A current optimization method for a simplified model of MMC-DCT system

Abstract

The application provides a current optimization method for a simplified model of an MMC-DCT system, and comprises the following steps: 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; S2, setting an expression of an output power effective value by a circuit calculation method; S3, under the premise of constant output power, obtaining a minimum modulus value of equivalent inductance current by changing the level number of output voltage input and the included angle between the output voltage and the equivalent inductance current; S4, obtaining the modulus value of the equivalent inductance voltage under the minimum modulus value of the equivalent inductance current; S5, obtaining the input level number closest to the ideal optimized current by the relationship between the input level number and the modulus value; and S6, the application can realize the optimization of the internal current of the MMC-DCT system, reduce the influence of the internal excessive circulating current on the transmission power accuracy of the MMC-DCT system and the internal loss.

Description

Technical Field

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

[0002] In recent years, with the gradual development of photovoltaic and wind power grid connection technologies, especially offshore wind power as an important branch of renewable energy, the demand for large-scale, long-distance power transmission has been increasing, highlighting the growing importance of DC grid connection technology in my country's power technology field. Offshore wind farms are typically located in waters far from the coast, requiring power transmission systems to not only possess efficient and stable energy transmission capabilities but also overcome the losses and voltage stability issues associated with long-distance transmission. Against this backdrop, the Direct Current Solid State Transformer Based on Modular Multilevel Converter (MMC-DCT) has become a key component of offshore wind power grid connection technology due to its unique technological advantages.

[0003] Offshore wind farms typically generate electricity at extremely high voltage levels, and the unique marine environment places even greater demands on the reliability and durability of the equipment. The modular design of the MMC-DCT not only enables it to withstand the high voltage levels output by offshore wind farms, but also effectively reduces energy loss during power transmission and improves the overall system efficiency through its high-level output and low harmonic content. Furthermore, the MMC-DCT's high integration and large-capacity design allow for a reduction in the number of converters in offshore wind power grid-connected systems, simplifying the system structure and lowering operation and maintenance costs.

[0004] More importantly, offshore wind farms are typically located far from land, making grid connection inconvenient. The MMC-DCT, through precise control of the switching cycles of power electronic devices, can output high-quality multi-level waveforms on both the primary and secondary sides of a high-frequency transformer, achieving effective voltage level conversion and efficient energy transfer. This characteristic is crucial for constructing DC transmission channels between offshore wind power and the onshore power grid, not only reducing energy losses during transmission but also improving grid stability and reliability.

[0005] With the continuous increase in offshore wind power installed capacity, the demand for MMC-DCT is becoming increasingly urgent. Properly allocating the voltage levels of the medium and low voltage side modules not only affects power transmission efficiency but also directly impacts system stability and operation and maintenance costs. Therefore, when designing and applying MMC-DCT, it is necessary to fully consider the actual needs and operating conditions of offshore wind farms, optimize control strategies, ensure voltage balance in each submodule, and reduce the complexity of controlling submodule capacitor voltages, thereby promoting the continuous development and innovation of offshore wind power grid connection technology. Summary of the Invention

[0006] This 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, this method optimizes the internal current of the MMC-DCT system by changing the number of input voltage levels, thereby reducing the impact of excessive internal circulating current on the transmission power accuracy of the MMC-DCT system and reducing internal losses.

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

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

[0009] Step S1: Based on the working principle of the MMC-DCT system, set the power equivalence principle equivalent value as the simplified equivalent power model of the MMC-DCT system;

[0010] Step S2: Set the expression for the effective value of the output power P using circuit calculation methods;

[0011] Step S3: Under the premise of constant output power, change the level of the output voltage and the angle between the output voltage and the equivalent inductor current. Obtain the equivalent inductance current at this time The minimum modulus;

[0012] Step S3: Calculate the equivalent inductance current. The magnitude of the equivalent inductance voltage at its minimum magnitude

[0013] Step S4, from and The phasor relationship between them at this point is obtained. The modulus;

[0014] Step S5: From the input level number and at this time Based on the relationship between the modulus and the input current, the number of input levels 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 input voltage levels, so as to reduce the impact of excessive internal circulating current on the power transmission efficiency of the MMC-DCT system and internal losses.

