Power conversion device

Abstract

The purpose of this invention is to improve the efficiency of power conversion devices and suppress shocks or unstable operation during mode switching. The power conversion device 1 of this invention comprises: a chopper 4 having switching elements 41 and 42, which bidirectionally converts the battery voltage VBAT to the DC link voltage VDC; an inverter 5 that bidirectionally converts the DC link voltage VDC to an AC voltage; and a control device 8 that controls the inverter 5 and the chopper 4. The control device 8 can select either a SW mode, which controls the DC link voltage VDC according to the voltage command value VDCref by turning the switching elements 41 and 42 on and off, or a TH mode, in which the switching elements 41 are always on, as the operating mode for controlling the chopper 4. When the operating mode of the chopper 4 changes from the SW mode to the TH mode, the voltage command value VDCref is reduced.

Description

Technical Field

[0001] This invention relates to an electric power conversion device. Prior Technology

[0002] As a means to achieve a decarbonized society, the introduction of renewable energy sources such as solar power generation is employed. In solar power generation, batteries are often installed to temporarily store the generated electricity. Hereinafter, both batteries and solar cells will be referred to as "batteries." When the power generation system of these batteries is interconnected with an AC system, a power conversion device is used to convert the DC voltage of the batteries into AC voltage.

[0003] Battery voltage is not constant. In the case of a rechargeable battery, the voltage varies depending on the charge level; in the case of a solar cell, the voltage varies depending on the amount of electricity generated. To address this variation in battery voltage, a power conversion device using an inverter and a chopper is previously known. The chopper converts the battery voltage to a DC-DC voltage, and the inverter converts the DC-DC voltage to an AC voltage. The DC-DC 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-DC voltage to a level higher than this minimum value.

[0004] On the other hand, when the battery voltage is high, as described in Patent Document 1, it is considered to set the switching elements (hereinafter referred to as elements) of the chopper to always be on. In this chopper operating mode, there is a DC short circuit between the input and output of the chopper, and the battery voltage and the DC link voltage are almost equal. In this way, since there are no switching losses of the elements or iron losses of the reactor, the efficiency of the power conversion device is improved. [Previous Technical Documents] [Patent Literature]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2015-82882 Summary of the Invention

[0006] [The problem the invention aims to solve] When switching between a mode that allows the chopper components to be switched on and off, and a mode that keeps them always on, there is a risk of overvoltage or overcurrent due to the switching shock. Furthermore, when the battery voltage is slightly lower than the DC link voltage, the switching time of the components is extremely short, which may cause the chopper's operation to become unstable. Additionally, consideration is given to maintaining this state for extended periods, depending on the battery capacity or charging / discharging pattern. Due to these issues, the increased peak voltage or current will lead to larger and more expensive power conversion devices.

[0007] In view of the above-mentioned problems, the present invention aims to suppress the impact or unstable operation during switching when using a power conversion device having an inverter and a chopper in an operating mode that switches the chopper components on and off and an operating mode that sets it to always on. [Technical means to solve the problem]

[0008] The power conversion device of the present invention comprises: a chopper having a switching element for bidirectional conversion of battery voltage and DC link voltage; an inverter for bidirectional conversion of DC link voltage and AC voltage; and a control device for controlling the inverter and the chopper; wherein the control device can select either a first mode in which the DC link voltage is controlled 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, as the operating mode for controlling the chopper; and the control device reduces the voltage command value when the operating mode changes 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 that turns the chopper's components on and off and an operating mode that sets it to always on, the impact or unstable operation during mode switching can be suppressed and the power conversion device can be miniaturized and reduced in cost. Simple Explanation of the Diagram

[0010] Figure 1 is a system configuration diagram of the power conversion device according to the first embodiment of the present invention. Figures 2(a) and (b) show examples of the operating waveforms of the chopper. Figure 3 is a block diagram of the control device in the first embodiment of the present invention. Figure 4 is a flowchart showing the processing sequence of the control device in the first embodiment of the present invention. Figure 5 is a diagram illustrating the operation of a power conversion device in the case of charging a battery according to the first embodiment of the present invention. Figure 6 is a diagram illustrating the operation of a power conversion device in the case of discharging a battery according to the first embodiment of the present invention. Figure 7 shows another example of the circuit configuration of the chopper in the power conversion device of the first embodiment of the present invention. Figures 8(a) and (b) show the operating waveforms of another example of a chopper. Figure 9 is a diagram illustrating the operation during discharge when the second voltage command value Vref2 is set to the same value as the third threshold value VTH3. Figure 10 is a block diagram of the control device in the second embodiment of the present invention. Figure 11 is a diagram illustrating the charging process during chopper mode switching, showing the changes in charging power. Figure 12 is a block diagram of the control device in the third embodiment of the present invention. Implementation

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

[0012] (First Implementation) (System configuration of power conversion device) Figure 1 is a system configuration diagram of the power conversion device according to the first embodiment of the present invention. The power conversion device 1 shown in Figure 1 is a bidirectional power conversion device that can convert the DC voltage of the battery 2 into AC voltage and output it to the AC system 3, and convert the AC voltage of the AC system 3 into DC voltage and output it to the battery 2. Through the power conversion function of this power conversion device 1, the battery 2 and the AC system 3 can be interconnected.

[0013] Battery 2 is a rechargeable battery that can be both charged and discharged. The voltage of battery 2 (battery voltage VBAT) varies depending on the state of battery 2. In Figure 1, the internal structure of battery 2 is shown, including an electromotive force 21 with a starting voltage VBO and an internal impedance 22 that causes a voltage drop VBR. The battery voltage VBAT can vary as the values ​​of the starting voltage VBO and the voltage drop VBR change according to the state of battery 2.

[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 components, the power conversion device 1 may further include protective components such as switches, fuses or surge protectors, and noise filters.

[0015] Chopper 4 has switching elements 41 and 42, which are used to convert the battery voltage VBAT into a DC link voltage VDC. Inverter 5 converts the DC link voltage VDC into an AC voltage and outputs it to AC system 3 via filter 7. Alternatively, inverter 5 can also convert the AC voltage input from AC system 3 via filter 7 into a DC link voltage VDC and output DC power to chopper 4. In this case, chopper 4 uses switching elements 41 and 42 to convert the DC link voltage VDC into the battery voltage VBAT and output DC power to battery 2.

