Current optimization method for MMC-DCT system simplified model

Abstract

The invention provides a current optimization method for a simplified model of an MMC-DCT (modular multilevel converter-discrete cosine transform) system. The method comprises the steps that S1, according to the working principle of the MMC-DCT system, the power equivalence principle is set to be equivalent to an equivalent power simplified model of the MMC-DCT system; s2, setting an expression of an effective value of the output power # imgabs0 # by a circuit calculation method; s3, on the premise that the output power is not changed, by changing the input level number of the output voltage and the included angle # imgabs1 # between the output voltage and the equivalent inductive current, the minimum module value of the equivalent inductive current # imgabs2 # at the moment is obtained; s3, the module value # imgabs4 # of the equivalent inductance voltage under the minimum module value of the equivalent inductance current # imgabs3 # is obtained; s4, according to the phasor relation among the # imgabs 5 #, the # imgabs 6 # and the # imgabs 7 # at the moment, the module value of the # imgabs 8 # is obtained; s5, according to the relation between the input level number and the modulus value of # imgabs9 # at the moment, the input level number of the input end closest to the ideal optimized current is calculated; according to the method, the internal current of the MMC-DCT system can be optimized, and the influence on the transmission power precision of the MMC-DCT system and the internal loss of the MMC-DCT system caused by internal overlarge circulating current are reduced.

Description

Technical Field

[0001] The invention relates to the technical field of power transmission, and in particular to a current optimization method for a simplified model of an MMC-DCT system. Background Art

[0002] In recent years, with the gradual development of photovoltaic and wind power grid-connected technology, especially offshore wind power as an important branch of the renewable energy field, the demand for large-scale and long-distance power transmission is growing, and the importance of DC grid-connected technology in my country's power technology field has become increasingly prominent. Offshore wind farms are usually located in sea areas far from the shore, which requires the power transmission system to not only have efficient and stable energy transmission capabilities, but also to overcome the loss and voltage stability problems caused by long-distance power transmission. In this context, Direct Current Solid State Transformer Based on Modular Multilevel Converter (MMC-DCT) has become a key component of offshore wind power grid-connected technology due to its unique technical advantages.

[0003] The electricity generated by offshore wind farms often has extremely high voltage levels, and due to the particularity of the marine environment, higher requirements are placed on the reliability and durability of the equipment. The modular design of MMC-DCT not only enables it to withstand the high voltage level output by offshore wind farms, but also effectively reduces energy losses during power transmission through its high-level output and low harmonic content, thereby improving the overall efficiency of the system. In addition, the high integration and large-capacity design of MMC-DCT enable it to reduce the number of converters, simplify the system structure, and reduce operation and maintenance costs in offshore wind power grid-connected systems.

[0004] More importantly, offshore wind farms are usually far away from land and grid connection is inconvenient. MMC-DCT can output high-quality multi-level waveforms on the primary and secondary sides of high-frequency transformers by accurately controlling the switching cycle of power electronic devices, achieving effective transformation of voltage levels and efficient energy transmission. This feature is crucial for building a DC transmission channel between offshore wind power and land power grids. It can not only reduce energy loss during transmission, but also improve the stability and reliability of the power grid.

[0005] With the continuous increase in offshore wind power installed capacity, the demand for MMC-DCT is becoming increasingly urgent. The correct allocation of medium and low voltage side level input modules is not only related to the efficiency of power transmission, but also directly affects the stability and operation and maintenance costs of the system. Therefore, when designing and applying MMC-DCT, it is necessary to fully consider the actual needs and operating conditions of offshore wind farms, optimize the control strategy, ensure the voltage balance of each submodule, and reduce the complexity of submodule capacitor voltage control, so as to promote the continuous development and innovation of offshore wind power grid-connected technology. Summary of the invention

[0006] The present invention proposes a current optimization method for a simplified model of an MMC-DCT system. Based on the simplified model of the MMC-DCT system, the method can optimize the internal current of the MMC-DCT system by changing the number of levels of input voltage, thereby reducing the influence of excessive internal circulating current on the transmission power accuracy of the MMC-DCT system and the internal loss.

[0007] The present invention adopts the following technical solutions.

