JP7472602B2 - Energy storage system and energy storage device - Google Patents

Description

The present invention relates to a power storage system and a power storage device that exchanges power between multiple distributed power storage devices.

In recent years, the use of natural energy is expected to shift from grid-connected systems to self-consumption (self-consumption) systems. One example of a self-consumption system is a power storage device at a disaster prevention base that stores electricity generated by a solar cell device.

Therefore, the applicant has proposed an inexpensive system that can control the power interchange between each storage device by simply switching the bidirectional DC/DC converter connected to the DC connection bus between discharging and charging, with high energy utilization efficiency and reduced risk (see, for example, Patent Document 1).

Patent No. 6458891

However, in conventional technology, when all the power storage devices are operating with charging characteristics, power is not exchanged between the power storage devices. Therefore, there is a risk that the device with the larger load (power consumption) will go down sooner, which makes it difficult to operate the power storage system stably.

The object of the present invention is to provide a power storage system and a power storage device that solves the above problems of the conventional technology, reduces the risk of a device with a large load going down even when all the power storage devices are operating with charging characteristics, and enables stable operation over the long term.

The energy storage system of the present invention is an energy storage system in which a basic unit is an energy storage device connected to a storage battery that stores DC power, and a plurality of the energy storage devices connected to a DC connection bus individually control charging or discharging by controlling the grid voltage of the DC connection bus to perform power interchange, and each of the plurality of energy storage devices includes a bidirectional DC/DC converter connected to the DC connection bus, an interchange control unit that sets the bidirectional DC/DC converter for charging or discharging, and a storage amount detection unit that detects the amount of stored energy in the storage battery, and when the bidirectional DC/DC converter is set for the charging use, it controls charging based on a charging characteristic in which a charging current increases with an increase in the grid voltage, and when the amount of stored energy in the storage battery is within a predetermined shift range, it shifts the charging characteristic in a direction in which an offset voltage that makes the charging current zero increases, and when the grid voltage is equal to or greater than the offset voltage, it operates as a charging region in which the current in the charging direction is positive, and when the grid voltage is less than the offset voltage, it operates as a discharging region in which the current in the charging direction is negative .
The energy storage device of the present invention is an energy storage device that is connected between a storage battery that stores DC power and a DC connection bus, and that individually controls charging or discharging by controlling the grid voltage of the DC connection bus to perform power interchange with other energy storage devices connected to the DC connection bus, and is characterized in that it includes a bidirectional DC/DC converter connected to the DC connection bus, an interchange control unit that sets the bidirectional DC/DC converter for charging or discharging, and a storage amount detection unit that detects the amount of stored energy in the storage battery, and when the bidirectional DC/DC converter is set for the charging use, it controls charging based on a charging characteristic in which a charging current increases as the grid voltage increases, and when the amount of stored energy in the storage battery is within a predetermined shift range, it shifts the charging characteristic in a direction in which an offset voltage that makes the charging current zero increases , and when the grid voltage is equal to or greater than the offset voltage, it operates as a charging region in which the current in the charging direction is positive, and when the grid voltage is less than the offset voltage, it operates as a discharging region in which the current in the charging direction is negative .

According to the present invention, even if all the storage devices are operating with charging characteristics, power can be transferred from a storage device with a high SOC to a storage device with a low SOC, which reduces the risk of a storage device with a high load going down and enables stable operation over the long term.

1 is a configuration diagram showing a configuration example of an embodiment of a power storage system according to the present invention; 2 is a block diagram showing a configuration of the power storage device shown in FIG. 1 . 3 is a diagram showing an output voltage characteristic of the first bidirectional DC/DC converter shown in FIG. 2; 3 is a flowchart illustrating a setting operation of the first bidirectional DC/DC converter by the interchange control unit shown in FIG. 2 . 2 is an explanatory diagram illustrating a power interchange operation by the power storage device shown in FIG. 1 . FIG. 6 is a diagram showing operating points of output voltage characteristics in the power interchange operation shown in FIGS. FIG. 6 is a diagram showing operating points of output voltage characteristics in the power interchange operation shown in FIG. 5( c ).

