CN107994573A - A kind of Multi-end flexible direct current transmission system multi-objective optimization design of power method - Google Patents

Abstract

The invention discloses a multi-objective optimization design method for a multi-terminal flexible direct current transmission system, comprising: building a multi-terminal flexible direct current transmission system model; starting the simulation of the multi-terminal flexible direct current transmission system model; detecting in real time the DC capacitance voltage of each sub-module of the multi-terminal flexible direct current transmission system ;Real-time monitoring of the current direction of each bridge arm of the multi-terminal flexible DC power transmission system is used as the basis for judging the charging and discharging of the capacitor; calculate the number of sub-modules m _ij that need to be opened for the j-th bridge arm of the i-th VSC in each switching cycle, and pass the to-be-optimized Parameters correct the capacitor voltage; determine the input and removal status of the sub-module and send it to the control system, and establish the objective function of voltage fluctuation and the number of switching times during stable operation; use multi-objective genetic algorithm to solve the optimal solution set of optimized variables. The invention satisfies better voltage balance effect and lower switching frequency at the same time, so that both can better meet the requirements, and has higher calculation efficiency, and is especially suitable for the occasion of multi-terminal flexible direct current transmission.

Description

A multi-objective optimization design method for multi-terminal flexible direct current transmission system

technical field

The invention relates to the technical field of new energy power generation, in particular to a multi-objective optimization design method for a multi-terminal flexible direct current transmission system.

Background technique

Multi-terminal flexible DC transmission system (Voltage Source Converter Multi-terminal HVDC, VSC-MTDC) refers to a flexible DC transmission system that contains three or more voltage source converter stations (voltage source converter, VSC) under the same DC network frame. Its most notable feature is that it can realize multi-power supply and multi-drop power receiving. As a more flexible and fast power transmission method, the multi-terminal flexible DC transmission system will have broad application prospects in the fields of wind power and other new energy grid integration, and the construction of urban DC distribution networks. With the rapid development of the voltage source converter, the DC transmission system based on the voltage source converter has become the mainstream trend in the development of the DC transmission system. Among the numerous voltage source converter topologies, the modular multilevel converter (MMC) based on H half-bridge cascading has become a One of the preferred topologies for future VSC-MTDC projects.

In the MMC DC transmission project, each converter bridge arm has many sub-modules (SM), and the balance of SM capacitor voltage has become an unavoidable problem in the application of MMC engineering. In actual engineering, the number of SMs is very large. The traditional voltage equalization method usually adopts the SM capacitor voltage sorting method, but the simple capacitor voltage sorting will cause high switching loss, and when the number of bridge arm SMs is too large, the number of pairs reaches It is undoubtedly a huge engineering difficulty to sort the voltages of hundreds or even thousands of scattered SMs, and the energy balance between SMs also becomes very difficult. Moreover, the multi-terminal flexible DC transmission system has more terminals than the traditional DC transmission system, and the number of sub-modules has doubled, requiring more complex calculations.

Some invented equalization methods actually set a set of upper and lower voltage limits near the rated value of the capacitor voltage, and focus the balance control on the SM whose capacitor voltage exceeds the limit, so that according to the limit situation, it will be processed first and then sorted. It can reduce the fluctuation of the DC capacitor voltage to a very small range, but it will inevitably cause frequent switching on and off of the switching frequency in the SM, thereby increasing the switching frequency of the MMC. The capacitive voltage equalization algorithm that maintains sorting is added to introduce state-holding parameters, which effectively avoids unnecessary repeated switching of the same insulated gate bipolar transistor (IGBT) caused by the sorting algorithm, and reduces losses.

It can be seen that in the traditional voltage equalization method, the capacitor voltage balance of the SM and the switching frequency of the MMC are a pair of contradictions, and the two goals are mutually exclusive and cannot be satisfied at the same time. There is still no effective solution to the above problems in the prior art.

Contents of the invention

In order to solve the deficiencies of the prior art, the purpose of the present invention is to provide a multi-objective optimization design method for a multi-terminal flexible direct current transmission system. The present invention can satisfy better voltage balance at the same time through the optimal design of voltage limit and state maintenance parameters effect and lower switching frequency, so that both can better meet the requirements.

