Parallel-off-grid switching method, controller and energy storage system of multi-machine parallel energy storage system

Abstract

The application provides a kind of parallel and off-grid switching method of multiple machine parallel energy storage system, controller and energy storage system, belongs to the field of energy storage.Energy storage system includes multiple parallel energy storage converters, the method includes: when energy storage system works in off-grid mode, if it is detected that power grid recovers normal, then control each energy storage converter to track grid voltage, so that the off-grid voltage of each energy storage converter is kept synchronous with grid voltage;When it is detected that the off-grid voltage of each energy storage converter is kept synchronous with grid voltage, control grid-connected switch to close;Each energy storage converter is connected in parallel, and is connected with grid through grid-connected switch;The working loop of each energy storage converter is switched from off-grid loop to grid-connected loop.The application can realize the automatic switching from off-grid to grid-connected when multiple energy storage converters are connected in parallel, and can realize the non-power-down switching from off-grid to grid-connected.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of energy storage, and in particular to a parallel and off-grid switching method of a multi-machine parallel energy storage system, a controller and an energy storage system. BACKGROUND

[0002] The power conversion system (PCS) can convert the direct current stored in the battery into alternating current, or convert the alternating current from the power grid into direct current to charge the battery. The power conversion system can operate in parallel with the grid, but when the grid fails, it needs to switch to off-grid operation, and when the grid returns to normal, it needs to restore parallel operation.

[0003] In related technologies, parallel and off-grid switching control is usually performed for a single power conversion system, and there is a lack of parallel and off-grid switching method for a multi-machine parallel energy storage system. SUMMARY

[0004] The embodiments of the present application provide a parallel and off-grid switching method of a multi-machine parallel energy storage system, a controller and an energy storage system to solve the problem that the prior art lacks a parallel and off-grid switching method for a multi-machine parallel energy storage system.

[0005] In a first aspect, the embodiments of the present application provide a parallel and off-grid switching method of a multi-machine parallel energy storage system, the energy storage system comprising a plurality of parallel power conversion systems, and the parallel and off-grid switching method of the multi-machine parallel energy storage system comprising:

[0006] When the energy storage system is working in off-grid mode, if it is detected that the grid has returned to normal, the grid voltage is tracked by each power conversion system, so that the off-grid voltage of each power conversion system is synchronized with the grid voltage;

[0007] When it is detected that the off-grid voltage of each power conversion system is synchronized with the grid voltage, the grid switch is closed; after the parallel connection of each power conversion system, the grid is connected through the grid switch;

[0008] The working loop of each power conversion system is switched from the off-grid loop to the grid-connected loop.

[0009] In a possible implementation, the above method further comprises:

[0010] When the energy storage system is working in grid-connected mode, if the grid fails, the drive of each power conversion system is blocked;

[0011] If it is determined that all power conversion systems of the energy storage system have been blocked, the grid switch is turned off;

[0012] The working loop of each power conversion system is switched from the grid-connected loop to the off-grid loop.

[0013] In a possible implementation, the method for controlling each energy storage converter to track the grid voltage comprises:

[0014] acquiring a real-time grid voltage and sending the real-time grid voltage to each energy storage converter; the real-time grid voltage is used to instruct each energy storage converter to perform off-grid loop control and positive and negative sequence compensation, so as to keep the off-grid voltage of each energy storage converter synchronized with the real-time grid voltage.

[0015] In a possible implementation, the method is applied to the first controller and the second controller.

[0016] When the first controller or the second controller sends an opening signal to the grid switch, the grid switch is opened.

[0017] When the first controller and the second controller both send a closing signal to the grid switch, the grid switch is closed.

[0018] In a possible implementation, the grid switch comprises a first switch, a closing coil, an opening coil, a second switch, a third switch, a fourth switch and a fifth switch.

[0019] The first switch is connected to the grid and the energy storage converter at two ends respectively.

[0020] The closing coil, the second switch and the third switch are connected in series; the opening coil, the fourth switch and the fifth switch are connected in parallel.

[0021] The closing signal of the first controller acts on the second switch, and the closing signal of the second controller acts on the third switch; the opening signal of the first controller acts on the fourth switch, and the opening signal of the second controller acts on the fifth switch.

[0022] In a possible implementation, when the energy storage system works in the off-grid mode, each energy storage converter adopts a dq control mode to perform off-grid loop control.

[0023] In a possible implementation, after the method for controlling each energy storage converter to block the drive, the method further comprises:

[0024] controlling the corresponding output switch of each energy storage converter to be opened; each energy storage converter is connected to the grid switch through the corresponding output switch.

[0025] If it is determined that all the energy storage converters of the energy storage system have blocked the drive, the grid switch is controlled to be opened, comprising:

[0026] If it is determined that all the energy storage converters of the energy storage system have blocked the drive and all the corresponding output switches of the energy storage converters have been opened, the grid switch is controlled to be opened.

[0027] In a possible implementation, the method further comprises:

[0028] When the energy storage system is in standby state, if the power grid is in normal state, the energy storage system is controlled to work in grid-connected mode, otherwise, the energy storage system is controlled to work in off-grid mode.

[0029] In a second aspect, an embodiment of the present application provides a grid-connected / off-grid switching device of a multi-machine parallel energy storage system, the energy storage system comprising a plurality of parallel energy storage converters, the grid-connected / off-grid switching device of the multi-machine parallel energy storage system comprising:

[0030] a synchronization module, configured to, when the energy storage system works in off-grid mode, if it is detected that the power grid returns to normal, control each energy storage converter to track the grid voltage, so that the off-grid voltage of each energy storage converter is kept synchronous with the grid voltage;

[0031] a control module, configured to, when it is detected that the off-grid voltage of each energy storage converter is kept synchronous with the grid voltage, control the grid-connected switch to be closed; each energy storage converter is connected in parallel and connected to the grid through the grid-connected switch;

[0032] a loop switching module, configured to control the working loop of each energy storage converter to be switched from an off-grid loop to a grid-connected loop.

[0033] In a third aspect, an embodiment of the present application provides a controller, comprising a processor and a memory, the memory being configured to store a computer program, and the processor being configured to call and run the computer program stored in the memory, so as to execute the grid-connected / off-grid switching method of the multi-machine parallel energy storage system as described in the first aspect or any possible implementation manner of the first aspect.