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

[0017] The control device 8 controls the chopper 4 or inverter 5 based on the voltage or current within the power conversion device 1. The voltage detector 9 detects the three-phase AC voltages VR, VS, and VT input and output between the AC system 3 and the power conversion device 1.

[0018] Figure 1 shows an example of the specific circuit configuration of chopper 4. In addition to the aforementioned switching elements 41 and 42, chopper 4 also includes a reactor 43, a capacitor 44, voltage detectors 45 and 46, and a current detector 47. With this configuration, chopper 4 can bidirectionally convert between the battery voltage VBAT and the DC link voltage VDC. Furthermore, Figure 1 shows a MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) as the switching elements 41 and 42, but other types of elements such as IGBT (Insulated Gate Bipolar Transistor) can also be used.

[0019] Voltage detectors 45 and 46 detect the DC link voltage VDC and the battery voltage VBAT, respectively, and output the detection results to the control device 8. Current detector 47 detects the current flowing to reactor 43 (reactor current IL). In the following description, the direction of charging battery 2 (the direction of the arrow in Figure 1) is set as positive, and the polarity of reactor current IL is defined.

[0020] In Figure 1, the signals transmitted and received by the control device 8 are shown as follows: the DC link voltage VDC of voltage detectors 45 and 46, the battery voltage VBAT, the reactor current IL, and the AC voltages VR, VS, and VT of current detectors 47 and 9; the gate voltages G1 and G2 of switching elements 41 and 42; and the control signal CSINV and the detection signal DSINV related to the inverter 5. The control signal CSINV and the detection signal DSINV are each represented by an arrow, but each may contain multiple signals. The implementation method of the control device 8 is arbitrary, but as an example, it can be implemented as an electronic circuit mounted on a substrate equipped with a microcomputer or FPGA (Field-Programmable Gate Array).

[0021] (Chopper Action Mode) The chopper 4 has two main operating modes (Mode 1 and Mode 2). Figure 2 shows an example of the operating waveforms of the chopper 4. Specifically, Figure 2 shows the waveforms of the gate voltages G1 and G2 of the switching elements 41 and 42 and the reactor current IL.

[0022] Figure 2(a) shows a waveform example of chopper 4 operating in mode 1. Mode 1 is an operating mode in which the battery voltage VBAT is converted into DC link voltage VDC by the switching elements 41 and 42 in chopper 4 alternately turning on and off with a predetermined switching cycle TSW. Hereinafter, mode 1 will also be referred to as SW mode.

[0023] As shown in Figure 2(a), in SW mode, the gate voltages G1 and G2 become pulse waveforms of the switching period TSW, and the reactor current IL becomes a triangular wave with ripples of the switching period TSW. In the example of Figure 2(a), the operating period of the gate voltage G1 is set to d. Furthermore, in a steady state, it can be expressed as d = VBAT / VDC. The control device 8 performs feedback control in a manner where the DC link voltage VDC is consistent with the specified voltage command value VDCref, resulting in a change in the operating period d, thereby controlling the DC link voltage VDC. Alternatively, in the secondary control loop, there is a case where feedback control is performed on the reactor current IL instead of the DC link voltage VDC. In this case, the control device 8 generates a current command value ILref as a result of VDC control, and controls the reactor current IL in a manner where it is consistent with this current command value ILref, thereby making the operating period d of the gate voltage G1 variable.

[0024] Figure 2(b) shows a waveform example of chopper 4 operating in mode 2. Mode 2 is an operating mode in which switching elements 41 and 42 in chopper 4 are set to always on and always off, respectively. Hereinafter, mode 2 will also be referred to as TH mode.

[0025] In TH mode, by DC short-circuiting the input and output of chopper 4, the DC link voltage VDC is almost identical to the battery voltage VBAT. Furthermore, TH mode can also be described as an operation where the duty cycle d in the aforementioned SW mode is kept constant at 1. In TH mode, the waveform of the reactor current IL does not contain ripple. Also, feedback control of the DC link voltage VDC or the reactor current IL is not performed.

[0026] In TH mode, since no switching operation is performed in switching elements 41 and 42, no switching losses of switching elements 41 and 42 or iron losses of reactor 43 are generated. When the battery voltage VBAT is high enough that the inverter 5 can output the desired AC voltage even if VDC=VBAT, the conversion efficiency of the power conversion device 1 can be improved by operating the chopper 4 in TH mode. Therefore, the above two operating modes are basically switched according to the level of battery voltage VBAT.

[0027] Here, the following describes the problems with the switching of the operating mode of the chopper 4. First, due to the impact during the switching of the operating mode, vibration occurs in the voltage or current of the power conversion device 1, which may increase the peak value of the voltage or current, leading to the risk of the power conversion device 1 becoming larger or more expensive.

[0028] Next, when the battery voltage VBAT is slightly lower than the DC link voltage VDC, since d = VBAT / VDC in a steady state, d becomes a value slightly less than 1. At this time, the off-state of component 41 and the on-state of component 42 become extremely short, potentially causing instability in the operation of chopper 4. It is also considered that this state might persist for an extended period depending on the capacity or charging / discharging pattern of battery 2. Therefore, in this invention, these problems are solved by performing control as described below in the control device 8, thus avoiding the large size or high cost of the power conversion device 1.

[0029] (Block diagram and flowchart of the control device) 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 or mode switching of the chopper 4 are shown, while the block controlling the inverter 5 is omitted. Figure 4 is a flowchart showing the processing sequence of the control device 8 in the first embodiment of the present invention. Hereinafter, the mode switching control of the chopper 4 in the control device 8 of this embodiment will be described using these figures.

[0030] As shown in Figure 3, the control device 8 includes a voltage command switching signal generating unit (hereinafter referred to as Svref generating unit) 81 that generates a voltage command switching signal Svref, a voltage command generating unit (hereinafter referred to as VDCref generating unit) 82 that generates a voltage command value VDCref, a voltage difference determining signal generating unit (hereinafter referred to as Svdif generating unit) 83 that generates a voltage difference determining signal Svdif, a mode switching signal generating unit (hereinafter referred to as Smode generating unit) 84 that generates a mode switching signal Smode, and a gate signal generating unit 85 that generates gate voltages G1 and G2.

