Full-direct-current energy storage system with multi-level balancing capability and control method of full-direct-current energy storage system

Abstract

The invention provides a full-direct-current energy storage system with multi-level equalization capability and a control method thereof, the full-direct-current energy storage system comprises a modularized multi-level converter, an energy storage battery, a battery string power management module and an energy state equalization module, the modularized multi-level converter is connected with an alternating-current power grid; the energy storage battery comprises a plurality of battery strings connected in parallel; each battery string comprises battery clusters connected in series; an alternating-current input end provided by the battery string power management module is connected in series with the modular multi-level converter, and a direct-current input end is connected with a battery string of the energy storage battery; and the energy state balancing module is positioned in each battery string of the energy storage battery to realize energy state balancing of the battery clusters in the battery strings. According to the invention, on the battery string level, independent management and control of the parallel battery strings can be realized by designing the battery string power management module, and the problem of parallel circulating current is solved; and on the battery cluster level, inter-cluster energy state equalization in each string of batteries is realized through an energy state equalization module.

Description

Technical Field

[0001] The present invention relates to the fields of battery energy storage and power electronics technology, and in particular to a full DC energy storage system with multi-level balancing capability and a control method thereof. Background Art

[0002] With the continuous development of the power system, the importance of energy storage technology has become increasingly prominent, and it may become a key driving force for promoting innovation in power production and consumption patterns and adjusting the energy structure.

[0003] On the grid side, energy storage systems can be used for transient power quality management, quickly adjust and compensate for the voltage and power of the power system, eliminate regional oscillations caused by sudden load changes and grid interconnection, and improve the transient stability of the grid. In addition, large-capacity energy storage systems with fast response capabilities act as energy buffers, achieving the decoupling of power generation and power consumption in time and space, which not only enhances the self-regulation ability of regional power grids, but also improves the overall efficiency of power transmission and substation, helping to alleviate the problem of cross-regional supply and demand imbalances.

[0004] On the user side, with the development of science and technology, more and more new types of electrical equipment have emerged, such as high-speed railways and electric vehicles. Such loads usually have short-term high power output and unpredictability, which puts pressure on the stability of the power grid. In addition, modern electrical equipment has increasingly stringent requirements on the performance of the power grid, especially in terms of voltage and frequency stability, no surge and peak interference, and low harmonic content. Energy storage technology can not only compensate for peak currents, eliminate surge voltages, and significantly improve power quality, but also act as an independent power source in the event of a power grid failure, providing a continuous and stable power supply.

[0005] Energy storage technologies of different carriers have their own characteristics and therefore have different application scenarios. Battery Energy Storage System (BESS) has a wide range of applications due to its various types of carrier forms, power range from a few kilowatts to tens of megawatts, and charging and discharging time from a few minutes to a few hours. Compared with traditional energy storage technology, BESS has the advantages of low installation site requirements, low maintenance costs, environmental friendliness, and fast response speed. More importantly, battery technology is developing rapidly, and the focus in the future will be low-cost, high cycle life, and high power density battery energy storage technology.

[0006] Battery energy storage system (BESS) generally includes battery system, battery management system (BMS), power conversion system (PCS) and energy management system (EMS). PCS, as the interface between battery and power grid, directly determines the charging and discharging efficiency of battery and the reliability of the whole system, and is the core component of BESS. Existing high-voltage direct-mounted battery energy storage PCS is divided into two directions: one is the high-voltage battery pack energy storage solution, mainly modular multilevel converter (MMC), active neutral-point clamped (ANPC) and multilevel push-pull converter (MPC). The second is the low-voltage battery pack energy storage solution, represented by cascaded H-bridge converter (CHBC), MMC, etc.

[0007] The following problems exist for PCS in high-voltage battery energy storage solutions: 1) When a high-voltage battery system is used, multiple battery clusters need to be connected in series to form a high-voltage battery string, so the energy state balance between the battery clusters needs to be solved; 2) In order to achieve power expansion, high-voltage battery strings need to be connected in parallel on the DC side, so the circulation problem caused by parallel connection needs to be solved. Summary of the invention

[0008] In view of the defects in the prior art, an object of the present invention is to provide a full DC energy storage system with multi-level balancing capability and a control method thereof.

