Bridge arm energy balance control method and system of bridge arm multiplexing type MMC (Modular Multilevel Converter)

Abstract

The invention provides an inter-bridge-arm energy balance control method and system of a bridge arm multiplexing type MMC, and the method comprises the steps: analyzing the combination relation between the reference voltage of an AM-MMC equivalent bridge arm and the reference voltage of an actual bridge arm; an equivalent lower bridge arm voltage distribution coefficient when a phase unit operates in a lower bridge arm multiplexing mode and an equivalent upper bridge arm voltage distribution coefficient when the phase unit operates in an upper bridge arm multiplexing mode are introduced, and inter-bridge arm energy balance control actively and controllably adjusts distribution of reference waves in real time. And the deviation between the capacitor voltage and the rated value of each bridge arm sub-module is obviously reduced. According to the invention, the problem of unbalanced energy between the bridge arms of the AM-MMC is effectively solved, the method is suitable for different operation conditions, and the dynamic performance and operation stability of the AM-MMC are further improved.

Description

Technical Field

[0001] The present invention belongs to the technical field related to bridge arm multiplexing MMC control, and in particular relates to a method and system for controlling energy balance between bridge arms of a bridge arm multiplexing MMC. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] With the advantages of high output voltage quality, easy expansion and redundant fault-tolerant operation, modular multilevel converter (MMC) has been widely used in the field of flexible DC transmission. With the continuous improvement of MMC voltage level and capacity, the disadvantages of high cost, large weight and volume of MMC commutation platform are becoming increasingly prominent. Patent CN112152496A discloses an arm multiplexing MMC (AM-MMC) topology that can improve the utilization rate of submodules. By dividing the multiplexing bridge arm to time-division multiplex some submodules, AM-MMC can reduce the number of submodules assembled by at least 25% compared with MMC of the same voltage and capacity level, thereby achieving lightweight.

[0004] Different from the symmetrical electrical structure and control objectives of the upper and lower bridge arms in the traditional MMC phase unit, the three bridge arms in the AM-MMC phase unit do not have good symmetry in terms of electrical structure and control objectives. Therefore, AM-MMC may face the problem of energy imbalance between the bridge arms of the phase unit during normal operation, which will greatly affect the stable output voltage waveform of the converter, and further affect the normal power transmission function and dynamic performance of the converter. The various modulation methods that have been proposed or disclosed for AM-MMC cannot solve the energy balance problem between the bridge arms of AM-MMC well. Summary of the invention

[0005] In order to overcome the deficiencies of the above-mentioned prior art, the present invention provides a method and system for controlling energy balance between bridge arms of a bridge arm multiplexing type MMC, which effectively solves the problem of energy imbalance between bridge arms of AM-MMC and is suitable for different operating conditions, further improving the dynamic performance and operating stability of AM-MMC.

[0006] In order to achieve the above object, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a method for controlling energy balance between bridge arms of a bridge arm multiplexing MMC, comprising:

[0008] Determine the multiplexing mode of the bridge arm multiplexing type MMC phase unit;

[0009] According to the multiplexing mode, combined with the corresponding relationship between the reference voltage of each bridge arm in the phase unit and the reference voltage of the equivalent bridge arm of the bridge arm multiplexing MMC, the distribution of the reference voltage of each bridge arm in the phase unit in the corresponding multiplexing mode is independently controlled by the equivalent lower bridge arm voltage distribution coefficient when the phase unit operates in the lower bridge arm multiplexing mode or the equivalent upper bridge arm voltage distribution coefficient when the phase unit operates in the upper bridge arm multiplexing mode;

[0010] According to the deviation degree of the average value of the capacitor voltage of different bridge arm submodules of the phase unit, the equivalent upper bridge arm voltage distribution coefficient and the equivalent lower bridge arm voltage distribution coefficient are dynamically adjusted, thereby adjusting the distribution relationship of the equivalent bridge arm reference voltage in the three bridge arms, thereby achieving energy balance between the bridge arms.

