Modular multilevel AC-AC converter and method of controlling the same

Abstract

The application belongs to the technical field of power electronics, and proposes a modular multilevel AC-AC converter and a control method thereof.The converter in the application comprises multiple circulating current paths connected in series, each of the circulating current paths comprises three cascade bridge arms connected in series, and an inductor and an output port are arranged between each two adjacent cascade bridge arms; each cascade bridge arm comprises multiple sub-modules, each of the sub-modules comprises multiple anti-parallel diodes and a capacitor connected in parallel with at least one diode; the number of the sub-modules is determined according to a reference value of an output port phase voltage, an amplitude of the output port phase voltage and a reference value of an average value of all sub-module capacitor voltages, the number of the sub-modules in each bridge arm is allowed to be flexibly configured, the internal bridge arms only have three circulating current paths and the paths are clear, and the bridge arms can be used to improve the control freedom.

Description

Technical Field

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

[0002] Modular multilevel AC-AC converters, due to their modular topology, offer advantages 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. Furthermore, they can eliminate intermediate DC-DC conversion stages, achieving direct AC-AC conversion. Compared to AC-DC-AC converters, this not only improves conversion efficiency but also reduces topology costs. Currently, modular multilevel AC-AC converters are receiving widespread attention in applications such as high-voltage high-power frequency conversion drives, flexible low-frequency transmission, wind power integration, flexible interconnection of distribution networks, and flexible AC transmission.

[0003] The inventors discovered that existing modular multilevel AC-AC converters mainly include, for example: Figure 1 The modular multilevel matrix converter shown, such as Figure 2 The hexagonal modular multilevel AC-AC converter shown, and such as Figure 3The Y-shaped modular multilevel AC-AC converter is shown. The modular multilevel matrix converter contains nine bridge arms, requiring a large number of submodules, resulting in a larger converter size, higher cost and operating losses. It also has nine internal loop current paths, leading to a complex control strategy. Furthermore, the AC systems connected to both sides are highly coupled, with any phase of one AC system coupled to all three phases of the other AC system. The hexagonal modular multilevel AC-AC converter can be considered a simplified modular multilevel matrix converter, reducing the number of bridge arms by one-third to only six, thus offering certain cost and size advantages. Simultaneously, the hexagonal modular multilevel AC-AC converter reduces the coupling between the two AC systems, with any phase of one AC system only coupled to two phases of the other AC system. However, because the six bridge arms of the hexagonal modular multilevel AC-AC converter are connected end-to-end to form a hexagonal shape, its topology has only one loop current path, resulting in lower control freedom. Moreover, during normal operation, the reactive power of the AC systems on both sides must remain strictly equal; otherwise, additional internal power compensation control is required. The Y-shaped modular multilevel AC-AC converter has three Y-shaped bridge arms. The two upper branches of each Y-shaped bridge arm are connected to one phase of the AC system on each side. Therefore, any phase of the AC system on one side is coupled to only one phase of the AC system on the other side, resulting in the lowest coupling degree between the two AC systems. In addition, the upper branch of the Y-shaped bridge arm is called the auxiliary bridge arm, and the lower branch is called the main bridge arm. The number of sub-modules in the auxiliary bridge arm is half that of the main bridge arm, so the Y-shaped modular multilevel AC-AC converter requires fewer sub-modules. At the same time, it has no internal circulating current channel, so there is no circulating current problem. However, the Y-shaped modular multilevel AC-AC converter requires additional design of the reactive power distribution coefficient between the main and auxiliary bridge arms, and there is a frequency leakage problem in the AC systems on both sides, which will affect the waveform quality of the AC output on both sides. Summary of the Invention

[0004] To address the aforementioned problems, this invention proposes a modular multilevel AC-AC converter and its control method. It requires fewer sub-modules, allows for flexible configuration of the number of sub-modules in each bridge arm, has fewer internal loop paths with clearer paths, thus improving control freedom. Each phase of the AC system on one side is coupled only to one phase of the AC system on the other side, resulting in low coupling between the two AC systems. Furthermore, the frequency variation range of the connected AC systems on both sides is wide, enabling same-frequency and different-frequency power conversion without additional control strategies.

