Modulation and control method for modular multilevel dc transformer

Abstract

The present application relates to the field of direct current transformer, in particular to a kind of modular multilevel direct current transformer modulation and control method, by controlling the phase shift angle size between different half-bridge module drive signals of half-bridge module valve group, modulation generates the asymmetric ladder wave with two rising slopes and falling slopes, and by detecting the output voltage size, the rising slope and the falling slope of the asymmetric ladder wave of modular multilevel direct current transformer half-bridge module valve group output are changed and the duty cycle of full-bridge module switch tube, so as to adjust the output voltage of modular multilevel direct current transformer.The modulation and control method realizes the fixed-frequency operation of resonant circuit, reduces the design difficulty of magnetic element, while realizing the zero-current turn-off of full-bridge module switch tube on the low-voltage side of modular multilevel direct current transformer, can reduce current stress and improve the working efficiency of converter.

Description

Technical Field

[0001] This invention relates to the field of DC transformers, specifically a modular multilevel DC transformer modulation and control method. Background Technology

[0002] With the popularization of renewable energy and the development of DC loads, DC transmission and distribution technologies are rapidly developing. DC transformers can interconnect DC transmission and distribution networks of different voltage levels, improving the flexibility of DC transmission and distribution, but they suffer from large size and low power density in practical applications. MMC (Modular Multilevel Converter) structures can reduce device voltage stress and have advantages such as modularity and good fault handling capabilities, and are widely used in high-voltage DC transmission and power electronic transformers.

[0003] By combining the MMC structure and resonant characteristics, a resonant modular multilevel DC transformer can be obtained, achieving high-voltage conversion while ensuring operational efficiency. However, traditional resonant converters typically adjust the output voltage by changing the frequency, and the highest efficiency of a resonant converter usually occurs near the resonant point. Therefore, adjusting the output voltage by changing the frequency makes it difficult to guarantee high operational efficiency simultaneously. Furthermore, when using frequency adjustment to control the output voltage, the magnetic component design needs to accommodate all operating frequencies, which leads to design difficulties for the magnetic components. Summary of the Invention

[0004] To address the shortcomings mentioned in the background section, the present invention aims to provide a modular multilevel DC transformer modulation and control method. This invention achieves constant-frequency output voltage regulation of the modular multilevel resonant converter by controlling the output of an asymmetric stepped wave with two rising and falling slopes from the modular multilevel structure.

[0005] The objective of this invention can be achieved through the following technical solutions:

[0006] A modular multilevel DC transformer modulation and control method, wherein the modular multilevel DC transformer consists of a high-voltage filter inductor L HV Half-bridge modular valve assembly, resonant inductor L R Resonant capacitor C R It consists of a transformer T and a full-bridge module. The half-bridge module valve group comprises N half-bridge modules and a valve group inductor L. g The two modules are connected in series, where N is an integer and N≥1. The upper end of the half-bridge module valve group is connected to the upper end of the primary winding of the transformer via a resonant capacitor and a resonant inductor; the lower end of the half-bridge module valve group is connected to the lower end of the primary winding of the transformer, and the secondary winding of the transformer is connected to the input port of the full-bridge module; half-bridge module valve group A and half-bridge module valve group B are connected in series and then connected to the high-voltage filter inductor L. HVThe outputs of full-bridge module A and full-bridge module B are connected in parallel to form a low-voltage DC port, thus forming a high-voltage DC port.

[0007] The modular multilevel DC transformer modulation method is as follows: the half-bridge module valve group modulates and generates an asymmetric stepped wave with two rising slopes and falling slopes by changing the phase shift angle between the half-bridge modules; half-bridge module valve group A and half-bridge module valve group B operate with phase shifting; the full-bridge module adopts duty cycle control, and full-bridge module A and full-bridge module B operate with phase shifting.

[0008] Furthermore, in the modular multilevel DC transformer modulation method, the phase difference between different half-bridge module valve groups can be configured to be 0 to 360°, and the corresponding phase difference between different full-bridge modules can be configured to be 0 to 360°.