The following describes in detail the embodiments of the present invention with reference to the drawings. Note that in the following embodiments, components that have similar functions are given the same reference numerals and descriptions are omitted as appropriate.

Referring to FIG. 1, this embodiment is a power storage system in which a power storage device 1 connected to a storage battery 2 is treated as one basic unit, and multiple power storage devices 1 are connected in parallel via a DC connection bus 20 to exchange power between the power storage devices 1. Note that the DC connection bus 20 forms a DC grid that is not connected to a stable power source (such as a power system) in an unelectrified area or disaster prevention area. Also, while FIG. 1 shows an example in which three power storage devices 1 are connected to the DC connection bus 20, there is no limit to the number of power storage devices 1 that can be connected to the DC connection bus 20.

The energy storage device 1 has a power interchange terminal T1, a storage battery connection terminal T2, a PV connection terminal T3, and a load connection terminal T4.

The power interchange terminal T1 is connected to the DC connection bus 20 and is a terminal for transmitting and receiving DC power. The battery connection terminal T2 is a terminal for charging and discharging the connected storage battery 2. The PV connection terminal T3 is a terminal for receiving DC power generated by the solar cell 3 connected via the connection box 31. The load connection terminal T4 is a terminal for supplying power to the connected load 4.

2, the energy storage device 1 includes a first bidirectional DC/DC converter 11 (hereinafter referred to as the first DC/DC 11), a second bidirectional DC/DC converter 12 (hereinafter referred to as the second DC/DC 12), an MPPT DC/DC converter 13, a PCS 14, current sensors 15 and 16, a storage amount detection unit 17, and an interchange control unit 18. The first DC/DC 11, the second DC/DC 12, the MPPT DC/DC converter 13, and the PCS 14 are connected via a DC link 19.

The second DC/DC 12 is a charger/discharger that is connected between the battery connection terminal T2 and the DC link 19 and charges/discharges the storage battery 2 with 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 to a voltage suitable for charging the storage battery 2, and charges the storage battery 2. When discharging the storage battery 2, the second DC/DC 12 converts the DC voltage of the storage battery 2 to the DC voltage of the DC link 19, and discharges the storage battery 2.

The MPPT DC/DC converter 13 is a maximum power point tracking (MPPT) DC/DC converter that is connected between the PV connection terminal T3 and the DC link 19 and receives DC power generated by the solar cell 3. The MPPT DC/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 that is connected between the load connection terminal T4 and the DC link 19, converts the DC voltage of the DC link 19 into power suitable for the load 4, and supplies it to the load 4.

The current sensor 15 outputs to the charge amount detection unit 17, which detects the amount of stored electricity based on the charge/discharge current detected when the storage battery 2 is charged or discharged.

The current sensor 16 detects the generated current Ipv of the solar cell 3 and outputs it to the interchange control unit 18.

The power storage amount detection unit 17 detects the amount of power stored in the storage battery 2 and calculates the SOC (State of charge) of the storage battery 2 based on the charge and discharge current of the storage battery 2 detected by the current sensor 15. The power storage amount detection unit 17 integrates the charge current and discharge current of the storage battery 2 and calculates the SOC based on the difference between the two. The amount of power stored may also be detected based on the terminal voltage of the storage battery 2, etc.

The interchange 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), etc. A control program is stored in the ROM. The CPU of the interchange control unit 18 operates by reading the control program stored in the ROM and expanding the control program in the RAM.

The interchange control unit 18 sets the first DC/DC 11 for discharging or charging based on the power generation state of the solar cell 3 and the SOC. When the interchange control unit 18 sets the first DC/DC 11 for charging and the SOC is within a preset shift range, it determines the amount of shift of the output voltage characteristics (droop characteristics) according to the SOC and notifies the first DC/DC 11.

The first DC/DC 11 is connected between the power interchange terminal T1 and the DC link 19, and is individually controlled by controlling the voltage of the DC connection bus 20 (hereinafter referred to as the grid voltage) based on a predetermined output voltage characteristic (droop characteristic).

