Design and control method of modular multilevel converter

Abstract

The application discloses a design and control method of a modular multilevel converter, and belongs to the technical field of converters. Firstly, parameters of the converter are set, and then a unified model of the converter under various conditions and a difference expression of capacitor voltage and output voltage of a sub-module under the consideration of a margin are derived, and then a loop current suppression method of the converter is judged. For the passive loop current suppression, it is further judged whether a double-frequency voltage component control method is adopted to further improve the reduction effect. According to the selection result, the direct current component operation value of the sub-module of the converter, the capacitor value of the sub-module and other parameters are recalculated, and the converter is designed according to the recalculation result. The method does not affect the voltage and current stress of the switching device, does not affect the system efficiency and the active and reactive operation range of the converter, and the capacitor value of the sub-module of the converter is significantly reduced, and the overall volume and cost of the converter are significantly reduced.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of converters, and relates to a design and control method of a modular multilevel converter. BACKGROUND

[0002] The modular multilevel converter is a key equipment in the field of high-voltage direct current transmission due to its advantages of modularity, realization of a large number of output levels, and a common DC bus. The modular multilevel converter can be further divided into an active circulating current suppression method based on control and a passive circulating current suppression method based on an additional bridge arm circulating current filter according to different circulating current suppression methods. Each bridge arm of the modular multilevel converter is composed of series-connected sub-modules, and is a single-phase structure. During operation, the bridge arm needs to withstand large power fluctuations at a fundamental frequency and a double fundamental frequency, thereby causing large fluctuations of capacitor voltages of the sub-modules and endangering the safe operation of switching devices. The existing scheme often needs to use a capacitor with a large capacitance value to reduce the amplitude of capacitor voltage fluctuations to a permissible range (generally 5% to 10% of the DC component of the sub-module capacitor voltage). The cost of the capacitor in the existing sub-module is comparable to that of the switching device, and the volume of the capacitor generally accounts for 70% to 80% of the total volume of the sub-module. The large sub-module capacitor seriously affects the volume and cost of the converter. Therefore, reducing the capacitance value required by the sub-module in the modular multilevel converter has obvious benefits in reducing the overall cost and volume of the converter. SUMMARY

[0003] To solve the problems in the prior art, the application provides a design and control method of a modular multilevel converter. The method does not affect the voltage and current stress of the switching device, does not affect the system efficiency and the active and reactive operation range of the converter, significantly reduces the capacitance value of the sub-module of the converter, and significantly reduces the overall volume and cost of the converter.

[0004] The application is implemented by adopting the following technical scheme to achieve the above application purposes.

[0005] In a first aspect, the application provides a design method of a modular multilevel converter, including:

[0006] S1, setting converter parameters, including: converter AC / DC side rated voltage, active and reactive operation range, actual value of connection reactance of the converter to the power grid, and number of bridge arm sub-modules;

[0007] S2, on the basis of the set converter parameters, deriving a unified model of the converter under various conditions and a difference expression of the sub-module capacitor voltage and the sub-module output voltage in consideration of a margin;

[0008] S3, judging the circulating current suppression method of the converter: active circulating current suppression or passive circulating current suppression;

[0009] S4, active circulating current suppression is adopted, step S5 is directly performed, passive circulating current suppression is adopted, and it is judged whether the double-frequency voltage component control needs to be adopted to further increase the capacitance reduction effect; if not, step S5 is directly performed, and if yes, the corresponding double-frequency voltage component control is added in the converter control, and step S6 is performed;

[0010] S5, according to the selection result, the DC component operating value of the converter submodule, the submodule capacitance value and other parameters in the converter design are calculated, and the converter is designed according to the calculation result;

[0011] S6, if the double-frequency voltage component control is selected in step S4 to further improve the capacitance reduction effect, the double-frequency voltage component output by the three-phase bridge arm submodule is detected in the converter control structure, and the double-frequency voltage component is controlled to a target value through a control loop.

[0012] As a further improvement of the application, the converter is formed by parallel connection of three-phase phase units, each phase contains two upper and lower bridge arms, each bridge arm is composed of N submodules and a bridge reactor in series, and the two bridge arms are connected in series to form a phase unit structure. The midpoint of the two bridge arms after series connection is an AC output point, and the end points at the upper and lower ends are DC output points. The DC output points of the three-phase phase units are connected in parallel to a DC bus, and the AC output points of the three-phase phase units are connected to an AC power grid.

[0013] Among them, the bridge reactor is in the form of two reactors L1, L2 in series, and the bridge reactor midpoint appears in each bridge arm; a resonance capacitor C0 is connected across the bridge reactor midpoint of the upper and lower bridge arms of the phase unit; the resonance capacitor C0 value and the reactance value 2L1 are designed to be a double-frequency resonance state about the AC side, which can make each phase contain a double-frequency parallel resonance cavity on the DC side. The resonance cavity withstands the double-frequency voltage on the AC side in any case and blocks the flow of double-frequency circulating current in the converter.

[0014] As a further improvement of the application, the derivation of the unified model of the converter under various conditions and the difference expression of the submodule capacitor voltage and the submodule output voltage considering the margin includes:

[0015] Derivation of the unified model of the converter compatible with active circulating current suppression, passive circulating current suppression not added, and passive circulating current suppression added, and calculation of the difference expression of the submodule capacitor voltage and the submodule output voltage considering the margin.

