Power Conversion Equipment

Abstract

This power conversion device (1) includes a plurality of unit converters (5) connected in series, and includes arms (A1-A3) connected to an AC power source (8), and a control device (7) that controls the arms. Each unit converter includes a capacitor (15). The control device is provided corresponding to each unit converter, and includes an inspection unit (53) that inspects the corresponding capacitor. When the voltage (VPP) of the AC component (Vac) included in the terminal voltage (VDC) of the corresponding capacitor is higher than a threshold voltage (Vt), the inspection unit determines that the corresponding capacitor needs to be replaced with a new one. Therefore, it is possible to determine whether or not each of the plurality of capacitors needs to be replaced with a new one while the power conversion device is operating.

Description

[Technical field]

[0001] The present disclosure relates to a power conversion device, and more particularly to a power conversion device including a plurality of unit converters connected in series. [Background technology]

[0002] For example, International Publication No. 2018 / 193606 (Patent Document 1) discloses a power conversion device including a power converter connected to an AC power source and a control device that controls the power converter. The power converter includes a plurality of unit converters connected in series. Each unit converter includes a capacitor that stores DC power, a rectifier element that converts AC power supplied from the AC power source into DC power and supplies it to the capacitor, and a switching element that converts the DC power of the capacitor into AC power and supplies it to the AC power source. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2018 / 193606 Summary of the Invention [Problem to be solved by the invention]

[0004] However, conventional power conversion devices have a problem in that the multiple capacitors included in the multiple unit converters deteriorate over time, resulting in a decrease in device performance.

[0005] One solution to this problem is to periodically stop the operation of the power conversion device, measure the capacitance of each capacitor, and replace deteriorated capacitors with new ones. However, this method has the problem that it takes a long time to measure the capacitance of multiple capacitors, which results in a long downtime of the device.

[0006] Therefore, a primary object of the present disclosure is to provide a power conversion device capable of determining whether or not each of a plurality of capacitors included in a plurality of unit converters needs to be replaced with a new one while the power conversion device is operating. [Means for solving the problem]

[0007] The power conversion device of the present disclosure includes a power converter and a control device. The power converter includes a plurality of unit converters connected in series and is connected to an AC power source. The control device transmits a control signal to each of the plurality of unit converters.

[0008] Each unit converter includes a main circuit, a voltage sensor, and a drive circuit. The main circuit includes a capacitor that stores DC power, a rectifier element that converts AC power supplied from an AC power source into DC power and supplies it to the capacitor, and a switching element that converts the DC power of the capacitor into AC power and supplies it to the AC power source. The voltage sensor detects the voltage between the terminals of the capacitor. The drive circuit drives the switching element in accordance with a control signal. The voltage between the terminals of the capacitor includes a DC component and an AC component.

[0009] The control device includes a control unit and an inspection unit. The control unit generates a control signal so that the terminal voltage of the capacitor detected by the voltage sensor becomes a reference DC voltage and the AC output voltage of the power converter becomes a reference AC voltage. The inspection unit is provided corresponding to each unit converter and inspects the capacitor included in the corresponding unit converter. The inspection unit includes a determination unit that determines that the corresponding capacitor needs to be replaced with a new one when the voltage of the AC component included in the terminal voltage of the corresponding capacitor is higher than a threshold voltage. Effect of the Invention

[0010] In the power conversion device of the present disclosure, an inspection unit is provided corresponding to each unit converter, and this inspection unit determines that the corresponding capacitor needs to be replaced with a new one when the voltage of the AC component included in the terminal voltage of the corresponding capacitor is higher than a threshold voltage. Therefore, while operating the power conversion device, it is possible to determine whether each of the multiple capacitors included in the multiple unit converters needs to be replaced with a new one. [Brief description of the drawings]

[0011] [Figure 1] 1 is a circuit block diagram showing a configuration of a power conversion device according to an embodiment of the present disclosure. [Diagram 2] 2 is a circuit block diagram showing a configuration of a unit converter shown in FIG. 1. [Diagram 3] 3 is a time chart showing a waveform of a voltage between terminals of the capacitor shown in FIG. 2. [Figure 4] 3 is another time chart showing the waveform of the voltage between the terminals of the capacitor shown in FIG. 2. [Diagram 5] FIG. 2 is a block diagram showing a configuration of a control device shown in FIG. [Figure 6] FIG. 6 is a block diagram showing a configuration of a control circuit shown in FIG. 5. [Figure 7] 3 is a circuit block diagram for explaining the initial charging of the capacitor shown in FIG. 2. FIG. [Figure 8] 7 is a block diagram showing the configuration of an inspection unit shown in FIG. 6. [Figure 9] 9 is a time chart for explaining the operation of the estimation unit shown in FIG. 8. [Figure 10] 4 is a flowchart showing a method for inspecting a capacitor. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Fig. 1 is a circuit block diagram showing a configuration of a power conversion device 1 according to an embodiment of the present disclosure. In Fig. 1, the power conversion device 1 is connected to power transmission lines 9u, 9v, and 9w for supplying three-phase AC power from a commercial AC power source 8 to a load, and is used as a reactive power compensator that compensates for reactive power generated by the load or the like.