[0031] The Svref generation unit 81 compares the battery voltage VBAT with a predetermined threshold and generates a voltage command switching signal Svref based on the result. Furthermore, it is preferable to set a hysteresis for the threshold used in the Svref generation unit 81. Specifically, for example, when the voltage command switching signal Svref is at level L, if the battery voltage VBAT is higher than a predetermined first threshold VTH1, the Svref generation unit 81 switches the voltage command switching signal Svref to level H. Similarly, for example, when the voltage command switching signal Svref is at level H, if the battery voltage VBAT is lower than a predetermined third threshold VTH3, the Svref generation unit 81 switches the voltage command switching signal Svref to level L. Furthermore, the third threshold VTH3 is set lower than the first threshold VTH1. Thus, by setting a hysteresis characteristic for the output of the voltage command switching signal Svref in the Svref generation unit 81, the logic value of the voltage command switching signal Svref can be prevented from frequently switching due to detection noise such as battery voltage VBAT.

[0032] The VDCref generation unit 82 generates a voltage command value VDCref for the DC link voltage VDC based on the voltage command switching signal Svref input from the Svref generation unit 81. A signal selection switch is provided in the VDCref generation unit 82, which outputs either a first voltage command value Vref1 or a second voltage command value Vref2 based on the voltage command switching signal Svref. Furthermore, the second voltage command value Vref2 is set lower than the first voltage command value Vref1. The first voltage command value Vref1 and the second voltage command value Vref2 correspond to the upper and lower limits of the voltage command value VDCref, respectively.

[0033] In the VDCref generation unit 82, the signal selection switch outputs a first voltage command value Vref1 when the voltage command switching signal Svref is at level L, and outputs a second voltage command value Vref2 when the voltage command switching signal Svref is at level H. An LPF (Low Pass Filter) is provided on the output side of the signal selection switch. The VDCref generation unit 82 outputs the voltage command value VDCref after processing the output of the signal selection switch with the LPF. That is, when the voltage command switching signal Svref changes from level L to level H, the voltage command value VDCref output by the VDCref generation unit 82 gradually decreases from the first voltage command value Vref1 to the second voltage command value Vref2. Conversely, when the voltage command switching signal Svref changes from level H to level L, the voltage command value VDCref output by the VDCref generation unit 82 gradually increases from the second voltage command value Vref2 to the first voltage command value Vref1. The rate at which the voltage command value VDCref decreases or increases under these conditions can be adjusted by the time constant of the LPF. Specifically, it is preferable to set the time constant of the LPF such that the rate at which the voltage command value VDCref decreases or increases is the same as or greater than the time constant of the feedback control of the DC link voltage VDC performed by the control device 8.

[0034] Alternatively, a signal selection switch or LPF can be omitted, and the VDCref generation unit 82 can be configured to increase or decrease the voltage command value VDCref between the first voltage command value Vref1 and the second voltage command value Vref2 at a predetermined rate. For example, in the VDCref generation unit 82, the voltage command value VDCref can be increased or decreased by a predetermined amount at constant calculation cycles according to the change of the voltage command switching signal Svref, so that the voltage command value VDCref changes in stages between the first voltage command value Vref1 and the second voltage command value Vref2. Furthermore, the rate at which the voltage command value VDCref increases or decreases can be a constant value or a value that changes according to the passage of time. In the control device 8, since the voltage command value VDCref can be continuously or in stages changed between the first voltage command value Vref1 and the second voltage command value Vref2 with only a certain amount of time, the VDCref generation unit 82 can be implemented by any configuration.

[0035] The SVDIF generation unit 83 compares the voltage difference VDIF between the DC link voltage VDC and the battery voltage VBAT with a predetermined threshold, and generates a voltage difference determination signal SVDIF based on the result. Specifically, when the voltage difference VDIF is lower than a predetermined second threshold VTH2, the SVDIF generation unit 83 outputs an H-level signal as the voltage difference determination signal SVDIF; when the voltage difference VDIF is higher than or equal to the second threshold VTH2, the SVDIF generation unit 83 outputs an L-level signal as the voltage difference determination signal SVDIF.

[0036] The Smode generation unit 84 generates a mode switching signal Smode based on the voltage command switching signal Svref input from the Svref generation unit 81 and the voltage difference determination signal Svdif input from the Svdif generation unit 83. The mode switching signal Smode represents the operating mode of the chopper 4 through its logic value. That is, the L level of the mode switching signal Smode indicates the SW mode, and the H level indicates the TH mode. In addition, Figure 3 shows an example in which the Smode generation unit 84 is constructed using a logic product (AND) gate, a rising detector, a falling detector, and an RS flip-flop. However, the internal configuration of the Smode generation unit 84 is arbitrary as long as the following operations can be achieved.

[0037] When the mode switching signal Smode is at level L, if both the voltage command switching signal Svref and the voltage difference determination signal Svdif become level H, then the Smode generation unit 84 switches the mode switching signal Smode to level H. This switches the chopper 4 from SW mode to TH mode. Furthermore, when the mode switching signal Smode is at level H, if the voltage command switching signal Svref becomes level L, then the Smode generation unit 84 changes the mode switching signal Smode to level L. This switches the chopper 4 from TH mode to SW mode.

[0038] Here, when the second threshold VTH2 is set to a value of around zero in the Svdif generation unit 83, under the condition that the voltage difference VDIF is sufficiently small, that is, under the condition that the DC link voltage VDC and the battery voltage VBAT are almost the same, the mode switching signal Smode switches from the L level to the H level, and the chopper 4 switches from the SW mode to the TH mode.

[0039] The gate signal generation unit 85 generates gate voltages G1 and G2 for switching elements 41 and 42 based on the voltage command value VDCref input from the VDCref generation unit 82 or the mode switching signal Smode input from the Smode generation unit 84. When the mode switching signal Smode is at level L, that is, when the chopper 4 operates in SW mode, the gate signal generation unit 85 performs feedback control calculations on the DC link voltage VDC based on the voltage command value VDCref, and calculates the operating cycle d of the switching element 41. Then, the gate voltages G1 and G2 are generated by repeatedly turning on and off the switching elements 41 and 42 according to the calculated operating cycle d. At this time, as described above, the operating cycle d of the switching element 41 can also be calculated by using the current command value ILref to perform feedback control calculations on the reactor current IL. On the other hand, when the mode switching signal Smode is H-level, that is, when the chopper 4 operates in TH mode, the gate signal generating unit 85 generates gate voltages G1 and G2 by switching elements 41 and 42 being always on and always off, respectively.