A multi-objective optimization design method for a multi-terminal flexible direct current transmission system, comprising the following steps:

Step 1: Build a multi-terminal flexible DC transmission system model in the power system transient simulation software according to the actual project;

Step 2: Set the initial parameters of the model operation and the initial values of the parameters to be optimized;

Step 3: Start the simulation of the multi-terminal flexible direct current transmission system model;

Step 4: Real-time detection and real-time storage of the DC capacitor voltage of each sub-module of the multi-terminal flexible DC transmission system;

Step 5: Real-time monitoring of the current direction of each bridge arm of the multi-terminal flexible direct current transmission system as the judgment basis for capacitor charging and discharging and storing in real time;

Step 6: Calculate the number of sub-modules m _ij that need to be turned on for the j-th bridge arm of the i-th terminal VSC in each switching cycle according to the nearest level approach debugging method, and the VSC is a voltage source converter station;

Step 7: Correct the capacitor voltage through the parameters to be optimized;

Step 8: Determine the switching status of the sub-modules according to the sorting result of the capacitor voltage and send it to the control system;

Step 9: Determine whether the model operation is over, if so, go to step 10, otherwise, perform the next switching cycle calculation, return to step 4, until the model operation is completed;

Step 10: Establish voltage fluctuation objective function and switching times objective function during stable operation;

Step 11: Use the multi-objective genetic algorithm to solve the optimal solution set of the optimization variable; judge whether the maximum algebra is reached, if so, obtain the optimal value of the variable parameter optimal solution set and apply it to the actual project of the multi-terminal flexible direct current transmission system, otherwise , add 1 to the optimization algebra and return to step 3 for the next cycle calculation.

Further, in the step 2, the initial parameters of model operation include the model running time, and the initial values of the parameters to be optimized include the initial value of the state maintenance parameters, state Change the initial value of the parameter, the initial value of the voltage rise parameter, and the initial value of the voltage drop parameter.

Further, in the second step, an iterative optimization algebra of the multi-objective genetic algorithm is also set.

Further, in the step seven, the capacitor voltage is corrected by the parameters to be optimized as follows:

The jth bridge arm of the i-th terminal VSC, when the current flows to the SM, that is, when the SM is charged, the SM capacitor voltage in the input state is multiplied by the state change parameter h _2ij , and the SM capacitor in the cut-off state and the capacitor voltage is lower than the voltage lower limit The voltage is multiplied by the voltage drop parameter l _2ij .

Further, in the step seven, when the jth bridge arm of the i-th terminal VSC, when the current flows out of the SM, that is, the SM is discharged, the capacitance voltage of the SM in the discharging state is multiplied by the state maintenance parameter h _1ij ; it will be in the cutting state And the SM capacitor voltage whose capacitor voltage is higher than the voltage upper limit is multiplied by the voltage rise parameter l _1ij .

Further, in the eighth step, the SMs of the j-th bridge arm of the i-th end VSC of the multi-terminal flexible direct current transmission system are sorted according to the capacitance voltage value, and if the current flows to the SM, that is, the SM is charged, the capacitor voltage is changed from high to high. Input m _ij SMs in low order, and cut off the remaining SMs; if the current flows out of the SMs, that is, the SMs are discharged, put in m _ij SMs in the order of capacitor voltage from low to high, and cut off the remaining SMs;

Send the SM switch status of each bridge arm of the flexible DC transmission system to the control system to control the switch status of the sub-modules.

Further, in the tenth step, the voltage fluctuation objective function and the switching times objective function during stable operation are established: the state maintenance parameter h _1ij , the state change parameter h _2ij , the voltage rise parameter l _1ij , the voltage drop The parameter l _2ij is used as the optimization variable [h _1ij h _2ij l _1ij l _2ij ]; the objective function F ₁ of voltage fluctuation and the objective function F ₂ of switching times during stable operation are established.

