A non-isolated energy storage modular multilevel converter and a control method thereof

Abstract

This invention discloses a non-isolated modular multilevel converter for energy storage and its control method, relating to the field of medium-voltage direct transmission of energy storage. The converter includes a bridge arm composed of multiple cascaded basic module units and a three-winding coupled inductor array. The basic module unit includes a half-bridge and capacitor unit and an additional half-bridge unit. The three-winding coupled inductor array interconnects the basic module units at corresponding positions in the three-phase horizontal direction and connects them to a battery pack. This invention eliminates the need for complex circulating current suppression strategies; it only requires basic voltage and current dual closed-loop control to naturally eliminate low-frequency voltage fluctuations in submodule capacitors and second-harmonic circulating currents in the bridge arm. This significantly reduces the size of the submodule capacitors, increases the system power density, reduces switching device losses and current stress, simplifies the control system, and improves dynamic response speed and fault safety.

Description

Technical Field

[0001] This invention relates to the field of medium-voltage direct transmission of energy storage, and specifically to a non-isolated modular multilevel converter for energy storage and its control method. Background Technology

[0002] As the global energy structure accelerates its transition to renewable energy, the large-scale grid connection of intermittent and fluctuating power sources such as wind and solar power poses a severe challenge to the stable operation of the power system and power quality. Energy storage systems, as a key technology for mitigating fluctuations in renewable energy output, enhancing the grid's peak-shaving and valley-filling capabilities, and ensuring power supply reliability, are becoming increasingly important. However, traditional centralized energy storage management models have inherent limitations in terms of capacity scalability, redundancy and fault tolerance, energy conversion efficiency, and system flexibility, making it difficult to meet the actual needs of large-scale, high-reliability energy storage applications.

[0003] Modular multilevel converters (MMCs), with their modular design, high voltage withstand ratings, low harmonic output, strong scalability, and excellent redundancy, have become the mainstream topology in the field of high-voltage, high-capacity power electronic conversion. Integrating MMC technology with energy storage systems to construct distributed energy storage MMC systems can effectively overcome the drawbacks of traditional centralized management. By embedding energy storage units (such as battery packs) into various sub-modules of the MMC, not only is the connection structure between the energy storage system and the converter simplified, reducing the difficulty of charge-discharge equalization and thermal management risks in high-power centralized energy storage, but the system's redundancy and fault ride-through capability are also significantly enhanced. Furthermore, the distributed architecture supports fine-grained energy scheduling based on sub-module states, optimizing the system's dynamic response characteristics and overall operating efficiency, providing an important technical path for building a highly reliable and efficient next-generation energy storage system.

[0004] Despite the significant advantages of distributed energy storage MMC, suppressing voltage fluctuations in its submodule capacitors remains a key technical challenge limiting system performance improvement. Existing research mainly focuses on the following types of solutions and their limitations: 1. Voltage ripple can be suppressed by adjusting the power distribution of the bridge arms through AC-side common-mode voltage injection or phase unit circulating current injection. However, such methods often lead to an increase in the effective value of the bridge arm current, increasing device losses and easily causing distortion of the DC-side output waveform, thus affecting power quality.

[0005] 2. A power transfer path between the upper and lower bridge arms is constructed using a flying capacitor to achieve dynamic power balance between the bridge arms. Although this solution can significantly reduce the voltage ripple amplitude of the submodule capacitors, it introduces additional passive components, increasing the system size, weight, and cost.

