A submodule capacitance optimization control method for MMC-MAB topology

Abstract

The present invention discloses a submodule capacitance optimization control method of an MMC-MAB topology structure, which belongs to the field of solid-state transformers of medium and low voltage hybrid AC-DC networking systems. The control method is based on a multi-active bridge converter structure including a modular level converter structure, a half-bridge and capacitor structure, a full-bridge structure and a high-frequency link, and an MMC-MAB topology structure of a three-phase bridge arm, including a carrier phase shift control of the MMC level and a double closed-loop phase angle control of the MAB level; the carrier phase shift control of the MMC level will generate a fluctuating power of a base frequency and a double frequency superposition at the half-bridge structure; the double closed-loop phase angle control of the MAB level is used to transfer this fluctuating power to the transformer for coupling and offset, and the DC power is transferred to the low-voltage DC port formed by the full-bridge parallel connection on the secondary side through the transformer. The present invention not only eliminates the base frequency and double frequency fluctuating power on the submodule capacitor, optimizes the capacitor, but also maintains the voltage stability of the low-voltage DC port.

Description

Technical Field

[0001] The invention relates to the field of solid-state transformers for medium and low voltage hybrid AC / DC networking systems, and in particular to a submodule capacitance optimization control method for an MMC-MAB topology structure. Background Art

[0002] Due to the continuous growth of load, renewable energy continues to penetrate and its share in total energy consumption continues to increase. The construction of energy Internet, including large-scale renewable energy, requires the power grid to have flexible flow control and multi-mode energy access functions. In order to meet the requirements of the construction of new energy Internet, related research focuses on a new type of transformer-solid-state transformer that can provide power flow control and has functions such as reactive power, harmonics and unbalance compensation.

[0003] SST usually includes a medium voltage AC / DC stage and a DC / DC isolation stage. The AC / DC stage can convert medium voltage AC into medium voltage DC and low voltage DC, so that the SST can be directly connected to the distribution network. Currently, MMC is widely used in the AC / DC stage due to its modular structure, easy control, low redundancy and fault tolerance operation cost, standard components, and excellent output waveform quality. The typical solution of the DC / DC isolation stage is a combination of active bridge and high-frequency transformer, which mainly plays the role of voltage conversion and electrical isolation. The voltage fluctuation of the sub-module capacitor of the MMC structure is an inherent problem in the SST based on MMC. It usually requires a larger capacitor to suppress it, which obviously restricts the improvement of the power density of the SST system. Therefore, reducing the size of the MMC capacitor is of great significance for the practical engineering application of SST.

[0004] At present, for the strategy to solve the voltage fluctuation of the capacitor of the MMC structure submodule, some scholars have replaced the half-bridge structure in the traditional MMC submodule with a full-bridge structure by adjusting the MMC submodule topology, and used the better controllability and negative level output capability of the full-bridge SM to achieve overmodulation and reduce the ripple voltage of the submodule capacitor. However, the replacement of the half-bridge to the full-bridge undoubtedly increases the number of devices and system costs. Based on the mutual influence between the SM voltage fluctuation and the interphase circulating current, some scholars have adopted the scheme of circulating current injection to indirectly affect the voltage fluctuation of the SM capacitor by reducing the interphase circulating current, but the suppression effect on the SM voltage fluctuation is relatively limited. There is also a scheme that uses a double half-bridge module to connect the upper and lower bridge arms of the same phase to the SM, realizing the decoupling of the fundamental frequency power; however, since the frequency components of the harmonic currents of the upper and lower bridge arms are not all in phase or inverse phase, the establishment of a channel between them cannot make all the fluctuation components cancel each other, and the effect on the voltage fluctuation of the SM capacitor is limited. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a sub-module capacitor optimization control method of an MMC-MAB topology structure, which can eliminate the low-frequency ripple voltage of the sub-module capacitor, reduce the volume of the sub-module capacitor, achieve capacitor optimization, and maintain the stability of the low-voltage DC port voltage.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0007] A submodule capacitance optimization control method of an MMC-MAB topology structure, wherein the MMC-MAB topology structure includes a modular level converter structure, a half-bridge and capacitor structure, a full-bridge structure, and a multi-active bridge converter structure of a high-frequency link and a three-phase bridge arm; the control method includes carrier phase shift control of the MMC level and dual closed-loop phase angle control of the MAB level;

