Medium-voltage bidirectional direct-current converter based on three-level resonant DAB and control method

Abstract

The invention belongs to a power electronic electric energy conversion technology, and particularly relates to a medium-voltage bidirectional direct-current converter based on a three-level resonance type DAB and a control method, and the medium-voltage bidirectional direct-current converter comprises N branch three-level resonance type DAB direct-current converter units and a modularized multi-level unit, the three-level resonance type DAB direct-current converter units of all branches are connected with a low-voltage direct-current bus side, and the modularized multi-level units are connected with a medium-voltage direct-current bus. A low-voltage port adopts a parallel structure of an NPC topology of a resonance type DAB to be matched with a direct-current low-voltage bus of the new energy motor, and the characteristics of low voltage and large current of the low-voltage direct-current bus are adapted; the current of the low-voltage direct-current bus is uniformly distributed on the input port of each resonance type DAB circuit, so that the current stress of a switching tube is reduced; a high-frequency isolation frequency converter is arranged in the three-level resonance type DAB structure, so that a low-voltage direct-current bus can be electrically isolated from a medium-voltage direct-current bus, and the electricity utilization safety of the low-voltage side is improved.

Description

Technical Field

[0001] The invention belongs to a power electronic electric energy conversion technology, and in particular relates to a medium voltage bidirectional direct current converter based on a three-level resonant DAB and a control method. Background Art

[0002] As the installed capacity of renewable energy power generation continues to grow, the collection technology of renewable energy, as the collection link of renewable energy power generation system, plays a great role in the stable operation of the entire system. Traditional MVAC collection technology has many problems, such as the harmonic pollution generated by the photovoltaic inverter and the grid, and the need for additional rectifiers to power DC loads. Compared with photovoltaic inverters, DC converters do not have low-order harmonic pollution problems, and have the characteristics of simple control, modularization and redundancy. Therefore, the new energy DC boost collection system has gradually attracted the attention of industry and academia.

[0003] The capacity of the new energy DC boost collection system is usually large (MW level), which can be mainly divided into two categories: grid-connected transmission and transformation and off-grid consumption. Grid-connected transmission and transformation is represented by medium-voltage DC collection of offshore wind power and high-voltage DC transmission, with the characteristics of high boost ratio and single-phase transmission. Off-grid consumption is represented by hydrogen production, ammonia production, and DC microgrid distribution, which also has a high boost ratio. Since the system contains energy storage units, most of them are bidirectional transmission. For the DC boost topology with off-grid consumption function, single-module large-capacity topologies such as three-phase DAB, this type of topology is easier to build and debug due to its single-module characteristics, but it does not have redundancy and is not easy to maintain. A high-ratio isolation transformer will be used inside the converter, and there are high requirements for the voltage and current tolerance of the switch tube.

[0004] The researchers also proposed a modular DC converter with parallel input and series output (IPOS) architecture. The wind turbine and photovoltaic array are connected to the input end of the DC boost topology through series and parallel connection, and are connected to the MVDC bus through series output. In terms of branch module selection, there are non-resonant and resonant DAB, LLC, and T2-DAB topologies according to engineering and technical requirements. This modular DC converter with IPOS architecture is mainly suitable for centralized renewable energy power generation. For distributed power generation, another modular DC converter with independent input and series output (IIOS) architecture uses multi-port independent connection of renewable energy motors, avoiding the series and parallel connection of a large number of photovoltaic arrays. The converter unit can realize MPPT and boost functions at the same time, which can significantly reduce system costs. The modules it constitutes are similar to IPOS. Due to the inherent multi-port characteristics of the IIOS architecture, the collection system is more prone to DC side short-circuit failures. Summary of the invention

[0005] The purpose of the present invention is to address the deficiencies of the DC conversion devices of the existing medium-voltage DC collection system, and to propose a medium-voltage bidirectional DC converter and control method based on a three-level resonant DAB that connects a medium-voltage DC bus and a low-voltage DC bus, thereby realizing forward and reverse flow control of energy and improving the flexibility of energy control of the medium-voltage DC collection system.

