Power Conversion Equipment

Abstract

To provide a power conversion device capable of continuously outputting a zero voltage.SOLUTION: A power conversion device comprises a plurality of cell converters connected in series via a pair of AC output terminals, each cell converter including a pair of AC output terminals. Each of the plurality of cell converters comprises a capacitor, a power conversion circuit connected between the capacitor and the pair of AC output terminals, a driving circuit which drives the power conversion circuit, and a feeder circuit which supplies power to the driving circuit on the basis of electric power from the capacitor. The power conversion circuit is a full-bridge circuit including a plurality of upper arms and a plurality of lower arms. The driving circuit keeps a first state in which the upper arm is on and the lower arm is off or a second state in which the upper arm is off and the lower arm is on longer than a carrier cycle of the power conversion circuit on the basis of a voltage value of the capacitor of the own cell converter among the plurality of cell converters.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present disclosure relates to a power conversion device. [Background technology]

[0002] One of the circuit configurations of a power conversion device is a modular multilevel converter (MMC). The MMC includes a number of cells connected in series, each of which has a full-bridge conversion circuit to which a capacitor is connected, and a drive circuit that turns on or off a number of arms in the full-bridge conversion circuit. The cell has a number of operation modes, one of which is an operation mode in which all upper arms are turned on to cause the cell to output zero voltage (see, for example, mode 1 in FIG. 4C of Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 6416411 Summary of the Invention [Problem to be solved by the invention]

[0004] When all the upper arms or all the lower arms are turned on to output zero voltage, the capacitor cannot be charged, and the voltage of the capacitor gradually drops. However, in a configuration in which the drive circuit that drives each arm uses power supplied from a capacitor, if the power supplied from the capacitor to the drive circuit is insufficient, it is not possible to maintain all the upper arms or all the lower arms in the on state. As a result, there is a risk that it will not be possible to continuously output zero voltage.

[0005] The present disclosure provides a power conversion device capable of continuously outputting zero voltage. [Means for solving the problem]

[0006] The present disclosure relates to a plurality of cell converters each having a pair of AC output terminals and connected in series via the pair of AC output terminals; Each of the plurality of cell converters comprises: a capacitor; a power conversion circuit connected between the capacitor and the pair of AC output terminals; a drive circuit that drives the power conversion circuit; and a power supply circuit that supplies power to the drive circuit based on power from the capacitor; the power conversion circuit is a full-bridge circuit having a plurality of upper arms and a plurality of lower arms, The drive circuit provides a power conversion device that maintains a first state in which the upper arm is on and the lower arm is off, or a second state in which the upper arm is off and the lower arm is on, for a period longer than the carrier period of the power conversion circuit, based on the voltage value of the capacitor of the cell converter itself among the multiple cell converters. Effect of the Invention

[0007] According to the present disclosure, it is possible to provide a power conversion device capable of continuously outputting zero voltage. [Brief description of the drawings]

[0008] [Figure 1] 1 is a diagram illustrating a configuration example of a power conversion device according to an embodiment; [Diagram 2] FIG. 2 is a diagram illustrating a first configuration example of a cell converter. [Diagram 3] 4A to 4C are diagrams illustrating operation modes in the first and second states. [Figure 4] FIG. 13 is a diagram showing an operation mode in which one of the two arms that are turned on is turned off when the voltage value of the capacitor falls below a predetermined voltage value in the first state or the second state. [Diagram 5] 5 is a diagram showing an operation mode in which one arm is alternately switched from on to off according to the operation mode of FIG. 4 when the voltage value of the capacitor falls below a predetermined voltage value. FIG. [Figure 6]FIG. 13 is a diagram showing an operation mode in which both of the on-state arms are turned off when the voltage value of the capacitor falls below a predetermined voltage value in the first state or the second state. [Figure 7] This shows an operation mode in which, when the output voltage Vout is set to zero voltage from a state in which the upper and lower arms are all off, the arm opposite to the arm that was on before the upper and lower arms were all turned off is turned on. [Figure 8] 1 is a diagram showing the relationship between the current flowing from the grid to the power conversion device, the on / off state of the upper and lower arms, and the output voltage of the cell converter. [Figure 9] 1 is a diagram showing the relationship between the current flowing from the grid to the power conversion device, the on / off state of the upper and lower arms, and the output voltage of the cell converter. [Figure 10] 13 is a diagram illustrating an example of functional blocks of a control unit that derives a period during which the upper arm and the lower arm are all in an off state. FIG. [Figure 11] 13 is a diagram illustrating an example of functional blocks of a control unit that derives a period during which the upper arm and the lower arm are all in an off state. FIG. [Figure 12] 11 is a diagram illustrating an example of a functional block of a control unit that determines the timing when all of the upper arms and the lower arms are turned off. FIG. [Figure 13] FIG. 2 is a diagram illustrating a configuration example of a power supply circuit. [Figure 14] FIG. 13 is a diagram illustrating a second configuration example of a cell converter. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0010] Fig. 1 is a diagram showing an example of the configuration of a power conversion device in one embodiment, and shows an example of the circuit configuration of an MMC. The MMC is applicable to, for example, a static static compensation (STATCOM) or a high voltage direct current (HVDC) transmission system. The power conversion device 400 shown in Fig. 1 includes a plurality of clusters 50 (50UV, 50VW, 50WU), a plurality of reactors 51 (51UV, 51VW, 51WU), and a control unit 401.

