Solid-state transformer topology based on MMC lightweight submodule and its mixing frequency modulation method

Abstract

The present invention discloses a solid-state transformer topology based on an MMC lightweight submodule and a frequency mixing modulation method thereof, wherein the topology includes: an input stage, an intermediate stage, and an output stage, wherein: the input stage includes a first voltage AC port and a first current DC port, the first voltage AC port being used to connect to three-phase AC power; the output stage includes a second voltage AC port and a second current DC port; the intermediate stage includes a front intermediate stage and a rear intermediate stage, wherein: the front intermediate stage includes a three-phase bridge arm; in the same single-phase bridge arm, the number of submodules of the upper bridge arm is the same as the number of submodules of the lower bridge arm, and the submodules of the upper bridge arm and the submodules of the lower bridge arm are connected to the primary side of the same intermediate stage transformer; the rear intermediate stage includes multiple rectifier circuits. By adopting the above technical solution, the high-frequency and low-frequency pulse currents received by the submodule can be reduced, the capacitance value of the capacitor in the submodule can be reduced, and the lightweight of the submodule can be achieved.

Description

Technical Field

[0001] The present invention relates to the field of power electronics technology, and in particular to a solid-state transformer topology based on an MMC lightweight submodule and a frequency mixing modulation method thereof. Background Art

[0002] The large-scale integration of distributed energy resources into the power grid and the increase in DC loads present significant challenges to traditional AC distribution networks. AC / DC hybrid distribution networks feature ports with multiple voltage levels and configurations, enabling the transmission and conversion of electrical energy between AC / DC, high-voltage, and low-voltage ports. These networks are suitable for integrating renewable energy and DC loads, and represent a key development direction for future distribution networks. Solid-state transformers (SSTs), with their flexible controllability and multi-port access capabilities, have become a core component of AC / DC hybrid distribution networks, particularly in the modular multilevel converter (MMC) topology.

[0003] The invention patent application entitled “MMC-based solid-state transformer topology, mixed frequency modulation and soft switching control method”, which was previously proposed by the applicant of the present invention and has application number CN2024111635327, provides a solid-state transformer topology that can improve the power density of SST (reduce the number of switching devices), and can achieve decoupling of the low-frequency port and the high-frequency port of the input stage of the solid-state transformer topology without the need for a frequency selection network. However, due to the connection method between the submodule and the intermediate stage transformer in the three-phase bridge arm of the solid-state transformer, as well as the related circuit structure, during operation, the solid-state transformer inputs a modulation signal to the submodule to control switching, power transmission, fault crossing, etc., and the high-frequency and low-frequency pulse currents in the modulation signal are a heavy burden on the submodule, especially there will be tripled and doubled frequency pulse currents, which further increase the burden on the submodule. Therefore, a capacitor with a higher capacitance value needs to be set in the submodule for buffering. Summary of the Invention

[0004] Purpose of the invention: The present invention provides a solid-state transformer topology based on an MMC lightweight sub-module and a mixing modulation method thereof, aiming to improve the SST power density (reduce the number of switching devices) and decouple the low-frequency port and the high-frequency port of the solid-state transformer topology input stage without the need for a frequency selection network, and can reduce the high-frequency and low-frequency pulse currents received by the sub-module, reduce the capacitance value of the capacitor in the sub-module, and achieve lightweight sub-module.

[0005] Technical solution: The present invention provides a solid-state transformer topology based on an MMC lightweight submodule, comprising: an input stage, an intermediate stage, and an output stage, wherein: the input stage comprises a first voltage AC port and a first current DC port, the first voltage AC port being used to connect to three-phase AC power; the output stage comprises a second voltage AC port and a second current DC port; the intermediate stage comprises a front intermediate stage and a rear intermediate stage, wherein: the front intermediate stage comprises a three-phase bridge arm, wherein the single-phase bridge arm comprises an upper bridge arm and a lower bridge arm connected in series, a connecting line is drawn from the series connection point of the upper bridge arm and the lower bridge arm, for connecting to the first voltage AC port; three single-phase bridge arms are connected in parallel, and connecting lines are drawn from two parallel connection points, for connecting to the third bridge arm; A current DC port is connected, and the submodule includes two leads, which are connected to the primary side of the intermediate stage transformer; in the same single-phase bridge arm, the number of submodules in the upper bridge arm is the same as the number of submodules in the lower bridge arm, and the submodules in the upper bridge arm and the submodules in the lower bridge arm are connected to the primary side of the same intermediate stage transformer; the post-intermediate stage includes multiple rectifier circuits, and the rectifier circuit includes two leads, a first connecting line and a second connecting line, and the two leads are connected to the secondary side of the intermediate stage transformer, the first connecting lines of the multiple rectifier circuits are connected to each other at a first connection point, and the second connecting lines are connected to each other at a second connection point, the first connection point and the second connection point are connected to the second voltage AC port, and the first connection point and the second connection point are connected to the second current DC port.

