Patent ID: 12199524

DESCRIPTION OF EMBODIMENTS

With reference to the drawings, embodiments of the present disclosure will be described below. In the following description, the same component is denoted by the same reference numeral. Those names and functions are the same. Thus, the detailed description thereof will not be repeated.

<Schematic Configuration of Power Conversion Device>

FIG.1is a schematic configuration diagram illustrating a power conversion device. With reference toFIG.1, a power conversion device1is configured of a modular multilevel converter including a plurality of converter cells (corresponding to “cell” inFIG.1) connected in series to each other. Power conversion device1performs power conversion between a DC circuit14and an AC circuit12. Power conversion device1includes a power conversion circuit unit2and a control device3.

Power conversion circuit unit2includes a plurality of leg circuits4u,4v,4w(hereinafter, also collectively referred to as “leg circuits4”) connected in parallel to each other between a positive-side DC terminal (that is, a high-potential-side DC terminal) Np and a negative-side DC terminal (that is, a low-potential-side DC terminal) Nn.

Leg circuit4is provided in each of a plurality of phases constituting an alternating current. Leg circuit4is connected between AC circuit12and DC circuit14, and performs the power conversion between both the circuits.FIG.1illustrates a case where AC circuit12is a three-phase AC system, and three leg circuits4u,4v,4ware provided corresponding to a U-phase, a V-phase, and a W-phase, respectively.

AC terminals Nu, Nv, Nw provided in leg circuits4u,4v,4ware connected to AC circuit12through a transformer13. For example, AC circuit12is an AC power system including an AC power supply and the like. InFIG.1, connection between AC terminals Nv, Nw and transformer13is not illustrated for ease of illustration.

A positive-side DC terminal Np and a negative-side DC terminal Nn that are commonly connected to each leg circuit4are connected to DC circuit14. For example, DC circuit14is a DC terminal of a DC power system including a DC power supply network or the like or another power conversion device. In the latter case, a back to back (BTB) system connecting AC power systems having different rated frequencies or the like is configured by coupling two power conversion devices.

Instead of use of transformer13inFIG.1, power conversion device1may be connected to AC circuit12through an interconnection reactor. A primary winding may be provided in each of leg circuits4u,4v,4winstead of AC terminals Nu, Nv, Nw, and leg circuits4u,4v,4wmay be connected to transformer13or the interconnection reactor in terms of AC through a secondary winding magnetically coupled to the primary winding. In this case, the primary winding may be set to following reactors8A,8B. That is, leg circuit4is electrically (that is, in terms of DC or AC) connected to AC circuit12through a connection portion provided in each of leg circuits4u,4v,4w, such as AC terminals Nu, Nv, Nw or the primary winding.

Leg circuit4uincludes two arms connected in series. Specifically, leg circuit4uincludes a positive-side arm5from positive-side DC terminal Np to AC terminal Nu and a negative-side arm6from negative-side DC terminal Nn to AC terminal Nu. The positive-side arm is also referred to as an upper arm, and the negative-side arm is also referred to as a lower arm. AC terminal Nu that is a connection point between positive-side arm5and negative-side arm6is connected to transformer13. Positive-side DC terminal Np and negative-side DC terminal Nn are connected to DC circuit14. Hereinafter, leg circuit4uwill be described below as a representative because leg circuits4v,4whave the same configuration.

Positive-side arm5includes a cell group51in which a plurality of converter cells7aare cascade-connected, a cell group52in which a plurality of converter cells7bare cascade-connected, and reactor8A. Cell group51, cell group52, and reactor8A are connected in series to each other. Negative-side arm6includes a cell group61in which the plurality of converter cells7aare cascade-connected, a cell group62in which the plurality of converter cells7bare cascade-connected, and reactor8B. Cell group61, cell group62, and reactor8B are connected in series to each other.

In the following description, the number of converter cells7aincluded in each of cell group51and cell group61is set to N1. Where, N1≥2. The number of converter cells7bincluded in each of cell group52and cell group62is set to N2. Where, N2≥1. In the following description, sometimes converter cells7aand7bare collectively referred to as a converter cell7. For ease of illustration, the plurality of converter cells7aare disposed adjacent to each other and the plurality of converter cells7bare disposed adjacent to each other in each arm, but limitation to the configuration is not intended. The plurality of converter cells7amay be disposed in a dispersed manner, and the plurality of converter cells7bmay be disposed in a dispersed manner. Each of the plurality of converter cells7included in each leg circuit4is converter cell7aor converter cell7b.

A position where reactor8A is inserted may be any position of positive-side arm5of leg circuit4u, and a position where reactor8B is inserted may be any position of negative-side arm6of leg circuit4u. A plurality of reactors8A and a plurality of reactors8B may be provided. Inductance values of the reactors may be different from each other. Only reactor8A of positive-side arm5or only reactor8B of negative-side arm6may be provided.

Although details will be described later, cell groups51,61and the cell groups52,62have different roles. Specifically, converter cell7aof cell groups51,61is not used for controlling the circulating current, but is in charge of controlling (that is, AC-DC conversion control) an AC electric quantity and a DC electric quantity, and converter cell7bof cell groups52,62is in charge of controlling the circulating current.

Power conversion device1includes an AC voltage detector10, an AC current detector16, DC voltage detectors11A,11B, and arm current detectors9A,9B provided in each leg circuit4as detectors that measure an electric quantity of (for example, current and voltage) used for control. Signals detected by these detectors are input to control device3.

InFIG.1, for ease of illustration, a signal line of the signal input from each detector to control device3and a signal line of the signal input and output between control device3and each converter cell7are partially collectively illustrated, but are actually provided for each detector and each converter cell7. The signal line between each converter cell7and control device3may be provided separately for transmission and for reception. For example, the signal line is formed of an optical fiber.

AC voltage detector10detects a U-phase AC voltage Vacu, a V-phase AC voltage Vacv, and a W-phase AC voltage Vacw of AC circuit12. AC current detector16detects a U-phase AC current Iacu, a V-phase AC current Iacv, and a W-phase AC current Iacw of AC circuit12. DC voltage detector11A detects a DC voltage Vdcp of positive-side DC terminal Np connected to DC circuit14. DC voltage detector11B detects a DC voltage Vdcn of negative-side DC terminal Nn connected to DC circuit14.

Arm current detectors9A,9B provided in U-phase leg circuit4udetect a positive-side arm current Ipu flowing through positive-side arm5and a negative-side arm current Inu flowing through negative-side arm6. Arm current detectors9A,9B provided in V-phase leg circuit4vdetect a positive-side arm current Ipv and a negative-side arm current Inv. Arm current detectors9A,9B provided in W-phase leg circuit4wdetect a positive-side arm current Ipw and a negative-side arm current Inw. In the following description, positive-side arm currents Ipu, Ipv, Ipw are collectively referred to as a positive-side arm current Iarmp, negative-side arm currents Inu, Inv, Inw are collectively referred to as a negative-side arm current Iarmn, and positive-side arm current Iarmp and negative-side arm current Iarmn are collectively referred to as an arm current Iarm. In arm current Iarm, a current flowing from positive-side DC terminal Np toward negative-side DC terminal Nn is set to positive.