[0034] In a fourth aspect, an embodiment of the present application provides an energy storage system, comprising a first controller, a second controller, a grid-connected switch and a plurality of parallel energy storage converters; the energy storage converters and the grid-connected switch are controlled by the first controller and the second controller; the first controller and the second controller are both the controller as described in the third aspect.

[0035] Each energy storage converter is connected in parallel and connected to the grid through the grid-connected switch.

[0036] In a fifth aspect, an embodiment of the present application provides a computer readable storage medium, the computer readable storage medium storing a computer program, the computer program being executed by a processor to implement the steps of the grid-connected / off-grid switching method of the multi-machine parallel energy storage system as described in the first aspect or any possible implementation manner of the first aspect.

[0037] This invention provides a method, controller, and energy storage system for switching between grid connection and off-grid operation in a multi-unit parallel energy storage system. When the energy storage system is operating in off-grid mode, if the grid is detected to have returned to normal, the method controls each energy storage converter to track the grid voltage, ensuring that the off-grid voltage of each energy storage converter is synchronized with the grid voltage. Once the off-grid voltage of each energy storage converter is synchronized with the grid voltage, the grid connection switch is closed, thus achieving a smooth switchover. Finally, the operating loop of each energy storage converter is switched from an off-grid loop to a grid-connected loop. This method enables automatic switching from off-grid to grid connection when multiple energy storage converters are operating in parallel, and also allows for seamless switching without power loss. Attached Figure Description

[0038] To more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0039] Figure 1 This is a schematic diagram of the structure of a multi-machine parallel energy storage system provided in an embodiment of the present invention;

[0040] Figure 2 This is a schematic diagram of the structure of a multi-machine parallel energy storage system provided in another embodiment of the present invention;

[0041] Figure 3 This is a schematic diagram of the structure of a multi-machine parallel energy storage system provided in another embodiment of the present invention;

[0042] Figure 4 This is a flowchart illustrating the implementation of a grid-connected / off-grid switching method for a multi-machine parallel energy storage system according to an embodiment of the present invention.

[0043] Figure 5 This is a schematic diagram of a state switching machine provided in an embodiment of the present invention;

[0044] Figure 6 This is a schematic diagram of the structure of a grid-connected switch provided in an embodiment of the present invention;

[0045] Figure 7 This is a schematic diagram of a grid-connected / off-grid switching device for a multi-machine parallel energy storage system provided in an embodiment of the present invention;

[0046] Figure 8 This is a schematic diagram of a controller provided in an embodiment of the present invention. Detailed Implementation

[0047] In the following description, for purposes of explanation and not limitation, specific details are set forth such as particular architectures, techniques, etc. in order to provide a thorough understanding of the embodiments of the present application. However, it will be apparent to those skilled in the art that the present application can be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods, devices, circuits, and

[0048] In order to make the objects, technical solutions and advantages of the present application clearer, the following will be described in conjunction with the accompanying drawings and specific embodiments.

[0049] Referring to Figure 1 , a structure schematic diagram of a multi-machine parallel energy storage system provided by an embodiment of the present application is shown. The multi-machine parallel energy storage system can be referred to as an energy storage system.

[0050] As shown in Figure 1 , the energy storage system can include a first controller 11, a second controller 12, a grid-connected switch 13 and a plurality of parallel energy storage converters 16; the energy storage converters 16 and the grid-connected switch 13 are controlled by the first controller 11 and the second controller 12; the first controller 11 and the second controller 12 are both used to execute the grid-connected / off-grid switching method of the multi-machine parallel energy storage system mentioned later;

[0051] Each energy storage converter 16 is connected in parallel and then connected with a power grid through the grid-connected switch 13.

[0052] Among them, referring to Figure 1 , the first controller 11, the second controller 12 and the grid-connected switch 13 can be located in a switching cabinet.

[0053] In an implementation, the first controller 11 and the second controller 12 can be in communication connection with each energy storage converter 16, such as through CAN communication connection. Figure 1 The connection relationship between the second controller 12 and the energy storage converter 16 is not shown in , but they can have communication connection. The first controller 11 and the second controller 12 can both execute the grid-connected / off-grid switching method of the multi-machine parallel energy storage system mentioned later, and can both acquire information of each energy storage converter 16 and send control instructions to each energy storage converter 16. Alternatively, the first controller 11 can be in communication connection with the second controller 12 for information interaction, such as through 485 communication connection.

[0054] In another embodiment, the first controller 11 and the second controller 12 can be communicatively connected. One of the first controller 11 and the second controller 12 serves as a master controller, and the other serves as a slave controller. The master controller is communicatively connected to each energy storage converter 16, and the slave controller is not connected or is connected to each energy storage converter 16. After the master controller obtains the information of each energy storage converter 16, the master controller can send the information to the slave controller, and the slave controller does not need to obtain the information separately. The master controller can execute the on-grid and off-grid switching method of the multi-machine parallel energy storage system described above. The slave controller can also send control instructions to each energy storage converter 16 through the master controller or directly, or when the master controller is not faulty, the master controller sends control instructions to each energy storage converter 16, and the slave controller only runs the on-grid and off-grid switching method of the multi-machine parallel energy storage system described above and does not send control instructions to each energy storage converter 16. When the master controller is faulty, the slave controller sends control instructions to each energy storage converter 16.

[0055] In some possible implementations, the master controller can also be communicatively connected to a cloud platform and / or an APP through a communication data acquisition unit. The master controller and the communication data acquisition unit can be communicatively connected through 485 communication, the communication data acquisition unit is communicatively connected to the cloud platform and / or the APP through WIFI or a USB interface, and the communication data acquisition unit can also be communicatively connected to each energy storage converter 16 through LAN communication.

[0056] Referring to Figure 1 , each energy storage converter 16 can be connected to a load after being connected in parallel.

[0057] Referring to Figure 2 , a structure diagram of a multi-machine parallel energy storage system provided by another embodiment of the application is shown, and referring to Figure 3 , a structure diagram of a multi-machine parallel energy storage system provided by still another embodiment of the application is shown. It should be noted that Figure 3 only one energy storage converter 16 is shown in the figure, and in actual applications, the number of energy storage converters 16 can be set according to actual needs, which is not limited herein. Figure 2 and Figure 3 , the energy storage systems shown in the figures can be incomplete, and therefore, the energy storage system is introduced jointly in combination with Figure 2 and Figure 3 .

