Cascade multi-level energy storage system and charging and discharging control method thereof

Abstract

The present invention belongs to the field of energy storage systems, and specifically relates to a cascaded multi-level energy storage system and a control method thereof, which aims to solve the problem that the voltage balancing control of the supporting capacitor and the energy balancing control of the energy storage element are seriously coupled in the prior art, resulting in a complex control module of the cascaded multi-level energy storage system, and it is difficult to take into account multiple control targets simultaneously in the system charge and discharge control. The present invention includes: a cascaded multi-level energy storage converter composed of three-phase bridge arms, each phase bridge arm includes N submodule units; an energy storage system controller composed of an AC control unit, a modulation unit and N submodule control units, the AC control unit operates in a grid-connected or off-grid state, controls the voltage or power on the high-voltage AC side, and the modulation unit generates a switch control signal for each submodule according to the charge state of the energy storage element in each submodule, and the submodule control unit controls the supporting capacitor voltage. The control system of the present invention is simple, and the charge and discharge control strategy can take into account multiple control targets simultaneously.

Description

Background Art

[0002] With the large-scale application of renewable energy power generation and the development of high-speed electromagnetic drive technology, the demand for high-voltage and large-capacity energy storage systems is becoming increasingly urgent. Cascaded multi-level energy storage systems can expand the voltage and capacity of energy storage through cascading, which is one of the important trends in the development of future energy storage systems.

[0003] The existing cascaded multi-level energy storage system is mainly based on modular multi-level converters and cascaded H-bridge multi-level converters. Generally, electrochemical energy storage elements are directly connected in parallel to the support capacitors in the original converter and connected to the semiconductor switch circuit, such as invention patents CN102185487A, CN108923518A and CN112803476A. However, the voltage of the energy storage element in this type of technical solution is limited by the output voltage range of the AC side, and its voltage operating range is generally narrow, which limits the discharge depth of the energy storage element and makes the energy utilization efficiency low. During the charging and discharging process, the large current ripple causes the health of the battery pack to deteriorate. In addition, there are also problems such as the difficulty in simultaneously satisfying the voltage balance and SOC balance of the energy storage element, which also restricts the large-scale application of this technology.

[0004] Invention patents CN105897018A and CN110556852A use a two-stage conversion circuit topology, adding a DC-DC conversion stage to the submodule of the original converter, connecting the support capacitor and the energy storage element through a DC-DC converter, making the voltage operating range of the energy storage element wider and enabling stable control of the support capacitor voltage. However, the cascaded multi-level energy storage system must control both the voltage balancing of the support capacitor and the energy balance of the energy storage element, making the control complex and difficult to cope with complex working conditions such as power fluctuations. Summary of the invention

[0005] In order to solve the above-mentioned problems in the prior art, namely, the coupling between the voltage balancing control of the supporting capacitor and the energy balancing control of the energy storage element in the prior art is serious, resulting in the complexity of the control module of the cascaded multi-level energy storage system, and the problem that it is difficult to take into account multiple control targets at the same time in the system charge and discharge control, the present invention provides a cascaded multi-level energy storage system, which includes a cascaded multi-level energy storage converter and an energy storage system controller;

[0006] The cascaded multi-level energy storage converter comprises a three-phase bridge arm, each phase bridge arm comprises N cascaded submodule units, and is used for charging the energy storage elements in the submodule units through the energy storage converter when the system state is grid-connected, and for supplying power to the load through the energy storage converter when the system state is off-grid; wherein N is a positive integer;

[0007] The energy storage system controller includes an AC control unit, a modulation unit and N submodule control units distributed in the submodule units. The AC control unit sets control instructions according to the system state. After the modulation unit converts the control instructions into switch control signals, the submodule control unit performs drive control of the corresponding submodules.

[0008] Furthermore, when the system state is grid-connected, the system operation mode is a power control mode, the energy storage system controller calculates an AC current reference value based on the average voltage of the energy storage element, calculates a feedback value by sampling the AC side current, and implements closed-loop control of the AC side charging and discharging power of the cascaded multi-level energy storage converter through current loop feedback control.

