A control method and system for a nine-bridge-arm modular multilevel converter

Abstract

The application provides a nine-bridge-arm modular multilevel converter control method and system, wherein according to a direct current side current reference value, a compensation amount of each phase circulating current direct current component, a compensation component of each bridge-arm current, an upper alternating current port and a lower alternating current port phase current reference value, and a distribution coefficient of the upper alternating current port and the lower alternating current port phase current among bridge arms, the current reference value of each bridge arm is obtained, and the switch of a sub-module is controlled based on the current reference value and the modulation signal reference value of the sub-module in the bridge arm; the method can simultaneously realize direct current side current, alternating current side current and internal circulating current control, does not need additional circulating current suppression control, and the control method is simpler; and through the setting of the distribution coefficient, the upper alternating current port phase current and the lower alternating current port phase current can be flexibly distributed among the upper bridge arm, the middle bridge arm and the lower bridge arm in the same phase, and the control freedom degree of the operation performance improvement of the converter is increased.

Description

Technical Field

[0001] This invention belongs to the field of power electronics technology, and in particular relates to a control method and system for a nine-bridge-arm modular multilevel converter. Background Technology

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

[0003] like Figure 1 As shown, the nine-bridge modular multilevel converter, a compact dual-AC port modular multilevel converter, is integrated from two back-to-back modular multilevel converters. Compared to two back-to-back modular multilevel converters, the nine-bridge modular multilevel converter reduces the number of bridge arms by 25% and the number of bridge arm inductors by 50%, offering significant cost and size advantages. Simultaneously, the nine-bridge modular multilevel converter retains the advantages of traditional modular multilevel converters, such as flexible voltage and power level expansion, excellent AC output waveform, low power loss, and strong fault ride-through and fault-tolerant operation capabilities. Currently, the nine-bridge modular multilevel converter is receiving widespread attention in applications with two sets of AC ports, such as dual three-phase or six-phase induction motor drives, unified power flow controllers, flexible multi-state switches, and railway power flow controllers.

[0004] According to the inventors, existing control methods for nine-arm modular multilevel converters are based on upper and lower AC port current control, supplemented by submodule capacitor voltage balancing control and circulating current suppression control. Because upper and lower AC port current control treats the in-phase upper, middle, and lower arms as a whole, it ignores the distribution relationship of the phase currents at the upper and lower AC ports among the in-phase upper, middle, and lower arms. Therefore, upper and lower AC port current control cannot control the distribution of phase currents at the upper and lower AC ports among the in-phase upper, middle, and lower arms. Since the distribution relationship of phase currents at the upper and lower AC ports among the in-phase upper, middle, and lower arms is crucial to the operating performance of the nine-arm modular multilevel converter, upper and lower AC port current control cannot simultaneously improve the operating performance of the nine-arm modular multilevel converter. Furthermore, the upper and lower AC port phase currents cannot reflect the common-mode component (circulating current) information of the upper, middle, and lower arm currents. Therefore, upper and lower AC port current control alone cannot control the internal circulating current, requiring separate circulating current suppression control design. In summary, existing control methods for nine-arm modular multilevel converters cannot simultaneously improve the operational performance of the nine-arm modular multilevel converter, and require additional design of circulating current suppression control, making the control strategy relatively complex. Summary of the Invention

[0005] To overcome the shortcomings of the prior art, this invention provides a control method and system for a nine-arm modular multilevel converter, which can simultaneously realize the control of DC-side current, AC-side current and internal circulating current without the need for additional circulating current suppression control, making the control method simpler. Moreover, by setting the allocation coefficient, the phase current of the upper AC port and the phase current of the lower AC port can be flexibly allocated among the upper, middle and lower phase arms of the same phase, increasing the control freedom for improving the converter's operating performance.

[0006] To achieve the above objectives, a first aspect of the present invention provides a control method for a nine-arm modular multilevel converter, wherein the nine-arm modular multilevel converter includes an upper arm, a middle arm, and a lower arm, an upper AC port is provided between the upper arm and the middle arm, and a lower AC port is provided between the middle arm and the lower arm, comprising:

[0007] Based on the DC side current reference value, the compensation amount of the DC component of the circulating current in each phase, the current compensation component of each bridge arm, the phase current reference values ​​of the upper AC port and the lower AC port, and the distribution coefficient of the phase current of the upper AC port and the lower AC port among the bridge arms, the current reference value of each bridge arm is obtained.