[0016] The MMC-DCT is a DC solid-state transformer based on a modular multilevel converter. It adopts a two-phase full-bridge MMC-DCT main circuit topology, which consists of a medium- and low-voltage side and a high-frequency transformer. The low-voltage side ports are used to connect to DC buses of different voltage levels, providing grid connection interfaces on power generation lines or distribution interfaces on transmission lines, to meet the adaptation conditions under different environmental requirements. Figure 2 When the MMC-DCT shown is expressed using an equivalent model,

[0017] like Figure 3 As shown, let u Ao and u Bo The voltages at points A and B relative to the neutral point O are V, and the voltages at points P and N relative to the neutral point are V. dc1 and -V dc1 Ignoring the voltage generated across the bridge arm inductor by the bridge arm current, the system voltage relationship is expressed as:

[0018]

[0019] Subtracting the two equations, we get:

[0020]

[0021] From the above equation, changing the voltage difference between the upper and lower bridge arms will correspondingly change the voltage waveform on the AC side of the transformer. Assuming this topology is two-phase and strictly symmetrical, and each phase can be analyzed independently, then its equivalent circuit diagram for phase A is as follows: Figure 4 As shown, the input current I of the medium-voltage side DC bus dc1 The transformer primary current i will be evenly distributed between phases A and B. ac1 It will also be evenly distributed between the upper and lower bridge arms, while taking into account the circulating current i generated between the bridge arms. cira The system current relationship is then expressed as:

[0022]

[0023] When the bridge arm inductance suppresses the circulating current, the potentials of points A1 and A2 are approximately equal, and points A1 and A2 are considered to be virtually short-circuited. After the short circuit, the inductances of the upper and lower bridge arms are equivalent to a parallel relationship, that is, the inductance value becomes half of the original bridge arm inductance value. Similarly, when analyzing the B-phase bridge arm on the medium-voltage side and the a-phase and b-phase bridge arms on the low-voltage side, the analysis results are consistent with those of the A-phase bridge arm, thus obtaining the power transfer model of the MMC-DCT system, such as... Figure 5 As shown in the figure, K is the turns ratio of the primary and secondary sides of the transformer.

[0024] In the simplified equivalent power model of the MMC-DCT system, the expression for the effective value of the output power P, the number of levels at which the output voltage is switched, and the angle between the output voltage and the equivalent inductor current are all included. The specific quantitative relationship between them is as follows: To ensure that, while maintaining the effective output power P, the value of i in the simplified model is... L The amplitude is the smallest, and there is a formula...

[0025]

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

[0027] Under fundamental frequency conditions, at this time it is necessary to make It has a maximum value, and the following formula applies:

[0028]

[0029] In the formula, V(ωt) is the equivalent voltage value, V dcn θ represents the voltage magnitude of the submodule when the nth level is applied. n Let sin(ωt) = 1, where ωt is the angle obtained by the ratio of the nth level input time to a quarter of a switching cycle.

[0030] when In the formula V dcn The magnitude is based on the capacitor voltage averaging strategy in the MMC-DCT system. If the magnitudes are equal, then... There is a maximum value, and the number of input levels is 0, from which we obtain

[0031]

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

[0033] Equivalent inductance current The minimum magnitude of the equivalent inductance voltage and the magnitude of the equivalent inductance voltage The relation, expressed as:

[0034] In the formula, ω=2πf s f s Let L be the operating frequency of the entire MMC-DCT system, and L be the magnitude of the final equivalent inductance of the entire system, where L = L s1 +L k +K 2 L s2 L s1 For the equivalent inductance on the input-side bridge arm, L s2 L is the equivalent inductance on the output side bridge arm. k This is the leakage inductance equivalent to that of a high-frequency transformer.

[0035] and In the phasor relationship between them, if and The relationship is known, and it is obtained through both and From the angular relationship, we can know that and If the included angle 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 at this point.

[0038] In step S5, the number of input levels is the same as at this time. The relationship between the modulus values ​​is as follows: Similar to the relationship between the input level number and angle, we first divide it into n sub-modules, and then calculate a sequence of input level numbers starting from 0 and ending at n-1, denoted as b0, b1, b2...b n-1 Find the closest The ideal voltage level is the minimum current 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 with MATLAB / Simulink.

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

[0041] When performing system simulation using the MMC-DCT simulation model, the voltage balancing of the submodule capacitors at the bridge arm of the DC solid-state transformer MMC-DCT based on the modular multilevel converter adopts a dual-sorting voltage balancing strategy.