[0008] A current optimization method for a simplified model of an MMC-DCT system, the method comprising the following steps:

[0009] Step S1, according to the working principle of the MMC-DCT system, setting the power equivalent principle as an equivalent power simplified model of the MMC-DCT system;

[0010] Step S2, setting an expression for the effective value of the output power P by a circuit calculation method;

[0011] Step S3: Under the premise of keeping the output power unchanged, by changing the level of the output voltage and the angle between the output voltage and the equivalent inductor current The equivalent inductor current is obtained at this time The minimum modulus value of ;

[0012] Step S3, find the equivalent inductor current The modulus of the equivalent inductance voltage at the minimum modulus

[0013] Step S4: and The phasor relationship between them at this time is The modulus value of

[0014] Step S5: Based on the input level number and the current The relationship between the modulus value and the input level number that is closest to the ideal optimized current is calculated.

[0015] In step S5, the internal current of the MMC-DCT system is optimized by changing the number of levels of the input voltage, so as to reduce the influence of excessive internal circulating current on the transmission power efficiency of the MMC-DCT system and the internal loss.

[0016] The MMC-DCT is a DC solid-state transformer based on a modular multi-level converter, and adopts a two-phase full-bridge MMC-DCT main circuit topology. Its main body consists of a medium and low voltage side and a high-frequency transformer link, wherein the ports on both sides of the low voltage are respectively used to connect DC buses of different voltage levels, provide a grid-connected interface on the power generation line, or provide a distribution interface on the transmission line, to meet the adaptation conditions under different environmental requirements; Figure 2 When the MMC-DCT shown is expressed as an equivalent model,

[0017] like Figure 3 As shown, let u Ao and u Bo The voltages of points A and B relative to the neutral point o are V respectively, and the voltages of points P and N relative to the neutral point are V respectively. dc1 and -V dc1 , ignoring the voltage generated by the bridge arm current on the bridge arm inductance, the system voltage relationship is expressed as:

[0018]

[0019] Subtracting the two equations, we get:

[0020]

[0021] From the above formula, we can know that changing the voltage difference between the upper and lower bridge arms will change the voltage waveform on the AC side of the transformer accordingly. Assuming that the topology is two-phase and has strict symmetry, each phase can be analyzed separately. Then, in the equivalent circuit diagram of phase A, Figure 4 As shown, the input current I of the DC bus on the medium voltage side dc1 Will be evenly distributed between phases A and B, and the transformer primary current i ac1 It will also be evenly distributed between the upper and lower bridge arms, taking into account the circulating current i generated between the bridge arms cira , then the system current relationship is expressed as:

[0022]

[0023] When the bridge arm inductance has an inhibitory effect on the circulating current, it is approximately considered that the potentials of point A1 and point A2 are equal, and point A1 and point A2 are considered to be virtually short-circuited. After the short-circuit, the inductance of the upper and lower bridge arms is equivalent to a parallel relationship, that is, the inductance value becomes half of the original bridge arm inductance value; similarly, when the medium voltage side B phase bridge arm and the low voltage side a phase bridge arm and b phase bridge arm are analyzed separately, the analysis results are consistent with the A phase bridge arm, and the power transmission model of the MMC-DCT system is obtained, such as Figure 5 As shown, K is the turns ratio between the primary and secondary sides of the transformer.

[0024] In the equivalent power simplified model of the MMC-DCT system, the expression of the effective value of the output power P, the number of levels of the output voltage input, and the angle between the output voltage and the equivalent inductor current are The specific quantitative relationship between them is: Under the premise of keeping the output effective power P unchanged, the i in the simplified model L The amplitude is the smallest, and there is a formula

[0025]

[0026] when Reach the theoretical maximum value, and but and The angle between them is 0, and they are on the same horizontal line;

[0027] Under fundamental wave conditions, There is a maximum value, and the following formula is given:

[0028]

[0029] Where V(ωt) is the voltage equivalent value, V dcn is the submodule voltage when the nth level is turned on, θ n The corresponding angle is obtained by the ratio of the n-th level input time to one quarter of the switching period, and sin(ωt)=1.

[0030] when In the formula V dcn The size is based on the capacitor voltage averaging strategy in the MMC-DCT system. If the size is equal, then There is a maximum value, the number of input levels is 0, thus we get

[0031]

[0032] In the formula, n is the total number of input levels, V dc is the capacitor voltage of each submodule in the MMC-DCT system.