[0040] The control device 8 uses the components shown in the block diagram of Figure 3 to execute the processing of the flowchart in Figure 4 at predetermined operation cycles, thereby generating gate voltages G1 and G2 to control the operation of the chopper 4.

[0041] In step S10, the control device 8 obtains the DC link voltage VDC from the voltage detector 45 and the battery voltage VBAT from the voltage detector 46.

[0042] In step S20, the control device 8 calculates the difference between the DC link voltage VDC and the battery voltage VBAT obtained in step S10, and calculates the voltage difference VDIF.

[0043] In step S30, the control device 8 obtains the previous values ​​of the voltage command switching signal Svref and the mode switching signal Smode. Here, if the processing described in steps S70 or S130 (described later) was performed in the previous operation cycle, the value of the voltage command switching signal Svref determined by those processing is obtained as the previous value. On the other hand, if neither step S70 nor S130 was performed in the previous operation cycle, the value of the voltage command switching signal Svref, which is the same as that obtained in step S30 of the previous operation cycle, is obtained as the previous value. Similarly, if the processing described in steps S110 or S150 (described later) was performed in the previous operation cycle, the value of the mode switching signal Smode determined by those processing is obtained as the previous value. On the other hand, if neither of the processes in steps S110 and S150 in the previous operation cycle is implemented, the value of the mode switching signal Smode, which is the same as that obtained by step S30 in the previous operation cycle, can be used as the previous value.

[0044] In step S40, the control device 8 determines whether the previous value of the mode switching signal Smode obtained in step S30 was at the L level. If the previous value of the mode switching signal Smode was at the L level, the process proceeds to step S50; otherwise, if the previous value was at the H level, the process proceeds to step S120.

[0045] In step S50, the control device 8 determines whether the previous value of the voltage command switching signal Svref obtained in step S30 was at level L. If the previous value of the voltage command switching signal Svref was at level L, the process proceeds to step S60; otherwise, if the previous value was at level H, the process proceeds to step S90.

[0046] In step S60, the control device 8 compares the battery voltage VBAT obtained in step S10 with a predetermined first threshold VTH1. As a result, if the battery voltage VBAT does not reach the first threshold VTH1, the process proceeds to step S100; otherwise, if it exceeds the first threshold VTH1, the process proceeds to step S70. Furthermore, when transitioning from step S60 to step S100, the mode switching signal Smode is maintained at the same L level as the previous value.

[0047] In step S70, the control device 8 switches the logic value of the voltage command switching signal Svref from the L level of the previous value to the H level.

[0048] In step S80, the control device 8, based on the voltage command switching signal Svref that switched from L level to H level in step S70, causes the voltage command value VDCref to continuously or intermittently change from the first voltage command value Vref1 to the second voltage command value Vref2. Here, as described above, the voltage command value VDCref gradually decreases from the first voltage command value Vref1 toward the second voltage command value Vref2 at a predetermined decreasing rate.

[0049] In step S90, the control device 8 compares the voltage difference VDIF calculated in step S20 with a predetermined second threshold VTH2. If the voltage difference VDIF is greater than the second threshold VTH2, the process proceeds to step S100; otherwise, if the voltage difference is less than the second threshold VTH2, the process proceeds to step S110. Furthermore, when transitioning from step S90 to step S100, the mode switching signal Smode is maintained at the same L level as the previous value.

[0050] In step S100, the control device 8 performs feedback control calculations on the DC link voltage VDC based on the voltage command value VDCref. Here, as described above, the duty cycle d of the switching element 41 is calculated so that the DC link voltage VDC is close to the voltage command value VDCref. After the processing in step S100 is performed, the process proceeds to step S170.

[0051] In step S110, the control device 8 switches the logic value of the mode switching signal Smode from the L level of the previous value to the H level. This switches the operating mode of the chopper 4 from SW mode to TH mode. After performing step S110, the process proceeds to step S170.

[0052] In step S120, the control device 8 compares the battery voltage VBAT obtained in step S10 with a predetermined third threshold VTH3. As a result, if the battery voltage VBAT is greater than the third threshold VTH3, the process proceeds to step S170; otherwise, if it is less than the third threshold VTH3, the process proceeds to step S130. Furthermore, when transitioning from step S120 to step S170, the mode switching signal Smode is maintained at the same H level as the previous value.

[0053] In step S130, the control device 8 switches the logic value of the voltage command switching signal Svref from the H level of the previous value to the L level.

[0054] In step S140, the control device 8, based on the voltage command switching signal Svref that switched from the H level to the L level in step S130, causes the voltage command value VDCref to continuously or intermittently change from the second voltage command value Vref2 to the first voltage command value Vref1. Here, as described above, the voltage command value VDCref gradually increases from the second voltage command value Vref2 toward the first voltage command value Vref1 at a predetermined rate.

[0055] In step S150, the control device 8 switches the logic value of the mode switching signal Smode from the H level of the previous value to the L level. This switches the operating mode of the chopper 4 from TH mode to SW mode.

[0056] In step S160, the control device 8 performs feedback control calculations on the DC link voltage VDC based on the voltage command value VDCref. Here, similar to step S100, the operating cycle d of the switching element 41 is calculated so that the DC link voltage VDC approaches the voltage command value VDCref. After performing the processing in step S160, 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, when the mode switching signal Smode is at level L and the operating period d of the switching element 41 is calculated in step S100 or S160, the operation is in SW mode, and the switching elements 41 and 42 are turned on and off according to the calculated operating period d, generating gate voltages G1 and G2. On the other hand, when the mode switching signal Smode is at level H, the operation is in TH mode, and the switching elements 41 and 42 are always on and always off, generating gate voltages G1 and G2. After performing the processing in step S170, the control device 8 terminates the flowchart in FIG4.