[0009] Furthermore, the modulation method includes the following steps:

[0010] Step 1: Based on the required output voltage amplitude V of the half-bridge module valve group S Determine the number K of constant-input submodules required in one switching cycle, where K is an integer and 0 ≤ K < N. Also determine the amplitude V of the half-bridge module valve group output voltage. S Represented as:

[0011]

[0012] Step 2: The half-bridge module valve group modulates and generates an asymmetric stepped wave with two rising and falling slopes; the drive signal of the switching transistor on the half-bridge module in the half-bridge module valve group is denoted as g. i1 ~g iN i = 1, 2, the upper and lower switches of the half-bridge module work complementaryly; drive signal g i1 ~g iK The duty cycle is 100%, and the drive signal g i(K+1) ~g iN The duty cycle is 50%; the drive signal g of the half-bridge module i(K+1) ~g iN Introducing a phase shift angle, if NK-1 is even, then the half-bridge module drive signal g... i(K+1) ~g i((N+K+1) / 2) The phase shift angle is θ2, and the half-bridge module drive signal g i((N+K+1) / 2) ~g iN If the phase shift angle is θ1, and NK-1 is odd, then the half-bridge module drive signal g i(K+1) ~g i((N+K) / 2) The phase shift angle is θ2, and the half-bridge module drive signal g i((N+K) / 2) ~g i((N+K) / 2+1) The phase shift angle is (θ1+θ2) / 2, and the half-bridge module drive signal g i((N+K) / 2+1) ~g iNThe phase shift angle is θ1.

[0013] Step 3: The full-bridge module uses duty cycle control, denoted as D; the first switching transistor Q... i1 With the fourth switch Q i4 Simultaneously activated; second switch Q i2 With the third switch Q i3 Simultaneously activated, the third switch Q... i3 Lag first switch Q i1 Turn-on in half a switching cycle; first switch Q i1 With half-bridge submodule drive signal g i1 The phase shift angle between them is θ2(NK-1) / 2.

[0014] Step 4: Control the switching transistors of the half-bridge module and the full-bridge module according to the drive signal with the above characteristics.

[0015] Furthermore, the driving signal g i(K+1) ~g iN The duty cycle can be set to any value greater than 0 and less than 100%;

[0016] The modular multilevel DC transformer control method is as follows: the output voltage of the modular multilevel DC transformer is adjusted by changing the rising and falling slopes of the asymmetric stepped wave output by the half-bridge module valve group and the duty cycle of the full-bridge module switching transistor.

[0017] Furthermore, the control method includes the following steps:

[0018] Step 1: Measure the high-voltage DC port voltage V in the modular multilevel DC transformer. HV Low-voltage DC port voltage V LV Perform sampling.

[0019] Step 2: Sample the high-voltage DC port voltage V HV With reference voltage V HV_ref The values ​​are compared, and the output voltage difference is PI-regulated. The resulting output is used as the power transmission setpoint p of the modular multilevel DC transformer. t *

[0020] Step 3: Based on the given power transmission value p of the DC transformer t * High-voltage DC port voltage V HV and low-voltage DC port voltage V LV The duty cycle d of the rising edge of the output voltage of the half-bridge module valve group is obtained. N1 and the duty cycle d of the falling edge N2 And the duty cycle D of the full-bridge module switching transistors.

[0021] Step 4: Based on the number K of constant input sub-modules, the duty cycle d of the rising edge of the half-bridge module valve group output voltage obtained in Step 3. N1 and the duty cycle d of the falling edge N2 The full-bridge module switching transistors' duty cycle D is used for modulation and voltage equalization control to control the DC transformer's output voltage. The duty cycle d of the rising edge of the half-bridge module's valve group output voltage is also used. N1 and the duty cycle d of the falling edge N2 The phase shift angles θ1 and θ2 of the half-bridge module drive signal can be calculated:

[0022]

[0023]

[0024] Furthermore, in step 2, the given value p of the modular multilevel DC transformer transmission power... t *It can also be obtained by comparing the low-voltage DC port voltage, transmission power with the corresponding reference value, and then adjusting the comparison value using a PI controller.