[0042] This 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, by changing the number of input voltage levels, the internal current of the MMC-DCT system is optimized, reducing the impact of excessive internal circulating current on the transmission power accuracy of the MMC-DCT system and internal losses.

[0043] Compared with the prior art, the present invention has the following advantages: The present invention can optimize the current magnitude simply by changing the number of voltage levels applied on the input side without affecting the transmission power, thereby reducing the impact of excessive internal circulating current on the transmission power efficiency of the MMC-DCT system and internal losses. Moreover, this method has no limitation on the number of sub-modules in each bridge arm of the MMC-DCT system and can be applied to systems with different numbers of sub-modules. Attached Figure Description

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

[0045] Appendix Figure 1 This is a schematic diagram illustrating the principle and flow of the current optimization method for a simplified model of the MMC-DCT system according to an embodiment of the present invention.

[0046] Appendix Figure 2 This is a schematic diagram of the topology of the MMC-DCT system according to an embodiment of the present invention;

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

[0048] Appendix Figure 4 This is a schematic diagram of the equivalent circuit of the pressure-side A phase in the MMC-DCT system according to an embodiment of the present invention;

[0049] Appendix Figure 5 This is a simplified power transfer model diagram of the MMC-DCT system according to an embodiment of the present invention;

[0050] Appendix Figure 6 Embodiments of the present invention and A schematic diagram of the phasor diagram between them;

[0051] Appendix Figure 7 The input full-level output voltage U0, primary and secondary side voltages U1 and U2, and inductor current waveform i are given in this embodiment of the invention. L A schematic diagram;

[0052] Appendix Figure 8 The output voltage U0, primary and secondary voltages U1 and U2, and inductor current waveform i under the optimized input level number in this embodiment of the invention are shown. L A schematic diagram;

[0053] Appendix Figure 9 This is a schematic diagram of the external system circuit of the MMC-DCT system according to an embodiment of the present invention. Detailed Implementation

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

[0055] Step S1: Based on the working principle of the MMC-DCT system, set the power equivalence principle equivalent value as the simplified equivalent power model of the MMC-DCT system;

[0056] Step S2: Set the expression for the effective value of the output power P using circuit calculation methods;

[0057] Step S3: Under the premise of constant output power, change the level of the output voltage and the angle between the output voltage and the equivalent inductor current. Obtain the equivalent inductance current at this time The minimum modulus;

[0058] Step S3: Calculate the equivalent inductance current. The magnitude of the equivalent inductance voltage at its minimum magnitude

[0059] Step S4, from and The phasor relationship between them at this point is obtained. The modulus;

[0060] Step S5: From the input level number and at this time Based on the relationship between the modulus and the input current, the number of input levels 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 input voltage levels, so as to reduce the impact of excessive internal circulating current on the power transmission efficiency of the MMC-DCT system and internal losses.

[0062] The MMC-DCT is a DC solid-state transformer based on a modular multilevel converter. It adopts a two-phase full-bridge MMC-DCT main circuit topology, which consists of a medium- and low-voltage side and a high-frequency transformer. The low-voltage side ports are used to connect to DC buses of different voltage levels, providing grid connection interfaces on power generation lines or distribution interfaces on transmission lines, to meet the adaptation conditions under different environmental requirements. Figure 2 When the MMC-DCT shown is expressed using an equivalent model,

[0063] like Figure 3 As shown, let u Ao and u Bo The voltages at points A and B relative to the neutral point O are V, and the voltages at points P and N relative to the neutral point are V. dc1 and -V dc1 Ignoring the voltage generated across the bridge arm inductor by the bridge arm current, the system voltage relationship is expressed as:

[0064]

[0065] Subtracting the two equations, we get:

[0066]

[0067] From the above equation, changing the voltage difference between the upper and lower bridge arms will correspondingly change the voltage waveform on the AC side of the transformer. Assuming this topology is two-phase and strictly symmetrical, and each phase can be analyzed independently, then its equivalent circuit diagram for phase A is as follows: Figure 4 As shown, the input current I of the medium-voltage side DC bus dc1 The transformer primary current i will be evenly distributed between phases A and B. ac1 It will also be evenly distributed between the upper and lower bridge arms, while taking into account the circulating current i generated between the bridge arms. cira The system current relationship is then expressed as:

[0068]

[0069] When the bridge arm inductance suppresses the circulating current, the potentials of points A1 and A2 are approximately equal, and points A1 and A2 are considered to be virtually short-circuited. After the short circuit, the inductances of the upper and lower bridge arms are equivalent to a parallel relationship, that is, the inductance value becomes half of the original bridge arm inductance value. Similarly, when analyzing the B-phase bridge arm on the medium-voltage side and the a-phase and b-phase bridge arms on the low-voltage side, the analysis results are consistent with those of the A-phase bridge arm, thus obtaining the power transfer model of the MMC-DCT system, such as... Figure 5 As shown in the figure, K is the turns ratio of the primary and secondary sides of the transformer.