[0006] Specifically, the primary side of the same intermediate-stage transformer is only connected to the submodules of the upper bridge arm and the lower bridge arm of the same single-phase bridge arm, and the number of connected upper bridge arm submodules is the same as the number of connected lower bridge arm submodules.

[0007] Specifically, the primary side of the same intermediate-stage transformer is connected to a submodule of the upper bridge arm and a submodule of the lower bridge arm in the same single-phase bridge arm, and the corresponding secondary side of the intermediate-stage transformer is connected to a rectifier circuit.

[0008] Specifically, the submodule includes a first front-stage bridge arm, a second front-stage bridge arm and a capacitor bridge arm connected in parallel. Leads are drawn from the midpoint and parallel point of the first front-stage bridge arm, which are cascaded with the adjacent submodules on the single-phase bridge arm. Leads are drawn from the midpoint of the first front-stage bridge arm and the midpoint of the second front-stage bridge arm respectively, and the two leads are connected to the primary side of the intermediate-stage transformer; a capacitor is provided on the capacitor bridge arm.

[0009] Specifically, the rectifier circuit includes a first rear-stage bridge arm and a second rear-stage bridge arm, the first rear-stage bridge arm and the second rear-stage bridge arm are connected in parallel, a first connecting line and a second connecting line are led out from the two parallel points, leads are respectively led out from the midpoint of the first rear-stage bridge arm and the midpoint of the second rear-stage bridge arm, and the two leads are connected to the secondary side of the intermediate-stage transformer.

[0010] Specifically, the rectifier circuit includes a capacitor connected in parallel with the first rear-stage bridge arm.

[0011] Specifically, the second voltage AC port includes four rear-end bridge arms, each rear-end bridge arm includes two switching devices connected in series, the four rear-end bridge arms are connected in parallel, and the parallel points are respectively connected to the first connection point and the second connection point. A connecting line is led out from the midpoint of each rear-end bridge arm for signal interaction.

[0012] The present invention also provides a mixing modulation method, which applies the solid-state transformer topology based on the MMC lightweight sub-module provided by the present invention, including: superimposing a high-frequency square wave modulation signal with the same carrier frequency and phase angle as the base frequency modulation signal input to the first front-stage bridge arm.

[0013] Specifically, a high-frequency square wave modulation signal having the same frequency as the carrier wave and an opposite phase angle is superimposed on the fundamental frequency modulation signal input to the second front-stage bridge arm.

[0014] Specifically, the switching signal of the switching device in the rectifier circuit is obtained by phase-shifting the high-frequency square wave modulation signal of the input submodule; the delay time between the switching signal of the switching device in the rectifier circuit and the high-frequency square wave modulation signal of the submodule is calculated as follows: after subtracting the reference value and the actual value of the voltage of the second current DC port, the delay time is obtained through PI control.

[0015] Beneficial effects: Compared with the existing technology, the present invention has the following significant advantages: it can reduce the high-frequency and low-frequency pulse currents received by the sub-module, reduce the capacitance value of the capacitor in the sub-module, and achieve lightweight sub-modules. At the same time, it can also improve the SST power density (reduce the number of switching devices), and achieve decoupling of the low-frequency port and the high-frequency port of the solid-state transformer topology input stage without the need for a frequency selection network. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 A schematic diagram of the topology of a solid-state transformer based on an MMC lightweight submodule provided by the present invention;

[0017] FIG2( a ) and FIG2( b ) are schematic diagrams of signal decoupling of a low-frequency port and a high-frequency port of a solid-state transformer topology provided by the present invention, respectively;

[0018] Figure 3 The overall control block diagram of the solid-state transformer topology based on the MMC lightweight submodule provided by the present invention;

[0019] FIG4(a), FIG4(b), and FIG4(c) are simulation verification diagrams of the waveforms of the MVAC port current, MVDC port voltage, and submodule capacitor voltage of the mixing modulation method based on common-mode and differential-mode decoupling of the solid-state transformer topology provided by the present invention, respectively;