[0009] Furthermore, when the system state is off-grid, the system operation mode is a voltage control mode, and the energy storage system controller uses a voltage single-loop open-loop control strategy to control the voltage outputted on the AC side of the cascaded multi-level energy storage converter.

[0010] Further, the submodule unit includes an AC side H-bridge, a DC-DC converter, an energy storage element and a supporting capacitor;

[0011] The first input terminal and the second input terminal of the H bridge serve as the first input terminal and the second input terminal of the submodule unit;

[0012] The first output terminal of the H-bridge, the first connection terminal of the support capacitor and the first input terminal of the DC-DC converter are connected together;

[0013] The second output terminal of the H-bridge, the second connection terminal of the support capacitor and the second input terminal of the DC-DC converter are connected together;

[0014] The first output terminal and the second output terminal of the DC-DC converter are respectively connected to the positive electrode and the negative electrode of the energy storage element.

[0015] Further, the H-bridge includes IGBT device 1, IGBT device 2, IGBT device 3 and IGBT device 4;

[0016] The source of the IGBT device 1 and the drain of the IGBT device 3 are connected together as a first input terminal of the H bridge;

[0017] The source of the IGBT device 2 and the drain of the IGBT device 4 are connected together as the second input terminal of the H bridge;

[0018] The drain of the IGBT device 1 and the drain of the IGBT device 2 are connected together as the first output terminal x of the H bridge;

[0019] The source of the IGBT device 3 and the source of the IGBT device 4 are connected together as the second output terminal y of the H bridge.

[0020] Further, the DC-DC converter includes an IGBT device 5, an IGBT device 6 and an energy storage filter inductor;

[0021] The drain of the IGBT device 5 serves as the first input terminal of the DC-DC converter;

[0022] The source of the IGBT device 6 serves as the second input terminal and the second output terminal of the DC-DC converter respectively;

[0023] The source of the IGBT device 5 and the drain of the IGBT device 6 are connected together to the first connection terminal of the energy storage filter inductor;

[0024] The second connection end of the energy storage filter inductor serves as the first output end of the DC-DC converter.

[0025] Furthermore, the submodule control unit controls the corresponding submodule unit including:

[0026] The submodule control unit drives and controls the IGBT device in the submodule unit according to the switching signal of the IGBT device in the H-bridge;

[0027] The submodule control unit collects the support capacitor voltage and inductor current in the corresponding submodule unit, generates a switching signal of the IGBT device in the DC-DC converter after performing double closed-loop control, and drives and controls the corresponding IGBT device.

[0028] Another aspect of the present invention provides a charge and discharge control method for a cascade multi-level energy storage system. Based on the above-mentioned cascade multi-level energy storage system, the charge and discharge control method includes:

[0029] Step S10, obtaining the voltages of the N energy storage elements in the N submodule units, and calculating the real-time average voltage U of the energy storage elements s_avr ;

[0030] Step S20, based on the real-time average voltage U of the energy storage element s_avr , set the active component of the three-phase AC current reference value of the cascaded multi-level energy storage converter to i d_ref ;

[0031] Step S30, when U s_avr ≤U le1 When i d_ref =i ch , where i chRepresents the maximum allowable charging current set for the energy storage element;

[0032] When U le1 s_avr le2 When i d_ref =P ch / U s_avr , where P ch Represents the maximum permissible charging power set for the energy storage element;

[0033] When U s_avr ≥U le2 When i d_ref =K pu ×(U nom -U s_avr ), where K pu is a positive constant, representing the preset energy storage element voltage closed-loop control proportional coefficient, U nom Represents the rated charging voltage of the energy storage element, U le1 , U le2 , U le3 A voltage threshold for determining a charging mode switch of a set energy storage element;

[0034] Step S40: performing a three-phase alternating current i of the cascaded multi-level energy storage converter. ha 、i hb and i hc The D-axis current i of the AC side of the cascaded multi-level energy storage converter is obtained by park transformation. hd and Q axis current i hq ;