[0058] Referring to Figure 2 and Figure 3 , the energy storage system can further include an AC bus 14 and a neutral line switch 15. The neutral line switch 15 can be controlled by the first controller 11 and / or the second controller 12 described above.

[0059] Each energy storage converter 16 is connected in parallel and then connected with the AC bus 14, the AC bus 14 is connected with the power grid through the grid-connected switch 13, and the AC bus 14 is also connected with the load; the zero line of the AC bus 14 is also grounded through the zero line switch 15.

[0060] The control of the first controller 11 and the second controller 12 on the grid-connected switch 13 and the zero line switch 15 can also refer to the control mode of the energy storage converter 16, and details are not repeated. The first controller 11 can also be referred to as a first parallel machine control card, and the second controller 12 can also be referred to as a second parallel machine control card. Figure 3 L1, L2 and L3 in the formula are three phase lines of the AC bus 14 respectively, and N is the zero line of the AC bus 14, the zero line of the AC bus 14 is grounded through the zero line switch 15. Referring to Figure 2 and Figure 3 , the AC bus 14 is also connected with the power grid through the grid-connected switch 13, and the AC bus 14 is also connected with the load. Referring to Figure 3 , there can be other switches not mentioned in the switching cabinet, and these other switches are in the closed state in the case not mentioned. The number of grid-connected switches 13 is usually 1, but according to actual needs, it can also be 2, as shown in Figure 3 , two grid-connected switches 13 are connected in parallel and are opened and closed together.

[0061] Among them, the zero line switch 15 can be a zero line contactor, a zero line relay or a zero line circuit breaker, etc.

[0062] Referring to Figure 2 and Figure 3 , the energy storage system can also include a transformer 17. After each energy storage converter 16 is connected in parallel, it is connected with the AC bus 14 through the transformer 17.

[0063] Referring to Figure 4 , it shows the implementation flowchart of the parallel and off-grid switching method of the multi-machine parallel energy storage system provided by the embodiment, and the execution subject of the parallel and off-grid switching method of the multi-machine parallel energy storage system is a controller. The controller can be the first controller or the second controller. The energy storage system includes a plurality of parallel energy storage converters.

[0064] The method is described in detail as follows:

[0065] In S101, when the energy storage system works in the off-grid mode, if it is detected that the power grid is restored to normal, each energy storage converter is controlled to track the grid voltage, so that the off-grid voltage of each energy storage converter is kept synchronous with the grid voltage.

[0066] When the energy storage system works in the off-grid mode, the grid-connected switch is in the open state, and the energy storage system operates in the off-grid mode. When the energy storage system works in the off-grid mode, whether the power grid returns to normal can be monitored in real time. When it is monitored that the power grid returns to normal, the controller can send a grid-connected flag to each energy storage converter. The grid-connected flag is used to instruct each energy storage converter to track the grid voltage, so that the off-grid voltage of each energy storage converter is synchronized with the grid voltage, so that subsequent smooth switching can be realized. The off-grid voltage of the energy storage converter is the output voltage of the energy storage converter in the off-grid mode.

[0067] When monitoring whether the power grid returns to normal, the grid voltage and the grid frequency can be monitored in real time. When at least one of the grid voltage exceeding the preset voltage range and the grid frequency exceeding the preset frequency range is met, it is determined that the power grid still fails, and the power grid is in an abnormal state. Otherwise, it is determined that the power grid returns to normal.

[0068] In S102, when it is detected that the off-grid voltage of each energy storage converter is synchronized with the grid voltage, the grid-connected switch is closed; after each energy storage converter is connected in parallel, the grid-connected switch is connected with the power grid.

[0069] When it is detected that the off-grid voltage of each energy storage converter is synchronized with the grid voltage, the grid-connected switch can be controlled to be closed at this time, without causing a large impact current, so as to protect the safety of equipment and the power grid. After the grid-connected switch is controlled to be closed, the energy storage system is connected with the power grid.

[0070] The off-grid voltage of the energy storage converter can be considered to be synchronized with the grid voltage when the absolute value of the voltage difference between the off-grid voltage of the energy storage converter and the grid voltage is less than or equal to a preset voltage difference value. Based on this judgment mode, when it is determined that the off-grid voltage of all energy storage converters is synchronized with the grid voltage, the grid-connected switch is controlled to be closed. The preset voltage difference value can be a small value.

[0071] In S103, the working loop of each energy storage converter is switched from the off-grid loop to the grid-connected loop.

[0072] After the grid-connected switch is controlled to be closed, the working loop of each energy storage converter can be switched from the off-grid loop to the grid-connected loop, that is, the grid-connected loop of each energy storage converter inherits the off-grid loop, so that the energy storage system works in the grid-connected mode and operates in the grid-connected mode, thereby completing the automatic no power-off switching of the energy storage system from the off-grid mode to the grid-connected mode.

[0073] The embodiment of the application can realize automatic switching from off-grid to grid-connected when multiple energy storage converters are connected in parallel, and can realize no power-off switching from off-grid to grid-connected, that is, seamless switching.

[0074] In some embodiments, the off-grid and grid-connected switching method of the multiple energy storage system in parallel can further include:

[0075] When the energy storage system works in the grid-connected mode, if the power grid fails, the drive of each energy storage converter is blocked;

[0076] If it is determined that all energy storage converters of the energy storage system have blocked the drive, the grid-connected switch is turned off;

[0077] The working loop of each energy storage converter is switched from the grid-connected loop to the off-grid loop.

[0078] When the energy storage system works in the grid-connected mode, if the power grid fails, the off-grid flag can be sent to each energy storage converter; the off-grid flag is used to instruct each energy storage converter to block its own drive and report its working state.

[0079] Specifically, when the energy storage system works in the grid-connected mode, the grid-connected switch is in a closed state, and the energy storage system is grid-connected. When the energy storage system works in the grid-connected mode, whether the power grid fails can be monitored in real time. When it is monitored that the power grid fails, the controller sends an off-grid flag to each energy storage converter. When each energy storage converter receives the off-grid flag, it blocks its own drive, that is, blocks the pulse, so that it stops outputting quickly, and after blocking its own drive, it reports its working state to the controller. The working state reported by the energy storage converter is used to indicate whether it has blocked the drive. The energy storage converter in the embodiment of the application refers to an online energy storage converter.