[0020] FIG5(a), FIG5(b) and FIG5(c) are schematic diagrams of simulation results of the high-frequency port output voltage, the high-frequency transformer secondary voltage and the phase-shift duty cycle of the solid-state transformer topology provided by the present invention. DETAILED DESCRIPTION

[0021] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0022] See Figure 1 , which is a solid-state transformer topology based on an MMC lightweight submodule provided by the present invention, including: an input stage, an intermediate stage and an output stage, wherein: the input stage includes a first voltage AC port and a first current DC port, the first voltage AC port being used to connect to three-phase AC power; the output stage includes a second voltage AC port and a second current DC port; the intermediate stage includes a front intermediate stage and a rear intermediate stage, wherein: the front intermediate stage includes a three-phase bridge arm, wherein the single-phase bridge arm includes an upper bridge arm and a lower bridge arm connected in series, a connecting line is drawn from the series connection point of the upper bridge arm and the lower bridge arm, for connecting to the first voltage AC port; three single-phase bridge arms are connected in parallel, and connecting lines are drawn from two parallel connection points for connecting to the first current DC port The submodule includes two leads, which are connected to the primary side of the intermediate stage transformer; in the same single-phase bridge arm, the number of submodules in the upper bridge arm is the same as the number of submodules in the lower bridge arm, and the submodules in the upper bridge arm and the submodules in the lower bridge arm are connected to the primary side of the same intermediate stage transformer; the post-intermediate stage includes multiple rectifier circuits, the rectifier circuit includes two leads, a first connecting line and a second connecting line, the two leads are connected to the secondary side of the intermediate stage transformer, the first connecting lines of the multiple rectifier circuits are connected to each other at a first connection point, and the second connecting lines are connected to each other at a second connection point (that is, multiple rectifier circuits are connected in parallel), the first connection point and the second connection point are connected to the second voltage AC port, and the first connection point and the second connection point are connected to the second current DC port.

[0023] In a specific implementation, the input stage MMC provides an MVAC port (a first voltage AC port, i.e., a medium voltage AC port) and an MVDC port (a first current DC port, i.e., a medium voltage DC port). The voltage and current on the MVAC side are u a 、u b 、u c and i a 、i b 、i c , the voltage and current on the MVDC side are u mvdc and i mvdc The intermediate transformer realizes the functions of voltage conversion, power transmission and high-frequency isolation. The rear stage of the transformer provides a LVDC port (second current DC port, i.e. low voltage intermediate current port) in parallel. The port voltage and port current are u lvdc and i lvdc, the LVDC port can be connected to a three-phase / single-phase inverter according to actual needs, and provides an LVAC port (second voltage AC port, i.e. low voltage AC port), the port voltage is u lvac In a hybrid AC / DC distribution network, the MVAC port is used to connect to the three-phase AC distribution network for active and reactive power exchange. The MVDC port can be directly connected to the DC distribution network or connected to the MVDC port of another SST, thereby achieving flexible interconnection between regional power grids, flexible inter-regional power allocation, and new energy consumption at the distribution network level. The LVDC port and LVAC port can connect to low-voltage AC / DC loads, and can also realize the access of different forms of low-voltage distributed energy.

[0024] In a specific implementation, the low voltage and medium voltage of the low voltage port and the medium voltage port represent the voltage comparison between the two, that is, the voltage of the low voltage port is usually lower than the voltage of the medium voltage port, rather than limiting the specific values ​​of the voltages of the low voltage port or the medium voltage port.

[0025] In a specific implementation, the high-frequency port, high-frequency signal and low-frequency port, low-frequency signal, etc. in the present invention refer to the frequency comparison between high frequency and low frequency, that is, the frequency of the high-frequency port (high-frequency signal) is usually higher than the frequency of the low-frequency port (low-frequency signal), rather than referring to a limitation on the specific numerical value of the frequency.

[0026] In the embodiment of the present invention, the primary side of the same intermediate-stage transformer is only connected to the submodules of the upper bridge arm and the lower bridge arm of the same single-phase bridge arm, and the number of connected upper bridge arm submodules is the same as the number of connected lower bridge arm submodules.