[0013] This power conversion device 1 includes switches S1 to S6, transformers 2, 3, current-limiting resistors R1 to R3, AC lines UL, VL, WL, current transformers C1 to C3, reactors L1 to L3, arms A1 to A3, an operation unit 6, and a control device .

[0014] One terminal of each of the switches S1 to S3 is connected to the power transmission lines 9u, 9v, 9w, and the other terminals are connected to the three primary windings of the transformer 2. The switches S1 to S3 are controlled by the control device 7, and are turned on during normal operation and turned off during inspection and maintenance of the power conversion device 1. The transformer 2 includes three primary windings and three secondary windings, and supplies and receives three-phase AC power.

[0015] One terminal of each of the current-limiting resistors R1 to R3 is connected to the three secondary windings of the transformer 2, and the other terminal is connected to one end of each of the AC lines UL, VL, and WL. The current-limiting resistors R1 to R3 limit the current flowing from the commercial AC power supply 8 to each of the arms A1 to A3 when the power conversion device 1 is started up.

[0016] The switches S4 to S6 are connected in parallel to the current-limiting resistors R1 to R3, respectively. The switches S4 to S6 are controlled by the control device 7, and are turned on after the currents flowing through the arms A1 to A3 are stabilized when the power conversion device 1 is started up. The transformer 3 feeds back to the control device 7 three-phase AC voltages Vu, Vv, and Vw having values ​​corresponding to the AC voltages of the AC lines UL, VL, and WL.

[0017] The reactor L1 and the arm A1 are connected in series between the AC line UL and the AC line VL. The reactor L2 and the arm A2 are connected in series between the AC line VL and the AC line WL. The reactor L3 and the arm A3 are connected in series between the AC line WL and the AC line UL. That is, the arms A1 to A3 are delta connected. The arms A1 to A3 (power converters) are controlled by the control device 7 and generate three-phase AC power.

[0018] Each of the arms A1 to A3 includes N unit converters 5 connected in series. Each of the N unit converters 5 generates AC power in accordance with a control signal from the control device 7. N is an integer of 2 or more, for example, 60.

[0019] A first terminal 5a of the first-stage unit converter 5 of arm A1 is connected to one terminal of reactor L1. In arm A1, second terminals 5b of each unit converter 5 except the last-stage unit converter 5 are connected to the first terminals 5a of the adjacent unit converter 5. A second terminal 5b of the last-stage unit converter 5 of arm A1 is connected to one terminal of reactor L2.

[0020] The first terminal 5a of the first-stage unit converter 5 of arm A2 is connected to the other terminal of reactor L2. In arm A2, the second terminal 5b of each unit converter 5 except the last-stage unit converter 5 is connected to the first terminal 5a of the adjacent unit converter 5. The second terminal 5b of the last-stage unit converter 5 of arm A2 is connected to one terminal of reactor L3.

[0021] The first terminal 5a of the first-stage unit converter 5 of arm A3 is connected to the other terminal of reactor L3. In arm A3, the second terminal 5b of each unit converter 5 except the last-stage unit converter 5 is connected to the first terminal 5a of the adjacent unit converter 5. The second terminal 5b of the last-stage unit converter 5 of arm A3 is connected to the other terminal of reactor L1.

[0022] The reactors L1 to L3 suppress the circulating currents flowing through the arms A1 to A3, respectively. The reactors L1 to L3 may be provided separately from the arms A1 to A3, or may be inductance components of the arms A1 to A3. The current transformers C1 to C3 detect the AC currents Iuv, Ivw, Iwu flowing through the arms A1 to A3, respectively, and feed them back to the control device 7.

[0023] The operation unit 6 includes a plurality of buttons operated by a user of the power conversion device 1, an image display unit that displays various information, and the like. By operating the operation unit 6, the user can set various information and start and stop the power conversion device 1. The operation unit 6 outputs a signal indicating the operation content to the control device 7.

[0024] The control device 7 turns on and off the switches S1 to S3 in accordance with signals from the operation unit 6. The control device 7 also controls each of the three arms A1 to A3 (i.e., each of the 3×N unit converters 5) by generating control signals GC, GB, and on command signal Son, which will be described later, based on a reactive power command value Qr, three-phase AC voltages Vu, Vv, and Vw, three-phase AC currents Iuv, Ivw, and Iwu, a DC voltage VDC, which will be described later, and the like.

