JP2022062948A - Power storage system and power storage device - Google Patents

Abstract

To provide a power storage system and a power storage device that can make grid voltage follow load change of a DC connection bus in the controllable range.SOLUTION: A power storage system includes a first DC/DC 11 that is connected to a DC connection bus 20 and controls autonomously charging and discharging current by using a current-voltage droop characteristic, a power detection unit 16 that detects power converted in the first DC/DC 11, and a characteristic control unit 18 that, when the current-voltage droop characteristic where the offset voltage Vof with zero charging and discharging current is the intermediate voltage Vcmid in a control region is the reference characteristic, determines as the current-voltage droop characteristic used in the first DC/DC 11, the current-voltage droop characteristic in which the offset voltage Vof is moved from the reference characteristic in accordance with the deviation between the grid voltage Vgrid and a grid voltage command Vgrid* calculated based on the power detected by the power detection unit 16 and a preset power-voltage droop characteristic.SELECTED DRAWING: Figure 2

Description

The present invention relates to a power storage system and a power storage device that exchange power between a plurality of distributed power storage devices.

In recent years, it is expected that the method of using natural energy will shift to a self-consumption (self-consumption) system instead of grid interconnection. As a self-consumption system, for example, a power storage device that stores electric power generated by a solar cell device at a disaster prevention base can be considered.

Therefore, the applicant can control the power interchange between the power storage devices by simply switching the bidirectional DC / DC converter connected to the DC grid between discharging and charging and setting the energy. We have proposed an inexpensive system with high utilization efficiency and reduced risk (see, for example, Patent Document 1).

Japanese Patent No. 6458891

In the prior art, the charge droop characteristic and the discharge droop characteristic, which are different when the charge is set and when the discharge is set, are used to control the lower limit voltage at which the charge current becomes zero when the charge is set and the control where the discharge current becomes zero when the discharge is set. The area between the upper limit voltage and the grid voltage is used as the control range. However, there is a problem that it is not possible to cope with the load fluctuation of the DC grid due to the connection of the load device or the power generation device to the DC connection bus, and the grid voltage cannot be suppressed within the control range.

That is, the power storage device set to charge cannot cope with the load fluctuation of the DC grid where the grid voltage falls below the lower limit voltage, and the grid voltage cannot be suppressed within the control range. Further, if the grid voltage exceeds the upper limit voltage, the power storage device set to discharge cannot cope with the load fluctuation of the DC grid, and the grid voltage cannot be suppressed within the control range.

An object of the present invention is to solve the above-mentioned problems of the prior art and to provide a power storage system and a power storage device capable of tracking a grid voltage within a control range in response to load fluctuations of a DC connection bus.

The power storage system of the present invention has a power storage device to which a storage battery for storing DC power is connected as a basic unit, and a plurality of the power storage devices connected to the DC connection bus autonomously control the grid voltage of the DC connection bus. A power storage system that controls charging or discharging, wherein one or more of the power storage devices are connected to the DC connection bus and a bidirectional DC / DC converter that controls charge / discharge current using current-voltage droop characteristics. The current-voltage droop characteristic, in which the power detector that detects the power converted by the bidirectional DC / DC converter and the current-voltage droop characteristic in which the offset voltage at which the charge / discharge current becomes zero is the intermediate voltage in the control region, is used as a reference characteristic. The offset voltage is moved from the reference characteristic according to the deviation between the grid voltage command calculated based on the power detected by the power detection unit and the preset power-voltage droop characteristic and the grid voltage. It is characterized by comprising a characteristic control unit that determines the current-voltage droop characteristic as the current-voltage droop characteristic used in the bidirectional DC / DC converter.
Further, the power storage device of the present invention is connected to the DC connection bus, and the power storage device that autonomously controls charging or discharging by controlling the grid voltage of the DC connection bus is connected to the DC connection bus, and is current-voltage. A bidirectional DC / DC converter that autonomously controls the charge / discharge current using droop characteristics, a power detector that detects the power converted by the bidirectional DC / DC converter, and zero charge / discharge current. The grid voltage calculated based on the power detected by the power detector and the preset power-voltage droop characteristic, with the current-voltage droop characteristic in which the offset voltage is the intermediate voltage of the control region as the reference characteristic. A characteristic that determines the current-voltage droop characteristic in which the offset voltage is moved from the reference characteristic according to the deviation between the command and the grid voltage as the current-voltage droop characteristic used in the bidirectional DC / DC converter. It is characterized by comprising a control unit.