[0009] According to one aspect of the present invention, a full DC energy storage system with multi-level balancing capability is provided, comprising a modular multi-level converter, an energy storage battery, a battery string power management module and an energy state balancing module, wherein:

[0010] The modular multi-level converter is connected to an AC power grid;

[0011] The energy storage battery comprises a plurality of battery strings connected in parallel; each of the battery strings comprises a plurality of battery clusters connected in series;

[0012] The AC input end provided by the battery string power management module is connected in series to the modular multi-level converter, and the DC input end is connected to the battery string of the energy storage battery;

[0013] The energy state balancing module is located in each of the battery strings of the energy storage battery to achieve energy state balancing of all the battery clusters in the battery string.

[0014] Preferably, the modular multilevel converter includes a three-phase converter, each phase of the converter includes a symmetrical upper bridge arm and a lower bridge arm, and the upper bridge arm and the lower bridge arm each include a reactance and a plurality of cascaded SM sub-modules; the AC power grid is connected between the two reactances of the upper bridge arm and the lower bridge arm.

[0015] Preferably, the battery string power management module adopts a multi-port voltage source converter; two of the multi-port voltage source converters are symmetrically distributed up and down, one of the multi-port voltage source converters is connected to the upper bridge arm of the modular multi-level converter and the battery string of the energy storage battery, and the other multi-port voltage source converter is connected to the lower bridge arm of the modular multi-level converter and the battery string of the energy storage battery.

[0016] Preferably, the battery string power management module provides multiple DC ports to achieve power expansion of the energy storage battery.

[0017] Preferably, the battery string power management module controls the voltage of the ports to which each battery string in the energy storage battery is connected by adjusting its own duty cycle, thereby avoiding the circulation problem caused by direct parallel connection of the battery strings.

[0018] Preferably, the energy state balancing module comprises n switching devices IGBT, n-1 LC series branches and n voltage stabilizing capacitors, where n is the number of the battery clusters of a single battery string;

[0019] Each of the voltage-stabilizing capacitors is connected in parallel with one of the battery clusters; adjacent voltage-stabilizing capacitors are connected via the switching device IGBT and the LC series branch.

[0020] Preferably, the voltage-stabilizing capacitor and the switching device IGBT form a half-bridge sub-module, and the midpoints of adjacent half-bridge sub-modules are connected via the LC series branch.

[0021] Preferably, the energy state balancing module performs energy relocation through phase shift control to achieve energy state balancing of each battery cluster in a single battery string.

[0022] Preferably, the energy storage battery absorbs power from the AC power grid, and the energy storage battery emits power to the AC power grid; wherein the power is:

[0023] P dc =P ac

[0024]

[0025] Among them, P dc is the DC side power of the full DC energy storage system, U ok and I ok is the voltage and current of the kth battery string of the energy storage battery, P ac is the AC side power of the full DC energy storage system, U m ,I m and It is the AC phase voltage amplitude, phase current amplitude and power factor angle of the full DC energy storage system.

[0026] According to a second aspect of the present invention, there is provided a control method for a full DC energy storage system with multi-level balancing capability, comprising:

[0027] MMC control loop ensures the stability and accuracy of voltage and current of the full DC energy storage system;

[0028] The BSPRM adds a balancing control loop to adjust the amount of energy transfer between the modular multilevel converter and the battery string power management module;

[0029] The battery string output current control loop controls the output current of each battery string of the energy storage battery;

[0030] The internal energy state balance control loop of the battery string controls the state balance between the battery clusters within each battery string of the energy storage battery.