[0025] The reactive power command value Qr is given, for example, from a central control room (not shown) of a power system including a commercial AC power source 8. The power conversion device 1 supplies reactive power of a value according to the reactive power command value Qr to the power transmission lines 9u, 9v, and 9w.

[0026] Fig. 2 is a circuit block diagram showing the configuration of the unit converter 5 shown in Fig. 1. In Fig. 2, the unit converter 5 includes a main circuit 10, a resistive element R4, a power source 20, and a control circuit 30. The main circuit 10 is formed of a full-bridge circuit equipped with a capacitor.

[0027] Specifically, the main circuit 10 includes switching elements 11-14, diodes (rectifier elements) D1-D4, and a capacitor 15. The main circuit 10 converts DC power into AC power by outputting a voltage pulse having an amplitude corresponding to the inter-terminal voltage VDC of the capacitor 15 between the first terminal 5a and the second terminal 5b by turning on and off the switching elements 11-14.

[0028] Each of the switching elements 11 to 14 is a self-extinguishing power semiconductor element, and is formed of, for example, an IGBT (Insulated Gate Bipolar Transistor). The switching elements 11 and 13 are connected in series between the DC line PL and the DC line NL. The switching elements 12 and 14 are connected in series between the DC line PL and the DC line NL. The collectors of the switching elements 11 and 12 are both connected to the DC line PL, and the emitters of the switching elements 13 and 14 are both connected to the DC line NL. The connection point between the emitter of the switching element 11 and the collector of the switching element 13 is connected to the first terminal 5a. The connection point between the emitter of the switching element 12 and the collector of the switching element 14 is connected to the second terminal 5b.

[0029] The diodes D1 to D4 are connected in anti-parallel to the switching elements 11 to 14. The capacitor 15 is connected between the DC line PL and the DC line NL, and stores DC power.

[0030] The conductive states (on and off) of the switching elements 11 to 14 are controlled by a control circuit 30. The switching elements 11 and 13 are turned on and off in a complementary manner. The switching elements 12 and 14 are turned on and off in a complementary manner. As shown in FIG. 2, if the voltage from the second terminal 5b to the first terminal 5a is defined as a cell voltage Vcell, the cell voltage Vcell is controlled by turning on and off each of the switching elements 11 to 14.

[0031] Specifically, when switching elements 11 and 14 are both on and switching elements 12 and 13 are both off, the cell voltage Vcell is equal to the terminal voltage VDC of capacitor 15. When switching elements 11 and 12 are both on and switching elements 13 and 14 are both off, the cell voltage Vcell is 0V. When switching elements 11 and 12 are both off and switching elements 13 and 14 are both on, the cell voltage Vcell is 0V. When switching elements 11 and 14 are both off and switching elements 12 and 13 are both on, the cell voltage Vcell is equal to a voltage obtained by inverting the polarity of the terminal voltage VDC of capacitor 15.

[0032] 2, an example in which the main circuit 10 is configured as a full-bridge circuit has been described, but the present invention is not limited to this. For example, the main circuit 10 may be configured as a half-bridge circuit.

[0033] The total voltage of each of the arms A1 to A3 is the sum of the cell voltages Vcell of the N unit converters 5 included in each of the arms A1 to A3. Therefore, by controlling the on / off states of each of the switching elements 11 to 14 of each unit converter 5, the total voltage of each of the arms A1 to A3 can be controlled.

[0034] The main circuit 10 further includes a switch S7. The switch S7 is connected between the first terminal 5a and the second terminal 5b. The switch S7 is configured to be able to short-circuit the first terminal 5a and the second terminal 5b by closing in response to a command from the control circuit 30.

[0035] The resistive element R4 is connected between the DC line PL and the input terminal 20a of the power source 20. The resistive element R4 steps down the inter-terminal voltage VDC of the capacitor 15 and supplies it to the power source 20, and also limits the current flowing from the capacitor 15 to the power source 20.

[0036] An input terminal 20b of the power supply 20 is connected to the DC line NL. The power supply 20 further steps down the DC voltage Vin supplied from the capacitor 15 via a resistor element R4 to generate a power supply voltage for the control circuit 30. That is, each unit converter 5 forms a self-contained cell capable of supplying power from the main circuit 10 to the control circuit 30.

[0037] The control circuit 30 includes a communication circuit 31, drive circuits 32 and 33, a switch operation circuit 34, a voltage sensor 35, and a peak value sensor 36. The control circuit 30 is driven by a power supply voltage supplied from the power supply 20, and transmits and receives signals to and from the control device 7 and controls the main circuit 10.

[0038] The communication circuit 31 communicates with the control device 7 via a communication line (not shown). That is, the communication circuit 31 receives a control signal GC for controlling the switching elements 11-14 and a control signal GB for turning off all of the switching elements 11-14, and provides the received control signals GC and GB to the drive circuits 32 and 33. The communication circuit 31 also receives an on-command signal Son for turning on the switch S7, and provides the received on-command signal Son to the switch operation circuit 34.

