals 5 A) on the primary side as the first side; DC terminals 5 B (hereinafter, second-side terminals 5 B) on the secondary side as the second side; a DC/AC conversion unit 2 on the primary side; a DC/AC conversion unit 3 on the secondary side; and a single-phase transformer 4 as a transformer. It is noted that, if insulation is not needed, only inductance LS may be connected instead of the single-phase transformer 4 .

The second control units 120 are provided for the respective DC/DC converters 10 .

FIG. 3 shows an example of a circuit diagram of the DC/DC converter 10 a . It is noted that there are various types of converters as the DC/DC converters 10 , and an appropriate type is selected in accordance with, for example, rated voltage, conversion capacity, operation frequency, and the like. The other DC/DC converters 10 b to 10 h are the same as the DC/DC converter 10 a.

As shown in FIG. 3 , the DC/AC conversion unit 2 on the primary side includes: a DC capacitor 6 connected between both poles of the first-side terminals 5 A; and a first full-bridge circuit formed by two switching legs composed of semiconductor switching elements Q 11 to Q 14 as positive-side and negative-side semiconductor elements connected in series. The DC/AC conversion unit 3 on the secondary side includes: a DC capacitor 7 connected between both poles of the second-side terminals 5 B; and a second full-bridge circuit formed by two switching legs composed of semiconductor switching elements Q 21 to Q 24 as positive-side and negative-side semiconductor elements connected in series. The single-phase transformer 4 is connected between intermediate connection points of the switching legs of the DC/AC conversion unit 2 and intermediate connection points of the switching legs of the DC/AC conversion unit 3 .

As the semiconductor switching elements Q 11 to Q 24 , for example, semiconductor switching elements having a self-turn-off function, such as IGBTs (Insulated Gate Bipolar Transistors), to which diodes are connected in antiparallel, are used. Each semiconductor switching element Q 11 to Q 24 may be used with a plurality of semiconductor switching elements combined in parallel, depending on the current capacity.

In this case, a snubber capacitor CS is connected in parallel to each semiconductor switching element Q 11 to Q 24 . Owing to effects of the snubber capacitor CS and the inductance LS of the AC output line, zero voltage switching which is soft switching of each semiconductor switching element Q 11 to Q 24 becomes possible.

The second control unit 120 a generates gate signals G 10 a for the semiconductor switching elements Q 11 to Q 24 in the DC/DC converter 10 a , thereby controlling switching of the semiconductor switching elements Q 11 to Q 24 .

In this case, by soft switching of the semiconductor switching elements Q 11 to Q 24 , switching loss can be reduced, the operation frequency can be increased, and the size of the single-phase transformer 4 can be reduced. It is noted that soft switching is technology in which, using a resonance phenomenon, voltage or current applied to a semiconductor switching element during a switching transient period is reduced to reduce switching loss and electromagnetic noise. As the inductance LS, the leakage inductance of the single-phase transformer 4 may be used.

The DC/DC converter 10 a is a circuit that converts DC voltage V 1 a applied to the first-side terminals 5 A, to DC voltage V 2 a applied to the second-side terminals 5 B, via the DC/AC conversion unit 2 , the single-phase transformer 4 , and the DC/AC conversion unit 3 , and can be freely controlled so as to perform bidirectional power conversion. It is desirable that the turns ratio of the single-phase transformer 4 is matched with the ratio of the DC voltage V 1 a on the primary side and the DC voltage V 2 a on the secondary side. It is noted that the DC/DC converter 10 a , in normal operation, steps up the primary-side voltage V 1 a on the input voltage Vin side and outputs the secondary-side voltage V 2 a to the output voltage Vout side.

As the DC capacitors 6 , 7 , electrolytic capacitors, film capacitors, or the like are used. High-frequency current flows through the DC capacitors 6 , 7 , but in the case of using film capacitors, deterioration due to the high-frequency current can be suppressed and thus the life of the capacitors is prolonged.

As shown in FIG. 2 , the first-side terminals 5 A of the M number of DC/DC converters 10 a to 10 h are connected so that common current flows between both positive and negative terminals of the first DC terminals 100 A of the power conversion device 100 , and the second-side terminals 5 B of the DC/DC converters 10 a to 10 h are connected so that common current flows between both positive and negative terminals of the second DC terminals 100 B of the power conversion device 100 .

That is, when current Iin is flowing through both positive and negative terminals of the first DC terminals 100 A of the power conversion device 100 , current flowing through the first-side terminals 5 A of each DC/DC converter 10 a to 10 h is Iin. In addition, when current Iout is flowing through both positive and negative terminals of the second DC terminals 100 B of the power conversion device 100 , current flowing through the second-side terminals 5 B of each DC/DC converter 10 a to 10 h is Iout.

That is, the positive terminal of the first-side terminals 5 A of the DC/DC converter 10 a is connected to the positive terminal of the first DC terminals 100 A, and the negative terminal of the first-side terminals 5 A of the DC/DC converter 10 a is connected to the positive terminal of the first-side terminals 5 A of the DC/DC converter 10 b . Similarly, the negative terminals of the first-side terminals 5 A of the DC/DC converters 10 b to 10 g are respectively connected to the positive terminals of the first-side terminals 5 A of the DC/DC converters 10 c to 10 h . The negative terminal of the first-side terminals 5 A of the DC/DC converter 10 h is connected to the negative terminal of the first DC terminals 100 A.

The positive terminal of the second-side terminals 5 B of the DC/DC converter 10 a is connected to the positive terminal of the second DC terminals 100 B, and the negative terminal of the second-side terminals 5 B of the DC/DC converter 10 a is connected to the positive terminal of the second-side terminals 5 B of the DC/DC converter 10 b . Similarly, the negative terminals of the second-side terminals 5 B of the DC/DC converters 10 b to 10 g are respectively connected to the positive terminals of the second-side terminals 5 B of the DC/DC converters 10 c to 10 h . The negative terminal of the second-side terminals 5 B of the DC/DC converter 10 h is connected to the negative terminal of the second DC terminals 100 B.

Each auxiliary converter 20 performs conversion between DC powers having different voltages, and in this case, is connected between two pairs of first-side terminals 5 A of adjacent two DC/DC converters 10 , to receive and pass power between the two pairs of first-side terminals 5 A. For example, the auxiliary converter 20 a is connected between the first-side terminals 5 A of the DC/DC converter 10 a and the first-side terminals 5 A of the DC/DC converter 10 b.

As described above, without limitation to connection between the adjacent DC/DC converters 10 , each auxiliary converter 20 may be connected between the first-side terminals 5 A of two DC/DC converters 10 . By M-1 number of auxiliary converters 20 being connected between the DC/DC converters 10 , the M number of DC/DC converters 10 c to 10 h can be balanced via the M-1 number of auxiliary converters 20 .

Each auxiliary converter 20 has P terminals 25 A connected to the first-side terminals 5 A of the DC/DC converter 10 on the high-potential side, and N terminals 25 B connected to the first-side terminals 5 A of the DC/DC converter 10 on the low-potential side. The circuit configuration of each auxiliary converter 20 is the same as that of the DC/DC converter 10 a , and has the same DC/AC conversion units 2 , 3 and single-phase transformer 4 as those shown in FIG. 3 .

The third control units 130 are provided for the respective auxiliary converters 20 . For example, the third control unit 130 a that controls the auxiliary converter 20 a generates gate signals G 20 a for the semiconductor switching elements Q 11 to Q 24 in the auxiliary converter 20 a , thereby controlling switching of each semiconductor switching element Q 11 to Q 24 .

It is noted that, as shown in FIG. 2 , the first DC terminals 100 A and the second DC terminals 100 B of the power conversion device 100 , the first-side terminals 5 A and the second-side terminals 5 B of each DC/DC converter 10 , and the P terminals 25 A and the N terminals 25 B of each auxiliary converter 20 are all composed of two positive and negative terminals. Here, unless otherwise specified, voltage between the positive and negative terminals is referred to as voltage of the terminals. For example, voltage of the P terminals 25 A refers to voltage between the positive and negative terminals of the P terminals 25 A.

As described above, the first-side terminals 5 A of the M number of DC/DC converters 10 a to 10 h are connected so that common current flows between both positive and negative terminals of the first DC terminals 100 A of the power conversion device 100 , and the second-side terminals 5 B of the DC/DC converters 10 a to 10 h are connected so that common current flows between both positive and negative terminals of the second DC terminals 100 B of the power conversion device 100 .