[0008] Based on the current reference value of each bridge arm and the corresponding current measurement value of each bridge arm, the modulation signal reference value of each bridge arm submodule is obtained.

[0009] Based on the reference value of the submodule modulation signal, a multi-carrier modulation strategy is used to control the switching actions in each submodule.

[0010] A second aspect of the present invention provides a nine-arm modular multilevel converter control system, the nine-arm modular multilevel converter including an upper arm, a middle arm, and a lower arm, having an upper AC port between the upper arm and the middle arm, and a lower AC port between the middle arm and the lower arm, comprising:

[0011] Bridge arm current reference value calculation module: Based on the DC side current reference value, the compensation amount of the DC component of the circulating current in each phase, the current compensation component of each bridge arm, the phase current reference values ​​of the upper AC port and the lower AC port, and the distribution coefficient of the phase current of the upper AC port and the lower AC port among the bridge arms, the current reference value of each bridge arm is obtained.

[0012] The modulation signal reference value calculation module of the sub-module: Based on the current reference value of each bridge arm and the corresponding current measurement value of each bridge arm, the modulation signal reference value of the sub-module in each bridge arm is obtained.

[0013] Submodule control module: Based on the reference value of the submodule modulation signal, it controls the switching actions in each submodule through a multi-carrier modulation strategy.

[0014] A third aspect of the present invention provides a computer device, comprising: a processor, a memory, and a bus, wherein the memory stores machine-readable instructions executable by the processor, and when the computer device is running, the processor communicates with the memory via the bus, and when the machine-readable instructions are executed by the processor, a nine-arm modular multilevel converter control method is performed.

[0015] A fourth aspect of the present invention provides a computer-readable storage medium storing a computer program that, when executed by a processor, performs a nine-arm modular multilevel converter control method.

[0016] The above one or more technical solutions have the following beneficial effects:

[0017] In this invention, the current reference value of each bridge arm is obtained based on the DC-side current reference value, the compensation amount of the DC component of the circulating current in each phase, the current compensation component of each bridge arm, the reference values ​​of the phase currents at the upper and lower AC ports, and the distribution coefficients of the phase currents at the upper and lower AC ports among the bridge arms. The switching of the sub-modules is controlled based on the reference values ​​of each current and the modulation signal reference values ​​of the sub-modules within the bridge arm. The method of this invention can simultaneously realize the control of DC-side current, AC-side current, and internal circulating current without the need for additional circulating current suppression control, making the control method simpler. Moreover, by setting the distribution coefficients, the phase currents at the upper and lower AC ports can be flexibly distributed among the upper, middle, and lower bridge arms in the same phase, increasing the control freedom for improving the converter's operating performance.

[0018] Advantages of additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0019] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0020] Figure 1 A schematic diagram of the topology of an existing nine-bridge-arm modular multilevel converter;

[0021] Figure 2 This is a block diagram of the upper and lower AC port power control in Embodiment 1 of the present invention;

[0022] Figure 3 This is a block diagram of the overall sub-module capacitor voltage balance control in Embodiment 1 of the present invention;

[0023] Figure 4This is a block diagram of the capacitor voltage balance control of the phase unit submodule in Embodiment 1 of the present invention;

[0024] Figure 5 This is a block diagram of the inter-arm submodule capacitor voltage balance control in Embodiment 1 of the present invention;

[0025] Figure 6 This is a block diagram of the capacitor voltage balance control of the sub-module within the bridge arm in Embodiment 1 of the present invention;

[0026] Figure 7 This is a block diagram of the upper, middle, and lower bridge arm current tracking control in Embodiment 1 of the present invention;

[0027] Figure 8 This is a waveform diagram of the phase voltage and current at the upper AC output port in Embodiment 1 of the present invention;

[0028] Figure 9 This is a waveform diagram of the phase voltage and current at the lower AC output port in Embodiment 1 of the present invention;

[0029] Figure 10 The diagram shows the current waveforms of the upper, middle, and lower arms of phase A in Embodiment 1 of the present invention.

[0030] Figure 11 The waveform diagram shows the capacitor voltages of the upper, middle, and lower bridge arm submodules of phase A in Embodiment 1 of the present invention. Detailed Implementation

[0031] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0032] It should be noted that the terminology used herein is for the purpose of describing particular implementations only and is not intended to limit the exemplary implementations of the present invention.