[0005] To achieve the above objectives, in a first aspect, the present invention provides a modular multilevel AC-AC converter, employing the following technical solution:

[0006] A modular multilevel AC-AC converter includes multiple loop current paths connected in series. Each loop current path includes three cascaded bridge arms connected in series. An inductor and an output port are provided between every two adjacent cascaded bridge arms.

[0007] Each cascaded bridge arm includes multiple sub-modules, each sub-module including multiple anti-parallel diodes and a capacitor connected in parallel with at least one diode; the number of sub-modules is determined based on the reference value of the output port phase voltage, the amplitude of the output port phase voltage, and the reference value of the average value of the capacitor voltages of all sub-modules.

[0008] Furthermore, the submodule includes four power semiconductor switches, four anti-parallel diodes, and one capacitor; wherein the four power semiconductor switches and the four anti-parallel diodes respectively constitute four power semiconductor switch groups, which are connected in an H-bridge structure and connected in parallel with the capacitor.

[0009] Furthermore, the formula for determining the number of sub-modules in the three cascaded bridge arms is as follows:

[0010]

[0011]

[0012]

[0013] Where n1, n2, and n3 represent the number of sub-modules in different cascaded bridge arms; u j1_ref and u j2_ref U1 and U2 represent the reference values ​​of the output port phase voltage at different locations; U1 and U2 represent the amplitudes of the output port phase voltage at different locations; U c_ref This indicates the reference value for determining the average value of the capacitor voltages of all submodules; ceil is the function name.

[0014] To achieve the above objectives, in a second aspect, the present invention also provides a modular multilevel AC-AC converter control method, employing the following technical solution:

[0015] A control method for a modular multilevel AC-AC converter, employing the modular multilevel AC-AC converter as described in the first aspect, includes: calculating the active and reactive power of the two output ports based on the output voltage and current of the two output ports; obtaining reference values ​​for the active and reactive components of the phase current of the two output ports by comparing the measured and reference values ​​of the active and reactive power of the two output ports; obtaining reference values ​​for the active and reactive components of the phase voltage of the two output ports by comparing the measured and reference values ​​of the active and reactive components of the phase current of the two output ports; and obtaining reference values ​​for the phase voltage of the two output ports after coordinate transformation; the three cascaded bridge arms are the upper bridge arm, the middle bridge arm, and the lower bridge arm; and the two output ports are the upper AC port and the lower AC port.

[0016] Furthermore, based on the measured values ​​of the capacitor voltages of all submodules, the average value of the capacitor voltages of all submodules in the converter is calculated. By comparing the measured value and the reference value of the average value of the capacitor voltages of all submodules, the compensation amount of the active component of the phase current at the input port is obtained.

[0017] Furthermore, based on the measured values ​​of the capacitor voltages of all sub-modules within the same phase, the average value of the capacitor voltages of all sub-modules within the same phase is calculated. By comparing the measured value and the reference value of the average value of the capacitor voltages of all sub-modules within the same phase, the amplitude of the phase circulating current compensation component is obtained, and multiplied with the normalized phase voltage of the output side port to obtain the circulating current compensation component that is in phase and frequency with the phase voltage of the output side port.

[0018] Furthermore, based on the measured values ​​of the capacitor voltages of the corresponding bridge arm submodules, the average values ​​of the capacitor voltages of different bridge arm submodules are calculated. The amplitudes of the compensation components of the output voltage reference values ​​of the upper and lower bridge arm submodules are obtained by comparing the average values ​​of the capacitor voltages of the upper and middle bridge arm submodules, as well as the average values ​​of the capacitor voltages of the middle and lower bridge arm submodules. The compensation component of the upper bridge arm submodule output voltage reference value is obtained by multiplying the amplitude of the compensation component of the upper bridge arm submodule output voltage reference value by the normalized upper AC port phase voltage reference value. Similarly, the compensation component of the lower bridge arm submodule output voltage reference value is obtained by multiplying the amplitude of the compensation component of the lower bridge arm submodule output voltage reference value by the normalized lower AC port phase voltage reference value. Finally, the compensation component of the middle bridge arm submodule output voltage reference value is obtained by subtracting the compensation components of the upper and lower bridge arm submodule output voltage reference values.