[0011] In a second aspect, the present invention provides an inter-bridge arm energy balance control system of a bridge arm multiplexing MMC, comprising:

[0012] A multiplexing mode determination module, which is configured to: determine the multiplexing mode of the bridge arm multiplexing type MMC phase unit;

[0013] A distribution module is configured to: according to the multiplexing mode, in combination with the corresponding relationship between the reference voltage of each bridge arm in the phase unit and the reference voltage of the equivalent bridge arm of the bridge arm multiplexing type MMC, independently control the distribution of the reference voltage of each bridge arm in the phase unit in the corresponding multiplexing mode through the equivalent lower bridge arm voltage distribution coefficient when the phase unit operates in the lower bridge arm multiplexing mode or the equivalent upper bridge arm voltage distribution coefficient when the phase unit operates in the upper bridge arm multiplexing mode;

[0014] The energy balance control module is configured to dynamically adjust the equivalent upper bridge arm voltage distribution coefficient and the equivalent lower bridge arm voltage distribution coefficient according to the degree of deviation of the average value of the capacitor voltage of different bridge arm sub-modules of the phase unit, thereby adjusting the distribution relationship of the equivalent bridge arm reference voltage in the three bridge arms, thereby achieving energy balance between the bridge arms.

[0015] In a third aspect, the present invention provides an electronic device comprising a memory and a processor, and computer instructions stored in the memory and executed on the processor, wherein when the computer instructions are executed by the processor, the method described in the first aspect is performed.

[0016] In a fourth aspect, the present invention provides a computer-readable storage medium for storing computer instructions, wherein when the computer instructions are executed by a processor, the method described in the first aspect is performed.

[0017] One or more of the above technical solutions have the following beneficial effects:

[0018] In the present invention, by analyzing the combined relationship between the equivalent bridge arm and the actual bridge arm reference voltage of the AM-MMC, the equivalent lower bridge arm voltage distribution coefficient when the phase unit operates in the lower bridge arm reuse mode and the equivalent upper bridge arm voltage distribution coefficient when the phase unit operates in the upper bridge arm reuse mode are introduced, and the energy balance control between the bridge arms is added to actively and controlledly adjust the distribution of the reference wave in real time, which significantly reduces the deviation between the capacitor voltage and the rated value of each bridge arm submodule. The present invention effectively solves the problem of energy imbalance between the bridge arms of the AM-MMC, is applicable to different operating conditions, and further improves the dynamic performance and operating stability of the AM-MMC.

[0019] Advantages of additional aspects of the present invention will be given in part in the following description, and in part will become obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0021] Figure 1 It is the topological structure diagram of three-phase AM-MMC;

[0022] Figure 2 This is a schematic diagram of the AM-MMC energy balance control process in the first embodiment of the present invention;

[0023] Figure 3 Schematic diagram of the value of the allocation coefficient A used by the general modulation system in the first embodiment of the present invention;

[0024] Figure 4 A schematic diagram of a steady-state bridge arm reference voltage before and after the energy balance control link is added in the first embodiment of the present invention;

[0025] Figure 5 This is a schematic diagram of the average value of the capacitor voltage of the bridge arm submodule in the first embodiment of the present invention;

[0026] Figure 6 Schematic diagram of the DC component of the average value of the capacitor voltage of the bridge arm submodule in the first embodiment of the present invention. DETAILED DESCRIPTION

[0027] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0028] It should be noted that the terms used herein are for describing specific embodiments only and are not intended to be limiting of exemplary embodiments according to the present invention.

[0029] In the absence of conflict, the embodiments of the present invention and the features of the embodiments may be combined with each other.