[0011] The U-phase cluster 50UV includes a plurality of cell converters 52UV1, 52UV2, . . . 52UV that are connected in series via a pair of AC output terminals a and b. x The V-phase cluster 50VW has a plurality of cell converters 52VW1, 52VW2, . . . 52VW connected in series via a pair of AC output terminals a, b. x The W-phase cluster 50WU includes a plurality of cell converters 52WU1, 52WU2, . . . 52WU connected in series via a pair of AC output terminals a and b. x x represents the number of cell converters connected in series in each cluster, and is an integer of 2 or more.

[0012] Multiple cell converter 52 (52UV1~52UV x ,52VW1~52VW x ,52WU1~52WU x ) each have a pair of AC output terminals a, b and are connected in series via the pair of AC output terminals a, b. Each of the multiple cell converters 52 has its first AC output terminal a connected to the second AC output terminal b of one of the cell converters adjacent to it, and its second AC output terminal b connected to the first AC output terminal a of the other cell converter adjacent to it.

[0013] Cluster 50UV, cluster 50VW, and cluster 50WU are delta-connected via reactors 51UV, 51VW, and 51WU, and are connected to grid 300. Connection to grid 300 may be via a transformer (not shown). Since a circulating current flows in the delta connection, control unit 401 can adjust the negative-phase reactive current by controlling this circulating current by a plurality of cell converters 52. Note that, although FIG. 1 illustrates a delta connection, cluster 50UV, cluster 50VW, and cluster 50WU may be star-connected or may be connected by another connection method.

[0014] Each of the cell converters 52 has a power conversion circuit having a plurality of switching elements and a drive circuit unit that operates the power conversion circuit. The cell converters 52 have the same configuration. The switching element has, for example, a transistor and a diode connected in anti-parallel to the transistor. Specific examples of the transistor include an IGBT (Insulated Gate Bipolar Transistor) and a MOSFET (Metal Oxide Semiconductor Field Effect Transistor).

[0015] The power conversion device 400 can output a multilevel voltage waveform having a voltage equal to or higher than the withstand voltage of the switching elements and with reduced harmonics by causing each of the multiple cell converters 52 to output a voltage waveform with a different phase from each other by the control unit 401. Therefore, the power conversion device 400 is applicable to, for example, a reactive power compensation device or a DC transmission system directly connected to an extra-high voltage system.

[0016] FIG. 2 is a diagram showing a first configuration example of the cell converter, and includes a plurality of cell converters 52UV1 to 52UV x ,52VW1~52VW x ,52WU1~52WU x 2 has a function of converting DC power in the capacitor 600 into AC power and outputting it to a pair of AC output terminals a, b, and a function of converting AC power input from the pair of AC output terminals a, b into DC power and supplying it to the capacitor 600.

[0017] The cell converter 52 includes a pair of AC output terminals a, b, a capacitor 600, a power conversion circuit 53, GDUs (Gate Drive Units) 502a to 502d, and a power supply circuit 630.

[0018] The capacitor 600 is a capacitive element connected to a pair of AC output terminals a, b via the power conversion circuit 53. Edc represents the voltage value of the capacitor 600.

[0019] The power conversion circuit 53 is an inverter circuit connected between the capacitor 600 and a pair of AC output terminals a, b, and converts power bidirectionally between DC and AC. The power conversion circuit 53 is connected in parallel to the capacitor 600. FIG. 2 illustrates a full-bridge circuit having a plurality of switching elements 501 (501a to 501d). The power conversion circuit 53 has a full-bridge configuration in which a circuit in which switching elements 501a and 501b are connected in series and a circuit in which switching elements 501c and 501d are connected in series are connected in parallel. A first AC output terminal a is connected to a connection point between the switching element 501a of the first upper arm and the switching element 501b of the first lower arm. A second AC output terminal b is connected to a connection point between the switching element 501c of the second upper arm and the switching element 501d of the second lower arm.

[0020] The multiple switching elements 501 illustrated in FIG. 2 are IGBTs with diodes connected in anti-parallel, but may be switching elements having a switching function such as MOSFETs or thyristors.