[0025] The beneficial effects of this invention are:

[0026] 1. The modular multilevel DC transformer modulation and control method provided by the present invention can ensure that the low-voltage side switching tube of the DC transformer can achieve zero-current switching by controlling the slope of the asymmetric stepped half-wave output of the half-bridge module valve group, thereby realizing the constant frequency operation of the resonant converter and improving the working efficiency of the converter.

[0027] 2. The modular multilevel DC transformer modulation and control method provided by the present invention provides two control degrees of freedom for the modular multilevel DC transformer through the asymmetric stepped half-wave modulation of the half-bridge modular valve group, which can further reduce current stress, improve the working characteristics of the modular multilevel DC transformer, and achieve higher working efficiency. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 This is a schematic diagram of the modular multilevel DC transformer topology of the present invention;

[0030] Figure 2 This is a schematic diagram of the principle of the half-bridge module in the half-bridge module valve group of the present invention;

[0031] Figure 3This is a schematic diagram of the modulation waveform of the modular multilevel structure DC transformer of the present invention. Figure 1 ;

[0032] Figure 4 This is a schematic diagram of the basic operating waveforms of the multi-level DC transformer of the present invention. Figure 1 ;

[0033] Figure 5 This is a schematic diagram of the basic operating waveforms of the multi-level DC transformer of the present invention. Figure 2 ;

[0034] Figure 6 This is a schematic diagram of the modulation waveform of the modular multilevel structure DC transformer of the present invention. Figure 2 ;

[0035] Figure 7 This is a diagram of the modular multilevel DC transformer output voltage control architecture of the present invention;

[0036] Figure 8 This is a schematic diagram of another modular multilevel DC transformer topology according to the present invention.

[0037] Figure 9 This is a schematic diagram of the modulation waveform of the modular multilevel structure DC transformer of the present invention. Figure 3 ;

[0038] Figure 10 This is a schematic diagram of the modulation waveform of the modular multilevel structure DC transformer of the present invention. Figure 4 ; Detailed Implementation

[0039] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0040] A modular multilevel DC transformer topology, such as Figure 1-2 As shown, the topology consists of a high-voltage filter inductor L HV Half-bridge modular valve assembly, resonant inductor L R Resonant capacitor C R It consists of a transformer T and a full-bridge module. The half-bridge module valve group comprises N half-bridge modules and a valve group inductor L. g The series is formed, where N is an integer and N≥1.

[0041] The upper end of the half-bridge modular valve group A is connected to a resonant capacitor C. RA Resonant inductor L RA Connecting transformer TA The upper end of the primary winding; the lower end of the half-bridge module valve group A is connected to transformer T. A The lower end of the primary winding, transformer T A The secondary winding is connected to the input port of the full-bridge module A.

[0042] The upper end of the half-bridge modular valve group B is connected to a resonant capacitor C. RB Resonant inductor L RB Connecting transformer T B The upper end of the primary winding; the lower end of the half-bridge module valve group B is connected to the transformer T. B The lower end of the primary winding, transformer T B The secondary winding is connected to the input port of the full-bridge module B.

[0043] Half-bridge modular valve group A and half-bridge modular valve group B are connected in series and then connected to a high-voltage filter inductor L. HV The outputs of full-bridge module A and full-bridge module B are connected in parallel to form a low-voltage DC port, thus forming a high-voltage DC port.

[0044] like Figure 3 The image shows the working waveform 1 of the asymmetric step modulation method with two rising and falling slopes proposed in this invention. The specific modulation method is as follows:

[0045] (1) Based on the required output voltage amplitude V of the half-bridge module valve group S Determine the number K of constant-input submodules required in one switching cycle, where K is an integer and 0 ≤ K < N. Also determine the amplitude V of the half-bridge module valve group output voltage. S Represented as:

[0046]