[0027] In the embodiment of the present invention, the primary side of the same intermediate stage transformer is connected to a submodule of the upper bridge arm and a submodule of the lower bridge arm in the same single-phase bridge arm, and the corresponding secondary side of the intermediate stage transformer is connected to a rectifier circuit, such as Figure 1 As shown, the transformer primary side au1 interface is connected to the au1 interface of the submodule, and the transformer primary side al2 interface is connected to the al2 interface of the submodule.

[0028] In an embodiment of the present invention, the submodule includes a first front-stage bridge arm, a second front-stage bridge arm and a capacitor bridge arm connected in parallel, and leads are led out from the midpoint and the parallel point (on the side of the switch tube S2) of the first front-stage bridge arm (the two leads constitute an MMC port, that is, a low-frequency port), which is cascaded with the adjacent submodules on the single-phase bridge arm, and leads are led out from the midpoint of the first front-stage bridge arm and the midpoint of the second front-stage bridge arm respectively (the two leads constitute a high-frequency port), and the two leads are connected to the primary side of the intermediate-stage transformer; a capacitor is provided on the capacitor bridge arm.

[0029] In an embodiment of the present invention, the rectifier circuit includes a first rear-stage bridge arm and a second rear-stage bridge arm, the first rear-stage bridge arm and the second rear-stage bridge arm are connected in parallel, a first connecting line and a second connecting line are led out from the two parallel points, leads are respectively led out from the midpoint of the first rear-stage bridge arm and the midpoint of the second rear-stage bridge arm, and the two leads are connected to the secondary side of the intermediate-stage transformer.

[0030] In an embodiment of the present invention, the rectifier circuit includes a capacitor connected in parallel with the first rear-stage bridge arm.

[0031] In an embodiment of the present invention, the second voltage AC port includes four rear-end bridge arms, each rear-end bridge arm includes two switching devices connected in series, the four rear-end bridge arms are connected in parallel, and the parallel points are respectively connected to the first connection point and the second connection point. A connecting line is led out from the midpoint of each rear-end bridge arm for signal interaction.

[0032] In a specific implementation, the rear-stage circuit structure of the submodule and intermediate-stage transformer group consists of a full-bridge rectifier circuit composed of four switching transistors Q1 to Q4 and an output parallel capacitor C0. The front-stage circuit of the submodule and intermediate-stage transformer group consists of four switching transistors S1 to S4 and a capacitor. Switches S1 and S2 are connected in series to form the first front-stage bridge arm, and switches S3 and S4 are connected in series to form the second front-stage bridge arm. Capacitor C1 is connected in parallel to both the first and second front-stage bridge arms. Connecting wires are drawn from the midpoint and end points of the first front-stage bridge arm (the parallel point with the second front-stage bridge arm) to construct the submodule's MMC port, which is used to connect to the remaining submodule groups, thereby achieving input stage cascading. Connecting wires are drawn from the midpoint of the first and second front-stage bridge arms to construct the submodule's high-frequency port, which is connected to the multi-winding transformer to achieve power transmission, conversion, and isolation from the medium-voltage side to the low-voltage side.

[0033] In a specific implementation, in the solid-state transformer topology provided by the present invention, the sub-modules connected to the primary side of the same intermediate-stage transformer are respectively from the upper bridge arm and the lower bridge arm of the same single-phase bridge arm. This is different from the invention patent application previously proposed by the applicant of the present invention, entitled “Solid-state transformer topology, mixing modulation and soft switching control method based on MMC”, with application number CN2024111635327. In this prior invention application, the sub-modules connected to the primary side of the same intermediate-stage transformer are respectively from three single-phase bridge arms. Based on the difference in the above-mentioned circuit structure, a difference in the current flow path is further generated. Through the improvement of the circuit structure, the pulse current originally flowing to the sub-module capacitor is changed to a large amount flowing into the multi-winding transformer. Therefore, the capacitance value required by the sub-module will be greatly reduced. Ideally, the low-frequency pulse voltage on the sub-module capacitor can be completely eliminated, and at the same time, the influence of the high-frequency current of the system is reduced by circulating current control.