[0033] Equivalent inductor current The minimum modulus of the equivalent inductance voltage The relationship is expressed as:

[0034] Where ω = 2πf s , f s is the operating frequency of the entire MMC-DCT system, L is the final equivalent inductance of the entire system, L = L s1 +L k +K 2 L s2 , L s1 is the equivalent inductance of the bridge arm on the input side, L s2 is the equivalent inductance on the output side bridge arm, L k is the equivalent leakage inductance of the high-frequency transformer.

[0035] and In the phasor relationship between and The relationship is known, and through the two and The angle relationship is known, and The angle between is a right angle, then the above conditions can be obtained

[0036]

[0037] In the formula for and The angle between them can be obtained

[0038] In step S5, the input level number is equal to the current The relationship between the modulus value is as follows: The relationship is the same as that of the input level number angle. First, it is divided into n submodules, and then a set of numbers starting from 0 to n-1 are calculated, which are recorded as b0, b1, b2...b n-1 , find the closest The ideal value level number is the minimum current size corresponding to the input voltage side.

[0039] The current optimization method can be verified by system simulation of the corresponding topology using an MMC-DCT simulation model built based on MATLAB / Simulink.

[0040] When the system is simulated with the MMC-DCT simulation model, the modulation strategy adopted by the DC solid-state transformer MMC-DCT based on the modular multi-level converter is quasi-square wave modulation.

[0041] When the system is simulated with the MMC-DCT simulation model, the sub-module capacitor voltage balance at the bridge arm of the DC solid-state transformer MMC-DCT based on the modular multi-level converter adopts a double-sequencing voltage balance strategy.

[0042] The present invention proposes a current optimization method for a simplified model of an MMC-DCT system. Based on the simplified model of the MMC-DCT system, the internal current of the MMC-DCT system is optimized by changing the number of levels of input voltage, thereby reducing the influence of excessive internal circulating current on the transmission power accuracy of the MMC-DCT system and the internal loss.

[0043] Compared with the prior art, the present invention has the following beneficial effects: under the premise of not affecting the transmission power, the present invention can optimize the current size by only changing the number of levels of the input side voltage input, thereby reducing the influence of excessive internal circulating current on the transmission power efficiency of the MMC-DCT system and the internal loss, and such a method has no restriction on the number of sub-modules of each bridge arm in the MMC-DCT system, and can be applied to systems with different numbers of sub-modules. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments:

[0045] Attached Figure 1 A schematic diagram of the principle flow of a current optimization method for a simplified model of an MMC-DCT system according to an embodiment of the present invention;

[0046] Attached Figure 2 A schematic diagram of the topological structure of an MMC-DCT system according to an embodiment of the present invention;

[0047] Attached Figure 3 is a schematic diagram of an equivalent circuit of an MMC-DCT system according to an embodiment of the present invention;

[0048] Attached Figure 4 Schematic diagram of an equivalent circuit of phase A on the low-pressure side of an MMC-DCT system according to an embodiment of the present invention;

[0049] Attached Figure 5 A simplified power transmission model schematic diagram of an MMC-DCT system according to an embodiment of the present invention;

[0050] Attached Figure 6 For the embodiment of the present invention and Schematic diagram of the phasor diagram between;

[0051] Attached Figure 7 The full-level output voltage U0, the primary and secondary voltages U1, U2 and the inductor current waveform i of the embodiment of the present invention are shown in FIG. L Schematic diagram of

[0052] Attached Figure 8 The output voltage U0, the primary and secondary voltages U1, U2 and the inductor current waveform i under the optimized input level number of the embodiment of the present invention are shown in FIG. L Schematic diagram of

[0053] Attached Fig. 9 Schematic diagram of the external system circuit of the MMC-DCT system according to an embodiment of the present invention. DETAILED DESCRIPTION

[0054] As shown in the figure, a current optimization method for a simplified model of an MMC-DCT system, the method comprises the following steps;

[0055] Step S1, according to the working principle of the MMC-DCT system, setting the power equivalent principle as an equivalent power simplified model of the MMC-DCT system;

[0056] Step S2, setting an expression for the effective value of the output power P by a circuit calculation method;

[0057] Step S3: Under the premise of keeping the output power unchanged, by changing the level of the output voltage and the angle between the output voltage and the equivalent inductor current The equivalent inductor current is obtained at this time The minimum modulus value of ;

[0058] Step S3, find the equivalent inductor current The modulus of the equivalent inductance voltage at the minimum modulus

[0059] Step S4: and The phasor relationship between them at this time is The modulus value of

[0060] Step S5: Based on the input level number and the current The relationship between the modulus value and the input level number that is closest to the ideal optimized current is calculated.