[0030] Embodiment 1

[0031] This embodiment discloses a method for controlling energy balance between bridge arms of a bridge arm multiplexing MMC, including:

[0032] Determine the multiplexing mode of the bridge arm multiplexing type MMC phase unit;

[0033] According to the multiplexing mode, combined with the corresponding relationship between the reference voltage of each bridge arm in the phase unit and the reference voltage of the equivalent bridge arm of the bridge arm multiplexing MMC, the distribution of the reference voltage of each bridge arm in the phase unit in the corresponding multiplexing mode is independently controlled by the equivalent lower bridge arm voltage distribution coefficient when the phase unit operates in the lower bridge arm multiplexing mode or the equivalent upper bridge arm voltage distribution coefficient when the phase unit operates in the upper bridge arm multiplexing mode;

[0034] According to the degree of deviation of the average value of the capacitor voltage of different bridge arm submodules of the phase unit, the equivalent upper bridge arm voltage distribution coefficient and the equivalent lower bridge arm voltage distribution coefficient are dynamically adjusted, thereby adjusting the distribution relationship of the equivalent bridge arm reference voltage in the three bridge arms, thereby achieving energy balance among the bridge arms.

[0035] To explain this embodiment more clearly, the topological structure and basic working principle of AM-MMC are first described. Figure 1 As shown, each phase unit of the AM-MMC is composed of an upper bridge arm, a multiplexed bridge arm, a lower bridge arm, two bridge arm inductors L, and two bridge arm switching switches K1 and K2. Each bridge arm contains N / 2 series submodules, where N can be calculated by the following formula:

[0036]

[0037] Among them, U dc is the DC rated voltage, U cN is the rated voltage of the submodule capacitor, U dc , U cN All are determined by system design parameters and scale.

[0038] The design concept of AM-MMC is derived from the working characteristics of traditional MMC. Conventional MMCs always have N sub-modules in operation per phase unit. If the number of sub-modules in operation in the upper bridge arm is small, the number of sub-modules in operation in the lower bridge arm must be large. At this time, some idle sub-modules in the upper bridge arm can be divided into the lower bridge arm to participate in modulation, and the lower bridge arm can reduce the corresponding number of sub-modules. Similarly, if the number of sub-modules in operation in the upper bridge arm is large, the number of sub-modules in operation in the lower bridge arm must be small. At this time, some idle sub-modules in the lower bridge arm can be divided into the upper bridge arm to participate in modulation, and the upper bridge arm can reduce the corresponding number of sub-modules. The above-mentioned idle sub-modules are used as multiplexed bridge arms, and the multiplexed bridge arms are "time-division multiplexed" according to the number of sub-modules that need to be invested in the upper and lower bridge arms.

[0039] The following uses the A phase unit as an example to illustrate the operating principle of AM-MMC. By analogy with the upper and lower bridge arm voltages of conventional MMC, the voltage u between the positive polarity end of the upper bridge arm of AM-MMC and the AC end is epa It is called the equivalent upper bridge arm voltage, and the voltage u between the AC end and the negative polarity end of the lower bridge arm is ena It is called the equivalent lower bridge arm voltage. Ignoring the bridge arm inductance voltage drop, according to the voltage loop equation, we can get:

[0040]

[0041] From formula (2), it can be seen that AM-MMC can convert the modulation of the AC output voltage into the modulation of the equivalent upper and lower bridge arm voltages. Under the control of the bridge arm switching switch, the multiplexed bridge arm can be connected in series with the upper and lower bridge arms to form composite upper and lower bridge arms, and the AM-MMC phase unit can operate in the upper bridge arm multiplexing mode and the lower bridge arm multiplexing mode respectively. a1 Shutdown, K a2 When it is turned on, the upper bridge arm and the multiplexed bridge arm are output in series, and the phase unit operates in the upper bridge arm multiplexing mode; when K a1 Conductivity, K a2 When turned off, the multiplexing bridge arm and the lower bridge arm are output in series, and the phase unit operates in the lower bridge arm multiplexing mode.