[0033] Where there is no conflict, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0034] Example 1

[0035] This embodiment discloses a control method for a nine-arm modular multilevel converter. The nine-arm modular multilevel converter includes an upper arm, a middle arm, and a lower arm. An upper AC port is provided between the upper arm and the middle arm, and a lower AC port is provided between the middle arm and the lower arm.

[0036] Based on the DC side current reference value, the compensation amount of the DC component of the circulating current in each phase, the current compensation component of each bridge arm, the phase current reference values ​​of the upper AC port and the lower AC port, and the distribution coefficient of the phase current of the upper AC port and the lower AC port among the bridge arms, the current reference value of each bridge arm is obtained.

[0037] Based on the current reference value of each bridge arm and the corresponding current measurement value of each bridge arm, the basic modulation signal reference value of each bridge arm submodule is obtained. Then, it is added to the compensation component of the modulation signal of each bridge arm submodule to obtain the final modulation signal reference value of each bridge arm submodule.

[0038] Based on the reference value of the submodule modulation signal, a multi-carrier modulation strategy is used to control the switching actions in each submodule.

[0039] like Figure 2 As shown, the power control unit at the upper and lower AC ports uses reference values ​​(P) of the active and reactive power at the upper and lower AC ports. 1_ref Q 1_ref P 2_ref Q 2_ref The difference between the measured values ​​(P1, Q1, P2, Q2) and the corresponding values ​​is calculated, and the error is sent to the PI controller to obtain the reference values ​​(i) of the active and reactive components of the phase current at the upper and lower AC ports. d1_ref i q1_ref i d2_ref i q2_ref After coordinate transformation, the reference values ​​(i) of the phase currents at the upper and lower AC ports are obtained. j1_ref i j2_ref (j = a, b, c).

[0040] like Figure 3 As shown, the submodule capacitor voltage overall balance control unit uses the reference value (U) of the submodule capacitor voltage to balance the voltage. c_ref ) and the average value of the capacitor voltages of all submodules of the converter (U cavg_all The difference is calculated, and the error is sent to the PI controller to obtain the reference value (I) of the DC side current. dc_ref ).

[0041] like Figure 4 As shown, the submodule capacitor voltage phase unit balance control unit uses the reference value (U) of the submodule capacitor voltage to... c_ref ) and the average value of the capacitor voltages of all sub-modules in phase j (U cavg_j The difference is calculated, and the error is fed into the PI controller to obtain the compensation amount (ΔI) for the DC component of the j-phase circulating current. cir_dc_j ).

[0042] like Figure 5As shown, the submodule capacitor voltage bridge arm balancing control unit balances the average value (u) of the submodule capacitor voltage in the upper bridge arm and the middle bridge arm respectively. cavg_Uj ) and (u cavg_Mj The average value of the capacitor voltage of the submodule in the middle bridge arm and the lower bridge arm (u) cavg_Mj ) and (u cavg_Lj The difference is calculated, and the error is sent to the PI controller to obtain the amplitude (ΔI) of the current compensation components of the upper and lower bridge arms, respectively. _Uj ) and (ΔI _Lj Then, the amplitude ΔI of the upper bridge arm current compensation component is... _Uj The phase voltage u at the normalized upper AC port j1 Multiplying them yields the compensation component (Δi) of the upper arm current. arm_Uj The amplitude ΔI of the lower bridge arm current compensation component will be increased. j_L Multiplying this by the normalized phase voltage uj2 at the lower AC port yields the compensation component (Δi) of the lower arm current. arm_Lj ), will Δi arm_Uj and Δi arm_Lj After inverting and adding the components, the compensation component (Δi) of the middle bridge arm current is obtained. arm_Mj The phase voltage of the upper and lower AC ports is determined by the AC system or load to which the upper and lower AC ports are connected.

[0043] like Figure 6 As shown, the submodule capacitor voltage balancing control unit within the bridge arm uses the reference value (U) of the i-th submodule capacitor voltage. c_ref ) and measured value (u c_xj_i The difference between x = U, M, L; j = a, b, c; i = 1-k) is calculated, and the error is fed into the PI controller to obtain the compensation component (u) of the modulation signal reference value of the i-th submodule. xj_i_comp Then multiply by the direction sign of the bridge arm current to obtain the final compensation component (Δu) of the modulation signal reference value of the i-th submodule. _xj_i Where U, M, and L represent the upper arm, middle arm, and lower arm, respectively; k represents the number of submodules in the arm; if the arm current is positive, the direction sign of the arm current is 1; if the arm current is positive or negative, the direction sign of the arm current is -1.

