Power converter

Abstract

This improves the efficiency of the power converter while suppressing shocks and unstable operation during mode switching. [Solution] The power converter 1 has switching elements 41 and 42, and uses these switching elements to convert the battery voltage V BAT and DC link voltage V DC A chopper 4 that converts the voltage bidirectionally, and a DC link voltage V DC The system includes an inverter 5 that converts AC voltage bidirectionally, and a control device 8 that controls the inverter 5 and the chopper 4. The control device 8 controls the DC link voltage V by switching elements 41 and 42 on and off as an operating mode for controlling the chopper 4. DC The voltage command value V DCref It is possible to select either SW mode, which controls according to the voltage, or TH mode, which keeps the switching element 41 in a constantly ON state, and when the operation mode of the chopper 4 is switched from SW mode to TH mode, the voltage command value V DCref Reduce.

Description

[Technical Field]

[0001] This invention relates to a power conversion device. [Background technology]

[0002] One means of achieving a decarbonized society is the introduction of renewable energy sources such as solar power generation. In solar power generation, storage batteries are often installed to temporarily store the generated electricity. Hereafter, storage batteries and solar cells will be collectively referred to as batteries. When these battery-based power generation systems are connected to the AC grid, a power conversion device is used to convert the DC voltage of the batteries to AC voltage.

[0003] Battery voltage is not constant; in the case of rechargeable batteries, it changes depending on the remaining charge, and in the case of solar cells, it changes depending on the generated power. To cope with these changes in battery voltage, power conversion devices consisting of an inverter and a chopper have been known for some time. The chopper converts the battery voltage to a DC link voltage, and the inverter converts the DC link voltage to an AC voltage. The DC link voltage has a minimum value required for the inverter to output the desired AC voltage. When the battery voltage is low, the chopper controls the DC link voltage to a level higher than this minimum value.

[0004] On the other hand, when the battery voltage is high, it is conceivable to keep the chopper's switching element (hereinafter referred to as "element") constantly on, as described in Patent Document 1. In this chopper operating mode, the input and output of the chopper are DC short-circuited, and the battery voltage and the DC link voltage become approximately equal. As a result, switching losses of the element and iron losses of the reactor do not occur, and the efficiency of the power converter is improved. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2015-82882 [Overview of the project] [Problems that the invention aims to solve]

[0006] If the chopper elements can be switched between an on / off mode and a mode where they are always on, there is a risk of overvoltage or overcurrent occurring due to the shock of switching. Also, if the battery voltage is slightly lower than the DC link voltage, the on or off time of the elements may become extremely short, potentially causing unstable chopper operation. Depending on the battery capacity and charge / discharge pattern, this condition may persist for a long time. The increase in peak voltage and current values ​​due to these problems will lead to larger and more expensive power conversion devices.

[0007] In view of the above problems, the present invention aims to suppress shocks and unstable operation during switching when using a power conversion device having an inverter and a chopper, by switching between an operating mode in which the chopper elements are turned on and off and an operating mode in which they are kept in a constantly on state. [Means for solving the problem]

[0008] The power conversion device according to the present invention comprises a chopper having a switching element and using the switching element to convert the battery voltage and DC link voltage bidirectionally, an inverter converting the DC link voltage and AC voltage bidirectionally, and a control device that controls the inverter and the chopper, wherein the control device can select either a first mode as an operating mode for controlling the chopper, which controls the DC link voltage according to a voltage command value by turning the switching element on and off, or a second mode in which the switching element is always on, and the control device reduces the voltage command value when transitioning the operating mode from the first mode to the second mode. [Effects of the Invention]

[0009] According to the present invention, in a power conversion device having an inverter and a chopper, when switching between an operating mode in which the chopper elements are turned on and off and an operating mode in which they are kept in a constantly on state, shocks and unstable operation during mode switching can be suppressed, making the power conversion device smaller and lower cost. [Brief explanation of the drawing]

[0010] [Figure 1] This is a system configuration diagram of a power conversion device according to the first embodiment of the present invention. [Figure 2] This figure shows an example of the operation waveform of a chopper. [Figure 3] This is a block diagram of a control device according to a first embodiment of the present invention. [Figure 4] This is a flowchart showing the processing procedure of the control device in the first embodiment of the present invention. [Figure 5] This figure shows an example of an operation chart for a power conversion device when charging a battery in the first embodiment of the present invention. [Figure 6] This figure shows an example of an operation chart of a power conversion device when discharging a battery in the first embodiment of the present invention. [Figure 7] This figure shows another example of the circuit configuration of a chopper in a power conversion device according to the first embodiment of the present invention. [Figure 8] This figure shows an example of a chopper's operating waveform in a different case. [Figure 9] This figure shows an example of an operation chart during discharge when the second voltage command value Vref2 is set to the same value as the third threshold VTH3. [Figure 10] This is a block diagram of a control device according to a second embodiment of the present invention. [Figure 11] This diagram shows an example of a charging operation chart when the charging power changes while the chopper mode is being switched. [Figure 12] This is a block diagram of a control device according to a third embodiment of the present invention. [Modes for carrying out the invention]

[0011] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0012] (First Embodiment) (System Configuration of Power Conversion Device) FIG. 1 is a system configuration diagram of a power conversion device according to the first embodiment of the present invention. The power conversion device 1 shown in FIG. 1 is a bidirectional power conversion device capable of converting the DC voltage of the battery 2 into an AC voltage and outputting it to the AC system 3, and converting the AC voltage of the AC system 3 into a DC voltage and outputting it to the battery 2. With the power conversion function of such a power conversion device 1, the battery 2 and the AC system 3 can be connected.

[0013] The battery 2 is a storage battery capable of both charging and discharging. The voltage of the battery 2 (battery voltage V BAT ) varies according to the state of the battery 2. In FIG. 1, as the internal configuration of the battery 2, an electromotive force 21 having an electromotive voltage V BO and an internal impedance 22 that causes a voltage drop V BR are shown. The values of the electromotive voltage V BO and the voltage drop V BR vary according to the state of the battery 2, so that the battery voltage V BAT can vary.

[0014] The power conversion device 1 includes a chopper 4, an inverter 5, a capacitor 6, a filter 7, a control device 8, and a voltage detector 9. In addition to the above elements, the power conversion device 1 may further include switches, protection components such as fuses and surge protectors, noise filters, etc.