[0031] Preferably, the MMC control loop includes an MMC current inner loop and an MMC voltage outer loop;

[0032] Among them, the MMC current inner loop adopts feedforward decoupling control, and the actual control variable is:

[0033]

[0034] Among them, u diff is the differential voltage between the upper and lower bridge arms of the modular multilevel converter MMC, u s is the grid voltage, i v is the grid current, ω represents the AC grid frequency, L represents the inductance of the MMC bridge arm, subscript d represents the d-axis component, subscript q represents the q-axis component, superscript * represents the reference value, K p1 K is the proportional gain coefficient of the current loop proportional-integral controller, i1 is the integral link gain coefficient of the current loop proportional-integral controller;

[0035] Wherein, the MMC voltage outer loop is specifically:

[0036] Subtract the voltage reference value from the voltage feedback value;

[0037] The difference is input into the proportional-integral controller to obtain the d-axis current reference value of the current inner loop.

[0038] Preferably, the MMC control loop includes an MMC submodule capacitor voltage control loop, specifically:

[0039] Subtract the bridge arm SM submodule capacitor voltage feedback value from the bridge arm capacitor voltage average value;

[0040] The difference is passed through a proportional controller to obtain Δu;

[0041] Superimpose Δu on the modulation wave of the bridge arm SM submodule;

[0042] The modulated wave generates a driving signal through a carrier phase shift modulation strategy to achieve capacitor voltage balance of the SM submodule inside the bridge arm.

[0043] Preferably, the MMC control loop includes an MMC circulation suppression control loop, specifically:

[0044] The actual control variable using the proportional-integral controller is set as:

[0045]

[0046] Among them, u com is the common mode voltage of the upper and lower bridge arms of MMC, i cir is the arm circulation, ω represents the AC grid frequency, L represents the MMC arm inductance, subscript d represents the d-axis component, subscript q represents the q-axis component, superscript * represents the reference value, K p2 K is the proportional link gain coefficient of the proportional integral controller of the circulating current suppression loop, i2 It is the integral link gain coefficient of the circulating current suppression loop proportional-integral controller.

[0047] Preferably, the BSPRM has an additional balancing control loop, specifically:

[0048] For the upper bridge arm of phase a, the additional balancing control voltage is set to:

[0049] Δu ap =K ap sin(ωt+β)

[0050] Among them, K ap is the amplitude of the additional balancing control voltage, β is the phase angle of the additional balancing control voltage, ω represents the frequency of the AC power grid, and t is the time;

[0051] By adjusting the amplitude and phase angle of the additional balancing control voltage, the size of the transfer between the MMC bridge arm SM submodule and the battery string power management module BPRSM is adjusted.

[0052] Preferably, the battery string output current control loop is specifically:

[0053] The difference between the battery string output current reference value of the kth branch and the battery string output current feedback value is calculated;

[0054] The difference is passed through the proportional controller to obtain the duty cycle D of the battery string power management module BPRSM k ;

[0055] The duty cycle D k Through PWM modulation, a driving signal is obtained to achieve closed-loop control of the output current of the battery string.

[0056] Preferably, the battery string internal energy state balance control loop is specifically:

[0057] The battery cluster inside each battery string uses a series LC circuit;

[0058] Obtaining the energy status of each of the battery clusters;

[0059] Determining the balanced power required by each battery cluster according to the energy state-balanced power relationship;

[0060] According to the balancing power required by each battery cluster, the shift phase corresponding to each LC balancing branch is set to achieve simultaneous balancing of all the battery clusters.

[0061] Compared with the prior art, the embodiments of the present invention have at least one of the following beneficial effects:

[0062] The full DC energy storage system with multi-level balancing capability in the embodiment of the present invention, the energy state balancing module, is connected to all battery clusters inside the battery string, and uses phase shift control to achieve energy state balance between each battery cluster, thereby solving the "barrel effect" of series-connected batteries.

[0063] The full DC energy storage system with multi-level balancing capability implemented in the present invention, the battery string power management module, provides multiple DC ports, realizes the power expansion of the battery system, and can control the port voltage connected to each battery string by adjusting the duty cycle, solves the circulation problem caused by the direct parallel connection of the battery strings, realizes the independent control of each battery string, is beneficial to improve the efficiency of the battery system and realize the decoupled operation within the battery system. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0065] Figure 1A system schematic diagram of a full DC energy storage system with multi-level balancing capability provided by an embodiment of the present invention.