[0021] At least one of the switching element and the anti-parallel diode is preferably an element including a wide bandgap semiconductor such as SiC (silicon carbide), GaN (gallium nitride), Ga2O3 (gallium oxide), or diamond. By applying a wide bandgap semiconductor to the switching element, the effect of reducing loss in the switching element is enhanced. The switching element may be an element including a semiconductor such as Si (silicon). Similarly, by applying an element including a wide bandgap semiconductor to the diode, the effect of reducing loss in the diode is enhanced. The diode may be an element including a semiconductor such as Si (silicon).

[0022] The GDU 502 (502a to 502d) is a drive circuit that drives the power conversion circuit 53, and specifically, is a gate drive circuit that drives the gates of multiple switching elements 501a to 501d configured in the power conversion circuit 53. The GDU 502 drives the multiple switching elements 501a to 501d configured in the power conversion circuit 53 based on power supplied from the capacitor 600 via the power supply circuit 630.

[0023] The power supply circuit 630 is a self-powered circuit (power supply circuit) that supplies power to the GDUs 502a to 502d based on the power from the capacitor 600. In the example shown in Fig. 2, the power supply circuit 630 is connected in parallel to the capacitor 600. Power is supplied to the GDUs 502a to 502d via the power supply circuit 630. Power supplied from the capacitor 600 is supplied to the power supply circuit 630, and the power supply circuit 630 supplies the necessary power to each of the GDUs 502a to 502d.

[0024] The GDUs 502a to 502d turn on or off the corresponding switching elements among the multiple switching elements 501a to 501d by applying a voltage between the gate and emitter of the corresponding switching element in accordance with a control signal from the control unit 401 (see FIG. 1). By this operation, a square-wave voltage is generated between a pair of AC output terminals a and b of the cell converter 52.

[0025] The control unit 401 (see FIG. 1) is a controller that generates control signals (e.g., pulse-width modulated signals) that turn on or off the multiple switching elements 501a to 501d in accordance with a carrier period Tc (the inverse of the carrier frequency) common to the multiple cell converters 52. The control unit 401 has a memory and a processor (e.g., a CPU (Central Processing Unit)), and each function of the control unit 401 is realized by the processor operating according to a program stored in the memory.

[0026] The control unit 401 monitors the voltage value Edc of the capacitor 600 of each of the multiple cell converters 52, and selects one or more cell converters from the multiple cell converters 52 based on the voltage value Edc of the capacitor 600 of each of the multiple cell converters 52. For example, when the voltage value Edc of each of the capacitors 600 of the multiple cell converters 52 is normal, the control unit 401 determines that all of the multiple cell converters 52 are healthy, and selects one or more specific cell converters from the multiple cell converters 52. For example, when the voltage value Edc of the capacitor 600 is within a predetermined voltage range VA, the control unit 401 determines that the voltage value Edc of the capacitor 600 is normal, and determines that the cell converter having the capacitor 600 is healthy. In order to avoid fixing the selected cell converter, the control unit 401 may sequentially switch one or more cell converters selected as a specific cell converter from the multiple cell converters 52.

[0027] FIG. 3 is a diagram showing an operation mode of a specific cell converter. When the control unit 401 determines that all of the multiple cell converters 52 are healthy, it maintains one or more specific cell converters selected from the multiple cell converters 52 in a first state or a second state. The first state represents a state in which all upper arms are on and all lower arms are off, and the second state represents a state in which all upper arms are off and all lower arms are on. In this example, all upper arms are switching elements 501a and 501c, and all lower arms are switching elements 501b and 501d. The GDU 502 of one or more specific cell converters selected by the control unit 401 maintains the first state or the second state according to the operation mode shown in FIG. 3 instructed by a control signal from the control unit 401. By maintaining the specific cell converter in the first state or the second state, the output voltage Vout from the pair of AC output terminals a and b becomes zero voltage, and the pair of AC output terminals a and b is short-circuited via all upper arms or all lower arms in the on state. As a result, the power conversion device 400 can continue to operate using the multiple cell converters 52 excluding the specific cell converter.

[0028] In many cases, each of the multiple clusters 50 configured in the power conversion device 400 is provided with one or more redundant cell converters in preparation for the occurrence of an abnormality in any of the cell converters. The presence of one or more redundant cell converters allows the power conversion device 400 to output the rated reactive power even if any of the cell converters fails. However, when all of the multiple cell converters 52 are healthy, switching the multiple switching elements in the one or more redundant cell converters also results in unnecessary switching loss in those redundant cell converters.

[0029] In contrast, in this embodiment, when all of the multiple cell converters 52 are healthy, one or more specific cell converters are maintained in the first state or the second state, thereby suppressing unnecessary switching losses in one or more redundant cell converters.