[0061] In step S5, the internal current of the MMC-DCT system is optimized by changing the number of levels of the input voltage, so as to reduce the influence of excessive internal circulating current on the transmission power efficiency of the MMC-DCT system and the internal loss.

[0062] The MMC-DCT is a DC solid-state transformer based on a modular multi-level converter, and adopts a two-phase full-bridge MMC-DCT main circuit topology. Its main body consists of a medium and low voltage side and a high-frequency transformer link, wherein the ports on both sides of the low voltage are respectively used to connect DC buses of different voltage levels, provide a grid-connected interface on the power generation line, or provide a distribution interface on the transmission line, to meet the adaptation conditions under different environmental requirements; Figure 2 When the MMC-DCT shown is expressed as an equivalent model,

[0063] like Figure 3 As shown, let u Ao and u Bo The voltages of points A and B relative to the neutral point o are V respectively, and the voltages of points P and N relative to the neutral point are V respectively. dc1 and -V dc1 , ignoring the voltage generated by the bridge arm current on the bridge arm inductance, the system voltage relationship is expressed as:

[0064]

[0065] Subtracting the two equations, we get:

[0066]

[0067] From the above formula, we can know that changing the voltage difference between the upper and lower bridge arms will change the voltage waveform on the AC side of the transformer accordingly. Assuming that the topology is two-phase and has strict symmetry, each phase can be analyzed separately, then in the equivalent circuit diagram of phase A; Figure 4 As shown, the input current I of the DC bus on the medium voltage side dc1 Will be evenly distributed between phases A and B, and the transformer primary current i ac1 It will also be evenly distributed between the upper and lower bridge arms, taking into account the circulating current i generated between the bridge arms cira , then the system current relationship is expressed as:

[0068]

[0069] When the bridge arm inductance has an inhibitory effect on the circulating current, it is approximately considered that the potentials of point A1 and point A2 are equal, and point A1 and point A2 are considered to be virtually short-circuited. After the short-circuit, the inductance of the upper and lower bridge arms is equivalent to a parallel relationship, that is, the inductance value becomes half of the original bridge arm inductance value; similarly, when the medium voltage side B phase bridge arm and the low voltage side a phase bridge arm and b phase bridge arm are analyzed separately, the analysis results are consistent with the A phase bridge arm, and the power transmission model of the MMC-DCT system is obtained, such as Figure 5 As shown, K is the turns ratio between the primary and secondary sides of the transformer.

[0070] In the equivalent power simplified model of the MMC-DCT system, the expression of the effective value of the output power P, the number of levels of the output voltage input, and the angle between the output voltage and the equivalent inductor current are The specific quantitative relationship between them is: Under the premise of keeping the output effective power P unchanged, the i in the simplified model L The amplitude is the smallest, and there is a formula

[0071]

[0072] when Reach the theoretical maximum value, and but and The angle between them is 0, and they are on the same horizontal line;

[0073] Under fundamental wave conditions, There is a maximum value, and the following formula is given:

[0074]

[0075] Where V(ωt) is the voltage equivalent value, V dcn is the submodule voltage when the nth level is turned on, θ n The corresponding angle is obtained by the ratio of the n-th level input time to one quarter of the switching period, and sin(ωt)=1.

[0076] when In the formula V dcn The size is based on the capacitor voltage averaging strategy in the MMC-DCT system. If the size is equal, then There is a maximum value, the number of input levels is 0, thus we get

[0077]

[0078] In the formula, n is the total number of input levels, V dc is the capacitor voltage of each submodule in the MMC-DCT system.

[0079] Equivalent inductor current The minimum modulus of the equivalent inductance voltage The relationship is expressed as:

[0080] Where ω = 2πf s , f s is the operating frequency of the entire MMC-DCT system, L is the final equivalent inductance of the entire system, L = L s1 +L k +K 2 L s2 , Ls1 is the equivalent inductance of the bridge arm on the input side, L s2 is the equivalent inductance on the output side bridge arm, L k is the equivalent leakage inductance of the high-frequency transformer.