[0042] The following is a detailed description of the general split bridge arm modulation method of AM-MMC:

[0043] In MMC, the output voltages of the upper and lower bridge arms are symmetrical. Similarly, the output voltages of the equivalent upper and lower bridge arms of AM-MMC also have corresponding symmetry. The reference voltages of the equivalent upper and lower bridge arms are and Can be expressed as

[0044]

[0045] Where m is the differential mode reference voltage modulation ratio of the converter, is the phase angle of the differential mode voltage.

[0046] The corresponding relationship between the bridge arm reference voltage and the equivalent bridge arm reference voltage when the phase unit operates in different modes can be expressed as:

[0047]

[0048] in, and They are the reference voltages of the upper bridge arm, multiplexed bridge arm and lower bridge arm in the phase unit respectively.

[0049] On the basis of satisfying equations (4) and (5), the reference voltages of the equivalent upper and lower bridge arms can be distributed to the three bridge arms according to a certain rule, and then the sub-modules can be controlled separately and independently to approach the reference voltage wave. One of the innovations of this embodiment is that a universal reference voltage distribution method is designed. This method can modify the distribution of the reference voltage by adjusting the distribution coefficient, thereby increasing the control flexibility of the system.

[0050] Assume A and C are DC voltage distribution coefficients, and B and D are AC voltage distribution coefficients, then A+C=1, B+D=1. When the phase unit operates in the lower bridge arm multiplexing mode, the expression of the bridge arm reference voltage is:

[0051]

[0052] When the phase unit operates in the upper bridge arm multiplexing mode, the expression of the bridge arm reference voltage is:

[0053]

[0054] From formula (1), we can see that each bridge arm of AM-MMC contains N / 2 submodules. During normal operation, the output voltage range of each bridge arm is [0,U dc / 2]. In order to ensure that the bridge arm reference voltage does not exceed the output voltage range, the distribution coefficients should satisfy A+B≤1, C+D≤1 and A, B, C, D are all in the range of [0,1], where the modulation ratio m is not greater than 1.

[0055] According to A+C=1, B+D=1, we can further infer that:

[0056]

[0057] Representing B, C, and D with A, the expressions of the reference voltages of the three bridge arms of the phase unit are:

[0058]

[0059] Among them, "①" and "②" represent the phase unit operating in the lower bridge arm multiplexing mode and the upper bridge arm multiplexing mode, respectively. The same applies to the following formula. It can be noted that after all the distribution coefficients are unified as variable A, the value of A not only determines the distribution of DC voltage, but also affects the distribution of AC voltage. For the sake of ease of explanation and understanding, variable A is specifically referred to as "symmetrical distribution coefficient" in the subsequent instructions.

[0060] Formulas (9) to (11) actually determine a symmetrical universal column bridge arm modulation method. Different multiplexing modes have the same distribution relationship for the equivalent bridge arm voltage. At this time, the upper and lower bridge arm reference voltages are symmetrical. In fact, the distribution relationship between the multiplexed bridge arm and the lower bridge arm for the equivalent lower bridge arm voltage can be different from the distribution relationship between the multiplexed bridge arm and the upper bridge arm for the equivalent upper bridge arm voltage. Thus, a more universal column bridge arm modulation expression is obtained, namely:

[0061]

[0062] Among them, x is the equivalent lower bridge arm voltage distribution coefficient when the phase unit operates in the lower bridge arm multiplexing mode, y is the equivalent upper bridge arm voltage distribution coefficient when the phase unit operates in the upper bridge arm multiplexing mode, and 0≤x, y≤1.

[0063] The general column-by-column bridge arm modulation method determined by equations (12) to (14) has higher control flexibility. x and y can independently control the distribution relationship of the equivalent bridge arm voltage under different multiplexing modes. When x≠y, the upper and lower bridge arm reference voltages are no longer symmetrical. When x=y, equations (9) to (11) are equivalent to equations (12) to (14).