[0070] In the simplified equivalent power model of the MMC-DCT system, the expression for the effective value of the output power P, the number of levels at which the output voltage is switched, and the angle between the output voltage and the equivalent inductor current are all included. The specific quantitative relationship between them is as follows: To ensure that, while maintaining the effective output power P, the value of i in the simplified model is... L The amplitude is the smallest, and there is a formula...

[0071]

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

[0073] Under fundamental frequency conditions, at this time it is necessary to make It has a maximum value, and the following formula applies:

[0074]

[0075] In the formula, V(ωt) is the equivalent voltage value, V dcn θ represents the voltage magnitude of the submodule when the nth level is applied. n Let sin(ωt) = 1, where ωt is the angle obtained by the ratio of the nth level input time to a quarter of a switching cycle.

[0076] when In the formula V dcn The magnitude is based on the capacitor voltage averaging strategy in the MMC-DCT system. If the magnitudes are equal, then... There is a maximum value, and the number of input levels is 0, from which we obtain

[0077]

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

[0079] Equivalent inductance current The minimum magnitude of the equivalent inductance voltage and the magnitude of the equivalent inductance voltage The relation, expressed as:

[0080] In the formula, ω=2πf s f s Let L be the operating frequency of the entire MMC-DCT system, and L be the magnitude of the final equivalent inductance of the entire system, where L = L s1 +L k +K 2 L s2 Ls1 For the equivalent inductance on the input-side bridge arm, L s2 L is the equivalent inductance on the output side bridge arm. k This is the leakage inductance equivalent to that of a high-frequency transformer.

[0081] and In the phasor relationship between them, if and The relationship is known, and it is obtained through both and From the angular relationship, we can know that and If the included angle 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 at this point.

[0084] In step S5, the number of input levels is the same as at this time. The relationship between the modulus values ​​is as follows: Similar to the relationship between the input level number and angle, we first divide it into n sub-modules, and then calculate a sequence of input level numbers starting from 0 and ending at n-1, denoted as b0, b1, b2...b n-1 Find the closest The ideal voltage level is the minimum current 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 with MATLAB / Simulink.

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

[0087] When performing system simulation using the MMC-DCT simulation model, the voltage balancing of the submodule capacitors at the bridge arm of the DC solid-state transformer MMC-DCT based on the modular multilevel converter adopts a dual-sorting voltage balancing strategy.

[0088] Example:

[0089] This embodiment uses MATLAB / Simulink to build an MMC-DCT simulation model and performs simulation verification on the above topology. The simulation parameters are shown in Table 1.

[0090] Table 1. Simulation System Parameters of MMC-DCT System

[0091]

[0092] In this embodiment, by further substituting the values ​​from the table above into the calculation formula, we can 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, the further known active power P and We can obtain:

[0097]

[0098] In this embodiment, the and After obtaining them, since they are perpendicular to each other, then...

[0099]

[0100] According to the phasor diagram of the three, we can see that

[0101]

[0102] In this embodiment, further, as can be seen from the above analysis, the number of input levels and the current... The relationship between the modulus values ​​is specifically expressed in the following mathematical formula.

[0103]

[0104] Divide it into n sub-modules and substitute the 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)] nThe voltage levels in the diagram are uniformly distributed, with θ1 set to 10°. This is divided into 9 modules. The value n closest to the theoretical ideal value is calculated and denoted as b0, b1, b2, ..., b... n-1 V dc =1.1×10 3 V,

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

[0108] b0 = V dc1 ·9=1×10 4 V;

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

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

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

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

[0113] When n = 3, the number of voltage levels input is 3.

[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 voltage levels input is 4.

[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 is easy to conclude from the above that when four levels are applied, the value is closest to the ideal value.