[0030] The control unit 401 maintains one or more specific cell converters selected from the multiple cell converters 52 in the first state or the second state for at least longer than the carrier period Tc. This allows the GDU 502 to maintain the output voltage Vout at zero voltage for a period longer than the carrier period Tc, unless the power supplied from the capacitor 600 to the GDU 502 via the power supply circuit 630 is insufficient.

[0031] In Figure 3, the polarity of current i is "positive" indicates a situation in which the current i flows out from one of the AC output terminals a in the direction shown in Figure 2, and the polarity of current i is "negative" indicates a situation in which the current i flows in from one of the AC output terminals a in the direction opposite to the direction shown in Figure 2 (the same applies to Figure 4 described below, etc.).

[0032] FIG. 3 is a diagram showing an operation mode in the first state or the second state. For example, when the voltage value Edc of the capacitor 600 is equal to or greater than the threshold value Vth, the control unit 401 causes one or more specific cell converters selected from the multiple cell converters 52 to be maintained in the first state or the second state for a period longer than the carrier period Tc. The threshold value Vth is an example of a predetermined voltage value, and is set to the above-mentioned predetermined voltage range VA (for example, the lower limit value of the voltage range VA). When the voltage value Edc of the capacitor 600 is equal to or greater than the threshold value Vth, the GDU 502 of the one or more specific cell converters maintains the first state or the second state for a period longer than the carrier period Tc. As a result, the power supplied to the GDU 502 via the power supply circuit 630 relatively remains in the capacitor 600, so that the GDU 502 can continuously maintain the output voltage Vout at zero voltage.

[0033] In the first state, the positive current i circulates through the terminal b, switching element 501c, switching element 501a, and terminal a (operation mode A), and the negative current i circulates through the terminal a, switching element 501a, switching element 501c, and terminal b (operation mode B). On the other hand, in the second state, the positive current i circulates through the terminal b, switching element 501d, switching element 501b, and terminal a (operation mode C), and the negative current i circulates through the terminal a, switching element 501b, switching element 501d, and terminal b (operation mode D). In any of the operation modes A to D, the current i circulates in this manner, so that the capacitor 600 is not charged by the current i.

[0034] 4 is a diagram showing an operation mode in which one of the two arms that are turned on is turned off when the capacitor voltage value falls below a predetermined voltage value in the first state or the second state. When the voltage value Edc falls below a threshold value Vth in the first state or the second state, the control unit 401 outputs a control signal to the GDU 502 of a specific cell converter to turn off one of the two arms that are turned on. When the voltage value Edc falls below the threshold value Vth, the GDU 502 turns off one of the two arms that are turned on.

[0035] For example, in FIG. 4, when the voltage value Edc falls below the threshold Vth in the first state, the GDU 502 turns off one arm (element 501a) of the two arms (elements 501a and 501b) that are on in the state shown in FIG. 3 (previous operation mode). As a result, a positive current i flows in the following order: terminal b, element 501c, capacitor 600, the parallel diode of element 501b, and terminal a. In operation mode A1, the capacitor 600 can be charged, and the output voltage Vout becomes −Edc. As a result, the capacitor 600 can be charged to a voltage value equal to or higher than the threshold Vth. Similarly, when the voltage value Edc falls below the threshold Vth in the second state, the GDU 502 turns off one arm (element 501b) of the two arms (elements 501b and 501d) that are on in the state shown in FIG. 3 (previous operation mode). As a result, the negative current i flows in the following order: terminal a, the parallel diode of element 501a, capacitor 600, element 501d, and terminal b, so that in operation mode C2, capacitor 600 can be charged and output voltage Vout becomes Edc. As a result, capacitor 600 can be charged to a voltage value equal to or higher than threshold Vth. Similarly, in operation modes B2 and D1, capacitor 600 can be charged to a voltage value equal to or higher than threshold Vth. Note that there are also modes such as operation modes A2, B1, C1, and D2 in which capacitor 600 is not charged depending on the polarity of current i.

[0036] Fig. 5 is a diagram showing an operation mode in which, when the voltage value of the capacitor drops below a predetermined voltage value, one arm is alternately switched from on to off according to the operation mode of Fig. 4. The GDU 502 of one or more specific cell converters selected by the control unit 401 turns multiple switching elements 501 to the on or off state in the operation mode shown in Fig. 5 according to a control signal from the control unit 401.

[0037] The GDU 502 switches the semiconductor element, which is changed from an ON state to an OFF state according to the operation mode of Fig. 4, according to the operation mode of Fig. 5. If one of the upper arms is in the OFF state in state STEP1 before the first state, the GDU 502 changes the other of the upper arms to the OFF state in state STEP3 after the first state. Alternatively, if one of the lower arms is in the OFF state in state STEP1 before the second state, the GDU 502 changes the other of the lower arms to the OFF state in state STEP3 after the second state.