[0066] Figure 2 A schematic diagram showing an example of typical topology of internal modules of a full DC energy storage system with multi-level balancing capability provided by an embodiment of the present invention.

[0067] Figure 3 A schematic diagram of an application scenario of a full DC energy storage system with multi-level balancing capability provided by an embodiment of the present invention.

[0068] Figure 4 A control block diagram of a full DC energy storage system with multi-level balancing capability provided by an embodiment of the present invention.

[0069] Figure 5 A system schematic diagram of a full DC energy storage system with multi-level balancing capability provided for an embodiment of the present invention, in which the MMC adopts a half-bridge submodule, the BPRSM adopts a multi-DC port two-level converter, and the battery system adopts three groups of battery strings and an energy state balancing circuit.

[0070] Figure 6 Current waveform diagrams of each group of battery strings under four working conditions based on a full DC energy storage system with multi-level balancing capability provided by one embodiment of the present invention.

[0071] Figure 7 A current waveform diagram of four battery clusters within a battery string #1 of a full DC energy storage system with multi-level balancing capability provided by an embodiment of the present invention.

[0072] Figure 8 A voltage waveform diagram of a BPRSM module of a full DC energy storage system with multi-level balancing capability provided by an embodiment of the present invention.

[0073] Fig. 9 A capacitor voltage waveform diagram of an MMC submodule of a full DC energy storage system with multi-level balancing capability provided by an embodiment of the present invention.

[0074] Fig.10 A waveform diagram of the AC side current of a full DC energy storage system with multi-level balancing capability provided by one embodiment of the present invention. DETAILED DESCRIPTION

[0075] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several variations and improvements may be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0076] like Figure 1 As shown, in one embodiment of the present invention, a full DC energy storage system with multi-level balancing capability is provided, including a modular multilevel converter, an energy storage battery, a battery string power management module and an energy state balancing module, wherein: the modular multilevel converter is connected to an AC power grid; the energy storage battery includes a plurality of parallel battery strings; each battery string includes a battery cluster connected in series; the AC input terminal provided by the battery string power management module is connected in series to the modular multilevel converter, and the DC input terminal is connected to the battery string of the energy storage battery; the energy state balancing module is located in each battery string of the energy storage battery to achieve energy state balancing of the battery cluster in the battery string.

[0077] In the above embodiment, at the battery string level, the battery string power management module is designed to achieve independent control of parallel battery strings and solve the parallel circulation problem; at the battery cluster level, the energy state balance module is used to achieve inter-cluster energy state balance within each battery string.

[0078] In a preferred embodiment, the modular multilevel converter includes a three-phase converter, each phase converter includes a symmetrical upper bridge arm and a lower bridge arm, and the upper bridge arm and the lower bridge arm each include an inductor and a plurality of cascaded SM sub-modules; the AC power grid is connected between the two inductors of the upper bridge arm and the lower bridge arm.

[0079] In a preferred embodiment of the present invention, the battery string power management module adopts a multi-port voltage source converter; two multi-port voltage source converters are symmetrically distributed and connected to the modular multi-level converter and the energy storage battery.

[0080] Furthermore, in a preferred embodiment, the battery string power management module provides multiple DC ports to achieve power expansion of the energy storage battery; by adjusting the duty cycle to control the port voltage connected to each battery string in the energy storage battery, the commutation problem caused by direct parallel connection of the battery strings is avoided.

[0081] In order to better achieve energy balance of the battery cluster, in a preferred embodiment of the present invention, Figure 2 As shown, the energy state balancing module includes n switching devices IGBT, n-1 LC series branches and n voltage stabilizing capacitors, where n is the number of battery clusters in each battery string;

[0082] Each voltage stabilizing capacitor is connected in parallel with a battery cluster; adjacent voltage stabilizing capacitors are connected through switching devices IGBT and LC series branches to achieve energy relocation. The voltage stabilizing capacitor and the switching device IGBT form a half-bridge submodule, and the midpoints of adjacent half-bridge submodules are connected through LC series branches.

[0083] The energy state balancing module realizes the energy state balancing of each battery cluster by performing energy relocation through phase shift control. Phase shift control is a control method in power electronics that controls the transmitted power by generating a phase difference between a corresponding set of switch tubes.