[0006] To achieve the above-mentioned object, the present invention provides a medium-voltage bidirectional DC converter based on a three-level resonant DAB, comprising three-level resonant DAB DC converter units of N branches and a modular multi-level unit, wherein the three-level resonant DAB DC converter units of all branches are connected to the low-voltage DC bus side, and the modular multi-level unit is connected to the medium-voltage DC bus.

[0007] Furthermore, the primary side of the three-level resonant DAB DC converter unit of each branch adopts a three-level midpoint clamped NPC circuit and the secondary side adopts an H-bridge structure, and the primary side and the secondary side are electrically isolated and converted into electric energy through a resonant cavity; the resonant cavity includes a high-frequency transformer and two series resonant circuits composed of an inductor and a capacitor in series, and the two series resonant circuits are respectively connected to the same-name ends of the primary and secondary sides of the high-frequency transformer; the midpoints of the NPC full bridge are respectively points a and b, and the midpoints of the H bridge are respectively points c and d, wherein points a and c are respectively connected to the same-name end branches of the high-frequency transformer, and points b and d are connected to the non-same-name end branches; the three-level midpoint clamped NPC circuits of all branches are connected in parallel on the low-voltage DC bus side.

[0008] Furthermore, the modular multi-level unit includes N sub-modules, each of which is a half-bridge circuit, a full-bridge circuit, a diode clamped circuit, a hybrid bridge circuit or a double half-bridge clamped circuit; the energy storage capacitor port of each sub-module is cascaded with a three-level resonant DAB DC converter unit, and two adjacent sub-modules are connected in series.

[0009] Furthermore, the medium voltage bidirectional DC converter has two working modes, the first working mode is: the low voltage DC bus supplies power to the medium voltage DC bus, that is, the forward working mode, and the second working mode is: the medium voltage DC bus supplies power to the low voltage DC bus, that is, the reverse working mode; in the forward working mode, the three-level resonant DAB DC converter unit controls the submodule capacitor voltage to be constant, and the submodule controls the medium voltage output voltage to reach the medium voltage bus set value; in the reverse working mode, the submodule controls the capacitor voltage to be constant, and the three-level resonant DAB DC converter unit controls the output voltage to reach the low voltage bus set value.

[0010] Furthermore, the three-level resonant DAB DC converter unit adopts a four-phase shift modulation strategy, and the four phase shifts are: ① the zero-level angle α1 of the three-level waveform of each phase of the primary three-level mid-point clamped NPC circuit; ② the inner phase shift angle α2 between the two phases of the full-bridge of the primary three-level mid-point clamped NPC circuit; ③ the inner phase shift angle α3 between the two phases of the secondary H-bridge topology full-bridge; ④ the outer phase shift angle β between the primary and secondary sides.

[0011] Furthermore, each of the submodules is a half-bridge circuit, adopts a phase shift modulation strategy, and the carrier phase shift is 2π / N, where N is the number of submodules.

[0012] A control method for the medium voltage bidirectional DC converter based on the three-level vibration mode DAB as described above is also provided, including a control method for forward transmission power and a control method for reverse transmission power.

[0013] Furthermore, the forward transmission power control method includes the control of a three-level resonant DAB DC converter unit and the control of a modular multi-level unit; the control of the three-level resonant DAB DC converter unit is the submodule voltage U smk The reference value U of the submodule voltage smref The difference is entered into the PI1 controller, and the output result is the external shift of the three-level resonant DAB DC converter unit of the corresponding branch compared with β k Each branch controls its own output voltage to form a voltage closed-loop control; the control of the modular multi-level unit is divided into voltage mode and current mode. In the voltage mode, the voltage reference value of the medium voltage side is U smref and the medium voltage side sampling value U MV The difference is entered into the PI2 controller, and the closed-loop output result is used as the reference value I of the medium voltage side current MVref ;Reference value of medium voltage side current I MVref And the sampling value of medium voltage current I MV The difference is entered into the PI3 controller, and its output result is used as the common duty cycle D of the submodules of the modular multi-level unit, which is a dual closed-loop control structure of the medium voltage side voltage and current; in the current mode, the reference value I of the medium voltage side current MVref The sampled value of the medium voltage current I MV The difference is entered into the PI4 controller, and its output result is used as the common duty cycle D of the sub-modules of the modular multi-level unit, which is a current closed-loop control structure.