[0039] The drive circuit 32 turns on or off each of the switching elements 11 and 13 in response to the control signal GC, and turns off the switching elements 11 and 13 in response to the control signal GB.

[0040] The drive circuit 33 turns on or off each of the switching elements 12 and 14 in response to the control signal GC, and turns off the switching elements 12 and 14 in response to the control signal GB.

[0041] In response to the on-command signal Son, the switch operation circuit 34 supplies current to the exciting coil 18 to turn on the switch S7. During normal operation, the supply of current to the exciting coil 18 is stopped, and therefore the switch S7 is in the off (open) state.

[0042] The voltage sensor 35 detects the instantaneous value of the voltage VDC across the terminals of the capacitor 15, and outputs a signal indicating the detected value to the communication circuit 31. The communication circuit 31 transmits a signal indicating the detected value of the voltage VDC across the terminals of the capacitor 15 to the control device 7.

[0043] Fig. 3 is a time chart showing the waveform of the terminal voltage VDC of capacitor 15. In Fig. 3, (A) shows the waveform of the terminal voltage VDC of capacitor 15, and (B) is an enlarged view of part α of (A). Fig. 3 shows a case where the capacitance value of capacitor 15 is relatively large, that is, a case where the deterioration level of capacitor 15 is small.

[0044] The control device 7 generates the control signal GC so that the terminal voltage VDC of the capacitor 15 becomes the reference DC voltage VDR. When the voltage VDC is lower than the reference DC voltage VDR, the control device 7 generates the control signal GC so that the voltage VDC increases, and when the voltage VDC is higher than the reference DC voltage VDR, the control device 7 generates the control signal GC so that the voltage VDC decreases.

[0045] Therefore, the voltage VDC does not have a constant value, but oscillates between a minimum value VL lower than the reference DC voltage VDR and a maximum value VH higher than the reference DC voltage VDR. In other words, the voltage VDC includes a DC component Vdc and an AC component Vac. The DC component Vdc is equal to the reference DC voltage VDR. The AC component Vac is called a ripple voltage and is superimposed on the DC component Vdc.

[0046] If the difference voltage VPP=VH−VL between the maximum value VH and the minimum value VL of the voltage VDC is the voltage of the AC component Vac, then the voltage VPP of the AC component Vac changes depending on the capacitance of the capacitor 15, that is, the deterioration level of the capacitor 15.

[0047] Fig. 4 is another time chart showing the waveform of the terminal voltage VDC of capacitor 15, and is to be compared with Fig. 3. In Fig. 4, (A) shows the waveform of the terminal voltage VDC of capacitor 15, and (B) is an enlarged view of the β portion of (A). Fig. 4 shows a case where the capacitance value of capacitor 15 is relatively small, that is, a case where the deterioration level of capacitor 15 is large.

[0048] The voltage VPP=VH-VL of the AC component Vac in Fig. 4 is higher than the voltage VPP=VH-VL of the AC component Vac in Fig. 3. When the degradation level of the capacitor 15 increases, the capacitance value of the capacitor 15 decreases and the voltage VPP of the AC component Vac increases. Therefore, the degradation level of the capacitor 15 can be determined from the voltage VPP of the AC component Vac.

[0049] 2 again, the peak value sensor 36 detects the maximum value VH and minimum value VL of the inter-terminal voltage VDC of the capacitor 15, and outputs signals indicative of these detected values ​​to the communication circuit 31. The communication circuit 31 transmits signals indicative of the maximum value VH and minimum value VL to the control device 7.

[0050] The peak value sensor 36 may detect the maximum value VH and the minimum value VL directly from the capacitor 15, or may detect them from the output signal of the voltage sensor 35. The peak value sensor 36 may also extract the AC component Vac from the terminal voltage VDC of the capacitor 15 using a high-pass filter, detect the positive peak value and the negative peak value of the extracted AC component Vac, and set the detected positive peak value and negative peak value as the maximum value VH and the minimum value VL, respectively.

[0051] Fig. 5 is a block diagram showing the configuration of the control device 7. In Fig. 5, the control device 7 includes a power source 40, a communication circuit 41, and a control circuit 42. When the power conversion device 1 is connected to the power transmission lines 9u, 9v, and 9w (Fig. 1), the power source 40 constantly generates a power supply voltage VC11 for the communication circuit 41 and a power supply voltage VC12 for the control circuit 42. Therefore, the communication circuit 41 and the control circuit 42 are operable even when the power conversion device 1 is stopped.