[0064] AM-MMC contains three phases and a total of 9 bridge arms, among which the status of the phase unit is equal, and the status of the upper bridge arm and the lower bridge arm of each phase unit is also equal, but the status of the multiplexed bridge arm is unequal to that of the upper and lower bridge arms. Therefore, AM-MMC has the problem of energy balance between bridge arms. The split bridge arm modulation requires an additional energy balance control link to achieve uniform distribution of phase unit energy among the three bridge arms. Furthermore, when the parameters and status of the upper and lower bridge arms of the bridge arm multiplexing MMC are completely equal, the modulation method represented by equations (9) to (11) can achieve energy balance; on the contrary, when the parameters and status of the upper and lower bridge arms of the bridge arm multiplexing MMC are unequal, the general split bridge arm modulation method shown in equations (12) to (14) should be used to achieve energy balance.

[0065] It is more common that the parameters and status of the upper and lower bridge arms of the bridge arm multiplexing type MMC are completely equal, so the general split bridge arm modulation method determined by equations (9) to (11) is more practical and universal. It can be seen from equations (9) to (11) that the symmetrical distribution coefficient A is the only controllable variable of the proposed general split bridge arm modulation. By adjusting the distribution coefficient, an energy balance control strategy between AM-MMC bridge arms is proposed, such as Figure 2 As shown, taking the A phase unit as an example, assuming u ca is the average capacitor voltage of the A-phase submodule, and its expression is:

[0066]

[0067] Among them, u cua 、u cma and u cda are the average values ​​of the submodule capacitor voltages of the upper bridge arm, the reused bridge arm, and the lower bridge arm. Since the upper bridge arm and the lower bridge arm are equal and symmetrical, u cua and u cda The DC components of are equal. Select u ca and u cma As the tracking object and the controlled object respectively, the difference between the DC components of the two is passed through the PI controller to obtain the adjustment value ΔA of the symmetrical distribution coefficient A, and A=A0+ΔA, where A0 is the initial value of the symmetrical distribution coefficient A. In order to ensure that the bridge arm reference voltage is within the bridge arm output voltage range, the value of A still needs to be between 0 and 1. After the energy balance control between the additional bridge arms is added, the split bridge arm modulation can actively and controllably adjust the distribution coefficient adaptively.

[0068] In order to verify the effectiveness of the proposed AM-MMC bridge arm energy balance control method, a single-ended AM-MMC system was built based on the electromagnetic transient simulation platform. The specific parameters of the system are shown in Table 1. The dynamic characteristics of AM-MMC before and after the addition of the bridge arm energy balance control method were verified by simulation. AM-MMC adopted the general split bridge arm modulation method corresponding to equations (9) to (11). At the beginning of the simulation, the symmetric distribution coefficient A was fixed to 0.5. When the operation reached 4 seconds, the proposed bridge arm energy balance control method was used to dynamically adjust the symmetric distribution coefficient A.

[0069] Table 1 AM-MMC simulation parameters

[0070]

[0071] Figure 3 The value of the symmetrical distribution coefficient A used by the modulation system before and after the energy balancing system is started is shown. After the 4-second additional energy balancing link is put into use, the symmetrical distribution coefficient A is adjusted from 0.5 to about 0.41 under the control of the control system. Figure 4 By comparing the steady-state bridge arm reference voltage of the A-phase unit before and after the additional energy balancing control link, it can be seen that the reference voltage distribution results of the composite bridge arm are different under the action of different distribution coefficients; Figure 5 The average value of the capacitor voltage of the bridge arm submodule is shown. After the additional energy balancing control is started, the average value of the capacitor voltage is controlled back to near the rated voltage, and the capacitor voltage fluctuation is also reduced accordingly. Figure 6 It can be seen from the extracted DC component of the average value of the capacitor voltage that the additional energy balance control link effectively solves the problem of energy imbalance between the bridge arms, and the DC components of the capacitor voltages of the three bridge arms are adjusted to be consistent under the action of the control system.