[0034] In a specific implementation, within the submodule and intermediate-stage transformer assembly, the first front-stage bridge arm can be considered part of the input-stage MMC submodule, used to enable energy exchange between the MVAC port and the MVDC port, maintaining the stability of the capacitor voltage within the submodule. The first and second front-stage bridge arms can be considered part of the primary H-bridge circuit of the intermediate-stage multi-winding transformer, required to enable energy transfer from the input stage to the intermediate stage. As can be seen from the solid-state transformer topology, the first front-stage bridge arm belongs to both the input-stage MMC and the intermediate-stage multi-winding transformer. During normal operation of the MMC-SST, the input-stage MMC receives a low-frequency AC current and voltage. Therefore, within the submodule and intermediate-stage transformer assembly, the first front-stage bridge arm contains both a high-frequency modulation signal and a low-frequency modulation signal. For the submodule and intermediate-stage transformer assembly, this ensures both the output of the input-stage MMC at a low-frequency AC voltage and the normal operation of the high-frequency transformer in the intermediate-stage DAB (Dual Active Bridge) converter. Given this premise, the MMC-SST provided by the present invention must simultaneously meet the following two conditions.

[0035] (1) For the input stage MMC of SST, the output voltage of the MMC port is only determined by the low-frequency modulation signal, or the output voltage of the MMC bridge arm is only determined by the low-frequency modulation signal.

[0036] (2) For the intermediate stage DAB of SST, the DAB port must output a high-frequency voltage, and the voltage signal of this port is only determined by the high-frequency modulation signal.

[0037] The present invention also provides a mixing modulation method, which applies the solid-state transformer topology based on the MMC lightweight sub-module provided by the present invention, including: superimposing a high-frequency square wave modulation signal with the same carrier frequency and phase angle as the base frequency modulation signal input to the first front-stage bridge arm.

[0038] In the embodiment of the present invention, a high-frequency square wave modulation signal having the same frequency and opposite phase angle as the carrier wave is superimposed on the fundamental frequency modulation signal input to the second front-stage bridge arm.

[0039] In the specific implementation, the baseband modulation signal is obtained by controlling the input stage MMC. The control method is consistent with the existing MMC control method, which specifically includes three aspects: circulating current suppression, overall energy control, and capacitor voltage balance control.

[0040] In an embodiment of the present invention, the switching signal of the switching device in the rectifier circuit is obtained by phase shifting the high-frequency square wave modulation signal of the input submodule; the delay time between the switching signal of the switching device in the rectifier circuit and the high-frequency square wave modulation signal of the submodule is calculated as follows: after taking the difference between the reference value and the actual value of the voltage of the second current DC port, the delay time is obtained through PI control.

[0041] In the specific implementation, by analyzing the input current of the submodules of the upper and lower bridge arms in the conventional MMC single-phase bridge arm, it can be seen that: (1) the two submodules corresponding to the upper and lower bridge arms of the same single-phase bridge arm have the same amplitude of the fundamental frequency and triple frequency pulse current and opposite phases, and the amplitude and phase of the double frequency pulse current are the same; (2) the submodules corresponding to the same horizontal position but not in the same single-phase bridge arm have the same pulse current amplitude, the fundamental frequency current is arranged in positive sequence, the double frequency current is arranged in negative sequence, and the triple frequency current is arranged in zero sequence. It can be further seen that the fundamental frequency and double frequency pulse current amplitudes of the submodules of different phases are the same, and the phase difference is only 120°, while the triple frequency pulse current amplitudes of the submodules of the same phase are the same, and the upper and lower bridge arms have opposite phases. In the present invention, the interconnection of submodules (the upper and lower bridge arm submodules are connected to the same intermediate transformer) allows the pulse current that originally flowed through the submodule capacitor to flow into the multi-winding transformer in large quantities. Based on the characteristics of the same signal amplitude and opposite phase, they can cancel each other out. Therefore, the capacitance required by the submodule is greatly reduced. Ideally, the low-frequency pulse voltage on the submodule capacitor can be completely eliminated, and the influence of the system's high-frequency current can be reduced by circulating current control.

[0042] Therefore, the topology provided by this invention achieves mutual cancellation of the submodule baseband currents, significantly reducing the submodule capacitor voltage pulses. Only a small portion of the current is diverted to the submodule capacitors, reducing the required capacitance value of the submodule capacitors and achieving lightweight submodules.

[0043] In a specific implementation, the topology structure provided by the present invention utilizes the switching capacitor conversion of the sub-modules and the three-phase fluctuating power offset to achieve autonomous balancing of the sub-module voltages and minimization of the capacitance requirements, thereby greatly simplifying the control scheme design and improving the power density of the system.