According to the present invention, the current-voltage droop characteristic in which the droop control reference is set at the center of the control region can be seamlessly and responsively controlled in the charge / discharge direction. It has the effect of being able to follow the grid voltage within the control range.

It is a block diagram which shows the structural example of the embodiment of the power storage system which concerns on this invention. It is a block diagram which shows the structure of the power storage device shown in FIG. It is a figure which modeled the power storage device connected to the DC connection bus shown in FIG. 1. It is a graph which shows the fluctuation of the grid voltage with the fluctuation of the load and the offset voltage in the model shown in FIG. It is explanatory drawing explaining the operation of the power storage device shown in FIG. It is a block diagram which shows the structure of the characteristic control part shown in FIG. It is a figure explaining the droop characteristic determination operation by the characteristic control part shown in FIG. It is a figure which shows the calculation example of K _soc = shown in FIG. It is a figure explaining the operation in the discharge direction in the power storage system which concerns on this invention. It is a figure explaining the operation in the charging direction in the power storage system which concerns on this invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the following embodiments, the same reference numerals are given to the configurations showing the same functions, and the description thereof will be omitted as appropriate.

In this embodiment, referring to FIG. 1, the power storage device 1 to which the storage battery 2 is connected is used as one basic unit, and a plurality of power storage devices 1 are connected in parallel by the DC connection bus 20 to generate electric power between the power storage devices 1. It is a power storage system that implements flexibility. The DC connection bus 20 is not connected to a stable power source (power system or the like), assuming a non-power source area, a disaster prevention area, or the like, and forms a low impedance DC grid. Further, FIG. 1 shows an example in which N power storage devices 1 are connected to the DC connection bus 20, and there is no limit to the number of power storage devices 1 connected to the DC connection bus 20.

Referring to FIG. 2, the power storage device 1 includes a grid connection terminal T1, a storage battery connection terminal T2, a PV connection terminal T3, and a load connection terminal T4, respectively.

The grid connection terminal T1 is a terminal connected to the DC connection bus 20 to transmit and receive DC power. The storage battery connection terminal T2 is a terminal for charging / discharging the connected storage battery 2. The PV connection terminal T3 is a terminal that receives DC power generated by the solar cell 3 connected via the junction box 31. The load connection terminal T4 is a terminal that supplies electric power to the connected load 4.

With reference to FIG. 2, the power storage device 1 is referred to as a first bidirectional DC / DC converter 11 (hereinafter referred to as a first DC / DC 11) and a second bidirectional DC / DC converter 12 (hereinafter referred to as a second DC / DC 12). ), An MPPTDC / DC converter 13, a PCS14, a current sensor 15, a power detection unit 16, a storage amount detection unit 17, and a characteristic control unit 18. The first DC / DC11, the second DC / DC12, the MPPTDC / DC converter 13, and the PCS14 are connected via the DC link 19.

The second DC / DC 12 is a charger / discharger connected between the storage battery connection terminal T2 and the DC link 19 and charging / discharging the storage battery 2 by constant DC link voltage control (fixed value control) of the DC link 19. When charging the storage battery 2, the second DC / DC 12 converts the DC voltage of the DC link 19 into a voltage suitable for charging the storage battery 2 to charge the storage battery 2. Further, when discharging from the storage battery 2, the second DC / DC 12 converts the DC voltage of the storage battery 2 into the DC voltage of the DC link 19 and discharges from the storage battery 2.

The MPPTDC / DC converter 13 is connected between the PV connection terminal T3 and the DC link 19, and is a DC / DC / of the maximum power point tracking system (MPPT: Maximum Power Point Tracking) that receives the direct current power generated by the solar cell 3. It is a DC converter. The MPPTDC / DC converter 13 outputs the DC power received from the solar cell 3 to the DC link 19.

The PCS 14 is a power conditioner system connected between the load connection terminal T4 and the DC link 19 and converting the DC voltage of the DC link 19 into electric power suitable for the load 4 and supplying the DC link 19 to the load 4.

The current sensor 15 detects the charge / discharge current when the storage battery 2 is charged / discharged and outputs the charge / discharge current to the storage amount detection unit 17.