[0081] and In the phasor relationship between and The relationship is known, and through the two and The angle relationship is known, and The angle between is a right angle, then the above conditions can be obtained

[0082]

[0083] In the formula for and The angle between them can be obtained

[0084] In step S5, the input level number is equal to the current The relationship between the modulus value is as follows: The relationship is the same as that of the input level number angle. First, it is divided into n submodules, and then a set of numbers starting from 0 to n-1 are calculated, which are recorded as b0, b1, b2...b n-1 , find the closest The ideal value level number is the minimum current size corresponding to the input voltage side.

[0085] The current optimization method can be verified by system simulation of the corresponding topology using an MMC-DCT simulation model built based on MATLAB / Simulink.

[0086] When the system is simulated with the MMC-DCT simulation model, the modulation strategy adopted by the DC solid-state transformer MMC-DCT based on the modular multi-level converter is quasi-square wave modulation.

[0087] When the system is simulated with the MMC-DCT simulation model, the sub-module capacitor voltage balance at the bridge arm of the DC solid-state transformer MMC-DCT based on the modular multi-level converter adopts a double-sequencing voltage balance strategy.

[0088] Example:

[0089] In this embodiment, an MMC-DCT simulation model is built based on MATLAB / Simulink, and simulation verification is performed on the above topology. The simulation parameters are shown in Table 1.

[0090] Table 1MMC-DCT system simulation system parameters

[0091]

[0092] In this embodiment, the values ​​in the above table are further substituted into the calculation formula to obtain:

[0093]

[0094] ω=2πf s =6.28×10 4 rad / s;

[0095] L=L s1 +L k +K 2 L s2 =1.2×10 -3 H;

[0096] In this embodiment, it is further known that the active power P and You can get:

[0097]

[0098] In this embodiment, the and After getting, since the two are at right angles,

[0099]

[0100] According to the three phase diagrams, we can know

[0101]

[0102] In this embodiment, further, from the above analysis, it can be seen that the number of input levels is related to the current The specific mathematical relationship is as follows:

[0103]

[0104] Divide it into n submodules and enter the data values:

[0105] [V dc1 cos(θ1)+V dc2 cos(θ2)+···+V dcn cos(θ n )]=7597.35V;

[0106] In this embodiment, it is assumed that [cos(θ1)+cos(θ2)+···+cos(θ n)], the level distribution is uniform, where θ1 is set to 10°, and it is divided into 9 modules. The n closest to the theoretical ideal value is calculated and set to b0, b1, b2...b n-1 , where V dc =1.1×10 3 V,

[0107] When n=0, the number of input levels is 0, then

[0108] b0=V dc1 9 = 1 × 10 4 V;

[0109] When n=1, the number of input levels is 1, then

[0110] b1=V dc1 ·cos(θ1)+8V dc1 ·cos(2θ1)=9447.04V;

[0111] When n=2, the number of input levels is 2, then

[0112] b2=V dc1 ·cos(θ1)+V dc1 ·cos(2θ1)+7V dc1 ·cos(3θ1)=8874.07V;

[0113] When n=3, the number of levels put into operation is 3, then

[0114] b3=V dc1 ·cos(θ1)+V dc1 ·cos(2θ1)+V dc1 ·cos(3θ1)+6V dc1 ·cos(4θ1)=8207.53V;

[0115] When n=4, the number of input levels is 4, then

[0116] b4=V dc1 ·cos(θ1)+V dc1 ·cos(2θ1)+V dc1 ·cos(3θ1)+V dc1 ·cos(4θ1)+5V dc1 ·cos(5θ1)=7522.78V;

[0117] In this embodiment, it can be easily concluded from the above that when four levels are input, it can be closest to the ideal value.

[0118] It can be seen that in order to achieve theoretical minimization of the inductor current, 4 levels need to be invested in the primary side and 0 level needs to be invested in the secondary side. The simulation result shows that the primary side outputs an 8-level step wave and the secondary side outputs a rectangular wave. The present invention can achieve the simulation effect of the corresponding parameters by changing the trapezoidal wave modulation strategy.

[0119] The modulation strategy adopted in the MMC-DCT system simulation is quasi-square wave modulation, and the submodule capacitor voltage balance adopts a double-sequencing voltage balance strategy, such as Figure 7 As shown in the figure, the output voltage waveform of the primary and secondary sides of the MMC-DCT system is fully put into 9 levels. The voltage waveforms on both sides are 10-level step waves. The peak voltage on the primary side is 10kV, the peak voltage on the secondary side is 6kV, the output voltage is 6kV, and the transmitted power is 1MW. At this time, the inductor current is measured under different input levels, such as Figure 7 As shown, when the full voltage level is applied, the output inductor current is approximately 120A.