[0035] Step S50, i d_ref and i hd As the reference value and feedback value of the D-axis current, 0 and i hq As the reference value and feedback value of the Q axis current respectively, a closed-loop control strategy is used to control the current, and based on the output result of the control loop, the three-phase modulation voltage u of the cascaded multi-level energy storage converter is obtained. ma 、u mb and u mc ;

[0036] Step S60, the three-phase modulation voltage u is performed by the modulation unit ma 、u mb and u mc Modulation to obtain switch control signals of N submodule units;

[0037] ​​Step S70 , based on the switch control signals of the N submodule units, the submodule control unit performs drive control of the corresponding submodules to achieve charge and discharge control of the cascade multi-level energy storage system.

[0038] Further, step S60 includes:

[0039] Step S61, calculate the state of charge SOC of the energy storage element in each submodule unit of the bridge arm of the current phase i , 1≤i≤N;

[0040] Step S62, respectively convert the three-phase modulation voltage u ma 、u mb and u mc Performing base frequency step wave modulation to obtain N step wave control signals of the A phase bridge arm, the B phase bridge arm and the C phase bridge arm respectively;

[0041] Step S63, for any phase in the three-phase bridge arm, sorting N step wave control signals from large to small according to the duty cycle of the step wave control signal;

[0042] Step S64: when the system is in a charging state, the state of charge SOC i Sorting the N submodule units from low to high, and sequentially allocating N step wave control signals sorted in descending order of duty cycle to the sorted N submodule units;

[0043] When the system is in the discharge state, according to the state of charge SOC i The N submodule units are sorted in order from high to low, and N step wave control signals sorted in order from large to small according to the duty cycle are sequentially allocated to the sorted N submodule units.

[0044] Furthermore, when the cascaded multi-level energy storage system issues a command to start charging the energy storage element, the control process of the submodule control unit includes:

[0045] The submodule control unit uses a preset support capacitor voltage target value U dc_nom is a given value to support the capacitor voltage u ci is the voltage outer loop feedback value, and performs closed-loop control of the voltage outer loop;

[0046] The submodule control unit uses the output u of the voltage loop clo As the given value of the current loop, the inductor current i Li is the feedback value of the current loop, and performs closed-loop control of the current inner loop;

[0047] Based on the output i of the current inner loop controller clo, obtaining control signals of each IGBT device of the DC-DC converter.

[0048] Beneficial effects of the present invention:

[0049] (1) The cascaded multi-level energy storage system of the present invention performs a decoupling and simplified design for the controller of the system, and uses a submodule control unit in the control system to independently control the support capacitor voltage of each submodule, so that the support capacitor voltage can be maintained near the rated value under any working condition. At the same time, a modulation unit is used to adjust the energy balance of the energy storage element, thereby achieving decoupling of the submodule support capacitor voltage balancing and the energy balance control of the energy storage element. This effectively reduces the complexity of the system controller and improves the accuracy and precision of the control strategy.

[0050] (2) In the cascaded multi-level energy storage system of the present invention, the sub-module control unit in the energy storage system controller corresponds one-to-one to the sub-module unit in the cascaded multi-level energy storage converter, and the closed-loop control of the supporting capacitor voltage is performed through the sub-module control unit, without the need to add an additional supporting capacitor voltage equalization control strategy, thereby effectively improving the control efficiency and reducing the control complexity.