[0044] like Figure 7 As shown, the bridge arm current tracking control unit tracks the reference values ​​(i) of the upper, middle, and lower bridge arm currents. arm_Uj_ref i arm_Mj_ref i arm_Lj_ref ) and the corresponding measured value (i arm_Uj i arm_Mj i arm_LjThe difference is calculated, and the error is sent to the PIR controller to obtain the basic modulation signal reference value (u) of the i-th submodule in the upper, middle, and lower in-phase bridge arms. _Uj_i_ref u _Mj_i_ref u _Lj_i_ref ).

[0045] Among them, the reference values ​​of the upper, middle and lower bridge arm currents are based on the reference values ​​of the upper and lower AC port phase currents (i) generated by the power control units of the upper and lower AC ports. j1_ref i j2_ref The distribution coefficients (λ, μ) of the phase currents at the upper and lower AC ports between the bridge arms, and the reference value (I) of the DC-side current generated by the submodule capacitor voltage balance control unit. dc_ref The compensation amount (ΔI) of the DC component of the j-phase circulating current cir_dc_j ), compensation amount of upper, middle and lower bridge arm currents (Δi) arm_Uj , Δi arm_Mj , Δi arm_Lj ) was calculated.

[0046] The allocation coefficient can be flexibly set according to the control objectives, such as reducing operating losses, reducing circulating current, and reducing voltage fluctuations in the submodule block capacitors.

[0047] Specifically, the reference values ​​for the currents of the upper, middle, and lower bridge arms can be expressed as follows:

[0048]

[0049]

[0050]

[0051] Among them, i arm_Uj_ref This is a reference value for the upper bridge arm current; I dc_ref This is the reference value for the DC-side current; ΔI cir_dc_j is the compensation amount for the DC component of the j-phase circulating current; (λ, μ) are the distribution coefficients of the phase currents at the upper and lower AC ports between the in-phase bridge arms; i j1_ref i is the reference value for the phase current at the upper AC port; j2_ref Δi is the reference value for the phase current at the lower AC port; arm_Uj This is the compensation component of the upper bridge arm current; i arm_Mj_ref This is a reference value for the current in the middle bridge arm; Δi arm_Mj This is the compensation component of the intermediate bridge arm current; i arm_Lj_ref This is the reference value for the lower bridge arm current; Δi arm _ Lj This is the compensation component for the lower bridge arm current.

[0052] The submodule modulation signal reference value calculation unit calculates the basic modulation signal reference value (u) of the i-th submodule in the upper, middle, and lower arms generated by the arm current tracking control unit. _Uj_i_ref u _Mj_i_ref u _Lj_i_ref The compensation component (Δu) of the reference value of the i-th submodule modulation signal generated by the submodule capacitor voltage balance control unit within the bridge arm. _xj_i Adding these together, we obtain the final modulation signal reference value (u) for the i-th submodule. _Uj_i u _Mj_i u _Lj_i ), which is sent into the modulation stage as a reference wave.

[0053] The modulation reference wave of the i-th submodule in the upper, middle, and lower arms of phase j can be represented as:

[0054] u Uj_i =u Uj_i_ref +Δu Uj_i (4)

[0055] u Mj_i =u Mj_i_ref +Δu Mj_i (5)

[0056] u Lj_i =u Lj_i_ref +Δu Lj_i (6)

[0057] In the formula, u _Uj_i The modulation reference wave for the i-th submodule in the j-phase upper arm; u _Uj_i_ref Let Δu be the basic modulation signal reference value for the i-th submodule in the j-phase upper arm; _Uj_i The compensation component is the reference value of the modulation signal of the i-th submodule in the j-phase upper arm; u _Mj_i The modulation reference wave for the i-th submodule in the j-phase intermediate bridge arm; u _Mj_i_ref Let Δu be the basic modulation signal reference value for the i-th submodule in the j-phase intermediate bridge arm; _Mj_i The compensation component is the reference value of the modulation signal of the i-th submodule in the j-phase intermediate bridge arm; u _Lj_i The modulation reference wave for the i-th submodule in the j-phase lower arm; u _Lj_i_ref Let Δu be the basic modulation signal reference value for the i-th submodule in the j-phase lower arm; _Lj_i The compensation component is the modulation signal reference value of the i-th submodule in the j-phase lower bridge arm. After obtaining the modulation reference waves of each submodule in the upper, middle and lower bridge arms, a multi-carrier modulation strategy is adopted to determine the trigger pulse of the semiconductor switch in each submodule.