Control device3may be configured of a dedicated circuit, and a part or all of the dedicated circuit may be configured of a field programmable gate array (FPGA), a microprocessor, or the like. Typically, control device3includes an auxiliary transformer, an analog to digital (AD) converter, an arithmetic unit, and the like as a hardware configuration. The arithmetic unit includes a central processing unit (CPU), a random access memory (RAM), and a read only memory (ROM). The AD converter includes an analog filter, a sample hold circuit, and a multiplexer. For example, control device3may be configured of a digital protection control device.

<Configuration Example of Converter Cell>

FIG.2is a circuit diagram illustrating an example of the converter cell constituting the cell group. Converter cell7inFIG.2(a)has a circuit configuration called a half-bridge configuration. Converter cell7includes a series body formed by connecting two switching elements31p,31nin series, a capacitor32as an energy accumulator, a bypass switch34, and a voltage detector33. The series body and capacitor32are connected in parallel. Voltage detector33detects a capacitor voltage Vc that is the voltage at both ends of capacitor32.

Converter cell7inFIG.2(b)has a circuit configuration called a full-bridge configuration. Converter cell7includes a first series body formed by connecting two switching elements31p1,31n1in series, a second series body formed by connecting two switching elements31p2,31n2in series, capacitor32, bypass switch34, and voltage detector33. The first series body, the second series body, and capacitor32are connected in parallel. Voltage detector33detects capacitor voltage Vc.

Two switching elements31p,31ninFIG.2(a)and four switching elements31p1,31n1,31p2,31n2inFIG.2(b)are configured such that a freewheeling diode (FWD) is connected in antiparallel to a self-arc-extinguishing semiconductor switching element such as an insulated gate bipolar transistor (IGBT), a gate commutated turn-off (GCT) thyristor, or a metal oxide semiconductor field effect transistor (MOSFET). InFIGS.2(a) and2(b), a capacitor such as a film capacitor is mainly used as capacitor32.

In the following description, switching elements31p,31n,31p1,31n1,31p2,31n2are also collectively referred to as a switching element31. In addition, on and off of the semiconductor switching element in switching element31will be simply referred to as “on and off of switching element31”.

With reference toFIG.2(a), both terminals of switching element31nare referred to as input and output terminals P1, P2. Voltage across capacitor32and zero voltage are output by switching operations of switching elements31p,31n. For example, when switching element31pis turned on and when switching element31nis turned off, the voltage across capacitor32is output. When switching element31pis turned off and when switching element31nis turned on, zero voltage is output. InFIG.2(a), both terminals of switching element31nare set as input and output terminals P1, P2, but both terminals of switching element31pmay be set as input and output terminals P1, P2, and in this case, the operation is reversed.

Bypass switch34is connected between input and output terminals P1, P2. InFIG.2(a), bypass switch34is connected in parallel to switching element31n. However, when both terminals of switching element31pare input and output terminals P1, P2, bypass switch34is connected in parallel to switching element31p. Converter cell7is short-circuited by turning on bypass switch34.

With reference toFIG.2(b), a midpoint between switching element31p1and switching element31n1and a midpoint between switching element31p2and switching element31n2are set to input and output terminals P1, P2of converter cell7. Converter cell7inFIG.2(b)outputs positive voltage or zero voltage by constantly turning on switching element31n2, constantly turning off switching element31p2, and alternately turning on switching elements31p1,31n1. In addition, converter cell7inFIG.2(b)can output zero voltage or negative voltage by constantly turning off switching element31n2, constantly turning on switching element31p2, and alternately turning on switching elements31p1,31n1.

Bypass switch34is connected between input and output terminals P1, P2. Bypass switch34is connected in parallel to the series body of switching elements31n1,31n2. Converter cell7is short-circuited by turning on bypass switch34.

In the following description, the case where converter cells7a,7bare configured as a half-bridge cell inFIG.2(a)and the semiconductor switching element and the capacitor as the energy accumulation element are used will be described as an example. However, converter cells7a,7bmay have a full-bride configuration inFIG.2(b). A converter cell other than the configuration described above, for example, a converter cell to which a circuit configuration called a clamped double cell or the like is applied may be used, and the switching element and the energy accumulation element are not limited to those described above.

<Configuration of Control Device3>

FIG.3is a view illustrating an internal configuration of control device3. With reference toFIG.3, control device3includes switching controllers501a,501b(hereinafter, also collectively referred to as a “switching controller501”). Switching controller501acontrols on and off of each switching element31of converter cell7a. Switching controller501bcontrols on and off of each switching element31of converter cell7b.

Switching controller501aincludes a basic controller502a, a U-phase positive-side cell group controller503UPa, a U-phase negative-side cell group controller503UNa, a V-phase positive-side cell group controller503VPa, a V-phase negative-side cell group controller503VNa, a W-phase positive-side cell group controller503WPa, and a W-phase negative-side cell group controller503WNa. Switching controller501bincludes a basic controller502b, a U-phase positive-side cell group controller503UPb, a U-phase negative-side cell group controller503UNb, a V-phase positive-side cell group controller503VPb, a V-phase negative-side cell group controller503VNb, a W-phase positive-side cell group controller503WPb, and a W-phase negative-side cell group controller503WNb.

In the following description, U-phase positive-side cell group controller503UPa, V-phase positive-side cell group controller503VPa, and W-phase positive-side cell group controller503WPa are also collectively referred to as a positive-side cell group controller503Pa. U-phase negative-side cell group controller503UNa, V-phase negative-side cell group controller503VNa, and W-phase negative-side cell group controller503WNa are also collectively referred to as a negative-side cell group controller503Na. Positive-side cell group controller503Pa and negative-side cell group controller503Na are also collectively referred to as a cell group controller503a. Positive-side cell group controller503Pa controls the operation of cell group51, and negative-side cell group controller503Na controls the operation of cell group61.

U-phase positive-side cell group controller503UPb, V-phase positive-side cell group controller503VPb, and W-phase positive-side cell group controller503WPb are also collectively referred to as a positive-side cell group controller503Pb. U-phase negative-side cell group controller503UNb, V-phase negative-side cell group controller503VNb, and W-phase negative-side cell group controller503WNb are also collectively referred to as a negative-side cell group controller503Nb. Positive-side cell group controller503Pb and negative-side cell group controller503Nb are also collectively referred to as a cell group controller503b. Positive-side cell group controller503Pb controls the operation of cell group52, and negative-side cell group controller503Nb controls the operation of cell group62.

Furthermore, basic controller502aand basic controller502bare also collectively referred to as a basic controller502, and cell group controller503aand cell group controller503bare also collectively referred to as a cell group controller503.

FIG.4is a view illustrating a configuration of basic controller502. With reference toFIG.4, control device3includes basic controllers502a,502b, a current arithmetic unit521, a voltage arithmetic unit522, positive-side cell group controllers503Pa,503Pb, and negative-side cell group controllers503Na,503Nb. Basic controller502aincludes an AC controller523, a DC controller524, and an instruction generation unit525. Basic controller502bincludes a circulating current controller526, a capacitor voltage controller527, an auxiliary voltage generation unit528, adders5i,5j,5k,5m, and gain circuits5g,5h.