[0038] For example, in operation mode A3, GDU 502 turns off element 501a in STEP 1 according to operation modes A1 and A2 in Fig. 4, then turns on element 501a in STEP 2 according to operation modes A and B in Fig. 3, and then turns off element 501c in STEP 3 according to operation modes B1 and B2 in Fig. 4. As a result, between operation mode STEP 1 before last and operation mode STEP 3 of the current, the element through which current i flows for charging capacitor 600 is switched between element 501a and element 501d. Therefore, heat generation due to current concentration in a specific semiconductor element can be reduced, and deterioration or damage of the specific semiconductor element can be suppressed.

[0039] Fig. 6 is a diagram showing an operation mode in which both of the two arms that are on are turned off when the capacitor voltage value drops below a predetermined voltage value in the first state or the second state. The GDU 502 of one or more specific cell converters selected by the control unit 401 turns multiple switching elements 501 on or off in the operation mode shown in Fig. 6 according to a control signal from the control unit 401. The control unit 401 maintains one or more specific cell converters selected from the multiple cell converters 52 in a third state. The third state refers to a state in which all upper arms are off and all lower arms are off.

[0040] In the operation modes of Figs. 4 and 5, the GDU 502 turns off one of the two arms that were on in the previous operation mode. In contrast, in the operation mode of Fig. 6, the GDU 502 turns off both of the two arms that were on in the previous operation mode. As a result, in the current operation mode, all of the four elements 501a to 501d are turned off, and one or more specific cell converters selected by the control unit 401 operate as rectifiers using diodes connected in parallel to each of the four elements 501a to 501d. As a result, the capacitor 600 can be charged regardless of the polarity of the current i.

[0041] 7 shows an operation mode in which, when the output voltage Vout is set to zero voltage from a state in which the upper and lower arms are all off, the arm opposite to the arm that was on before the upper and lower arms were all turned off is turned on. GDU 502 of one or more specific cell converters selected by control unit 401 turns multiple switching elements 501 on or off in the operation mode shown in FIG. 7 according to a control signal from control unit 401.

[0042] When the state STEP1 before the upper and lower arms all enter the off state STEP2 is the first state, and the voltage value Edc becomes equal to or greater than the threshold value Vth when the upper and lower arms are all in the off state STEP2, the GDU 502 switches to the second state in STEP3. On the other hand, when the state STEP1 before the upper and lower arms all enter the off state STEP2 is the second state, and the voltage value Edc becomes equal to or greater than the threshold value Vth when the upper and lower arms are all in the off state STEP2, the GDU 502 switches to the first state in STEP3.

[0043] For example, in operation mode A5, the GDU 502 turns on the upper arm (elements 501a, 501c) in STEP 1, turns off all semiconductor elements in STEP 2, and then turns on the lower arm (elements 501b, 501d) in STEP 3. This prevents heat generation due to current concentration in a specific semiconductor element, and avoids deterioration or damage to the element. As a result, between the operation mode STEP 1 before last and the operation mode STEP 3 of the current, the element through which the current i for charging the capacitor 600 flows is switched between the upper arm (elements 501a, 501c) and the lower arm (elements 501b, 501d). This makes it possible to reduce heat generation due to current concentration in a specific semiconductor element, and to suppress deterioration or damage to the specific semiconductor element.

[0044] For example, the control unit 401 periodically determines whether the voltage value Edc is equal to or greater than the threshold value Vth. When the condition that the voltage value Edc is equal to or greater than the threshold value Vth is satisfied, the control unit 401 commands the GDU 502 of the selected one or more specific cell converters to maintain the first state or the second state. On the other hand, when the condition that the voltage value Edc is less than the threshold value Vth is satisfied, the control unit 401 commands the GDU 502 of the selected one or more specific cell converters to operate in any of the above-mentioned operation modes for charging the capacitor 600.

[0045] 3 to 7 in real time, the number of times the semiconductor element switches to charge the capacitor 600 increases, resulting in increased switching loss. In response to this, by periodically determining whether the voltage value Edc is equal to or greater than the threshold value Vth, an excessive increase in switching loss can be suppressed.

[0046] The output voltage Vout in the third state in which all the semiconductor elements are off is determined by the polarity of the current i, and is −Edc when the polarity of the current i is positive, and is Edc when the polarity of the current i is negative (see, for example, FIG. 6). Therefore, depending on the timing of determining whether the voltage value Edc of the capacitor 600 increases or decreases, or depending on the length of the period of the third state, the polarity of the output voltage Vout in the state in which all the semiconductor elements are off may be biased toward either positive or negative. In other words, the output voltage Vout is −Edc or a voltage in which an offset voltage is superimposed on Edc. The offset voltage may cause a biased magnetization of the transformer, which may cause system disturbance. In general, an AC current i having the same frequency as that of the system 300 flows through the power conversion device 400.