[0006] Therefore, there is an urgent need to develop a new energy storage MMC topology and control method with a simpler topology, which can effectively suppress capacitor voltage fluctuations without complex additional control and has high integration and non-isolation characteristics, in order to solve the problems of complex control, high cost and limited waveform quality in the existing technology. Summary of the Invention

[0007] To address the aforementioned issues, this invention discloses a non-isolated modular multilevel converter for energy storage, comprising: a modular multilevel converter body composed of multiple bridge arms, each phase of which includes an upper bridge arm and a lower bridge arm; Multiple basic module units are cascaded to form the bridge arm; the basic module unit consists of a half-bridge and capacitor unit and an additional half-bridge unit. A three-winding coupled inductor array, which serves as a high-frequency link, is used to interconnect the three basic module units at corresponding positions in the transverse three phases; The battery pack, together with the three-winding coupled inductor array and the basic module unit of the transverse three phases, constitutes a combined module unit; and a common busbar is connected to the end of the bridge arm to access the medium-voltage DC side. Each phase of the upper bridge arm includes The cascaded combined module units and the upper bridge arm inductor; Each phase of the lower bridge arm includes The cascaded combined module units and the lower bridge arm inductor.

[0008] The basic module unit includes a first power switch transistor. Second power switching transistor Third power switching transistor Fourth power switching transistor and the first capacitor ; The three-winding coupled inductor array T includes phase A coupled inductor. B-phase coupled inductor Coupled inductor with phase C ; The first power switch The collector and the third power switch The collector electrical connection; Second power switch The emitter and the fourth power switch The emitter is electrically connected; The first power switch The emitter and the second power switch The collectors are electrically connected to form the first midpoint; The third power switch The emitter and the fourth power switch The collectors are electrically connected to form a second midpoint; The first capacitor Connected in parallel to the first power switch The collector and the second power switch Between the emitters; The A-phase coupled inductor The same-named terminal is electrically connected to the second midpoint of the A-phase basic module unit at the same horizontal position, and its opposite-named terminal is electrically connected to the positive terminal of the battery pack of the combined module unit at the same horizontal position. The B-phase coupled inductor The same-named terminal is electrically connected to the second midpoint of the B-phase basic module unit at the same horizontal position, and its opposite-named terminal is electrically connected to the positive terminal of the battery pack of the combined module unit at the same horizontal position. C-phase coupled inductor The same-named terminal is electrically connected to the second midpoint of the C-phase basic module unit at the same horizontal position, and its opposite-named terminal is electrically connected to the positive terminal of the battery pack of the combined module unit at the same horizontal position. The negative terminal of the battery pack is electrically connected to the fourth power switch in one of the three basic module units located laterally at the same position. The emitter; The three-winding coupled inductor array T is coupled to the A-phase coupled inductor. B-phase coupled inductor Coupled inductor with phase C Achieve high-frequency interconnection of combined module units of horizontal ABC three-phase bridge arms.

[0009] In each phase of the upper bridge arm, the first midpoint of the first combined module unit is electrically connected to the positive terminal of the medium-voltage DC bus; The second power switch of the combined module unit described above The emitter is electrically connected to the first midpoint of the next-level combined module unit; In each phase of the upper bridge arm, the second power switch of the last combined module unit The emitters are connected to phases A, B, and C of the medium-voltage AC bus via corresponding upper bridge arm inductors; In each phase of the lower bridge arm, the first midpoint of the first combined module unit is respectively connected through the corresponding lower bridge arm inductor. , , Electrically connected to phases A, B, and C of the medium-voltage AC busbar; Inside each phase of the lower bridge arm, the second power switch of the upper-level combined module unit... The emitter is electrically connected to the first midpoint of the next-level combined module unit; In each phase of the lower bridge arm, the second power switch of the last combined module unit The emitter is electrically connected to the negative terminal of the medium-voltage DC bus.

[0010] The A-phase, B-phase, and C-phase coupled inductors of the three-winding coupled inductor array have the same number of turns and coupling coefficient, and are connected in the same way with the same terminals, so that the resultant magnetic flux in the core is zero under the action of the symmetrical components of the three-phase current.