[0058] Furthermore, in the flowchart of Figure 4 described above, steps S50-S70 and S120-S130 correspond to the Svref generation unit 81 in Figure 3. Steps S80 and S140 correspond to the VDCref generation unit 82 in Figure 3, and step S90 corresponds to the Svdif generation unit 83 in Figure 3. Steps S40, S110, and S150 correspond to the Smode generation unit 84 in Figure 3, and steps S100, S160, and S170 correspond to the gate signal generation unit 85.

[0059] (Illustration of charging process) Figure 5 is a diagram illustrating the operation of the power conversion device 1 during the charging of the battery 2 in the first embodiment of the present invention. In Figure 5, the following are examples of the operation of the voltage command switching signal Svref, the voltage difference determination signal Svdif, the mode switching signal Smode, the DC link voltage VDC, the battery voltage VBAT, the voltage command value VDCref, the voltage difference VDIF between the DC link voltage VDC and the battery voltage VBAT, the operating cycle d of the switching element 41, and the reactor current IL. Furthermore, in Figure 5, the DC link voltage VDC and the battery voltage VBAT are displayed superimposed on the same graph using solid and dashed lines, respectively. Also, the fluctuations (ripples) generated when the chopper 4 operates in SW mode are ignored in the reactor current IL.

[0060] At time t0, because the battery voltage VBAT is lower than the first threshold VTH1, and the voltage command switching signal Svref generated by the Svref generation unit 81 is at level L, the voltage command value VDCref generated by the VDCref generation unit 82 becomes the first voltage command value Vref1. Furthermore, because the voltage difference VDIF is higher than the second threshold VTH2, the voltage difference determination signal Svdif generated by the Svdif generation unit 83 is at level L. Thus, at time t0, both the voltage command switching signal Svref and the voltage difference determination signal Svdif are at level L, therefore the mode switching signal Smode output from the Smode generation unit 84 also becomes at level L. At this time, the chopper 4 operates in SW mode, and the DC link voltage VDC is controlled by the chopper 4 in conjunction with the voltage command value VDCref (the first voltage command value Vref1).

[0061] After time t0, by charging battery 2, the battery voltage VBAT gradually increases, but the DC link voltage VDC remains controlled by chopper 4 at the voltage command value VDCref, i.e., the first voltage command value Vref1. As a result, the operating cycle d of switching element 41 increases in accordance with the battery voltage VBAT. On the other hand, the voltage difference VDIF gradually decreases.

[0062] When the battery voltage VBAT increases to the first threshold VTH1 at time t1, the voltage command switching signal Svref changes from level L to level H due to the operation of the Svref generation unit 81. Correspondingly, the VDCref generation unit 82 operates in accordance with the change in the voltage command switching signal Svref, causing the voltage command value VDCref to decrease at a predetermined rate from the first voltage command value Vref1 towards the second voltage command value Vref2. At this point, because the voltage difference VDIF remains above the second threshold VTH2, i.e., the voltage difference determination signal Svdif is at level L, the mode switching signal Smode also remains at level L. Therefore, the chopper 4 continues to operate in SW mode, and through the chopper 4, the DC link voltage VDC decreases as the voltage command value VDCref changes.

[0063] When the voltage difference VDIF decreases to the second threshold VTH2 at time t2, the voltage difference determination signal Svdif changes from the L level to the H level due to the operation of the Svdif generation unit 83. Correspondingly, the mode switching signal Smode changes from the L level to the H level due to the operation of the Smode generation unit 84, which generates the voltage difference determination signal Svdif. Thus, the chopper 4 switches from SW mode to TH mode, and the operating period d of the switching element 41 remains constant at 1. The voltage command value VDCref continues to decrease towards the second voltage command value Vref2, while the DC link voltage VDC increases in conjunction with the battery voltage VBAT.

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

[0065] (Diagram of the action during discharge) Figure 6 is a diagram illustrating the operation of the power conversion device 1 when discharging the battery 2 in the first embodiment of the present invention. Unlike Figure 5, in Figure 6, in addition to the DC link voltage VDC and the battery voltage VBAT, the voltage command value VDCref and the battery voltage VBAT are also displayed superimposed on the same graph using solid and dashed lines, respectively.

[0066] At time t3, the voltage command switching signal Svref, the voltage difference determination signal Svdif, and the mode switching signal Smode are all at level H. At this time, since the chopper 4 operates in TH mode, the operating period d of the switching element 41 is constant at 1, so the DC link voltage VDC is almost the same as the battery voltage VBAT.

[0067] After time t3, by discharging battery 2, the battery voltage VBAT gradually decreases, and the DC link voltage VDC decreases in accordance with the battery voltage VBAT.

[0068] When the battery voltage VBAT decreases to the third threshold VTH3 at time t4, the voltage command switching signal Svref changes from level H to level L due to the operation of the Svref generation unit 81. Correspondingly, the VDCref generation unit 82 operates in accordance with this change in the voltage command switching signal Svref, causing the voltage command value VDCref to increase at a predetermined rate from the second voltage command value Vref2 towards the first voltage command value Vref1. Furthermore, as the voltage command switching signal Svref becomes level L, the mode switching signal Smode also switches from level H to level L according to the operation of the Smode generation unit 84. This switches the operating mode of the chopper 4 from TH mode to SW mode. However, at this point, the battery voltage VBAT is higher than the voltage command value VDCref, therefore the DC link voltage VDC continues to be almost identical to the battery voltage VBAT.

[0069] When the voltage command value VDCref increases to the level of the battery voltage VBAT at time t5, the DC link voltage VDC begins to increase along with the voltage command value VDCref under the control of chopper 4. That is, the actual operation of SW mode begins from time t5. When the voltage difference VDIF increases to the second threshold VTH2 at time t6, the voltage difference determination signal Svdif changes from the H level to the L level through the operation of Svdif generation unit 83.

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

[0071] (Effect) As previously described, a problem with the operating mode switching of chopper 4 is that when the battery voltage VBAT remains slightly lower than the DC link voltage VDC for an extended period, the operation of chopper 4 may become unstable. In this invention, as described above, when the battery voltage VBAT increases to the first threshold VTH1, the voltage command value VDCref for the DC link voltage VDC is reduced at a predetermined rate, causing the operating mode of chopper 4 to switch from SW mode to TH mode. This prevents chopper 4 from operating continuously for extended periods when its operating mode is SW mode and the voltage difference VDIF between the DC link voltage VDC and the battery voltage VBAT is less than the first threshold VTH1.