[0052] The communication circuit 41 is driven by a power supply voltage VC11 supplied from a power supply 40, and transmits and receives signals between the 3×N unit converters 5 included in the three arms A1-A3 (FIG. 1) and a control circuit 42. That is, the communication circuit 41 receives a signal indicating the inter-terminal voltage VDC of the capacitor 15 transmitted from each unit converter 5, a signal indicating a maximum value VH of the voltage VDC, and a signal indicating a minimum value VL of the voltage VDC, and provides these received signals to the control circuit 42. The communication circuit 41 also transmits 3×N sets of signals GC, GB, Son generated by the control circuit 42 to each of the 3×N unit converters 5.

[0053] Fig. 6 is a block diagram showing the configuration of the control circuit 42. In Fig. 6, the control circuit 42 includes control units 50, 51, a timer unit 52, 3 × N inspection units 53, and an image display unit 54. The control unit 50 controls the switches S1 to S6 in accordance with a signal from the operation unit 6. That is, when the user of the power conversion device 1 operates the operation unit 6 (Fig. 1) and instructs the start-up of the power conversion device 1, the operation unit 6 outputs a signal commanding the start-up of the power conversion device 1.

[0054] In response to the signal, the control unit 50 turns on the switches S1 to S3. When the switches S1 to S3 are turned on, AC power is supplied from the commercial AC power source 8 to the arms A1 to A3 via the switches S1 to S3, the transformer 2, the current-limiting resistors R1 to R3, and the reactors L1 to L3, and initial charging of the capacitor 15 begins.

[0055] Fig. 7 is a circuit block diagram for explaining the initial charging of the capacitor 15. Fig. 7 shows two adjacent unit converters 5. The unit converter 5 on the left side in Fig. 7 is the unit converter 5 in the first stage of the arm A1, and the unit converter 5 on the right side in Fig. 7 is the unit converter 5 in the second stage of the arm A1. During the initial charging, the switch S7 is opened, and the switching elements 11 to 14 are turned off.

[0056] When the AC voltage Vu of the AC line UL (Fig. 1) is higher than the AC voltage Vv of the AC line VL, a current Ic flows through a path from the first terminal 5a of the left-side unit converter 5 to the second terminal 5b via diode D1, capacitor 15, and diode D4, as shown by the arrow in Fig. 7. This current Ic charges capacitor 15, and the inter-terminal voltage VDC of capacitor 15 rises.

[0057] This current Ic flows from the second terminal 5b of the left unit converter 5 to the first terminal 5a of the right unit converter 5, and flows to the second terminal 5b via diode D1, capacitor 15, and diode D4. This current Ic charges capacitor 15 of the right unit converter 5, and the terminal voltage VDC of capacitor 15 rises.

[0058] Conversely, when the AC voltage Vu of the AC line UL is lower than the AC voltage Vv of the AC line VL, a current Ic flows through a path from the second terminal 5b of the right-side unit converter 5 to the first terminal 5a via the diode D2, the capacitor 15, and the diode D3. The current Ic charges the capacitor 15, and the inter-terminal voltage VDC of the capacitor 15 rises.

[0059] This current Ic flows from the first terminal 5a of the unit converter 5 on the right side to the second terminal 5b of the unit converter 5 on the left side, and then flows through a path to the first terminal 5a via diode D2, capacitor 15, and diode D3. This current Ic charges capacitor 15 of the unit converter 5 on the left side, and the inter-terminal voltage VDC of capacitor 15 rises. In this way, capacitors 15 of all unit converters 5 in arms A1 to A3 are charged.

[0060] Since N unit converters 5 are connected in series in each of arms A1 to A3, the voltage VDC of capacitor 15 of each unit converter 5 rises to a value obtained by dividing the peak value of the voltage (inter-phase voltages Vuv, Vvw, Vwu) applied to each of arms A1 to A3 by the number N of unit converters 5 included in each of arms A1 to A3.

[0061] 6, when the initial charging of capacitor 15 is completed, control unit 50 turns on switches S4 to S6, whereby terminals of current-limiting resistors R1 to R3 (FIG. 1) are short-circuited, and the three secondary windings of transformer 2 are directly connected to AC lines UL, VL, and WL, respectively.

[0062] Furthermore, when a user of the power conversion device 1 operates the operation unit 6 (FIG. 1) to instruct the power conversion device 1 to be stopped, the operation unit 6 outputs a signal commanding the stopping of the power conversion device 1. In response to the signal, the control unit 50 turns off the switches S1 to S6. This stops the power supply from the commercial AC power source 8 to the arms A1 to A3.

[0063] In addition, when the initial charging of capacitor 15 is completed, control unit 51 generates 3×N sets of control signals GC, GB based on the three-phase AC voltages Vu, Vv, Vw from transformer 3, the three-phase AC currents Iuv, Ivw, Iwu detected by current transformers C1 to C3, and signals indicating 3×N DC voltages VDC provided from the 3×N unit converters 5 via communication circuit 41, and outputs the signals to communication circuit 41.