[0072] The present invention designs a universal split-bridge-arm modulation method by analyzing the combined relationship between the equivalent bridge arm and the actual bridge arm reference voltage of the AM-MMC. Based on the proposed modulation method, the energy balance control between bridge arms is added to actively and controlledly adjust the distribution of reference waves in real time, which significantly reduces the deviation between the capacitor voltage and the rated value of each bridge arm submodule. The present invention effectively solves the problem of energy imbalance between bridge arms of AM-MMC, is applicable to different operating conditions, and further improves the dynamic performance and operating stability of AM-MMC.

[0073] Embodiment 2

[0074] The purpose of this embodiment is to provide an energy balance control system between bridge arms of a bridge arm multiplexing MMC, including:

[0075] A multiplexing mode determination module, which is configured to: determine the multiplexing mode of the bridge arm multiplexing type MMC;

[0076] A distribution module is configured to: according to the multiplexing mode, in combination with the corresponding relationship between the reference voltage of each bridge arm in the phase unit and the reference voltage of the equivalent bridge arm of the bridge arm multiplexing type MMC, independently control the distribution of the reference voltage of each bridge arm in the phase unit in the corresponding multiplexing mode through the equivalent lower bridge arm voltage distribution coefficient when the phase unit operates in the lower bridge arm multiplexing mode or the equivalent upper bridge arm voltage distribution coefficient when the phase unit operates in the upper bridge arm multiplexing mode;

[0077] The energy balance control module is configured to dynamically adjust the distribution of reference voltages of corresponding bridge arms according to the degree of deviation of the average values ​​of capacitor voltages of different bridge arm submodules to achieve energy balance between bridge arms.

[0078] In further embodiments, there is also provided:

[0079] An electronic device includes a memory and a processor, and computer instructions stored in the memory and executed on the processor, wherein when the computer instructions are executed by the processor, the method described in Embodiment 1 is performed. For the sake of brevity, no further description is given here.

[0080] It should be understood that in this embodiment, the processor may be a central processing unit CPU, and the processor may also be other general-purpose processors, digital signal processors DSP, application-specific integrated circuits ASIC, off-the-shelf programmable gate arrays FPGA or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. The general-purpose processor may be a microprocessor or the processor may also be any conventional processor, etc.

[0081] The memory may include a read-only memory and a random access memory, and provide instructions and data to the processor. A portion of the memory may also include a non-volatile random access memory. For example, the memory may also store information about the device type.

[0082] A computer-readable storage medium is used to store computer instructions. When the computer instructions are executed by a processor, the method described in embodiment 1 is completed.

[0083] The method in the first embodiment can be directly embodied as a hardware processor, or a combination of hardware and software modules in the processor. The software module can be located in a mature storage medium in the field such as a random access memory, a flash memory, a read-only memory, a programmable read-only memory, or an electrically erasable programmable memory, a register, etc. The storage medium is located in the memory, and the processor reads the information in the memory and completes the steps of the above method in combination with its hardware. To avoid repetition, it will not be described in detail here.

[0084] A computer program product includes a computer program, and when the computer program is executed by a processor, the method described in the first embodiment is implemented.

[0085] The present invention also provides at least one computer program product tangibly stored on a non-transitory computer-readable storage medium. The computer program product includes computer executable instructions, such as instructions included in a program module, which are executed in a device on a real or virtual processor of the target to perform the process / method as described above. Typically, program modules include routines, programs, libraries, objects, classes, components, data structures, etc. that perform specific tasks or implement specific abstract data types. In various embodiments, the functions of program modules can be combined or divided between program modules as needed. Machine executable instructions for program modules can be executed in local or distributed devices. In distributed devices, program modules can be located in local and remote storage media.