Further, the voltage fluctuation objective function _F1 is defined as the maximum value of the capacitor voltage fluctuations of all sub-modules within the statistical stable operation time:

F ₁ ＝max(|U _Cijz -U _C |)

Among them, U _Cijk is the capacitive voltage of the z-th sub-module of the j-th bridge arm of the i-th VSC of the multi-terminal flexible DC transmission system within a switching cycle; U _C is the rated capacitive voltage of the sub-module.

Further, the switching times objective function _F2 is defined as counting the switching times of all sub-modules within the stable running time:

Among them, X _ij (k) is the switching times of all sub-modules on the j-th bridge arm of the i-th terminal VSC in the k-th switching cycle; S _ijz (k) is the j-th bridge arm of the i-th VSC in the k-th switching cycle The working state of the z-th sub-module of the bridge arm, S _ijz (k+1) is the working state of the z-th sub-module of the j-th bridge arm of the VSC at the i-th terminal VSC in the k+1th switching cycle.

Further, in the eleventh step, a Pareto optimal solution is selected using a multi-objective decision-making model based on Nash equilibrium points.

Further, the multi-objective optimization method of the multi-terminal flexible direct current transmission system, the applied multi-terminal flexible direct current transmission system is composed of four-terminal voltage source converter station VSC, and the one-terminal voltage source converter station VSC of the multi-terminal flexible direct current transmission system adopts modular level The topological structure of connected multi-level MMC, each phase in VSC includes upper and lower bridge arms, each phase has the same structure, AC side series reactance L _s and resistance R _s ; L ₀ is connected in series between the upper and lower bridge arms;

Each bridge arm is composed of n identical sub-modules SM in series, and each sub-module contains two IGBTs, freewheeling diodes and a DC capacitor; when the upper IGBT of the sub-module is turned on and the lower IGBT is turned off, the sub-module is switched on; When the lower IGBT of the module is turned on and the upper IGBT is turned off, the sub-module is cut off, and the output voltage of the sub-module can be controlled by switching the switch state.

Compared with prior art, the beneficial effect of the present invention is:

(1) The traditional method can only meet the requirements of reducing the fluctuation of the capacitor voltage or reducing the switching frequency, and the two goals are mutually exclusive and cannot be satisfied at the same time. The invention can better modify the SM capacitor voltage, optimize the capacitor voltage sorting effect, and satisfy better voltage balance effect and lower Switching frequency, so that both can better meet the requirements.

(2) The traditional method can only calculate the switching state of one bridge arm sub-module at a time. The multi-terminal flexible DC transmission system includes at least nine bridge arms of three-terminal VSC, and the optimization calculation of capacitor voltage equalization is huge. The present invention takes the minimum voltage fluctuation and the minimum number of switching times as the optimization target, adopts an intelligent parallel algorithm to simultaneously optimize the state maintenance parameters, state change parameters, voltage rise parameters, and voltage drop parameters, and can simultaneously obtain the switch states of sub-modules on multiple VSC bridge arms , which is more computationally efficient.

(3) The present invention builds a multi-terminal flexible DC transmission system model in the power system transient simulation software according to the actual engineering, detects the electrical quantity in the multi-terminal flexible DC transmission system in real time, and calculates the needs of each bridge arm according to the nearest level approximation debugging method The number of opened sub-modules is run multiple times through the multi-terminal flexible DC transmission system model, that is, multiple iterative optimization of the multi-objective genetic algorithm NSGA-II, and the selection of state maintenance parameters, state change parameters, voltage rise parameters, and voltage drop parameters is continuously optimized. Value, to achieve the minimum fluctuation of sub-module capacitor voltage and the minimum number of switching times during stable operation, and apply the multi-objective optimization design results to the actual multi-terminal flexible DC project. This method satisfies better voltage balance effect and lower switching frequency at the same time, so that both can better meet the requirements, and the calculation efficiency is higher, especially suitable for the occasion of multi-terminal flexible direct current transmission.

Description of drawings

The accompanying drawings constituting a part of the present application are used to provide further understanding of the present application, and the schematic embodiments and descriptions of the present application are used to explain the present application, and do not constitute improper limitations to the present application.