When the first DC/DC converter 11 is set for discharging, it is individually controlled by grid voltage control based on the discharge characteristics shown in Fig. 3(a). The discharge characteristics shown in Fig. 3(a) have a negative slope in which the current in the discharging direction is positive and the discharge current decreases with an increase in the grid voltage, and are set so that the discharge current becomes zero at a grid voltage _V2 .

The first DC/DC converter 11 is set for charging, and when there is no notification of the shift amount, it is individually controlled by grid voltage control based on the reference charging characteristic shown in Fig. 3(b). The charging characteristic shown in Fig. 3(b) is set to a positive slope in which the current in the charging direction is positive, the charging current is zero at a grid voltage _V1 lower than the grid voltage _V2, and the charging current increases as the grid voltage of the DC connection bus 20 increases.

The first DC/DC converter 11 is set for charging, and when a shift amount _VX is notified, the first DC/DC converter 11 is individually controlled by grid voltage control based on a shift charging characteristic in which the reference charging characteristic is shifted in the direction in which the offset voltage _V1 increases by the notified shift amount, as shown in Fig. 3(c). The charging characteristic shown in Fig. 3(c) is set to a positive slope in which the current in the charging direction is positive, the charging current is zero at a grid voltage ( _V1 + _VX ) obtained by adding the shift amount _VX to the grid voltage _V1 , and the charging current increases with an increase in the grid voltage of the DC connection bus 20. Note that, in the shift charging characteristic, when the grid voltage is ( _V1 + _VX ) or more, the current in the charging direction is positive, and when the grid voltage is less than ( _V1 + _VX ), the current in the charging direction is negative, which is a discharge region.

Hereinafter, the setting operation of the first DC/DC 11 by the interchange control unit 18 will be described in detail with reference to FIG.
4, the interchange control unit 18 determines whether the generated current Ipv detected by the current sensor 16 is equal to or less than 0 (A) in order to determine whether the solar cell 3 is generating power (step S101). Note that the presence or absence of power generation may be determined based on the output of a separately provided illuminometer instead of the generated current Ipv, or the presence or absence of power generation may be determined based on the time of day.

The interchange control unit 18 determines that the solar cell 3 is not generating power when the power generation current Ipv is 0 (A) or less in step S101. Then, the interchange control unit 18 determines whether the SOC calculated by the power storage amount detection unit 17 is equal to or greater than a lower threshold value _TH1 (e.g., 20%) for determining whether the remaining battery charge is sufficient (step S102).

The interchange control unit 18 determines that the solar cell 3 is in a power generating state when the power generation current Ipv exceeds 0 (A) in step S101. The interchange control unit 18 then determines whether the SOC calculated by the charge storage detection unit 17 is equal to or greater than an upper threshold _TH2 (e.g., 70%) for determining whether the battery remaining capacity is approaching full charge (step S103). The upper threshold _TH2 is set to a value greater than the lower threshold _TH1 .

In the case where the SOC is equal to or greater than the lower threshold _TH1 in step S102, and in the case where the SOC is equal to or greater than the upper threshold _TH2 in step S103, the interchange control unit 18 determines that the remaining battery charge is sufficient, sets the first DC/DC converter 11 for discharging (step S104), and ends the setting operation. As a result, the first DC/DC converter 11 operates based on the discharge characteristics shown in Fig. 3(a) and is connected to the DC connection bus 20, and enters a dischargeable state in which it interchanges power with another storage device 1 set for charging.

When the SOC is less than the lower threshold _TH1 in step S102 and when the SOC is less than the upper threshold _TH2 in step S103, the interchange control unit 18 determines that the remaining battery level is insufficient and sets the first DC/DC converter 11 for charging (step S105).

As a result, during the day, when the solar cell 3 generates power and charges the storage battery 2, the storage device 1 with a lightly charged storage battery 2 and an SOC below the upper threshold value _TH2 waits in a chargeable state, and the storage device 1 with an SOC equal to or greater than the upper threshold value _TH2 transitions from a chargeable state to a dischargeable state and supplies power to the storage device 1 waiting in a chargeable state.

When the solar cell 3 is not generating electricity, the storage battery 2 has little remaining charge and the storage device 1 with an SOC less than the lower threshold value _TH1 is in a chargeable state, while the storage device ₁ with an SOC equal to or greater than the lower threshold value TH1 is in a dischargeable state, and electricity is supplied to the storage device 1 in the chargeable state.