[0047] Therefore, the GDU 502 switches all of the upper and lower arms to the off state for each half cycle of the AC flowing through the system 300 to which the multiple cell converters 52 are connected (i.e., half cycle of the current i) in accordance with a control signal from the control unit 401 (see FIG. 8). As a result, in one cycle of the current i, the output voltage Vout with all the semiconductor elements in the off state is cancelled out, and the offset voltage can be made close to zero.

[0048] For example, as shown in Fig. 8, the GDU502 sets the period during which the upper arm and the lower arm are all off to the same length (t1 = t2) in the first and second half cycles of the current i. This improves the accuracy of canceling the output voltage Vout when all the semiconductor elements are off, and makes it possible to bring the offset voltage close to zero with high accuracy.

[0049] 9, the GDU 502 drives the upper arm and the lower arm so that the absolute value of the effective voltage output from a pair of AC output terminals a, b in a half cycle of a current i coincides with the absolute value of the effective voltage Vout in the first half cycle of the current i during the first and second half cycles. That is, the control unit 401 controls the GDU 502 so that the absolute value of the effective value of the output voltage Vout in the first half cycle of the current i (effective voltage Vout1) coincides with the absolute value of the effective value of the output voltage Vout in the second half cycle of the current i (effective voltage Vout2). This improves the accuracy of offsetting the output voltage Vout in the third state in which all the semiconductor elements are off, and the offset voltage can be brought close to zero with high accuracy.

[0050] For example, the period when the output voltage Vout is zero voltage is mode X, the period in the third state in the first half cycle of the current i is mode Y, and the period in the third state in the second half cycle of the current i is mode Z. As shown in FIG. 10, the control unit 401 can derive the period T1 of the third state in mode Y by performing a PI adjustment calculation of the difference ΔE between the command value Er of the capacitor voltage of the capacitor 600 and the average value Ea of the detection value Edc by the PI adjuster 412. FIG. 10 illustrates a filter 411 that calculates the average value Ea by moving the detection value Edc of the capacitor voltage of the capacitor 600 in one cycle of the AC i, but the control unit 401 may calculate the average value Ea by another means such as a first-order lag filter. The control unit 401 has a function of limiting the lower limit of the PI adjustment calculation output value calculated by the PI adjuster 412, thereby preventing the polarity of the period T1 from becoming negative.

[0051] On the other hand, the control unit 401 may determine the length of the period T2 of the third state in mode Z so that the absolute value of the effective voltage value Vout1 in the period T1 is equal to the absolute value of the effective voltage value Vout2 in the period T2. For example, as shown in FIG. 11, the control unit 401 determines in the determination unit 415 whether or not the result of integrating the detection value Edc in the period T1 in mode Y by the integrator 413 matches the result of integrating the detection value Edc in the period T2 in mode Z by the integrator 414. The control unit 401 ends the period T2 (determines the end timing of the period T2) when both results match. The control unit 401 can determine the output voltage mode based on the pulse mode or the reference result of the output voltage Vout.

[0052] By controlling in this manner by the control unit 401, the GDU 502 drives the upper arm and the lower arm so that a first integral value, which is a value obtained by integrating the voltage value Edc during the third state when the upper arm and the lower arm are all off in the first half cycle of the current i, is equal to a second integral value, which is a value obtained by integrating the voltage value Edc during the third state when the upper arm and the lower arm are all off in the second half cycle of the current i. This improves the accuracy of canceling out the output voltage Vout in the third state when all the semiconductor elements are off, and the offset voltage can be brought close to zero with high accuracy.

[0053] Moreover, the capacitor 600 is charged in the third state in which the upper arm and the lower arm are all off. However, since the output voltage Vout in the third state becomes a disturbance to the control of the power conversion circuit 53, it is desirable to shorten the periods T1 and T2 during which the capacitor 600 is charged as much as possible. The amount of charge stored in the capacitor 600 is determined by the magnitude of the charging current to the capacitor 600. In other words, the periods T1 and T2 can be shortened by charging the capacitor 600 during a period in which the absolute value of the current flowing through the AC output terminals a and b is equal to or greater than a predetermined value.

[0054] For example, as shown in FIG. 12, the control unit 401 calculates the absolute value of the current value of the current i in the absolute value circuit 416 and stores the calculated absolute value in the memory 417. The control unit 401 compares the previous value stored in the memory 417 with the absolute value (current value) calculated this time in the absolute value circuit 416 by the comparator 418. If the previous value is smaller than the current value, the comparator 418 outputs a first judgment value (for example, 1) indicating that the previous value is smaller than the current value, and if the previous value is larger than the current value, the comparator 418 outputs a second judgment value (for example, 0) indicating that the previous value is larger than the current value. The timing of the maximum value or minimum value of the current i is when the first judgment value switches to the second judgment value or when the second judgment value switches to the first judgment value. Therefore, the control unit 401 may start the period T1 and the period T2 when it detects that the first judgment value switches to the second judgment value or when the second judgment value switches to the first judgment value.