Therefore, input voltage Vin which is voltage of the first DC terminals 100 A is the sum of voltages V 1 (V 1 a to V 1 h ) of the first-side terminals 5 A of the DC/DC converters 10 ( 10 a to 10 h ). In addition, output voltage Vout which is voltage of the second DC terminals 100 B is the sum of voltages V 2 (V 2 a to V 2 h ) of the second-side terminals 5 B of the DC/DC converters 10 ( 10 a to 10 h ). Thus, input and output voltages of the power conversion device 100 can be represented by Expression (1) and Expression (2).

V in= V 1 a+V 1 b+ . . . +V 1 h   (1)

V out= V 2 a+V 2 b+ . . . +V 2 h   (2)

Control of the power conversion device 100 configured as described above will be described below, with reference to FIG. 4 to FIG. 6 .

FIG. 4 is a block diagram showing the entire configuration of the controller of the power conversion device 100 , i.e., the entire configuration including the first control unit 110 for controlling the entire main circuit, the second control units 120 ( 120 a to 120 h ) for controlling the respective DC/DC converters 10 , and the third control units 130 ( 130 a to 130 g ) for controlling the respective auxiliary converters 20 .

As shown in FIG. 4 , each second control unit 120 ( 120 a to 120 h ) has means for detecting input and output voltages V 1 (V 1 a to V 1 h ), V 2 (V 2 a to V 2 h ), and the detected input and output voltages V 1 , V 2 are transmitted to the first control unit 110 . The first control unit 110 transmits voltage V 1 of the P terminals 25 A and voltage V 1 of the N terminals 25 B of each auxiliary converter 20 to the respective third control units 130 ( 130 a to 130 g ).

The first control unit 110 includes a Vin calculation unit 111 , a Vout calculation unit 112 , a Vin control unit 113 , and an average value calculation unit 114 . The Vin calculation unit 111 calculates input voltage Vin, using the above Expression (1), and the Vout calculation unit 112 calculates output voltage Vout, using the above Expression (2). The Vin control unit 113 receives given target voltage Vin* and the input voltage Vin, and generates a power command P* as a first command so that the input voltage Vin is controlled to be the target voltage Vin*, that is, the input voltage Vin approaches the target voltage Vin*. The average value calculation unit 114 receives the output voltage Vout and the power command P*, and divides them by the number M of the DC/DC converters 10 a to 10 h . Thus, command voltage V 2 * (=(Vout/M)) for voltage V 2 of the second-side terminals 5 B of each DC/DC converter 10 , and a power command value P 10 * (=(P*/M)) used for control of each DC/DC converter 10 , are generated. The first control unit 110 transmits the command voltage V 2 * and the power command value P 10 * to the second control unit 120 of each DC/DC converter 10 .

Each second control unit 120 ( 120 a to 120 h ) detects the input and output voltages V 1 (V 1 a to V 1 h ), V 2 (V 2 a to V 2 h ) and transmits them to the first control unit 110 , and generates the gate signal G 10 (G 10 a to G 10 h ) for controlling each DC/DC converter 10 , on the basis of the command voltage V 2 * and the power command value P 10 *.

FIG. 5 shows an example of a control block diagram illustrating the second control unit 120 a for controlling the DC/DC converter 10 a . It is noted that the second control units 120 b to 120 h are the same as the second control unit 120 a.

As shown in FIG. 5 , in the second control unit 120 a , a subtractor 121 a calculates a deviation ΔV 2 a between the command voltage V 2 * and the voltage V 2 a . A controller 121 b calculates a first control command value 121 c by, for example, proportional control so that the deviation ΔV 2 a approaches 0. The power command value P 10 * from the first control unit 110 is added to the first control command value 121 c , whereby a control command P 10 a * for performing output control of the DC/DC converter 10 a is generated.

A phase difference generator 122 outputs a difference θ 10 a between switching phases of the DC/AC conversion unit 2 and the DC/AC conversion unit 3 in the DC/DC converter 10 a , on the basis of the control command P 10 a *. It is noted that output power is controlled by delaying the switching phase of the semiconductor switching elements Q 21 to Q 24 in the DC/AC conversion unit 3 on the secondary side by a predetermined phase difference θ [rad] relative to the switching phase of the semiconductor switching elements Q 11 to Q 14 in the DC/AC conversion unit 2 on the primary side.

A PWM signal generator 123 generates and outputs the gate signal G 10 a which is a PWM signal for performing switching control of each semiconductor switching element Q 11 to Q 24 in the DC/DC converter 10 a , on the basis of the phase difference θ 10 a.

As shown in FIG. 4 , each third control unit 130 ( 130 a to 130 g ) generates the gate signal G 20 (G 20 a to G 20 g ) for controlling each auxiliary converter 20 , on the basis of voltage V 1 of the P terminals 25 A and voltage V 1 of the N terminals 25 B of each auxiliary converter 20 . For example, the third control unit 130 a generates the gate signal G 20 a for controlling the auxiliary converter 20 a , on the basis of voltage V 1 a of the P terminals 25 A and voltage V 1 b of the N terminals 25 B of the auxiliary converter 20 a.

FIG. 6 shows an example of a control block diagram illustrating the third control unit 130 a for controlling the auxiliary converter 20 a . It is noted that the third control units 130 b to 130 g are the same as the third control unit 130 a.

As shown in FIG. 6 , in the third control unit 130 a , a subtractor 131 a calculates a voltage difference ΔV 1 ab between voltage V 1 a of the P terminals 25 A and voltage V 1 b of the N terminals 25 B. A V 1 controller 131 b generates a power command (control command) 131 c for performing output control of the auxiliary converter 20 a , by, for example, proportional integral control so that the half of voltage difference ΔV 1 ab is compensated and voltage difference ΔV 1 ab approaches 0.

Here, a power transmission direction from the P terminals 25 A to the N terminals 25 B is defined as positive.

In the case of V 1 a >V 1 b , a positive power command 131 c is generated so that the voltage V 1 a of the P terminals 25 A approaches (V 1 a −(ΔV 1 ab/ 2)). At this time, DC power is transmitted from the P terminals 25 A to the N terminals 25 B, voltage V 1 a of the P terminals 25 A gradually decreases, and voltage V 1 b of the N terminals 25 B gradually increases, so that the voltage difference ΔV 1 ab between voltage V 1 a and voltage V 1 b decreases.

In the case of V 1 a <V 1 b , a negative power command 131 c is generated so that voltage V 1 b of the N terminals 25 B approaches (V 1 b −(ΔV 1 ab/ 2)). At this time, DC power is transmitted from the N terminals 25 B to the P terminals 25 A, voltage V 1 b of the N terminals 25 B gradually decreases, and voltage V 1 a of the P terminals 25 A gradually increases, so that the voltage difference ΔV 1 ab between voltage V 1 a and voltage V 1 b decreases.

A phase difference generator 132 outputs a difference θ 20 a between switching phases of the DC/AC conversion unit 2 and the DC/AC conversion unit 3 in the auxiliary converter 20 a , on the basis of the power command 131 c . A PWM signal generator 133 generates and outputs the gate signal G 20 a which is a PWM signal for performing switching control of each semiconductor switching element Q 11 to Q 24 in the auxiliary converter 20 a , on the basis of the phase difference θ 20 a.

As described above, each auxiliary converter 20 is subjected to output control such that, on the basis of comparison between voltage V 1 of the P terminals 25 A and voltage V 1 of the N terminals 25 B, power is received and passed from the high-voltage side to the low-voltage side so as to equalize the two DC voltages. In this case, the third control unit 130 a generates command voltage so that voltage of the terminals on the high-voltage side decreases by half the difference voltage, thereby controlling the corresponding auxiliary converter 20 .

The P terminals 25 A and the N terminals 25 B of each auxiliary converter 20 are connected to the respective first-side terminals 5 A of the two DC/DC converters 10 , and two voltages of the first-side terminals 5 A are controlled to be equalized by the auxiliary converter 20 . Thus, voltages V 1 (V 1 a to V 1 h ) of the first-side terminals 5 A of the M number of DC/DC converters 10 are controlled to be equalized.

As described above, the M number of DC/DC converters 10 and the M-1 number of auxiliary converters 20 are controlled by the first control unit 110 , the second control units 120 , and the third control units 130 .

In each DC/DC converter 10 , the second control unit 120 performs output control of the DC/DC converter 10 on the basis of the command voltage V 2 * for voltage V 2 and the power command value P 10 *. The power command value P 10 * is calculated by dividing the power command P* for controlling the input voltage Vin of the entire power conversion device 100 to be the target voltage Vin*, by the number M of the DC/DC converters 10 . Therefore, the DC/DC converters 10 operate so that each voltage V 2 is controlled to be the command voltage V 2 * and the input voltage Vin of the entire power conversion device 100 is controlled to be the target voltage Vin*.