[0015] The chopper 4 has switching elements 41 and 42, and uses these switching elements 41 and 42 to convert the battery voltage V BAT into a DC link voltage V DC . The inverter 5 converts the DC link voltage V DC into an AC voltage and outputs it to the AC system 3 via the filter 7. The inverter 5 also converts the AC voltage input from the AC system 3 via the filter 7 into a DC link voltage VDC It can also be converted to a DC link voltage V and output to chopper 4. In this case, chopper 4 uses switching elements 41 and 42 to output a DC link voltage V DC Battery voltage V BAT It converts the signal to a specific type and outputs DC power to battery 2.

[0016] A smoothing capacitor 6 is connected to the line connecting the DC side of the chopper 4 and the inverter 5. A filter 7 is inserted between the inverter 5 and the AC system 3 to reduce harmonic components in the AC output of the inverter 5. In other words, the inverter 5 is connected to the AC system 3 via the filter 7.

[0017] The control device 8 controls the chopper 4 and inverter 5 based on the voltage and current in the power converter 1. The voltage detector 9 detects the three-phase AC voltage V that is input and output between the AC system 3 and the power converter 1. R ,V S ,V T It detects.

[0018] Figure 1 shows an example of a specific circuit configuration of chopper 4. In addition to the aforementioned switching elements 41 and 42, chopper 4 includes, for example, a reactor 43, a capacitor 44, voltage detectors 45 and 46, and a current detector 47. With this configuration, chopper 4 can handle the battery voltage V BAT and DC link voltage V DC It can convert in both directions. Note that although MOSFETs (Metal-Oxide-Semiconductor Field Effect Transistors) are shown as switching elements 41 and 42 in Figure 1, other types of elements such as IGBTs (Insulated Gate Bipolar Transistors) may also be used.

[0019] Voltage detectors 45 and 46 detect the DC link voltage V DC and battery voltage V BAT The current detector 47 detects the current flowing through the reactor 43 (reactor current I) and outputs these detection results to the control device 8. L) is detected. In the following explanation, the direction in which battery 2 is charged (the direction of the arrow in Figure 1) is taken as positive, and the reactor current I L Define the polarity.

[0020] In Figure 1, the DC link voltage V is transmitted and received by the control device 8, as measured by voltage detectors 45, 46, current detector 47, and voltage detector 9. DC Battery voltage V BAT , reactor current I L and AC voltage V R ,V S ,V T Each detection signal, the gate voltages G1 and G2 of the switching elements 41 and 42, and the control signal CS related to the inverter 5. INV and detection signal DS INV This indicates that. Furthermore, the control signal CS INV and detection signal DS INV Each of these is indicated by a single arrow, but each of these may contain multiple signals. The method of implementing the control device 8 is arbitrary, but one example is to implement it as an electronic circuit on a board equipped with devices such as a microcomputer or FPGA (Field-Programmable Gate Array).

[0021] (Chopper operation mode) Chopper 4 has two main operating modes (first mode and second mode). Figure 2 shows an example of the operating waveform of chopper 4. Specifically, the gate voltages G1 and G2 of switching elements 41 and 42 and the reactor current I L The waveform is shown in Figure 2.

[0022] Figure 2(a) shows an example waveform when the chopper 4 operates in the first mode. The first mode is when the switching elements 41 and 42 in the chopper 4 are switched at a predetermined switching period T. SW By switching it on and off alternately, the battery voltage V BAT DC link voltage V DC This is the operating mode for conversion. Below, the first mode will also be referred to as SW mode.

[0023] As shown in Figure 2(a), in SW mode, the gate voltages G1 and G2 are equal to the switching period T. SW This results in a pulse waveform, and the reactor current I L The switching period is T. SW This results in a triangular wave with ripple. In the example in Figure 2(a), the on-duty cycle of the gate voltage G1 is denoted as d. Note that in the steady state, d = V BAT / V DC It can be expressed as follows. The control device 8 controls the DC link voltage V DC The predetermined voltage command value V DCref By performing feedback control to match this, and as a result changing the on-duty cycle d, the DC link voltage V DC The control is performed. Note that the DC link voltage V is in the minor loop of the control. DC Rather, reactor current I L In some cases, feedback control is applied. In this case, the control device 8 controls V DC As a result of the control, the current command value I Lref This current command value I is generated. Lref In the reactor current I L The on-duty cycle d of the gate voltage G1 is varied by controlling it so that it matches.

[0024] Figure 2(b) shows an example waveform when chopper 4 is operating in the second mode. The second mode is an operating mode in which the switching elements 41 and 42 in chopper 4 are kept on and kept off, respectively. Hereafter, the second mode will also be referred to as the TH mode.

[0025] In TH mode, the input and output of chopper 4 are DC short-circuited, resulting in a DC link voltage V DC The battery voltage is V BAT This is almost identical. Furthermore, TH mode can be described as operation with the on-duty cycle d fixed to 1 in the aforementioned SW mode. In TH mode, the reactor current I L The waveform does not contain ripple. Also, the DC link voltage V DC or reactor current I LFeedback control is not performed.

[0026] In TH mode, no switching operation is performed in switching elements 41 and 42, so no switching losses occur in switching elements 41 and 42, and no iron losses occur in reactor 43. Battery voltage V BAT The value is sufficiently high, V DC = V BAT However, if the inverter 5 can output the desired AC voltage, the conversion efficiency of the power converter 1 can be improved by operating the chopper 4 in TH mode. Therefore, the above two operating modes are basically based on the battery voltage V BAT It can be switched depending on the level.

[0027] Here, we will discuss the problems with switching the operating mode of the chopper 4. First, the shock during the switching of the operating mode causes fluctuations in the voltage and current of the power converter 1, which may lead to an increase in the peak values ​​of the voltage and current, potentially resulting in a larger and more expensive power converter 1.

[0028] Next, the battery voltage V BAT DC link voltage V DC If it is slightly lower, then d = V in the steady state. BAT / V DC As a result, d will be a value slightly less than 1. In this case, the off period of element 41 and the on period of element 42 will each become extremely short, which may cause the operation of the chopper 4 to become unstable. Depending on the capacity of the battery 2 and the charge / discharge pattern, this condition may continue for a long time. Therefore, in this invention, these problems are solved by performing the control described below in the control device 8, thereby avoiding the need to increase the size and cost of the power conversion device 1.

[0029] (Block diagram and flowchart of the control unit) Figure 3 is a block diagram of the control device 8 in the first embodiment of the present invention. In Figure 3, blocks related to the control and mode switching of the chopper 4 are shown, while the block for controlling the inverter 5 is omitted. Figure 4 is a flowchart of the processing procedure of the control device 8 in the first embodiment of the present invention. The mode switching control of the chopper 4 by the control device 8 of this embodiment will be explained below using these figures.