[0019] Furthermore, by comparing the measured and reference values ​​of the capacitor voltage of a single submodule, the compensation component of the output voltage reference signal of a single submodule is obtained. Then, depending on the positive or negative direction of the bridge arm current, it is multiplied by +1 or -1 respectively to obtain the final compensation component of the output voltage reference value of a single submodule.

[0020] Furthermore, based on the distribution coefficients of the phase currents at the upper and lower AC ports in the upper, middle, and lower bridge arms, the feedforward component of the circulating current fundamental frequency component reference value is calculated. Then, it is added to the circulating current compensation component of that phase to finally obtain the reference value of the three-phase circulating current. By comparing the reference value and the measured value of the circulating current, the compensation component of the submodule output voltage reference value in the upper, middle, and lower bridge arms is obtained.

[0021] Furthermore, based on the phase voltage reference values ​​of the upper and lower AC ports, the reference values ​​of the voltages of the upper, middle, and lower bridge arms are calculated and divided by the number of sub-modules and the reference value of the sub-module capacitor voltage in the corresponding bridge arm. Then, these values ​​are added to the compensation amount of the active component of the phase current of the input AC port, the final compensation component of the reference value of the output voltage of a single sub-module, and the compensation component of the reference value of the output voltage of the sub-modules in the upper, middle, and lower bridge arms, respectively, to obtain the reference value of the output voltage of each sub-module, which serves as the reference wave for the sub-module modulation signal. The modulation strategy adopts multi-carrier modulation, ultimately obtaining the trigger pulse of the semiconductor switch of each sub-module.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] 1. The converter in this invention includes multiple loop paths connected in series. Each loop path includes three cascaded bridge arms connected in series. An inductor and an output port are provided between every two adjacent cascaded bridge arms. Each cascaded bridge arm includes multiple sub-modules. Each sub-module includes multiple anti-parallel diodes and a capacitor connected in parallel with at least one diode. The number of sub-modules is determined based on the reference value of the phase voltage at the output port, the amplitude of the phase voltage at the output port, and the reference value of the average value of the capacitor voltages of all sub-modules. The number of sub-modules in each bridge arm can be flexibly configured. The internal loop paths are few and the paths are clear, which can be used to improve the degree of control freedom. In the converter, any phase of the AC system on one side is coupled to only one phase of the AC system on the other side, and the coupling degree between the two AC systems is low.

[0024] 2. The converter of this invention has a wide frequency variation range of the AC systems connected to both sides, and can realize same-frequency and different-frequency power conversion without additional control strategies;

[0025] 3. This invention can accurately control the active power exchange of the AC systems on both sides, realize independent reactive power control of the AC systems on both sides, and effectively ensure the balance of the capacitor voltage of the submodule under the same frequency and different frequency power conversion. Moreover, the circulating current control is simple and can suppress circulating current harmonic components of multiple frequencies at the same time. Attached Figure Description

[0026] The accompanying drawings, which form part of this embodiment, are used to provide a further understanding of this embodiment. The illustrative embodiments and their descriptions are used to explain this embodiment and do not constitute an improper limitation of this embodiment.

[0027] Figure 1 This is a schematic diagram of the modular multilevel matrix converter topology in the background technology of this invention;

[0028] Figure 2 This is a schematic diagram of the hexagonal modular multilevel AC-AC converter topology in the background art of this invention;

[0029] Figure 3 This is a schematic diagram of the Y-shaped modular multilevel AC-AC converter topology in the background art of this invention;

[0030] Figure 4 This is a schematic diagram of the submodule topology of Embodiment 1 of the present invention;

[0031] Figure 5 This is a schematic diagram of the cascaded bridge arm topology of Embodiment 1 of the present invention;

[0032] Figure 6 This is a schematic diagram of the single-phase modular multilevel AC-AC converter topology of Embodiment 1 of the present invention;

[0033] Figure 7 This is a schematic diagram of the topology of the three-phase modular multilevel AC-AC converter according to Embodiment 1 of the present invention;