[0014] Furthermore, the reverse transmission power control method includes the control of a three-level resonant DAB DC converter unit and the control of a modular multi-level unit; the control of the three-level resonant DAB DC converter unit is that the low-voltage side voltage sampling value U LV The reference value of the low voltage side voltage is U LVrefThe difference is entered into the PI5 controller, and the output result is the common external shift comparison β of the three-level resonant DAB DC converter unit. c , the shift ratio of each branch uses the common external shift ratio β c , forming a voltage closed-loop control; the control of the modular multi-level unit includes two loops: the total voltage control of the submodule voltage and the voltage balancing control. In the first loop, the reference value of the total submodule voltage is N×U smref , the sum of the submodule voltage sampling is ∑U smk The difference between the sampled value and the reference value is multiplied by -1 and then outputted by the PI6 controller as the medium voltage side current reference value I MVref , medium voltage side current reference value I MVref And the sampling value of medium voltage current I MV The difference is entered into the PI7 controller, and its output result is used as the common duty cycle Dc of the submodules of the modular multi-level unit; in the second loop, that is, the voltage balancing control loop, the submodule voltage U of each branch smk The average value of the submodule voltage U smave After the difference is entered into the PI8 controller, the output result is used as the modified duty cycle ΔD of all submodules. The expression of the average value of the submodule voltage is U smave =∑U smk / N, therefore, the duty cycle of each submodule is D C +ΔD.

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] 1) The low-voltage port adopts the parallel structure of the NPC topology of the resonant DAB to match the DC low-voltage bus of new energy motors (such as wind turbines or photovoltaic arrays), which adapts to the low voltage and high current characteristics of the low-voltage DC bus; the current of the low-voltage DC bus is evenly distributed on the input port of each resonant DAB circuit, reducing the current stress of the switch tube; the three-level resonant DAB structure has a high-frequency isolation inverter inside, so that the low-voltage DC bus can be electrically isolated from the medium-voltage DC bus, improving the power safety on the low-voltage side.

[0017] 2) The medium voltage port adopts modular multi-level units to match the medium voltage DC bus. The number of sub-modules is determined according to the voltage level of the medium voltage DC bus. Multiple sub-modules share the voltage of the medium voltage DC bus, avoiding the use of a single high-ratio isolation transformer. It has the characteristics of redundancy and modularity, and is easy to inspect and maintain.

[0018] 3) An integrated medium-voltage bidirectional DC converter based on three-level resonant DAB can realize the medium-voltage DC collection function of new energy, improving the integration of the new energy power generation and transmission system; the degree of modularization is very high. If some branches fail, they can be bypassed through bypass switches to ensure the reliability of the system and power supply continuity; at the same time, the highly modular design method greatly shortens the design cycle, reduces the design cost, and shows strong flexibility.

[0019] 4) The medium-voltage bidirectional DC converter based on the three-level resonant DAB of the present invention has two working modes, which can be switched according to the energy management requirements of the system; the converter as a whole adopts a master-slave control architecture, which can realize bidirectional power transmission, so as to cooperate with the energy management system for flexible scheduling, and overall improve the stable operation capability of the medium-voltage DC collection system.

[0020] 5) The three-level resonant DAB DC converter unit adopts a four-phase shift modulation strategy, which has four control variables and can be adjusted with multiple degrees of freedom to achieve optimization functions such as optimization of the effective value and amplitude of the high-frequency current and widening the soft switching range; the modular multi-level unit adopts a carrier phase shift modulation strategy, which can effectively multiply the current ripple of the medium-voltage DC by N times, thereby achieving the function of reducing the current ripple.