Each second control unit 120 causes the DC/DC converter 10 to operate so that the voltage V 2 is controlled to be the command voltage V 2 *, and when the voltage V 2 is determined, the voltage V 1 is automatically determined in accordance with the power to be transmitted.

The DC/DC converters 10 have the same configuration, and each voltage V 1 determined by operation thereof becomes, ideally, a value (Vin*/M) obtained by dividing the target voltage Vin* for input voltage Vin by the number M of the DC/DC converters 10 . However, actually, error occurs by factors such as variation in components and detection error of detectors. Therefore, although the input voltage Vin which is the sum of voltages V 1 can be controlled to be the target voltage Vin*, variation occurs among the voltages V 1 .

Through operations of the auxiliary converters 20 , variation among voltages V 1 is eliminated and the voltages V 1 are controlled to be equalized.

It is noted that, since the target voltages for input voltage Vin and output voltage Vout of the power conversion device 100 are set as the power transmission system 1 , the target voltages are controlled to be constant. Therefore, the step-up ratio (Vout/Vin) of the power conversion device 100 also becomes a constant value, and the input and output voltages V 1 , V 2 of each DC/DC converter 10 becomes approximately constant values.

In the present embodiment, voltages of generated powers sent from the plurality of electric generators 210 a , 201 b via the power conditioners 220 a , 220 b are stepped up by the respective step-up DC/DC conversion units 230 a , 230 b and the resultant powers are collected together to be inputted to the power conversion device 100 . The power conversion device 100 further steps up voltage of the inputted DC power and transmits the resultant power to the electrical substation 300 .

The power conversion device 100 has a plurality of DC/DC converters 10 of which the input terminals (first-side terminals 5 A) are connected so that common current flows between both positive and negative terminals of the input terminals (first DC terminals 100 A) of the power conversion device 100 and the output terminals (second-side terminals 5 B) are connected so that common current flows between both positive and negative terminals of the output terminals (second DC terminals 100 B) of the power conversion device 100 . Therefore, voltage assignment per one DC/DC converter 10 can be reduced. This enables application to high-voltage usage in which the input voltage Vin is equal to or higher than the withstand voltages of elements such as the semiconductor switching elements in the power conversion device 100 . Each DC/DC converter 10 , normally, further steps up the input voltage and outputs the resultant voltage, thus enabling output of DC power having further high voltage.

In addition, since the collected power obtained by collecting generated powers can be increased to be high voltage, it becomes possible to collect powers with high efficiency and over a long distance. The power conversion device 100 further steps up voltage of the inputted high-voltage DC power and transmits the resultant power, whereby the power transmission system 1 enables power transmission with high efficiency and over a long distance.

In addition, since the amount of power generation greatly varies in the wind power generation system 200 offshore, the input voltage Vin of the power conversion device 100 needs to be controlled to be constant by the power conversion device 100 .

In the present embodiment, the auxiliary converters 20 are each provided between two DC/DC converters 10 , on the input side.

The DC/DC converters 10 operate so that each voltage V 2 on the output voltage Vout side is controlled to be the command voltage V 2 *, the input voltage Vin is controlled to be the target voltage Vin*, and each voltage V 1 on the input voltage Vin side is automatically determined. Thus, through operations of the DC/DC converters 10 , the voltages V 2 are controlled to be equalized, and the input voltage Vin is controlled to be the target voltage Vin*. Then, through operations of the auxiliary converters 20 , variation among the voltages V 1 in the input voltage Vin is eliminated and the voltages V 1 are controlled to be equalized.

Thus, the power conversion device 100 allows input of high-voltage DC power from a power source having varying voltage and can reliably perform output control, and the input voltages V 1 , the output voltages V 2 , and the power loads can be equalized among the DC/DC converters 10 . Therefore, it becomes possible to reduce a margin provided in consideration of errors such as variation in components and detection errors of detectors, whereby size reduction and cost reduction of the power conversion device 100 can be achieved.

Each auxiliary converter 20 only has to transmit power corresponding to half the difference voltage between input voltages of two DC/DC converters 10 . Therefore, the power capacity of the auxiliary converters 20 can be made smaller than that of the DC/DC converters 10 . That is, the element capacity of the semiconductor switching elements Q in each auxiliary converter 20 can be reduced. It is noted that power received and passed by each auxiliary converter 20 is only transferred between the DC/DC converters 10 and therefore does not influence power transmitted by the power conversion device 100 .

Each DC/DC converter 10 controls only one voltage V 2 of the input and output voltages to be the command voltage V 2 *, and each auxiliary converter 20 controls only the high-voltage-side voltage which is one of the input and output voltages, to be the command voltage. Therefore, the DC/DC converters 10 and the auxiliary converters 20 can be realized with a simple configuration, whereby the size and the cost can be further reduced.

In the present embodiment, since the auxiliary converters 20 are provided on the input voltage side of the power conversion device 100 , lower voltage is used as compared to the case where the auxiliary converters 20 are provided on the output voltage side as described later. Therefore, the capacities of elements such as the semiconductor switching elements Q in each auxiliary converter 20 can be reduced, whereby size reduction and cost reduction can be achieved.

In the above embodiment, means for detecting the input and output voltages V 1 , V 2 of each DC/DC converter 10 is provided to each second control unit 120 , but may be provided to the first control unit 110 or may be provided to both control units.

The input voltage Vin and the output voltage Vout of the power conversion device 100 are calculated by the first control unit 110 . However, the first control unit 110 may have means for detecting the input voltage Vin and the output voltage Vout.

In the above embodiment, the controller has the first control unit 110 , the second control units 120 , and the third control units 130 , and performs output control of the DC/DC converters 10 and the auxiliary converters 20 , but the configuration is not limited thereto. In the case where each balancing circuit connected between two DC/DC converters 10 is not the auxiliary converter 20 for which switching control is performed, the controller is configured without the third control unit 130 , and thus only the DC/DC converters 10 are control targets.

Next, embodiment 2 of the present invention will be described.

In the above embodiment 1, the auxiliary converters 20 are provided on the input voltage side of the power conversion device 100 , whereas in the present embodiment, the auxiliary converters 20 are provided on the output voltage side. The configuration of the power transmission system 1 is the same as that described in FIG. 1 in the above embodiment 1.

FIG. 7 is a diagram showing the configuration of a power conversion device 101 according to the present embodiment 2.

As shown in FIG. 7 , the power conversion device 101 has first DC terminals 101 B and second DC terminals 101 A each pair of which is composed of both positive and negative terminals. In the present embodiment 2, DC power from the wind power generation system 200 is inputted to the second DC terminals 101 A. That is, the second DC terminals 101 A serve as the input terminals of the main circuit (power conversion unit) of the power conversion device 101 , and the input voltage Vin is voltage of the second DC terminals 101 A. In addition, the first DC terminals 101 B serve as the output terminals of the main circuit of the power conversion device 101 , and the output voltage Vout is voltage of the first DC terminals 101 B.

The power conversion device 101 includes, between the input terminals (second DC terminals 101 A) and the output terminals (first DC terminals 101 B), a plurality (M number) of DC/DC converters 10 ( 10 a to 10 h ) and M-1 number of auxiliary converters 20 ( 20 a to 20 g ).

In addition, the power conversion device 101 includes a controller for performing output control of the DC/DC converters 10 and the auxiliary converters 20 of the main circuit. The controller includes: a first control unit 110 a for controlling the entire main circuit; second control units 120 ( 120 a to 120 h ) for controlling the respective DC/DC converters 10 ; and third control units 130 ( 130 a to 130 g ) for controlling the respective auxiliary converters 20 .

Each DC/DC converter 10 includes: DC terminals 5 A (first-side terminals 5 A) on the secondary side as the first side; DC terminals 5 B (second-side terminals 5 B) on the primary side as the second side; a DC/AC conversion unit 2 on the first side; a DC/AC conversion unit 3 on the second side; and a single-phase transformer 4 . The circuit configuration of each DC/DC converter 10 is the same as that shown in FIG. 3 in the above embodiment 1, but in the present embodiment 2, the input/output relationship between the first-side terminals 5 A and the second-side terminals 5 B is reversed. It is noted that the circuit of each DC/DC converter 10 is right-left symmetric and thus is capable of bidirectional operations, and therefore, merely the first side and the second side as the terms for input and output sides are reversed.

The second control units 120 are provided for the respective DC/DC converters 10 .

The first-side terminals 5 A of the M number of DC/DC converters 10 a to 10 h are connected so that common current flows between both positive and negative terminals of the first DC terminals 101 B of the power conversion device 101 , and the second-side terminals 5 B of the DC/DC converters 10 a to 10 h are connected so that common current flows between both positive and negative terminals of the second DC terminals 101 A of the power conversion device 101 .