Basic controller502asupplies voltage instruction values Vpref1, Vnref1to positive-side cell group controller503Pa and negative-side cell group controller503Na, respectively. Basic controller502bsupplies voltage instruction values Vpref2, Vnref2to positive-side cell group controller503Pb and negative-side cell group controller503Nb, respectively.

Voltage instruction values Vpref1, Vnref1supplied to positive-side cell group controller503Pa and negative-side cell group controller503Na for controlling AC-DC conversion are not based on a detection value of a circulating current Icc. Voltage instruction values Vpref2, Vnref2supplied to the positive-side cell group controller503Pb and negative-side cell group controller503Nb for controlling the circulating current are based on the detection value of circulating current Icc. From this, it can be said that converter cells7aof cell groups51,61are controlled not based on the circulating current, and converter cells7bof cell groups52,62are controlled based on the circulating current.

Current arithmetic unit521takes in the positive-side arm currents Ipu, Ipv, Ipw detected by arm current detector9A and the negative-side arm currents Inu, Inv, Inw detected by arm current detector9B. Current arithmetic unit521operates AC currents Iacu, Iacv, Iacw (hereinafter, also collectively referred to as an “AC current Iac”), a DC current Idc, and circulating currents Iccu, Iccv, Iccw (hereinafter, also collectively referred to as a “circulating current Icc”) from the taken arm current. Current arithmetic unit521outputs each AC current Iac to AC controller523and auxiliary voltage generation unit528, outputs DC current Idc to DC controller524and auxiliary voltage generation unit528, and outputs circulating current Icc to circulating current controller526.

U-phase AC current Iacu, V-phase AC current Iacv, and W-phase AC current Iacw are defined such that a current flowing from AC terminals Nu, Nv, Nw of each leg circuit4toward transformer13is set to positive. DC current Idc is defined such that a direction from DC circuit14toward positive-side DC terminal Np and a direction from negative-side DC terminal Nn toward DC circuit14are set to positive. Circulating currents Iccu, Iccv, Iccw flowing through leg circuits4u,4v,4ware defined such that the direction from positive-side DC terminal Np toward negative-side DC terminal Nn is set to positive.

U-phase, V-phase, W-phase AC voltages Vacu, Vacv, Vacw (hereinafter, also collectively referred to as an “AC voltage Vac”) detected by AC voltage detector10are further input to AC controller523. AC controller523generates U-phase, V-phase, W-phase AC voltage instruction values Vacrefu, Vacrefv, Vacrefw (hereinafter, also collectively referred to as an “AC voltage instruction value Vacref”) based on AC current Iac and AC voltage Vac.

DC voltages Vdcp, Vdcn detected by DC voltage detectors11A,11B are further input to DC controller524. DC controller524generates a DC voltage instruction value Vdcref based on DC voltage (that is, the voltage between the DC terminals) Vdc and the DC current Idc of DC circuit14calculated from DC voltages Vdcp, Vdcn.

Instruction generation unit525generates voltage instruction values Vpref1u, Vnref1uused for U-phase cell groups51,61based on U-phase AC voltage instruction value Vacrefu and DC voltage instruction value Vdcref. Instruction generation unit525generates voltage instruction values Vpref1v, Vnref1vused for V-phase cell groups51,61based on a V-phase AC voltage instruction value Vacrefv and DC voltage instruction value Vdcref. Instruction generation unit525generates voltage instruction values Vpref1w, Vnref1wused for W-phase cell groups51,61based on a W-phase AC voltage instruction value Vacrefw and DC voltage instruction value Vdcref.

Voltage instruction values Vpref1u, Vpref1v, Vpref1w(also collectively referred to as a “voltage instruction value Vpref1”) are supplied to positive-side cell group controller503Pa. Voltage instruction values Vnref1u, Vnref1v, Vnref1w(also collectively referred to as a “voltage instruction value Vnref1”) are supplied to negative-side cell group controller503Na.

Voltage arithmetic unit522receives information about capacitor voltage Vc from each converter cell7bprovided in cell groups52,62of each leg circuit4. Voltage arithmetic unit522calculates a representative value Vcp2of the plurality of capacitor voltages of cell group52and calculates a representative value Vcn2of the plurality of capacitor voltages of cell group62for each phase based on the information about each capacitor voltage Vc. Representative values Vcp2of the U phase, the V phase, and the W phase are described as Vcpu2, Vcpv2, and Vcpw2, respectively, and representative values Vcn2of the U phase, the V phase, and the W phase are described as Vcnu2, Vcnv2, and Vcnw2, respectively.

An average value, a median value, a maximum value, a minimum value, or the like of capacitor voltage Vc of each cell group can be appropriately applied for the arithmetic operation of the representative value. Voltage arithmetic unit522outputs representative values Vcpu2, Vcpv2, Vcpw2of the capacitor voltages of the respective cell groups52and representative values Vcnu2, Vcnv2, Vcnw2of the capacitor voltages of the respective cell groups62to capacitor voltage controller527.

Capacitor voltage controller527receives information about each arm current Iarm, and receives information about capacitor voltages Vcpu2, Vcpv2, Vcpw2, Vcnu2, Vcnv2, Vcnw2from voltage arithmetic unit522.

Capacitor voltage controller527generates a correction value Vpcorr in order to correct voltage instruction value Vpref2for cell group52based on each arm current Iarm and capacitor voltages Vcpu2, Vcpv2, Vcpw2, and outputs generated correction value Vpcorr to adder5i. Capacitor voltage controller527generates a correction value Vncorr in order to correct a voltage instruction value Vnref2for cell group62based on each arm current Iarm and capacitor voltages Vcnu2, Vcnv2, Vcnw2, and outputs generated correction value Vncorr to adder5j.

Circulating current controller526generates circulating voltage instruction values Vccrefu, Vccrefv, Vccrefw (hereinafter, also collectively referred to as a “circulating voltage instruction value Vccref”) for the circulating current control of each phase based on circulating currents Iccu, Iccv, Iccw. Adder5iadds voltage instruction value Vccref, a value obtained by multiplying voltage instruction value Vpref1for cell group51by a gain k in gain circuit5g, and correction value Vpcorr for each phase to generate a voltage value Vxp. Adder5jadds voltage instruction value Vccref, a value obtained by multiplying voltage instruction value Vnref1for cell group61by gain k in gain circuit5h, and correction value Vncorr for each phase to generate a voltage value Vxn.

When the capacitor voltages at cell groups52,62decrease, auxiliary voltage generation unit528generates auxiliary voltage instruction values Vssp, Vssn in order to assist the charge of the capacitors of cell groups52,62based on DC current Idc and AC current Iac.

Voltage value Vxp is added to auxiliary voltage instruction value Vssp for each phase in adder5k. As a result, voltage instruction value Vpref2for cell group52is generated in order to control the circulating current. Voltage instruction value Vpref2is supplied to positive-side cell group controller503Pb. Voltage value Vxn is added to auxiliary voltage instruction value Vssn for each phase in adder5m. As a result, voltage instruction value Vnref2for cell group62is generated in order to control the circulating current. Voltage instruction value Vnref2is supplied to negative-side cell group controller503Nb.