The power detection unit 16 is connected to the first DC / DC 11, detects the power P _OUT converted by the first DC / DC 11, and outputs the detected power P _OUT and the grid voltage Vgrid to the characteristic control unit 18. In the present embodiment, the power detection unit 16 detects the power P _{OUT in the charging direction as positive and the power P OUT in} the discharging direction as negative.

The storage amount detection unit 17 calculates the SOC (SOC: State of charge) based on the charge / discharge current of the storage battery 2 detected by the current sensor 15 as the detection of the storage amount of the storage battery 2. The storage amount detection unit 17 integrates the charge current and the discharge current of the storage battery 2, respectively, and calculates the SOC based on the difference value. The amount of stored electricity may be detected based on the terminal voltage of the storage battery 2 or the like.

The characteristic control unit 18 is an information processing unit such as a microcomputer equipped with a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), and the like. The control program is stored in the ROM. The CPU of the characteristic control unit 18 operates by reading the control program stored in the ROM and expanding the control program in the RAM.

The characteristic control unit 18 obtains a current-voltage droop characteristic having a drooping characteristic according to a linear function calculation formula based on the power P _OUT detected by the power detection unit 16 and the SOC calculated by the storage amount detection unit 17. decide.

The first DC / DC 11 is connected between the grid connection terminal T1 and the DC link 19, and is the voltage of the DC connection bus 20 (hereinafter referred to as the grid voltage) based on the current-voltage droop characteristic determined by the characteristic control unit 18. The charge / discharge current is autonomously controlled according to (referred to as).

First, the control model in the power storage system of the present embodiment will be described in detail with reference to FIGS. 3 to 5.
FIG. 3A is a model of N power storage devices 1 connected to the DC connection bus 20. In FIG. 3A, i ₁ to i _N are charge / discharge currents of each of the N power storage devices 1, and R is a virtual resistance value having a slope of the current-voltage droop characteristic. And Vof ₁ to Vof _N are offset voltages of each of N power storage devices 1 derived from the grid voltage Vgrid of the DC connection bus 20 and i ₁ to i _N and R.

According to the model of FIG. 3A, the charge / discharge current ik in the _kth power storage device 1 is
ik _{= Vgrid / R-Vof k /} R
And
The total charge / discharge current iΣ of the N power storage devices 1 is based on the assumption that the average of Vof ₁ to Vof _N is Vof _avr .
iΣ = N × Vgrid / RN × Vof _avr / R
Will be.

Therefore, the model of the whole system can be represented by FIG. 3 (b), and the whole system droop characteristic is Vgrid = (R / N) × iΣ + Vof _avr as shown in the block diagram of the whole system shown in FIG. 3 (c).
Will be. The i _LOAD shown in FIG. 3B is a load current connected to the DC connection bus 20.
iΣ = -i _LOAD
Is the relationship.

As a result, the change amount ΔVgrid of the grid voltage Vgrid given by the change amount Δi _LOAD of the load current i _LOAD becomes
ΔVgrid = (R / N) × ΔiΣ
As shown in FIG. 4A, the fluctuation amount ΔVgrid generated by the change amount Δi _LOAD appears as the same amount of voltage fluctuation in the operating characteristics of the single machine.

Further, the fluctuation amount ΔVgrid of the grid voltage Vgrid given by the change amount Vof _avr of the offset voltage Vof of the entire system is
ΔVgrid = ΔVof _avr
Therefore, the fluctuation amount ΔVgrid generated by the change amount Vof _avr appears as the same amount of voltage fluctuation in the operating characteristics of the single machine as shown in FIG. 4 (b).

Therefore, in the present embodiment, as shown by the dotted line in FIG. 5A, the offset voltage Vof at which the charge / discharge current becomes zero is an intermediate voltage in the droop control region (control lower limit voltage Vc _min -control upper limit voltage Vc _max ). Using the current-voltage droop characteristic, which is Vc _mid , as a reference characteristic, it is possible to seamlessly respond to changes in both polarities of the load current. Then, as shown in FIG. 5 (b), the change amount Δi _LOAD due to the disturbance (load and system fluctuation) and the fluctuation amount ΔVgrid given by the change amount Vof _avr are set to the current-voltage droop characteristic in the single power storage device 1. The offset voltage Vof is controlled to an arbitrary grid voltage Vgrid by moving and controlling the reference characteristic (intermediate voltage Vc _mid ) as shown by the solid line in FIG. 5 (a). The alternate long and short dash line shown in FIG. 5A is the droop characteristic of the entire system, which is the sum of the droop characteristics of the single machine whose movement is controlled, and the grid voltage Vgrid is the intersection of the droop characteristic of the entire system and the load current i _LOAD . Is controlled by.