[0011] The control method for the non-isolated energy storage modular multilevel converter includes the following steps: Step S1: Execute the first control strategy on the half-bridge and capacitor unit; Step S2: Execute the second control strategy on the additional half-bridge unit; The first control strategy employs a dual closed-loop control of voltage and current in the abc coordinate system, including an outer loop control of the output DC voltage and an inner loop control of the AC current, and uses carrier phase-shift pulse width modulation to generate the drive signal. The second control strategy utilizes the circuit formed by the additional half-bridge unit and the three-winding coupled inductor, and employs current loop pulse width modulation control.

[0012] Step S1 specifically includes: S11: Calculate the DC voltage setpoint With actual DC voltage The deviation is input to the first PI regulator, which outputs an AC current amplitude command. ; S12: The AC current amplitude command With input voltage Phase obtained via phase-locked loop (PLL) Multiplication generates the instantaneous value of the inner loop current. ; S13: Calculate the instantaneous value of the inner current loop given. With actual alternating current The deviation is input to the second PI regulator to obtain the modulation voltage command. ; S14: Command the modulation voltage Carrier phase-shift pulse width modulation (CPWM) is performed to generate drive signals for the half-bridge and capacitor units. ; Step S2 specifically includes: S21: Calculate the battery pack current setpoint With actual battery current The deviation is input to the third PI regulator to obtain the modulation voltage command. ; S22: Command the modulation voltage Pulse width modulation processing is performed to generate the drive signal for the additional half-bridge unit. .

[0013] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention achieves high-frequency interconnection of horizontal three-phase basic module units through a unique three-winding coupled inductor array topology. This allows the modular multilevel converter to achieve stable power transmission and voltage level regulation by simply performing basic AC / DC conversion voltage and current dual closed-loop control at the MMC level. It eliminates the need for a bridge arm 2-fold frequency harmonic circulating current suppression strategy, thus enabling both output power and voltage level regulation and saving system detection and control resources.

[0014] 2. Utilizing the magnetic flux coupling characteristics of the A-phase, B-phase, and C-phase coupled inductors in the three-winding coupled inductor array, the fundamental frequency component and the second harmonic ripple power component in the three-phase bridge arms cancel each other out at the coupled inductors. This naturally eliminates the voltage ripple power of the submodule capacitors, and the capacitors only need to absorb the sub-voltage harmonics of the switching frequency during operation, significantly reducing the size of the capacitors in the submodules and thus improving the system power density.

[0015] 3. Because the three-winding coupled inductor array naturally blocks the path of the second-harmonic circulating current, this invention eliminates the voltage fluctuation of the submodule capacitors and also completely removes the second-harmonic circulating current in the bridge arms. This reduces the switching and conduction losses of the switching devices, lowers the current stress requirements on the switching devices, and facilitates further optimization of the system design.

[0016] 4. On the one hand, since the control strategy of this invention does not require complex real-time detection and decoupling calculation processes, the control loop delay is extremely low; on the other hand, due to the significant reduction in the capacitance value of the first capacitor, the electrostatic energy stored inside the basic module unit is significantly reduced. Because no complex detection and calculation processes are required, the reduced capacitance in the system lowers the system's energy storage, improves the system's response speed, and simultaneously mitigates the harm caused by capacitor discharge due to short circuits in the factor modules, which leads to an increase in fault current. Attached Figure Description