Next, the interchange control unit 18 judges whether the SOC calculated by the stored power detection unit 17 is within a shift range (THn _or more and _THP or less) set between a lower threshold value _TH1 and an upper threshold value _TH2 (step S106). The lower limit value _THn of the shift range is set to a value equal to or greater than the lower threshold value _TH1 , and the upper limit value _THP of the shift range is set to a value equal to or less than the upper threshold value _TH2 .

If the SOC is outside the shift range in step S106, the interchange control unit 18 ends the setting operation without notifying the shift amount V _X. As a result, the first DC/DC 11 operates based on the reference charging characteristics shown in Fig. 3(b), is connected to the DC connection bus 20, and is in a chargeable state in which power is interchanged from another power storage device 1 set for discharging.

If the SOC is within the shift range in step S106, the interchange control unit 18 determines the shift amount _VX (step S107), notifies the first DC/DC 11 of the determined shift amount _VX (step S108), and ends the setting operation. As a result, the first DC/DC 11 operates based on the shift charging characteristics shown in Fig. 3(c), is connected to the DC connection bus 20, and is in a chargeable state in which power is interchanged from another storage device 1 set for discharging.

The shift amount _VX is set to a value smaller than (grid voltage _V2 -grid voltage _V1 ). The shift amount _VX may be a fixed value, but is preferably set to a larger value as the SOC increases. For example, the shift amount VX is calculated by the following formula.
_Vx = ( _V0 - _V1 ) x (SOC - _THn ) / ( _THP - _THn )
Here, _V0 is a grid voltage set between grid voltages _V1 and _V2 . Note that the shift amount _VX calculated by the above formula increases linearly with the SOC increasing up to an upper limit of ( _V0 - _V1 ), but the shift amount _VX may be increased in stages.

The interchange control unit 18 periodically performs the judgments in steps S101 to S103 and S106, and executes the above-mentioned setting operation when the judgments in steps S101 to S103 and S106 change. This makes it possible to respond to changes in the power generation state of the solar cell 3 and the amount of electricity stored in the storage battery 2.

As a result, at night, when the solar cell 3 is not generating power, the power storage device 1 with a sufficient stored power amount and an SOC equal to or greater than the lower threshold TH1 waits in a dischargeable state. When the SOC of the power storage device ₁ becomes less than the lower threshold _TH1 , the power storage device 1 transitions to a chargeable state and can receive power.

During the day, when the solar cell 3 generates power and charges the storage battery 2, the storage battery 2 has a shallow charge amount and the power storage device 1 with an SOC below the upper threshold _TH2 is in a standby state in which it can be charged. When the SOC of the power storage device 1 becomes equal to or greater than the lower threshold _TH2 , the power storage device 1 transitions from a chargeable state to a dischargeable state and supplies power to the storage device 1 in a standby state in which it can be charged.

Furthermore, during the day, when the solar cell 3 generates power and charges the storage battery 2, the storage device 1 whose storage amount of the storage battery 2 is within the shift range (THn or more and THp or less) operates with the shift charging characteristics increased by the shift amount _VX , and can be charged with a storage amount different from that of the storage device 1 operating with the reference charging characteristics.

Figure 5(a) shows a state in which two storage devices 1 are connected to the DC connection bus 20, with the first first DC/DC 11 set for discharging (discharge characteristics) and the second first DC/DC 11 set for charging (reference charge characteristics). In this state, as shown in Figure 6(a), the intersection of the reference charge characteristics and the discharge characteristics is the operating point A. Note that in Figure 6(a), the current in the charge direction is positive in the reference charge characteristics, and the current in the discharge direction is positive in the discharge characteristics.

The grid voltage V _A at the operating point A has a value greater than the grid voltage V ₁ and lower than the grid voltage V _2. Then, the discharge current from the first DC/DC 11 set for discharge (discharge characteristic) to the DC connection bus 20 becomes I _A (A), and the charge current from the DC connection bus 20 to the first DC/DC 11 set for charge (reference charge characteristic) becomes I _A (A), and power interchange is executed from one power storage device 1 set for discharge (discharge characteristic) to one power storage device 1 set for charge (reference charge characteristic).