[0064] That is, the control unit 51 determines three-phase AC currents Iu, Iv, Iw at levels corresponding to the AC currents flowing through the AC lines UL, VL, WL, based on the AC currents Iuv, Ivw, Iwu from the current transformers C1 to C3, where Iu=Iuv-Iwu, Iv=Ivw-Iuv, and Iw=Iwu-Ivw.

[0065] The control unit 51 calculates reactive power Q based on the three-phase AC voltages Vu, Vv, Vw from the transformer 3 and the above-mentioned three-phase AC currents Iu, Iv, Iw, and calculates the deviation ΔQ=Qr−Q between the reactive power command value Qr and the reactive power Q.

[0066] The control unit 51 generates 3×N reference DC voltages VDR corresponding to the 3×N unit converters 5 based on the AC currents Iuv, Ivw, Iwu from the current transformers C1 to C3, the three-phase AC voltages Vu, Vv, Vw from the transformer 3, etc.

[0067] The control unit 51 obtains a deviation ΔVDC between each reference DC voltage VDR and the corresponding DC voltage VDC. The control unit 51 generates three-phase reference AC voltages Vuvr, Vvwr, and Vwur so that the integral value of the voltage deviation ΔVDC becomes zero and the integral value of the reactive power deviation ΔQ becomes zero.

[0068] In other words, the control unit 51 controls the active current of each unit converter 5 so that the integral value of the voltage deviation ΔVDC becomes 0, and also controls the reactive current of each unit converter 5 so that the integral value of the reactive power deviation ΔQ becomes 0. Based on the three-phase reference AC voltages Vuvr, Vvwr, Vwur, each unit converter 5 of arms A1 to A3 is operated, the DC voltage VDC is set to the reference DC voltage VDR, and the reactive power Q is set to the reactive power command value Qr.

[0069] Specifically, the control unit 51 generates 3×N sets of control signals GC, GB so that the three-phase AC voltages Vuv, Vvw, Vwu output from the arms A1 to A3 become the three-phase reference AC voltages Vuvr, Vvwr, Vwur, for example, according to PWM (Pulse Width Modulation) control.

[0070] The communication circuit 41 transmits 3×N sets of control signals GC, GB generated by the control unit 51 to the 3×N unit converters 5 included in the arms A1-A3. In each unit converter 5, the control signals GC, GB are received by the communication circuit 31 and provided to the drive circuits 32, 33. The drive circuits 32, 33 turn the switching elements 11-14 on and off in accordance with the control signals GC, GB. This converts the inter-terminal voltage VDC of the capacitor 15 into an AC voltage.

[0071] Furthermore, when the control unit 51 detects an abnormality, such as a short-circuit failure of a switching element, in any of the 3×N unit converters 5 based on the 3×N voltages VDC or the like, it outputs an on-command signal Son for the switch S7 to the failed unit converter 5. This on-command signal Son is provided to the switch operating circuit 34 of the failed unit converter 5 via the communication circuits 41, 31.

[0072] In response to the on-command signal Son, the switch operation circuit 34 supplies current to the exciting coil 18 to turn on the switch S7. This shorts the first terminal 5a and the second terminal 5b of the faulty unit converter 5. In addition, information identifying the faulty unit converter 5 is displayed on, for example, the image display unit of the operation unit 6.

[0073] Furthermore, when a user of the power conversion device 1 operates the operation unit 6 (FIG. 1) to instruct the power conversion device 1 to be stopped, the operation unit 6 outputs a signal commanding the stop of the power conversion device 1. In response to the signal, the operation of the unit converter 5 by the control unit 51 is stopped, and the switching elements 11 to 14 of all the unit converters 5 are turned off.

[0074] The timer unit 52 measures the time during which the unit converter 5 is operated by the control unit 51, and outputs a signal φ52 based on the measurement result. The signal φ52 is usually set to an inactivation level of "L". The signal φ52 is set to an activation level of "H" for a predetermined period of time at a predetermined cycle (for example, one year). The signal φ52 is provided to each inspection unit 53.

[0075] Furthermore, the inspection unit 53 inspects the corresponding capacitor 15 according to the signal indicating the maximum value VH and the signal indicating the minimum value VL from the corresponding unit converter 5 and the signal φ52 from the timer unit 52.

[0076] Fig. 8 is a block diagram showing the configuration of inspection unit 53. In Fig. 8, inspection unit 53 includes a calculation unit 61, a judgment unit 62, a storage unit 63, and an estimation unit 64. The calculation unit 61 finds a differential voltage VPP=VH-VL between a maximum value VH and a minimum value VL indicated by a signal from a corresponding unit converter 5, and provides a signal DVPP indicating the found voltage VPP to the judgment unit 62 and the storage unit 63.