[0058] Figure 8 , Figure 9 , Figure 10 and Figure 11 The waveforms of the phase voltage and current at the upper and lower AC output ports, as well as the currents of the upper, middle, and lower bridge arms of phase A and the voltage of the submodule capacitors, are displayed respectively.

[0059] The control method proposed in this embodiment can simultaneously control DC-side current, AC-side current, and internal circulating current without requiring separate circulating current suppression control. This simplifies the control method, and by setting different allocation coefficients, it can easily achieve flexible distribution of phase currents at the upper and lower AC ports in the same phase upper, middle, and lower bridge arms, increasing the control freedom for improving converter performance. Furthermore, it can integrate submodule capacitor voltage balance control at different levels to ensure that the submodule capacitor voltage remains stable near the reference value.

[0060] Example 2

[0061] The purpose of this embodiment is to provide a nine-arm modular multilevel converter control system. The nine-arm modular multilevel converter includes an upper arm, a middle arm, and a lower arm. An upper AC port is provided between the upper arm and the middle arm, and a lower AC port is provided between the middle arm and the lower arm.

[0062] Bridge arm current reference value calculation module: Based on the DC side current reference value, the compensation amount of the DC component of the circulating current in each phase, the current compensation component of each bridge arm, the phase current reference values ​​of the upper AC port and the lower AC port, and the distribution coefficient of the phase current of the upper AC port and the lower AC port among the bridge arms, the current reference value of each bridge arm is obtained.

[0063] The modulation signal reference value calculation module of the sub-module: Based on the current reference value of each bridge arm and the corresponding current measurement value of each bridge arm, the modulation signal reference value of the sub-module in each bridge arm is obtained.

[0064] Submodule control module: Based on the reference value of the submodule modulation signal, it controls the switching actions in each submodule through a multi-carrier modulation strategy.

[0065] Example 3

[0066] The purpose of this embodiment is to provide a computing device, including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the program to implement the steps of the above-described method.

[0067] Example 4

[0068] The purpose of this embodiment is to provide a computer-readable storage medium.

[0069] A computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, performs the steps of the above method.

[0070] The steps and methods involved in the apparatuses of Embodiments 2, 3, and 4 above correspond to those in Embodiment 1. For specific implementation details, please refer to the relevant description section of Embodiment 1. The term "computer-readable storage medium" should be understood as a single medium or multiple media including one or more instruction sets; it should also be understood as including any medium capable of storing, encoding, or carrying an instruction set for execution by a processor and enabling the processor to perform any of the methods in this invention.

[0071] Those skilled in the art will understand that the modules or steps of the present invention described above can be implemented using general-purpose computer devices. Optionally, they can be implemented using computer-executable program code, thereby allowing them to be stored in a storage device for execution by a computer device, or they can be fabricated as separate integrated circuit modules, or multiple modules or steps can be fabricated as a single integrated circuit module. The present invention is not limited to any particular combination of hardware and software.

[0072] While the specific embodiments of the present invention have been described above in conjunction with the accompanying drawings, this 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 without creative effort based on the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims

1. A control method for a nine-arm modular multilevel converter, wherein the nine-arm modular multilevel converter includes an upper arm, a middle arm, and a lower arm, an upper AC port is provided between the upper arm and the middle arm, and a lower AC port is provided between the middle arm and the lower arm, characterized in that, include: Based on the DC-side current reference value, the compensation amount of the DC component of the circulating current in each phase, the current compensation components of each bridge arm, the phase current reference values ​​of the upper and lower AC ports, and the distribution coefficients of the phase currents of the upper and lower AC ports among the bridge arms, the current reference values ​​of each bridge arm are obtained. The determination of the current reference values ​​of each bridge arm is as follows: The ratio of the DC side current reference value to three, the compensation amount of the DC component of the circulating current in each phase, the product of the upper AC port allocation coefficient and the upper AC port phase current reference value, the product of the difference between 1 and the lower AC port allocation coefficient and the lower AC port phase current reference value, and the sum of the upper arm current compensation component, yield the upper arm current reference value. The current reference value of the middle bridge arm is obtained by summing the ratio of the DC side current reference value to three, the compensation amount of the DC component of the circulating current of each phase, the negative product of the difference between the distribution coefficient of 1 and the upper AC port and the phase current reference value of the upper AC port, the product of the difference between the distribution coefficient of 1 and the lower AC port and the phase current reference value of the lower AC port, and the compensation component of the middle bridge arm current. The current reference value of the lower bridge arm is obtained by summing the ratio of the DC side current reference value to three, the compensation amount of the DC component of the circulating current of each phase, the negative product of the difference between the distribution coefficient of 1 and the upper AC port and the phase current reference value of the upper AC port, the negative product of the difference between the distribution coefficient of 1 and the lower AC port and the phase current reference value of the lower AC port, and the compensation component of the lower bridge arm current. By setting different distribution coefficients for the phase currents of the upper and lower AC ports among the bridge arms, the phase currents of the upper and lower AC ports are distributed among the upper, middle, and lower bridge arms of the same phase. Based on the current reference value of each bridge arm and the corresponding current measurement value of each bridge arm, the modulation signal reference value of each bridge arm submodule is obtained. Based on the reference value of the submodule modulation signal, a multi-carrier modulation strategy is used to control the switching actions in each submodule; Based on the reference value and the corresponding measured value of the capacitor voltage of the submodule in the bridge arm, the compensation component of the submodule modulation signal reference value is obtained; Based on the compensation component of the modulation signal reference value of each submodule in each bridge arm and the modulation signal reference value of the corresponding submodule, the final modulation signal reference value of the corresponding submodule is obtained.