[0030] As shown in Figure 3, the control device 8 receives a voltage command switching signal S vref Voltage command switching signal generation unit (hereinafter referred to as S) that generates the voltage command switching signal. vref Generation unit) 81 and voltage command value V DCref Voltage command generation unit (hereinafter referred to as V) that generates the voltage command (V DCref Generation unit) 82 and voltage difference determination signal S vdif Voltage difference determination signal generation unit (hereinafter referred to as S) that generates the voltage difference determination signal. vdif Generation unit) 83 and mode switching signal S mode The mode switching signal generation unit (hereinafter referred to as S) generates the mode switching signal. mode The system includes a generation unit 84 and a gate signal generation unit 85 that generates gate voltages G1 and G2.

[0031] S vref The generation unit 81 generates the battery voltage V BAT The voltage command switching signal S is generated based on the result of comparing it with a predetermined threshold. vref This generates S. vref It is preferable to provide hysteresis to the threshold used in the generation unit 81. Specifically, for example, the voltage command switching signal S vref When the voltage is at the L level, the battery voltage V BAT is a predetermined first threshold V TH1 As it gets higher, S vref The generation unit 81 generates a voltage command switching signal S vref Switch it to the H level. Also, for example, the voltage command switching signal S vref When the voltage is at the H level, the battery voltage V BAT The predetermined third threshold V TH3 As it gets lower, S vref The generation unit 81 generates a voltage command switching signal S vref Switch to L level. Note that the third threshold V TH3is set lower than the first threshold value V TH1 . Thus, in the S vref generation unit 81, by providing a hysteresis characteristic with respect to the output of the voltage command switching signal S vref , it is possible to prevent the logical value of the voltage command switching signal S BAT from frequently switching due to detection noise or the like of the battery voltage V vref .

[0032] V DCref generation unit 82 generates a voltage command value V vref for the DC link voltage V vref based on the voltage command switching signal S DC input from the S DCref generation unit 81. A signal selection switch is provided in the V DCref generation unit 82, and by this signal selection switch, either the first voltage command value V vref ]>or the second voltage command value V ref1 is output. Note that the second voltage command value V ref2 is set lower than the first voltage command value V ref2 . The first voltage command value V ref1 and the second voltage command value V ref1 correspond to the upper limit value and the lower limit value of the voltage command value V ref2 respectively. DCref

[0033] V DCref In the generation unit 82, the signal selection switch outputs the first voltage command value V vref when the voltage command switching signal S ref1 is at the L level, and outputs the second voltage command value V vref when the voltage command switching signal S ref2 is at the H level. An LPF (Low Pass Filter) is provided on the output side of the signal selection switch. The V DCref generation unit 82 outputs the value obtained by performing LPF processing on the output of the signal selection switch as the voltage command value V DCref . That is, when the voltage command switching signal S vref changes from the L level to the H level, the voltage command value V DCref output from the V DCref generation unit 82DCref The first voltage command value V ref1 From the second voltage command value V ref2 It gradually decreases to the voltage command switching signal S. vref When V changes from H level to L level, DCref Voltage command value V output from generation unit 82 DCref This is the second voltage command value V ref2 From the first voltage command value V ref1 It gradually increases to the voltage command value V in these states. DCref The rate of decrease or increase can be adjusted by the time constant of the LPF. Specifically, for example, the voltage command value V DCref The rate of decrease or increase of the DC link voltage V is controlled by the control device 8. DC It is preferable to set the time constant of the LPF to be the same as, or greater than, the time constant of the feedback control.

[0034] Furthermore, without using a signal selection switch or LPF, the voltage command value V DCref The first voltage command value V ref1 and the second voltage command value V ref2 V increases or decreases at a predetermined rate between these two points. DCref The generation unit 82 can also be configured. For example, V DCref In the generation unit 82, the voltage command switching signal S vref The voltage command value V changes at regular intervals according to the change in voltage command value. DCref By increasing or decreasing by a predetermined amount, the voltage command value V DCref The first voltage command value V ref1 and the second voltage command value V ref2 It may also be made to change in steps between these values. Furthermore, the voltage command value V DCref The rate at which the voltage command value V increases or decreases may be a constant value or a value that changes over time. The control device 8 increases or decreases the voltage command value V over a certain period of time. DCref The first voltage command value V ref1 and the second voltage command value V ref2 If it can be changed continuously or stepwise between them, then V can be changed by any configuration. DCref The generation unit 82 can be realized.

[0035] S vdif The generation unit 83 generates a DC link voltage V DC and battery voltage V BAT Voltage difference V DIF The voltage difference is compared with a predetermined threshold, and based on the result, a voltage difference determination signal S is issued. vdif It generates a voltage difference V. DIF is a predetermined second threshold V TH2 When it is lower, S vdif The generation unit 83 generates a voltage difference determination signal S vdif It outputs an H-level signal, and the voltage difference V DIF The second threshold V TH2 In the above case, S vdif The generation unit 83 generates a voltage difference determination signal S. vdif It outputs a low-level signal.

[0036] S mode The generating unit 84 is S vref Voltage command switching signal S input from generation unit 81 vref And, S vdif Voltage difference determination signal S input from generation unit 83 vdif Based on this, the mode switching signal S mode Generates a mode switching signal S. mode The logical value of this signal represents the operating mode of chopper 4. That is, the mode switching signal S mode The L level indicates SW mode, and the H level indicates TH mode. In Figure 3, S is generated by an AND gate, rising edge detector, falling edge detector, and RS flip-flop. mode An example of the configuration of the generation unit 84 is shown, but if the following operation can be achieved, S mode The internal configuration of the generation unit 84 is arbitrary.

[0037] Mode switching signal S mode When the voltage command switching signal S is at the L level vref and voltage difference determination signal S vdif When both reach level H, S mode The generation unit 84 generates a mode switching signal S mode Switch to the H level. This switches chopper 4 from SW mode to TH mode. Also, the mode switching signal Smode When the voltage command switching signal S is at the H level vref When it reaches L level, S mode The generation unit 84 generates a mode switching signal S mode Change the level to L. This switches Chopper 4 from TH mode to SW mode.

[0038] Here, S vdif In the generation unit 83, the second threshold V TH2 If it is set to a value near zero, the aforementioned voltage difference V DIF When it is sufficiently small, that is, DC link voltage V DC and battery voltage V BAT When they are nearly identical, the mode switching signal S mode The level is switched from L to H, and Chopper 4 is switched from SW mode to TH mode.