[0047] (2) The half-bridge module valve group modulates and generates an asymmetric stepped half-wave with two rising and falling slopes. The switching transistor drive signal on the half-bridge module in the half-bridge module valve group is denoted as g. i1 ~g iN With i = 1 and 2, the upper and lower switches of the half-bridge module work complementaryly. Drive signal g 11 ~g 1K and drive signal g 21 ~g 2K The duty cycle is 100%, and the drive signal g 1(K+1) ~g 1N and drive signal g 2(K+1) ~g 2N It is 50%; in the half-bridge module drive signal g 1(K+1) ~g 1N Interval and drive signal g 2(K+1) ~g 2N A phase shift angle is introduced; if NK-1 is even, then the half-bridge module drive signal gi(K+1) ~g i((N+K+1) / 2) The phase shift angle is θ2, and the half-bridge module drive signal g i((N+K+1) / 2) ~g iN The phase shift angle is θ1; if NK-1 is odd, then the half-bridge module drive signal g i(K+1) ~g i((N+K) / 2) The phase shift angle is θ2, and the half-bridge module drive signal g i((N+K) / 2) ~g i((N+K) / 2+1) The phase shift angle is (θ1+θ2) / 2, and the half-bridge module drive signal g i((N+K) / 2+1) ~g iN The phase shift angle is θ1. The drive signal g for the half-bridge module valve group A. 11 ~g 1K The drive signal g corresponding to the half-bridge module valve group B 21 ~g 2K Phase shift 180°;

[0048] (3) The full-bridge module adopts duty cycle control, denoted as D; the first switching transistor Q i1 With the fourth switch Q i4 Simultaneously activated; second switch Q i2 With the third switch Q i3 Simultaneously activated, the third switch Q... i3 Lag first switch Q i1 Turn-on after half a switching cycle. First switch Q... i1 With half-bridge module drive signal g i1 The phase shift angle between them is θ1(NK-1) / 2.

[0049] Full-bridge module A drive signal Q 11 ~Q 14 The drive signal Q corresponding to the full-bridge module B 21 ~Q 24 Interphase shift 180°

[0050] like Figure 4 The figure shows the basic operating waveform 1 of the modulation method provided by the present invention, and the operating timing is as follows:

[0051] (1) Before time t0, all switches of the full-bridge module are locked and the resonant current is 0.

[0052] (2) At time t0, the first switching transistor Q of the full-bridge module 11 and the fourth switch Q 14 Turn on; the output voltage of the half-bridge module valve group is equal to V. HV / 2, the output voltage of the half-bridge module valve group rises, and the slope of the rising edge of the output voltage of the half-bridge module valve group is V. S / (d N1 T sThe duty cycle of the voltage rising at this slope is d. N1 The resonant current begins to increase.

[0053] (3) At time t1, the output voltage of the half-bridge module valve group rises to the set value;

[0054] (4) At time t2, the resonant current is less than 0, the output voltage of the half-bridge module valve group decreases, and the slope of the voltage drop is -V. S / (d N2 T s The duty cycle of the voltage decreasing at this slope is d. N2 ;

[0055] (5) At time t3, the resonant current is equal to 0, and the first switching transistor Q of the full-bridge module is switched. 11 and the fourth switch Q 14 Achieve zero-current shutdown;

[0056] (6) At time t4, the second switch Q of the full-bridge module... 12 and the third switch Q 13 When turned on, the output voltage of the half-bridge module valve group is equal to V. HV / 2, the output voltage of the half-bridge module valve group continues to decrease, and the slope of the voltage drop is -V. S / (d N1 T s The duty cycle of the voltage decreasing at this slope is d. N1 ;

[0057] (7) At time t5, the output voltage of the half-bridge module valve group drops to the set value;

[0058] (8) At time t6, the resonant current is greater than 0, the output voltage of the half-bridge module valve group rises, and the slope of the rising edge of the output voltage of the half-bridge module valve group is V. S / (d N2 T s The duty cycle of the voltage rising at this slope is d. N2 The resonant current begins to increase.