[0008] The carrier phase shift control of the MMC level specifically refers to the voltage and current double closed-loop carrier phase shift pulse width modulation strategy adopted by the half-bridge and capacitor structure in the dq rotating coordinate system;

[0009] The dual closed-loop phase angle control of the MAB level specifically refers to the output voltage and input current dual closed-loop phase shift control strategy of the multi-active bridge converter of the full-bridge structure and the high-frequency link.

[0010] A further improvement of the technical solution of the present invention is that the voltage and current dual closed-loop carrier phase-shift pulse width modulation strategy specifically includes the following steps:

[0011] 1.1, three-phase grid voltage u a ,u b ,u c The phase ωt and three-phase grid voltage u are obtained through the PLL phase-locked loop link. a ,u b ,u c Perform dq transformation with ωt to obtain the d-axis component u d , q-axis component u q , three-phase grid current i a ,i b ,i c Perform dq transformation with ωt to obtain the d-axis component i d ,q-axis component i q ;

[0012] 1.2, first set the MVDC bus voltage to U Mdcr The actual MVDC bus voltage value U Mdc The difference between the output signal obtained by the PI controller and the component of the MVAC current on the d-axis i d The output value obtained by a PI controller is the component of the MVAC voltage on the d axis. ud Add, then subtract the q-axis component of the MVAC current i q * ωL The output value of i d Feedforward decoupling;

[0013] 1.3, given value of q-axis component current of MVDC current i qr The component of the actual MVDC current in the q axis i q The output value obtained after the PI controller is added to the q-axis component of the MVAC voltage, and then the d-axis component of the MVAC current is subtracted. i d * ωL To achieve i q Feedforward decoupling;

[0014] 1.4, input the decoupled values ​​of step 1.2 and step 1.3 into the dq / abc inverse transformation to obtain the three-phase modulation components u a_r , u b_r , u c_r , the driving signal Q of the half-bridge and capacitor structure is obtained through carrier phase shift modulation simulation sm .

[0015] A further improvement of the technical solution of the present invention is that the output voltage and input current double closed-loop phase shift control strategy specifically includes the following steps:

[0016] 2.1, set the voltage setting value U of the low voltage DC bus voltage LVDC_ref Subtract the actual voltage U of the low voltage DC bus LVDC The average value of , and the output value obtained by the PI regulator is used as the given signal of the inner loop;

[0017] 2.2, the low-frequency fluctuation component of the base frequency and the double frequency superimposed in the output current of the half-bridge structure in the half-bridge and capacitor structure, the low-frequency pulsating signal is extracted by a bandpass filter, and superimposed with the PI regulator output control signal obtained in step 2.1 to obtain a control signal containing a fluctuation component;

[0018] 2.3, take the control signal with fluctuation component obtained in step 2.2 as the given current of inner loop input, and adjust its waveform to make it the same as the low-frequency fluctuation current waveform of SM. At this time, the actual MAB input current feedback should add the same band-pass filter as that used by SM;

[0019] 2.4, the difference between the MAB input current obtained in step 2.3 and the given value is used as the input signal of the inner loop PIR regulator. The PIR regulator should include two oscillation links, and the two oscillation links respectively select the fundamental frequency fluctuation component and the double frequency fluctuation component;

[0020] 2.5, the output signal obtained by the PIR regulator in step 2.4 is used to obtain the final phase shift angle modulation signal, and the driving signal S of the primary and secondary full bridge structure of the multi-active bridge converter is obtained through square wave phase shift modulation. kj_1-4 , S kj_5-8 , where k is top, low, and x is a, b, c.