[0039] The gate signal generation unit 85 is V DCref Voltage command value V input from generation unit 82 DCref Yes, S mode Mode switching signal S input from generation unit 84 mode Based on these factors, the gate voltages G1 and G2 of the switching elements 41 and 42 are generated. Mode switching signal S mode When the voltage is at an L level, that is, when the chopper 4 is operated in SW mode, the gate signal generation unit 85 generates a voltage command value V DCref Based on the DC link voltage V DC A feedback control calculation is performed to calculate the on-duty cycle d of the switching element 41. Then, gate voltages G1 and G2 are generated so that the switching elements 41 and 42 repeatedly switch on and off according to the calculated on-duty cycle d. At this time, as described above, the current command value I Lref Using the reactor current I L The on-duty cycle d of the switching element 41 may be calculated by performing a feedback control calculation. On the other hand, the mode switching signal S modeWhen the signal is at an H level, that is, when the chopper 4 is operated in TH mode, the gate signal generation unit 85 generates gate voltages G1 and G2 such that the switching elements 41 and 42 are always on and always off, respectively.

[0040] The control device 8 generates gate voltages G1 and G2 and controls the operation of the chopper 4 by executing the flowchart process shown in Figure 4 at predetermined calculation cycles using the components shown in the block diagram in Figure 3.

[0041] In step S10, the control device 8 receives the DC link voltage V from the voltage detector 45. DC The voltage detector 46 measures the battery voltage V BAT Obtain each of them.

[0042] In step S20, the control device 8 receives the DC link voltage V acquired in step S10. DC and battery voltage V BAT Calculate the difference and the voltage difference V DIF Calculate.

[0043] In step S30, the control device 8 receives a voltage command switching signal S vref and mode switching signal S mode The previous value is obtained. Here, if the process in step S70 or S130 described later was performed in the previous calculation cycle, the voltage command switching signal S determined by these processes is obtained. vref The value of is obtained as the previous value. On the other hand, if neither the process in steps S70 and S130 was performed in the previous calculation cycle, the same voltage command switching signal S obtained in step S30 in the previous calculation cycle is obtained. vref The value of should be obtained as the previous value. Similarly, if the process in step S110 or S150 described later was performed in the previous calculation cycle, the mode switching signal S determined by these processes should be obtained. modeThe value of is obtained as the previous value. On the other hand, if neither the process in steps S110 and S150 was performed in the previous calculation cycle, the same mode switching signal S obtained in step S30 in the previous calculation cycle is obtained. mode You can simply retrieve the value of the previous value.

[0044] In step S40, the control device 8 receives the mode switching signal S acquired in step S30. mode Determine whether the previous value is at the L level. Mode switching signal S mode If the previous value is at the L level, proceed to step S50; otherwise, i.e., if it is at the H level, proceed to step S120.

[0045] In step S50, the control device 8 receives the voltage command switching signal S acquired in step S30. vref Determine whether the previous value is at the L level. Voltage command switching signal S vref If the previous value is at the L level, proceed to step S60; otherwise, i.e., if it is at the H level, proceed to step S90.

[0046] In step S60, the control device 8 receives the battery voltage V acquired in step S10. BAT a predetermined first threshold V TH1 Compare it to the battery voltage V. BAT The first threshold V TH1 If it is less than V, proceed to step S100; otherwise, i.e., the first threshold V TH1 In the above cases, proceed to step S70. Note that if you proceed from step S60 to step S100, the mode switching signal S mode It will be maintained at the same L level as the previous value.

[0047] In step S70, the control device 8 receives a voltage command switching signal S vref Switch the logical value from the previous L level to the H level.

[0048] In step S80, the control device 8 receives the voltage command switching signal S, which was switched from L level to H level in step S70. vref Depending on the voltage command value V DCref The first voltage command value V ref1 From the second voltage command value V ref2 The voltage command value V is changed continuously or in steps. DCref The first voltage command value V ref1 From the second voltage command value V ref2 The goal is to gradually decrease it at a predetermined rate towards that point.

[0049] In step S90, the control device 8 uses the voltage difference V calculated in step S20. DIF a predetermined second threshold V TH2 Compare it to the following. As a result, the voltage difference V DIF The second threshold V TH2 If it is greater, proceed to step S100; otherwise, i.e., the second threshold V TH2 In the following cases, proceed to step S110. Note that if you proceed from step S90 to step S100, the mode switching signal S mode It will be maintained at the same L level as the previous value.

[0050] In step S100, the control device 8 controls the voltage command value V DCref Based on the DC link voltage V DC The feedback control calculation is performed. Here, as mentioned above, the DC link voltage V DC The voltage command value V DCref The on-duty cycle d of the switching element 41 is calculated to approach this value. After the processing in step S100 is completed, the process proceeds to step S170.

[0051] In step S110, the control device 8 receives a mode switching signal S mode The logical value of is switched from the previous L level to H level. This switches the operating mode of chopper 4 from SW mode to TH mode. After the processing in step S110 is completed, proceed to step S170.

[0052] In step S120, the control device 8 receives the battery voltage V acquired in step S10. BAT a predetermined third threshold V TH3 Compare it to the battery voltage V. BAT The third threshold V TH3 If it is greater, proceed to step S170; otherwise, i.e., the third threshold V TH3 In the following cases, proceed to step S130. Note that if you proceed from step S120 to step S170, the mode switching signal S mode It will be maintained at the same H level as the previous value.

[0053] In step S130, the control device 8 receives a voltage command switching signal S vref Switch the logical value from the previous H level to the L level.

[0054] In step S140, the control device 8 receives the voltage command switching signal S, which was switched from H level to L level in step S130. vref Depending on the voltage command value V DCref The second voltage command value V ref2 From the first voltage command value V ref1 The voltage command value V is changed continuously or in steps. DCref The second voltage command value V ref2 From the first voltage command value V ref1 The value is gradually increased towards a predetermined rate of increase.

[0055] In step S150, the control device 8 receives a mode switching signal S mode The logical value is switched from the previous H level to L level. This switches the operating mode of chopper 4 from TH mode to SW mode.

[0056] In step S160, the control device 8 controls the voltage command value V DCref Based on the DC link voltage V DC The feedback control calculation is performed. Here, similar to step S100, the DC link voltage V DC The voltage command value V DCrefThe on-duty cycle d of the switching element 41 is calculated to approach this value. After the processing in step S160 is completed, the process proceeds to step S170.

[0057] In step S170, the control device 8 generates gate voltages G1 and G2 for the switching elements 41 and 42 of the chopper 4. Here, the mode switching signal S mode If the signal is at an L level and the on-duty cycle d of the switching element 41 is calculated in step S100 or S160, then in SW mode operation, gate voltages G1 and G2 are generated so that the switching elements 41 and 42 operate on and off respectively according to the calculated on-duty cycle d. Meanwhile, the mode switching signal S mode If the signal level is high, the gate voltages G1 and G2 are generated so that switching elements 41 and 42 are always on and always off, respectively, in TH mode operation. After the process in step S170 is completed, the control device 8 terminates the flowchart shown in Figure 4.