[0080] When monitoring whether the power grid fails, the grid voltage and the grid frequency can be monitored in real time. When at least one of the grid voltage exceeding the preset voltage range and the grid frequency exceeding the preset frequency range is met, it is determined that the power grid fails and the power grid is in an abnormal state, otherwise it is determined that the power grid does not fail and the power grid is in a normal state.

[0081] When the power grid fails, the embodiments of the present application control each energy storage converter to block its own drive so that it stops outputting, which can avoid back-feeding and improve safety, and can ensure that the switching process from grid-connected mode to off-grid mode is smooth and avoids system instability caused by sudden changes in electric energy during the switching process.

[0082] If it is determined according to the working states reported by each energy storage converter that all the energy storage converters have blocked the drive, the grid-connected switch is controlled to be turned off.

[0083] The embodiments of the present application determine whether all the energy storage converters of the energy storage system have blocked the drive according to the working states reported by each energy storage converter. In order to ensure safety, the grid-connected switch is controlled to be turned off only after all the energy storage converters of the energy storage system have blocked the drive, so that the energy storage system is disconnected from the power grid.

[0084] After the grid-connected switch is controlled to be turned off, the working loop of each energy storage converter is controlled to be switched from the grid-connected loop to the off-grid loop, that is, the off-grid loop of each energy storage converter inherits the grid-connected loop, so that the energy storage system works in off-grid mode and runs off-grid.

[0085] In the embodiments of the present application, the above-mentioned control of the working loop of each energy storage converter from the grid-connected loop to the off-grid loop can include: issuing an off-grid start-up instruction to each energy storage converter; the off-grid start-up instruction is used to instruct each energy storage converter to switch from the grid-connected loop to the off-grid loop and perform a parallel off-grid soft start-up, and after starting up, work in off-grid mode.

[0086] When the energy storage system works in grid-connected mode, if the power grid fails, the embodiments of the present application control each energy storage converter to block its own drive; when it is determined that all the energy storage converters have blocked the drive, the grid-connected switch is controlled to be turned off to disconnect the energy storage system from the power grid; finally, each energy storage converter is controlled to work in off-grid mode, so that automatic switching from grid-connected mode to off-grid mode of multiple energy storage converters in parallel can be realized, and the safety and reliability of the multiple energy storage converters in parallel are ensured.

[0087] In some possible implementation manners, when it is determined that all the energy storage converters of the energy storage system have blocked the drive, the grid-connected switch is controlled to be turned off, and at the same time, the zero-line switch is controlled to be turned on. The power grid is connected to the AC bus through the grid-connected switch, each energy storage converter is connected to the AC bus, and the zero line of the AC bus is grounded through the zero-line switch. The AC bus is also connected to a load.

[0088] Since the AC bus is also connected to a load, after the grid-connected switch is controlled to be turned off, in order to ensure the safety of the load, the zero-line switch also needs to be controlled to be turned on.

[0089] Correspondingly, in S102, the zero line switch can be controlled to be turned off while the grid-connected switch is turned on.

[0090] It should be noted that the embodiments of the present application can also be compatible with single-machine on-off grid switching, that is, also applicable to the case where the energy storage system only contains one energy storage converter.

[0091] In some embodiments, the on-off grid switching method of the multi-machine parallel energy storage system described above can further include:

[0092] When the energy storage system is in standby state, if the power grid is in normal state, the energy storage system is controlled to work in grid-connected mode, otherwise, the energy storage system is controlled to work in off-grid mode.

[0093] The embodiments of the present application are in automatic on-off grid switching mode, when the energy storage system is in standby state, the energy storage system can be automatically judged to enter off-grid mode or grid-connected mode according to the grid condition.

[0094] In some possible implementation manners, the energy storage system can be controlled to automatically switch on-off grid by a switching state machine.

[0095] Exemplarily, referring to Figure 5 When the energy storage system is in standby state, it can be determined whether the energy storage system operates in grid-connected mode or off-grid mode according to the grid state. When the grid state is normal, that is, the grid has not failed, the energy storage system is controlled to work in grid-connected mode and operates in grid-connected mode; when the grid state is abnormal, that is, the grid has failed, the energy storage system is controlled to work in off-grid mode and operates in off-grid mode.

[0096] When the energy storage system operates in grid-connected mode, if the grid fails, the grid-connected switch is triggered to switch from on to off, and the energy storage system is blocked and driven, and after all the energy storage converters of the energy storage system are blocked and driven, the grid-connected switch is controlled to be turned off, and the energy storage system operates in off-grid mode. Figure 4 Rly on_to_off in the above formula represents that the grid-connected switch is switched from on to off.

[0097] When the energy storage system operates in off-grid mode, if the grid returns to normal, after the output voltage of the energy storage converter is synchronized with the grid voltage, the grid-connected switch is controlled to be turned on, and the energy storage system operates in grid-connected mode. Figure 4 Rly off_to_on in the above formula represents that the grid-connected switch is switched from off to on.

[0098] In some embodiments, the above control of each energy storage converter to track the grid voltage includes:

[0099] The real-time grid voltage is acquired and sent to each energy storage converter. The real-time grid voltage is used to instruct each energy storage converter to perform off-grid loop control and to perform positive and negative sequence compensation, so that the off-grid voltage of the energy storage converter is kept synchronized with the real-time grid voltage.

[0100] In the embodiments of the present application, the real-time grid voltage can include the frequency and the effective value of the real-time grid voltage, wherein the frequency of the grid voltage can be acquired by phase-locked loop. By sending the real-time grid voltage to each energy storage converter, the energy storage converter can keep the output voltage synchronized with the real-time grid voltage through off-grid loop control when receiving the real-time grid voltage.

[0101] Since the grid voltage may be unbalanced, positive and negative sequence compensation is also needed when performing off-grid loop control, so that the output voltage of the energy storage converter can be kept synchronized with the real-time grid voltage even when the grid voltage is unbalanced.