[0021] 6) Forward power transmission can adopt output voltage control and output current control modes to realize parallel connection of multiple integrated three-level resonant DAB medium-voltage bidirectional DC converters on the medium-voltage DC bus. One of them provides voltage support for the medium-voltage bus through voltage control, and the others work in current control mode to supply energy to the medium-voltage bus. Reverse power transmission is achieved through low-voltage bus voltage control, and the medium-voltage side adopts sub-module voltage balancing and voltage control, which can realize reverse power transmission to supply energy to the energy storage unit of the low-voltage bus and maintain the stability of the low-voltage bus voltage. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a topological structure diagram of a medium voltage bidirectional DC converter based on a three-level resonant DAB according to the present invention;

[0023] Figure 2 for Figure 1 Middle modular multi-level unit topology diagram;

[0024] Figure 3 for Figure 1 The topological structure diagram of the three-level resonant DAB DC converter unit;

[0025] Figure 4 It is the four-degree-of-freedom modulation diagram of the three-level resonant DAB DC converter unit;

[0026] Figure 5Carrier phase shift modulation for modular multi-level units;

[0027] Figure 6 for Figure 1 A method for controlling forward transmission power;

[0028] Figure 7 for Figure 1 A method for controlling reverse transmission power. DETAILED DESCRIPTION

[0029] The specific implementation modes of the present invention are described below in conjunction with the accompanying drawings. The following description is only for example purposes and does not limit the application scope of the present invention.

[0030] like Figure 1 The medium-voltage bidirectional DC converter based on the three-level resonant DAB shown in the figure includes three-level resonant DAB DC converter units of N branches and a modular multi-level unit (i.e., MMC circuit). The three-level resonant DAB DC converter units of all branches are connected to the low-voltage DC bus, and the modular multi-level unit is connected to the medium-voltage DC bus. The medium-voltage bidirectional DC converter has the ability of bidirectional energy conversion.

[0031] Combination Figure 3 The primary side of the three-level resonant DAB DC converter unit of each branch shown in the figure adopts a neutral three-level midpoint clamped NPC circuit, and the secondary side adopts an H-bridge structure. The primary and secondary sides are electrically isolated and converted into electric energy through a resonant cavity. The resonant cavity includes a high-frequency transformer and two series resonant circuits composed of an inductor and a capacitor in series. The two series resonant circuits are connected to the same-named ends of the primary and secondary sides of the high-frequency transformer respectively. The midpoints of the NPC full bridge are points a and b, and the midpoints of the H bridge are points c and d, respectively. Points a and c are connected to the same-named end branches of the high-frequency transformer, and points b and d are connected to the non-same-named end branches. The three-level midpoint clamped NPC circuits of all branches are connected in parallel on the low-voltage DC bus side. Figure 2 The modular multi-level unit shown includes N sub-modules, each of which is a half-bridge circuit, a full-bridge circuit, a diode clamped circuit, a hybrid bridge circuit or a double half-bridge clamped circuit; the energy storage capacitor port of each sub-module is cascaded with a three-level resonant DAB DC converter unit, and two adjacent sub-modules are connected in series.

[0032] That is, the medium voltage bidirectional DC converter has two working modes. The first working mode is: the low voltage DC bus supplies power to the medium voltage DC bus, which is the forward working mode. The second working mode is: the medium voltage DC bus supplies power to the low voltage DC bus, which is the reverse working mode. In the forward working mode, the three-level resonant DAB DC converter unit controls the submodule capacitor voltage to be constant, and the submodule controls the medium voltage output voltage to reach the set value of the medium voltage bus. In the reverse working mode, the submodule controls the capacitor voltage to be constant, and the three-level resonant DAB DC converter unit controls the output voltage to reach the set value of the low voltage bus.

[0033] The three-level resonant DAB DC converter unit adopts a four-phase shift modulation strategy, such as Figure 4 The four phase shifts shown are: ① the zero level angle α1 of each phase of the three-level waveform of the primary three-level midpoint clamped NPC circuit; ② the internal phase shift angle α2 between the two phases of the primary three-level midpoint clamped NPC circuit full bridge; ③ the internal phase shift angle α3 between the two phases of the secondary H-bridge topology full bridge; ④ the external phase shift angle β between the primary and secondary sides. Figure 5 As shown, when each submodule is a half-bridge circuit, the carrier phase shift using the phase shift modulation strategy is 2π / N, where N is the number of submodules.