[0059] (9) At time t7, the resonant current is equal to 0, and the second switch Q of the full-bridge module is... 12 and the third switch Q 14 Achieve zero-current shutdown;

[0060] (10) At time t8, the output voltage of the half-bridge module valve group rises to V. HV / 2;

[0061] like Figure 5The figure shows the basic operating waveform 2 of the modulation method provided by the present invention, and the operating timing is as follows:

[0062] (1) Before time t0, all switches of the full-bridge module are locked and the resonant current is 0.

[0063] (2) At time t0, the output voltage of the half-bridge module valve group is equal to V. HV / 2, the output voltage of the half-bridge module valve group rises, and the slope of the rising edge of the output voltage of the half-bridge module valve group is V. S / (d N1 T s ), with a duty cycle of d N1 ;

[0064] (3) At time t1, the first switch Q of the full-bridge module 11 and the fourth switch Q 14 Once switched on, the resonant current begins to increase;

[0065] (4) At time t2, the output voltage of the half-bridge module valve group rises to the set value;

[0066] (5) At time t3, the output voltage of the half-bridge module valve group decreases, and the slope of the voltage drop is -V. S / (d N2 T s ), with a duty cycle of d N2 ;

[0067] (6) At time t4, the resonant current is equal to 0, and the first switching transistor Q of the full-bridge module is... 11 and the fourth switch Q 14 Achieve zero-current shutdown;

[0068] (7) At time t5, the output voltage of the half-bridge module valve group drops to V. HV / 2, the output voltage of the half-bridge module valve group continues to decrease, and the slope of the voltage drop is -V. S / (d N1 T s ), with a duty cycle of d N1 ;

[0069] (8) At time t6, the second switch Q of the full-bridge module... 12 and the third switch Q 13 Once switched on, the resonant current begins to increase;

[0070] (9) At time t7, the output voltage of the half-bridge module valve group drops to the set value;

[0071] (10) At time t8, the output voltage of the half-bridge module valve group rises, and the slope of the rising edge of the output voltage of the half-bridge module valve group is V.S / (d N2 T s ), with a duty cycle of d N2 ;

[0072] (11) At time t9, the resonant current is equal to 0, and the second switch Q of the full-bridge module is... 12 and the third switch Q 14 Achieve zero-current shutdown;

[0073] (12) In t 10 At that moment, the output voltage of the half-bridge module valve group rises to V. HV / 2;

[0074] like Figure 6 The figure shows the working waveform 2 of the asymmetric stepped modulation method with two rising and falling slopes proposed in this invention. Among them, the half-bridge drive signal g... 11 ~g 1K and drive signal g 21 ~g 2K The duty cycle is 100%, and the drive signal g 1(K+1) ~g 1N and drive signal g 2(K+1) ~g 2N 40%; Half-bridge module valve group A drive signal g 11 ~g 1K The drive signal g corresponding to the half-bridge module valve group B 21 ~g 2K Interphase shift 180°

[0075] like Figure 7 The diagram shown is a schematic of the control method provided by the present invention.

[0076] (1) First, the high-voltage DC port voltage V in the modular multilevel DC transformer is... HV Low-voltage DC port voltage V LV Perform sampling.

[0077] (2) The sampled high-voltage DC port voltage V HV With reference voltage V HV_ref The values ​​are compared, and the output voltage difference is PI-regulated. The resulting output is used as the power transmission setpoint p of the modular multilevel DC transformer. t *

[0078] (3) Based on the given value p of the DC transformer transmission power t * High-voltage DC port voltage V HV and low-voltage DC port voltage V LV The duty cycle d of the rising edge of the output voltage of the half-bridge module valve group is obtained. N1and the duty cycle d of the falling edge N2 And the duty cycle D of the full-bridge module switching transistors.