[0034] Figure 8 This is a block diagram of the external AC port power control according to Embodiment 1 of the present invention;

[0035] Figure 9 This is a block diagram of the overall sub-module capacitor voltage balance control in Embodiment 1 of the present invention;

[0036] Figure 10 This is a block diagram of the capacitor voltage balance control of the phase unit submodule in Embodiment 1 of the present invention;

[0037] Figure 11 This is a block diagram of the inter-arm submodule capacitor voltage balance control in Embodiment 1 of the present invention;

[0038] Figure 12 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;

[0039] Figure 13 This is a block diagram of the circulation control in Embodiment 1 of the present invention;

[0040] Figure 14 This is a waveform diagram of the phase voltage and current at the upper AC output port during the same-frequency power conversion in Embodiment 1 of the present invention;

[0041] Figure 15 The diagram shows the phase voltage and current waveforms at the AC output port during the same-frequency power conversion in Embodiment 1 of the present invention.

[0042] Figure 16 This is a waveform diagram of the phase voltage and current at the upper AC output port during the heterogeneous power conversion in Embodiment 1 of the present invention;

[0043] Figure 17 The diagram shows the phase voltage and current waveforms of the AC output port during the different frequency power conversion in Embodiment 1 of the present invention. Detailed Implementation

[0044] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0045] It should be noted that the following detailed descriptions are exemplary and intended to provide further explanation of this application. 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 application pertains.

[0046] Example 1:

[0047] Among existing converters, the modular multilevel matrix converter (MMC) contains nine bridge arms, requiring a large number of submodules, resulting in a large converter size, high cost, and high operating losses. It also has nine internal loop current paths, leading to complex control strategies. The hexagonal modular multilevel AC-AC converter has six bridge arms connected end-to-end to form a hexagonal shape, thus its topology has only one internal loop current path, resulting in low control freedom. Furthermore, during normal operation, the reactive power of the AC systems on both sides must be strictly equal; otherwise, additional internal power compensation control is required. The Y-shaped modular multilevel AC-AC converter requires additional design of reactive power distribution coefficients between the main and auxiliary bridge arms, and the frequency leakage of the AC systems on both sides affects the waveform quality of the AC output. To address these issues, such as... Figure 7 As shown, this embodiment provides a modular multilevel AC-AC converter, including multiple loop current paths connected in series. Each loop current path includes three cascaded bridge arms connected in series, and an inductor and an output port are provided between every two adjacent cascaded bridge arms.

[0048] Each cascaded bridge arm includes multiple sub-modules, each sub-module including multiple anti-parallel diodes and a capacitor connected in parallel with at least one diode; the number of sub-modules is determined based on the reference value of the output port phase voltage, the amplitude of the output port phase voltage, and the reference value of the average value of the capacitor voltages of all sub-modules.

[0049] Specifically, the converter in this embodiment requires fewer submodules, and the number of submodules in each bridge arm can be flexibly configured. It has only three clear loop paths internally, which can improve control freedom. Each phase of the AC system on one side is coupled only to one phase of the AC system on the other side, resulting in low coupling between the two AC systems. Furthermore, the frequency variation range of the connected AC systems on both sides is wide, enabling same-frequency and different-frequency power conversion without additional control strategies.

[0050] Optionally, the converter in this embodiment is a single-phase modular multilevel AC-AC converter or a three-phase topology modular multilevel AC-AC converter; wherein, as shown... Figure 6 As shown, each phase of the single-phase modular multilevel AC-AC converter contains three cascaded bridge arms, referred to as the upper bridge arm, middle bridge arm, and lower bridge arm. The upper and lower bridge arms are each connected in series with a bridge arm inductor, thus comprising a total of six cascaded bridge arms and four bridge arm inductors; Figure 7 As shown, the three-phase modular multilevel AC-AC converter also includes three cascaded bridge arms per phase, referred to as the upper bridge arm, middle bridge arm and lower bridge arm respectively. The upper bridge arm and the lower bridge arm are connected in series with a bridge arm inductor. The three-phase topology includes a total of nine cascaded bridge arms and six bridge arm inductors.