[0058] Furthermore, in the flowchart of Figure 4 described above, the processes in steps S50-S70 and S120-S130 are the same as in Figure 3 S vref This corresponds to the generation unit 81. Also, the processing in steps S80 and S140 is shown in V in Figure 3. DCref This corresponds to the generation unit 82, and the processing in step S90 is shown in Figure 3. vdif This corresponds to the generation unit 83. Also, the processing in steps S40, S110 and S150 is shown in Figure 3 S mode The generation unit 84 corresponds to the gate signal generation unit 85, and the processing in steps S100, S160, and S170 corresponds to the gate signal generation unit 85.

[0059] (Charging operation chart) Figure 5 is a diagram showing an example of the operation chart of the power conversion device 1 when charging the battery 2 in the first embodiment of the present invention. In Figure 5, the target item is the voltage command switching signal S. vref Voltage difference determination signal S vdif Mode switching signal S mode DC link voltage V DC Battery voltage V BAT, voltage command value V DCref DC link voltage V DC and battery voltage V BAT Voltage difference V DIF , the on-duty cycle d of the switching element 41, and the reactor current I L Examples of the operation charts are shown. Also, Figure 5 shows the DC link voltage V DC and battery voltage V BAT These are shown superimposed on the same graph using solid and dotted lines, respectively. Note that the reactor current I L Therefore, we ignored the fluctuations (ripple) that occur when Chopper 4 operates in SW mode.

[0060] At time t0, the battery voltage V BAT The first threshold V TH1 Lower, S vref Voltage command switching signal S generated by the generation unit 81 vref Since it is at L level, V DCref Voltage command value V generated by the generation unit 82 DCref The first voltage command value V ref1 It is as follows. Also, the voltage difference V DIF The second threshold is V TH2 Because it is higher, S vdif Voltage difference determination signal S generated by the generation unit 83 vdif It is at the L level. Thus, at time t0, the voltage command switching signal S vref and voltage difference determination signal S vdif Since both are at L level, S mode Mode switching signal S output from generation unit 84 mode The voltage is also at an L level. At this time, chopper 4 operates in SW mode, and the DC link voltage V is controlled by chopper 4. DC The voltage command value is V DCref (First voltage command value V ref1 It is controlled accordingly.

[0061] From time t0 onward, battery 2 is charged, causing the battery voltage V BAT The DC link voltage V gradually increases, but DC The voltage command value V is determined by chopper 4. DCref , that is, the first voltage command value V ref1It maintains a controlled state. As a result, the on-duty cycle d of the switching element 41 is set to the battery voltage V BAT It increases accordingly. On the other hand, the voltage difference V DIF It gradually decreases.

[0062] At time t1, the battery voltage V BAT The first threshold V TH1 When it increases to S vref The operation of the generation unit 81 generates a voltage command switching signal S vref This changes from L level to H level. This voltage command switching signal S vref V in response to changes DCref The operation of the generation unit 82 generates the voltage command value V DCref The first voltage command value V ref1 From the second voltage command value V ref2 It begins to decrease towards a predetermined rate. At this point, the voltage difference V DIF The second threshold V TH2 A higher state, i.e., the voltage difference determination signal S. vdif Because it remains at an L level, the mode switching signal S mode The voltage remains at the L level. Therefore, chopper 4 continues to operate in SW mode, and the DC link voltage V is controlled by chopper 4. DC The voltage command value is V DCref It decreases in accordance with the change.

[0063] Voltage difference V at time t2 DIF The second threshold V TH2 When it decreases to S vdif The operation of the generation unit 83 generates a voltage difference determination signal S vdif The voltage changes from L level to H level. This voltage difference judgment signal S vdif S in response to changes mode The operation of the generation unit 84 generates the mode switching signal S mode The voltage can also be switched from L level to H level. This switches chopper 4 from SW mode to TH mode, and the on-duty cycle d of switching element 41 is fixed at 1. Voltage command value V DCref The second voltage command value is V ref2 While the DC link voltage V continues to decrease toward DCThe battery voltage is V BAT It increases accordingly.

[0064] In the power converter 1 of this embodiment, the switching from SW mode to TH mode is performed as described above.

[0065] (Operation chart during discharge) Figure 6 is a diagram showing an example of the operation chart of the power converter 1 when the battery 2 is discharged according to the first embodiment of the present invention. The difference from Figure 5 is that in Figure 6 the DC link voltage V DC and battery voltage V BAT In addition, the voltage command value V DCref and battery voltage V BAT These are shown superimposed on the same graph using solid and dotted lines, respectively.

[0066] At time t3, the voltage command switching signal S vref Voltage difference determination signal S vdif and mode switching signal S mode All are at the H level. At this time, chopper 4 operates in TH mode, and the on-duty cycle d of switching element 41 is fixed at 1, so the DC link voltage V DC The battery voltage is V BAT It is in a state that is almost identical to that.

[0067] From time t3 onward, battery 2 is discharged, causing the battery voltage V BAT The DC link voltage V gradually decreases. DC The battery voltage is V BAT It decreases accordingly.

[0068] At time t4, the battery voltage V BAT The third threshold V TH3 When it decreases to S vref The operation of the generation unit 81 generates a voltage command switching signal S vref This changes from a high level to a low level. This voltage command switching signal S vref V in response to changes DCref The operation of the generation unit 82 generates the voltage command value V DCref The second voltage command value is V ref2 From the first voltage command value V ref1It begins to increase at a predetermined rate toward [a certain point]. Also, the voltage command switching signal S vref When it reaches L level, S mode The operation of the generation unit 84 generates a mode switching signal S mode The voltage can also be switched from H level to L level. This switches the operating mode of chopper 4 from TH mode to SW mode. However, at this point the battery voltage V BAT The voltage command value V DCref Because it is higher, DC link voltage V DC Battery voltage V BAT The state remains almost identical to that.

[0069] At time t5, the voltage command value V DCref Battery voltage V BAT When it increases to this level, the DC link voltage V is controlled by chopper 4. DC The voltage command value V DCref It begins to increase accordingly. That is, the effective SW mode operation starts from this time t5. At time t6, the voltage difference V DIF The second threshold V TH2 When it increases to S vdif The operation of the generation unit 83 generates a voltage difference determination signal S vdif It changes from H level to L level.

[0070] In the power converter 1 of this embodiment, the switching from TH mode to SW mode is performed as described above.