As described above, basic controller502acontrols the output voltages at the plurality of converter cells7ain each arm based on voltage instruction values Vpref1, Vnref1generated by DC current Idc and DC voltage Vdc of DC circuit14and AC current Iac and AC voltage Vac of each phase of AC circuit12.

When the capacitor voltage at converter cell7bdecreases, basic controller502blinearly combines auxiliary voltage instruction values Vssp, Vssn, voltage instruction value Vccref, voltage instruction values Vpref1, Vnref1, and correction values Vpcorr, Vncorr to generate voltage instruction values Vpref2, Vnref2controlling the output voltages of the plurality of converter cells7b. When the capacitor voltage at converter cell7bdoes not decrease, auxiliary voltage instruction values Vssp, Vssn are not generated. In this case, basic controller502blinearly combines voltage instruction value Vccref, voltage instruction values Vpref1, Vnref1, and correction values Vpcorr, Vncorr to generate voltage instruction values Vpref2, Vnref2controlling the output voltages of the plurality of converter cells7b.

<Detailed Operation of Control Device3>

(Operation of Current Arithmetic Unit)

With reference toFIG.1, the connection point between positive-side arm5and negative-side arm6of U-phase leg circuit4uis AC terminal Nu, and AC terminal Nu is connected to transformer13. Accordingly, AC current Iacu flowing from AC terminal Nu toward transformer13is a current value obtained by subtracting negative-side arm current Inu from positive-side arm current Ipu as in the following Equation (1).
Iacu=Ipu−Inu(1)

Assuming that the average value of positive-side arm current Ipu and negative-side arm current Inu is a common current flowing through positive-side arm5and negative-side arm6, this current is a leg current Icomu flowing through the DC terminal of leg circuit4u. Leg current Icomu is expressed by the following Equation (2).
Icomu=(Ipu−Inu)/2  (2)

Also in the V phase, AC current Iacv and a leg current Icomv are calculated using positive-side arm current Ipv and negative-side arm current Inv, and also in the W phase, AC current Iacw and a leg current Icomw are calculated using positive-side arm current Ipw and negative-side arm current Inw. Specifically, they are represented by the following Equations (3) to (6).
Iacv=Ipv−Inv(3)
Icomv=(Ipv−Inv)/2  (4)
Iacw=Ipw−Inw(5)
Icomw=(Ipw+Inw)/2  (6)

The positive-side DC terminals of leg circuits4u,4v,4wof the respective phases are commonly connected as positive-side DC terminal Np, and the negative-side DC terminals are commonly connected as negative-side DC terminal Nn. From this configuration, the current value obtained by adding leg currents Icomu, Icomv, Icomw of the respective phases becomes DC current Idc that flows in from the positive-side terminal of DC circuit14and feeds back to DC circuit14through the negative-side terminal. Accordingly, DC current Idc is expressed as the following Equation (7).
Idc=Icomu+Icomv+Icomw(7)

When the DC current components included in the leg current are equally shared by the respective phases, the current capacity of the converter cell can be equalized. With this taken into consideration, the difference between the leg current and ⅓ of the DC current value can be operated as the current value of the circulating current that does not flow through DC circuit14but flows between the legs of each phase. Consequently, circulation currents Iccu, Iccv, Iccw of the U phase, the V phase, and the W phase are expressed as the following Equations (8), (9), (10).
Iccu=Icomu−Idc/3  (8)
Iccv=Icomv−Idc/3  (9)
Iccw=Icomw−Idc/3  (10)

Current arithmetic unit521inFIG.4operates AC currents Iacu, Iacv, Iacw, the DC current Idc, and the circulation currents Iccu, Iccv, Iccw from positive-side arm currents Ipu, Ipv, Ipw and negative-side arm currents Inu, Inv, Inw according to the above equation.

(Operation of AC Controller523)

From AC voltages Vacu, Vacv, Vacw detected by AC voltage detector10and AC currents Iacu, Iacv, Iacw output from current arithmetic unit521, AC controller523outputs the AC voltages to be output from converter cells7constituting power conversion device1as AC voltage instruction values Vacrefu, Vacrefv, Vacrefw.

For example, AC controller523is configured of an AC current controller that performs feedback control such that the AC current value is matched with the AC current instruction value, an AC voltage controller that performs feedback control such that the AC voltage value is matched with the AC voltage instruction value, and the like according to a function required for power conversion device1. Alternatively, AC controller523may be configured of a power controller that obtains power from the AC current value and the AC voltage value and performs feedback control such that the power value becomes a desired value. In practice, one or a plurality of the AC current controllers, the AC voltage controllers, and the power controllers are combined to configure and operate AC controller523.

Because the AC current controller described above controls the current output to AC circuit12through transformer13, the voltage component controlling the current is a positive phase component and a reversed phase component of the multi-phase AC voltage or a component known as a normal mode component. Similarly, the AC voltage controller outputs the positive phase component and the reversed phase component to AC circuit12through transformer13.

When the three-phase AC voltage is output to AC circuit12, it is also conceivable to output a voltage component common to the three phases, which are known as a zero-phase component or a common mode component, to AC circuit12in addition to these positive and negative phase components. For example, when a third harmonic wave having a frequency three times the fundamental wave frequency is superimposed on the zero-phase component, it is known that the fundamental AC component that can be output by converter cell7can be increased by about 15%.

Furthermore, by outputting a constant zero-phase component, the following effects can be obtained. In power conversion device1having the configuration inFIG.1, the AC voltage component output from cell group51and the AC voltage component output from cell group61have opposite polarities, and the DC voltage component output from cell group51and the DC voltage component output from cell group61have the same polarity. Accordingly, when a certain zero-phase component is included in the AC voltage component, the zero-phase component is superimposed on the DC voltage component output from cell group51and the DC voltage component output from cell group61in the positive and negative opposite directions. As a result, because the difference is generated between the DC power output from cell group51and the DC power output from cell group61, the energy accumulated in capacitor32included in each converter cell7can be exchanged between cell group51and cell group61. Thus, the voltage value of capacitor32of each converter cell7constituting cell group51and the voltage value of capacitor32of converter cell7constituting cell group61can be balanced, and the zero-phase voltage can be used for such balance control.

(Operation of DC Controller524)

DC controller524operates a DC inter-terminal voltage Vdc from the difference voltage between DC voltages Vdcp, Vdcn detected by DC voltage detectors11A,11B, and is expressed as the following Equation (11).
Vdc=Vdcp−Vdcn(11)

DC controller524generates and outputs the DC voltage that should be output by converter cell7as DC voltage instruction value Vdcref from DC inter-terminal voltage Vdc and DC current Idc.

For example, DC controller524is configured by combining any one or a plurality of the DC current controllers that control the DC current, the DC voltage controllers that control the DC voltage, and the DC power controllers that control the DC power. The DC voltage component output from cell group51and the DC voltage component output from cell group61have the same polarity according to DC voltage instruction value Vdcref output from the DC voltage controller, the DC current controller, and the DC power controller. Because cell groups51,61are connected in series, the output voltages of cell groups51,61are combined, and the combined voltage becomes a voltage component generated between the positive-side DC terminal and the negative-side DC terminal of leg circuit4. DC voltage instruction value Vdcref is given to positive-side cell group controller503Pa and negative-side cell group controller503Na as components common to the respective phases.