[0120] The trapezoidal wave modulation strategy and related parameters of the low-voltage side of the modulation make the simulation output an 8-level and symmetrical step wave, and the secondary side is a rectangular wave. The corresponding output peak voltage and output voltage are consistent with the full input level. At this time, the inductor current size is measured under different input levels, such as Figure 8 As shown in the figure, when the full level is input, the output inductor current is 105A. Compared with the full level input, the inductor current is reduced by 13%, achieving current optimization.

[0121] In order to implement the above embodiment, this example also proposes an external circuit system, when the instruction processor in the circuit system is executed, the energy transfer of the MMC-DCT system circuit proposed in the above embodiment of the present invention is performed. Fig. 9 A block diagram of an exemplary electronic device suitable for implementing embodiments of the present invention is shown. Fig. 9 The external circuit system arrangement diagram shown is only an example and should not bring any limitation to the functions and scope of use of the embodiments of the present invention. Fig. 9As shown, the external circuit layout design diagram is presented in the form of a general computing device. The components of the external circuit layout design diagram may include, but are not limited to: one or more processors or processing units, system memory, manual or automatic switching switches, network adapters, displays, external devices, and buses connecting different system components (including system memory and processing units). The memory may include a computer system readable medium in the form of a volatile memory, such as a random access memory (Random Access Memory; hereinafter referred to as: RAM) and / or a cache memory. The memory may include at least one program product, which has a set (e.g., at least one) of program modules, which are configured to perform the functions of various embodiments of the present invention. The electronic device may also communicate with one or more external devices (e.g., keyboards, pointing devices, displays, etc.), and may also communicate with one or more devices that enable a user to interact with the electronic device / or communicate with any device that enables the electronic device to communicate with one or more other computing devices (e.g., network cards, modems, etc.). Such communication may be performed through an input / output (I / O) interface. The processing unit executes various functional applications by running programs stored in the system storage, such as implementing the MMC-DCT system circuit control strategy method mentioned in the aforementioned embodiment.

[0122] The above is only a preferred embodiment of the present invention, and does not limit the present invention in other forms. Any technician familiar with the profession may use the above disclosed technical content to change or modify it into an equivalent embodiment with equivalent changes. However, any simple modification, equivalent change and modification made to the above embodiment according to the technical essence of the present invention without departing from the technical solution of the present invention still belongs to the protection scope of the technical solution of the present invention.

Claims

1. A current optimization method for a simplified model of an MMC-DCT system, characterized in that: The method comprises the following steps: Step S1, according to the working principle of the MMC-DCT system, setting the power equivalent principle as an equivalent power simplified model of the MMC-DCT system; Step S2, setting an expression for the effective value of the output power P by a circuit calculation method; Step S3: Under the premise of keeping the output power unchanged, by changing the level of the output voltage and the angle between the output voltage and the equivalent inductor current The equivalent inductor current is obtained at this time The minimum modulus value of ; Step S3, find the equivalent inductor current The modulus of the equivalent inductance voltage at the minimum modulus Step S4: and The phasor relationship between them at this time is The modulus value of Step S5: Based on the input level number and the current The relationship between the modulus value and the input level number that is closest to the ideal optimized current is calculated.

2. The current optimization method for the simplified model of the MMC-DCT system according to claim 1, characterized in that: In step S5, the internal current of the MMC-DCT system is optimized by changing the number of levels of the input voltage, so as to reduce the influence of excessive internal circulating current on the transmission power efficiency of the MMC-DCT system and the internal loss.