[0079] (4) Based on the number K of constant input sub-modules, the duty cycle d of the rising edge of the half-bridge module valve group output voltage obtained in step 3. N1 and the duty cycle d of the falling edge N2 The full-bridge module switching transistors' duty cycle D is used for modulation and voltage equalization control to control the DC transformer's output voltage. The duty cycle d of the rising edge of the half-bridge module's valve group output voltage is also used. N1 and the duty cycle d of the falling edge N2 The phase shift angles θ1 and θ2 of the half-bridge module drive signal can be calculated:

[0080]

[0081]

[0082] like Figure 8 The diagram shows another topology of the present invention, which consists of a high-voltage filter inductor L. HV Half-bridge modular valve assembly, resonant inductor L R Resonant capacitor C R The system consists of a transformer T and a full-bridge module. Half-bridge module valve groups A, B, and C are connected in series and then connected to a high-voltage filter inductor to form a high-voltage DC port. The outputs of full-bridge modules A, B, and C are connected in parallel to form a low-voltage DC port. Each half-bridge module valve group has its own resonant capacitor C at its upper end. R Resonant inductor L R Connect the upper end of the primary winding of each transformer T to the lower end of the primary winding of each transformer T. The full-bridge module is connected to the secondary winding of each transformer T.

[0083] like Figure 9 The figure shows the working waveform 3 of the asymmetric stepped modulation method with two rising and falling slopes proposed in this invention. Among them, the half-bridge drive signal g... 11 ~g 1K drive signal g 21 ~g 2K and drive signal g 31 ~g 3K The duty cycle is 100%, and the drive signal g 1(K+1) ~g 1N drive signal g 2(K+1) ~g 2N and drive signal g 3(K+1) ~g 3N The phase shift is 120° between the drive signals of half-bridge modular valve group A, half-bridge modular valve group B, and half-bridge modular valve group C, which is 50%;

[0084] like Figure 10 The figure shows the working waveform 4 of the asymmetric stepped modulation method with two rising and falling slopes proposed in this invention. Among them, the half-bridge drive signal g... 11 ~g 1K drive signal g 21 ~g 2K and drive signal g 31 ~g 3K The duty cycle is 100%, and the drive signal g 1(K+1) ~g 1N drive signal g 2(K+1) ~g 2N and drive signal g 3(K+1) ~g 3N The percentage is 33%; the drive signals for half-bridge valve group A, half-bridge valve group B, and half-bridge valve group C are phase-shifted by 120°.

[0085] The modular multilevel DC transformer modulation and control method provided by this invention can ensure that the low-voltage side switching transistors of the DC transformer can achieve zero-current switching by controlling the slope of the asymmetric stepped half-wave output of the half-bridge module valve group, thereby achieving constant frequency operation of the resonant converter and improving the working efficiency of the converter.

[0086] The modular multilevel DC transformer modulation and control method provided by this invention provides two degrees of control freedom for the modular multilevel DC transformer through asymmetric stepped half-wave modulation of the half-bridge modular valve group, which can further improve the working characteristics of the modular multilevel DC transformer and achieve higher working efficiency.