[0077] The determination unit 62 compares the voltage VPP indicated by the signal DVPP from the calculation unit 61 with a threshold voltage Vt, and outputs a signal φ62 indicating the comparison result. When the voltage VPP is lower than the threshold voltage Vt, the signal φ62 is set to an "L" level. When the voltage VPP is higher than the threshold voltage Vt, the signal φ62 is set to an "H" level. The threshold voltage Vt is previously determined by an experiment.

[0078] The signal φ62 at the "L" level indicates that the deterioration level of the corresponding capacitor 15 is small and that there is no need to replace the corresponding capacitor 15 with a new one. The signal φ53 at the "H" level indicates that the deterioration level of the corresponding capacitor 15 is large and that the corresponding capacitor 15 needs to be replaced with a new one. The signal φ62 is provided to the estimation unit 64 and the image display unit 54.

[0079] The storage unit 63 judges whether the output signal φ52 of the timer unit 52 (FIG. 6) is at the “H” level. When the signal φ52 is at the “H” level, the storage unit 63 takes in the output signal DVPP of the arithmetic unit 61 and stores the taken-in signal DVPP. When the signal φ52 is at the “L” level, the storage unit 63 does not store the output signal DVPP of the arithmetic unit 61. The storage unit 63 sequentially stores a plurality of signals DVPP.

[0080] When the output signal φ62 of the judgment unit 62 is at the “L” level, i.e., when it is determined that the corresponding capacitor 15 does not need to be replaced with a new one, the estimation unit 64 estimates the time to replace the capacitor 15 based on the multiple signals DVPP stored in the memory unit 63, and outputs a signal indicating the estimation result.

[0081] Fig. 9 is a diagram for explaining the operation of the estimation unit 64. It is assumed that the output signal φ52 of the timer unit 52 is set to the "H" level m times, and m signals DVPP are stored in the storage unit 63. m is an integer equal to or greater than 2. Fig. 9 shows the case where m=3.

[0082] The estimation unit 64 reads out m signals DVPP from the storage unit 63, and converts the read out m signals DVPP into voltages VPP1-VPPm of the AC component Vac. Next, the estimation unit 64 writes the voltages VPP1-VPPm into Fig. 9, in which the horizontal axis indicates time and the vertical axis indicates voltage VPP.

[0083] Next, the estimation unit 64 draws an approximation curve B1 passing through the voltages VPP1 to VPPm, and obtains the time tX at which the curve B1 exceeds the threshold voltage Vt. The estimation unit 64 estimates that when the operation time of the power conversion device 1 reaches tX, it is time to replace the capacitor 15 with a new one. The estimation unit 64 outputs a signal φ65 indicating the estimation result to the image display unit 54.

[0084] The image display unit 54 displays the determination result indicated by the output signal φ62 of the determination unit 62 and the estimation result indicated by the output signal φ65 of the estimation unit 64. The image display unit 54 displays the determination result and the estimation result for each of the 3×N capacitors 15.

[0085] Note that information identifying the capacitors 15 that have been determined to need to be replaced with new ones, or information identifying the capacitors 15 that are estimated to soon need to be replaced, may be displayed on the image display unit 54. Furthermore, the image display unit 54 may be included in the operation unit 6 (FIG. 1).

[0086] Fig. 10 is a flowchart showing a method for inspecting capacitor 15. In step ST1, peak value sensor 36 (Fig. 2) detects maximum value VH and minimum value VL of inter-terminal voltage VDC of capacitor 15. Signals indicating maximum value VH and minimum value VL are transmitted to calculation unit 61 (Fig. 8) of inspection unit 53 via communication circuits 31, 41 (Figs. 2 and 5).

[0087] In step ST2, the calculation unit 61 calculates the difference voltage VPP=VH-VL between the maximum value VH and the minimum value VL. In step ST3, the storage unit 63 (FIG. 8) determines whether the output signal φ52 of the timer unit 52 (FIG. 6) is at the "H" level. The signal φ52 is a signal that is set to the "H" level for a predetermined period of time at a predetermined cycle.

[0088] If the signal φ52 is at the “H” level, in step ST4, the signal DVPP indicating the differential voltage VPP is stored in the storage unit 63. If the signal φ52 is not at the “H” level, in step ST5, the determination unit 62 determines whether or not the voltage VPP indicated by the signal DVPP is higher than the threshold voltage Vt.

[0089] If VPP>Vt, in step ST6, the signal φ62 is set to the "H" level by the determination unit 62. The "H" level signal φ62 indicates that the corresponding capacitor 15 needs to be replaced with a new one.

[0090] If VPP>Vt is not true, in step ST7, the signal φ62 is set to the "L" level by the determination unit 62. The "L" level signal φ62 indicates that the corresponding capacitor 15 does not need to be replaced with a new one.