Each auxiliary converter 20 performs conversion between DC powers having different voltages, and is connected between two pairs of first-side terminals 5 A of two DC/DC converters 10 , to receive and pass power between the two pairs of first-side terminals 5 A.

Each auxiliary converter 20 has P terminals 25 A connected to the first-side terminals 5 A of the DC/DC converter 10 on the high-potential side, and N terminals 25 B connected to the first-side terminals 5 A of the DC/DC converter 10 on the low-potential side. The circuit configuration of each auxiliary converter 20 is the same as that of the DC/DC converter 10 a , and has the same DC/AC conversion units 2 , 3 and single-phase transformer 4 as those shown in FIG. 3 .

The third control units 130 are provided for the respective auxiliary converters 20 .

As described above, the first-side terminals 5 A of the M number of DC/DC converters 10 a to 10 h are connected so that common current flows between both positive and negative terminals of the first DC terminals 101 B of the power conversion device 101 , and the second-side terminals 5 B thereof are connected so that common current flows between both positive and negative terminals of the second DC terminals 101 A of the power conversion device 101 .

Therefore, input voltage Vin which is voltage of the second DC terminals 101 A is the sum of voltages V 2 (V 2 a to V 2 h ) of the second-side terminals 5 B of the DC/DC converters 10 ( 10 a to 10 h ). In addition, output voltage Vout which is voltage of the first DC terminals 101 B is the sum of voltages V 1 (V 1 a to V 1 h ) of the first-side terminals 5 A of the DC/DC converters 10 ( 10 a to 10 h ). Thus, input and output voltages of the power conversion device 101 can be represented by Expression (3) and Expression (4).

V in= V 2 a+V 2 b+ . . . +V 2 h   (3)

V out= V 1 a+V 1 b+ . . . +V 1 h   (4)

Control of the power conversion device 101 configured as described above will be described below, with reference to FIG. 8 .

FIG. 8 is a block diagram showing the entire configuration of the controller of the power conversion device 101 , i.e., the entire configuration including the first control unit 110 a for controlling the entire main circuit, the second control units 120 ( 120 a to 120 h ) for controlling the respective DC/DC converters 10 , and the third control units 130 ( 130 a to 130 g ) for controlling the respective auxiliary converters 20 .

As shown in FIG. 8 , each second control unit 120 ( 120 a to 120 h ) has means for detecting input and output voltages V 2 (V 2 a to V 2 h ), V 1 (V 1 a to V 1 h ), and the detected input and output voltages V 2 , V 1 are transmitted to the first control unit 110 a . The first control unit 110 a transmits voltage V 1 of the P terminals 25 A and voltage V 1 of the N terminals 25 B of each auxiliary converter 20 to the respective third control units 130 ( 130 a to 130 g ).

The first control unit 110 a includes a Vin calculation unit 111 a , a V 1 distribution unit 112 a , a Vin control unit 113 , and an average value calculation unit 114 . The V 1 distribution unit 112 a transmits voltages V 1 of the P terminals 25 A and the N terminals 25 B of the auxiliary converters 20 to the respective third control units 130 . The Vin calculation unit 111 a calculates input voltage Vin, using the above Expression (3). The Vin control unit 113 receives given target voltage Vin* and the input voltage Vin, and generates a power command P* as a first command so that the input voltage Vin is controlled to be the target voltage Vin*, that is, the input voltage Vin approaches the target voltage Vin*. The average value calculation unit 114 receives the input voltage Vin and the power command P*, and divides them by the number M of the DC/DC converters 10 a to 10 h . Thus, command voltage V 2 * (=(Vout/M)) for voltage V 2 of the second-side terminals 5 B of each DC/DC converter 10 , and a power command value P 10 * (=(P*/M)) used for control of each DC/DC converter 10 , are generated. The first control unit 110 a transmits the command voltage V 2 * and the power command value P 10 * to the second control unit 120 of each DC/DC converter 10 .

Each second control unit 120 ( 120 a to 120 h ) detects the input and output voltages V 2 (V 2 a to V 2 h ), V 1 (V 1 a to V 1 h ) and transmits them to the first control unit 110 a , and generates the gate signal G 10 (G 10 a to G 10 h ) for controlling each DC/DC converter 10 , on the basis of the command voltage V 2 * and the power command value P 10 *.

For example, the second control unit 120 a that controls the DC/DC converter 10 a will be described below. As in the above embodiment 1 shown in FIG. 5 , in the second control unit 120 a , a subtractor 121 a calculates a deviation ΔV 2 a between the command voltage V 2 * and the voltage V 2 a . A controller 121 b calculates a first control command value 121 c by, for example, proportional control so that the deviation ΔV 2 a approaches 0. The power command value P 10 * from the first control unit 110 a is added to the first control command value 121 c , whereby a control command P 10 a * for performing output control of the DC/DC converter 10 a is generated.

A phase difference generator 122 outputs a difference θ 10 a between switching phases of the DC/AC conversion unit 2 and the DC/AC conversion unit 3 in the DC/DC converter 10 a , on the basis of the control command P 10 a *. Then, a PWM signal generator 123 generates and outputs the gate signal G 10 a which is a PWM signal for performing switching control of each semiconductor switching element Q 11 to Q 24 in the DC/DC converter 10 a , on the basis of the phase difference θ 10 a.

Each third control unit 130 ( 130 a to 130 g ) generates the gate signal G 20 (G 20 a to G 20 g ) for controlling each auxiliary converter 20 , on the basis of voltage V 1 of the P terminals 25 A and voltage V 1 of the N terminals 25 B of each auxiliary converter 20 . For example, the third control unit 130 a generates the gate signal G 20 a for controlling the auxiliary converter 20 a , on the basis of voltage V 1 a of the P terminals 25 A and voltage V 1 b of the N terminals 25 B of the auxiliary converter 20 a.

The third control unit 130 is the same as in the above embodiment 1, and each auxiliary converter 20 is subjected to output control such that, on the basis of comparison between voltage V 1 of the P terminals 25 A and voltage V 1 of the N terminals 25 B, power is received and passed from the high-voltage side to the low-voltage side so as to equalize the two DC voltages.

In the above embodiment 1, voltage V 2 controlled by each DC/DC converter 10 is output voltage, whereas in the present embodiment 2, voltage V 2 is input voltage. However, each voltage V 2 and the input voltage Vin of the power conversion device 101 can be controlled in the same manner as in the above embodiment 1.

That is, in each DC/DC converter 10 , the second control unit 120 performs output control of the DC/DC converter 10 on the basis of the command voltage V 2 * for voltage V 2 and the power command value P 10 *. The power command value P 10 * is calculated by dividing the power command P* for controlling the input voltage Vin of the entire power conversion device 101 to be the target voltage Vin*, by the number M of the DC/DC converters 10 . Therefore, the DC/DC converters 10 operate so that each voltage V 2 is controlled to be the command voltage V 2 * and the input voltage Vin of the entire power conversion device 101 is controlled to be the target voltage Vin*.

Each second control unit 120 causes the DC/DC converter 10 to operate so that the voltage V 2 is controlled to be the command voltage V 2 *, and when the voltage V 2 is determined, the voltage V 1 is automatically determined in accordance with the power to be transmitted.

The DC/DC converters 10 have the same configuration, and the voltage V 1 determined by operation thereof becomes, ideally, a value (Vout/M) obtained by dividing the output voltage Vout by the number M of the DC/DC converters 10 . However, actually, error occurs by factors such as variation in components and detection error of detectors. Therefore, variation occurs among the voltages V 1 .

Through operations of the auxiliary converters 20 , variation among voltages V 1 is eliminated and the voltages V 1 are controlled to be equalized.

Also in the present embodiment 2, since the power conversion device 101 is configured with a plurality of DC/DC converters 10 connected, voltage assignment per one DC/DC converter 10 can be reduced, and the input voltage Vin can be increased to be high voltage.

The auxiliary converters 20 are each provided between two DC/DC converters 10 , on the output side, thus enabling control of both input and output voltages of each DC/DC converter 10 . Therefore, as in the above embodiment 1, the power conversion device 101 allows input of high-voltage DC power from a power source having varying voltage and can reliably perform output control, and the input voltages V 1 , the output voltages V 2 , and the power loads can be equalized among the DC/DC converters 10 . Therefore, it becomes possible to reduce a margin provided in consideration of errors such as variation in components and detection errors of detectors, whereby size reduction and cost reduction of the power conversion device 101 can be achieved.

Each auxiliary converter 20 only has to transmit power corresponding to half the difference voltage between input voltages of two DC/DC converters 10 . Therefore, the power capacity of the auxiliary converters 20 can be made smaller than that of the DC/DC converters 10 .