[0072] Furthermore, after the voltage command value VDCref begins to decrease, when the voltage difference VDIF decreases to the second threshold VTH2, the chopper 4 switches from SW mode to TH mode. Therefore, when the second threshold VTH2 is set to a value around zero, mode switching occurs when the DC link voltage VDC and the battery voltage VBAT are almost identical. This suppresses the impact of mode switching and prevents voltage or current fluctuations or excessive fluctuations in the power conversion device 1.

[0073] Furthermore, by considering the time constant of the feedback control of the DC link voltage VDC, the reduction rate of the voltage command value VDCref is set. This prevents overshoot or oscillation of the DC link voltage VDC during the period when it is reduced according to the voltage command value VDCref, thus ensuring a smooth reduction of VDC. Moreover, feedback control of the DC link voltage VDC can be performed according to the voltage command value VDCref until just before the chopper 4 switches to TH mode. Therefore, even in the event of disturbances such as changes in charging power, the voltage or current of the power conversion device 1 can be prevented from oscillating or becoming excessive.

[0074] As described above, in this embodiment, in the power conversion device 1 equipped with inverter 5 and chopper 4, by using the TH mode, in which the switching element 41 of chopper 4 is always on, the efficiency of power conversion device 1 is improved, and the impact or unstable operation during mode switching is suppressed. This allows for miniaturization and cost reduction of power conversion device 1.

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

[0076] (1) The power conversion device 1 includes: a chopper 4 having switching elements 41 and 42, which are used to bidirectionally convert the battery voltage VBAT and the DC link voltage VDC; an inverter 5, which bidirectionally converts the DC link voltage VDC and the AC voltage; and a control device 8, which controls the inverter 5 and the chopper 4. The control device 8 can select either a first mode (SW mode) in which the DC link voltage VDC is controlled according to the voltage command value VDCref by turning the switching elements 41 and 42 on and off, or a second mode (TH mode) in which the switching element 41 is always on, as the operating mode of the chopper 4. When the control device 8 changes the operating mode of the chopper 4 from SW mode to TH mode, it reduces the voltage command value VDCref (step S80). As a result, the efficiency of the power conversion device 1 can be improved, and the impact or unstable operation during mode switching can be suppressed.

[0077] (2) When the battery voltage VBAT increases to a predetermined first threshold VTH1 or higher (step S60: No), the control device 8 reduces the voltage command value VDCref at a predetermined rate (step S80). Therefore, when the operating mode of the chopper 4 is changed from SW mode to TH mode, the voltage command value VDCref can be reliably reduced.

[0078] (3) After the control device 8 causes the voltage command value VDCref to begin decreasing in step S80, when the difference between the DC link voltage VDC and the battery voltage VBAT (voltage difference VDIF) decreases to the specified second threshold VTH2 (step S90: no), the switching element 41 is set to the always-on state, causing the chopper 4 to operate in TH mode (step S110). Therefore, the operating mode of the chopper 4 can be switched from SW mode to TH mode at appropriate timing. As a result, the voltage or current vibration of the power conversion device 1 during mode switching can be suppressed, and excessive voltage or current can be prevented from flowing in the power conversion device 1.

[0079] (4) When the chopper 4 operates in TH mode (step S40: No), if the battery voltage VBAT decreases to the specified third threshold VTH3 (step S120: No), the control device 8 increases the voltage command value VDCref (step S140), and the switching elements 41 and 42 are turned on and off respectively, causing the chopper 4 to operate in SW mode (step S150). The third threshold VTH3 is a lower value than the first threshold VTH1. Therefore, it can prevent the chopper 4 from switching its operating mode from TH mode to SW mode and then switching back to TH mode in a short time, thus stabilizing the operation of the chopper 4.

[0080] (Another example related to the circuit configuration of a chopper) Figure 7 shows another example of the circuit configuration of the chopper in the power conversion device 1 according to the first embodiment of the present invention. In the power conversion device 1 of Figure 1, the chopper 4A of Figure 7 can be provided instead of the chopper 4. In addition to the elements provided by the chopper 4 of Figure 1, the chopper 4A shown in Figure 7 further includes a switching element 48. In the chopper 4A, the switching element 48 and the portion that connects the switching element 41 and the reactor 43 in series are connected in parallel.

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

[0082] Figure 8(a) shows a waveform example of chopper 4A operating in SW mode. As shown in Figure 8(a), in SW mode, chopper 4A causes switching elements 41 and 42 to alternately turn on and off with a specified switching cycle TSW, and sets switching element 48 to always be on.

[0083] Figure 8(b) shows a waveform example of chopper 4 operating in TH mode. As shown in Figure 8(b), in TH mode, chopper 4A sets switching elements 41, 42, and 48 to 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. Therefore, when using chopper 4A, since there is no loss in reactor 43, the efficiency of power conversion device 1 can be improved accordingly.

[0084] (Another example related to the method of setting the voltage command value and the operation diagram during discharge) In Figure 6 above, an example of the operation during discharge is shown for the case where the second voltage command value Vref2 in the power conversion device 1 is set lower than the third threshold VTH3. In this operation diagram, after the chopper 4 switches from TH mode to SW mode, a delay occurs because there is a period during which the DC link voltage VDC decreases in conjunction with the battery voltage VBAT (during time t4~t5), until the DC link voltage VDC is controlled according to the voltage command value VDCref. To avoid this situation, in the power conversion device 1 of Figure 1, the third threshold VTH3 can be set to the same value as the lower limit of the voltage command value VDCref, i.e., the second voltage command value Vref2.

[0085] Figure 9 shows an example of the discharge operation when the third threshold VTH3 is set to the same value as the second voltage command value Vref2. In Figure 9, times t3, t4, and t5 are the same as in Figure 6. That is, as illustrated in Figure 6, when the battery voltage VBAT decreases to the third threshold VTH3 at time t4, the chopper 4 switches from TH mode to SW mode. Also, at the same time t4, the DC link voltage VDC also decreases to the third threshold VTH3. Here, since the second voltage command value Vref2 is the same as the third threshold VTH3, at time t4, the voltage command value VDCref starts to increase from the second voltage command value Vref2, which is the third threshold VTH3. Therefore, the DC link voltage VDC is controlled immediately in accordance with the voltage command value VDCref, without the control delay shown in Figure 6.