[0091] In step ST8, the estimation unit 64 estimates the time to replace the corresponding capacitor 15 with a new one based on the multiple signals DVPP stored in the storage unit 63. In step ST9, the determination result of the determination unit 62 and the estimation result of the estimation unit 64 are displayed by the image display unit 54. Steps ST1 to ST9 are repeatedly executed.

[0092] The user of the power conversion device 1 determines whether or not to replace the capacitor 15 based on the display result of the image display unit 54. That is, if it is determined that the capacitor 15 should be replaced with a new one, the user stops the power conversion device 1 and replaces the capacitor 15 with a new one. Also, if the user estimates the time when the capacitor 15 should be replaced with a new one, the user prepares a new capacitor 15 that will be required at that time.

[0093] As described above, in this embodiment, an inspection unit 53 is provided corresponding to each unit converter 5, and this inspection unit 53 determines that the corresponding capacitor 15 needs to be replaced with a new one when the voltage VPP of the AC component Vac included in the terminal voltage VDC of the corresponding capacitor 15 is higher than the threshold voltage Vt. Therefore, while the power conversion device 1 is operating, it can be determined whether or not each of the 3×N capacitors 15 included in the 3×N unit converters 5 needs to be replaced with a new one.

[0094] Even if it is determined that the capacitor 15 does not need to be replaced with a new one, the timing for replacing the capacitor 15 with a new one is estimated. Therefore, a new capacitor 15 can be prepared at the estimated time, and a shortage of new capacitors 15 can be prevented.

[0095] The embodiments disclosed herein should be considered to be illustrative and not restrictive in all respects. The present invention is defined by the claims rather than the above description, and is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0096] 1 power conversion device, S1 to S6 switches, 2, 3 transformer, R1 to R3 current limiting resistors, UL, VL, WL AC lines, C1 to C3 current transformers, L1 to L3 reactors, A1 to A3 arms, 5 unit converter, 6 operation unit, 7 control device, 8 commercial AC power source, 9u, 9v, 9w transmission lines, 10 main circuit, 11 to 14 switching elements, PL, NL DC lines, 15 capacitor, 18 exciting coil, R4 resistance element, 20, 40 power source, 30, 42 control circuit, 31, 41 communication circuit, 32, 33 drive circuit, 34 switch operation circuit, 35 voltage sensor, 36 peak value sensor, 50, 51 control unit, 52 timekeeping unit, 53 inspection unit, 54 image display unit, 61 timekeeping unit, 62 judgment unit, 63 Memory section, 64 Estimation section.

Claims

1. a power converter including a plurality of unit converters connected in series and connected to an AC power source; a control device that outputs a control signal to each of the plurality of unit converters; Each unit converter is a main circuit including a capacitor for storing DC power, a rectifying element for converting AC power supplied from the AC power source into DC power and supplying the DC power to the capacitor, and a switching element for converting the DC power of the capacitor into AC power and supplying the AC power to the AC power source; a voltage sensor for detecting a voltage between the terminals of the capacitor; a drive circuit that drives the switching element in accordance with the control signal; the voltage across the capacitor includes a DC component and an AC component; The control device includes: a control unit that generates the control signal so that the terminal voltage of the capacitor detected by the voltage sensor becomes a reference DC voltage and so that the AC output voltage of the power converter becomes a reference AC voltage; an inspection unit provided in correspondence with each of the unit converters and configured to inspect the capacitor included in the corresponding unit converter; the inspection unit includes a determination unit that determines that the corresponding capacitor needs to be replaced with a new one when a voltage of the AC component included in a voltage between the terminals of the corresponding capacitor is higher than a threshold voltage; The inspection unit includes: A storage unit that sequentially stores the voltage of the AC component at a predetermined cycle; an estimation unit that, when the voltage of the AC component is smaller than the threshold voltage, estimates a time to replace the corresponding capacitor with a new one based on the memory content of the memory unit.

2. The power conversion device according to claim 1 , wherein the AC component voltage is a difference voltage between a maximum value and a minimum value of the inter-terminal voltage of the capacitor.

3. Each of the unit converters further includes a peak value detector that detects maximum and minimum values ​​of the terminal voltage of the capacitor and outputs a signal indicating the detected values; the inspection unit further includes a calculation unit that calculates a difference voltage between the maximum value and the minimum value indicated by the output signal of the peak value detection unit, The power conversion device according to claim 1 , wherein the determination unit determines that the corresponding capacitor needs to be replaced with a new one when the differential voltage found by the calculation unit is higher than the threshold voltage.

4. The power conversion device according to claim 1 , further comprising a display unit that displays the determination results of the plurality of determination units.

5. The power conversion device according to claim 1 , further comprising a display unit that displays the estimation results of the plurality of estimators.

Images