[0051] (3) The charge and discharge control method of the cascaded multi-level energy storage system of the present invention fully combines the system component configuration, and allocates switch signals in sequence according to the charge state sorting results of the energy storage components in the modulation unit of the energy storage system controller in combination with the charge and discharge state of the energy storage system, thereby achieving decoupling from the voltage balancing control of the supporting capacitor, effectively improving the performance of the control strategy and improving the control precision and accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Other features, objects and advantages of the present application will become more apparent by reading the detailed description of non-limiting embodiments made with reference to the following drawings:

[0053] Figure 1 is a topological schematic diagram of the cascade multi-level energy storage system of the present invention;

[0054] Figure 2 It is a topological schematic diagram of a submodule unit in a cascaded multi-level energy storage converter of an embodiment of a cascaded multi-level energy storage system of the present invention;

[0055] Figure 3 It is a schematic diagram of connecting an energy storage system controller of an embodiment of a cascaded multi-level energy storage system of the present invention. DETAILED DESCRIPTION

[0056] The present application will be further described in detail below in conjunction with the accompanying drawings and embodiments. It is to be understood that the specific embodiments described herein are only used to explain the relevant invention, rather than to limit the invention. It is also necessary to explain that, for ease of description, only the parts related to the relevant invention are shown in the accompanying drawings.

[0057] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0058] A cascaded multi-level energy storage system of the present invention comprises a cascaded multi-level energy storage converter and an energy storage system controller;

[0059] The cascaded multi-level energy storage converter comprises a three-phase bridge arm, each phase bridge arm comprises N cascaded submodule units, and is used for charging the energy storage elements in the submodule units through the energy storage converter when the system state is grid-connected, and for supplying power to the load through the energy storage converter when the system state is off-grid; wherein N is a positive integer;

[0060] The energy storage system controller includes an AC control unit, a modulation unit and N submodule control units distributed in the submodule units. The AC control unit sets control instructions according to the system state. After the modulation unit converts the control instructions into switch control signals, the submodule control unit performs drive control of the corresponding submodules.

[0061] In order to more clearly explain the cascade multi-level energy storage system of the present invention, Figure 1 Each module in the embodiment of the present invention is described in detail.

[0062] The cascaded multilevel energy storage system of the first embodiment of the present invention includes a cascaded multilevel energy storage converter and an energy storage system controller. Each module is described in detail as follows:

[0063] The cascaded multilevel energy storage converter includes a three-phase bridge arm, each phase bridge arm includes N cascaded submodule units, which are used for charging the energy storage elements in the submodule units through the energy storage converter when the system state is grid-connected, and for supplying power to the load through the energy storage converter when the system state is off-grid; wherein N is a positive integer.

[0064] like Figure 2 , which is a topological schematic diagram of a submodule unit in a cascaded multilevel energy storage converter of an embodiment of a cascaded multilevel energy storage system of the present invention, wherein the submodule unit includes an AC side H-bridge, a DC-DC converter, an energy storage element, and a supporting capacitor;

[0065] The first input terminal and the second input terminal of the H-bridge serve as the first input terminal and the second input terminal of the submodule unit, the first output terminal of the H-bridge, the first connection terminal of the supporting capacitor and the first input terminal of the DC-DC converter are connected together, the second output terminal of the H-bridge, the second connection terminal of the supporting capacitor and the second input terminal of the DC-DC converter are connected together, and the first output terminal and the second output terminal of the DC-DC converter are respectively connected to the positive electrode and the negative electrode of the energy storage element.

[0066] The H bridge includes IGBT device 1, IGBT device 2, IGBT device 3 and IGBT device 4;

[0067] The source of IGBT device 1 and the drain of IGBT device 3 are connected together as the first input terminal of the H bridge, the source of IGBT device 2 and the drain of IGBT device 4 are connected together as the second input terminal of the H bridge, the drain of IGBT device 1 and the drain of IGBT device 2 are connected together as the first output terminal x of the H bridge, and the source of IGBT device 3 and the source of IGBT device 4 are connected together as the second output terminal y of the H bridge.

[0068] The DC-DC converter includes an IGBT device 5, an IGBT device 6 and an energy storage filter inductor;

[0069] The drain of the IGBT device 5 serves as the first input terminal of the DC-DC converter, the source of the IGBT device 6 serves as the second input terminal and the second output terminal of the DC-DC converter respectively, the source of the IGBT device 5 and the drain of the IGBT device 6 are connected together to the first connection terminal of the energy storage filter inductor, and the second connection terminal of the energy storage filter inductor serves as the first output terminal of the DC-DC converter.