[0055] By controlling in this manner by the control unit 401, the GDU 502 can turn off all of the upper and lower arms at a timing according to the value of the current i flowing through the AC output terminals a and b, thereby shortening the periods T1 and T2.

[0056] The control unit 401 may start the periods T1 and T2 when it detects that the absolute value of the current value of the current i has reached a predetermined value or more. The control unit 401 may end the periods T1 and T2 when it detects that the first judgment value has been switched to the second judgment value or that the second judgment value has been switched to the first judgment value. By controlling in this way by the control unit 401, the GDU 502 can turn off all of the upper and lower arms at a timing according to the value of the current i flowing through the AC output terminals a and b, thereby shortening the periods T1 and T2.

[0057] Fig. 13 is a diagram showing a configuration example of a power supply circuit. The power supply circuit 630 shown in Fig. 13 includes resistors 10a and 10b, a diode 10c, and a capacitance element 10d. The capacitance element 10d has a capacity sufficient to hold the power required for the GDU 502 to turn on the multiple switching elements 501, and is charged with power from a capacitor 600.

[0058] The method of feeding power to each GDU is not limited to the power feeding circuit 630 shown in Fig. 2. Fig. 14 is a diagram showing a second configuration example of a cell converter. The cell converter 52A shown in Fig. 14 includes power feeding circuits 21-24 provided for each GDU 502. Each of the power feeding circuits 21-24 is a circuit connected in parallel to the corresponding switching element 501, and receives power supply from the capacitor 600 via the element terminal of the corresponding switching element 501.

[0059] Thus, in this embodiment, the GDU 502 maintains the first state or the second state longer than the carrier period Tc of the power conversion circuit 53 based on the voltage value Edc of the capacitor 600 of its own cell converter among the multiple cell converters 52. As a result, the GDU 502 maintains the first state or the second state longer than the carrier period Tc of the power conversion circuit 53 by referring to the voltage value Edc, and can continuously output zero voltage taking into account the voltage value Edc.

[0060] Although the power conversion device has been described above by way of an embodiment, the present invention is not limited to the above embodiment. Various modifications and improvements, such as combinations or substitutions with part or all of other embodiments, are possible within the scope of the present invention. [Explanation of symbols]

[0061] 21~24 Power supply circuit 50, 50UV, 50VW, 50WU cluster 51, 51UV, 51VW, 51WU Reactor 52 Cell Converter 52UV1, 52UV2, 52UV x Cell Converter 52VW1, 52VW2, 52VW x Cell Converter 52WU1, 52WU2, 52WU x Cell Converter 53 Power Conversion Circuit 300 lines 400 Power Converter 401 Control section 501, 501a, 501b, 501c, 501d Switching elements 502, 502a, 502b, 502c, 502d GDU 600 Capacitor 630 Power Supply Circuit a,b AC output terminal

Claims

1. a plurality of cell converters each having a pair of AC output terminals and connected in series via the pair of AC output terminals; Each of the plurality of cell converters comprises: a capacitor; a power conversion circuit connected between the capacitor and the pair of AC output terminals; a drive circuit that drives the power conversion circuit; and a power supply circuit that supplies power to the drive circuit based on power from the capacitor; the power conversion circuit is a full-bridge circuit having a plurality of upper arms and a plurality of lower arms, the drive circuit maintains a first state in which the upper arm is on and the lower arm is off or a second state in which the upper arm is off and the lower arm is on for a period longer than a carrier period of the power conversion circuit based on a voltage value of the capacitor of the cell converter of the plurality of cell converters, The drive circuit turns off one of the upper arms when the voltage value drops below a predetermined voltage value in the first state, and turns off one of the lower arms when the voltage value drops below a predetermined voltage value in the second state.

2. a plurality of cell converters each having a pair of AC output terminals and connected in series via the pair of AC output terminals; Each of the plurality of cell converters comprises: a capacitor; a power conversion circuit connected between the capacitor and the pair of AC output terminals; a drive circuit that drives the power conversion circuit; and a power supply circuit that supplies power to the drive circuit based on power from the capacitor; the power conversion circuit is a full-bridge circuit having a plurality of upper arms and a plurality of lower arms, the drive circuit maintains a first state in which the upper arm is on and the lower arm is off or a second state in which the upper arm is off and the lower arm is on for a period longer than a carrier period of the power conversion circuit based on a voltage value of the capacitor of the cell converter of the plurality of cell converters, The drive circuit turns off both of the upper arms when the voltage value falls below a predetermined voltage value in the first state, and turns off both of the lower arms when the voltage value falls below a predetermined voltage value in the second state.