[0021] The further improvement of the technical solution of the present invention is that: in the MMC-MAB topology, the half-bridge structure outputs a fluctuating current component i with the fundamental frequency and the double frequency superimposed in the half-bridge and capacitor structure. f_kj , through the output voltage and input current control double closed-loop phase shift control strategy, it is transmitted to the transformer of the multi-active bridge high-frequency chain. The fluctuating current component presents a three-phase symmetrical characteristic, and is mutually coupled and offset at the high-frequency chain, thereby eliminating the fluctuating power; i f_kj Here k is top, low, and x is a, b, c.

[0022] A further improvement of the technical solution of the present invention is that the output current i of the half-bridge structure in the half-bridge and capacitor structure is sm_kj Contains DC component i dc The AC component of the fluctuation i f_kj ; Under the traditional control strategy, the fluctuating AC component i f_kj It is absorbed by the large capacitor C of the submodule, and only the DC component i dc The oscillating AC component i f_kj With DC component i dc All are transmitted to the full bridge structure of the subsequent multi-active bridge converter through the control method; i f_kj Here k is top, low, and x is a, b, c.

[0023] Due to the adoption of the above technical solution, the technical progress achieved by the present invention is:

[0024] 1. The present invention eliminates the low-frequency ripple voltage of the submodule capacitor and reduces the volume of the submodule capacitor, thereby achieving capacitor optimization; at the same time, the voltage stability of the low-voltage DC port connected to the MAB is not affected, the SST three-port function is maintained, and the stable state can be quickly restored when disturbed.

[0025] 2. The control strategy of the present invention for the multi-active bridge converter eliminates the low-frequency fluctuation voltage of the SM capacitor and simultaneously achieves full suppression of the double-frequency circulating current in the bridge arm of the MMC system. Therefore, the MMC-level circulating current suppression strategy is no longer required, thus eliminating some detection circuits and control resources. The elimination of the double-frequency circulating current in the bridge arm significantly reduces the switching loss and conduction loss of the switching device caused thereby.

[0026] 3. The present invention does not affect the steady-state operation of the system, and the size of the submodule capacitor is greatly reduced, which means that the energy stored in the system is reduced, which is conducive to improving the dynamic response capability of the system. At the same time, when a short-circuit fault occurs in the submodule, the current rise rate of the small-capacitance capacitor caused by discharge slows down, reducing the harm of the fault.

[0027] 4. In the MMC-MAB topology of the present invention, the complete elimination of the low-frequency fluctuation voltage of the SM capacitor voltage achieved by the MAB control method is a software method that does not require modification of the topology, avoids the difficulty of hardware circuit design, and solves the problem of unsatisfactory devices when operating under high-power conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0029] Figure 1 This is an electrical schematic diagram of the MMC-MAB topology structure in an embodiment of the present invention;

[0030] Figure 2 Schematic diagram of the current path of the SM-DC / DC isolation stage under the traditional control strategy in an embodiment of the present invention;

[0031] Figure 3 Schematic diagram of the current path of the SM-DC / DC isolation stage after the sub-module capacitance optimization control method based on MMC-MAB is adopted in an embodiment of the present invention;

[0032] Figure 4 1 is a schematic diagram of a carrier pulse width phase shift control strategy of a half-bridge and capacitor structure in an embodiment of the present invention;

[0033] Figure 5 It is a schematic diagram of the output voltage and input current double closed-loop phase-shift control strategy of the multi-active bridge converter in an embodiment of the present invention;

[0034] Figure 6Schematic diagram of the submodule fundamental frequency current ripple component of the conventional three-phase MMC-MAB topology structure in the embodiment of the present invention;

[0035] Figure 7 Schematic diagram of the double frequency current ripple component of the submodule of the conventional three-phase MMC-MAB topology structure in the embodiment of the present invention;