[0087] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A modular multilevel DC transformer, characterized in that, This includes high-voltage filter inductors, half-bridge module valve assemblies, resonant inductors, resonant capacitors, transformers, and full-bridge modules; The half-bridge module valve group consists of N It consists of a half-bridge module connected in series with a valve group inductor. N Integer and N ≥1; The upper end of the half-bridge module valve group is connected to the upper end of the primary winding of the transformer via a resonant capacitor and a resonant inductor; The lower end of the half-bridge module valve group is connected to the lower end of the primary winding of the transformer, and the secondary winding of the transformer is connected to the input port of the full-bridge module; The two half-bridge module valve groups are connected in series and then connected to a high-voltage filter inductor to form a high-voltage DC port; The outputs of the two full-bridge modules are connected in parallel to form a low-voltage DC port. The half-bridge module valve group modulation changes the phase shift angle between the half-bridge modules. One of the two half-bridge module valve groups generates an asymmetric stepped wave with an upward slope, and the other generates a stepped wave with a downward slope. The two generated stepped waves are asymmetric, and the two half-bridge module valve groups operate in phase shift mode. The full-bridge module uses duty cycle control, and the two full-bridge modules operate in phase shift mode. The modulation method of the modular multilevel DC transformer includes the following steps: Step 1: Based on the required output voltage amplitude of the half-bridge module valve group V S Determine the number of constant input submodules required within one switching cycle. K , K Integer and 0≤ K < N Half-bridge module valve group output voltage amplitude V S Represented as: Step 2: The half-bridge module valve group modulates and generates an asymmetric stepped wave with two rising and falling slopes; the switching transistor drive signal on the half-bridge module in the half-bridge module valve group is denoted as... g i1 ~ g iN , i =1, 2, the upper and lower switches of the half-bridge module work complementaryly; drive signal g i1 ~ g iK Duty cycle is 100%, drive signal g i(K+1) ~ g iN The duty cycle is 50%; the drive signal of the half-bridge module g i(K+1) ~ g iN Introducing a phase shift angle, if N - K -1 is an even number, so the half-bridge module drive signal g i(K+1) ~ g i((N+K+1) / 2) The phase angle of the shift is θ 2. Half-bridge module drive signals g i((N+K+1) / 2) ~ g iN The phase angle of the shift is θ 1. If N - K -1 is an odd number, so the half-bridge module drive signal g i(K+1) ~ g i((N+K) / 2) The phase angle of the shift is θ 2. Half-bridge module drive signals g i((N+K) / 2) ~ g i((N+K) / 2+1) The phase angle of the shift is ( θ 1+ θ 2) / 2, Half-bridge module drive signal g i((N+K) / 2+1) ~ g iN The phase angle of the shift is θ 1; Step 3: The full-bridge module uses duty cycle control, denoted as . D First switching transistor Q i1 With the fourth switch Q i4 Simultaneously launched; Second switching transistor Q i2 With the third switch Q i3 Simultaneously activated, the third switch Q... i3 Lag first switch Q i1 Turn-on in half a switching cycle; first switch Q i1 With half-bridge submodule drive signal g i1 The phase shift angle between them is θ 2( N - K -1) / 2; Step 4: Control the switching transistors of the half-bridge module and the full-bridge module according to the drive signal with the above characteristics.

2. A modular multilevel DC transformer according to claim 1, characterized in that, The phase difference between different half-bridge module valve groups is configured to be 0~360°, and the phase difference between different full-bridge modules is configured to be 0~360°.

3. A control method for a modular multilevel DC transformer, characterized in that, The modular multilevel DC transformer, as described in any one of claims 1-2, adjusts the output voltage of the modular multilevel DC transformer by changing the rising and falling slopes of the asymmetric stepped wave output by the half-bridge module valve group and the duty cycle of the full-bridge module switching transistor.

4. The control method for a modular multilevel DC transformer according to claim 3, characterized in that, The control method includes the following steps: Step 1: Measure the voltage at the high-voltage DC port of the modular multilevel DC transformer. V HV Low-voltage DC port voltage V LV Perform sampling; Step 2: Sample the high-voltage DC port voltage V HV With reference voltage V HV_ref The values ​​are compared, and the output voltage difference is PI-regulated. The resulting output is used as the power transmission setpoint for the modular multilevel DC transformer. p t ; Step 3: Based on the given power transmission value of the DC transformer p t High voltage DC port voltage V HV and low-voltage DC port voltage V LV The duty cycle of the rising edge of the output voltage of the half-bridge module valve group is obtained. d N1 and falling edge duty cycle d N2 and the duty cycle of the full-bridge module switching transistors D ; Step 4: Based on the number of constant input sub-modules K The duty cycle of the rising edge of the half-bridge module valve group output voltage obtained in step 3 d N1 and falling edge duty cycle d N2 and the duty cycle of the full-bridge module switching transistors D Modulation and voltage equalization control are performed to control the output voltage of the DC transformer; the duty cycle of the rising edge of the output voltage of the half-bridge module valve group is controlled. d N1 and falling edge duty cycle d N2 The phase shift angle of the half-bridge module drive signal was calculated. θ 1 and θ 2: 。 5. The control method for a modular multilevel DC transformer according to claim 4, characterized in that, In step 2 of the control method, the given value of the transmission power of the modular multilevel DC transformer is... p t It can also be obtained by comparing the low-voltage DC port voltage, transmission power and corresponding reference values, and by performing PI adjustment on the comparison value.

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