[0071] (effect) As already mentioned, one of the problems with switching the operating mode of Chopper 4 is the battery voltage V BAT DC link voltage V DC If the voltage remains slightly lower for an extended period, the operation of chopper 4 may become unstable. In this invention, as described above, the battery voltage V BAT The first threshold V TH1 When it increases to this point, the DC link voltage V DC Voltage command value V DCrefThe voltage is reduced at a predetermined rate, and the operating mode of chopper 4 is switched from SW mode to TH mode. As a result, the operating mode of chopper 4 is SW mode, and the DC link voltage V DC and battery voltage V BAT Voltage difference V DIF The first threshold V TH1 This reduces the size of the chopper 4, preventing it from operating continuously for extended periods.

[0072] Also, the voltage command value V DCref After the voltage difference V begins to decrease, DIF The second threshold V TH2 When it decreases to this point, chopper 4 switches from SW mode to TH mode. Therefore, the second threshold V TH2 Setting it to a value near zero results in a DC link voltage V DC and battery voltage V BAT Mode switching is performed when these conditions are nearly identical. This suppresses the shock of mode switching and prevents the voltage and current of the power converter 1 from fluctuating or becoming excessive.

[0073] Furthermore, the DC link voltage V DC Considering the time constant of the feedback control, the voltage command value V DCref By setting the reduction rate, the DC link voltage V DC The voltage command value V DCref During the period in which the DC link voltage V is reduced accordingly, DC Without causing overshoot or vibration, the DC link voltage V DC This allows for a smooth reduction of the DC link voltage V. And, until just before chopper 4 switches to TH mode, DC The voltage command value V DCref Feedback control can be performed accordingly. This prevents fluctuations in the voltage and current of the power converter 1, as well as excessive voltage and current, even if disturbances such as changes in charging power occur along the way.

[0074] As described above, in this embodiment, in a power converter 1 equipped with an inverter 5 and a chopper 4, by using the TH mode in which the switching element 41 of the chopper 4 is kept in a constantly on state, the efficiency of the power converter 1 is improved while suppressing shocks and unstable operation during mode switching. As a result, the power converter 1 can be made smaller and less expensive.

[0075] According to the first embodiment of the present invention described above, the following effects are achieved.

[0076] (1) The power converter 1 has switching elements 41 and 42, and uses these switching elements to convert the battery voltage V BAT and DC link voltage V DC A chopper 4 that converts the voltage bidirectionally, and a DC link voltage V DC The system includes an inverter 5 that converts AC voltage bidirectionally, and a control device 8 that controls the inverter 5 and the chopper 4. The control device 8 controls the DC link voltage V by switching elements 41 and 42 on and off as an operating mode for controlling the chopper 4. DC The voltage command value V DCref The control device 8 can select between a first mode (SW mode) that controls according to the voltage command value V DCref This reduces (step S80). As a result, it becomes possible to improve the efficiency of the power converter 1 while suppressing shocks and unstable operation during mode switching.

[0077] (2) The control device 8 controls the battery voltage V BAT is a predetermined first threshold V TH1 When it increases to the above (Step S60: No), the voltage command value V DCref The voltage is reduced at a predetermined rate (step S80). In this way, when the operating mode of the chopper 4 is switched from SW mode to TH mode, the voltage command value V DCref It can definitely be reduced.

[0078] (3) In step S80, the control device 8 controls the voltage command value V DCref After it began to decrease, the DC link voltage V DC and battery voltage V BAT The difference (voltage difference V) DIF ) is a predetermined second threshold V TH2 When the voltage decreases to (step S90: No), the switching element 41 is kept in the permanently ON state to operate the chopper 4 in TH mode (step S110). In this way, the switching of the chopper 4's operating mode from SW mode to TH mode can be performed at an appropriate timing. As a result, fluctuations in the voltage and current of the power converter 1 during mode switching are suppressed, and excessive voltage and current flow in the power converter 1 can be prevented.

[0079] (4) When the chopper 4 is operating in TH mode, the control device 8 operates with a battery voltage V (Step S40: No) BAT The predetermined third threshold V TH3 When it decreases to (Step S120: No), the voltage command value V DCref This third threshold V is increased (step S140), and the switching elements 41 and 42 are switched on and off respectively to operate the chopper 4 in SW mode (step S150). TH3 The first threshold V TH1 This is a lower value. By doing this, it is possible to stabilize the operation of the chopper 4 by preventing it from switching back to TH mode shortly after switching from TH mode to SW mode.

[0080] (Another example regarding the circuit configuration of a chopper) Figure 7 shows another example of the chopper circuit configuration in a power converter 1 according to the first embodiment of the present invention. In the power converter 1 of Figure 1, chopper 4A of Figure 7 may be provided instead of chopper 4. The chopper 4A shown in Figure 7 further includes a switching element 48 in addition to the elements of chopper 4 of Figure 1. In chopper 4A, the switching element 48 is connected in parallel with the part in which switching element 41 and reactor 43 are connected in series.

[0081] Figure 8 shows an example of the operating waveform of chopper 4A, as shown in Figure 7. In Figure 8, the gate voltages G1 and G2 of the switching elements 41 and 42 shown in Figure 2 and the reactor current I L In addition, the waveform of the gate voltage G3 of the switching element 48 is also shown.

[0082] Figure 8(a) shows an example waveform when chopper 4A is operating in SW mode. As shown in Figure 8(a), in SW mode, chopper 4A switches the switching elements 41 and 42 at a predetermined switching period T SW The switching element 48 is kept off while it is alternately turned on and off.

[0083] Figure 8(b) shows an example waveform when chopper 4 is operating in TH mode. As shown in Figure 8(b), in TH mode, chopper 4A keeps switching elements 41, 42, and 48 always off, always off, and always on, respectively. Unlike chopper 4 in Figure 1, in TH mode, reactor 43 is bypassed by switching element 48. Thus, when using chopper 4A, the efficiency of power converter 1 can be improved because the losses of reactor 43 are eliminated.

[0084] (Another example regarding the setting method of voltage command values ​​and the operation chart during discharge) In Figure 6 above, the second voltage command value V is shown in the power converter 1. ref2 The third threshold V TH3 An example of an operation chart during discharge is shown for cases where the setting is lower. In this operation chart, after chopper 4 is switched from TH mode to SW mode, the DC link voltage V DC Battery voltage V BAT Because there is a period of decrease in accordance with this (the period from time t4 to t5), the DC link voltage V DC The voltage command value V DCref A delay occurs before the control is implemented according to this. To avoid this, the power converter 1 in Figure 1 uses a third threshold V TH3 The voltage command value V DCref The lower limit of the second voltage command value Vref2 You can also set it to the same value.