[0086] Furthermore, as described above, when the second voltage command value Vref2 is set to the same value as the third threshold VTH3, the DC link voltage VDC can be reduced to the second voltage command value Vref2, which is the third threshold VTH3. Here, the DC link voltage VDC specifies the minimum value required for the inverter 5 to output the desired AC voltage and interconnect with the AC system 3. Therefore, by setting the third threshold VTH3 to a value higher than the minimum value of the DC link voltage VDC, regardless of the operating mode of the chopper 4 or the level of the battery voltage VBAT, the inverter 5 can output the desired AC voltage to the AC system 3.

[0087] (Second Implementation) (Block diagram of the control device) Figure 10 is a block diagram of the control device 8A in the second embodiment of the present invention. In this embodiment, the control device 8A shown in Figure 10 is provided in place of the control device 8 in the power conversion device 1 of Figure 1. 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 is equipped with the detection values ​​of AC voltages VR, VS, and VT from the input voltage detector 9. In addition to the elements of the control device 8, it also includes a system voltage detection unit (hereinafter, VGP detection unit) 86 and a threshold setting unit 87. Furthermore, in Figure 10, the internal components of the VDCref generation unit 82, the Svdif generation unit 83, and the Smode generation unit 84 are omitted.

[0088] The VGP detection unit 86 detects the voltage amplitude VGP of the AC system 3 from the detected values ​​of AC voltages VR, VS, and VT. The threshold setting unit 87 generates a first threshold VTH1 and a third threshold VTH3 based on the voltage amplitude VGP detected by the VGP detection unit 86. Specifically, the thresholds are set according to the voltage amplitude VGP such that the higher the voltage amplitude VGP, the higher the first threshold VTH1 and the third threshold VTH3, respectively. Alternatively, only one of the first threshold VTH1 or the third threshold VTH3 can be changed according to the voltage amplitude VGP, while the other remains unchanged. The first threshold VTH1 and the third threshold VTH3 generated by the threshold setting unit 87 are input to the Svref generation unit 81 for processing as described above.

[0089] (Effect) In the Svref generation unit 81, the lower the first threshold VTH1 and the third threshold VTH3 are set, the lower the battery voltage VBAT value when the voltage command switching signal Svref is set to the H level. Therefore, the range of battery voltage VBAT when the chopper 4 operates in TH mode is increased, which helps to improve the efficiency of the power conversion device 1. On the other hand, when the first threshold VTH1 and the third threshold VTH3 are too low, the minimum value of the DC link voltage VDC in TH mode operation also becomes smaller, which may prevent the inverter 5 from outputting the desired AC voltage. Here, the desired AC voltage that the inverter 5 should output and the minimum value of its required DC link voltage VDC are determined by the voltage amplitude VGP of the AC system 3. Therefore, in this embodiment, the voltage amplitude VGP is detected by the VGP detection unit 86, and based on the detection result, the threshold setting unit 87 sets the first threshold VTH1 and the third threshold VTH3 to be within the range where the third threshold VTH3 is higher than the minimum value of the DC link voltage VDC, as described above. In this way, the range of the battery voltage VBAT when the chopper 4 operates in TH mode can be expanded, provided that the inverter 5 can output the desired AC voltage.

[0090] (Third Implementation) (The Influence of Internal Impedance of the Battery) In Figure 1, the internal structure of battery 2 is shown, including electromotive force 21 and internal impedance 22. The voltages generated within battery 2 are also shown, including the starting voltage VBO of electromotive force 21 and the voltage drop VBR of internal impedance 22. Based on these internal structures, the battery voltage VBAT, detected by voltage detector 46 and input to control device 8, is represented as the sum (difference) of starting voltage VBO and voltage drop VBR. In battery 2, starting voltage VBO varies depending on the charge margin. On the other hand, voltage drop VBR varies depending on the magnitude of the charge / discharge current.

[0091] Figure 11 is a diagram illustrating the charging operation during the mode switching of chopper 4, showing the changes in charging power. In Figure 11, in addition to the operation diagrams of the signals shown in Figure 5, an operation diagram of the start-up voltage VBO is 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 VBAT increases to the first threshold VTH1, and the transition from SW mode to TH mode begins.

[0092] In chopper 4, during the transition from SW mode to TH mode at time t2, when the charging power of battery 2 decreases, the reactor current IL also decreases accordingly. In the operation diagram of Figure 11, at time t2, the slope of the decrease in charging power is sufficiently steep compared to the change in battery voltage VBAT or voltage command value VDCref. Furthermore, it is actually assumed that due to the sudden decrease in charging power, the duty cycle d of the DC link voltage VDC or gate voltage G1 also changes abruptly, but this is ignored in Figure 11.

[0093] When the charging power decreases at time t2 as described above, the voltage drop VBR decreases accordingly, and consequently the battery voltage VBAT also decreases. As explained in Figure 3, in the comparison between the battery voltage VBAT and the threshold in the Svref generation unit 81, a hysteresis is set between the first threshold VTH1 and the third threshold VTH3. However, based on the resistance value RB (the magnitude of the voltage drop VBR) of the internal impedance 22 in battery 2, as shown in Figure 11, at time t2, the battery voltage VBAT decreases according to the change in voltage drop VBR, resulting in a situation where it falls below the third threshold VTH3. At this time, in the chopper 4, by canceling the transition to TH mode and returning to SW mode, the voltage command value VDCref increases again. Thus, there is a situation where the change in battery voltage VBAT caused by the internal impedance 22 hinders smooth mode switching.