[0086] The computer program code for implementing the method of the present invention can be written in one or more programming languages. These computer program codes can be provided to the processor of a general-purpose computer, a special-purpose computer or other programmable data processing device, so that the program code, when executed by the computer or other programmable data processing device, causes the function / operation specified in the flow chart and / or block diagram to be implemented. The program code can be executed completely on a computer, partially on a computer, as an independent software package, partially on a computer and partially on a remote computer or completely on a remote computer or server.

[0087] In the context of the present invention, computer program codes or related data may be carried by any appropriate carrier to enable a device, apparatus or processor to perform the various processes and operations described above. Examples of carriers include signals, computer readable media, etc. Examples of signals may include electrical, optical, radio, acoustic or other forms of propagation signals, such as carrier waves, infrared signals, etc.

[0088] Those of ordinary skill in the art will appreciate that the units and algorithm steps of each example described in conjunction with this embodiment can be implemented in electronic hardware or a combination of computer software and electronic hardware. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professional and technical personnel can use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of this application.

[0089] Although the above describes the specific implementation mode of the present invention in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art on the basis of the technical solution of the present invention without creative work are still within the scope of protection of the present invention.

Claims

1. A method for controlling energy balance between bridge arms of a bridge arm multiplexing MMC, characterized in that: include: Determine the multiplexing mode of the bridge arm multiplexing type MMC phase unit; According to the multiplexing mode, combined with the corresponding relationship between the reference voltage of each bridge arm in the phase unit and the reference voltage of the equivalent bridge arm of the bridge arm multiplexing MMC, the distribution of the reference voltage of each bridge arm in the phase unit in the corresponding multiplexing mode is independently controlled by the equivalent lower bridge arm voltage distribution coefficient when the phase unit operates in the lower bridge arm multiplexing mode or the equivalent upper bridge arm voltage distribution coefficient when the phase unit operates in the upper bridge arm multiplexing mode; According to the degree of deviation of the average value of the capacitor voltage of different bridge arm submodules of the phase unit, the equivalent upper bridge arm voltage distribution coefficient and the equivalent lower bridge arm voltage distribution coefficient are dynamically adjusted, thereby adjusting the distribution relationship of the equivalent bridge arm reference voltage in the three bridge arms, thereby achieving energy balance among the bridge arms.

2. The method for controlling energy balance between bridge arms of a bridge arm multiplexing MMC according to claim 1, characterized in that: The distribution of the reference voltage of each bridge arm in the phase unit in the corresponding multiplexing mode is independently controlled by the equivalent lower bridge arm voltage distribution coefficient when the phase unit operates in the lower bridge arm multiplexing mode or the equivalent upper bridge arm voltage distribution coefficient when the phase unit operates in the upper bridge arm multiplexing mode, specifically: Among them, "①" and "②" represent the phase unit operating in the lower bridge arm multiplexing mode and the phase unit operating in the upper bridge arm multiplexing mode, respectively. dc is the DC rated voltage, m is the differential mode reference voltage modulation ratio of the converter, is the phase angle of the differential mode voltage, x is the equivalent lower arm voltage distribution coefficient when the phase unit operates in the lower arm multiplexing mode, and y is the equivalent upper arm voltage distribution coefficient when the phase unit operates in the upper arm multiplexing mode.

3. The method for controlling energy balance between bridge arms of a bridge arm multiplexing MMC according to claim 1 or 2, characterized in that: When the equivalent lower arm voltage distribution coefficient when the phase unit operates in the lower arm multiplexing mode and the equivalent upper arm voltage distribution coefficient when the phase unit operates in the upper arm multiplexing mode are not equal, the upper arm reference voltage and the lower arm reference voltage of the arm multiplexing type MMC are not symmetrical; when the equivalent lower arm voltage distribution coefficient when the phase unit operates in the lower arm multiplexing mode and the equivalent upper arm voltage distribution coefficient when the phase unit operates in the upper arm multiplexing mode are equal, the upper arm reference voltage and the lower arm reference voltage of the arm multiplexing type MMC are symmetrical.