[0102] In some possible implementations, the above-mentioned positive and negative sequence compensation can include:

[0103] The average value of the positive feedback current and the average value of the negative feedback current of the energy storage system are acquired;

[0104] Based on the average value of the positive feedback current and the average value of the negative feedback current, current sharing control is performed to obtain a positive average current control quantity and a negative average current control quantity;

[0105] The positive average current control quantity is superimposed on the given output voltage of the voltage loop in the off-grid loop to perform voltage loop control and obtain a given output current;

[0106] The negative average current control quantity is transformed to obtain a first positive sequence control quantity, and the first positive sequence control quantity is superimposed on the given output current.

[0107] The given output current is used as the input of the current loop of the off-grid loop.

[0108] The average value of the positive feedback current is the average value of the positive feedback current of each energy storage converter in the energy storage system; and the average value of the negative feedback current is the average value of the negative feedback current of each energy storage converter in the energy storage system.

[0109] The above-mentioned current sharing control based on the average value of the positive feedback current and the average value of the negative feedback current to obtain a positive average current control quantity and a negative average current control quantity can include:

[0110] The average value of the positive feedback current is controlled by positive sequence current sharing control to obtain a positive average current control quantity;

[0111] The negative sequence feedback current average value is subjected to negative sequence current sharing control to obtain a negative sequence average current control quantity.

[0112] In the embodiments of the present application, the positive sequence feedback current average value is subjected to positive sequence current sharing control to obtain a positive sequence average current control quantity. Specifically, the positive sequence feedback current corresponding to the energy storage converter is obtained, a first difference value between the positive sequence feedback current average value and the positive sequence feedback current is calculated, and the first difference value is input into a first preset proportional integral controller to obtain the positive sequence average current control quantity. The related parameters in the first preset proportional integral controller are pre-set and can be set according to actual requirements.

[0113] The negative sequence feedback current average value is subjected to negative sequence current sharing control to obtain a negative sequence average current control quantity. Specifically, the negative sequence feedback current corresponding to the energy storage converter is obtained, a second difference value between the negative sequence feedback current average value and the negative sequence feedback current is calculated, and the second difference value is input into a second preset proportional integral controller to obtain the negative sequence average current control quantity. The related parameters in the second preset proportional integral controller are pre-set and can be set according to actual requirements.

[0114] The negative sequence average current control quantity is transformed to obtain a first positive sequence control quantity, which can include: obtaining the negative sequence feedback voltage corresponding to the energy storage converter, calculating a fifth difference value between the negative sequence average current control quantity of the corresponding energy storage converter and the negative sequence feedback voltage of the corresponding energy storage converter, and inputting the fifth difference value into a fifth preset proportional integral controller to obtain a first negative sequence control quantity of the corresponding energy storage converter, and transforming the first negative sequence control quantity to a positive sequence time sequence to obtain a first positive sequence control quantity of the corresponding energy storage converter. The related parameters in the fifth preset proportional integral controller are pre-set and can be set according to actual requirements.

[0115] In some embodiments, the grid-connected / off-grid switching method of the multi-machine parallel energy storage system is applied to the first controller and the second controller, that is, the first controller and the second controller can both execute the grid-connected / off-grid switching method of the multi-machine parallel energy storage system, and both will send an opening signal or a closing signal to the grid-connected switch based on the execution result.

[0116] When the first controller or the second controller sends an opening signal to the grid-connected switch, the grid-connected switch is opened;

[0117] When the first controller and the second controller both send a closing signal to the grid-connected switch, the grid-connected switch is closed.

[0118] In the embodiments of the present application, in order to prevent misoperation, the parallel switch is closed only when the first controller and the second controller both send the closing signal to the parallel switch. Meanwhile, in order to prevent the parallel switch from being unable to be timely controlled to be opened due to the failure of the first controller or the second controller, the parallel switch is opened only when one of the first controller and the second controller sends the opening signal to the parallel switch.

[0119] That is, the parallel switch is closed only when the first controller and the second controller both control the parallel switch to be closed, and the parallel switch is opened when the first controller or the second controller controls the parallel switch to be opened. The parallel switch can be a parallel contactor, a parallel circuit breaker (for example, a parallel frame circuit breaker) or a parallel relay, etc.

[0120] For other devices controlled by the first controller and the second controller, such as a zero line switch, an energy storage converter, etc., the same logic as the parallel switch can be maintained for control. Alternatively, only the master controller sends control instructions to the zero line switch and the energy storage converter, and the slave controller does not send control instructions to the zero line switch and the energy storage converter when the master controller does not fail. When the master controller fails, the slave controller sends control instructions to the zero line switch and the energy storage converter.

[0121] The control mode of the first controller and the second controller on the parallel switch can be realized in the form of a software program, for example, the parallel switch has a corresponding control unit, and the control unit controls the parallel switch to be closed when receiving the closing signal of the first controller and the second controller at the same time, and controls the parallel switch to be opened when receiving the opening signal of one of the controllers.

[0122] The control mode of the first controller and the second controller on the parallel switch can also be realized in the form of hardware. Referring to Figure 6 The parallel switch 13 can include a first switch 131, a closing coil 132, an opening coil 133, a second switch 134, a third switch 135, a fourth switch 136 and a fifth switch 137;

[0123] The two ends of the first switch 131 are respectively connected to the power grid and the energy storage converter 16;

[0124] The closing coil 132, the second switch 134 and the third switch 135 are connected in series; the opening coil 133, the fourth switch 136 and the fifth switch 137 are connected in parallel;

[0125] The closing signal of the first controller 11 acts on the second switch 134, and the closing signal of the second controller 12 acts on the third switch 135; the opening signal of the first controller 11 acts on the fourth switch 136, and the opening signal of the second controller 12 acts on the fifth switch 137.

[0126] The closing coil 132, the second switch 134 and the third switch 135 are connected in series to form a series loop. The opening coil 133, the fourth switch 136 and the fifth switch 137 are connected in parallel. The closing coil 132 can also be referred to as a pull-in coil.

[0127] In some possible implementations, the series loop described above can further include a first power supply connected in series; the opening coil 133 can be connected in series with a second power supply, and then connected in parallel with the fourth switch 136 and the fifth switch 137.

[0128] When the first controller 11 sends a closing signal to the grid-connected switch 13, the second switch 134 is closed. When the second controller 12 sends a closing signal to the grid-connected switch 13, the third switch 135 is closed. When both the second switch 134 and the third switch 135 are closed, the closing coil 132 is powered, and at this time the closing coil 132 functions to close the first switch 131, i.e., the grid-connected switch 13 is closed, connecting the power grid and the energy storage converter 16.