[0070] like Figure 3 As shown, a connection diagram of an energy storage system controller of an embodiment of a cascaded multi-level energy storage system of the present invention is shown. The energy storage system controller includes an AC control unit, a modulation unit, and N submodule control units distributed in the submodule units. The AC control unit sets a control instruction according to the system state. After the modulation unit converts the control instruction into a switch control signal, the submodule control unit performs drive control of the corresponding submodule.

[0071] When the system is in grid-connected state, the system operates in power control mode. The energy storage system controller calculates the AC current reference value based on the average voltage of the energy storage element, calculates the feedback value by sampling the AC side current, and realizes closed-loop control of the AC side charging and discharging power of the cascaded multi-level energy storage converter through current loop feedback control.

[0072] When the system state is off-grid, the system operation mode is voltage control mode, and the energy storage system controller adopts a voltage single-loop open-loop control strategy to control the voltage outputted on the AC side of the cascaded multi-level energy storage converter.

[0073] The submodule control unit controls the corresponding submodule units including:

[0074] The submodule control unit drives the IGBT device in the control submodule unit according to the switching signal of the IGBT device in the H bridge;

[0075] The submodule control unit collects the support capacitor voltage and inductor current in the corresponding submodule unit, generates a switching signal of the IGBT device in the DC-DC converter after performing double closed-loop control, and drives and controls the corresponding IGBT device.

[0076] A charge and discharge control method for a cascaded multi-level energy storage system according to a second embodiment of the present invention is based on the above-mentioned cascaded multi-level energy storage system. The charge and discharge control method includes:

[0077] Step S10, obtaining the voltages of the N energy storage elements in the N submodule units, and calculating the real-time average voltage U of the energy storage elements s_avr , as shown in formula (1):

[0078]

[0079] Among them, u sa_i 、u sb_i and u sc_i are the supercapacitor voltages of the i-th energy storage element among the N energy storage elements in phase a, phase b, and phase c, respectively.

[0080] Step S20, based on the real-time average voltage U of the energy storage element s_avr , set the active component of the three-phase AC current reference value of the cascaded multi-level energy storage converter to i d_ref .

[0081] Step S30, when U s_avr ≤U le1 When i d_ref =i ch , where i ch Represents the maximum allowable charging current set for the energy storage element;

[0082] When U le1 s_avr le2 When i d_ref =P ch / U s_avr , where P ch ​​Represents the maximum permissible charging power set for the energy storage element;

[0083] When U s_avr ≥U le2 When i d_ref =K pu ×(U nom -U s_avr ), where K pu is a positive constant, representing the preset energy storage element voltage closed-loop control proportional coefficient, U nom Represents the rated charging voltage of the energy storage element, U le1 , U le2 , U le3 The voltage threshold for determining the charging mode switching of the energy storage element is set.

[0084] Step S40: performing a three-phase alternating current i of the cascaded multi-level energy storage converter. ha 、i hb and i hc The D-axis current i of the AC side of the cascaded multi-level energy storage converter is obtained by park transformation. hd and Q axis current i hq .

[0085] The park transformation is to transform i ha 、i hb and i hc The current is projected and equivalent to the rotating D-axis and Q-axis. The park transformation method is a relatively mature method in the prior art. When performing the park transformation, the conversion formula of the prior art can be referred to, which will not be described in detail in the present invention.

[0086] Step S50, i d_ref and i hd As the reference value and feedback value of the D-axis current, 0 and i hq As the reference value and feedback value of the Q axis current respectively, a closed-loop control strategy is used to control the current, and based on the output result of the control loop, the three-phase modulation voltage u of the cascaded multi-level energy storage converter is obtained. ma 、u mb and u mc .