[0085] Figure 9 shows the third threshold V TH3 The second voltage command value V ref2 This shows an example of the operation chart during discharge when set to the same value. In Figure 9, times t3, t4, and t5 are the same as in Figure 6. That is, as explained in Figure 6, at time t4 the battery voltage V BAT The third threshold V TH3 When it decreases to this level, chopper 4 switches from TH mode to SW mode. Also, at the same time t4, the DC link voltage V DC The third threshold V TH3 It has decreased to this point. Here, the second voltage command value V ref2 The third threshold is V TH3 Since it is the same value, the voltage command value V at time t4 DCref This is the second voltage command value V ref2 That is, the third threshold V TH3 It begins to increase from there. Therefore, the DC link voltage V DC The voltage command value V DCref Control is initiated accordingly, and no control delay occurs as shown in Figure 6.

[0086] Furthermore, as mentioned above, the second voltage command value V ref2 The third threshold V TH3 When setting it to the same value, DC link voltage V DC The second voltage command value is V ref2 That is, the third threshold V TH3 It can decrease to this extent. Here, the DC link voltage V DC The minimum value required for inverter 5 to output the desired AC voltage and connect to AC system 3 is defined. Therefore, the third threshold V TH3 This DC link voltage V DC By setting it to a value higher than the minimum value, the operating mode of chopper 4 and the battery voltage V can be changed. BAT Regardless of the level, the inverter 5 can output the desired AC voltage to the AC system 3.

[0087] (Second embodiment) (Block diagram of the control unit) Figure 10 is a block diagram of the control device 8A in a second embodiment of the present invention. In this embodiment, the power converter 1 of Figure 1 is equipped with the control device 8A of Figure 10 instead of the control device 8. The control device 8A shown in Figure 10 differs from the control device 8 of Figure 3 described in the first embodiment in that it uses a voltage detector 9 to measure the AC voltage V R ,V S ,V T The detected value is further input. In addition to the elements provided by the control device 8, the system voltage detection unit (hereinafter referred to as V GP It further includes a detection unit 86 and a threshold setting unit 87. Note that in Figure 10, V DCref Generation part 82, S vdif Generation unit 83 and S mode The internal elements of the generation unit 84 have been omitted.

[0088] V GP The detection unit 86 detects the AC voltage V R ,V S ,V T From the detected value, the voltage amplitude V of AC system 3 GP The threshold setting unit 87 detects V GP Voltage amplitude V detected by detection unit 86 GP Based on this, the first threshold V TH1 and the third threshold V TH3 It generates a voltage amplitude V. Specifically, GP The higher the value, the higher the first threshold V. TH1 and the third threshold V TH3 The voltage amplitude V increases as each of them increases. GP These thresholds are set accordingly. Note that the first threshold V TH1 or third threshold V TH3 Only one of the voltage amplitude V GP The first threshold V generated by the threshold setting unit 87 may be changed accordingly, while the other may remain unchanged. TH1 and the third threshold V TH3 is, S vref The S described above is input to the generation unit 81. vref It is used in the processing of the generation unit 81.

[0089] (effect) Svref In the generation unit 81, the first threshold V TH1 and the third threshold V TH3 The lower each of these is set, the lower the voltage command switching signal S vref Battery voltage V that sets the level to H BAT The value of becomes lower. Therefore, the battery voltage V required to operate chopper 4 in TH mode BAT The range becomes wider, leading to an improvement in the efficiency of the power converter 1. On the other hand, the first threshold V TH1 and the third threshold V TH3 If it is too low, the DC link voltage V in TH mode operation DC The minimum value of also decreases, and there is a risk that inverter 5 will not be able to output the desired AC voltage. Here, the desired AC voltage that inverter 5 should output and the DC link voltage V required for it DC The minimum value is the voltage amplitude V of AC system 3. GP It is determined by V. GP The voltage amplitude V is detected by the detection unit 86. GP The system detects the detection result, and the threshold setting unit 87 sets a third threshold V TH3 The above DC link voltage V DC Within the range that is higher than the minimum value, the first threshold V TH1 and the third threshold V TH3 We decided to change each of them as described above. In this way, assuming that inverter 5 can output the desired AC voltage, the chopper 4 operates in TH mode at a battery voltage V BAT The range can be expanded.

[0090] (Third embodiment) (Influence of the battery's internal impedance) Figure 1 shows the internal configuration of battery 2, including the electromotive force 21 and internal impedance 22. The voltage generated inside battery 2 is represented by the electromotive force V due to the electromotive force 21. BO And the voltage drop V across the internal impedance 22 BR This was shown. With these internal configurations, the battery voltage V detected by the voltage detector 46 and input to the control device 8 is BAT The electromotive force V BO and voltage drop V BRIt is expressed as the sum (difference). In battery 2, the electromotive force V BO This changes depending on the remaining charge. On the other hand, the voltage drop V BR This value changes depending on the magnitude of the charge and discharge current.

[0091] Figure 11 shows an example of an operation chart during charging when the charging power changes while the chopper 4 is switching modes. In addition to the operation charts of each signal shown in Figure 5, Figure 11 also shows the electromotive voltage V BO An example of the operation chart has been added. In Figure 11, the operation up to time t1 is the same as in Figure 5. That is, at time t1, the battery voltage V BAT The first threshold V TH1 The signal increases to a certain point, and the transition from SW mode to TH mode begins.

[0092] In chopper 4, if the charging power of battery 2 decreases at time t2 during the switching from SW mode to TH mode, this causes the reactor current I L It also decreases. In the example operation chart in Figure 11, the slope of the decrease in charging power at time t2 is equal to the battery voltage V BAT or voltage command value V DCref It was deemed sufficiently steep compared to the fluctuations. In reality, the DC link voltage V is affected by the rapid decrease in charging power. DC Although the on-duty cycle d of the gate voltage G1 is also expected to change abruptly, this is ignored in Figure 11.

[0093] As the charging power decreases as described above at time t2, the voltage drop V decreases accordingly. BR The voltage decreases, and consequently the battery voltage V BAT It also decreases. As explained in Figure 3, S vref Battery voltage V in the generation unit 81 BAT For comparison with the threshold, the first threshold V TH1 and the third threshold V TH3 Hysteresis is introduced by this. However, the resistance value R of the internal impedance 22 in battery 2 B (Voltage drop V BR Depending on the magnitude, as shown in Figure 11, the voltage drop V at time t2 may occur. BRBattery voltage V changes in response to changes in the battery voltage V BAT As a result of the decrease, the third threshold V TH3 It may become lower than this. In this case, chopper 4 cancels the transition to TH mode and returns to SW mode, resulting in a voltage command value V DCref It increases again. Thus, the battery voltage V due to the internal impedance 22 BAT Changes in this area may prevent smooth mode switching.