Consequently, according to DC voltage instruction value Vdcref, the voltage components output from cell groups51,61become DC voltage components output to DC circuit14.

(Operation of Instruction Generation Unit525)

Instruction generation unit525operates the voltage to be output from cell group51as voltage instruction value Vpref1, and operates the voltage to be output from cell group61as voltage instruction value Vnref1. Each of voltage instruction values Vpref1, Vnref1is obtained by combining DC voltage instruction value Vdcref and AC voltage instruction value Vacref for each phase.

Specifically, cell group51and cell group61are connected in series between positive-side DC terminal Np and negative-side DC terminal Nn that are connected to DC circuit14. Accordingly, when each of voltage instruction value Vpref1of cell group51and voltage instruction value Vnref1of cell group61is calculated, ½ of DC voltage instruction value Vdcref is added and combined. On the other hand, because each of AC terminals Nu, Nv, Nw are located at the connection point between positive-side arm5and the negative-side arm6, AC voltage instruction value Vacref is subtracted and combined when voltage instruction value Vpref1of cell group51is calculated, and AC voltage instruction value Vacref is added and combined when voltage instruction value Vnref1of cell group61is calculated. Specifically, voltage instruction values Vpref1u, Vpref1v, Vpref1w, Vnref1u, Vnref1v, Vnref1ware expressed as the following Equations (12) to (17).
Vpref1u=Vdcref/2−Vacrefu(12)
Vpref1v=Vdcref/2−Vacrefv(13)
Vpref1w=Vdcref/2−Vacrefw(14)
Vnref1u=Vdcref/2+Vacrefu(15)
Vnref1v=Vdcref/2+Vacrefv(16)
Vnref1w=Vdcref/2+Vacrefw(17)

Further, a zero-phase potential Vn is expressed by the following Equation (18).
Vn=Vacrefu+Vacrefv+Vacrefw(18)

For example, in leg circuit4uofFIG.1, when cell group51outputs the AC voltage having a relatively small value and when cell group61outputs the AC voltage having a relatively large value, the potential of AC terminal Nu approaches the potential of positive DC terminal Np, and a high voltage is output to AC terminal Nu. Specifically, cell group61outputs the AC voltage having the same polarity as the AC voltage to be output from AC terminal Nu, and cell group51outputs the AC voltage having the opposite polarity to the AC voltage to be output from AC terminal Nu.

In power conversion device1, instruction generation unit525combines the positive and negative phase components and the zero-phase component included in the AC voltage instruction value Vacref with the DC voltage instruction value Vdcref by the above operation, but does not combine voltage components that cause the circulating current to flow to achieve energy balance between phases, and does not combine a voltage components that control the circulating current.

(Operation of Circulating Current Controller526)

U-phase, V-phase, W-phase circulating currents Iccu, Iccv, Iccw operated by current arithmetic unit521are sent to circulating current controller526. Circulating current controller526performs feedback control such that the circulating current value is matched with the circulating current instruction value. That is, a compensator that amplifies a deviation between the circulating current instruction value and the circulating current value is provided in circulating current controller526. At this point, a zero current is usually given as the circulating current instruction value, but a non-zero value may be given when imbalance is generated in the power system. Circulating current controller526outputs voltage components to be output by cell groups52,62for the circulating current control as voltage instruction value Vccref.

The circulating current is a current flowing between legs of different phases. Cell groups51,61and reactors8A,8B exist in a current path of the circulating current, and the circulating current is generated by applying the potential difference generated by switching of cell groups51,61to reactors8A,8B. Accordingly, when voltages of opposite polarities are applied to the reactor by cell groups52,62provided in the same path, the circulating current is prevented.

For example, in the case where circulating current Iccu flows from the positive-side DC terminal to the negative-side DC terminal of leg circuit4u, the voltage in the direction in which the circulating current is decreased is applied to the reactors8A,8B when the positive voltage is output in each of cell groups52,62of leg circuit4u. When the current flows in the reverse direction of the above, the circulating current is attenuated when the voltages at cell groups52,62are also applied in the reverse direction. Circulating current controller526executes feedback control such that the circulating current instruction value and the circulating current value are matched with each other.

(Operation of Capacitor Voltage Controller527)

The voltage at capacitor32of each converter cell7bconstituting each of cell groups52,62is detected by voltage detector33. Voltage arithmetic unit522operates capacitor voltages Vcpu2, Vcpv2, Vcpw2of converter cells7bof cell group52and capacitor voltages Vcnu2, Vcnv2, Vcnw2(simply referred to as “capacitor voltage”) of converter cells7bof cell group62.

The compensator provided in capacitor voltage controller527performs control operation such that the capacitor voltages at cell groups52,62of the respective phases follow the capacitor voltage instruction value. Capacitor voltage controller527outputs a result obtained by multiplying the control arithmetic result by the polarity (for example, 1 or −1) of arm current Iarm to adders5i,5jas the correction value for the circulating current control.

Specifically, capacitor voltage controller527performs the control operation such that capacitor voltages Vcpu2, Vcpv2, Vcpw2follow the capacitor voltage instruction value, and multiplies the control arithmetic result by the polarities of the positive arm currents Ipu, Ipv, Ipw to generate correction values Vpcorru, Vpcorrv, Vpcorrw (hereinafter, also collectively referred to as a “correction value Vpcorr”) for the U-phase, the V-phase, and the W-phase. In addition, capacitor voltage controller527performs the control operation such that capacitor voltages Vcnu2, Vcnv2, Vcnw2follow the capacitor voltage instruction value, and multiplies the control arithmetic result by the polarities of negative arm currents Inu, Inv, Inw to generate correction values Vncorru, Vncorrv, Vncorrw (hereinafter, also collectively referred to as a “correction values Vncorr”) for the U-phase, the V-phase, and the W-phase.

(Operation of Adders5i,5j)

Adder5iadds circulating voltage instruction value Vccref for circulating current control, a value proportional to voltage instruction value Vpref1for cell group51, and correction value Vpcorr for each phase. The addition result of adder5iis output to adder5kas voltage values Vxpu, Vxpv, Vxpw (hereinafter, also collectively referred to as “voltage value Vxp”) in the U-phase, the V-phase, and the W-phase. Adder5jadds voltage instruction value Vccref2for the circulating current control, a value proportional to voltage instruction value Vnref1for cell group61, and correction value Vncorr for each phase. The addition result of adder5jis output to adder5mas voltage values Vxnu, Vxnv, Vxnw (hereinafter, also collectively referred to as “voltage value Vxn”) in the U-phase, the V-phase, and the W-phase.