[0051] like Figure 7 As shown, the three-phase modular multilevel AC-AC converter in this embodiment includes nine cascaded bridge arms 200 and six bridge arm inductors; the bridge arm inductors include a first inductor 407, a second inductor 408, a third inductor 409, a fourth inductor 410, a fifth inductor 411, and a sixth inductor 412; correspondingly, the output ports include a first output port 401, a second output port 402, a third output port 403, a fourth output port 404, a fifth output terminal 405, and a sixth output port 406, wherein the first output port 401, the second output port 402, and the third output port 403 constitute the upper AC output port, and the fourth output port 404, the fifth output terminal 405, and the sixth output port 406 constitute the lower AC output port; as shown Figure 5 As shown, each cascaded bridge arm includes multiple sub-modules 100; such as Figure 4 As shown, each submodule 100 consists of four IGBT power semiconductor switches and one capacitor 105. Ports 106 and 107 serve as the AC output ports of the submodule. The four IGBT power semiconductor switches are designated as first switch 101, second switch 102, third switch 103, and fourth switch 104. Furthermore, Figure 7 The number of submodules in each phase upper, middle, and lower arm of the three-phase modular multilevel AC-AC converter shown is determined according to the following formula:

[0052]

[0053]

[0054]

[0055] In the formula, n1, n2, and n3 represent the number of sub-modules in the upper arm, middle arm, and lower arm, respectively; u j1_ref and u j2_ref U1 and U2 represent the reference values ​​of the phase voltages at the upper and lower AC ports, respectively; U1 and U2 represent the amplitudes of the phase voltages at the upper and lower AC ports, respectively; U c_ref This indicates the reference value for determining the average value of the capacitor voltages of all submodules; ceil is the function name.

[0056] In this embodiment, all cascaded bridge arms include multiple sub-modules, and the output ports of the sub-modules are connected in series sequentially; optionally, such as Figure 4 As shown, the submodule can include four IGBT power semiconductor switches, four anti-parallel diodes, and one capacitor. The four IGBT power semiconductor switches and four anti-parallel diodes each form four power semiconductor switch groups, connected in an H-bridge structure, and connected in parallel with the capacitor.

[0057] Example 2:

[0058] This embodiment provides a modular multilevel AC-AC converter control method, and correspondingly, a modular multilevel AC-AC converter control system, including:

[0059] The external AC port power control unit is used to control the active and reactive power flowing into or out of the AC system on both sides;

[0060] The internal submodule capacitor voltage balance control includes an overall balance control unit, a phase unit balance control unit, an inter-arm balance control unit, and an intra-arm balance control unit. The overall balance control unit controls the average value of the capacitor voltages of all submodules in the converter to be stable near a reference value. The phase unit balance control unit controls the average value of the capacitor voltages of all submodules within the same phase to be stable near a reference value. The inter-arm balance control unit controls the average value of the capacitor voltages of submodules between the upper and middle arms of the same phase, and between the middle and lower arms, to remain equal. The intra-arm balance control unit controls the capacitor voltage of a single submodule within the same arm to be stable near a reference value.

[0061] The circulating current control unit is used to control the harmonic components in the circulating current of each phase to be zero;

[0062] The submodule output voltage reference value calculation unit is used to synthesize the final reference value of the output voltage of each submodule, which is then normalized and used as the reference wave for the modulation signal of each submodule.

[0063] In this embodiment, the external AC port power control unit includes a power calculation unit PC, a power regulation unit PA, and a current regulation unit IA. Wherein, as... Figure 8 As shown, the power calculation unit PC calculates the active and reactive power of the AC ports on both sides based on the output voltage and current of the AC ports on both sides; the power adjustment unit PA compares the measured values ​​and reference values ​​of the active and reactive power of the AC ports on both sides with the PI regulator to obtain the reference values ​​of the active and reactive components of the phase current of the AC ports on both sides; the current adjustment unit IA compares the measured values ​​and reference values ​​of the active and reactive components of the phase current of the AC ports on both sides with the PI regulator to obtain the reference values ​​of the active and reactive components of the phase voltage of the AC ports on both sides, and obtains the reference values ​​of the phase voltage of the AC ports on both sides after coordinate transformation.