[0044] Figure 2(a) shows the modulation analysis of the low-frequency port (MMC port) (based on the first front-stage bridge arm). Due to the superposition of a high-frequency square wave signal with the same frequency and phase as the carrier, the switching times t1 and t2 both have a lag related to the amplitude of the high-frequency square wave signal, while the switching time remains unchanged. Therefore, the output voltage of a single submodule at the low-frequency port only has a time lag with respect to the high-frequency signal. Furthermore, because traditional PWM modulation involves phase shifting the carrier, the IGBT operation time in a single-phase inverter also has a lag. Therefore, the output voltage of the low-frequency port is the same as that of the traditional PWM modulation method with phase shifted carrier, indicating that voltage decoupling of the low-frequency port has been achieved.

[0045] As shown in Figure 2(b), the modulation analysis of the high-frequency port (based on the second front-stage bridge arm) is shown. Since the fundamental frequency is superimposed on the high-frequency signal opposite to that of bridge arm 1, the output voltage at both ends of bridge arm 2 is similar to that of bridge arm 1, lagging behind the time related to the high-frequency signal. Since the high-frequency port belongs to the full-bridge output, the fundamental frequency output signals of the high-frequency port cancel each other out, and only the high-frequency signal is output, realizing the decoupled output of the high-frequency port.

[0046] like Figure 3 As shown, this is the overall control block diagram of the solid-state transformer topology based on the MMC sub-module bridge arm multiplexing type provided by the present invention. The baseband modulation signal is obtained by controlling the input stage MMC. The control method is consistent with the existing MMC control method, specifically including three aspects: circulating current suppression, overall energy control, and capacitor voltage balance control. The switching signals of the rear-stage switching tubes Q1 to Q4 of the sub-module and the intermediate-stage multi-winding transformer group are obtained after time delay from the high-frequency modulation signal of the sub-module of the three-phase bridge arm of the front stage. After the voltage reference value and the actual value of the LVDC port are subtracted, the delay time t of the front- and rear-stage high-frequency modulation signals can be obtained through PI control.

[0047] FIG4 is a simulation diagram for verifying the effectiveness of the MMC-SST topology structure and the corresponding mixing modulation method based on common-mode and differential-mode decoupling provided by the present invention. Figure 4(a) to Figure 4(c) The waveforms of the MVAC port current, MVDC port voltage, and submodule capacitor voltage are given respectively. According to the simulation results, the hybrid frequency modulation method provided by the present invention can enable all ports of the solid-state transformer topology provided by the present invention to operate normally, with symmetrical distribution of three-phase current and stable operation. In addition, the output voltage of the MVDC port can be stabilized at 200V, and the fluctuation is almost negligible. When the capacitance of the submodule capacitor is only 5e to 5F, the maximum fluctuation of the capacitor voltage is 360V and the minimum is 250V, realizing the lightweight design of the submodule. Therefore, combined with the simulation results of Figure 4, it can be concluded that the hybrid frequency PWM modulation method provided by the present invention effectively prevents the interference of high-frequency signals on low-frequency ports.

[0048] like Figure 5(a) to Figure 5(c) As shown, the high-frequency port output voltage u is given hal , high frequency transformer secondary voltage u s The results show that the primary voltage u hxi (Figure 5(a) shows a three-level voltage waveform, in which the occurrence time of the positive and negative levels varies in different carrier cycles. However, the duration of these levels remains unchanged within each carrier cycle, which is consistent with the analysis results proposed above. In addition, due to the fundamental frequency fluctuation of the capacitor voltage, it can be hxiThe fundamental frequency fluctuation component is observed in the secondary side of the high-frequency transformer. The secondary side presents a two-level voltage waveform with a phase shift duty cycle d relative to the superimposed high-frequency square wave.

[0049] In a specific implementation, the topology provided by the present invention reduces the number of switching tubes used by 1 / 5 and 1 / 3 respectively compared with the traditional half-bridge topology and full-bridge topology by fully utilizing the structure of the input stage MMC.