Next, the droop characteristic determination operation by the characteristic control unit 18 will be described in detail with reference to FIGS. 6 to 8.
Referring to FIG. 6, the characteristic control unit 18 calculates the grid voltage command Vgrid ^* based on the power P _OUT detected by the power detection unit 16 and the SOC calculated by the storage amount detection unit 17. It includes a calculation unit 181 and an offset voltage calculation unit 182 that calculates an offset voltage command Vof ^* based on the deviation between the grid voltage command Vgrid ^* and the fed-back grid voltage Vgrid.

As shown in FIG. 7, the grid voltage calculation unit 181 has the minimum value Pc _min and the maximum value Pc _max of the output power as the control lower limit voltage Vc _min and the control upper limit voltage Vc _max in the control region in the current-voltage droop characteristic. The power-voltage droop characteristic having the drooping characteristic is set by the arithmetic expression of the linear function. The grid voltage calculation unit 181 uses this power-voltage droop characteristic to set the grid voltage command Vgrid ^* to Vgrid ^* =.
(1 / K _soc ) x [(Vc _max -Vc _min ) / (Pc _max -Pc _min) ] x P _OUT + Vc _mid
Calculated by. As a result, according to the power P _OUT detected by the power detection unit 16, the grid voltage command Vgrid ^* is set in the direction from the intermediate voltage Vc _mid to the control upper limit voltage Vc _max during charging, and the grid voltage command Vgrid ^* is set during discharging. It is operated from the intermediate voltage Vc _mid to the control lower limit voltage Vc _min .

(1 / K _soc ) is a coefficient for weighting the power-voltage droop characteristic with the SOC calculated by the storage amount detection unit 17. In this embodiment, K _soc is calculated to a value of .555 or more and 1.0 or less based on SOC, as shown in FIG. Then, K _soc changes the calculation formula using the polarity of the power P _OUT , and when the power P _OUT is positive and the charging direction, the smaller the amount of electricity stored, the smaller the value, and the power P _OUT is negative and the discharging direction. In that case, the larger the amount of electricity stored, the smaller the value. Therefore, when the power P _OUT is positive and the charging direction is set, the smaller the storage amount, the larger the gradient of the power-voltage droop characteristic. Therefore, the calculated grid voltage command Vgrid ^* is larger than the storage device 1 having a large storage amount. The value of the power storage device 1 having a small amount of storage is larger. Further, when the power P _OUT is negative and the discharge direction is large, the gradient of the power-voltage droop characteristic becomes larger as the amount of storage increases. Therefore, the calculated grid voltage command Vgrid ^* is larger than that of the power storage device 1 having a large amount of storage. The value of the power storage device 1 having a small amount of power storage is larger.

The offset voltage calculation unit 182 determines the current-voltage droop characteristic by adding the intermediate voltage Vc _mid to the integrated value of the deviation between the grid voltage command Vgrid ^* and the fed-back grid voltage Vgrid. ^* Calculate.

The offset voltage command Vof ^* calculated by the offset voltage calculation unit 182 is output to the first DC / DC 11, and the first DC / DC 11 is the voltage of the DC connection bus 20 based on the determined current-voltage droop characteristic (hereinafter referred to as the voltage). , Called grid voltage) autonomously controls the charge / discharge current.

In this way, by setting the reference for droop control based on the current-voltage droop characteristic at the center of the control area and controlling it seamlessly and responsively in the charge / discharge direction, the grid voltage corresponds to the load fluctuation of the DC connection bus 20. The Vgrid can be made to follow within the control range.

Then, by weighting the power-voltage droop characteristic with SOC, the charge / discharge power amount can be controlled according to the power storage amount of the power storage device 1, and the power storage between the power storage devices 1 connected to the DC connection bus 20 can be controlled. The amount can be averaged to optimally operate the power storage system.