3. a plurality of cell converters each having a pair of AC output terminals and connected in series via the pair of AC output terminals; Each of the plurality of cell converters comprises: a capacitor; a power conversion circuit connected between the capacitor and the pair of AC output terminals; a drive circuit that drives the power conversion circuit; and a power supply circuit that supplies power to the drive circuit based on power from the capacitor; the power conversion circuit is a full-bridge circuit having a plurality of upper arms and a plurality of lower arms, the drive circuit maintains a first state in which the upper arm is on and the lower arm is off or a second state in which the upper arm is off and the lower arm is on for a period longer than a carrier period of the power conversion circuit based on a voltage value of the capacitor of the cell converter of the plurality of cell converters, The drive circuit includes: When a state before the upper arm and the lower arm are all turned off is the first state, when the upper arm and the lower arm are all turned off and the voltage value becomes equal to or greater than a predetermined voltage value, the state is switched to the second state; A power conversion device that switches to the first state when the state before the upper arm and the lower arm are all turned off is the second state and when the upper arm and the lower arm are all turned off and the voltage value becomes equal to or greater than a predetermined voltage value.

4. a plurality of cell converters each having a pair of AC output terminals and connected in series via the pair of AC output terminals; Each of the plurality of cell converters comprises: a capacitor; a power conversion circuit connected between the capacitor and the pair of AC output terminals; a drive circuit that drives the power conversion circuit; and a power supply circuit that supplies power to the drive circuit based on power from the capacitor; the power conversion circuit is a full-bridge circuit having a plurality of upper arms and a plurality of lower arms, the drive circuit maintains a first state in which the upper arm is on and the lower arm is off or a second state in which the upper arm is off and the lower arm is on for a period longer than a carrier period of the power conversion circuit based on a voltage value of the capacitor of the cell converter of the plurality of cell converters, The drive circuit switches all of the upper arms and the lower arms to an off state for each half cycle of an alternating current flowing through a system to which the multiple cell converters are connected.

5. a plurality of cell converters each having a pair of AC output terminals and connected in series via the pair of AC output terminals; Each of the plurality of cell converters comprises: a capacitor; a power conversion circuit connected between the capacitor and the pair of AC output terminals; a drive circuit that drives the power conversion circuit; and a power supply circuit that supplies power to the drive circuit based on power from the capacitor; the power conversion circuit is a full-bridge circuit having a plurality of upper arms and a plurality of lower arms, the drive circuit maintains a first state in which the upper arm is on and the lower arm is off or a second state in which the upper arm is off and the lower arm is on for a period longer than a carrier period of the power conversion circuit based on a voltage value of the capacitor of the cell converter of the plurality of cell converters, A power conversion device that, when the voltage values ​​of the capacitors of each of the multiple cell converters are all normal, selects one or more specific cell converters from the multiple cell converters, and maintains the first state or the second state of the selected specific cell converter for a period longer than the carrier period.

6. The power conversion device according to claim 1 , wherein the drive circuit maintains the first state or the second state for a period longer than the carrier period when the voltage value is equal to or higher than a predetermined voltage value.

7. The power conversion device according to claim 1 , wherein the drive circuit alternately switches the one arm from on to off when the voltage value falls below the predetermined voltage value.

8. The power conversion device according to claim 4 , wherein the drive circuit sets a period during which the upper arm and the lower arm are all in an off state to have the same length in a first half cycle and a second half cycle of the AC.

9. 5. The power conversion device according to claim 4, wherein the drive circuit drives the upper arm and the lower arm so that an absolute value of an effective voltage output from the pair of AC output terminals in a half cycle of the AC is equal to an absolute value in a first half cycle of the AC and an absolute value in a second half cycle of the AC.

10. 10. The power conversion device according to claim 4 or 9, wherein the drive circuit drives the upper arm and the lower arm so that a result of integrating the voltage value during a period in which the upper arm and the lower arm are all off in a first half cycle of the AC is equal to a result of integrating the voltage value during a period in which the upper arm and the lower arm are all off in a second half cycle of the AC.

11. The power conversion device according to claim 4 , wherein the drive circuit turns off all of the upper arms and the lower arms at a timing according to a value of a current flowing through the AC output terminal.

12. a control unit that selects one or more cell converters from among the plurality of cell converters based on a voltage value of each of the capacitors of the plurality of cell converters; The power conversion device according to claim 1 , wherein the control unit causes the first state or the second state of the selected cell converter to be maintained for a period longer than the carrier period.

13. The power conversion device according to claim 12, wherein when the voltage values ​​of the capacitors of the plurality of cell converters are all normal, the control unit selects one or more specific cell converters from the plurality of cell converters and maintains the first state or the second state of the selected specific cell converter for a period longer than the carrier period.

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