[0118] Therefore, to theoretically minimize the inductor current, the primary side needs to be supplied with 4 voltage levels, and the secondary side with 0 voltage levels. The simulation results demonstrate that the primary side output should be an 8-level stepped wave, and the secondary side should output a rectangular wave. This invention can achieve the simulation effect of the corresponding parameters by changing the trapezoidal wave modulation strategy.

[0119] The modulation strategy used in the simulation of this MMC-DCT system is quasi-square wave modulation, and the voltage balancing of the submodule capacitors adopts a dual-order voltage balancing strategy, such as... Figure 7 The diagram shows the primary and secondary output voltage waveforms of the MMC-DCT system with all nine voltage levels engaged. Both waveforms are 10-level stepped waves. The primary peak voltage is 10kV, the secondary peak voltage is 6kV, the output voltage is 6kV, and the transmitted power is 1MW. The inductor current is measured under different voltage levels. Figure 7 As shown, when the circuit is at full level, the output inductor current is approximately 120A.

[0120] The trapezoidal wave modulation strategy and related parameters on the low-voltage side of the modulation are used to make the simulation output an 8-level symmetrical stepped wave, while the secondary side is a rectangular wave. The corresponding output peak voltage is consistent with the voltage at full voltage level. The inductor current is then measured under different voltage levels. Figure 8 As shown, when the circuit is at full level, the output inductor current is 105A. Compared to the full-level circuit, the inductor current is reduced by 13%, achieving current optimization.

[0121] To implement the above embodiments, this example also proposes an external circuit system that, when the instruction processor in the circuit system is executed, performs energy transfer as proposed in the MMC-DCT system circuit of the foregoing embodiments of the present invention. Figure 9 A block diagram of an exemplary electronic device suitable for implementing embodiments of the present invention is shown. Figure 9 The illustrated external circuit system layout diagram is merely an example and should not impose any limitations on the functionality and scope of use of the embodiments of the present invention. Figure 9As shown, the external circuit layout design is represented in the form of a general-purpose computing device. Components of the external circuit layout design 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 computer system-readable media in the form of volatile memory, such as random access memory (RAM) and / or cache memory. The memory may include at least one program product having a set (e.g., at least one) of program modules configured to perform the functions of the embodiments of the present invention. The electronic device may also communicate with one or more external devices (e.g., keyboard, pointing device, display, etc.), and may also communicate with one or more devices that enable a user to interact with the electronic device / or with any device that enables the electronic device to communicate with one or more other computing devices (e.g., network interface card, modem, etc.). Such communication may be performed via an input / output (I / O) interface. The processing unit executes various functional applications by running programs stored in the system memory, such as implementing the MMC-DCT system circuit control strategy method mentioned in the foregoing embodiments.

[0122] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A current optimization method for a simplified model of an MMC-DCT system, characterized in that: The method includes the following steps; Step S1: Based on the working principle of the MMC-DCT system, set the power equivalence principle equivalent value as the simplified equivalent power model of the MMC-DCT system; Step S2: Set the expression for the effective value of the output power P using circuit calculation methods; Step S3: Under the premise of constant output power, change the voltage level of the transformer secondary output voltage and the angle between the transformer secondary output voltage and the equivalent inductor current. Obtain the equivalent inductance current at this time The minimum modulus; Find the equivalent inductance current. The magnitude of the equivalent inductance voltage at its minimum magnitude Step S4, from and The phasor relationship between them at this point is obtained. The modulus; Step S5: From the input level number and at this time Based on the relationship between the modulus and the input current, the number of input levels closest to the ideal optimized current is calculated.

2. The current optimization method for a simplified model of an 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 input voltage levels, so as to reduce the impact of excessive internal circulating current on the power transmission efficiency of the MMC-DCT system and internal losses.