Each DC/DC converter 10 controls only one voltage V 2 of the input and output voltages to be the command voltage V 2 *, and each auxiliary converter 20 controls only the high-voltage-side voltage which is one of the input and output voltages, to be the command voltage. Therefore, the DC/DC converters 10 and the auxiliary converters 20 can be realized with a simple configuration, whereby the size and the cost can be further reduced.

Next, embodiment 3 of the present invention will be described.

In the present embodiment 3, each DC/DC converter 10 shown in the above embodiment 1 is configured by connecting a plurality (N number) of converter cells 30 ( 30 a to 30 x ). Also in this case, the controller of the power conversion device 100 includes: a first control unit 110 for controlling the entire main circuit; second control units 120 ( 120 a to 120 h ) for controlling the respective DC/DC converters 10 ; and third control units 130 ( 130 a to 130 g ) for controlling the respective auxiliary converters 20 . Each second control unit 120 includes a control unit for the DC/DC converter 10 and cell control units 140 for controlling the respective converter cells 30 .

The configurations other than the DC/DC converter 10 and the second control unit 120 are the same as in the above embodiment 1.

FIG. 9 shows an example of a circuit diagram of the DC/DC converter 10 a according to the present embodiment 3. It is noted that the other DC/DC converters 10 b to 10 h are the same as the DC/DC converter 10 a.

As shown in FIG. 9 , the DC/DC converter 10 a includes a plurality of converter cells 30 ( 30 a to 30 x ) between the first-side terminals 5 A and the second-side terminals 5 B.

FIG. 10 shows an example of a circuit diagram of the converter cell 30 a , and FIG. 11 shows the second control unit 120 a for controlling the DC/DC converter 10 a.

In this case, the second control unit 120 a for controlling the DC/DC converter 10 a is composed of a control unit 120 aa for controlling the entire circuit of the DC/DC converter 10 a , and the cell control units 140 ( 140 a to 140 x ) for controlling the respective converter cells 30 ( 30 a to 30 x ).

Each converter cell 30 includes, between primary-side DC terminals 15 A as A terminals and secondary-side DC terminals 15 B as B terminals, a DC/AC conversion unit 12 on the primary side, a DC/AC conversion unit 13 on the secondary side, and a single-phase transformer 14 . The cell control units 140 are provided for the respective converter cells 30 .

Regarding the plurality of converter cells 30 ( 30 a to 30 x ) in the DC/DC converter 10 a , the DC terminals 15 A on the primary side are connected in parallel to the first-side terminals 5 A of the DC/DC converter 10 a , and the DC terminals 15 B on the secondary side are connected so that common current flows between both poles of the second-side terminals 5 B of the DC/DC converter 10 a . That is, the input sides of the plurality of converter cells 30 composing the DC/DC converter 10 a are connected in parallel and the output sides thereof are connected so that common current flows.

The converter cell 30 a is configured as shown in FIG. 10 . It is noted that there are various types of converter cells as the converter cells 30 , and an appropriate type is selected in accordance with, for example, rated voltage, conversion capacity, operation frequency, and the like. The other converter cells 30 b to 30 x are the same as the converter cell 30 a . It is noted that, in this case, the circuit configuration of the converter cell 30 a is the same as that of the DC/DC converter 10 a shown in FIG. 3 in the above embodiment 1.

As shown in FIG. 10 , the DC/AC conversion unit 12 on the primary side includes: a DC capacitor 6 a connected between both poles of the DC terminals 15 A; and a first full-bridge circuit formed by two switching legs composed of semiconductor switching elements Q 11 a to Q 14 a as positive-side and negative-side semiconductor elements connected in series. The DC/AC conversion unit 13 on the secondary side includes: a DC capacitor 7 a connected between both poles of the DC terminals 15 B; and a second full-bridge circuit formed by two switching legs composed of semiconductor switching elements Q 21 a to Q 24 a as positive-side and negative-side semiconductor elements connected in series. The single-phase transformer 14 as a transformer is connected between intermediate connection points of the switching legs of the DC/AC conversion unit 12 and intermediate connection points of the switching legs of the DC/AC conversion unit 13 . It is noted that, if insulation is not needed, only inductance LS may be connected instead of the single-phase transformer 14 .

As the semiconductor switching elements Q 11 a to Q 24 a , for example, semiconductor switching elements having a self-turn-off function, such as IGBTs, to which diodes are connected in antiparallel are used. Each semiconductor switching element Q 11 a to Q 24 a may be used with a plurality of semiconductor switching elements combined in parallel, depending on the current capacity.

In this case, a snubber capacitor CS is connected in parallel to each semiconductor switching element Q 11 a to Q 24 a . Owing to effects of the snubber capacitor CS and the inductance LS of the AC output line, zero voltage switching which is soft switching of each semiconductor switching element Q 11 a to Q 24 a becomes possible.

The cell control unit 140 a generates gate signals G 30 a for the semiconductor switching elements Q 11 a to Q 24 a in the converter cell 30 a , thereby controlling switching of the semiconductor switching elements Q 11 a to Q 24 a.

In this case, by soft switching of the semiconductor switching elements Q 11 a to Q 24 a , switching loss can be reduced, the operation frequency can be increased, and the size of the single-phase transformer 14 can be reduced.

In the converter cell 30 a , since the DC terminals 15 A are connected in parallel to the first-side terminals 5 A of the DC/DC converter 10 a , the DC voltage V 1 a applied to the first-side terminals 5 A is applied to the DC terminals 15 A. Further, the converter cell 30 a is a circuit that converts the DC voltage V 1 a to DC voltage V 21 a applied to the DC terminals 15 B, via the DC/AC conversion unit 12 , the single-phase transformer 14 , and the DC/AC conversion unit 13 , and can be freely controlled so as to perform bidirectional power conversion. It is desirable that the turns ratio of the single-phase transformer 14 is matched with the ratio of the DC voltage V 1 a on the primary side and the DC voltage V 21 a on the secondary side.

It is noted that the DC/DC converter 10 a composed of the plurality of converter cells 30 , in normal operation, steps up the primary-side voltage V 1 a and outputs the secondary-side voltage V 21 a , but the operation of each converter cell 30 is not limited to step-up operation.

As the DC capacitors 6 a , 7 a , electrolytic capacitors, film capacitors, or the like are used. High-frequency current flows through the DC capacitors 6 a , 7 a , but in the case of using film capacitors, deterioration due to the high-frequency current can be suppressed and thus the life of the capacitors is prolonged.

As described above, the DC terminals 15 B of the N number of converter cells 30 a to 30 x in the DC/DC converter 10 a are connected so that common current flows between both positive and negative terminals of the second-side terminals 5 B of the DC/DC converter 10 a.

Therefore, voltage V 2 a of the second-side terminals 5 B of the DC/DC converter 10 a is the sum of voltages V 21 (V 21 a to V 21 x ) of the DC terminals 15 B on the output side of the converter cells 30 ( 30 a to 30 x ). Thus, voltage V 2 a of the second-side terminals 5 B which is output voltage of the DC/DC converter 10 a is represented by Expression (5).

V 2 a=V 21 a+V 21 b+ . . . +V 21 x   (5)

Control of the DC/DC converter 10 a configured as described above will be described below.

As shown in FIG. 11 , the control unit 120 aa detects input and output voltages V 1 a , V 2 a of the DC/DC converter 10 a , and transmits them to the first control unit 110 (not shown). It is noted that the detection of voltage V 2 a is performed through calculation as described later. Each cell control unit 140 has means for detecting voltage V 21 (V 21 a to V 21 x ) of the DC terminals 15 B which is output voltage of each converter cell 30 , and transmits the detected voltage V 21 to the control unit 120 aa.

In addition, as in the above embodiment 1, the command voltage V 2 * (=(Vout/M)) and the power command value P 10 * (=(P*/M)) are generated by the first control unit 110 , and are transmitted to the second control units 120 of the respective DC/DC converters 10 . In the second control unit 120 a , the received command voltage V 2 * and power command value P 10 * are inputted to the control unit 120 aa.

The control unit 120 aa includes a V 2 control unit 121 , a V 2 calculation unit 125 , and an average value calculation unit 126 . The V 2 calculation unit 125 calculates voltage V 2 a using the above Expression (5) on the basis of the voltages V 21 received from the cell control units 140 . The V 2 control unit 121 receives the command voltage V 2 * from the first control unit 110 and the voltage V 2 a , and calculates a first control command value 121 c so that the voltage V 2 a approaches the command voltage V 2 *. The power command value P 10 * from the first control unit 110 is added to the first control command value 121 c , whereby a control command P 10 a * for performing output control of the DC/DC converter 10 a is generated.