2. The control method for a nine-arm modular multilevel converter as described in claim 1, characterized in that, Based on the relationship between the capacitor voltages of the upper, middle, and lower bridge arms, the current compensation components of the upper, middle, and lower bridge arms are obtained respectively, as follows: The average value of the capacitor voltage of the submodule in the upper bridge arm is subtracted from the average value of the capacitor voltage of the submodule in the middle bridge arm, and the average value of the capacitor voltage of the submodule in the middle bridge arm is subtracted from the average value of the capacitor voltage of the submodule in the lower bridge arm. The difference results are used to obtain the current compensation components of the upper bridge arm and the lower bridge arm based on the PI controller. The middle bridge arm current compensation component is obtained by inverting and adding the upper and lower bridge arm current compensation components.

3. The control method for a nine-arm modular multilevel converter as described in claim 1, characterized in that, Based on the power reference values ​​and measured values ​​of the upper and lower AC ports, the phase current reference values ​​of the upper and lower AC ports are obtained, specifically: The active and reactive power reference values ​​of the upper and lower AC ports are subtracted from the corresponding measured values. The subtraction results are then used to obtain the reference values ​​of the active and reactive components of the phase current of the upper and lower AC ports based on the PI controller. The reference values ​​of the active and reactive components of the phase current at the upper and lower AC ports are obtained by coordinate transformation.

4. The control method for a nine-arm modular multilevel converter as described in claim 1, characterized in that, The compensation amount for the DC component of the circulating current in each phase is obtained based on the reference value of the capacitor voltage of the sub-module in the bridge arm and the average value of the capacitor voltage of all sub-modules in each phase in the bridge arm.

5. A control system for a nine-arm modular multilevel converter employs a control method for a nine-arm modular multilevel converter as described in any one of claims 1-4, wherein the nine-arm modular multilevel converter includes an upper arm, a middle arm, and a lower arm, an upper AC port is provided between the upper arm and the middle arm, and a lower AC port is provided between the middle arm and the lower arm, characterized in that, include: Bridge arm current reference value calculation module: Based on the DC side current reference value, the compensation amount of the DC component of the circulating current in each phase, the current compensation component of each bridge arm, the phase current reference values ​​of the upper AC port and the lower AC port, and the distribution coefficient of the phase current of the upper AC port and the lower AC port among the bridge arms, the current reference value of each bridge arm is obtained. The modulation signal reference value calculation module of the sub-module: Based on the current reference value of each bridge arm and the corresponding current measurement value of each bridge arm, the modulation signal reference value of the sub-module in each bridge arm is obtained. Submodule control module: Based on the reference value of the submodule modulation signal, it controls the switching actions in each submodule through a multi-carrier modulation strategy.

6. A computer device, characterized in that, include: The system includes a processor, a memory, and a bus. The memory stores machine-readable instructions executable by the processor. When the computer device is running, the processor communicates with the memory via the bus. When the machine-readable instructions are executed by the processor, they perform a nine-arm modular multilevel converter control method as described in any one of claims 1 to 4.

7. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores a computer program, which is executed by a processor to perform a nine-bridge-arm modular multilevel converter control method as described in any one of claims 1 to 4.

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