[0084] like Figure 3 As shown, the energy storage battery absorbs power from the AC grid, and the energy storage battery sends power to the AC grid. In order to meet the energy balance at the system level, in a preferred embodiment, the multi-AC port power electronic transformer satisfies the following power equation:

[0085] P dc =P ac

[0086]

[0087] Among them, P dc is the DC side power of the system, U ok and I ok is the voltage and current of the kth battery string of the energy storage battery, P ac is the system AC side power, U m ,I m and is the system AC phase voltage amplitude, phase current amplitude and power factor angle.

[0088] Based on the same inventive concept, in other embodiments of the present invention, a control method for a full DC energy storage system with multi-level balancing capability is provided, comprising:

[0089] MMC control loop ensures the stability and accuracy of voltage and current of the full DC energy storage system;

[0090] The BSPRM adds a balancing control loop to adjust the amount of energy transfer between the modular multilevel converter and the battery string power management module;

[0091] The battery string output current control loop controls the output current of each battery string of the energy storage battery;

[0092] The internal energy state balance control loop of the battery string controls the state balance between the battery clusters within each battery string of the energy storage battery.

[0093] In a preferred embodiment, Figure 4 As shown, the MMC control loop includes:

[0094] MMC voltage and current control loop, in which the current inner loop adopts feedforward decoupling control, and the actual control variables are set as follows:

[0095]

[0096] Among them, udiff is the differential voltage of the upper and lower bridge arms of MMC, u s is the grid voltage, i v is the grid current, ω represents the AC grid frequency, L represents the inductance of the MMC bridge arm, subscript d represents the d-axis component, subscript q represents the q-axis component, superscript * represents the reference value, K p1 K is the proportional gain coefficient of the current loop proportional-integral controller, i1 It is the integral link gain coefficient of the current loop proportional-integral controller.

[0097] The voltage outer loop outputs the d-axis current reference value of the current inner loop after the voltage reference value is subtracted from the voltage feedback value through a proportional-integral controller.

[0098] The MMC submodule capacitor voltage control loop obtains Δu by subtracting the bridge arm submodule capacitor voltage feedback value from the bridge arm capacitor voltage average value through a proportional controller, which is then superimposed on the modulation wave of the bridge arm submodule. Finally, a driving signal is generated through a carrier phase shift modulation strategy to achieve capacitor voltage balance of the submodules inside the bridge arm.

[0099] The MMC circulation suppression control loop uses a proportional-integral controller, and the actual control variables are set as follows:

[0100]

[0101] Among them, u com is the common mode voltage of the upper and lower bridge arms of MMC, i cir is the arm circulation, ω represents the AC grid frequency, L represents the MMC arm inductance, subscript d represents the d-axis component, subscript q represents the q-axis component, superscript * represents the reference value, K p2 K is the proportional link gain coefficient of the proportional integral controller of the circulating current suppression loop, i2 It is the integral link gain coefficient of the circulating current suppression loop proportional-integral controller.

[0102] In a preferred embodiment, the BSPRM has an additional balancing control loop, specifically:

[0103] Taking the upper bridge arm of phase a as an example, the additional balancing control voltage is set as follows:

[0104] Δu ap =K ap sin(ωt+β)

[0105] Among them, K ap is the amplitude of the additional balancing control voltage, and β is the phase angle of the additional balancing control voltage. By adjusting the amplitude and phase angle of the additional balancing control voltage, the energy transfer between the MMC bridge arm SM submodule and the BPRSM module can be adjusted.

[0106] In a preferred embodiment, the battery string output current control loop is specifically:

[0107] The duty cycle D of the BPRSM module is obtained by subtracting the reference value of the battery string output current of the kth branch from the feedback value of the battery string output current through the proportional controller. k , through PWM modulation, the driving signal is obtained to achieve closed-loop control of the battery string output current.