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

[0095] The RB estimation unit 88 estimates the resistance value RB of the internal impedance 22 based on the battery voltage VBAT and the reactor current IL. Specifically, the estimated resistance value RB is calculated using the battery voltage VBAT (start-up voltage VBO) when the battery 2 is not being charged or discharged, and the battery voltage VBAT and reactor current IL during charging and discharging. Furthermore, in SW mode, since the reactor current IL contains ripple, when the estimated resistance value RB is calculated during chopper 4's SW mode operation, it is preferable to install an LPF inside the RB estimation unit 88 to remove the ripple from the reactor current IL before calculating the estimated resistance value RB. However, if the estimated resistance value RB is calculated in TH mode, an LPF is not needed in the RB estimation unit 88. In this case, there is no detection noise caused by the on / off operation of switching elements 41 and 42, thus improving the estimation accuracy of the resistance value RB. Therefore, in the control device 8B of this embodiment, it is preferable that the RB estimation unit 88 performs the estimation calculation of the resistance value RB when the chopper 4 operates in TH mode.

[0096] The threshold setting unit 89 generates a third threshold VTH3 based on the resistance value RB of the internal impedance 22 estimated by the RB estimation unit 88. Specifically, when the resistance value RB is less than a predetermined fourth threshold RTH, the third threshold VTH3 is adjusted so that the larger the resistance value RB is, the lower the third threshold VTH3 is. Therefore, the larger the estimated resistance value RB is, the greater the difference between the hysteresis width in the Svref generation unit 81, i.e., the first threshold VTH1 and the third threshold VTH3. Furthermore, when the resistance value RB is greater than the fourth threshold RTH, by setting the third threshold VTH3 constant without adjustment, the third threshold VTH3 can be prevented from becoming too small.

[0097] (Effect) In this embodiment, the hysteresis characteristic of the Svref generation unit 81 can be appropriately set based on the resistance value RB. This allows for smooth mode switching of the chopper 4 even when the charging power of the battery 2 fluctuates. Furthermore, even when the resistance value RB changes due to battery 2 degradation, the hysteresis characteristic of the Svref generation unit 81 can be appropriately maintained based on the changes in the resistance value RB by periodically estimating the resistance value RB. Additionally, the estimated resistance value RB can also be used for battery 2 degradation diagnosis.

[0098] This invention is not limited to the embodiments or variations described above, and can be implemented using any constituent elements without departing from its spirit. Furthermore, each embodiment or variation can be used individually, or multiple embodiments can be arbitrarily combined. That is, in this invention, the aforementioned effects can be achieved by arbitrarily combining the features of each embodiment.

[0099] The above-described embodiments or variations are merely examples, and the invention is not limited to these contents as long as they do not impair the features of the invention. Furthermore, while various embodiments or variations have been described above, the invention is not limited to these contents. Other forms that can be considered within the scope of the technical concept of the invention are also included within the scope of the invention.

[0100] 1: Power conversion device 2: Battery 3: Communication System 4,4A: Chopper 5: Inverter 6: Capacitor 7: Filter 8, 8A, 8B: Control devices 9: Voltage detector 21: Electromotive force 22: Internal impedance 41: Switching element 42: Switching element 43: Reactor 44: Capacitor 45: Voltage Detector 46: Voltage Detector 47: Current Detector 48: Switching elements 81: Voltage command switching signal generation unit (Svref generation unit) 82: Voltage command generation unit (VDCref generation unit) 83: Voltage Difference Determination Signal Generation Unit (SVDIF Generation Unit) 84: Mode Switching Signal Generation Unit (Smode Generation Unit) 85: Gate signal generation unit 86: System Voltage Detection Unit (VGP Detection Unit) 87: Threshold setting section 88: Internal Impedance Estimation Section (RB Estimation Section) 89: Threshold setting section CSINV: Control Signal DSINV: Detection signal d: Work cycle G1: Gate voltage G2: Gate voltage G3: Gate voltage H: Level IL: Reactor current L: Level RB: Resistance value RTH: 4th Threshold S10: Steps S20: Steps S30: Steps S40: Steps S50: Steps S60: Steps S70: Steps S80: Steps S90: Steps S100: Steps S110: Steps S120: Steps S130: Steps S140: Steps S150: Steps S160: Steps S170: Steps Smode: Mode switching signal Svdif: Voltage Difference Detection Signal Svref: Voltage command switching signal SW: Mode TH: Pattern TSW: Switching cycle t0~t6: Time points VBAT: Battery voltage VBO: Startup voltage VBR: Voltage Drop VDC: DC link voltage VDCref: Voltage command value VDIF: Voltage Difference VGP: Voltage Amplitude VR: AC voltage Vref1: First voltage command value Vref2: Second voltage command value VS: AC voltage VT: AC voltage VTH1: First threshold VTH2: Second Threshold VTH3: Third Threshold

Claims

1. A power conversion device comprising: a chopper having a switching element for bidirectional conversion of battery voltage and DC-DC link voltage using the switching element; an inverter for bidirectional conversion of the DC-DC link voltage and AC voltage; and a control device for controlling the inverter and the chopper; wherein the control device is selectable as an operating mode for controlling the chopper by switching the switching element on and off according to a voltage command value, or a second mode in which the switching element is always on; the control device reduces the voltage command value when the operating mode changes from the first mode to the second mode; and the control device reduces the voltage command value at a predetermined rate when the battery voltage increases to or exceeds a predetermined first threshold value.

2. The power conversion device of claim 1, wherein after the control device causes the voltage command value to begin to decrease, when the difference between the DC link voltage and the battery voltage decreases to a predetermined second threshold, the control device sets the switching element to a always-on state so that the chopper operates in the second mode.

3. The power conversion device of claim 1, wherein when the chopper operates in the second mode, if the battery voltage decreases to a predetermined third threshold, the control device increases the voltage command value and causes the switching element to turn on and off, thereby causing the chopper to operate in the first mode, wherein the third threshold is lower than the first threshold.

4. The power conversion device as requested in item 3, wherein the third threshold is the same as the lower limit of the voltage command value.

5. The power conversion device as claimed in claim 4, wherein the inverter is connected to an AC system via a filter, and the third threshold is a higher value than the minimum value of the DC link voltage required for the inverter to output the desired AC voltage.

6. As in the power conversion device of claim 5, the higher the voltage of the AC system, the higher the value of one or both of the first threshold and the third threshold is set.

7. The power conversion device of claim 4, wherein the internal impedance of the battery that outputs the battery voltage estimated by the control device is less than a predetermined fourth threshold, and the greater the internal impedance, the greater the difference between the first threshold and the third threshold.

8. The power conversion device of claim 7, wherein the control device estimates the internal impedance when the chopper operates in the second mode.

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