Fig. 5(b) shows a state in which two storage devices 1 are connected to the DC connection bus 20, with the first first DC/DC 11 set for charging (reference charging characteristic) and the second first DC/DC 11 set for charging (shift charging characteristic: shift amount _VXB ). In this state, as shown in Fig. 6(b), the operating point B is where the positive charging current in the reference charging characteristic and the negative charging current in the shift charging characteristic are equal in absolute value. Note that in Fig. 6(b), the current in the charging direction is positive in the reference charging characteristic and the shift charging characteristic. Therefore, in the negative region, this means that the current flows in the discharging direction.

When the grid voltage and the reference charging characteristic are in _a directly proportional relationship, the grid voltage _VB at the operating point B is a value obtained by adding the shift amount _VXb /2 to the grid voltage _V1 . Then, the discharge current from the first DC/DC 11 set for charging (shift charging characteristic) to the DC connection bus 20 is _IB (A), and the charge current from the DC connection bus 20 to the first DC/DC 11 set for charging (reference charging characteristic) is _IB (A). As a result, even when all the storage devices 1 connected to the DC connection bus 20 are set for charging, power interchange is performed from the storage device 1 with a high SOC to the storage device 1 with a low SOC, the amount of electricity stored in each storage device 1 is averaged, the risk of the storage device 1 with a large load 4 going down is reduced, and stable operation over the long term is possible.

5(c) shows a state in which three storage devices 1 are connected to the DC connection bus 20, with the first first DC/DC 11 set for charging (reference charging characteristics), the second first DC/DC 11 set for charging (shift charging characteristics: shift amount _VXB ), and the third first DC/DC 11 set for discharging (discharging characteristics). In this state, as shown in FIG. 7(a), the two storage devices 1 set for charging operate with a composite charging characteristic having a gradient twice that of the reference charging characteristic (shift charging characteristic) with a grid voltage _VB ( _V1 + _VXB /2) and a composite charging characteristic in which the reference charging characteristic and the shift charging characteristic are combined, and the charging current is zero at the grid voltage VB (V1+VXB/2). The intersection of the composite charging characteristic and the discharge characteristic is the operating point C. In FIG. 7(a), the current in the charging direction is positive in the composite charging characteristic, and the current in the discharging direction is positive in the discharge characteristic.

When N energy storage devices 1 set for charging are connected to the DC connecting bus 20, the composite charging characteristic has a gradient N times that of the reference charging characteristic (shift charging characteristic) with the charging current being zero at a grid voltage obtained by adding a value obtained by dividing the sum of the shift amounts _VX by N to the grid voltage _V1 . When M energy storage devices 1 set for discharging are connected to the DC connecting bus 20, the composite discharging characteristic has a gradient M times that of the discharging characteristic with the discharging current being zero at a grid voltage _V2 .

The grid voltage V _C at the operating point C is greater than the grid voltage V ₁ and less than the grid voltage V _2. The discharge current from the first DC/DC 11 set for discharging (discharge characteristic) to the DC connection bus 20 is I _C (A), and the total charge current from the DC connection bus 20 to the first DC/DC 11 set for charging (reference charge characteristic) and the first DC/DC 11 set for charging (shift charge characteristic) is I _C (A).

7B, at the grid voltage V _C of the operating point C, the charging current I _{C1 (A) from the DC connection bus 20 to the first DC/DC 11 set for charging (reference charging characteristics) is greater than the charging current I C2 (} A) from the DC connection bus 20 to the first DC/DC 11 set for charging (shift charging characteristics). Therefore, priority is given to power interchange to the power storage device 1 with a low SOC, the amount of stored power in each power storage device 1 is averaged, the risk of the power storage device 1 with a large load 4 going down is reduced, and stable operation over the long term is possible.