[0017] Figure 1 This is an electrical schematic diagram of the non-isolated energy storage modular multilevel converter in the embodiments of this application; Figure 2 This is an electrical schematic diagram of the basic module unit in the embodiments of this application; Figure 3 This is an electrical schematic diagram of the combined module unit in the embodiments of this application; Figure 4 This is a schematic diagram of the AC component of the input current in an embodiment of this application; Figure 5 This is a schematic diagram of the control method for the non-isolated modular multilevel converter for energy storage in this application embodiment; In the diagram, MVAC represents the medium-voltage AC bus, MVDC represents the medium-voltage DC bus, and U... MVDC This refers to the medium-voltage DC bus voltage; SM n +HB n For module units; Q i_1 Q i_2 S i_3 S i_4 These are the first, second, third, and fourth power switches of the module unit; C i L is the first capacitor of the module unit; g1 L g3 L g5 For the upper bridge arm inductor, L g2 L g4 L g6 For the lower bridge arm inductor; i ua i da These are the currents of the upper and lower bridge arms of phase A, i ub i db These are the currents of the upper and lower arms of phase B, i. uc i dc These represent the currents of the upper and lower bridge arms of phase C, respectively; T is the three-winding coupled inductor array, and L... i_a L i_b L i_c These are the coupling inductors for phases A, B, and C, respectively; u ci u cj The first capacitor C of the i-th module unit is respectively i The capacitor voltage, the first capacitor C of the j-th module unit j The capacitor voltage; SM i +HB i SM j +HB j For the i-th and j-th module units, the high-frequency link module of the full bridge; i af1 i bf1 i cf1 These are the fundamental frequency components of phases A, B, and C of the bridge arm, i. af2 i bf2 i cf2 These are the second harmonic components of phases A, B, and C of the bridge arm, respectively; u a u b u c These are the phase voltages of the three-phase AC input voltage; i a i b ic These are the phase currents of the three-phase AC input current, respectively; ω t For the phase-locked loop output phase; jω0C i The first capacitor C i Equivalent admittance; n is the number of module combinations; u x This refers to the three-phase AC input voltage value; i x This represents the actual input current value; i x * ω is the given value for the inner current loop; t This is the output phase of the phase-locked loop; V is the output value of the outer loop PI regulator. m The modulated wave is a half-bridge and capacitor structure (SM); V m The modulated wave is a half-bridge and capacitor structure (SM); V b For the modulated wave of the additional half-bridge structure (HB); u dp The upper arm modulation signal of the half-bridge and capacitor structure (SM) is u. dn Modulate the signal for the lower bridge arm submodule of the half-bridge and capacitor structure (SM); u bp For the upper arm modulation signal of the additional half-bridge structure (HB), u bn The modulation signal for the lower bridge arm submodule of the additional half-bridge structure (HB); S sm The driving signal for the half-bridge and capacitor structure; i bat * The setpoint for the battery pack current; i bat This represents the actual battery current; v b For the additional half-bridge (HB) structure modulation voltage command; S hb is the drive signal for the additional half-bridge structure (HB); f is the switching frequency; D is the open-loop duty cycle. Detailed Implementation

[0018] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that the component names, connection relationships and technical features involved in the following embodiments are strictly as described in the claims.

[0019] Example 1: As Figure 1-3 As shown, this embodiment provides a non-isolated modular multilevel converter for energy storage. The converter mainly includes a modular multilevel converter body, multiple basic module units, a three-winding coupled inductor array, a battery pack, and a common bus.

[0020] The modular multilevel converter is composed of multiple bridge arms, each phase of which includes an upper bridge arm and a lower bridge arm. In this embodiment, taking a three-phase system as an example, it includes three bridge arms: phase A, phase B, and phase C.

[0021] The multiple basic module units are cascaded to form the bridge arm. Specifically, the basic module unit consists of a half-bridge and a capacitor unit, and an additional half-bridge unit. Figure 2 As shown, the basic module unit includes a first power switch transistor. Second power switching transistor Third power switching transistor Fourth power switching transistor and the first capacitor .

[0022] The specific connection relationships are as follows: The first power switch The collector and the third power switch The collector electrical connection; The second power switch transistor The emitter and the fourth power switch The emitter is electrically connected; The first power switch The emitter and the second power switch The collectors are electrically connected to form the first midpoint; The third power switch The emitter and the fourth power switch The collectors are electrically connected to form a second midpoint; The first capacitor Connected in parallel to the first power switch The collector and the second power switch Between the emitters.