[0064] like Figure 9 As shown, the overall submodule capacitor voltage balance control unit includes an overall submodule capacitor voltage average value calculation unit WCC and an adjustment unit WCA. The overall submodule capacitor voltage average value calculation unit WCC calculates the average value ucavg_all of all submodule capacitor voltages based on the measured values ​​of all submodule capacitor voltages in the converter. The adjustment unit WCA compares the measured value ucavg_all of the average value of all submodule capacitor voltages with a reference value Uc_ref, and then uses a PI regulator to obtain the compensation amount id1_comp of the active component of the phase current at the input AC port.

[0065] like Figure 10 As shown, the phase unit balance control unit for submodule capacitor voltage includes a phase unit submodule capacitor voltage average value calculation unit (PCC) and an adjustment unit (PCA). The PCC calculates the average value ucavg_j of all submodule capacitor voltages within the same phase based on the measured values ​​uc_xj_i. The PCA compares the measured value ucavg_j with a reference value Uc_ref, then uses a PI controller to obtain the amplitude ΔIcir_j of the circulating current compensation component for that phase. This amplitude Δicir_j is then multiplied by the normalized output-side port phase voltage uj2 to obtain the circulating current compensation component Δicir_j, which is in phase and frequency with uj2.

[0066] like Figure 11As shown, the submodule capacitor voltage bridge arm balance control unit includes an average value calculation unit ACC for the upper, middle, and lower bridge arm submodule capacitor voltages, an upper bridge arm and middle bridge arm submodule capacitor voltage adjustment unit ACA1, a middle bridge arm and lower bridge arm submodule capacitor voltage balance adjustment unit ACA2, and a submodule output voltage reference value compensation component calculation unit SCC. Specifically, the average value calculation unit ACC for the upper, middle, and lower bridge arm submodule capacitor voltages calculates the average values ​​of the upper, middle, and lower bridge arm submodule capacitor voltages based on the measured values ​​uc_xj_i of the submodule capacitor voltages in the corresponding bridge arms. The submodule capacitor voltage adjustment units ACA1 and ACA2 for the upper and middle bridge arms, and the middle and lower bridge arms, respectively, compare the average values ​​of the upper and middle bridge arm submodule capacitor voltages (ucavgj_U and ucavgj_M) and the middle and lower bridge arm submodule capacitor voltages (ucavgj_M and uca_i). vgj_L), and then the amplitude of the compensation component of the submodule output voltage reference value is obtained by the PI regulator. The submodule output voltage reference value compensation component calculation unit SCC obtains the compensation component Δuj_U of the upper bridge arm submodule output voltage reference value by multiplying ΔUj_U with the normalized uj1, obtains the compensation component Δuj_L of the lower bridge arm submodule output voltage reference value by multiplying ΔUj_L with the normalized uj2, and obtains the compensation component Δuj_M of the middle bridge arm submodule output voltage reference value by subtracting Δuj_U and Δuj_L.

[0067] like Figure 12 As shown, the submodule capacitor voltage balance control unit (ICA) in the bridge arm compares the measured value uc_xj_i of the individual submodule capacitor voltage with the reference value Uc_ref, and uses a PI regulator to obtain the compensation component uxj_i_comp of the individual submodule output voltage reference value. Then, depending on the positive or negative direction of the bridge arm current, it multiplies by +1 or -1 to obtain the final compensation component Δuxj_i of the individual submodule output voltage reference value.

[0068] like Figure 13 As shown, the circulating current control unit includes a circulating current reference value calculation unit (CRC) and a circulating current suppression unit (CSA). The CRC first calculates the feedforward component icir1_j of the circulating current fundamental frequency component reference value based on the distribution coefficients λ and μ of the phase currents at the upper and lower AC ports in the upper, middle, and lower bridge arms. Then, it adds this component to the corresponding phase circulating current compensation component Δicir_j to obtain the reference value icirj_ref of the three-phase circulating current. The CSA compares the circulating current reference value icirj_ref with the measured value icirj, and then uses a PIR regulator to obtain the compensation component Δuj of the submodule output voltage reference value in the upper, middle, and lower bridge arms.