As shown in FIG. 9A, the power storage device 1a having a small amount of electricity stored (K _soc = 1.000 in the discharge direction) and the power storage device 1b having a large amount of electricity stored (K _soc = 0.555 in the discharge direction) are connected by DC. A simulation was carried out in which the current source 30 was connected to the DC connection bus 20 and the current _icu flowing in the direction of the current source 30 was applied from the DC connection bus 20 in the state of being connected to the bus 20. The current i _cu was changed to 1.0 A at 30 [sec] by passing 2.7 A at 10 [sec].

As a result, as shown in FIG. 9 (b), the fluctuation of the grid voltage Vgrid caused by the application of the current i _cu is suppressed, and as shown in FIG. 9 (c), the share ratio depending on K _soc (SOC) ( It can be seen that the discharge is carried out at 1 / 1.0000: 1 / 0.555).

Further, as shown in FIG. 10A, a power storage device 1a having a small amount of electricity stored (K _soc = 0.555 in the charging direction) and a power storage device 1b having a large amount of electricity stored (K _soc = 1.000 in the charging direction) are used. A simulation was carried out in which a current source 30a was connected to the DC connection bus 20 and a current _icu flowing in the direction of the DC connection bus 20 was applied from the current source 30a in a state of being connected to the DC connection bus 20. The current i _cu was changed to 1.0 A at 30 [sec] by passing 2.6 A at 10 [sec].

As a result, as shown in FIG. 10 (b), the fluctuation of the grid voltage Vgrid caused by the application of the current i _cu is suppressed, and as shown in FIG. 10 (c), the share ratio depending on K _soc (SOC) ( It can be seen that charging is performed at 1 / 0.555: 1 / 1.000).

That is, the grid voltage Vgrid is also common to the power storage devices 1a and 1b. Therefore, the power sharing ratio is also the current sharing ratio (power = voltage × current). Therefore, by varying the slope of the power-voltage droop characteristic according to the K _soc (SOC), the power sharing ratio is changed.

In the power storage system of the present embodiment, if one or more power storage devices 1 are connected to the DC connection bus 20, a predetermined effect can be obtained. That is, the power storage device 1 of the present embodiment may be used by being connected in parallel with a power storage device having another configuration (for example, the power storage device of Patent Document 1) that autonomously controls the charge / discharge current according to the grid voltage. can.

As described above, according to the present embodiment, the power storage device 1 to which the storage battery 2 for storing DC power is connected is a basic unit, and the plurality of power storage devices 1 connected to the DC connection bus 20 are DC connection buses. A power storage system that autonomously controls charging or discharging by controlling the grid voltage Vgrid of 20. One or more power storage devices 1 are connected to a DC connection bus 20 and charged / discharged using a current-voltage droop characteristic. The control region includes a first DC / DC 11 (bidirectional DC / DC converter) that controls the current, a power detection unit 16 that detects the power converted by the first DC / DC 11, and an offset voltage Vof that makes the charge / discharge current zero. The grid voltage command Vgrid ^* and the grid voltage calculated based on the power detected by the power detection unit 16 and the preset power-voltage droop characteristic with the current-voltage droop characteristic as the intermediate voltage Vc _mid as the reference characteristic. It is provided with a characteristic control unit 18 that determines a current-voltage droop characteristic in which an offset voltage Vof is moved from a reference characteristic according to a deviation from Vgrid as a current-voltage droop characteristic used in the first DC / DC 11.
With this configuration, the current-voltage droop characteristic in which the droop control reference is set in the center of the control area can be seamlessly and responsively controlled in the charge / discharge direction. Therefore, the grid corresponds to the load fluctuation of the DC connection bus 20. The voltage Vgrid can be made to follow within the control range.

Further, according to the present embodiment, in the current-voltage droop characteristic, the minimum value Pc _min and the maximum value Pc _max of the output power are the control lower limit voltage Vc _min and the control upper limit voltage Vc in the control region in the current-voltage droop characteristic. It has a drooping characteristic according to the calculation formula of the linear function that becomes _max .
With this configuration, it is possible to respond to load fluctuations with simple control.

Further, according to the present embodiment, the storage amount detection unit 17 for calculating the SOC as the storage amount of the storage battery 2 is provided, and the characteristic control unit 18 is powered by the power-voltage according to the SOC calculated by the storage amount detection unit 17. By changing the gradient of the droop characteristic and weighting the power-voltage droop characteristic with SOC, the charge / discharge power amount is controlled according to the charge amount of the power storage device 1.
With this configuration, the charge / discharge current can be controlled according to the amount of electricity stored, and the amount of electricity stored between the electricity storage devices 1 connected to the DC connection bus 20 can be averaged.

