Charging and discharging system and method, and photovoltaic power generation system

1. A charging and discharging system comprising a unidirectional converter, a unidirectional switch, an energy storage device and a controller, wherein

an input terminal of the unidirectional converter is connected to an output terminal of a photovoltaic device of a photovoltaic power generation system to which the charging and discharging system is applied, and an output terminal of the unidirectional converter is connected to an input terminal of the energy storage device;

the unidirectional switch is connected between an output terminal of the energy storage device and an input terminal of a bidirectional inverter of the photovoltaic power generation system to which the charging and discharging system is applied; and

the controller is connected to the unidirectional converter, the unidirectional switch, the energy storage device and the bidirectional inverter,

wherein in a case that output power of the photovoltaic device is less than preset minimum output power of the bidirectional inverter, sending, by the controller, a voltage-reducing-signal to the bidirectional inverter, and in a case that a voltage of the input terminal of the bidirectional inverter is detected to be lower than or equal to an open-circuit voltage of the energy storage device, controlling, by the controller, the unidirectional switch to close, and

wherein the voltage-reducing-signal is used to instruct the bidirectional inverter to reduce the voltage of the input terminal of the bidirectional inverter to be lower than or equal to the open-circuit voltage of the energy storage device.

2. The charging and discharging system according to claim 1 , wherein the unidirectional switch comprises a direct current contactor.

3. The charging and discharging system according to claim 1 , wherein the unidirectional switch comprises a direct current contactor and a diode.

4. The charging and discharging system according to claim 1 , wherein the unidirectional converter comprises a single-stage DCDC converter.

5. The charging and discharging system according to claim 1 , wherein the unidirectional converter comprises a multiple-stage DCDC converter.

6. A photovoltaic power generation system comprising a photovoltaic device, a bidirectional inverter and a charging and discharging system, the charging and discharging system comprising a unidirectional converter, a unidirectional switch, an energy storage device and a controller, wherein

an input terminal of the unidirectional converter is connected to an output terminal of a photovoltaic device of a photovoltaic power generation system to which the charging and discharging system is applied, and an output terminal of the unidirectional converter is connected to an input terminal of the energy storage device;

the unidirectional switch is connected between an output terminal of the energy storage device and an input terminal of a bidirectional inverter of the photovoltaic power generation system to which the charging and discharging system is applied; and

the controller is connected to the unidirectional converter, the unidirectional switch, the energy storage device and the bidirectional inverter; and

the charging and discharging system is connected between the photovoltaic device and the bidirectional inverter,

in a case that output power of the photovoltaic device is greater than preset maximum output power of the bidirectional converter, controlling, by the controller, the unidirectional converter to charge the energy storage device with first charging power, and

wherein the first charging power is a difference between the output power of the photovoltaic device and the preset maximum output power of the bidirectional inverter.

7. The photovoltaic power generation system according to claim 6 , wherein the unidirectional switch comprises a direct current contactor.

8. The photovoltaic power generation system according to claim 6 , wherein the unidirectional switch comprises a direct current contactor and a diode.

9. The photovoltaic power generation system according to claim 6 , wherein the unidirectional converter comprises a single-stage DCDC converter.

10. The photovoltaic power generation system according to claim 6 , wherein the unidirectional converter comprises a multiple-stage DCDC converter.

11. A charging and discharging method applied to a charging and discharging system comprising: a unidirectional converter, a unidirectional switch, an energy storage device and a controller, wherein

an input terminal of the unidirectional converter is connected to an output terminal of a photovoltaic device of a photovoltaic power generation system to which the charging and discharging system is applied, and an output terminal of the unidirectional converter is connected to an input terminal of the energy storage device;

the unidirectional switch is connected between an output terminal of the energy storage device and an input terminal of a bidirectional inverter of the photovoltaic power generation system to which the charging and discharging system is applied; and

the controller is connected to the unidirectional converter, the unidirectional switch, the energy storage device and the bidirectional inverter;

and the method comprising:

in a case that output power of the photovoltaic device is greater than preset maximum output power of the bidirectional inverter, the controller controls the unidirectional converter to charge the energy storage device with first charging power, wherein the first charging power is a difference between the output power of the photovoltaic device and the preset maximum output power of the bidirectional inverter; wherein the controller controls unidirectional converter to charge the energy storage device with first charging power, comprises:

comparing a voltage of the input terminal of the unidirectional converter with the open-circuit voltage of the energy storage device; and

in a case that the voltage of the input terminal of the unidirectional converter is higher than the open-circuit voltage of the energy storage device, controlling the unidirectional converter to reduce the voltage of the input terminal of the unidirectional converter to be equal to the open-circuit voltage of the energy storage device, and in a case that the voltage of the input terminal of the unidirectional converter is equal to the open-circuit voltage of the energy storage device, controlling the unidirectional converter to charge the energy storage device with the first charging power; or

in a case that the voltage of the input terminal of the unidirectional converter is lower than the open-circuit voltage of the energy storage device, controlling the unidirectional converter to raise the voltage of the input terminal of the unidirectional converter to be equal to the open-circuit voltage of the energy storage device, and in a case that the voltage of the input terminal of the unidirectional converter is equal to the open-circuit voltage of the energy storage device, controlling the unidirectional converter to charge the energy storage device with the first charging power; or

in a case that the voltage of the input terminal of the unidirectional converter is equal to the open-circuit voltage of the energy storage device, controlling the unidirectional converter to charge the energy storage device with the first charging power; or

in a case that output power of the photovoltaic device is less than preset minimum output power of the bidirectional inverter, the controller sends a voltage-reducing-signal to the bidirectional inverter, and in a case that a voltage of the input terminal of the bidirectional inverter is detected to be lower than or equal to an open-circuit voltage of the energy storage device, the controller controls the unidirectional switch to close, wherein the voltage-reducing-signal is used to instruct the bidirectional inverter to reduce the voltage of the input terminal of the bidirectional inverter to be lower than or equal to the open-circuit voltage of the energy storage device.

12. The charging and discharging method according to claim 11 , wherein the unidirectional switch comprises a direct current contactor.

13. The charging and discharging method according to claim 11 , wherein the unidirectional switch comprises a direct current contactor and a diode.

14. The charging and discharging method according to claim 11 , wherein the unidirectional converter comprises a single-stage DCDC converter.

15. The charging and discharging method according to claim 11 , wherein the unidirectional converter comprises a multiple-stage DCDC converter.