The reason why the proportional values of the voltage instruction values are added in adders5i,5jis that the half bridge type inFIG.2(a)is used for converter cells7bconstituting cell groups52,62for the circulating current control. Because the half-bridge type converter cell can output only the zero voltage or the positive voltage, in order to increase or decrease the output voltage of converter cell7according to the increase or decrease in the circulating current, the output voltage is required to increase or decrease based on a certain voltage value. However, when the voltage serving as the reference is fixed to a constant value, undesirably capacitor32continues to be charged by DC current Idc flowing between DC circuit14and leg circuit4. In order to avoid this problem, k times of voltage instruction values Vpref1, Vnref1nfor cell groups51,61are added to voltage instruction values Vpref2, Vnref2for cell groups52,62as the reference voltages.

Thus, under the current conditions corresponding to the voltage instruction values Vpref1, Vnref1, the deviation between the AC power and the DC power generated in converter cells7bconstituting cell groups52,62can be reduced (that is, the active power flowing into or out of converter cell7bapproaches zero), so that the voltage fluctuation of capacitors32of converter cells7bcan be prevented. Gain k is set to an arbitrary value such that the output voltage of converter cell7bis not saturated when voltage instruction value Vccref for the circulating current control is given.

When converter cell7bof cell groups52,62for the circulating current control is configured of converter cell7having the full-bridge configuration inFIG.2(b), converter cell7bcan output the voltage at both poles, so that gain k can also be set to zero. In this case, when the capacitor voltage at converter cells7bdoes not decrease, basic controller502bcontrols the output voltages at the plurality of converter cells7bin each arm based on voltage instruction value Vccref and voltage instruction values Vpref2, Vnref2generated by correction values Vpcorr, Vncorr. When the capacitor voltage at converter cells7bdecreases, basic controller502blinearly combines auxiliary voltage instruction values Vssp, Vssn, voltage instruction value Vccref, and correction values Vpcorr, Vncorr to generate voltage instruction values Vpref2, Vnref2.

Furthermore, the reason why the correction value is added in adders5i,5jwill be described. Because the voltages output from cell groups52,62for the circulating current control have a function of controlling the currents flowing through reactors8A,8B, the output power of cell groups52,62becomes substantially reactive power. However, when the active power due to the loss exists in reactors8A,8B cannot be ignored, the active power is required to be supplied to cell groups52,62. This is because the voltages at capacitors32of cell groups52,62cannot be maintained only by providing proportional values of voltage instruction values Vpref1, Vnref1to the cell groups52,62.

According to the above configuration, (i) when arm current Iarm is positive (polarity=1) and when the capacitor voltage is smaller than the instruction value, the compensator outputs the positive signal. Accordingly, by multiplying the output of the compensator by the polarity (=1) of arm current Iarm, the correction value for the circulating current control becomes the signal having the positive component. The signal of the correction value lengthens the period during which switching element31pis conductive, so that the period during which arm current Iarm flows into capacitor32increases. As a result, the deviation between the capacitor voltage instruction value and the detection value of the capacitor voltage is eliminated because capacitor32is charged.

(ii) When arm current Iarm is positive (polarity=1) and when the capacitor voltage is larger than the instruction value, the compensator outputs the negative signal. Accordingly, by multiplying the output of the compensator by the polarity (=1) of arm current Iarm, the correction value for the circulating current control becomes the signal having the negative component. The signal of the correction value shortens the period during which switching element31pis conductive, so that the deviation between the capacitor voltage instruction value and the detection value of the capacitor voltage is eliminated.

(iii) When arm current Iarm is negative (polarity=−1) and when the capacitor voltage is smaller than the instruction value, the compensator outputs the positive signal. Accordingly, by multiplying the output of the compensator by the polarity (=−1) of arm current Iarm, the correction value for the circulating current control becomes the signal having the negative component. The signal of the correction value shortens the period during which switching element31pis conductive, so that the period during which arm current Iarm flows out of capacitor32decreases. As a result, the deviation between the capacitor voltage instruction value and the detection value of the capacitor voltage is eliminated because the discharge time of capacitor32decreases.

(iv) When arm current Iarm is negative (polarity=−1) and when the capacitor voltage is larger than the instruction value, the compensator outputs the negative signal. Accordingly, by multiplying the output of the compensator by the polarity (=−1) of arm current Iarm, the correction value for the circulating current control becomes the signal having the positive component. The discharge time of capacitor32increases because the signal of the correction value lengthens the period during which switching element31pis conductive, so that the deviation between the capacitor voltage instruction value and the detection value of the capacitor voltage is eliminated.

(Operation of Auxiliary Voltage Generation Unit528)

In the above description, the capacitor voltage is maintained by the correction value output from capacitor voltage controller527. However, when the magnitude of arm current Iarm is small and when the active power and the reactive power output from power conversion device1are small, converter cells7bof cell groups52,62that do not perform the AC-DC conversion control cannot sufficiently charge capacitor32even with the correction value by capacitor voltage controller527. In this case, the voltage at capacitor32of converter cell7bcannot be maintained but decreases. Accordingly, when the voltage at capacitor32of converter cell7bis less than a certain value, auxiliary voltage generation unit528generates such an auxiliary voltage instruction value that the voltage at capacitor32is maintained (that is, capacitor32is charged).

Specifically, auxiliary voltage generation unit528receives capacitor voltages Vcpu2, Vcpv2, Vcpw2at cell group52of each phase and the capacitor voltages Vcnu2, Vcnv2, Vcnw2at cell group62of each phase. Auxiliary voltage generation unit528determines whether at least one of capacitor voltages Vcpu2, Vcpv2, Vcpw2, Vcnu2, Vcnv2, Vcnw2is less than a threshold Th1.

When at least one capacitor voltage is less than threshold Th1, auxiliary voltage generation unit528determines that the capacitor voltage decreases, and generates the auxiliary voltage instruction value including at least one of a DC voltage component (hereinafter, referred to as a “DC component”) and a fundamental frequency component (hereinafter, referred to as a “fundamental AC component”) of the AC voltage of AC circuit12based on DC current Idc and AC current Iac. Specifically, auxiliary voltage instruction values Vsspu, Vsspv, Vsspw (hereinafter, also collectively referred to as “Vssp”) for the U-phase, V-phase, and W-phase cell groups52and auxiliary voltage instruction values Vssnu, Vssnv, Vssnw (hereinafter, also collectively referred to as “Vssn”) for the U-phase, V-phase, and W-phase cell groups62are generated. For example, threshold Th1 is set to about 90% of the rated value of the capacitor voltage.

(i) When the DC component is included in auxiliary voltage instruction value Vss, auxiliary voltage generation unit528sets the sign of the DC component based on the direction of DC current Idc flowing into power conversion circuit unit2. Specifically, when DC current Idc flows from DC circuit14to positive-side DC terminal Np, auxiliary voltage generation unit528sets the sign of the DC component to positive (that is, the direction in which converter cell7boutputs the positive voltage). On the other hand, when DC current Idc flows from DC circuit14to negative-side DC terminal Nn, auxiliary voltage generation unit528sets the sign of the DC component to negative (that is, the direction in which converter cell7boutputs the negative voltage).

When the voltage instruction value to which auxiliary voltage instruction value Vss is added is provided to cell groups52,62, cell groups52,62can give and receive the DC power. Consequently, the charge of capacitor32of converter cell7bis promoted, and the voltage at capacitor32can be maintained or increased.