[0036] Figure 8 Schematic diagram of the phase shift control modulation principle of a multi-active bridge converter in an embodiment of the present invention;

[0037] Among them, MMC is a modular multi-level converter; SM kj_i is the i-th (i=1~N) half-bridge module of the j-phase (j is a, b, c) of the k (top, low) bridge arm; N is the number of sub-modules; Q sm_1 , Q sm_2 They are the first and second power switch tubes of the half submodule respectively; S kj_1 , S kj_2 , S kj_3 , S kj_4 They are the first, second, third and fourth power switch tubes on the primary side of the MAB high-frequency link transformer of the k-bridge arm and the j-phase respectively; S5, S6, S7 and S8 are the fifth, sixth, seventh and eighth power switch tubes on the secondary side of the MAB high-frequency link transformer respectively; L r is the phase-shifting inductor on the primary side of the MAB high-frequency link transformer; MVAC is the medium-voltage AC bus, LVDC is the low-voltage DC bus formed by the parallel connection of the output sides of the multi-active bridge converters; u a 、u b 、u c is the three-phase AC input voltage, i a 、i b 、i c is the three-phase AC input current; L g is the bridge arm inductance; C sm is the capacitance; i SMkj is the output current of the half-bridge structure in the x-phase half-bridge and capacitor structure; i dc is the DC component of the output current of a half-bridge structure in a half-bridge and capacitor structure; i f_kj is the superposition of the fundamental frequency and double frequency components of the output current of the half-bridge structure in the k-bridge arm j-phase half-bridge and capacitor structure; U Mdcr is the voltage reference value of the medium voltage DC bus voltage; U Mdc is the actual output value of the medium voltage DC bus voltage; ωt is the phase-locked loop (PLL) output phase; u d 、u q are the d-axis component and q-axis component after the three-phase input voltage is transformed into dq; i d 、i qare the d-axis component and q-axis component after the three-phase input current is transformed into dq; i qr is the given value of the input current in the q-axis component; L is the filter inductance; u a_r 、u b_r 、u c_r is the three-phase voltage modulation signal component; CPS-SPWM is carrier phase shift modulation; Q sm is the driving signal of the cascaded half-bridge submodule; U LVDC_ref is the voltage setting value of the low voltage DC bus voltage; U LVDC is the actual output value of the low-voltage DC bus voltage; LPF is a low-pass filter; BPF is a band-pass filter; I smkj is the j-phase submodule current of the k-bridge arm, I MABkj is the input current of the primary side full bridge structure of MAB in phase j of the k-bridge arm, I * kj Given for it; S MABkj It is the control signal of the primary and secondary full-bridge structure of the multi-active bridge converter; i kjq1 is the fundamental frequency current component of the j-phase SM in the k-bridge arm; i kjq2 is the double frequency current component of the j-phase SM in the k-bridge arm; V1 and V2 are the primary and secondary voltages of the converter respectively; δ is the phase shift angle between the primary and secondary control signals of the multi-active bridge converter; M1 and M2 are the modulation signals for triggering the primary and secondary sides of the multi-active bridge converter. DETAILED DESCRIPTION

[0038] The embodiment of the present invention solves the problem of large sub-module capacitance in the prior art by providing a sub-module capacitance optimization control method of an MMC-MAB topology structure. The low-frequency ripple voltage of the sub-module capacitance is eliminated and the volume of the sub-module capacitance is reduced by adopting carrier phase shift control for the MMC level and double closed-loop phase angle control for the MAB level, thereby achieving capacitance optimization.

[0039] The technical solution of the present invention will be clearly and completely described below through specific implementation methods. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0040] like Figure 1As shown, the applied MMC-MAB topology structure includes: a modular level converter structure, a half-bridge and capacitor structure, a full-bridge structure and a multi-active bridge converter structure of a high-frequency link and a three-phase bridge arm; wherein the MMC topology is a traditional structure, the three full-bridge structures on the primary side of the MAB are respectively connected to the SMs of the three phases of the MMC, and the secondary side of the MAB is a full-bridge structure connected to the LVDC bus; all MABs are connected in parallel to the LVDC bus.