Fig. 1 is a flowchart of a multi-objective optimization design method for a multi-terminal flexible direct current transmission system;

Figure 2 is a schematic diagram of the main wiring of a multi-terminal flexible direct current transmission system project;

Fig. 3 is a schematic diagram of a single-ended main circuit of a multi-terminal flexible direct current transmission system;

Fig. 4 is a schematic diagram of a Pareto optimal solution set for optimizing parameters of a multi-terminal flexible direct current transmission system.

Detailed ways

It should be pointed out that the following detailed description is exemplary and intended to provide further explanation to the present application. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

It should be noted that the terminology used here is only for describing specific implementations, and is not intended to limit the exemplary implementations according to the present application. As used herein, unless the context clearly dictates otherwise, the singular is intended to include the plural, and it should also be understood that when the terms "comprising" and/or "comprising" are used in this specification, they mean There are features, steps, operations, means, components and/or combinations thereof.

As introduced in the background technology, there is a contradiction between the capacitor voltage balance of SM and the switching frequency of MMC in the prior art, and the two goals are mutually exclusive and cannot be satisfied at the same time. In order to solve the above technical problems, this application proposes a A multi-objective optimization design method for a multi-terminal flexible direct current transmission system.

In a typical implementation of the present application, Figure 2 is provided as a schematic diagram of the main wiring of a multi-terminal flexible direct current transmission system project, which consists of a four-terminal voltage source converter station (VSC).

Figure 3 shows a schematic diagram of the main circuit of one-terminal voltage source converter station (VSC) of the multi-terminal flexible DC transmission system, which adopts the topology structure of modular cascaded multilevel (MMC). Each phase in VSC includes upper and lower two The bridge arm has the same structure for each phase, and the AC side has a series reactance L _s and a resistance R _s ; L ₀ is connected in series between the upper and lower bridge arms, which can suppress the circulating current to a certain extent. Each bridge arm is composed of n identical sub-modules (SM) connected in series, and each sub-module contains two IGBTs, freewheeling diodes and a DC capacitor. When the upper IGBT of the sub-module is turned on and the lower IGBT is turned off, the sub-module is switched on; when the lower IGBT of the sub-module is turned on and the upper IGBT is turned off, the sub-module is cut off. By switching the switch state, the control of the output voltage of the sub-module can be realized.

In actual engineering, the multi-terminal flexible DC transmission system includes more than 3 voltage source converter stations (VSC), and the number of SM is very large. The traditional voltage equalization method usually adopts the method of sorting and gating the voltage of SM capacitors, and the sorting of simple capacitor voltage equalization It will cause high switching loss, and when there are too many bridge arm SMs, it is undoubtedly a huge engineering difficulty to sort the voltages of hundreds or even thousands of scattered SMs, and the energy balance between SMs also becomes very difficult.

As shown in Figure 1, a multi-objective optimization design method for a multi-terminal flexible direct current transmission system of the present invention, the specific steps are:

(1) Build the multi-terminal flexible DC transmission system model in the power system transient simulation software according to the actual project, and set the model running time to 1 second; set the initial value of the parameters to be optimized: the jth value of the i-th VSC of the multi-terminal flexible DC transmission system The initial value of the state maintenance parameter of each bridge arm h _1ij =1.02, the range of change is [1.0-1.05]; the initial value of the state change parameter h _2ij =0.98, the range of change is [0.95-1.0]; the initial value of the voltage rise parameter is l _1ij = 1.05, change range [1.0～1.1]; initial value of voltage drop parameter l _2ij =0.95, change range [0.9～1.0]; set multi-objective genetic algorithm NSGA-II iterative optimization algebra Gen=50;

(2) Start the simulation of the multi-terminal flexible direct current transmission system in the power system transient simulation software;

(3) For each switching cycle, the DC capacitor voltage of each sub-module in the multi-terminal flexible direct current transmission system is detected in real time and stored in real time; the direction of the current of each bridge arm of the multi-terminal flexible direct current transmission system is monitored in real time as a judgment basis for capacitor charging and discharging , and stored in real time.