[0087] Step S60, the three-phase modulation voltage u is performed by the modulation unit ma 、u mb and u mc Modulation, obtain the switch control signal of N sub-module units:

[0088] Step S61, calculate the state of charge SOC of the energy storage element in each submodule unit of the bridge arm of the current phase i , 1≤i≤N;

[0089] Step S62, respectively convert the three-phase modulation voltage u ma 、u mb and u mc Perform base frequency step wave modulation to obtain N step wave control signals of the A phase bridge arm, the B phase bridge arm and the C phase bridge arm respectively (that is, N step wave control signals of the A phase bridge arm, N step wave control signals of the B phase bridge arm and N step wave control signals of the C phase bridge arm are obtained here, a total of 3×N step wave control signals);

[0090] Step S63, for any phase in the three-phase bridge arm, sorting N step wave control signals from large to small according to the duty cycle of the step wave control signal;

[0091] Step S64: when the system is in a charging state, the state of charge SOC i Sorting the N submodule units from low to high, and sequentially allocating N step wave control signals sorted in descending order of duty cycle to the sorted N submodule units;

[0092] When the system is in the discharge state, according to the state of charge SOC i The N submodule units are sorted in order from high to low, and N step wave control signals sorted in order from large to small according to the duty cycle are sequentially allocated to the sorted N submodule units.

[0093] Step S70: Based on the switch control signals of the N submodule units, the submodule control unit performs drive control of the corresponding submodules to achieve charge and discharge control of the cascade multi-level energy storage system.

[0094] When the cascaded multi-level energy storage system issues a command to start charging the energy storage element, the control process of the submodule control unit includes:

[0095] The submodule control unit uses a preset support capacitor voltage target value U dc_nom is a given value to support the capacitor voltage u ci is the voltage outer loop feedback value, and performs closed-loop control of the voltage outer loop;

[0096] The submodule control unit uses the output u of the voltage loop clo As the given value of the current loop, the inductor current i Li is the feedback value of the current loop, and performs closed-loop control of the current inner loop;

[0097] Based on the output i of the current inner loop controller clo , obtain the control signal of each IGBT device of the DC-DC converter.

[0098] Those skilled in the art can clearly understand that, for the convenience and brevity of description, the specific working process and related instructions of the method described above can refer to the corresponding process in the aforementioned system embodiment, and will not be repeated here.

[0099] It should be noted that the cascaded multi-level energy storage system and control method thereof provided in the above embodiments are only illustrated by the division of the above functional modules. In practical applications, the above functions can be assigned to different functional modules as needed, that is, the modules or steps in the embodiments of the present invention can be decomposed or combined. For example, the modules in the above embodiments can be combined into one module, or further divided into multiple sub-modules to complete all or part of the functions described above. The names of the modules and steps involved in the embodiments of the present invention are only for distinguishing the modules or steps, and are not regarded as improper limitations of the present invention.

[0100] A device according to a third embodiment of the present invention includes:

[0101] at least one processor; and

[0102] a memory communicatively connected to at least one of the processors; wherein,

[0103] The memory stores instructions that can be executed by the processor, and the instructions are used to be executed by the processor to implement the above-mentioned charge and discharge control method of the cascade multi-level energy storage system.

[0104] A fourth embodiment of the present invention provides a computer-readable storage medium, wherein the computer-readable storage medium stores computer instructions, and the computer instructions are used to be executed by the computer to implement the above-mentioned charge and discharge control method of the cascade multi-level energy storage system.

[0105] Those skilled in the art can clearly understand that, for the convenience and brevity of description, the specific working process and related instructions of the storage device and processing device described above can refer to the corresponding process in the aforementioned method embodiment and will not be repeated here.

[0106] Those skilled in the art should be able to appreciate that the modules and method steps of each example described in conjunction with the embodiments disclosed herein can be implemented with electronic hardware, computer software or a combination of the two, and the programs corresponding to the software modules and method steps can be placed in random access memory (RAM), internal memory, read-only memory (ROM), electrically programmable ROM, electrically erasable programmable ROM, registers, hard disks, removable disks, CD-ROMs, or any other form of storage medium known in the technical field. In order to clearly illustrate the interchangeability of electronic hardware and software, the composition and steps of each example have been generally described in the above description according to the function. Whether these functions are performed in electronic hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered to exceed the scope of the present invention.