The average value calculation unit 126 receives the voltage V 2 a and the control command P 10 a * and divides them by the number N of the converter cells 30 a to 30 x . Thus, command voltage V 21 * (=(V 2 a /N)) which is B voltage for voltage V 21 of the DC terminals 15 B of each converter cell 30 , and a power command value P 30 * (=(P 10 a */N)) as a second control command value used for control of each converter cell 30 , are generated. Then, the control unit 120 aa transmits the command voltage V 21 * and the power command value P 30 * to the cell control unit 140 of each converter cell 30 .

Each cell control unit 140 ( 140 a to 140 x ) generates the gate signal G 30 (G 30 a to G 30 x ) for controlling each converter cell 30 , on the basis of the command voltage V 21 * and the power command value P 30 *.

Next, the entire control of the power conversion device 100 according to the present embodiment 3 will be described below, with reference to FIG. 12 to FIG. 16 .

FIG. 12 is a block diagram showing the entire configuration of the controller of the power conversion device 100 , i.e., the entire configuration including the first control unit 110 for controlling the entire main circuit, the second control units 120 ( 120 a to 120 h ) for controlling the respective DC/DC converters 10 , and the third control units 130 ( 130 a to 130 g ) for controlling the respective auxiliary converters 20 . Each second control unit 120 includes the control unit 120 aa for controlling the entire circuit of the corresponding DC/DC converter 10 , and the cell control units 140 ( 140 a to 140 x ) for controlling the N number of converter cells 30 in the DC/DC converter 10 .

FIG. 13 is a control block diagram showing the first control unit 110 . FIG. 14 is a control block diagram showing the control unit 120 aa for controlling the DC/DC converter 10 a , and FIG. 15 is a control block diagram showing the cell control unit 140 a for controlling the converter cell 30 a in DC/DC converter 10 a . FIG. 16 is a control block diagram showing the third control unit 130 a for controlling the auxiliary converter 20 a.

It is noted that FIG. 14 shows the details of the control unit 120 aa shown in FIG. 11 .

As shown in FIG. 12 and FIG. 13 , the first control unit 110 receives voltages V 1 (V 1 a to V 1 h ) and voltages V 2 (V 2 a to V 2 h ) of the DC/DC converters 10 from the control units 120 aa of the DC/DC converters 10 . Then, the first control unit 110 transmits the voltages V 1 (V 1 a to V 1 h ) to the respective third control units 130 .

The Vin calculation unit 111 calculates input voltage Vin, using the above Expression (1), and the Vout calculation unit 112 calculates output voltage Vout, using the above Expression (2). In the Vin control unit 113 , a subtractor 113 a calculates a deviation Vin between given target voltage Vin* and the input voltage Vin. A controller 113 b generates a power command P* as a first command by proportional integral control so that the deviation ΔVin approaches 0.

The average value calculation unit 114 receives the output voltage Vout and the power command P*, and divides them by the number M of the DC/DC converters 10 a to 10 h . Thus, the command voltage V 2 * (=(Vout/M)) for voltage V 2 of the second-side terminals 5 B of each DC/DC converter 10 , and the power command value P 10 * (=(P*/M)) used for control of each DC/DC converter 10 , are generated. The generated command voltage V 2 * and power command value P 10 * are transmitted via a signal distributor 115 to the control units 120 aa to 120 ha in the respective second control units 120 ( 120 a to 120 h ).

Next, the control unit 120 aa in the second control unit 120 a for controlling the DC/DC converter 10 a will be described. It is noted that the control units 120 ba to 120 ha of the other second control units 120 b to 120 h are the same as the control unit 120 aa.

As shown in FIG. 12 and FIG. 14 , in the control unit 120 aa , the command voltage V 2 * from the first control unit 110 and the voltage V 2 a are inputted and the voltages V 21 (V 21 a to V 21 x ) of the converter cells 30 are received from the respective cell control units 140 ( 140 a to 140 x ). In addition, the control unit 120 aa transmits the input and output voltage V 1 a , V 2 a of the DC/DC converter 10 a to the first control unit 110 .

As described above, the V 2 calculation unit 125 calculates the voltage V 2 a using the above Expression (5) on the basis of the voltages V 21 received from the respective cell control units 140 .

In the V 2 control unit 121 , a subtractor 121 a calculates a deviation ΔV 2 a between the command voltage V 2 * from the first control unit 110 and the voltage V 2 a . A controller 121 b generates a first control command value 121 c by proportional control so that the deviation ΔV 2 a approaches 0. The power command value P 10 * from the first control unit 110 is added to the first control command value 121 c , whereby a control command P 10 a * for performing output control of the DC/DC converter 10 a is generated.

The average value calculation unit 126 receives the voltage V 2 a and the control command P 10 a * and divides them by the number N of the converter cells 30 a to 30 x . Thus, command voltage V 21 * (=(V 2 a /N)) for voltage V 21 of the DC terminals 15 B of each converter cell 30 , and a power command value P 30 * (=(P 10 a */N)) used for control of each converter cell 30 , are generated. The generated command voltage V 21 * and power command value P 30 * are transmitted via a signal distributor 127 to the cell control units 140 ( 140 a to 140 x ).

Next, the cell control unit 140 a for controlling the converter cell 30 a in the DC/DC converter 10 a will be described. It is noted that the other cell control units 140 b to 140 x are the same as the cell control unit 140 a.

As shown in FIG. 12 and FIG. 15 , the cell control unit 140 a receives the command voltage V 21 * and the power command value P 30 * from the control unit 120 aa . In addition, the cell control unit 140 a detects voltage V 21 a of the DC terminals 15 B which is output voltage of the converter cell 30 a , and transmits the detected voltage V 21 a to the control unit 120 aa.

In the cell control unit 140 a , a subtractor 141 a calculates a deviation ΔV 21 a between the command voltage V 21 * from the control unit 120 aa and the voltage V 21 a . A controller 141 b calculates a third control command value 141 c by proportional control so that the deviation ΔV 21 a approaches 0. The power command value P 30 * (second control command value) from the control unit 120 aa is added to the third control command value 141 c , whereby a control command P 30 a * for performing output control of the converter cell 30 a is generated.

A phase difference generator 142 outputs a difference θ 30 a between switching phases of the DC/AC conversion unit 12 and the DC/AC conversion unit 13 in the converter cell 30 a , on the basis of the control command P 30 a *. A PWM signal generator 143 generates and outputs the gate signal G 30 a which is a PWM signal for performing switching control of each semiconductor switching element Q 11 a to Q 24 a in the converter cell 30 a , on the basis of the phase difference θ 30 a.

In the cell control unit 140 a , the control command P 30 a * for performing output control of the converter cell 30 a is generated by adding the third control command value 141 c and the power command value P 30 *, and the power command value P 30 * is generated by adding the first control command value 121 c and the power command value P 10 *. That is, the control command P 30 a * for performing output control of the converter cell 30 a is a power command for compensating variation in the input voltage Vin of the power conversion device 100 , compensating variation in the output voltage V 2 a of the DC/DC converter 10 a , and compensating variation in the output voltage V 21 a of the converter cell 30 a.

Next, the third control unit 130 a for controlling the auxiliary converter 20 a will be described. It is noted that the other third control units 130 b to 130 g are the same as the third control unit 130 a.

As shown in FIG. 12 and FIG. 16 , the third control unit 130 a receives the voltage V 1 a of the P terminals 25 A and the voltage V 1 b of the N terminals 25 B of the auxiliary converter 20 a from the first control unit 110 .

The third control unit 130 a operates in the same manner as in the above embodiment 1. That is, in the third control unit 130 a , a subtractor 131 a calculates a voltage difference ΔV 1 ab between voltage V 1 a of the P terminals 25 A and voltage V 1 b of the N terminals 25 B. A V 1 controller 131 b generates a power command (control command) 131 c for performing output control of the auxiliary converter 20 a by proportional integral control so that the half of voltage difference ΔV 1 ab is compensated and voltage difference ΔV 1 ab approaches 0. Also in this case, control is performed such that, on the basis of comparison between voltage V 1 a of the P terminals 25 A and voltage V 1 b of the N terminals 25 B, command voltage is generated so as to decrease voltage of the high-voltage-side terminal by half the voltage difference ΔV 1 ab and thus power is received and passed from the high-voltage side to the low-voltage side so as to equalize the two DC voltages.

A phase difference generator 132 outputs a difference θ 20 a between switching phases of the DC/AC conversion unit 2 and the DC/AC conversion unit 3 in the auxiliary converter 20 a , on the basis of the power command 131 c . A PWM signal generator 133 generates and outputs the gate signal G 20 a which is a PWM signal for performing switching control of each semiconductor switching element Q 11 to Q 24 in the auxiliary converter 20 a , on the basis of the phase difference θ 20 a.