(4) Calculate the number of sub-modules m _ij that need to be turned on for the j-th bridge arm of the i-th terminal VSC in each switching cycle according to the nearest level approach debugging method;

(5) The jth bridge arm of the i-th terminal VSC, when the current flows to the SM, that is, when the SM is charged, the SM capacitor voltage in the input state is multiplied by the state change parameter h _2ij , and it will be in the cut-off state and the capacitor voltage is lower than the lower voltage limit Multiply the SM capacitance voltage of the SM by the voltage drop parameter l _2ij ; when the current flows out of the SM, that is, when the SM is discharged, multiply the capacitor voltage of the SM in the discharge state by the state maintenance parameter h _1ij ; the SM in the cutting state and the capacitor voltage is higher than the upper voltage limit The capacitor voltage is multiplied by the voltage rise parameter l _1ij ;

(6) In this switching cycle, sort the SMs of the j-th bridge arm of the i-th VSC of the multi-terminal flexible direct current transmission system according to the capacitance voltage value. Put in m _ij SMs in the order of and cut off the remaining SMs; if the current flows out of the SMs, that is, the SMs are discharged, put in m _ij SMs in the order of capacitor voltage from low to high, and cut off the rest of the SMs;

(7) In this switching period, the SM switching state of each bridge arm of the flexible direct current transmission system is sent to the control system to control the switching state of the sub-module.

(8) Repeat step (2) to step (7) for the next switch cycle until the end of the model operation;

(9) The state maintenance parameters h _1ij , state change parameters h _2ij , voltage rise parameters l _1ij , and voltage drop parameters l _2ij of the multi-terminal flexible DC transmission system are used as optimization variables [h _1ij h _2ij l _1ij l _2ij ]; stable running time Generally take the time period between 0.5 seconds and 0.7 seconds; establish the voltage fluctuation target function F ₁ and the switching times target function F ₂ during stable operation;

The voltage fluctuation objective function _F1 is defined as the maximum value of the capacitor voltage fluctuations of all sub-modules within the statistical stable operation time:

F ₁ ＝max(|U _Cijz -U _C |)

Among them, U _Cijk is the capacitive voltage of the z-th sub-module of the j-th bridge arm of the i-th VSC of the multi-terminal flexible DC transmission system within a switching cycle; U _C is the rated capacitive voltage of the sub-module;

The switching times objective function _F2 is defined as counting the switching times of all sub-modules within the stable running time:

Among them, X _ij (k) is the switching times of all sub-modules on the j-th bridge arm of the i-th terminal VSC in the k-th switching cycle; S _ijz (k) is the j-th bridge arm of the i-th VSC in the k-th switching cycle The working state of the zth submodule of the bridge arm, S _ijz (k+1) is the working state of the zth submodule of the jth bridge arm of the jth bridge arm of the k+1 switch period VSC;

(10) Use the multi-objective genetic algorithm NSGA-II to solve the Pareto optimal solution set of the optimization variable [h _1ij h _2ij l _1ij l _2ij ], and add 1 to the iterative optimization algebra Gen;

(11) If the iterative optimization algebra Gen is less than or equal to 50, repeat steps (2) to (10);

(12) The Pareto optimal solution [h _1ij h _2ij l _1ij l _2ij ] is selected using the multi-objective decision-making model based on Nash equilibrium points, as shown in Figure 4, and applied to the actual project of the multi-terminal flexible DC transmission system.

The invention aims at the problem that the traditional method can only meet the requirements of reducing the fluctuation of the capacitor voltage or reducing the switching frequency, and the two goals are mutually exclusive and cannot be satisfied at the same time. The present invention obtains the optimal working state S ^* of the three bridge arms of the flexible direct current transmission system through the above-mentioned method, can simultaneously reduce sub-modules and switching frequency, and solves the fundamental problem of capacitor voltage equalization control of the flexible direct current transmission system.

In addition, the traditional method can only calculate the switching state of one bridge arm sub-module at a time, and the computational complexity of the traditional sorting voltage equalization control method is The computational complexity of the present invention is n×n _gen , where n _gen is the evolution algebra of the intelligent algorithm. Since the number of bridge arms SM is very large, reaching hundreds or even thousands, n _gen <n. The flexible direct current transmission system includes three bridge arms. If the multi-terminal flexible direct current system has more bridge arms, the algorithm efficiency of the present invention is more obvious and the calculation complexity is lower.