3. The current optimization method for the simplified model of the MMC-DCT system according to claim 1, characterized in that: The MMC-DCT is a DC solid-state transformer based on a modular multi-level converter, and adopts a two-phase full-bridge MMC-DCT main circuit topology. Its main body consists of a medium and low voltage side and a high-frequency transformer link, wherein the ports on both sides of the low voltage are respectively used to connect DC buses of different voltage levels, provide a grid-connected interface on the power generation line, or provide a distribution interface on the transmission line, to meet the adaptation conditions under different environmental requirements; When MMC-DCT is expressed as an equivalent model, Let u Ao and u Bo The voltages of points A and B relative to the neutral point o are V respectively, and the voltages of points P and N relative to the neutral point are V respectively. dc1 and -V dc1 , ignoring the voltage generated by the bridge arm current on the bridge arm inductance, the system voltage relationship is expressed as: Subtracting the two equations, we get: From the above formula, we can know that changing the voltage difference between the upper and lower bridge arms will change the voltage waveform on the AC side of the transformer accordingly. Assuming that the topology is two-phase and has strict symmetry, each phase can be analyzed separately. In the equivalent circuit diagram of phase A, the input current I of the DC bus on the medium voltage side is dc1 Will be evenly distributed between phases A and B, and the transformer primary current i ac1 It will also be evenly distributed between the upper and lower bridge arms, taking into account the circulating current i generated between the bridge arms cira , then the system current relationship is expressed as: When the bridge arm inductance has an inhibitory effect on the circulating current, the potentials of point A1 and point A2 are approximately considered to be equal, and point A1 and point A2 are considered to be virtually short-circuited. After the short-circuit, the inductance of the upper and lower bridge arms is equivalent to a parallel relationship, that is, the inductance value becomes half of the original bridge arm inductance value; similarly, when the medium-voltage side B-phase bridge arm and the low-voltage side a-phase bridge arm and b-phase bridge arm are analyzed separately, the analysis results are consistent with the A-phase bridge arm, and the power transmission model of the MMC-DCT system is obtained.

4. The current optimization method for the simplified model of the MMC-DCT system according to claim 1, characterized in that: In the equivalent power simplified model of the MMC-DCT system, the expression of the effective value of the output power P, the number of levels of the output voltage input, and the angle between the output voltage and the equivalent inductor current are The specific quantitative relationship between them is: Under the premise of keeping the output effective power P unchanged, the i in the simplified model L The amplitude is the smallest, and there is a formula when Reach the theoretical maximum value, and but and The angle between them is 0, and they are on the same horizontal line; Under fundamental wave conditions, There is a maximum value, and the following formula is given: Where V(ωt) is the voltage equivalent value, V dcn is the submodule voltage when the nth level is turned on, θ n The corresponding angle is obtained by the ratio of the n-th level input time to one quarter of the switching period, and sin(ωt)=1.

5. The current optimization method for the simplified model of the MMC-DCT system according to claim 4, characterized in that: when In the formula V dcn The size is based on the capacitor voltage averaging strategy in the MMC-DCT system. If the size is equal, then There is a maximum value, the number of input levels is 0, thus we get In the formula, n is the total number of input levels, V dc is the capacitor voltage of each submodule in the MMC-DCT system.

6. The current optimization method for the simplified model of the MMC-DCT system according to claim 1, characterized in that: Equivalent inductor current The minimum modulus of the equivalent inductance voltage The relationship is expressed as: Where ω = 2πf s , f s is the operating frequency of the entire MMC-DCT system, L is the final equivalent inductance of the entire system, L = L s1 +L k +K 2 L s2 , L s1 is the equivalent inductance of the bridge arm on the input side, L s2 is the equivalent inductance on the output side bridge arm, L k is the equivalent leakage inductance of the high-frequency transformer.

7. The current optimization method for the simplified model of the MMC-DCT system according to claim 1, characterized in that: and In the phasor relationship between and The relationship is known, and through the two and The angle relationship is known, and The angle between is a right angle, then the above conditions can be obtained In the formula for and The angle between them can be obtained 8. The current optimization method for the simplified model of the MMC-DCT system according to claim 1, characterized in that: In step S5, the input level number is equal to the current The relationship between the modulus value is as follows: The relationship is the same as that of the input level number angle. First, it is divided into n submodules, and then a set of numbers starting from 0 to n-1 are calculated, which are recorded as b0, b1, b2...b n-1 , find the closest The ideal value level number is the minimum current size corresponding to the input voltage side.

9. The current optimization method for the simplified model of the MMC-DCT system according to claim 1, characterized in that: The current optimization method can be verified by system simulation of the corresponding topology using an MMC-DCT simulation model built based on MATLAB / Simulink.

10. The current optimization method for the simplified model of the MMC-DCT system according to claim 9, characterized in that: When the system is simulated with the MMC-DCT simulation model, the modulation strategy adopted by the DC solid-state transformer MMC-DCT based on the modular multi-level converter is quasi-square wave modulation; When the system is simulated with the MMC-DCT simulation model, the sub-module capacitor voltage balance at the bridge arm of the DC solid-state transformer MMC-DCT based on the modular multi-level converter adopts a double-sequencing voltage balance strategy.

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