[0108] In a preferred embodiment, the energy state balance control loop within the battery string is specifically:

[0109] The battery cluster balancing adopts a series LC circuit. After the battery management system monitors the battery status to obtain the energy status of the battery cluster, the balancing power required for each battery cluster is determined according to the energy status-balancing power (PB) relationship, thereby setting the corresponding shift ratio of each LC balancing branch to achieve the purpose of balancing all battery clusters at the same time.

[0110] In order to verify the feasibility and effect of the all-DC energy storage system with multi-level balancing capability and the control method thereof in the above-mentioned embodiment, the present invention provides a specific embodiment, referring to Figure 5 As shown, a high-voltage direct-mounted battery energy storage power conversion system based on MMC is used to connect to the AC power grid; in this embodiment, the high-voltage direct-mounted battery energy storage power conversion system based on MMC is composed of a modular multi-level converter, two battery string power management modules and a battery module including three groups of battery strings and an energy state balancing module (SOC balancing circuit). Independent control of the battery string can be achieved by adjusting the duty cycle differential mode component of the battery power management module, and energy state balancing between clusters can be achieved by adjusting the shift ratio of the balancing circuit within the battery string. The specific control strategy can be referred to Figure 4 The following is a further explanation in conjunction with a specific simulation example.

[0111] In combination with the above implementation cases, MATLAB / Simulink software is used to simulate and verify the full DC energy storage system with multi-level balancing capability according to the embodiment of the present invention. The simulation parameters are shown in Table 1.

[0112] Table 1: Simulation parameters of high-voltage direct-mounted battery energy storage power conversion system based on MMC

[0113]

[0114]

[0115] The MMC-based high-voltage direct-mounted battery energy storage power conversion system connects three battery strings. The connection diagram is as follows: Figure 5 , the control method of the simulation example is referenced Figure 4The MMC-based high-voltage direct-mounted battery energy storage power conversion system consists of a modular multi-level converter, two battery string power management modules, and a battery module containing three battery strings and an energy state balancing circuit. The AC side grid-connected current is controlled by the MMC current inner loop, the DC side bus voltage is controlled by the MMC voltage outer loop, and the battery string port voltage is controlled by the BPRSM module, thereby controlling the differential mode current of each battery string. The energy state balance between each battery cluster in the string is controlled by the energy state balancing module.

[0116] In order to verify the control capability of the MMC-based high-voltage direct-mounted battery energy storage power conversion system, four operating conditions were set in the simulation.

[0117] Working condition 1: The battery system is in the charging state, the absorbed power of battery string #1 is 0.67MW, the absorbed power of battery string #2 is 0.67MW, and the absorbed power of battery string #3 is 0.67MW. The balanced power of battery cluster 1 in battery string #1 is 25kW, the balanced power of battery cluster 2 is -25kW, and battery clusters 3 and 4 do not perform energy state balancing.

[0118] Working condition 2: The battery system is in the charging state, the absorbed power of battery string #1 is 0.9MW, the absorbed power of battery string #2 is 0.7MW, and the absorbed power of battery string #3 is 0.4MW. The balanced power of battery cluster 1 in battery string #1 is 25kW, the balanced power of battery cluster 4 is -25kW, and battery cluster 2 and battery cluster 3 are not balanced in energy state.

[0119] Working condition 3: The battery system is in the discharge state, the output power of battery string #1 is 0.67MW, the output power of battery string #2 is 0.67MW, and the output power of battery string #3 is 0.67MW. The balanced power of battery cluster 1 in battery string #1 is -5kW, the balanced power of battery cluster 2 is 25kW, the balanced power of battery cluster 32 is -10kW, and the balanced power of battery cluster 4 is -10kW.

[0120] Condition 4: The battery system is in a discharging state, the output power of battery string #1 is 1MW, the absorbed power of battery string #2 is 0.5MW, and the absorbed power of battery string #3 is 0.5MW. The battery clusters within battery string #1 are not energy-balanced.

[0121] Figure 6 The current waveforms of each battery string under four working conditions are shown in Figure 2. Figure 7 The current waveform of the four battery clusters in battery string #1. Figure 8 It is the voltage waveform of the BPRSM module. Fig. 9 It is the capacitor voltage waveform of the MMC submodule. Fig.10 is the AC side current waveform.