The above descriptions are only preferred embodiments of the present application, and are not intended to limit the present application. For those skilled in the art, various modifications and changes may be made to the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of this application shall be included within the protection scope of this application.

Claims (10)

1. A method for multi-objective optimal design of a multi-terminal flexible direct current transmission system, characterized in that it comprises the following steps: Step 1: Build a multi-terminal flexible DC transmission system model in the power system transient simulation software according to the actual project; Step 2: Set the initial parameters of the model operation and the initial values of the parameters to be optimized; Step 3: Start the simulation of the multi-terminal flexible direct current transmission system model; Step 4: Real-time detection and real-time storage of the DC capacitor voltage of each sub-module of the multi-terminal flexible DC transmission system; Step 5: Real-time monitoring of the current direction of each bridge arm of the multi-terminal flexible direct current transmission system as the judgment basis for capacitor charging and discharging and storing in real time; Step 6: Calculate the number of sub-modules m _ij that need to be turned on for the j-th bridge arm of the i-th terminal VSC in each switching cycle according to the nearest level approach debugging method, and the VSC is a voltage source converter station; Step 7: Correct the capacitor voltage through the parameters to be optimized; Step 8: Determine the switching status of the sub-modules according to the sorting result of the capacitor voltage and send it to the control system; Step 9: Determine whether the model operation is over, if so, go to step 10, otherwise, perform the next switching cycle calculation, return to step 4, until the model operation is completed; Step 10: Establish voltage fluctuation objective function and switching times objective function during stable operation; Step 11: Use the multi-objective genetic algorithm to solve the optimal solution set of the optimized variable; judge whether the maximum algebra is reached, if so, obtain the optimal value of the variable parameter optimal solution set and apply it to the actual project of the multi-terminal flexible direct current transmission system, otherwise , add 1 to the optimization algebra and return to step 3 for the next cycle calculation. 2. A multi-terminal flexible direct current transmission system multi-objective optimization design method as claimed in claim 1, characterized in that, in said step 2, the initial parameters of model operation include model running time, and the initial values of said parameters to be optimized include The initial value of the state maintenance parameter, the initial value of the state change parameter, the initial value of the voltage rise parameter and the initial value of the voltage drop parameter of the jth bridge arm of the i-th end VSC of the multi-terminal flexible direct current transmission system. 3. A method for multi-objective optimization design of a multi-terminal flexible direct current transmission system according to claim 2, characterized in that, in said step 2, an iterative optimization algebra of the multi-objective genetic algorithm is also set. 4. A multi-terminal flexible direct current transmission system multi-objective optimization design method as claimed in claim 1, characterized in that, in said step seven, the correction of the capacitor voltage by the parameters to be optimized is specifically: The jth bridge arm of the i-th terminal VSC, when the current flows to the SM, that is, when the SM is charged, the SM capacitor voltage in the input state is multiplied by the state change parameter h _2ij , and the SM capacitor in the cut-off state and the capacitor voltage is lower than the voltage lower limit The voltage is multiplied by the voltage drop parameter l _2ij . 5. The method for multi-objective optimization design of a multi-terminal flexible direct current transmission system as claimed in claim 4, wherein in said step seven, the j-th bridge arm of the i-th end VSC discharges when the current flows out of the SM , the capacitor voltage of the SM in the discharge state is multiplied by the state maintenance parameter h _1ij ; the capacitor voltage of the SM in the cut-off state and the capacitor voltage is higher than the voltage upper limit is multiplied by the voltage rise parameter l _1ij . 