[0041] like Figure 2 , 3 As shown, the output current i of the half-bridge structure in the half-bridge and capacitor structure is SMkj Contains DC component i dc The AC component i f_kj ; Under the traditional control strategy, the low-frequency AC component i f_kj It is absorbed by the large capacitor C of the submodule, and only the DC component i dc The low-frequency AC component i f_kj With DC component i dc All of them are transmitted to the full-bridge structure of the subsequent multi-active bridge converter through a control method.

[0042] The control method includes carrier phase shift control of the MMC level and double closed-loop phase angle control of the MAB level. The MMC level is responsible for medium voltage AC and DC conversion to meet the basic functions of the solid-state transformer; the output voltage loop in the MAB level double closed-loop phase shift control controls the LVDC port voltage to be stable, and the input current is set to the low-frequency fluctuating current component of the submodule, thereby eliminating the low-frequency fluctuations in the SM and greatly reducing the size of the capacitor; and the three-phase submodules are connected horizontally with the help of the MAB multi-winding transformer structure, so that the fluctuating power is coupled and offset in the high-frequency link transformer according to the three-phase symmetry, and no longer continues to be transmitted to the LVDC port formed by the parallel connection of the secondary side of the transformer, and does not affect the LVDC voltage stability.

[0043] like Figure 4 As shown, the carrier phase shift control of the MMC level specifically refers to the voltage and current double closed-loop carrier phase shift pulse width modulation strategy adopted by the half-bridge and capacitor structure in the dq rotating coordinate system; specifically includes the following steps:

[0044] 1.1, three-phase grid voltage u a ,u b ,u c The phase ωt and three-phase grid voltage u are obtained through the PLL phase-locked loop link. a ,u b ,u c Perform dq transformation with ωt to obtain the d-axis component u d , q-axis component uq , three-phase grid current i a ,i b ,i c Perform dq transformation with ωt to obtain the d-axis component i d ,q-axis component i q ;

[0045] 1.2, first set the MVDC bus voltage to U Mdcr The actual MVDC bus voltage value U Mdc The difference between the output signal obtained by the PI controller and the component of the MVAC current on the d-axis i d The output value obtained by a PI controller is the component of the MVAC voltage on the d axis. u d Add, then subtract the q-axis component of the MVAC current i q * ωL The output value of i d Feedforward decoupling;

[0046] 1.3, given value of q-axis component current of MVDC current i qr The component of the actual MVDC current in the q axis i q The output value obtained after the PI controller is added to the q-axis component of the MVAC voltage, and then the d-axis component of the MVAC current is subtracted. i d * ωL To achieve i q The feed-forward decoupling of

[0047] 1.4, input the decoupled values ​​of step 1.2 and step 1.3 into the dq / abc inverse transformation to obtain the three-phase modulation components u a_r , u b_r , u c_r , the driving signal Q of the half-bridge and capacitor structure is obtained through carrier phase shift modulation simulation sm .

[0048] like Figure 5As shown, the dual closed-loop phase angle control of the MAB level specifically refers to the dual closed-loop phase shift control strategy for the output voltage and input current of the multi-active bridge converter with full-bridge structure and high-frequency link. The dual closed-loop phase shift control strategy for the output voltage and input current control of the multi-active bridge converter with full-bridge structure and high-frequency link, the outer loop is the MAB output voltage control, and the inner loop is the MAB input current pulsation control. In order to avoid the influence of the incomplete coupling and cancellation of the fluctuating power under the control strategy, the outer loop controls the average value of the output voltage; the given of the inner loop shares the same MA output voltage control signal, thereby realizing the stable sharing of the low-voltage DC load power between the MABs; the output signal of the outer loop PI regulator superimposed on the fluctuation of the SM current is used as the given of the inner loop MAB input current, and the MAB input current waveform is adjusted, and the low-frequency fluctuation component containing the base frequency and the double frequency is tracked by the PIR regulator; the control signal output by the regulator is converted into a phase-shifted square wave signal to control the MAB; wherein LPF is a low-pass filter and BPF is a band-pass filter; specifically includes the following steps:

[0049] 2.1, set the voltage setting value U of the low voltage DC bus voltage LVDC_ref Subtract the actual voltage U of the low voltage DC bus LVDC The average value of , and the output value obtained by the PI regulator is used as the given signal of the inner loop;

[0050] 2.2, the low-frequency fluctuation component of the base frequency and the double frequency superimposed in the output current of the half-bridge structure in the half-bridge and capacitor structure, the low-frequency pulsating signal is extracted by a bandpass filter, and superimposed with the PI regulator output control signal obtained in step 2.1 to obtain a control signal containing a fluctuation component;

[0051] 2.3, take the control signal with fluctuation component obtained in step 2.2 as the given current of inner loop input, and adjust its waveform to make it the same as the low-frequency fluctuation current waveform of SM. At this time, the actual MAB input current feedback should add the same band-pass filter as that used by SM;

[0052] 2.4, the difference between the MAB input current obtained in step 2.3 and the given value is used as the input signal of the inner loop PIR regulator. The PIR regulator should include two oscillation links, and the two oscillation links respectively select the fundamental frequency fluctuation component and the double frequency fluctuation component;

[0053] 2.5, Step 2.4 The output signal obtained by the PIR regulator obtains the final phase shift angle modulation signal, and the driving signal S of the primary and secondary full bridge structure of the multi-active bridge converter is obtained through square wave phase shift modulation. kj_1-4 , S kj_5-8 , where k is top, low, and x is a, b, c.

[0054] like Figure 6 , Figure 7 As shown, the input current of the MMC structure sub-module includes a DC component and an AC component. The AC part mainly includes the fundamental frequency and the second harmonic component. Between the lateral SMs of the three-port MMC-MAB topology, the phase of the fundamental frequency component in the fluctuating current is three-phase positive sequence, and the phase of the second harmonic component in the fluctuating current is three-phase negative sequence, both showing three-phase symmetry characteristics.

[0055] As Figure 8 shown, the two modulation signals M1 and M2 are complementary, fixing the duty cycle of the full-bridge control signal at 50%; the magnitude relationship between M1 and M2 determines the positive or negative of the phase shift angle, that is, determines the direction of power transfer. When M1 < M2, the power is transferred forward, otherwise the power is transferred backward. The output signal of the closed-loop control regulator is converted into a modulation wave M. The change of the regulator output signal causes the change of M, thereby realizing the control of the direction and magnitude of the power transfer of the converter.

[0056] The embodiments described above are only descriptions of the preferred embodiments of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention device.