[0069] The submodule output voltage reference value calculation unit first calculates the reference values ​​of the upper, middle, and lower bridge arm voltages based on the upper and lower AC port phase voltage reference values ​​obtained from the external AC port power control unit. These values ​​are then divided by the number of submodules in the corresponding bridge arm and the reference value Uc of the submodule capacitor voltage. Finally, these values ​​are added to the compensation components generated by the inter-bridge arm submodule capacitor voltage balancing control unit, the intra-bridge arm submodule capacitor voltage balancing control unit, and the circulating current control unit to obtain the reference value of each submodule output voltage, which serves as the reference wave for the submodule modulation signal. The modulation strategy employs multi-carrier modulation to ultimately obtain the trigger pulse for each submodule semiconductor switch.

[0070]

[0071]

[0072]

[0073] Among them, u U_i(i＝1～n1) u is the reference wave for the modulation signal of the i-th submodule in the upper bridge arm; j1_ref U is the reference value for the phase voltage at the upper AC port; n is the number of submodules in the bridge arm; U c This is the reference value for the capacitor voltage of the submodule; Δu j_U The compensation component for the output voltage reference value of each submodule in the upper bridge arm; Δu Uj_i Δu is the compensation component of the output voltage reference value of the i-th submodule in the upper bridge arm. j This refers to the compensation components for the output voltage reference values ​​of each submodule in the upper, middle, and lower bridge arms of phase j; u M_i(i＝1～n2) u is the reference wave for the modulation signal of the i-th submodule in the middle bridge arm; j2_ref The reference value for the phase voltage at the lower AC port; Δu j_M The compensation component for the output voltage reference value of each submodule in the middle bridge arm; Δu Mj_i This is the compensation component of the output voltage reference value of the i-th submodule in the middle bridge arm; u L_i(i＝1n3) Δu is the reference wave for the modulation signal of the i-th submodule in the lower bridge arm. j_L The compensation component for the output voltage reference value of each submodule in the lower bridge arm; Δu Lj_i This is the compensation component of the output voltage reference value of the i-th submodule in the lower bridge arm.

[0074] After obtaining the modulation signals of each submodule in the upper, middle, and lower bridge arms, a multi-carrier modulation strategy is used to determine the trigger pulses of the semiconductor switches in each submodule. Figure 14 , Figure 15 , Figure 16 and Figure 17The waveforms of phase voltage and current at the upper AC port and lower AC output port are shown respectively during same-frequency and different-frequency power conversion.

[0075] The above description is merely a preferred embodiment of this practice and is not intended to limit the scope of this practice. Various modifications and variations can be made to this practice by those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this practice should be included within the protection scope of this practice.