Claims

1. A solid-state transformer topology based on MMC lightweight submodule, characterized in that: include: Input stage, intermediate stage, and output stage, where: The input stage includes a first voltage AC port and a first current DC port, wherein the first voltage AC port is used to connect to three-phase AC power; The output stage includes a second voltage AC port and a second current DC port; The intermediate stage includes a front intermediate stage and a rear intermediate stage, wherein: The front intermediate stage includes three-phase bridge arms, wherein the single-phase bridge arm includes an upper bridge arm and a lower bridge arm connected in series, and a connecting line is drawn from the series connection point of the upper bridge arm and the lower bridge arm for connecting to the first voltage AC port; the three single-phase bridge arms are connected in parallel, and connecting lines are drawn from the two parallel connection points for connecting to the first current DC port, and the submodule includes two leads connected to the primary side of the intermediate stage transformer; In the same single-phase bridge arm, the number of submodules in the upper bridge arm is the same as the number of submodules in the lower bridge arm, and the submodules in the upper bridge arm and the submodules in the lower bridge arm are connected to the primary side of the same intermediate-stage transformer; The post-intermediate stage includes multiple rectifier circuits, which include two leads, a first connecting line and a second connecting line. The two leads are connected to the secondary side of the intermediate stage transformer. The first connecting lines of the multiple rectifier circuits are connected to each other at a first connecting point, and the second connecting lines are connected to each other at a second connecting point. The first connecting point and the second connecting point are connected to the second voltage AC port, and the first connecting point and the second connecting point are connected to the second current DC port.

2. The solid-state transformer topology based on the MMC lightweight submodule according to claim 1 is characterized in that: The primary side of the same intermediate-stage transformer is only connected to the sub-modules of the upper bridge arm and the lower bridge arm of the same single-phase bridge arm, and the number of connected upper bridge arm sub-modules is the same as the number of connected lower bridge arm sub-modules.

3. The solid-state transformer topology of the MMC lightweight submodule according to claim 2 is characterized in that: The primary side of the same intermediate-stage transformer is connected to a submodule of the upper bridge arm and a submodule of the lower bridge arm in the same single-phase bridge arm, and the corresponding secondary side of the intermediate-stage transformer is connected to a rectifier circuit.

4. The solid-state transformer topology based on the MMC lightweight submodule according to claim 1 is characterized in that: The submodule includes a first front-stage bridge arm, a second front-stage bridge arm and a capacitor bridge arm connected in parallel. Leads are led out from the midpoint and parallel point of the first front-stage bridge arm, and are cascaded with the adjacent submodules on the single-phase bridge arm. Leads are led out from the midpoint of the first front-stage bridge arm and the midpoint of the second front-stage bridge arm respectively, and the two leads are connected to the primary side of the intermediate-stage transformer; a capacitor is provided on the capacitor bridge arm.

5. The solid-state transformer topology based on the MMC lightweight submodule according to claim 1 is characterized in that: The rectifier circuit includes a first rear-stage bridge arm and a second rear-stage bridge arm, the first rear-stage bridge arm and the second rear-stage bridge arm are connected in parallel, a first connecting line and a second connecting line are led out from the two parallel points, leads are respectively led out from the midpoint of the first rear-stage bridge arm and the midpoint of the second rear-stage bridge arm, and the two leads are connected to the secondary side of the intermediate-stage transformer.

6. The solid-state transformer topology based on the MMC lightweight submodule according to claim 5 is characterized in that: The rectifier circuit includes a capacitor connected in parallel with the first rear-stage bridge arm.

7. The solid-state transformer topology based on the MMC lightweight submodule according to claim 1 is characterized in that: The second voltage AC port includes four rear-end bridge arms, each rear-end bridge arm includes two switching devices in series, the four rear-end bridge arms are connected in parallel, and the parallel points are respectively connected to the first connection point and the second connection point. A connecting line is led out from the midpoint of each rear-end bridge arm for signal interaction.

8. A frequency mixing modulation method, characterized in that: The solid-state transformer topology based on the MMC lightweight submodule according to any one of claims 1 to 7 comprises: A high-frequency square wave modulation signal having the same frequency and phase angle as the carrier wave is superimposed on the fundamental frequency modulation signal input to the first front-stage bridge arm.

9. The frequency mixing modulation method according to claim 8, wherein: include: A high-frequency square wave modulation signal with the same frequency and opposite phase angle as the carrier wave is superimposed on the fundamental frequency modulation signal input to the second front-stage bridge arm.

10. The frequency mixing modulation method according to claim 9, characterized in that: include: The switching signal of the switching device in the rectifier circuit is obtained by phase shifting the high-frequency square wave modulation signal of the input submodule; the delay time between the switching signal of the switching device in the rectifier circuit and the high-frequency square wave modulation signal of the submodule is calculated as follows: after taking the difference between the reference value and the actual value of the voltage of the second current DC port, the delay time is obtained through PI control.

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