As described above, the grid voltages V _A to V _c at the operating points A to C are all greater than the grid voltage V ₁ and less than the grid voltage V _2. Therefore, if the grid voltage V ₂ at which the discharge current becomes zero in the discharge characteristic is set as the upper limit voltage of the grid voltage, and the grid voltage V ₁ at which the charge current becomes zero in the reference charge characteristic is set as the lower limit voltage of the grid voltage, the grid voltage will be operated within an appropriate range.

In this embodiment, the shift amount _VX is calculated according to the SOC, but further, the power margin in the power storage device 1 may be determined based on the power generation _PPV of the solar cell 3 and the power consumption _POUT of the load 4, depending on whether the value obtained by subtracting the power consumption _POUT from the power generation _PPV is large or small, and when the value obtained by subtracting the power consumption _POUT from the power generation _PPV is smaller than a threshold value (e.g., the rated power _Ptyp of the solar cell 3) and the power margin is low, the shift amount _VX calculated according to the SOC may be reduced according to the degree of the power margin. In this case, power interchange according to the power margin in the power storage device 1 may be realized.

For example, the shift amount V _x ' taking into consideration the power margin is calculated by the following formula.
_Vx ' = Vx _x ( _PPV - _POUT ) / _Ptyp
Here, P _typ is the rated power of the solar cell 3 .
When the generated power _PPV falls below the consumed power _POUT , the shift amount _VX ' is not made negative, but is limited to _VX ' = 0. This allows operation within an appropriate range without the grid voltage falling below _V1 .

Furthermore, an abnormality detection circuit for detecting an abnormality requiring protection such as overcharging or overcurrent may be provided in the storage device 1, and when an abnormality is detected by the abnormality detection circuit, the reference charging characteristic may be shifted in a direction in which the offset voltage _V1 increases. In this case, it becomes possible to suppress or stop charging the storage device 1 without shutting it down.

In this embodiment, the slopes of the discharge characteristic, reference charging characteristic, and shift charging characteristic of the first DC/DC 11 in each storage device 1 have been described as being the same. However, the discharge characteristic, reference charging characteristic, and shift charging characteristic can be weighted and controlled by controlling the slopes. That is, when first DC/DC 11 with different slopes of the discharge characteristic are connected in parallel, the discharge amount of the first DC/DC 11 with the larger slope of the discharge characteristic becomes larger. Similarly, when first DC/DC 11 with different slopes of the reference charging characteristic and shift charging characteristic are connected in parallel, the storage amount of the first DC/DC 11 with the larger slope of the reference charging characteristic and shift charging characteristic becomes larger.

As described above, according to the present embodiment, the energy storage system performs power interchange by controlling charging or discharging individually by controlling the grid voltage of the DC connection bus 20, with the energy storage device 1 connected to the storage battery 2 that stores DC power as a basic unit, and the multiple energy storage devices 1 connected to the DC connection bus 20, and each of the multiple energy storage devices 1 includes a first DC/DC 11 which is a bidirectional DC/DC converter connected to the DC connection bus 20, an interchange control unit 18 which sets the first DC/DC 11 for charging or discharging, and a storage amount detection unit 17 which detects the amount of stored energy in the storage battery 2, and the first DC/DC 11 set for charging controls charging based on a charging characteristic in which the charging current increases with an increase in the grid voltage of the DC connection bus 20, and when the amount of stored energy in the storage battery 2 is within a preset shift range (not less than TH _n and not more than TH _P ), the charging characteristic is shifted in the direction in which the offset voltage V ₁ at which the charging current becomes zero increases.
With this configuration, even when all the storage devices 1 are operating with charging characteristics, power can be transferred from the storage device 1 with a higher SOC to the storage device 1 with a lower SOC, thereby reducing the risk of the storage device 1 with a large load 4 going down and enabling stable operation over the long term.

Furthermore, according to this embodiment, the first DC/DC converter 11 increases the shift amount _VX for shifting the charging characteristics as the SOC is higher (as the amount of charge stored in the storage battery 3 is greater).
This configuration makes it possible to efficiently exchange electric power from the power storage device 1 with a higher SOC to the power storage device 1 with a lower SOC.