Claims

1. A submodule capacitance optimization control method for an MMC-MAB topology structure, characterized in that: The MMC-MAB topology structure includes a modular level converter structure, a half-bridge and capacitor structure, a full-bridge structure and a multi-active bridge converter structure of a high-frequency link and a three-phase bridge arm; the control method includes carrier phase shift control for the MMC level and dual closed-loop phase angle control for the MAB level; The carrier phase shift control of the MMC level specifically refers to the voltage and current double closed-loop carrier phase shift pulse width modulation strategy adopted by the half-bridge and capacitor structure in the dq rotating coordinate system; The voltage and current dual closed-loop carrier phase-shift pulse width modulation strategy specifically includes the following steps: 1.1, three-phase grid voltage u a ,u b ,u c The phase ωt and three-phase grid voltage u are obtained through the PLL phase-locked loop link. a ,u b ,u c Perform dq transformation with ωt to obtain the d-axis component u d , q-axis component u q , three-phase grid current i a ,i b ,i c Perform dq transformation with ωt to obtain the d-axis component i d ,q-axis component i q ; 1.2, first set the MVDC bus voltage to U Mdcr The actual MVDC bus voltage value U Mdc The difference between the output signal obtained by the PI controller and the component i of the MVAC current on the d-axis is d The output value obtained by a PI controller is the difference between the MVAC voltage component u on the d axis and the d Add them together and subtract the q-axis component i of the MVAC current q *ωL output value to achieve i d Feedforward decoupling; 1.3, MVDC current at a given value i of the q-axis component current qr The actual MVDC current component i in the q axis q The output value obtained after the PI controller is added to the q-axis component of the MVAC voltage, and then the d-axis component of the MVAC current is subtracted. d *ωL to achieve i q Feedforward decoupling; 1.4, input the decoupled values ​​of step 1.2 and step 1.3 into the dq / abc inverse transformation to obtain the three-phase modulation component u a_r 、u b_r 、u c_r , the driving signal Q of the half-bridge and capacitor structure is obtained through carrier phase shift modulation simulation sm ; The dual closed-loop phase angle control of the MAB level specifically refers to the output voltage and input current dual closed-loop phase shift control strategy of the multi-active bridge converter of the full-bridge structure and the high-frequency link; The output voltage and input current dual closed-loop phase-shift control strategy specifically includes the following steps: 2.1, set the voltage setting value U of the low voltage DC bus voltage LVDC_ref Subtract the actual voltage U of the low voltage DC bus LVDC The average value of , and the output value obtained by the PI regulator is used as the given signal of the inner loop; 2.2, the low-frequency fluctuation component of the base frequency and the double frequency superimposed in the output current of the half-bridge structure in the half-bridge and capacitor structure, the low-frequency pulsating signal is extracted by a bandpass filter, and superimposed with the PI regulator output control signal obtained in step 2.1 to obtain a control signal containing a fluctuation component; 2.3, take the control signal with fluctuation component obtained in step 2.2 as the given current of inner loop input, and adjust its waveform to make it the same as the low-frequency fluctuation current waveform of SM. At this time, the actual MAB input current feedback should add the same band-pass filter as that used by SM; 2.4, the difference between the MAB input current obtained in step 2.3 and the given value is used as the input signal of the inner loop PIR regulator. The PIR regulator should include two oscillation links, and the two oscillation links respectively select the fundamental frequency fluctuation component and the double frequency fluctuation component; 2.5, the output signal obtained by the PIR regulator in step 2.4 is used to obtain the final phase shift angle modulation signal, and the driving signal S of the primary and secondary full bridge structure of the multi-active bridge converter is obtained through square wave phase shift modulation. kj_1-4 , S kj_5-8 , where k is top, low, and j is a, b, c.

2. The submodule capacitance optimization control method of a MMC-MAB topology structure according to claim 1, characterized in that: In the MMC-MAB topology, the fluctuating current component i of the half-bridge output fundamental frequency and double frequency superimposed in the half-bridge and capacitor structure f_kj , through the output voltage and input current control double closed-loop phase shift control strategy, it is transmitted to the transformer of the multi-active bridge high-frequency chain. The fluctuating current component presents a three-phase symmetrical characteristic, and is mutually coupled and offset at the high-frequency chain, thereby eliminating the fluctuating power; i f_kj Here k is top and low, and j is a, b, and c.

3. The submodule capacitance optimization control method of a MMC-MAB topology structure according to claim 1, characterized in that: The output current i of the half-bridge structure in the half-bridge and capacitor structure sm_kj Contains DC component i dc The AC component of the fluctuation i f_kj ; Under the traditional control strategy, the fluctuating AC component i f_kj It is absorbed by the large capacitor C of the submodule, and only the DC component i dc The oscillating AC component i f_kj With DC component i dc All are transmitted to the full bridge structure of the subsequent multi-active bridge converter through the control method; i f_kj Here k is top and low, and j is a, b, and c.

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