[0107] The terms "first", "second", etc. are used to distinguish similar objects rather than to describe or indicate a particular order or sequence.

[0108] The term "comprise" or any other similar term is intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus / device that includes a list of elements includes not only those elements but also other elements not expressly listed, or also includes elements inherent to such process, method, article, or apparatus / device.

[0109] So far, the technical solutions of the present invention have been described in conjunction with the preferred embodiments shown in the accompanying drawings. However, it is easy for those skilled in the art to understand that the protection scope of the present invention is obviously not limited to these specific embodiments. Without departing from the principle of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will fall within the protection scope of the present invention.

Claims

1. A cascaded multi-level energy storage system, characterized in that: The system includes a cascaded multi-level energy storage converter and an energy storage system controller; The cascaded multi-level energy storage converter comprises a three-phase bridge arm, each phase bridge arm comprises N cascaded submodule units, and is used for charging the energy storage elements in the submodule units through the energy storage converter when the system state is grid-connected, and for supplying power to the load through the energy storage converter when the system state is off-grid; wherein N is a positive integer; The energy storage system controller includes an AC control unit, a modulation unit, and N submodule control units distributed in the submodule units. The AC control unit sets a control instruction according to the system state. After the modulation unit converts the control instruction into a switch control signal, the submodule control unit performs drive control of the corresponding submodule. The charging and discharging control steps of the cascade multi-level energy storage system include: Step S10, obtaining the voltages of the N energy storage elements in the N submodule units, and calculating the real-time average voltage U of the energy storage elements s_avr ; Step S20, based on the real-time average voltage U of the energy storage element s_avr , set the active component of the three-phase AC current reference value of the cascaded multi-level energy storage converter to i d_ref ; Step S30, when U s_avr ≤U le1 When i d_ref =i ch , where i ch Represents the maximum allowable charging current set for the energy storage element; When U le1 s_avr le2 When i d_ref =P ch / U s_avr , where P ch Represents the maximum permissible charging power set for the energy storage element;​​ When U s_avr ≥U le2 When i d_ref =K pu ×(U nom -U s_avr ), where K pu is a positive constant, representing the preset energy storage element voltage closed-loop control proportional coefficient, U nom Represents the rated charging voltage of the energy storage element, U le1 , U le2 , U le3 A voltage threshold for determining a charging mode switch of a set energy storage element; Step S40: performing a three-phase alternating current i of the cascaded multi-level energy storage converter. ha 、i hb and i hc The D-axis current i of the AC side of the cascaded multi-level energy storage converter is obtained by park transformation. hd and Q axis current i hq ; Step S50, i d_ref and i hd As the reference value and feedback value of the D-axis current, 0 and i hq As the reference value and feedback value of the Q axis current respectively, a closed-loop control strategy is used to control the current, and based on the output result of the control loop, the three-phase modulation voltage u of the cascaded multi-level energy storage converter is obtained. ma 、u mb and u mc ; Step S60, the three-phase modulation voltage u is performed by the modulation unit ma 、u mb and u mc Modulation to obtain switch control signals of N submodule units; Step S70: Based on the switch control signals of the N submodule units, the submodule control unit performs drive control of the corresponding submodules to achieve charge and discharge control of the cascade multi-level energy storage system.

2. The cascade multi-level energy storage system according to claim 1, characterized in that: When the system state is grid-connected, the system operation mode is power control mode. The energy storage system controller calculates the AC current reference value according to the average voltage of the energy storage element, calculates the feedback value by sampling the AC side current, and realizes closed-loop control of the AC side charging and discharging power of the cascaded multi-level energy storage converter through current loop feedback control.

3. The cascade multi-level energy storage system according to claim 1, characterized in that: When the system state is off-grid, the system operation mode is voltage control mode, and the energy storage system controller adopts a voltage single-loop open-loop control strategy to control the voltage outputted on the AC side of the cascaded multi-level energy storage converter.