[0129] When the first controller 11 sends an opening signal to the grid-connected switch 13, the fourth switch 136 is closed, and at this time the opening coil 133 is powered. The opening coil 133 functions to open the first switch 131, i.e., the grid-connected switch 13 is opened, disconnecting the power grid and the AC bus 14. Similarly, when the second controller 12 sends an opening signal to the grid-connected switch 13, the fifth switch 137 is closed, and at this time the opening coil 133 is powered. The opening coil 133 functions to open the first switch 131, i.e., the grid-connected switch 13 is opened, disconnecting the power grid and the energy storage converter 16.

[0130] When the first controller 11 sends a closing signal to the grid-connected switch 13, the second switch 134 is closed while the fourth switch 136 is opened. When the second controller 12 sends a closing signal to the grid-connected switch 13, the third switch 135 is closed while the fifth switch 137 is opened. Similarly, when the first controller 11 sends an opening signal to the grid-connected switch 13, the fourth switch 136 is closed while the second switch 134 and / or the third switch 135 are opened. When the second controller 12 sends an opening signal to the grid-connected switch 13, the fifth switch 137 is closed while the second switch 134 and / or the third switch 135 are opened.

[0131] When the closing coil 132 functions, the first switch 131 is closed, i.e., the grid-connected switch 13 is closed. When the opening coil 133 functions, the first switch 131 is opened, i.e., the grid-connected switch 13 is opened.

[0132] In some embodiments, when the energy storage system operates in an off-grid mode, each energy storage converter adopts a dq control mode for off-grid loop control.

[0133] Since each energy storage converter in the energy storage system is a three-phase output without a neutral line, each energy storage converter adopts a dq control mode for off-grid loop control in the off-grid mode.

[0134] In some embodiments, after the control of blocking the drive of each energy storage converter, further comprising:

[0135] controlling the corresponding output switch of each energy storage converter to be turned off; each energy storage converter is connected with the grid-connected switch through the corresponding output switch;

[0136] If it is determined that all energy storage converters of the energy storage system have been blocked from driving, the grid-connected switch is controlled to be turned off, comprising:

[0137] If it is determined that all energy storage converters of the energy storage system have been blocked from driving and all corresponding output switches of the energy storage converters have been turned off, the grid-connected switch is controlled to be turned off.

[0138] Specifically, the off-grid flag is also used to instruct each energy storage converter to control the corresponding output switch to be turned off.

[0139] If it is determined that all energy storage converters have been blocked from driving and all corresponding output switches of the energy storage converters have been turned off according to the working states reported by each energy storage converter, the grid-connected switch is controlled to be turned off.

[0140] The output switch corresponding to the energy storage converter can be an internal output switch of the energy storage converter, and when it is turned off, the energy storage converter has no output.

[0141] In the embodiments of the present application, in order to further ensure that the energy storage system has no output when switching from grid-connected to off-grid, in addition to being able to control each energy storage converter of the energy storage system to be blocked from driving, the output switch of each energy storage converter can be further turned off.

[0142] Correspondingly, the working state reported by each energy storage converter is used to indicate whether it has been blocked from driving and whether the corresponding output switch has been turned off. When all energy storage converters of the energy storage system have been blocked from driving and all corresponding output switches have been turned off, the grid-connected switch is controlled to be turned off, so as to further improve the safety and avoid system instability caused by sudden changes in electric energy during the switching process.

[0143] Correspondingly, the above-mentioned switching of the working loop of each energy storage converter from the grid-connected loop to the off-grid loop can comprise: issuing an off-grid start-up instruction to each energy storage converter; the off-grid start-up instruction is used to instruct each energy storage converter to close the corresponding output switch, and to switch from the grid-connected loop to the off-grid loop, and to perform a grid-connected off-grid slow start-up, and after starting up, to work in the off-grid mode.

[0144] In some embodiments, referring to Figure 2 andFigure 3 Each energy storage converter 16 is connected to the AC bus 14 through a transformer 17.

[0145] Since each energy storage converter 16 in the energy storage system is a three-phase three-wire output without a neutral line, in order to meet the requirement that the output of the energy storage system can be single-phase load carrying, the above-mentioned transformer 17 is connected to the output end of each energy storage converter 16 to perform voltage system conversion, so that the output of the energy storage system has a neutral line.

[0146] It should be understood that the size of the serial number of each step in the above-mentioned embodiment does not mean the order of execution, and the execution order of each process should be determined according to its function and inherent logic, and should not constitute any limitation on the implementation process of the embodiment of the present application.

[0147] Figure 7 The structure schematic diagram of the on-off grid switching device of the multi-machine parallel energy storage system provided by the embodiment of the present application is shown, only the parts related to the embodiment of the present application are shown for the convenience of description, and the details are as follows:

[0148] The energy storage system includes a plurality of parallel energy storage converters. As shown in Figure 7 The on-off grid switching device 30 of the multi-machine parallel energy storage system includes:

[0149] The synchronization module 31 is configured to, when the energy storage system works in the off-grid mode, if it is detected that the power grid is restored to normal, control each energy storage converter to track the grid voltage, so that the off-grid voltage of each energy storage converter is kept synchronous with the grid voltage.

[0150] The control module 32 is configured to, when it is detected that the off-grid voltage of each energy storage converter is kept synchronous with the grid voltage, control the grid switch to be closed; after each energy storage converter is connected in parallel, the grid switch is connected with the grid.

[0151] The loop switching module 33 is configured to control the working loop of each energy storage converter to be switched from the off-grid loop to the grid-connected loop.

[0152] In a possible implementation, the on-off grid switching device 30 of the multi-machine parallel energy storage system further includes a grid-connected off-grid module.

[0153] The above-mentioned grid-connected off-grid module is configured to:

[0154] When the energy storage system works in the grid-connected mode, if the power grid fails, the grid-connected off-grid module is configured to control each energy storage converter to be blocked from driving.

[0155] If it is determined that all energy storage converters of the energy storage system have been blocked from driving, the grid-connected off-grid module is configured to control the grid switch to be opened.

[0156] The grid-connected off-grid module is configured to control the working loop of each energy storage converter to be switched from the grid-connected loop to the off-grid loop.