[0023] The three-winding coupled inductor array T serves as a high-frequency link, used to interconnect the three basic module units at corresponding positions in the transverse three-phase configuration. For example... Figure 3 As shown, the three-winding coupled inductor array includes an A-phase coupled inductor. B-phase coupled inductor Coupled inductor with phase C .

[0024] The specific interconnection method is as follows: The A-phase coupled inductor The same-named terminal is electrically connected to the second midpoint of the A-phase basic module unit at the same transverse position, i.e. emitter and The collector connection point, whose opposite-named end is electrically connected to the positive terminal of the battery pack of the combined module unit at the same transverse position; The B-phase coupled inductor The same-named terminal is electrically connected to the second midpoint of the B-phase basic module unit at the same horizontal position, and its opposite-named terminal is electrically connected to the positive terminal of the battery pack of the combined module unit at the same horizontal position. C-phase coupled inductor The same-named terminal is electrically connected to the second midpoint of the C-phase basic module unit at the same lateral position, and its opposite-named terminal is electrically connected to the positive terminal of the battery pack of the combined module unit at the same lateral position.

[0025] Furthermore, the negative terminal of the battery pack is electrically connected to the fourth power switch in one of the three basic module units located laterally at the same position. The emitter. Through the above connection, the three-winding coupled inductor array is connected via the A-phase coupled inductor. B-phase coupled inductor Coupled inductor with phase C High-frequency interconnection of the combined module units of the horizontal ABC three-phase bridge arms was achieved. The battery pack, the three-winding coupled inductor array, and the horizontal three-phase basic module units together constitute the combined module units.

[0026] The specific cascaded structure of the bridge arm: Each phase of the upper bridge arm includes The cascaded combined module units and the upper bridge arm inductor.

[0027] Each phase of the lower bridge arm includes The cascaded combined module units and the lower bridge arm inductor.

[0028] The specific electrical connection path is as follows, referring to claim 3 and... Figure 1 : In each phase of the upper bridge arm, the first midpoint of the first combined module unit is electrically connected to the positive terminal of the medium-voltage DC bus, i.e., the common bus.

[0029] The second power switch of the combined module unit described above The emitter is electrically connected to the first midpoint of the next-level combined module unit.

[0030] In each phase of the upper bridge arm, the second power switch of the last combined module unit The emitters are respectively connected to the corresponding upper bridge arm inductors. It is electrically connected to phases A, B, and C of the medium-voltage AC bus.

[0031] In each phase of the lower bridge arm, the first midpoint of the first combined module unit is respectively connected through the corresponding lower bridge arm inductor. Electrically connected to phases A, B, and C of the medium-voltage AC bus.

[0032] Inside each phase of the lower bridge arm, the second power switch of the upper-level combined module unit... The emitter is electrically connected to the first midpoint of the next-level combined module unit.

[0033] In each phase of the lower bridge arm, the second power switch of the last combined module unit The emitter is electrically connected to the negative terminal of the medium-voltage DC bus, i.e., the common bus.

[0034] Preferably, the A-phase, B-phase, and C-phase coupled inductors of the three-winding coupled inductor array have the same number of turns and coupling coefficient, and are connected in a consistent manner with corresponding terminals. This design ensures that the resultant magnetic flux in the core is zero under the action of the symmetrical components of the three-phase current. Figure 4 As shown, the input current contains DC and AC components. The AC component mainly includes the fundamental frequency component and the second harmonic component. Fundamental frequency component Exhibiting three-phase positive-sequence symmetry, it can cancel out at the three-winding coupled inductance; 2nd harmonic component. It exhibits three-phase negative sequence symmetry, which can also be canceled at the three-winding coupled inductance, thereby effectively reducing the core size and loss.

[0035] Example 2: This example provides a control method for the above-mentioned non-isolated energy storage modular multilevel converter. For example... Figure 5 As shown, the control method includes the following steps: Step S1: Execute the first control strategy on the half-bridge and capacitor unit.