(ii) When the fundamental AC component is included in auxiliary voltage instruction value Vss, auxiliary voltage generation unit528sets the phase of the fundamental AC component based on the phase of AC current Iac flowing into power conversion circuit unit2. Specifically, auxiliary voltage generation unit528sets the phase of the fundamental AC component such that the phase difference between AC current Iac flowing into power conversion circuit unit2and the fundamental AC component becomes less than ±90°. More preferably, auxiliary voltage generation unit528sets the phase of the fundamental AC component such that the phase of the fundamental AC component is in the same phase (that is, the phase difference is zero) as the phase of AC current Iac.

When the voltage instruction value to which auxiliary voltage instruction value Vss is added is given to cell groups52,62, cell groups52,62can give and receive the AC power. Consequently, the charge of capacitor32of converter cell7bis promoted, and the voltage at capacitor32can be maintained or increased.

(iii) In the DC component and the fundamental AC component, the component effective for charging capacitor32can also be included in auxiliary voltage instruction value Vss. Auxiliary voltage generation unit528calculates auxiliary DC power from DC current Idc flowing into power conversion circuit unit2and the maximum value of the DC component, and calculates auxiliary AC power (more specifically, auxiliary active power) from AC current Iac flowing into power conversion circuit unit2and the maximum amplitude value of the fundamental AC component. The maximum value of the DC component and the maximum amplitude value of the fundamental AC component are known and previously stored in an internal memory of control device3.

Auxiliary voltage generation unit528determines whether the auxiliary DC power is larger than the auxiliary AC power. Auxiliary voltage generation unit528generates auxiliary voltage instruction value Vss including the DC component when the auxiliary DC power is larger than the auxiliary AC power, and generates auxiliary voltage instruction value Vss including the fundamental AC component when the auxiliary DC power is smaller than the auxiliary AC power. When the auxiliary DC power and the auxiliary AC power are the same, auxiliary voltage generation unit528generates auxiliary voltage instruction value Vss including either the DC component or the fundamental AC component.

As described above, when the voltage instruction value to which auxiliary voltage instruction value Vss is added is given to cell groups52,62, cell groups52,62can efficiently give and receive the power, and the charge of capacitor32of converter cell7bis promoted.

(iv) As described above, auxiliary voltage generation unit528generates auxiliary voltage instruction value Vss. Thereafter, when the capacitor voltage is restored by the generation of auxiliary voltage instruction value Vss, auxiliary voltage generation unit528stops the generation and output of auxiliary voltage instruction value Vss. Specifically, when all the capacitor voltages (that is, each of capacitor voltages Vcpu2, Vcpv2, Vcpw2, Vcnu2, Vcnv2, Vcnw2) become equal to or larger than a threshold Th2, auxiliary voltage generation unit528stops the generation and output of auxiliary voltage instruction value Vss. In order to prevent chattering and the like, threshold Th2 is set to be larger than threshold Th1 and to be close to the rated value of the capacitor voltage (for example, 99% of the rated value). Auxiliary voltage generation unit528may be configured to generate auxiliary voltage instruction value Vss including both the DC component and the fundamental AC component as described above.

(Operations of Adders5k,5m)

Adder5kadds voltage value Vxp and auxiliary voltage instruction value Vssp for each phase. The addition result of adder5kis input to positive-side cell group controller503Pb as a voltage instruction value Vpref2(for U-phase: Vpref2u, for V-phase: Vpref2v, for W-phase: Vpref2w) representing the voltage component to be output from cell group52for the circulating current control. Adder5madds voltage value Vxn and auxiliary voltage instruction value Vssn for each phase. The addition result of adder5mis input to negative-side cell group controller503Nb as a voltage instruction value Vnref2(for U phase: Vnref2u, for V phase: Vnref2v, for W phase: Vnref2w) representing the voltage component to be output from cell group62for the circulating current control. In adders5k,5m, the voltage instruction value to which the auxiliary voltage instruction value is added is given to cell groups52,62, so that the charge of capacitor32of each converter cell7bcan be promoted.

At this point, because inter-DC terminal voltage Vdc is controlled by cell groups51,61, even when auxiliary voltage instruction value Vss including the DC component is added to the voltage instruction value for cell groups52,62, inter-DC terminal voltage Vdc does not greatly fluctuate. When starting the output of the auxiliary voltage instruction value Vss by determining that the capacitor voltage is lowered, auxiliary voltage generation unit528changes the effective value of auxiliary voltage instruction value Vss including the DC component in a ramp shape, so that the instantaneous fluctuation can also be prevented. Similarly, auxiliary voltage generation unit528may change the effective value of auxiliary voltage instruction value Vss in the ramp shape when stopping the output of auxiliary voltage instruction value Vss by determining that the capacitor voltage is returned.

In addition, because AC voltage Vac is controlled by cell groups51,61, the AC voltage and the AC current do not fluctuate even when auxiliary voltage instruction value Vss including the fundamental AC component is added to the voltage instruction value for cell groups52,62. When starting and stopping the output of auxiliary voltage instruction value Vss, auxiliary voltage generation unit528changes the effective value of auxiliary voltage instruction value Vss including the fundamental AC component in the ramp shape, so that auxiliary voltage generation unit528can prevent the instantaneous fluctuation.

(Configuration and Operation of Cell Group Controller503)

FIG.5is a view illustrating a configuration of cell group controller503. With reference toFIG.5, cell group controller503includes N individual controllers202_1to202_N (hereinafter, also collectively referred to as an “individual controllers202”). For example, N1 converter cells7aare included in cell groups51,61. Accordingly, each of positive-side cell group controller503Pa and negative-side cell group controller503Na corresponding to cell groups51,61includes N1 individual controllers202. Hereinafter, for the sake of description, voltage instruction values Vpref1, Vnref1, Vpref2, Vnref2will be collectively referred to as a voltage instruction value Vref.

Individual controller202_iindividually controls corresponding converter cells7. Individual controller202_ireceives voltage instruction value Vref, arm current Iarm, capacitor voltage instruction value Vcref, and a switching permission signal GEn from basic controller502. Capacitor voltage instruction value Vcref and switching permission signal GEn are generated by basic controller502. For example, capacitor voltage instruction value Vcref is a rated value of capacitors32of the plurality of converter cells7included in each cell group. Individual controller202_ireceives capacitor voltage Vc from corresponding converter cell7_i. Individual controller202_itransmits capacitor voltage Vc to basic controller502.

When switching permission signal GEn is “1”, each switching element31of converter cell7can perform on and off switching operation by gate signal ga. Switching permission signal GEn “1” means that converter cell7is in a deblocked state.

When switching permission signal GEn is “0”, all switching elements31of converter cell7are turned off by gate signal ga. Switching permission signal GEn “0” means that converter cell7is in a gate block state. For example, when an accident is generated in the power system or when transient operation is difficult, basic controller502generates switching permission signal GEn having the value of “0” and outputs switching permission signal GEn to individual controller202.