As described above, powers transmitted by each converter cell 30 and each auxiliary converter 20 can be controlled by calculating the phase differences θ [rad] between the switching phase of the DC/AC conversion unit 12 , 2 on the primary side and the switching phase of the DC/AC conversion unit 13 , 3 on the secondary side.

That is, in the insulation-type converter cell 30 that performs DC/DC conversion, transmitted power P can be controlled by generating the phase difference θ for controlling output power, and performing switching such that the phase difference θ [rad] is provided between the primary-side circuit and the secondary-side circuit.

In this case, description will be given using the converter cell 30 a shown in FIG. 10 , as an example. The power P transmitted from the primary-side DC terminals 15 A to the secondary-side DC terminals 15 B is represented by the following Expression (6).

P =( V 1 a−V 21 a/ωLS )·(θ−θ 2 /π)  (6)

Here, ω is a value obtained by multiplying a switching frequency fsw by 2π.

Further, if the input voltage V 1 a and the output voltage V 21 a of the converter cell 30 a are equal to each other and are set as V 1 a =V 21 a =Vdc, the above Expression (6) can be transformed into the following Expression (7).

P =( Vdc 2 /ωLS )·(θ−θ 2 /π)  (7)

From the above Expression (7), the phase difference θ for controlling the transmitted power can be calculated by the following Expression (8).

θ=π/2−√(π 2 /4−(π P/Vdc 2 )·ω LS )  (8)

Here, in the converter cell 30 a , when the DC voltage V 21 a applied to the DC capacitor 7 a on the secondary side varies by ΔV 21 a from the command voltage V 21 *, electrostatic energy W stored in the DC capacitor 7 a is calculated by the following Expression (9).

Here, Cdc is the capacitance of the DC capacitor 7 a.

Normally, variation ΔV 21 a of the DC voltage is slight as compared to the command voltage V 21 *. Therefore, the above Expression (9) can be deformed into the following Expression (10).

W ≈(1/2) Cdc·V 21* 2 ±Cdc·V 21*·Δ V 21  (10)

That is, a variation amount Δw in the electrostatic energy W that occurs due to the voltage variation ΔV 21 a of the DC voltage V 21 a applied to the DC capacitor 7 a on the secondary side is represented by the second term on the right-hand side of the above Expression (10).

Therefore, control of the output voltage V 21 a of the converter cell 30 a can be performed by increasing or decreasing the transmitted power of the converter cell 30 a by power Pa that compensates the variation amount Δw of the electrostatic energy W. The power Pa that compensates the variation amount Δw can be calculated by the following Expression (11).

The power Pa calculated by the above Expression (11) is power that compensates slight voltage variation ΔV 21 a occurring in the DC voltage V 21 a applied to the DC capacitor 7 a on the secondary side, and therefore the power Pa is smaller as compared to the transmitted power of the converter cell 30 a . In this case, the corresponding phase difference θ a also becomes small, and therefore the above Expression (7) can be approximated as shown by the following Expression (12).

Pa ≈( Vdc 2 /ωLS )·θ a   (12)

By deforming the above Expression (12), the following Expression (13) can be obtained.

θ a (ω LS/Vdc 2 )· Pa   (13)

By using Expression (13) for calculating the phase difference θ a for controlling the transmitted power Pa, it is possible to easily obtain the phase difference as compared to the above Expression (8).

In the present embodiment, in control of each converter cell 30 and control of each auxiliary converter 20 , when the phase difference is calculated from the power command (control command), the calculation is performed using the above Expression (13). That is, when the phase difference generator 142 in the cell control unit 140 and the phase difference generator 132 in the third control unit 130 each calculate the phase difference, a value obtained by calculation of the right-hand side of the above Expression (13) is used. Thus, the calculation becomes easy and the configurations of the control units 130 , 140 can be simplified.

As described above, in the present embodiment 3, the power conversion device 100 has a plurality of DC/DC converters 10 connected to each other, and has auxiliary converters 20 each provided between two DC/DC converters 10 , on the input side. Further, each DC/DC converter 10 is composed of a plurality of converter cells 30 , the input sides of the plurality of converter cells 30 in each DC/DC converter 10 are connected in parallel, and the output sides thereof are connected so that common current flows. Therefore, the same effects as in the above embodiment 1 are obtained, and in addition, the following effects are obtained.

Since the input sides of the plurality of converter cells 30 are connected in parallel and the output sides thereof are connected so that common current flows, each DC/DC converter 10 can have a great step-up ratio. In addition, the output voltages of the converter cells 30 are equally controlled by the cell control units 140 , so that power loads thereof can be equalized. Therefore, it becomes possible to reduce a margin provided in consideration of errors such as variation in components and detection errors of detectors, whereby size reduction and cost reduction of the converter cells 30 can be achieved.

Each auxiliary converter 20 only has to transmit power corresponding to half the difference voltage between the input voltages of two DC/DC converters 10 . Since the input voltage of each DC/DC converter 10 is the same as the input voltages of the converter cells 30 in the DC/DC converter 10 , the power capacity of the auxiliary converter 20 can be made smaller than the power capacities of the converter cells 30 .

In the above embodiment, the first control unit 110 uses proportional integral control for calculation of the power command P* for controlling the input voltage Vin, and the third control unit 130 uses proportional integral control for calculation for output control of each auxiliary converter 20 . Using the proportional integral control enables enhancement of accuracy of the voltage control.

Here, the input voltage Vin of the power conversion device 100 is voltage of the power transmission system 1 , and therefore needs to be controlled with high accuracy. In this case, since the Vin control unit 113 is configured with the proportional integral controller (controller 113 b ), high-accuracy control can be performed.

In the second control unit 120 , proportional control is used for calculations of the first control command value 121 c for output control of each DC/DC converter 10 and the third control command value 141 c for output control of each converter cell 30 .

The output voltage V 2 of the DC/DC converter 10 is controlled using the average value as the command voltage V 2 *. The first control command value 121 c calculated using proportional control compensates only an amount corresponding to variation relative to the average value. Similarly, the third control command value 141 c calculated using proportional control compensates only an amount corresponding to variation relative to the average value of output voltage of each converter cell 30 .

If output powers of all the converter cells 30 are summed, the powers for compensating only the amount corresponding to variation relative to the average value are cancelled with each other, and eventually, the power for controlling the input voltage Vin of the power conversion device 100 remains and is outputted. Thus, control of the input voltage Vin of the power conversion device 100 and output voltage control of each DC/DC converter 10 and each converter cell 30 can be performed without interfering with each other.

In the above embodiment 3, output voltages of the converter cells 30 in the DC/DC converter 10 a are detected by the respective cell control units 140 in the second control unit 120 a . However, these output voltages may be detected by the control unit 120 aa , or may be detected by the first control unit 110 .

In the present embodiment 3, the DC/DC converters 10 each composed of a plurality of converter cells 30 are applied to embodiment 1. However, such DC/DC converters 10 may be applied to the above embodiment 2. That is, each auxiliary converter 20 may be provided between two DC/DC converters 10 , on the output side, and also in this case, the same effects can be obtained.

The DC/DC converters 10 , the auxiliary converters 20 , and the converter cells 30 used in the above embodiments 1 to 3 have such a configuration that full-bridge circuits each formed by two switching legs are provided on both sides of a transformer. However, a configuration having three-phase bridge circuits may be employed.

In a power conversion device according to the present embodiment 4, the converter cells shown in the above embodiment 3 are configured such that three-phase bridge circuits each formed by three switching legs are provided on both sides of a transformer. The other configurations are the same as in the above embodiment 3.

Also in this case, each DC/DC converter 10 includes a plurality of converter cells 40 between the first-side terminals 5 A and the second-side terminals 5 B. The controller of the power conversion device 100 includes: a first control unit 110 for controlling the entire main circuit; second control units 120 for controlling the respective DC/DC converters 10 ; and third control units 130 for controlling the respective auxiliary converters 20 . Each second control unit 120 includes a control unit for the DC/DC converter 10 and cell control units 140 for controlling the respective converter cells 40 .

FIG. 17 is a circuit diagram of each converter cell 40 according to the present embodiment 4.

Each converter cell 40 includes, between primary-side DC terminals 15 A as A terminals and secondary-side DC terminals 15 B as B terminals, a DC/AC conversion unit 22 on the primary side, a DC/AC conversion unit 23 on the secondary side, and a three-phase transformer 24 as a transformer. The cell control units 140 are provided for the respective converter cells 40 .