[0036] The first control strategy employs dual closed-loop control of voltage and current in the abc coordinate system. Specifically, it includes: 1. Calculate the DC voltage setpoint. With actual DC voltage The deviation is input to the first PI regulator, which outputs an AC current amplitude command. .

[0037] 2. The AC current amplitude command With input voltage Phase obtained via phase-locked loop (PLL) Multiplication generates the instantaneous value of the inner loop current. Subsequently, the instantaneous value of the inner current loop is calculated. With actual alternating current The deviation is input to the second PI regulator to obtain the modulation voltage command. .

[0038] 3. The modulation voltage command Carrier phase-shift pulse width modulation (CPS-SPWM) processing is performed to generate the drive signals for the half-bridge and capacitor units. During this process, the modulation signal of the upper bridge arm submodule is opposite to that of the lower bridge arm submodule, and they are collectively referred to as the modulation wave. .

[0039] Step S2: Execute the second control strategy on the additional half-bridge unit.

[0040] The second control strategy utilizes a circuit formed by the additional half-bridge unit and the three-winding coupled inductor, which is essentially a Buck / Boost circuit, employing current loop pulse width modulation control. Specifically, it includes: 1. Calculate the current setpoint of the battery pack. With actual battery current The deviation is input to the third PI regulator to obtain the modulation voltage command. .

[0041] 2. The modulation voltage command Pulse width modulation processing is performed to generate the drive signal for the additional half-bridge unit. Similarly, the upper arm modulates the signal. Modulation signal with lower bridge arm Conversely, they are collectively referred to as modulated waves. .

[0042] Through the above control strategy, the non-isolated modular multilevel converter for energy storage implemented in this embodiment can independently control the DC side voltage and AC side current, while achieving precise management of battery pack charging and discharging, thus improving the system's response speed and stability.

[0043] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A non-isolated energy storage modular multilevel converter, characterized in that, include: The modular multilevel converter body is composed of multiple bridge arms, each phase of which includes an upper bridge arm and a lower bridge arm; Multiple basic modular units are cascaded together to form the bridge arm; The basic module unit consists of a half-bridge and capacitor unit and an additional half-bridge unit; A three-winding coupled inductor array, which serves as a high-frequency link, is used to interconnect the three basic module units at corresponding positions in the transverse three phases; The battery pack, together with the three-winding coupled inductor array and the lateral three-phase basic module unit, constitutes a combined module unit; and A common busbar is connected to the end of the bridge arm to access the medium-voltage DC side; Each phase of the upper bridge arm includes The cascaded combined module units and the upper bridge arm inductor; Each phase of the lower bridge arm includes The cascaded combined module units and the lower bridge arm inductor.

2. The non-isolated energy storage modular multilevel converter according to claim 1, characterized in that, The basic module unit includes a first power switch ( ), second power switch ( ), third power switch ( ), fourth power switch ( ) and the first capacitor ( The three-winding coupled inductor array (T) includes an A-phase coupled inductor (); B-phase coupled inductor ( ) and C-phase coupled inductor ( The first power switch ( The collector of the third power switch () The collector electrical connection of ); The second power switch ( The emitter of the fourth power switch () The emitter is electrically connected; The first power switch ( The emitter of ) and the second power switch ( The collectors of the electrodes are electrically connected to form the first midpoint; The third power switch ( The emitter of the fourth power switch () The collectors of the electrodes are electrically connected to form a second midpoint; The first capacitor ( ) connected in parallel to the first power switch ( The collector of the second power switch () and the second power switch () Between the emitters of ) The A-phase coupling inductor ( The same-named terminal of the A-phase basic module unit is electrically connected to the second midpoint of the A-phase basic module unit at the same horizontal position, and its opposite-named terminal is electrically connected to the positive terminal of the battery pack of the combined module unit at the same horizontal position. The B-phase coupling inductor ( The same-named terminal of the B-phase basic module unit is electrically connected to the second midpoint of the B-phase basic module unit at the same horizontal position, and its opposite-named terminal is electrically connected to the positive terminal of the battery pack of the combined module unit at the same horizontal position. The C-phase coupled inductor ( The same-named terminal of the C-phase basic module unit is electrically connected to the second midpoint of the C-phase basic module unit at the same horizontal position, and its opposite-named terminal is electrically connected to the positive terminal of the battery pack of the combined module unit at the same horizontal position. The negative terminal of the battery pack is electrically connected to the fourth power switch in one of the three basic module units located laterally at the same position. The emitter of ) The three-winding coupled inductor array (T) is connected to the A-phase coupled inductor (T). B-phase coupled inductor ( ) and C-phase coupled inductor ( This enables high-frequency interconnection of combined module units of the horizontal ABC three-phase bridge arms.