4. The method for controlling energy balance between bridge arms of a bridge arm multiplexing MMC according to claim 1, characterized in that: When the parameters and positions of the upper and lower bridge arms of the bridge arm multiplexing MMC are completely equal, a modulation method in which the upper bridge arm reference voltage and the lower bridge arm reference voltage of the bridge arm multiplexing MMC are symmetrical is adopted, and the symmetrical distribution coefficient is adjusted to achieve energy balance between the bridge arms of the bridge arm multiplexing MMC, specifically: Calculate the average value of the phase unit submodule capacitor voltage according to the average value of the submodule capacitor voltage of the upper bridge arm, the reused bridge arm and the lower bridge arm; The average value of the capacitor voltage of the submodule of the multiplexed bridge arm and the average value of the capacitor voltage of the phase submodule of the phase unit are respectively used as the tracking object and the controlled object; The difference between the average value of the submodule capacitor voltage of the reused bridge arm and the DC component of the average value of the phase unit submodule capacitor voltage is passed through a PI controller to obtain an adjustment amount of the symmetrical distribution coefficient; The universal column-split bridge arm is modulated according to the obtained adjustment amount of the symmetrical distribution coefficient.

5. The method for controlling energy balance between bridge arms of a bridge arm multiplexing MMC according to claim 4, characterized in that: The sum of the adjustment amount of the symmetric distribution coefficient and the initial value of the symmetric distribution coefficient ranges from 0 to 1.

6. The method for controlling energy balance between bridge arms of a bridge arm multiplexing MMC according to claim 1, characterized in that: When the phase unit operates in the lower bridge arm multiplexing mode, the equivalent lower bridge arm voltage distribution coefficient is greater than or equal to 0, and when the phase unit operates in the upper bridge arm multiplexing mode, the equivalent upper bridge arm voltage distribution coefficient is less than or equal to 1.

7. The method for controlling energy balance between bridge arms of a bridge arm multiplexing MMC according to claim 1, characterized in that: The voltage between the positive polarity end and the AC end of the upper bridge arm of the bridge arm multiplexing type MMC is taken as the equivalent upper bridge arm voltage, and the voltage between the AC end and the negative polarity end of the lower bridge arm of the bridge arm multiplexing type MMC is taken as the equivalent lower bridge arm voltage.

8. An energy balance control system between bridge arms of a bridge arm multiplexing MMC, characterized in that: include: A multiplexing mode determination module, which is configured to: determine the multiplexing mode of the bridge arm multiplexing type MMC phase unit; A distribution module is configured to: according to the multiplexing mode, in combination with the corresponding relationship between the reference voltage of each bridge arm in the phase unit and the reference voltage of the equivalent bridge arm of the bridge arm multiplexing type MMC, independently control the distribution of the reference voltage of each bridge arm in the phase unit in the corresponding multiplexing mode through the equivalent lower bridge arm voltage distribution coefficient when the phase unit operates in the lower bridge arm multiplexing mode or the equivalent upper bridge arm voltage distribution coefficient when the phase unit operates in the upper bridge arm multiplexing mode; The energy balance control module is configured to dynamically adjust the equivalent upper bridge arm voltage distribution coefficient and the equivalent lower bridge arm voltage distribution coefficient according to the degree of deviation of the average value of the capacitor voltage of different bridge arm sub-modules of the phase unit, thereby adjusting the distribution relationship of the equivalent bridge arm reference voltage in the three bridge arms, thereby achieving energy balance between the bridge arms.

9. An electronic device, characterized in that: The method comprises a memory and a processor and computer instructions stored in the memory and executed on the processor, wherein when the computer instructions are executed by the processor, the method according to any one of claims 1 to 7 is completed.

10. A computer-readable storage medium, characterized in that: Used to store computer instructions, which, when executed by a processor, complete the method described in any one of claims 1 to 7.

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