Furthermore, according to this embodiment, each of the multiple power storage devices 1 includes an MPPT DC/DC converter 13 which is a DC power receiving unit that receives power generated by the solar cells 3, and a PCS 14 which is a power supply unit that supplies the power consumed by the load 4, and when the value obtained by subtracting the power consumption P _OUT from the power generation power P _PV is smaller than a threshold value (for example, the rated power P _typ of the solar cells 3), the first DC/DC converter 11 reduces the shift amount _VX in accordance with the degree of the difference.
This configuration makes it possible to realize power interchange according to the power margin in the power storage device 1.

Furthermore, according to the present embodiment, the multiple power storage devices 1 each include an abnormality detection circuit that detects an abnormality that requires protection, and when an abnormality is detected by the abnormality detection circuit, the first DC/DC converter 11 shifts the charging characteristics in a direction in which the offset voltage _V1 increases.
This configuration makes it possible to suppress or stop charging the power storage device 1 without causing it to shut down.

The present invention has been described above using specific embodiments, but the above embodiments are merely examples, and it goes without saying that the present invention can be modified and implemented without departing from the spirit of the invention.

Reference Signs List 1 Energy storage device 2 Storage battery 3 Solar cell 4 Load 11 First bidirectional DC/DC converter 12 Second bidirectional DC/DC converter 13 MPPT DC/DC converter 14 PCS
15, 16 Current sensor 17 Storage amount detection unit 18 Interchange control unit 19 DC link 20 DC connection bus 31 Connection box T1 Power interchange terminal T2 Storage battery connection terminal T3 PV connection terminal T4 Load connection terminal

Claims (5)

A power storage system in which a basic unit is a power storage device connected to a storage battery that stores DC power, and a plurality of the power storage devices connected to a DC connection bus are individually controlled to charge or discharge by controlling a grid voltage of the DC connection bus to perform power interchange,
The plurality of power storage devices
a bidirectional DC/DC converter connected to the DC connection bus;
an interchange control unit that sets the bidirectional DC/DC converter for charging or discharging;
a storage amount detection unit that detects a storage amount of the storage battery,
the bidirectional DC/DC converter, when configured for charging, controls charging based on a charging characteristic in which a charging current increases as the grid voltage increases, and, when an amount of charge stored in the storage battery is within a preset shift range, shifts the charging characteristic in a direction in which an offset voltage increases so that the charging current becomes zero , and, when the grid voltage is equal to or greater than the offset voltage, operates in a charging region where the current in the charging direction is positive, and, when the grid voltage is less than the offset voltage, operates in a discharging region where the current in the charging direction is negative . The energy storage system according to claim 1, characterized in that the bidirectional DC/DC converter increases the amount of shift of the charging characteristics as the amount of stored electricity in the storage battery increases. The plurality of power storage devices
a power receiving unit that receives power generated by the solar cell;
a power supply unit that supplies the power consumed by the load;
3. The power storage system according to claim 2, wherein, when a value obtained by subtracting the power consumption from the generated power is smaller than a threshold value, the bidirectional DC/DC converter reduces the shift amount in accordance with the degree of the difference. Each of the plurality of power storage devices includes an abnormality detection circuit that detects an abnormality that requires protection;
4. The power storage system according to claim 1, wherein the bidirectional DC/DC converter shifts the charging characteristics in a direction in which the offset voltage increases when an abnormality is detected by the abnormality detection circuit. A power storage device that is connected between a storage battery that stores direct current power and a DC connection bus, and that controls charging or discharging individually by controlling a grid voltage of the DC connection bus to exchange power with another power storage device connected to the DC connection bus,
a bidirectional DC/DC converter connected to the DC connection bus;
an interchange control unit that sets the bidirectional DC/DC converter for charging or discharging;
a charge amount detection unit that detects a charge amount of the storage battery,
the bidirectional DC/DC converter, when set for charging, controls charging based on a charging characteristic in which a charging current increases as the grid voltage increases, and, when the amount of charge in the storage battery is within a predetermined shift range, shifts the charging characteristic in a direction in which an offset voltage at which the charging current becomes zero increases , and, when the grid voltage is equal to or greater than the offset voltage, operates in a charging region where the current in the charging direction is positive, and, when the grid voltage is less than the offset voltage, operates in a discharging region where the current in the charging direction is negative .

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