[0034] The control method of the medium voltage bidirectional DC converter based on the three-level vibration mode DAB includes a control method for forward transmission power and a control method for reverse transmission power. Among them, the control method of the energy forward transmission mode is divided into a medium voltage DC voltage control mode and a medium voltage DC current control mode; the control method of the energy reverse transmission mode is a low voltage voltage control mode. The following is further explained in conjunction with the accompanying drawings.

[0035] like Figure 6 The control method of the forward transmission power shown includes the control of the three-level resonant DAB DC converter unit and the control of the modular multi-level unit; the control of the three-level resonant DAB DC converter unit is the sub-module voltage U of each branch smk The reference value U of the submodule voltage smref The difference is entered into the PI1 controller, and the output result is the external shift of the three-level resonant DAB DC converter unit of the corresponding branch compared with β k Each branch controls its own output voltage to form a voltage closed-loop control; the control of the modular multi-level unit is divided into voltage mode and current mode. In the voltage mode, the voltage reference value of the medium voltage side is U smref and the medium voltage side sampling value U MV The difference is entered into the PI2 controller, and the closed-loop output result is used as the reference value I of the medium voltage side current MVref ;Reference value of medium voltage side current I MVref And the sampling value of medium voltage current I MVThe difference is entered into the PI3 controller, and its output result is used as the common duty cycle D of the submodules of the modular multi-level unit, which is a dual closed-loop control structure of the medium voltage side voltage and current; in the current mode, the reference value I of the medium voltage side current MVref The sampled value of the medium voltage current I MV The difference is entered into the PI4 controller, and its output result is used as the common duty cycle D of the sub-modules of the modular multi-level unit, which is a current closed-loop control structure.

[0036] like Figure 7 The reverse transmission power control method shown includes the control of a three-level resonant DAB DC converter unit and the control of a modular multi-level unit; the control of the three-level resonant DAB DC converter unit is that the low-voltage side voltage sampling value U LV The reference value of the low voltage side voltage is U LVref The difference is entered into the PI5 controller, and the output result is the common external shift comparison β of the three-level resonant DAB DC converter unit. c , the shift ratio of each branch uses the common external shift ratio β c , forming a voltage closed-loop control; the control of the modular multi-level unit includes two loops: the total voltage control of the submodule voltage and the voltage balancing control. In the first loop, the reference value of the total submodule voltage is N×U smref , the sum of the submodule voltage sampling is ∑U smk The difference between the sampled value and the reference value is multiplied by -1 and then outputted by the PI6 controller as the medium voltage side current reference value I MVref , medium voltage side current reference value I MVref And the sampling value of medium voltage current I MV The difference is entered into the PI7 controller, and its output result is used as the common duty cycle Dc of the submodules of the modular multi-level unit; in the second loop, that is, the voltage balancing control loop, the submodule voltage U of each branch smk The average value of the submodule voltage U smave After the difference is entered into the PI8 controller, the output result is used as the modified duty cycle ΔD of all submodules. The expression of the average value of the submodule voltage is U smave =∑U smk / N, therefore, the duty cycle of each submodule is D C +ΔD.

[0037] The following is a further description with reference to the accompanying drawings and specific embodiments.

[0038] The low voltage side of the medium voltage bidirectional DC converter based on three-level resonant DAB is 1800V, the medium voltage DC bus voltage is 20kV, and the number of branches is set to N = 26. The high frequency transformer ratio of each branch is 1.8:1, so the capacitor voltage value of the submodule is 1800 / 1.8 = 1000V. Figure 2 shown.