Regarding the plurality of converter cells 40 in each DC/DC converter 10 , the DC terminals 15 A on the primary side are connected in parallel to the first-side terminals 5 A of the DC/DC converter 10 , and the DC terminals 15 B on the secondary side are connected so that common current flows between both poles of the second-side terminals 5 B of the DC/DC converter 10 . That is, the input sides of the plurality of converter cells 40 composing the DC/DC converter 10 are connected in parallel and the output sides thereof are connected so that common current flows.

As shown in FIG. 17 , the DC/AC conversion unit 22 on the primary side includes: a DC capacitor 6 a connected between both poles of the DC terminals 15 A; and a first three-phase bridge circuit formed by three switching legs composed of semiconductor switching elements Q 11 a to Q 16 a as positive-side and negative-side semiconductor elements connected in series. The DC/AC conversion unit 23 on the secondary side includes: a DC capacitor 7 a connected between both poles of the DC terminals 15 B; and a second three-phase bridge circuit formed by three switching legs composed of semiconductor switching elements Q 21 a to Q 26 a as positive-side and negative-side semiconductor elements connected in series. The three-phase transformer 24 is connected between intermediate connection points of the switching legs of the DC/AC conversion unit 22 and intermediate connection points of the switching legs of the DC/AC conversion unit 23 .

As the semiconductor switching elements Q 11 a to Q 26 a , for example, semiconductor switching elements having a self-turn-off function, such as IGBTs, to which diodes are connected in antiparallel are used. Each semiconductor switching element Q 11 a to Q 26 a may be used with a plurality of semiconductor switching elements combined in parallel, depending on the current capacity.

In this case, a snubber capacitor CS is connected in parallel to each semiconductor switching element Q 11 a to Q 26 a . Owing to effects of the snubber capacitor CS and the inductance LS of the AC output line, zero voltage switching which is soft switching of each semiconductor switching element Q 11 a to Q 26 a becomes possible.

Each cell control unit 140 generates gate signals G 40 for the semiconductor switching elements Q 11 a to Q 26 a in the converter cell 40 , thereby controlling switching of the semiconductor switching elements Q 11 a to Q 26 a.

In this case, by soft switching of the semiconductor switching elements Q 11 a to Q 26 a , switching loss can be reduced, the operation frequency can be increased, and the size of the three-phase transformer 24 can be reduced.

It is noted that, as the three-phase transformer 24 , three single-phase transformers may be used instead of one three-phase transformer. As in the above embodiment 3, leakage inductance LS of the three-phase transformer 24 is shown, but the leakage inductance of the three-phase transformer 24 may not necessarily be used, and additional inductances may be used. It is noted that, if insulation is not needed, only inductance LS may be connected instead of the three-phase transformer 24 .

Since each converter cell 40 has a three-phase bridge circuit, ripple current flowing through the DC capacitors 6 a , 7 a can be reduced and the capacitances of the DC capacitors 6 a , 7 a can be reduced. Therefore, the size of the power conversion device 100 can be reduced.

Also in the present embodiment, in each converter cell 40 , since the DC terminals 15 A are connected in parallel to the first-side terminals 5 A of the DC/DC converter 10 , the DC voltage V 1 a applied to the first-side terminals 5 A is applied to the DC terminals 15 A. Further, the converter cell 40 is a circuit that converts the DC voltage V 1 a to DC voltage V 21 a applied to the DC terminals 15 B, via the DC/AC conversion unit 22 , the three-phase transformer 24 , and the DC/AC conversion unit 23 , and can be freely controlled so as to perform bidirectional power conversion. It is desirable that the turns ratio of the three-phase transformer 24 is matched with the ratio of the DC voltage V 1 a on the primary side and the DC voltage V 21 a on the secondary side.

It is noted that the DC/DC converter 10 composed of the plurality of converter cells 40 , in normal operation, steps up the primary-side voltage V 1 a and outputs the secondary-side voltage V 2 a , but the operation of each converter cell 40 is not limited to step-up operation.

As in the above embodiment 3, powers transmitted by each converter cell 40 can be controlled by calculating the phase differences θ [rad] between the switching phase of the DC/AC conversion unit 22 on the primary side and the switching phase of the DC/AC conversion unit 23 on the secondary side.

That is, in the insulation-type converter cell 40 that performs DC/DC conversion, transmitted power P can be controlled by generating the phase difference θ for controlling output power, and performing switching such that the phase difference θ [rad] is provided between the primary-side circuit and the secondary-side circuit.

In this case, description will be given using the converter cell 40 shown in FIG. 17 , as an example. The power P transmitted from the primary-side DC terminals 15 A to the secondary-side DC terminals 15 B is represented by the following Expression (14).

P =( V 1 a−V 21 a/ωLS )·[(2/3)·θ−θ 2 /(2π)]  (14)

Here, ω is a value obtained by multiplying a switching frequency fsw by 2π.

Further, if the input voltage V 1 a and the output voltage V 21 a of the converter cell 40 are equal to each other and are set as V 1 a =V 21 a =Vdc, the above Expression (14) can be transformed into the following Expression (15).

P =( Vdc 2 /ωLS )·[(2/3)·θ−θ 2 /(2π)]  (15)

From the above Expression (15), the phase difference θ for controlling the transmitted power can be calculated by the following Expression (16).

θ=2π/3−√(4π 2 /9−(2π P/Vdc 2 )·ω LS )  (16)

As in the above embodiment 3, normally, the variation ΔV 21 a of the DC voltage is slight as compared to the command voltage V 21 *. For performing the control of the output voltage V 21 a of the converter cell 40 , power that compensates slight voltage variation ΔV 21 a occurring in the DC capacitor 7 a on the secondary side is used and this power is smaller as compared to transmitted power of the converter cell 40 . A phase difference corresponding to the power Pa that compensates this slight voltage variation ΔV 21 a also becomes small, and therefore, as in the above embodiment 3, the above Expression (15) can be approximated as shown by the following Expression (17).

Pa ≈(2/3)·( Vdc 2 /ωLS )·θ a   (17)

By deforming the above Expression (17), the following Expression (18) can be obtained.

θ a ≈(3/2)·(ω LS/Vdc 2 )· Pa   (18)

The phase difference θ a for controlling the transmitted power Pa can be calculated using Expression (18), and thus the phase difference can be easily obtained as compared to the above Expression (16).

In the present embodiment, in control of each converter cell 40 , when the phase difference is calculated from the power command (control command), the calculation is performed using the above Expression (18). That is, when the phase difference generator 142 in each cell control unit 140 calculates the phase difference, a value obtained by calculation of the right-hand side of the above Expression (18) is used. Thus, the configuration of the cell control unit 140 can be simplified.

As described above, in the present embodiment 4, the power conversion device 100 has a plurality of DC/DC converters 10 connected to each other, and has auxiliary converters 20 each provided between the DC/DC converters 10 , on the input side. Further, each DC/DC converter 10 is composed of a plurality of converter cells 40 , the input sides of the plurality of converter cells 40 in each DC/DC converter 10 are connected in parallel, and the output sides thereof are connected so that common current flows. Therefore, the same effects as those in the above embodiment 3 are obtained.

In addition, since each converter cell 40 has a three-phase bridge circuit, ripple current flowing through the DC capacitors 6 a , 7 a can be reduced and the size of the power conversion device 100 can be reduced.

In the present embodiment, the DC/DC converters 10 each composed of a plurality of converter cells 40 are applied to embodiment 1. However, such DC/DC converters 10 may be applied to the above embodiment 2. That is, each auxiliary converter 20 may be provided between two DC/DC converters 10 , on the output side, and also in this case, the same effects can be obtained.

Also the auxiliary converters 20 may have such a configuration that a three-phase bridge circuit as shown in FIG. 17 is provided on both sides of a three-phase transformer 24 . In this case, when the phase difference generator 132 calculates the phase difference, a value obtained by calculation of the right-hand side of the above Expression (18) is used. Thus, the configuration of the third control unit 130 can also be simplified.

The same circuit configuration as shown in FIG. 17 may be applied to the DC/DC converters 10 in the above embodiments 1, 2, and also in this case, the same effects are obtained.

Normally, silicon is used for the semiconductor switching elements Q in the DC/DC converters 10 , the auxiliary converters 20 , and the converter cells 30 , 40 used in the above embodiments. However, semiconductor switching elements made of silicon carbide, gallium nitride, diamond, or the like as wide bandgap semiconductors having wider bandgaps than silicon may be used. In this case, usage at higher voltage becomes possible, and further, higher-speed switching can be performed, whereby size reduction of the transformers 4 , 14 , 24 can be achieved.

It is noted that, within the scope of the present invention, the above embodiments may be freely combined with each other, or each of the above embodiments may be modified or simplified as appropriate.