4. The cascade multi-level energy storage system according to claim 1, characterized in that: The submodule unit includes an AC side H-bridge, a DC-DC converter, an energy storage element and a supporting capacitor; The first input terminal and the second input terminal of the H bridge serve as the first input terminal and the second input terminal of the submodule unit; The first output terminal of the H-bridge, the first connection terminal of the support capacitor and the first input terminal of the DC-DC converter are connected together; The second output terminal of the H-bridge, the second connection terminal of the support capacitor and the second input terminal of the DC-DC converter are connected together; The first output terminal and the second output terminal of the DC-DC converter are respectively connected to the positive electrode and the negative electrode of the energy storage element.

5. The cascade multi-level energy storage system according to claim 4, characterized in that: The H-bridge includes IGBT device 1, IGBT device 2, IGBT device 3 and IGBT device 4; The source of the IGBT device 1 and the drain of the IGBT device 3 are connected together as a first input terminal of the H bridge; The source of the IGBT device 2 and the drain of the IGBT device 4 are connected together as the second input terminal of the H bridge; The drain of the IGBT device 1 and the drain of the IGBT device 2 are connected together as the first output terminal x of the H bridge; The source of the IGBT device 3 and the source of the IGBT device 4 are connected together as the second output terminal y of the H bridge.

6. The cascade multi-level energy storage system according to claim 5, characterized in that: The DC-DC converter includes an IGBT device 5, an IGBT device 6 and an energy storage filter inductor; The drain of the IGBT device 5 serves as the first input terminal of the DC-DC converter; The source of the IGBT device 6 serves as the second input terminal and the second output terminal of the DC-DC converter respectively; The source of the IGBT device 5 and the drain of the IGBT device 6 are connected together to the first connection terminal of the energy storage filter inductor; The second connection end of the energy storage filter inductor serves as the first output end of the DC-DC converter.

7. The cascade multi-level energy storage system according to claim 6, characterized in that: The submodule control unit controls the corresponding submodule unit including: The submodule control unit drives and controls the IGBT device in the submodule unit according to the switching signal of the IGBT device in the H-bridge; The submodule control unit collects the support capacitor voltage and inductor current in the corresponding submodule unit, generates a switching signal of the IGBT device in the DC-DC converter after performing double closed-loop control, and drives and controls the corresponding IGBT device.

8. The cascade multi-level energy storage system according to claim 1, characterized in that: Step S60 includes: Step S61, calculate the state of charge SOC of the energy storage element in each submodule unit of the bridge arm of the current phase i , 1≤i≤N; Step S62, respectively convert the three-phase modulation voltage u ma 、u mb and u mc Performing base frequency step wave modulation to obtain N step wave control signals of the A phase bridge arm, the B phase bridge arm and the C phase bridge arm respectively; Step S63, for any phase in the three-phase bridge arm, sorting N step wave control signals from large to small according to the duty cycle of the step wave control signal; Step S64: when the system is in a charging state, the state of charge SOC i Sorting the N submodule units from low to high, and sequentially allocating N step wave control signals sorted in descending order of duty cycle to the sorted N submodule units; When the system is in the discharge state, according to the state of charge SOC i The N submodule units are sorted in order from high to low, and N step wave control signals sorted in order from large to small according to the duty cycle are sequentially allocated to the sorted N submodule units.

9. The cascade multi-level energy storage system according to claim 7, characterized in that: When the cascaded multi-level energy storage system issues a command to start charging the energy storage element, the control process of the submodule control unit includes: The submodule control unit uses a preset support capacitor voltage target value U dc_nom is a given value to support the capacitor voltage u ci is the voltage outer loop feedback value, and performs closed-loop control of the voltage outer loop; The submodule control unit uses the output u of the voltage loop clo As the given value of the current loop, the inductor current i Li is the feedback value of the current loop, and performs closed-loop control of the current inner loop; Based on the output i of the current inner loop controller clo , obtaining control signals of each IGBT device of the DC-DC converter.

Images