6. A multi-objective optimization design method for a multi-terminal flexible direct current transmission system as claimed in claim 1, wherein in said step eight, the jth bridge arm of the i-th end VSC of the multi-terminal flexible direct current transmission system is The SMs are sorted according to the capacitor voltage value. If the current flows to the SM, that is, the SM is charged, m _ij SMs are input according to the order of the capacitor voltage from high to low, and the remaining SMs are cut off; if the current flows out of the SM, that is, the SM is discharged, then the capacitor Input m _ij SMs in order of voltage from low to high, and cut off the remaining SMs; Send the SM switch status of each bridge arm of the flexible DC transmission system to the control system to control the switch status of the sub-modules. 7. A multi-objective optimization design method for a multi-terminal flexible direct current transmission system as claimed in claim 1, wherein in said step ten, the objective function of voltage fluctuation and the number of switching times during stable operation are established: the multi-terminal flexible direct current The state maintenance parameters h _1ij , state change parameters h _2ij , voltage rise parameters l _1ij , and voltage drop parameters l _2ij of the transmission system are used as optimization variables [h _1ij h _2ij l _1ij l _2ij ]; establish a voltage fluctuation objective function F ₁ during stable operation and the switching times objective function F ₂ . 8. a kind of multi-terminal flexible direct current transmission system multi-objective optimal design method as claimed in claim 7, is characterized in that, described voltage fluctuation objective function F ₁ is defined as the capacitance voltage fluctuation amount of all sub-modules in statistical stable operation time Maximum value: F ₁ ＝max(|U _Cijz -U _C |) Among them, U _Cijk is the capacitive voltage of the z-th sub-module of the j-th bridge arm of the i-th VSC of the multi-terminal flexible DC transmission system within a switching cycle; U _C is the rated capacitive voltage of the sub-module. 9. a kind of multi-terminal flexible direct current transmission system multi-objective optimal design method as claimed in claim 7, is characterized in that, described switching times objective function F ₂ is defined as the switching times of all submodules in statistical stable operation time: <mrow><msub><mi>F</mi><mn>2</mn></msub><mo>=</mo><munder><mo>&amp;Sigma;</mo><mi>i</mi></munder><munder><mo>&amp;Sigma;</mo><mi>j</mi></munder><msub><mi>X</mi><mrow><mi>i</mi><mi>j</mi></mrow></msub><mrow><mo>(</mo><mi>k</mi><mo>)</mo></mrow></mrow> <mrow><msub><mi>X</mi><mrow><mi>i</mi><mi>j</mi></mrow></msub><mrow><mo>(</mo><mi>k</mi><mo>)</mo></mrow><mo>=</mo><mfenced open = "{" close = ""><mtable><mtr><mtd><mn>1</mn><mo>,</mo><msub><mi>S</mi><mrow><mi>i</mi><mi>j</mi><mi>z</mi></mrow></msub><mo>(</mo><mi>k</mi><mo>)</mo><mo>&amp;NotEqual;</mo><msub><mi>S</mi><mrow><mi>i</mi><mi>j</mi><mi>z</mi></mrow></msub><mo>(</mo><mi>k</mi><mo>+</mo><mn>1</mn><mo>)</mo></mtd></mtr><mtr><mtd><mn>0</mn><mo>,</mo><msub><mi>S</mi><mrow><mi>i</mi><mi>j</mi><mi>z</mi></mrow></msub><mo>(</mo><mi>k</mi><mo>)</mo><mo>=</mo><msub><mi>S</mi><mrow><mi>i</mi><mi>j</mi><mi>z</mi></mrow></msub><mo>(</mo><mi>k</mi><mo>+</mo><mn>1</mn><mo>)</mo></mtd></mtr></mtable></mfenced></mrow> Among them, X _ij (k) is the switching times of all sub-modules on the j-th bridge arm of the i-th terminal VSC in the k-th switching cycle; S _ijz (k) is the j-th bridge arm of the i-th VSC in the k-th switching cycle The working state of the z-th sub-module of the bridge arm, S _ijz (k+1) is the working state of the z-th sub-module of the j-th bridge arm of the VSC at the i-th terminal VSC in the k+1th switching cycle. 10. A method for multi-objective optimization design of a multi-terminal flexible direct current transmission system as claimed in claim 1, wherein said step eleven adopts a multi-objective decision-making model based on Nash equilibrium points to select a Pareto optimal solution.