Further, according to the present embodiment, instead of changing the inclination of the power-voltage droop characteristic, the characteristic control unit 18 makes the moving speed of the current-voltage droop characteristic faster in the charging direction as the SOC is smaller, and in the discharging direction. The movement speed of the current-voltage droop characteristic is controlled faster as the SOC increases.
With this configuration, it is possible to average the amount of electricity stored between the electricity storage devices 1 connected to the DC connection bus 20 by utilizing the charge / discharge current of the electricity storage device 1 connected to the DC connection bus 20.

Although the present invention has been described above with specific embodiments, it goes without saying that the above embodiment is an example and can be modified and implemented without departing from the spirit of the present invention.

1, 1a, 1b Power storage device 2 Storage battery 3 Solar cell 4 Load 11 1st DC / DC (1st bidirectional DC / DC converter)
12 2nd DC / DC (2nd bidirectional DC / DC converter)
13 MPPTDC / DC converter 14 PCS
15 Current sensor 16 Power detection unit 17 Power storage amount detection unit 18 Characteristic control unit 19 DC link 20 DC connection bus 30 30a Current source 31 Connection box 181 Grid voltage calculation unit 182 Offset voltage calculation unit T1 Grid connection terminal T2 Storage battery connection terminal T3 PV Connection terminal T4 Load connection terminal

Claims (5)

A storage device to which a storage battery for storing DC power is connected is used as a basic unit, and a plurality of the storage devices connected to the DC connection bus autonomously control charging or discharging by controlling the grid voltage of the DC connection bus. It ’s a system,
One or more of the power storage devices
A bidirectional DC / DC converter connected to the DC connection bus and controlling charge / discharge current using current-voltage droop characteristics.
A power detector that detects the power converted by the bidirectional DC / DC converter, and
The current-voltage droop characteristic, in which the offset voltage at which the charge / discharge current becomes zero is the intermediate voltage in the control region, is used as the reference characteristic, and the power detected by the power detector and the preset power-voltage droop characteristic are used. The current-voltage droop characteristic obtained by moving the offset voltage from the reference characteristic according to the deviation between the grid voltage command calculated based on the grid voltage command and the grid voltage is used in the bidirectional DC / DC converter. A power storage system including a characteristic control unit that determines voltage droop characteristics. The current-voltage droop characteristic has a drooping characteristic according to a linear function in which the minimum and maximum values of the output power are the control lower limit voltage and the control upper limit voltage in the control region in the current-voltage droop characteristic. The power storage system according to claim 1. It is provided with a storage amount detection unit that calculates SOC as the storage amount of the storage battery.
The characteristic control unit changes the inclination of the power-voltage droop characteristic according to the SOC calculated by the storage amount detection unit, and weights the power-voltage droop characteristic with the SOC, thereby storing the power. The power storage system according to claim 2, wherein the charge / discharge power amount is controlled according to the power storage amount of the device. It is provided with a storage amount detection unit that calculates SOC as the storage amount of the storage battery.
The characteristic control unit controls the moving speed of the current-voltage droop characteristic in the charging direction faster as the SOC is smaller, and controls the moving speed of the current-voltage droop characteristic faster in the discharging direction as the SOC is higher. The power storage system according to claim 2. A power storage device that is connected to a DC connection bus and autonomously controls charging or discharging by controlling the grid voltage of the DC connection bus
A bidirectional DC / DC converter connected to the DC connection bus and autonomously controlling the charge / discharge current using the current-voltage droop characteristic.
A power detector that detects the power converted by the bidirectional DC / DC converter, and
The current-voltage droop characteristic, in which the offset voltage at which the charge / discharge current becomes zero is the intermediate voltage in the control region, is used as the reference characteristic, and the power detected by the power detector and the preset power-voltage droop characteristic are used. The current-voltage droop characteristic obtained by moving the offset voltage from the reference characteristic according to the deviation between the grid voltage command calculated based on the grid voltage command and the grid voltage is used in the bidirectional DC / DC converter. A power storage device including a characteristic control unit that determines a voltage droop characteristic.

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