Claims

1. A control method for a modular multilevel AC-AC converter, characterized in that, It includes multiple loop circuits connected in series, each loop circuit includes three cascaded bridge arms connected in series, and an inductor and an output port are provided between every two adjacent cascaded bridge arms; Each cascaded bridge arm includes multiple sub-modules. The number of sub-modules is determined based on the reference value of the phase voltage at the output port, the amplitude of the phase voltage at the output port, and the reference value of the average value of the capacitor voltages of all sub-modules. The submodule includes four power semiconductor switches, four anti-parallel diodes, and one capacitor; wherein the four power semiconductor switches and four anti-parallel diodes respectively constitute four power semiconductor switch groups, which are connected in an H-bridge structure and connected in parallel with the capacitor. Based on the measured values ​​of the capacitor voltages of all sub-modules, the average value of the capacitor voltages of all sub-modules in the converter is calculated. By comparing the measured value and the reference value of the average value of the capacitor voltages of all sub-modules, the compensation amount of the active component of the phase current at the input port is obtained. This includes calculating the active and reactive power of the two output ports based on their output voltage and current; obtaining reference values ​​for the active and reactive components of the phase current of the two output ports by comparing the measured and reference values ​​of the active and reactive power of the two output ports; obtaining reference values ​​for the active and reactive components of the phase voltage of the two output ports by comparing the measured and reference values ​​of the active and reactive components of the phase current of the two output ports; and obtaining reference values ​​for the phase voltage of the two output ports after coordinate transformation. The three cascaded bridge arms are the upper bridge arm, the middle bridge arm, and the lower bridge arm; the two output ports are the upper AC port and the lower AC port. Based on the measured values ​​of the capacitor voltages of all sub-modules in the same phase, the average value of the capacitor voltages of all sub-modules in the same phase is calculated. By comparing the measured value and the reference value of the average value of the capacitor voltages of all sub-modules in the same phase, the amplitude of the phase circulating current compensation component is obtained, and multiplied with the normalized phase voltage of the lower AC port on the output side to obtain the circulating current compensation component that is in phase and frequency with the phase voltage of the lower AC port on the output side. Based on the measured values ​​of the capacitor voltages of the corresponding bridge arm submodules, the average values ​​of the capacitor voltages of different bridge arm submodules are calculated. The amplitudes of the compensation components of the output voltage reference values ​​of the upper and lower bridge arm submodules are obtained by comparing the average values ​​of the capacitor voltages of the upper and middle bridge arm submodules, and the average values ​​of the capacitor voltages of the middle and lower bridge arm submodules. The compensation component of the output voltage reference value of the upper bridge arm submodule is obtained by multiplying the amplitude of the compensation component of the output voltage reference value of the upper bridge arm submodule by the normalized upper AC port phase voltage reference value. Similarly, the compensation component of the output voltage reference value of the lower bridge arm submodule is obtained by multiplying the amplitude of the compensation component of the output voltage reference value of the lower bridge arm submodule by the normalized lower AC port phase voltage reference value. Finally, the compensation component of the output voltage reference value of the middle bridge arm submodule is obtained by subtracting the compensation components of the output voltage reference values ​​of the upper and lower bridge arm submodules.

2. The control method for a modular multilevel AC-AC converter as described in claim 1, characterized in that, The formula for determining the number of sub-modules in the three cascaded bridge arms is: in, n 1. n 2 and n 3 represents the number of sub-modules in different cascaded bridge arms; u j1_ref and u j2_ref These represent the reference values ​​of the phase voltage at the output port at different locations; U 1 and U 2 represents the amplitude of the phase voltage at the output port at different locations; U c_ref This represents a reference value indicating the average voltage of all submodule capacitors; ceil is the function name.

3. The control method for a modular multilevel AC-AC converter as described in claim 1, characterized in that, By comparing the measured and reference values ​​of the capacitor voltage of a single submodule, the compensation component of the output voltage reference value of a single submodule is obtained. Then, depending on the positive or negative direction of the bridge arm current, it is multiplied by +1 or -1 to obtain the final compensation component of the output voltage reference value of a single submodule.

4. The control method for a modular multilevel AC-AC converter as described in claim 3, characterized in that, First, based on the distribution coefficients of the phase currents at the upper and lower AC ports in the upper, middle, and lower bridge arms, the feedforward component of the circulating current fundamental frequency reference value is calculated. Then, it is added to the circulating current compensation component of that phase to finally obtain the reference value of the three-phase circulating current. By comparing the reference value and the measured value of the circulating current, the compensation component of the submodule output voltage reference value in the upper, middle, and lower bridge arms is obtained.

5. The control method for a modular multilevel AC-AC converter as described in claim 4, characterized in that, First, based on the phase voltage reference values ​​of the upper and lower AC ports, the reference values ​​of the voltages of the upper, middle, and lower bridge arms are calculated and divided by the number of sub-modules in the corresponding bridge arm and the reference value of the sub-module capacitor voltage. Then, these values ​​are added to the compensation amount of the active component of the phase current of the input AC port, the final compensation component of the reference value of the output voltage of a single sub-module, and the compensation component of the reference value of the output voltage of the sub-modules in the upper, middle, and lower bridge arms, respectively, to finally obtain the reference value of the output voltage of each sub-module, which serves as the reference wave for the sub-module modulation signal. The modulation strategy employs multi-carrier modulation, ultimately yielding the trigger pulses for the semiconductor switches of each submodule.

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