[0094] (Block diagram of the control unit) Figure 12 is a block diagram of the control device 8B in a third embodiment of the present invention. In this embodiment, the power converter 1 of Figure 1 is equipped with the control device 8B of Figure 12 instead of the control device 8. The control device 8B shown in Figure 12 includes, in addition to the elements of the control device 8 of Figure 3 described in the first embodiment, an internal impedance estimation unit (hereinafter referred to as R B The system further includes an estimation unit 88 and a threshold setting unit 89.

[0095] R B The estimation unit 88 estimates the battery voltage V BAT and reactor current I L Based on this, the resistance value R of the internal impedance 22 B We estimate the following: Specifically, the battery voltage V when battery 2 is not being charged or discharged. BAT That is, the electromotive force V BO And, the battery voltage V during charging and discharging. BAT and reactor current I L Using this, the resistance value R B The estimation calculation is performed. Note that in SW mode, the reactor current I L Because ripple is included, the resistance value R is when chopper 4 is operating in SW mode. B When performing estimation calculations, R B An LPF is provided inside the estimation unit 88, thereby causing the reactor current I L Remove the ripple and then the resistance value R B It is preferable to perform an estimation calculation of the resistance value R during TH mode. B If you want to perform an estimation calculation, then R BIn the estimation unit 88, an LPF is unnecessary. In this case, since there is no detection noise due to the on / off operation of the switching elements 41 and 42, the estimation accuracy of the resistance value R B is improved. Therefore, in the control device 8B of the present embodiment, the R B estimation unit 88 preferably performs an estimation calculation of the resistance value R B when the chopper 4 operates in the TH mode.

[0096] The threshold setting unit 89 generates a third threshold V B based on the resistance value R B of the internal impedance 22 estimated by the R TH3 estimation unit 88. Specifically, when the resistance value R B is smaller than a predetermined fourth threshold R TH , the third threshold V B is adjusted so that the larger the resistance value R TH3 , the lower the third threshold V TH3 . Thereby, the larger the estimated resistance value R B , the larger the hysteresis width in the S vref generation unit 81, that is, the difference between the first threshold V TH1 and the third threshold V TH3 . Further, when the resistance value R B ]>is larger than the fourth threshold R TH , by keeping the third threshold V TH3 constant without adjustment, it is possible to prevent the third threshold V TH3 from becoming too small.

[0097] (Effect) In the present embodiment, the hysteresis characteristics of the S B generation unit 81 can be appropriately set based on the resistance value R vref . Thereby, even when the charging power of the battery 2 fluctuates, smooth mode switching of the chopper 4 can be realized. Further, even when the resistance value R B changes due to deterioration of the battery 2, by periodically estimating the resistance value R B , the hysteresis characteristics of the S B generation unit 81 can be appropriately maintained according to the change in the resistance value R vref . Note that the resistance value RB The estimation result can also be used for diagnosing the degradation of the battery 2 and the like.

[0098] The present invention is not limited to the above-described embodiments and modifications, and can be implemented using any components without departing from the gist thereof. Also, each embodiment and modification may be adopted alone, or a plurality of them may be arbitrarily combined and adopted. That is, in the present invention, by arbitrarily combining the features of each embodiment, the above-described effects can be achieved.

[0099] The above-described embodiments and modifications are merely examples, and the present invention is not limited to these contents as long as the features of the invention are not impaired. Also, although various embodiments and modifications have been described above, the present invention is not limited to these contents. Other aspects conceivable within the scope of the technical idea of the present invention are also included in the scope of the present invention.

Explanation of Reference Numerals

[0100] 1 Power conversion device 2 Battery 3 AC system 4, 4A Chopper 5 Inverter 6 Capacitor 7 Filter 8, 8A, 8B Control device 9 Voltage detector 21 Electromotive force 22 Internal impedance 41, 42 Switching element 43 Reactor 44 Capacitor 45, 46 Voltage detector 47 Current detector 48 Switching element 81 Voltage command switching signal generation unit (S vref Generation unit) 82 Voltage command generation unit (V DCref Generation unit) 83 Voltage difference determination signal generation unit (S vdif [[ID=6s0]]Generation unit) 84 Mode switching signal generation unit (S mode generation part) 85 Gate signal generation unit 86 System voltage detection unit (V GP Detection unit) 87 Threshold setting section 88 Internal impedance estimation unit (R B Estimation part) 89. Threshold setting section

Claims

1. A chopper having a switching element, which uses the switching element to convert the battery voltage and DC link voltage bidirectionally, An inverter that converts the DC link voltage and AC voltage bidirectionally, The system comprises the inverter and a control device for controlling the chopper, The control device can select either a first mode for controlling the chopper, which controls the DC link voltage according to a voltage command value by switching the switching element on and off, or a second mode in which the switching element is kept in a constantly on state. The control device is a power converter that reduces the voltage command value when transitioning the operating mode from the first mode to the second mode.

2. A power conversion device according to claim 1, The control device is a power conversion device that reduces the voltage command value at a predetermined rate when the battery voltage increases to a predetermined first threshold or higher.

3. A power conversion device according to claim 2, The control device is a power conversion device that, after starting to decrease the voltage command value, keeps the switching element permanently on and operates the chopper in the second mode when the difference between the DC link voltage and the battery voltage decreases to a predetermined second threshold.

4. A power conversion device according to claim 2, When the chopper is operating in the second mode and the battery voltage decreases to a predetermined third threshold, the control device increases the voltage command value and switches the switching element on and off to operate the chopper in the first mode. The power converter wherein the third threshold value is lower than the first threshold value.

5. A power conversion device according to claim 4, A power converter in which the third threshold value is the same as the lower limit value of the voltage command value.

6. A power conversion device according to claim 5, The inverter is connected to the AC system via a filter. A power converter in which the third threshold value is higher than the minimum DC link voltage required for the inverter to output a desired AC voltage.

7. A power conversion device according to claim 6, A power converter in which the value of one or both of the first threshold and the third threshold are set higher the higher the voltage of the AC system.

8. A power conversion device according to claim 5, The control device estimates the internal impedance of the battery that outputs the battery voltage, and if the estimated internal impedance is smaller than a predetermined fourth threshold, the power conversion device increases the difference between the first threshold and the third threshold as the internal impedance increases.

9. A power conversion device according to claim 8, The control device is a power converter that estimates the internal impedance when the chopper operates in the second mode.

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