[0157] In a possible implementation, in the synchronization module 31, the control of each energy storage converter tracking the grid voltage comprises:

[0158] The real-time grid voltage is obtained, and the real-time grid voltage is sent to each energy storage converter; the real-time grid voltage is used to instruct each energy storage converter to perform off-grid loop control and positive and negative sequence compensation, so that the off-grid voltage of each energy storage converter is kept synchronous with the real-time grid voltage.

[0159] In a possible implementation, the first controller and the second controller are used to perform the method corresponding to the on-off grid switching device 30 of the multi-machine parallel energy storage system described above;

[0160] When the first controller or the second controller sends an opening signal to the on-grid switch, the on-grid switch is opened;

[0161] When the first controller and the second controller both send a closing signal to the on-grid switch, the on-grid switch is closed.

[0162] In a possible implementation, the on-grid switch comprises a first switch, a closing coil, an opening coil, a second switch, a third switch, a fourth switch and a fifth switch;

[0163] The first switch is connected to the grid and the energy storage converter at two ends respectively;

[0164] The closing coil, the second switch and the third switch are connected in series; the opening coil, the fourth switch and the fifth switch are connected in parallel;

[0165] The closing signal of the first controller acts on the second switch, and the closing signal of the second controller acts on the third switch; the opening signal of the first controller acts on the fourth switch, and the opening signal of the second controller acts on the fifth switch.

[0166] In a possible implementation, when the energy storage system works in the off-grid mode, each energy storage converter adopts a dq control mode for off-grid loop control.

[0167] In a possible implementation, in the on-off grid module, after the control of blocking the driving of each energy storage converter, the method further comprises:

[0168] The output switch corresponding to each energy storage converter is controlled to be opened; each energy storage converter is connected to the on-grid switch through the corresponding output switch;

[0169] If it is determined that all the energy storage converters of the energy storage system have been blocked and driven, the on-grid switch is controlled to be opened, comprising:

[0170] If it is determined that all the energy storage converters of the energy storage system have been blocked and driven, and the output switches corresponding to all the energy storage converters have been opened, the on-grid switch is controlled to be opened.

[0171] In one possible implementation, the grid-connected / off-grid switching device 30 of the multi-machine parallel energy storage system further includes a standby module.

[0172] The aforementioned standby module is used to: when the energy storage system is in standby mode, if the power grid is in normal condition, control the energy storage system to operate in grid-connected mode; otherwise, control the energy storage system to operate in off-grid mode.

[0173] Figure 8 This is a schematic diagram of the controller provided in an embodiment of the present invention. Figure 8 As shown, the controller 4 in this embodiment includes a processor 40 and a memory 41. The memory 41 stores a computer program 42, and the processor 40 calls and runs the computer program 42 stored in the memory 41 to execute the steps in the above embodiments of the grid-connected / off-grid switching methods for various multi-machine parallel energy storage systems, for example... Figure 4 S101 to S103 are shown. Alternatively, the processor 40 is used to call and run the computer program 42 stored in the memory 41 to implement the functions of each module / unit in the above-described device embodiments, for example... Figure 7 The functions of each module are shown.

[0174] For example, the computer program 42 can be divided into one or more modules / units, which are stored in the memory 41 and executed by the processor 40 to complete the present invention. The one or more modules / units can be a series of computer program instruction segments capable of performing a specific function, which describe the execution process of the computer program 42 in the controller 4. For example, the computer program 42 can be divided into... Figure 7 The modules shown.

[0175] The controller 4 may include, but is not limited to, a processor 40 and a memory 41. Those skilled in the art will understand that... Figure 8 This is merely an example of controller 4 and does not constitute a limitation on controller 4. It may include more or fewer components than shown, or combine certain components, or different components. For example, the controller may also include input / output devices, network access devices, buses, etc. For example, the controller may also include the aforementioned DMS and OMS, etc.

[0176] The processor 40 can be a central processing unit (CPU), and can also be other general-purpose processors, digital signal processors (DSP), application specific integrated circuits (ASIC), field-programmable gate arrays (FPGA) or other programmable logic devices, discrete gates or transistor logic components, discrete hardware components, etc. The general-purpose processor can be a microprocessor or the processor can also be any conventional processor.

[0177] The memory 41 can be an internal storage unit of the controller 4, such as a hard disk or a memory of the controller 4. The memory 41 can also be an external storage device of the controller 4, such as a plug-in hard disk, a smart media card (SMC), a secure digital (SD) card, a flash card, etc. Further, the memory 41 can include both the internal storage unit and the external storage device of the controller 4. The memory 41 is used to store the computer program and other programs and data required by the controller. The memory 41 can also be used to temporarily store data that has been output or will be output.

[0178] Corresponding to the above controller, the embodiment of the present application further provides an energy storage system, comprising a first controller, a second controller, a grid-connected switch and a plurality of parallel energy storage converters; the energy storage converters and the grid-connected switch are controlled by the first controller and the second controller; the first controller and the second controller are both the above controller;

[0179] After the energy storage converters are connected in parallel, the energy storage converters are connected with the power grid through the grid-connected switch.

[0180] The energy storage system can refer to the description in the foregoing embodiments, and will not be described here.

[0181] Those skilled in the art can clearly understand that, for the convenience and brevity of description, only the above-mentioned division of each functional unit and module is exemplified, and in actual application, the above-mentioned functions can be completed by different functional units and modules according to needs, that is, the internal structure of the device is divided into different functional units or modules to complete all or part of the functions described above. Each functional unit and module in the embodiment can be integrated in one processing unit, or each unit can be physically present separately, or two or more units can be integrated in one unit. The above-mentioned integrated unit can be realized in the form of hardware or software. In addition, the specific names of each functional unit and module are only for the convenience of mutual distinction, and do not limit the protection scope of the present application. The specific working process of the units and modules in the above system can refer to the corresponding process in the foregoing method embodiments, which will not be described here.

[0182] In the above embodiments, the description of each embodiment has its own emphasis, and the parts not described or recorded in detail in a certain embodiment can be referred to the related description of other embodiments.

[0183] Those of ordinary skill in the art can realize that the units and algorithm steps of each example described in combination with the embodiments disclosed herein can be realized in electronic hardware or a combination of computer software and electronic hardware. Whether the functions are executed in hardware or software depends on the specific application and design constraints of the technical solution. A person skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of the present application.