[0122] The simulation waveform results show that the battery string power management module of the MMC-based high-voltage direct-mounted battery energy storage power conversion system can provide multiple DC ports to achieve power expansion of the battery system, and can control the port voltage connected to each battery string by adjusting the duty cycle, solve the circulation problem caused by direct parallel connection of battery strings, and realize independent control of each battery string; the energy state balancing module can be connected to the battery cluster inside the battery string, and use phase shift control to achieve energy state balance between each battery cluster, thereby solving the "barrel effect" of series-connected batteries.

[0123] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various modifications or variations within the scope of the claims, which does not affect the essence of the present invention. The above preferred features can be used in any combination without conflicting with each other.

Claims

1. A full DC energy storage system with multi-level balancing capability, characterized in that: It includes a modular multi-level converter, an energy storage battery, a battery string power management module and an energy state balancing module, wherein: The modular multi-level converter is connected to an AC power grid; The energy storage battery comprises a plurality of battery strings connected in parallel; each of the battery strings comprises a plurality of battery clusters connected in series; The AC input end provided by the battery string power management module is connected in series to the modular multi-level converter, and the DC input end is connected to the battery string of the energy storage battery; The energy state balancing module is located in each of the battery strings of the energy storage battery to achieve energy state balancing of all the battery clusters in the battery string.

2. A full DC energy storage system with multi-level balancing capability according to claim 1, characterized in that: The modular multilevel converter comprises a three-phase converter, each phase of the converter comprises a symmetrical upper bridge arm and a lower bridge arm, the upper bridge arm and the lower bridge arm each comprise a reactance and a plurality of cascaded SM submodules; the AC power grid is connected between the two reactances of the upper bridge arm and the lower bridge arm.

3. A full DC energy storage system with multi-level balancing capability according to claim 2, characterized in that: The battery string power management module adopts a multi-port voltage source converter; two multi-port voltage source converters are symmetrically distributed up and down, one multi-port voltage source converter is connected to the upper bridge arm of the modular multi-level converter and the battery string of the energy storage battery, and the other multi-port voltage source converter is connected to the lower bridge arm of the modular multi-level converter and the battery string of the energy storage battery.

4. A full DC energy storage system with multi-level balancing capability according to claim 3, characterized in that: The battery string power management module provides multiple DC ports to achieve power expansion for the energy storage battery.

5. A full DC energy storage system with multi-level balancing capability according to claim 4, characterized in that: The battery string power management module controls the voltage of the ports to which the battery strings are connected in the energy storage battery by adjusting its own duty cycle, thereby avoiding the circulation problem caused by the direct parallel connection of the battery strings.

6. A full DC energy storage system with multi-level balancing capability according to claim 3, characterized in that: The energy state balancing module includes n switching devices IGBT, n-1 LC series branches and n voltage stabilizing capacitors, where n is the number of battery clusters in a single battery string; Each of the voltage-stabilizing capacitors is connected in parallel with one of the battery clusters; adjacent voltage-stabilizing capacitors are connected via the switching device IGBT and the LC series branch.

7. A full DC energy storage system with multi-level balancing capability according to claim 6, characterized in that: The voltage stabilizing capacitor and the switching device IGBT form a half-bridge sub-module, and the midpoints of adjacent half-bridge sub-modules are connected through the LC series branch.

8. A full DC energy storage system with multi-level balancing capability according to claim 7, characterized in that: The energy state balancing module performs energy relocation through phase shift control to achieve energy state balancing of each battery cluster in a single battery string.

9. A full DC energy storage system with multi-level balancing capability according to claim 1, characterized in that: The energy storage battery absorbs power from the AC power grid, and the energy storage battery sends power to the AC power grid; wherein the power is: P dc =P ac Among them, P dc is the DC side power of the full DC energy storage system, U ok and I ok is the voltage and current of the kth battery string of the energy storage battery, P ac is the AC side power of the full DC energy storage system, U m ,I m and It is the AC phase voltage amplitude, phase current amplitude and power factor angle of the full DC energy storage system.