Carrier signal generation unit203sets a reference phase of the carrier signal for each converter cell7, and generates the carrier signal having the set reference phase. Specifically, carrier signal generation unit203sets an interval between the reference phases (hereinafter, also referred to as a “carrier reference phase”) of the plurality of carrier signals CR(i) to an interval obtained by dividing 360 degrees by the number N of the plurality of converter cells7_i. The reference phase of carrier signal CR(i) represents a difference between the phase of carrier signal CR(i) and a reference phase. The phase of a carrier signal CR (0) can be used as the reference phase. Carrier signal generation unit203generates carrier signals CR (1) to CR (N) having the set carrier reference phase.

Individual controller202_ireceives a carrier signal CRi from carrier signal generation unit203. Individual controller202_iperforms pulse width modulation (PWM) control on converter cell7_iusing carrier signal CRi. Specifically, when switching enabling signal GEn is “1” (that is, the converter cell7_iis in the deblock state), individual controller202_imodulates voltage instruction value Vref and carrier signal CRi of converter cell7_iby the phase shift PWM method, thereby generating gate signal ga (for example, a PWM modulation signal) and outputting gate signal ga to converter cell7_i. Individual controller202_iperforms modulation according to the configuration of converter cell7_i. In the configuration of converter cell7_i, the number n of PWM modulation signals to be output also increases or decreases. For example, n=2 for the converter cell in the half-bridge configuration, and n=4 for the converter cell in the full-bridge configuration.

Advantages

According to the embodiment, in power conversion device1including the cell group for the AC-DC conversion control and the cell group for the circulating current control, the voltage at the capacitor can be appropriately controlled even when the arm current is small.

OTHER EMBODIMENTS

(1) In the above-described embodiments, in each of reactors8A,8B, only positive-side reactor8A or only negative-side reactor8B may be provided in each leg circuit4. When only negative-side reactor8B is provided, cell group52for the circulating current control becomes unnecessary, and positive-side cell group controller503Pb, adders5i,5k, and gain circuit5grelated to cell group52are also unnecessary. Therefore, there is an advantage that the configuration of control device3can be simplified. Similarly, when only positive-side reactor8A is provided, cell group62for the circulating current control becomes unnecessary, and negative-side cell group controller503Nb, adders5j,5m, and gain circuit5hrelated to cell group62are also unnecessary. Therefore, there is an advantage that the configuration of control device3can be simplified.(2) In the above-described embodiments, converter cells7aconstituting cell groups51,61not for the circulating current control and each converter cells7bconstituting cell groups52,62for the circulating current control have the same configuration. Alternatively, converter cells7aconstituting cell groups51,61and converter cells7bconstituting cell groups52,62may have different configurations.(3) An example in which capacitor voltage controller527multiplies the output of the compensator by the polarity of arm current Iarm has been described in the above-described embodiments. However, the similar effect can be obtained by multiplying the output of the compensator by the current value of arm current Iarm instead of the polarity of arm current Iarm.(4) The configuration in which it is determined that the capacitor voltage decreases when at least one of capacitor voltages Vcpu2, Vcpv2, Vcpw2, Vcnu2, Vcnv2, Vcnw2is less than threshold Th1 has been described in the above-described embodiments. However, the configuration is not limited to the embodiments. For example, when the capacitor voltage of at least one converter cell7bin all converter cells7bincluded in each leg circuit4is less than threshold Th1, it may be determined that the capacitor voltage decreases. In this case, when the capacitor voltages at all converter cells7bincluded in each leg circuit4become equal to or greater than threshold Th2, it may be determined that the capacitor voltage is returned.(5) In the embodiment described above, when converter cells7bof cell groups52,62have the full-bridge configuration, gain k is set to zero, and voltage instruction values Vpref2, Vnref2are generated using voltage instruction value Vccref and correction values Vpcorr, Vncorr. However, when converter cell7bhas the full-bridge configuration, basic controller502bmay control the output voltages of the plurality of converter cells7bin each arm using voltage instruction value Vccref and correction values Vpcorr, Vncorr without generating voltage instruction values Vpref2, Vnref2. In this case, switching elements31p1,31n1of converter cell7bare controlled based on voltage instruction value Vccref, and switching elements31p2,31n2of converter cell7bare controlled based on correction values Vpcorr, Vncorr.

Specifically, when the capacitor voltage at converter cell7bdoes not decrease, basic controller502boutputs voltage instruction value Vccref to positive-side cell group controller503Pb and negative-side cell group controller503Nb, outputs correction value Vpcorr to positive-side cell group controller503Pb, and outputs correction value Vncorr to negative-side cell group controller503Nb. Positive-side cell group controller503Pb controls switching elements31p1,31n1of each converter cell7bof cell group52based on voltage instruction value Vccref, and controls the switching elements31p2,31n2of each converter cell7bbased on correction value Vpcorr. Negative-side cell group controller503Nb controls switching elements31p1,31n1of each converter cell7bof cell group62based on voltage instruction value Vccref, and controls switching elements31p2,31n2of each converter cell7bbased on correction value Vncorr.

On the other hand, when the capacitor voltage at converter cell7bdecreases, basic controller502blinearly combines auxiliary voltage instruction value Vssp with voltage instruction value Vccref and correction value Vpcorr, and outputs the combined value to positive-side cell group controller503Pb. Basic controller502blinearly combines auxiliary voltage instruction value Vssn with voltage instruction value Vccref and correction value Vncorr, and outputs the combined value to negative-side cell group controller503Nb. Positive-side cell group controller503Pb controls switching elements31p1,31n1of each converter cell7bof cell group52based on linearly combined voltage instruction value Vccref, and controls switching elements31p2,31n2of each converter cell7bbased on linearly combined correction value Vpcorr. Negative-side cell group controller503Nb controls switching elements31p1,31n1of each converter cell7bof cell group62based on linearly combined voltage instruction value Vccref, and controls switching elements31p2,31n2of each converter cell7bbased on linearly combined correction value Vncorr.

It should be considered that the disclosed embodiment is an example in all respects and not restrictive. The scope of the present disclosure is defined by not the description above, but the claims, and it is intended that all modifications within the meaning and scope of the claims and their equivalents are included in the present disclosure.

REFERENCE SIGNS LIST

1: power conversion device,2: power conversion circuit unit,3: control device,4u,4v,4w: leg circuit,5: positive-side arm,5g,5h: gain circuit,5i,5j,5k,5m: adder,6: negative-side arm,7a,7b: converter cell,8A,8B: reactor,9A,9B: arm current detector,10: AC voltage detector,11A,11B: DC voltage detector,12: AC circuit,13: transformer,14: DC circuit,16: AC current detector,31n,31n2,31n1,31p2,31p,31p1: switching element,32: capacitor,33: voltage detector,34: bypass switch,51,52,61,62: cell group,202: individual controller,203: carrier signal generation unit,501a,501b: switching controller,502a,502b: basic controller,503Na,503Nb: negative-side cell group controller,503Pa,503Pb: positive-side cell group controller,521: current arithmetic unit,522: voltage arithmetic unit,523: AC controller,524: DC controller,525: instruction generation unit,526: circulating current controller,527: capacitor voltage controller,528: auxiliary voltage generation unit