3. The current optimization method for a simplified model of an MMC-DCT system according to claim 1, characterized in that: The MMC-DCT is a DC solid-state transformer based on a modular multilevel converter. It adopts a two-phase full-bridge MMC-DCT main circuit topology, which consists of a medium- and low-voltage side and a high-frequency transformer. The low-voltage side ports are used to connect to DC buses of different voltage levels, providing grid connection interfaces on power generation lines or distribution interfaces on transmission lines, to meet the adaptation conditions under different environmental requirements. When MMC-DCT is expressed using an equivalent model Let u Ao and u Bo The voltages at points A and B relative to the neutral point O are V, and the voltages at points P and N relative to the neutral point are V. dc1 and -V dc1 Ignoring the voltage generated across the bridge arm inductor by the bridge arm current, the system voltage relationship is expressed as: Subtracting the two equations, we get: From the above equation, changing the voltage difference between the upper and lower bridge arms will correspondingly change the voltage waveform on the AC side of the transformer. Assuming this topology is two-phase and strictly symmetrical, and each phase can be analyzed independently, then in the equivalent circuit diagram of phase A, the input current I of the DC bus on the medium voltage side is... dc1 The transformer primary current i will be evenly distributed between phases A and B. ac1 It will also be evenly distributed between the upper and lower bridge arms, while taking into account the circulating current i generated between the bridge arms. cira The system current relationship is then expressed as: When the inductance of the bridge arm has a suppressive effect on the circulating current, it is approximately assumed that the potentials of points A1 and A2 are equal, and points A1 and A2 are considered to be virtually short-circuited. After the short circuit, the inductances of the upper and lower bridge arms are equivalent to a parallel relationship, that is, the inductance value becomes half of the original bridge arm inductance value. Similarly, when the B-phase bridge arm on the medium-voltage side and the a-phase and b-phase bridge arms on the low-voltage side are analyzed separately, the results are consistent with the analysis of the A-phase bridge arm, thus obtaining the power transmission model of the MMC-DCT system.

4. The current optimization method for a simplified model of an MMC-DCT system according to claim 1, characterized in that: In the simplified equivalent power model of the MMC-DCT system, the expression for the effective value of the output power P, the number of levels at which the output voltage is switched, and the angle between the output voltage and the equivalent inductor current are all included. The specific quantitative relationship between them is as follows: To ensure that, while maintaining the effective output power P, the value of i in the simplified model is... L The amplitude is the smallest, and there is a formula... when Reaching the theoretical maximum value, at the same time but and The angle between them is 0, and they are on the same horizontal line; Under fundamental frequency conditions, at this time it is necessary to make It has a maximum value, and the following formula applies: In the formula, V(ωt) is the equivalent voltage value, V dcn θ represents the voltage magnitude of the submodule when the nth level is applied. n Let sin(ωt) = 1, where ωt is the angle obtained by the ratio of the nth level input time to a quarter of a switching cycle.

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

6. The current optimization method for a simplified model of an MMC-DCT system according to claim 1, characterized in that: Equivalent inductance current The minimum magnitude of the equivalent inductance voltage and the magnitude of the equivalent inductance voltage The relation, expressed as: In the formula, ω=2πf s f s Let L be the operating frequency of the entire MMC-DCT system, and L be the magnitude of the final equivalent inductance of the entire system, where L = L s1 +L k +K 2 L s2 L s1 For the equivalent inductance on the input-side bridge arm, L s2 L is the equivalent inductance on the output side bridge arm. k This is the leakage inductance equivalent to that of a high-frequency transformer.

7. The current optimization method for a simplified model of an MMC-DCT system according to claim 1, characterized in that: and In the phasor relationship between them, if and The relationship is known, and it is obtained through both and From the angular relationship, we can know that and If the included angle is a right angle, then the above conditions can be obtained. In the formula for and The angle between them can be obtained at this point.

8. The current optimization method for a simplified model of an MMC-DCT system according to claim 1, characterized in that: In step S5, the number of input levels is the same as at this time. The relationship between the modulus values ​​is as follows: Similar to the relationship between the input level number and angle, we first divide it into n sub-modules, and then calculate a sequence of input level numbers starting from 0 and ending at n-1, denoted as b0, b1, b2…b n-1 Find the closest The ideal voltage level is the minimum current corresponding to the input voltage side.

9. The current optimization method for a simplified model of an 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 with MATLAB / Simulink.

10. A current optimization method for a simplified model of an MMC-DCT system according to claim 9, characterized in that: When performing system simulation using the MMC-DCT simulation model, the modulation strategy adopted by the DC solid-state transformer MMC-DCT based on the modular multilevel converter is quasi-square wave modulation. When performing system simulation using the MMC-DCT simulation model, the voltage balancing of the submodule capacitors at the bridge arm of the DC solid-state transformer MMC-DCT based on the modular multilevel converter adopts a dual-sorting voltage balancing strategy.

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