10. A control method for a full DC energy storage system with multi-level balancing capability, characterized in that: include: MMC control loop ensures the stability and accuracy of voltage and current of the full DC energy storage system; The BSPRM adds a balancing control loop to adjust the amount of energy transfer between the modular multilevel converter and the battery string power management module; The battery string output current control loop controls the output current of each battery string of the energy storage battery; The internal energy state balance control loop of the battery string controls the state balance between the battery clusters within each battery string of the energy storage battery.

11. The control method of a full DC energy storage system with multi-level balancing capability according to claim 10, characterized in that: The MMC control loop includes an MMC current inner loop and an MMC voltage outer loop; Among them, the MMC current inner loop adopts feedforward decoupling control, and the actual control variable is: Among them, u diff is the differential voltage between the upper and lower bridge arms of the modular multilevel converter MMC, u s is the grid voltage, i v is the grid current, ω represents the AC grid frequency, L represents the inductance of the MMC bridge arm, subscript d represents the d-axis component, subscript q represents the q-axis component, superscript * represents the reference value, K p1 K is the proportional gain coefficient of the current loop proportional-integral controller, i1 is the integral link gain coefficient of the current loop proportional-integral controller; Wherein, the MMC voltage outer loop is specifically: Subtract the voltage reference value from the voltage feedback value; The difference is input into the proportional-integral controller to obtain the d-axis current reference value of the current inner loop.

12. The control method of a full DC energy storage system with multi-level balancing capability according to claim 10, characterized in that: The MMC control loop includes an MMC submodule capacitor voltage control loop, specifically: Subtract the bridge arm SM submodule capacitor voltage feedback value from the bridge arm capacitor voltage average value; The difference is passed through a proportional controller to obtain Δu; Superimpose Δu on the modulation wave of the bridge arm SM submodule; The modulated wave generates a driving signal through a carrier phase shift modulation strategy to achieve capacitor voltage balance of the SM submodule inside the bridge arm.

13. The control method of a full DC energy storage system with multi-level balancing capability according to claim 10, characterized in that: The MMC control loop includes an MMC circulation suppression control loop, specifically: The actual control variable using the proportional-integral controller is set as: Among them, u com is the common mode voltage of the upper and lower bridge arms of MMC, i cir is the arm circulation, ω represents the AC grid frequency, L represents the MMC arm inductance, subscript d represents the d-axis component, subscript q represents the q-axis component, superscript * represents the reference value, K p2 K is the proportional link gain coefficient of the proportional integral controller of the circulating current suppression loop, i2 It is the integral link gain coefficient of the circulating current suppression loop proportional-integral controller.

14. The control method of a full DC energy storage system with multi-level balancing capability according to claim 10, characterized in that: The BSPRM adds a balance control loop, specifically: For the upper bridge arm of phase a, the additional balancing control voltage is set to: Thu ap =K ap sin(ωt+β) Among them, K ap is the amplitude of the additional balancing control voltage, β is the phase angle of the additional balancing control voltage, ω represents the frequency of the AC power grid, and t is the time; By adjusting the amplitude and phase angle of the additional balancing control voltage, the size of the transfer between the MMC bridge arm SM submodule and the battery string power management module BPRSM is adjusted.

15. The control method of a full DC energy storage system with multi-level balancing capability according to claim 10, characterized in that: The battery string output current control loop is specifically: The difference between the battery string output current reference value of the kth branch and the battery string output current feedback value is calculated; The difference is passed through the proportional controller to obtain the duty cycle D of the battery string power management module BPRSM k ; The duty cycle D k Through PWM modulation, a driving signal is obtained to achieve closed-loop control of the output current of the battery string.

16. The control method of a full DC energy storage system with multi-level balancing capability according to claim 10, characterized in that: The battery string internal energy state balance control loop is specifically: The battery cluster inside each battery string uses a series LC circuit; Obtaining the energy status of each of the battery clusters; Determining the balanced power required by each battery cluster according to the energy state-balanced power relationship; According to the balancing power required by each battery cluster, the shift phase corresponding to each LC balancing branch is set to achieve simultaneous balancing of all the battery clusters.

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