3. The non-isolated energy storage modular multilevel converter according to claim 1, characterized in that, In each phase of the upper bridge arm, the first midpoint of the first combined module unit is electrically connected to the positive terminal of the medium-voltage DC bus; The second power switch of the combined module unit described above ( The emitter of the ) is electrically connected to the first midpoint of the next-level combined module unit; In each phase of the upper bridge arm, the second power switch of the last combined module unit ( The emitters of the components are connected to phases A, B, and C of the medium-voltage AC bus via corresponding upper bridge arm inductors. In each phase of the lower bridge arm, the first midpoint of the first combined module unit is respectively connected through the corresponding lower bridge arm inductor ( , , Electrically connected to phases A, B, and C of the medium-voltage AC busbar; Inside each phase of the lower bridge arm, the second power switch of the upper-level combined module unit ( The emitter of the ) is electrically connected to the first midpoint of the next-level combined module unit; In each phase of the lower bridge arm, the second power switch of the last combined module unit ( The emitter of the ) is electrically connected to the negative terminal of the medium-voltage DC bus.

4. The non-isolated energy storage modular multilevel converter according to any one of claims 1 to 3, characterized in that: The A-phase, B-phase, and C-phase coupled inductors of the three-winding coupled inductor array have the same number of turns and coupling coefficient, and are connected in the same way with the same terminals, so that the resultant magnetic flux in the core is zero under the action of the symmetrical components of the three-phase current.

5. The non-isolated energy storage modular multilevel converter according to claim 4, characterized in that, The control method for the non-isolated energy storage modular multilevel converter includes the following steps: Step S1: Execute the first control strategy on the half-bridge and capacitor units; Step S2: Execute the second control strategy on the additional half-bridge unit; The first control strategy employs a dual closed-loop control of voltage and current in the abc coordinate system, including an outer loop control of the output DC voltage and an inner loop control of the AC current, and uses carrier phase-shift pulse width modulation to generate the drive signal. The second control strategy utilizes the circuit formed by the additional half-bridge unit and the three-winding coupled inductor, and employs current loop pulse width modulation control.

6. The control method according to claim 5, characterized in that, Step S1 specifically includes: S11: Calculate the DC voltage setpoint With actual DC voltage The deviation is input to the first PI regulator, which outputs an AC current amplitude command. ; S12: The AC current amplitude command With input voltage Phase obtained via phase-locked loop (PLL) Multiplication generates the instantaneous value of the inner loop current. ; S13: Calculate the instantaneous value of the inner current loop given. With actual alternating current The deviation is input to the second PI regulator to obtain the modulation voltage command. ; S14: Command the modulation voltage Carrier phase-shift pulse width modulation (CPWM) is performed to generate drive signals for the half-bridge and capacitor units. ; Step S2 specifically includes: S21: Calculate the battery pack current setpoint With actual battery current The deviation is input to the third PI regulator to obtain the modulation voltage command. ; S22: Command the modulation voltage Pulse width modulation processing is performed to generate the drive signal for the additional half-bridge unit. .

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