[0039] The three-level resonant DAB structure is used to match the low-voltage DC bus. Figure 3 As shown, the first and third switches of the NPC module are complementarily turned on, and their duty cycles are 0.5-α1 / π and 0.5+α1 / π. The second and fourth switches are complementarily turned on, and their duty cycles are 0.5+α1 / π and 0.5-α1 / π. The upper and lower switches in the H-bridge are complementarily turned on, and the duty cycle of each H-bridge switch is fixed at 0.5. Due to the use of four-phase shift modulation, the high-frequency waveform output by the primary side can achieve up to five-level modulation, and the secondary side output waveform is three-level modulation. The power size and direction of the converter are controlled by controlling the external phase shift angle β. The internal phase shift angle can achieve optimized modulation. As shown Figure 5 As shown, the proposed quadruple phase-shift modulation method.

[0040] For MMC modules, phase-shift modulation is used. Phase-shift modulation means that each half-bridge submodule is given the same duty cycle, and the switching signal of each half-bridge submodule is phase-shifted by 2π / N (rad) in turn, where N is the number of branches in the system. When phase-shift modulation is used, the fluctuation frequency of the output voltage signal is a multiple of the switching signal frequency. That is, the switching frequency can be equivalently increased, thereby reducing current and voltage ripples. The modular multi-level topology structure that matches the medium-voltage DC bus, such as Figure 4 As shown, this is the carrier phase shift modulation method when N=4.

[0041] During forward transmission, the resonant DAB module of each branch absorbs energy from the low-voltage bus and establishes the voltage of the sub-module. The MMC circuit absorbs energy from the sub-module and transfers the energy to the medium-voltage DC bus to achieve the purpose of medium-voltage collection. During reverse transmission, each branch absorbs energy from the low-voltage bus and establishes the voltage of the sub-module. The MMC circuit absorbs energy from the sub-module and transfers the energy to the medium-voltage DC bus to achieve the purpose of medium-voltage collection. During reverse transmission, the medium-voltage bus transmits energy to the sub-module capacitor and establishes a stable voltage. The resonant DAB module absorbs energy from the sub-module of the previous level and transfers the energy to the low-voltage DC bus. The control method is as follows: Figure 5 and Figure 6 As shown, no further details are given.

Claims

1. A medium voltage bidirectional DC converter based on three-level resonant DAB, characterized in that: It comprises three-level resonant DAB DC converter units of N branches and a modular multi-level unit. The three-level resonant DAB DC converter units of all branches are connected to the low-voltage DC bus side, and the modular multi-level unit is connected to the medium-voltage DC bus.

2. The medium voltage bidirectional DC converter based on three-level vibration mode DAB according to claim 1, characterized in that: The primary side of the three-level resonant DAB DC converter unit of each branch adopts a three-level midpoint clamped NPC circuit and the secondary side adopts an H-bridge structure, and the primary side and the secondary side are electrically isolated and converted into electric energy through a resonant cavity; the resonant cavity includes a high-frequency transformer and two series resonant circuits composed of an inductor and a capacitor in series, and the two series resonant circuits are respectively connected to the same-named ends of the primary and secondary sides of the high-frequency transformer; the midpoints of the NPC full bridge are respectively points a and b, and the midpoints of the H bridge are respectively points c and d, wherein points a and c are respectively connected to the same-named end branches of the high-frequency transformer, and points b and d are connected to the non-same-named end branches; the three-level midpoint clamped NPC circuits of all branches are connected in parallel on the low-voltage DC bus side.

3. The medium voltage bidirectional DC converter based on three-level vibration mode DAB according to claim 2, characterized in that: The modular multi-level unit includes N sub-modules, each of which is a half-bridge circuit, a full-bridge circuit, a diode clamped circuit, a hybrid bridge circuit or a double half-bridge clamped circuit; the energy storage capacitor port of each sub-module is cascaded with a three-level resonant DAB DC converter unit, and two adjacent sub-modules are connected in series.

4. The medium voltage bidirectional DC converter based on three-level vibration mode DAB according to claim 3, characterized in that: The medium voltage bidirectional DC converter has two working modes, the first working mode is: the low voltage DC bus supplies power to the medium voltage DC bus, that is, the forward working mode, and the second working mode is: the medium voltage DC bus supplies power to the low voltage DC bus, that is, the reverse working mode; in the forward working mode, the three-level resonant DAB DC converter unit controls the submodule capacitor voltage to be constant, and the submodule controls the medium voltage output voltage to reach the medium voltage bus set value; in the reverse working mode, the submodule controls the capacitor voltage to be constant, and the three-level resonant DAB DC converter unit controls the output voltage to reach the low voltage bus set value.