[0184] In the embodiments provided by the present application, it should be understood that the disclosed devices / controllers and methods can be implemented in other ways. For example, the above-described device / controller embodiments are only schematic. For example, the division of the modules or units is only a logical function division, and there can be another division manner in actual implementation. For example, a plurality of units or components can be combined or integrated into another system, or some features can be ignored or not executed. In addition, the coupling or direct coupling or communication connection between the units shown or discussed can be indirect coupling or communication connection through some interface, device or unit, and can be electrical, mechanical or in other forms.

[0185] The units described as separate components can or can not be physically separated, and the components shown as units can or can not be physical units, that is, they can be located in one place, or can be distributed on a plurality of network units. Some or all of the units can be selected according to actual needs to achieve the purpose of the embodiment.

[0186] In addition, each function unit in each embodiment of the present application can be integrated in one processing unit, or each unit can be physically present separately, or two or more units can be integrated in one unit. The integrated unit can be realized in the form of hardware or in the form of a software function unit.

[0187] The integrated module / unit, if realized in the form of a software function unit and sold or used as an independent product, can be stored in a computer-readable storage medium. Based on this understanding, all or part of the processes in the above-mentioned embodiment methods can also be completed by a computer program instructing related hardware, and the computer program can be stored in a computer-readable storage medium. The computer program can implement the steps of each multi-machine parallel energy storage system parallel and off-grid switching method embodiment when executed by a processor. The computer program includes computer program code, which can be in the form of source code, object code, an executable file, or some intermediate form, etc. The computer-readable medium can include any entity or device capable of carrying the computer program code, a recording medium, a U disk, a mobile hard disk, a magnetic disk, an optical disk, a computer memory, a read-only memory (ROM), a random access memory (RAM), an electrical carrier signal, a telecommunication signal, and a software distribution medium, etc. It should be noted that the contents included in the computer-readable medium can be appropriately increased or decreased according to the requirements of legislation and patent practice in the jurisdiction, for example, in some jurisdictions, according to legislation and patent practice, the computer-readable medium does not include electrical carrier signals and telecommunication signals.

[0188] The above-described embodiments are only used to illustrate the technical solutions of the present application, rather than limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement for part of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of each embodiment of the present application, and should be included in the protection scope of the present application.

Claims

1. A method for switching between grid connection and off-grid operation in a multi-unit parallel energy storage system, characterized in that, The energy storage system includes multiple parallel-connected energy storage converters, and the method includes: When the energy storage system is operating in off-grid mode, if the grid is detected to have returned to normal, the system controls each energy storage converter to track the grid voltage, so that the off-grid voltage of each energy storage converter is synchronized with the grid voltage. When it is detected that the off-grid voltage of each energy storage converter is synchronized with the grid voltage, the grid-connected switch is closed; after the energy storage converters are connected in parallel, they are connected to the grid through the grid-connected switch. The operating loop of each energy storage converter is switched from an off-grid loop to a grid-connected loop. The method is applied to the first controller and the second controller; When the first controller or the second controller sends a disconnect signal to the grid-connected switch, the grid-connected switch is disconnected; when both the first controller and the second controller send a close signal to the grid-connected switch, the grid-connected switch is closed. The grid-connected switch includes a first switch, a closing coil, a disconnecting coil, a second switch, a third switch, a fourth switch, and a fifth switch; the two ends of the first switch are respectively connected to the power grid and the energy storage converter; the closing coil, the second switch, and the third switch are connected in series; the disconnecting coil, the fourth switch, and the fifth switch are connected in parallel. The closing signal of the first controller acts on the second switch, and the closing signal of the second controller acts on the third switch; the opening signal of the first controller acts on the fourth switch, and the opening signal of the second controller acts on the fifth switch.

2. The grid-connected / off-grid switching method for a multi-unit parallel energy storage system according to claim 1, characterized in that, The method further includes: When the energy storage system is operating in grid-connected mode, if a grid fault occurs, the drive of each energy storage converter will be blocked. If it is determined that all energy storage converters of the energy storage system are locked from driving, then the grid connection switch is controlled to disconnect. The operating loop of each energy storage converter is switched from a grid-connected loop to an off-grid loop.

3. The grid-connected / off-grid switching method for a multi-unit parallel energy storage system according to claim 1, characterized in that, The control of each energy storage converter to track the grid voltage includes: The system acquires the real-time grid voltage and sends it to each energy storage converter. The real-time grid voltage is used to instruct each energy storage converter to perform off-grid loop control and positive and negative sequence compensation so that its off-grid voltage is synchronized with the real-time grid voltage.

4. The grid-connected / off-grid switching method for a multi-unit parallel energy storage system according to claim 1, characterized in that, When the energy storage system operates in off-grid mode, each energy storage converter uses dq control method for off-grid loop control.

5. The grid-connected / off-grid switching method for a multi-unit parallel energy storage system according to claim 2, characterized in that, After controlling the blocking drive of each energy storage converter, the following is also included: The output switches corresponding to each energy storage converter are disconnected; each energy storage converter is connected to the grid-connected switch through its corresponding output switch. If it is determined that all energy storage converters of the energy storage system are locked from driving, then controlling the grid connection switch to disconnect includes: If it is determined that all energy storage converters of the energy storage system are locked from driving and all output switches corresponding to the energy storage converters are disconnected, then the grid connection switch is controlled to disconnect.

6. The grid-connected / off-grid switching method for a multi-unit parallel energy storage system according to any one of claims 1 to 5, characterized in that, The method further includes: When the energy storage system is in standby mode, if the power grid is in normal condition, the energy storage system is controlled to operate in grid-connected mode; otherwise, the energy storage system is controlled to operate in off-grid mode.

7. A controller, characterized in that, It includes a memory and a processor, wherein the memory is used to store computer programs, and the processor is used to call and run the computer programs stored in the memory to perform the grid-connected / off-grid switching method for a multi-machine parallel energy storage system as described in any one of claims 1 to 6.

8. An energy storage system, characterized in that, It includes a first controller, a second controller, a grid-connected switch, and multiple parallel-connected energy storage converters; the energy storage converters and the grid-connected switch are controlled by the first controller and the second controller; the first controller and the second controller are both controllers as described in claim 7; After the various energy storage converters are connected in parallel, they are connected to the power grid through the grid-connected switch.

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