5. The medium voltage bidirectional DC converter based on three-level vibration mode DAB according to claim 2, characterized in that: The three-level resonant DAB DC converter unit adopts a four-phase shift modulation strategy, and the four phase shifts are: ① the zero-level angle α1 of each phase of the three-level waveform of the primary three-level mid-point clamped NPC circuit; ② the inner phase shift angle α2 between the two phases of the full-bridge of the primary three-level mid-point clamped NPC circuit; ③ the inner phase shift angle α3 between the two phases of the secondary H-bridge topology full-bridge; ④ the outer phase shift angle β between the primary and secondary sides.

6. The medium voltage bidirectional DC converter based on three-level vibration mode DAB according to claim 3, characterized in that: Each of the submodules is a half-bridge circuit, adopts a phase shift modulation strategy, and the carrier phase shift is 2π / N, where N is the number of submodules.

7. A control method for a medium voltage bidirectional DC converter based on a three-level vibration mode DAB as claimed in claim 1, characterized in that: The control method includes a control method for forward transmission power and a control method for reverse transmission power.

8. The control method according to claim 7, characterized in that: The forward transmission power control method includes the control of a three-level resonant DAB DC converter unit and the control of a modular multi-level unit; the control of the three-level resonant DAB DC converter unit is to control the submodule voltage U of each branch smk The reference value U of the submodule voltage smref The difference is entered into the PI1 controller, and the output result is the external shift of the three-level resonant DAB DC converter unit of the corresponding branch compared with β k Each branch controls its own output voltage to form a voltage closed-loop control; the control of the modular multi-level unit is divided into voltage mode and current mode. In the voltage mode, the voltage reference value of the medium voltage side is U smref and the medium voltage side sampling value U MV The difference is entered into the PI2 controller, and the closed-loop output result is used as the reference value I of the medium voltage side current MVref ; Reference value of medium voltage side current I MVref And the sampling value of medium voltage current I MV The difference is entered into the PI3 controller, and its output result is used as the common duty cycle D of the submodules of the modular multi-level unit, which is a double closed-loop control structure of the voltage and current on the medium voltage side; In current mode, the reference value of the medium voltage side current I MVref The sampled value of the medium voltage current I MV The difference is entered into the PI4 controller, and its output result is used as the common duty cycle D of the sub-modules of the modular multi-level unit, which is a current closed-loop control structure.

9. The control method according to claim 7, characterized in that: The reverse transmission power control method includes the control of a three-level resonant DAB DC converter unit and the control of a modular multi-level unit; the control of the three-level resonant DAB DC converter unit is as follows: the low-voltage side voltage sampling value U LV The reference value of the low voltage side voltage is U LVref The difference is entered into the PI5 controller, and the output result is the common external shift comparison β of the three-level resonant DAB DC converter unit. c , the shift ratio of each branch uses the common external shift ratio β c , forming a voltage closed-loop control; the control of the modular multi-level unit includes two loops: the total voltage control of the submodule voltage and the voltage balancing control. In the first loop, the reference value of the total submodule voltage is N×U smref , the sum of the submodule voltage sampling is ∑U smk The difference between the sampled value and the reference value is multiplied by -1 and then outputted by the PI6 controller as the medium voltage side current reference value I MVref , medium voltage side current reference value I MVref And the sampling value of medium voltage current I MV The difference is entered into the PI7 controller, and its output result is used as the common duty cycle Dc of the submodules of the modular multi-level unit; in the second loop, that is, the voltage balancing control loop, the submodule voltage U of each branch smk The average value of the submodule voltage U smave After the difference is entered into the PI8 controller, the output result is used as the modified duty cycle ΔD of all submodules. The expression of the average value of the submodule voltage is U smave =∑U smk / N, therefore, the duty cycle of each submodule is D C +ΔD.

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