[0016] As a further improvement of the application, the derivation of the unified model of the converter under various conditions and the difference expression of the submodule capacitor voltage and the submodule output voltage considering the margin specifically includes:

[0017] According to the converter model under different steady-state conditions, circuit parameters, and ideal control results after applying control, the final difference expression is obtained by subtracting the submodule output voltage from the submodule capacitor voltage, and then subtracting the target margin setting value of the two. Then, by applying the substitution method, the difference expression is simplified to a form without trigonometric functions. For the formula component that only appears in the case of passive circulating current suppression without submodule output voltage double-frequency component control, multiply it by the coefficient λ N ; for the formula component that only appears in the case of passive circulating current suppression with submodule output voltage double-frequency component control, multiply it by the coefficient λ C .

[0018] As a further improvement of the application, the derivation of the unified converter model under various conditions and the difference expression of the submodule capacitor voltage and the submodule output voltage considering the margin comprises the following steps:

[0019] According to the AC side rated voltage U SN and the AC side rated current I N , the AC side connection reactance per unit X pu is solved as follows

[0020] X pu = X eq / (U SN / I N )

[0021] where X eq is the actual value of the set converter to grid connection reactance; according to the AC side rated voltage U SN and the DC side voltage U dc , the rated modulation ratio m0 is solved as follows

[0022]

[0023] Further according to the AC side current per unit and the rated modulation ratio m0, the coefficient is calculated as follows

[0024]

[0025] The coefficient K dc is defined as follows

[0026]

[0027] where ε ul(NR) is the set converter rated capacitor voltage fluctuation ratio value, and ε ul(HR) is the converter capacitor voltage fluctuation ratio value.

[0028] The variable time t and the power factor angle are replaced by variables x, y as follows:

[0029]

[0030] sign(x) = {1, ωt ∈ (0, π]; -1, ωt ∈ [-π, 0]}

[0031]

[0032] where ω is the alternating side angular frequency, it can be seen that for any t and x, y take values between -1 and 1;

[0033] According to the set sub-module output voltage lower limit design value u min , the coefficient the rated modulation ratio m0 and the variable y, the modulation ratio m h of the double frequency component can be expressed as follows:

[0034]

[0035] Further, the maximum value of the capacitor voltage fluctuation function (x, y) under any x and y conditions is calculated is the alternating side current per unit value, f rS (x, y) is the system fluctuation function, which is expressed as follows

[0036]

[0037] where m h is the double frequency component modulation ratio, K dc and are the calculated coefficients, m0 is the rated modulation ratio, X pu is the alternating side connection reactance per unit value, x and y are variables; further, the difference expression of the sub-module capacitor voltage and the sub-module output voltage under the consideration of a certain margin M diff can be obtained The result is

[0038]

[0039]

[0040] As a further improvement of the application, the S5 comprises:

[0041] According to the selection result, the coefficients λ N and λ C in the difference expression are set; for the case of using active circulating current suppression, the coefficients λ N = 0 and λC = 0, for the case of passive circulating current suppression and without the control of the second harmonic voltage component, set the coefficient λ N = 1 and λ C = 0, for the case of passive circulating current suppression and with the control of the second harmonic voltage component, set the coefficient λ N = 0 and λ C = 1; and then calculate the DC component operating value of the corresponding designed sub-module and the sub-module capacitance value of the converter at this time according to the difference value expression, and design the converter according to the same.

[0042] As a further improvement of the application, the S5 specifically comprises:

[0043] In the design phase of the converter, according to the selected case, the following processing is performed, the minimum value of the difference value expression with respect to the variables x and y is solved, and further the minimum value is set to 0, and the converter capacitance voltage fluctuation ratio value ε ul(HR) is calculated at this time; for the solution of the difference value expression At the minimum value point position of x and y belonging to the range of -1 to 1 and the converter capacitance voltage fluctuation ratio value ε ul(HR) , it is solved by the following method; first solve the equation

[0044]

[0045] Solve the solution that satisfies the definition interval, if there is a solution, further determine whether the extreme point is a maximum value, a minimum value or a saddle point by checking the second derivative, and only keep the solution of the minimum value point;

[0046] At the same time, for the solution of the first function extreme value of under the four boundary conditions of x = 1, x = -1, y = 1 and y = -1, find the extreme value points on the four boundaries, and finally compare the values of the boundary extreme value points and the extreme value points inside the definition domain to determine whether the coordinates x dmin and y dimin are located on the boundary or inside the definition; further solve the equation group and with the minimum value equal to 0 according to the minimum value point position;

[0047] If the minimum value is inside the definition domain, the equation group is solved as

[0048]

[0049] If the minimum value is on the boundary x = -1, the equation group is solved as

[0050]

[0051] Similarly, the results of the minimum value on the other boundaries; by solving the equation group The minimum point coordinate x dmin , y dimin , and the converter capacitor voltage fluctuation proportion value ε ul(HR) ;

[0052] Further, according to the converter rated capacitor voltage fluctuation proportion value ε ul(NR) and the converter capacitor voltage fluctuation proportion value ε ul(HR) , the DC component operating value U cap(HR) of the corresponding designed sub-module required by the converter at this time is calculated as follows:

[0053]

[0054] Wherein, U cap(NR) is the DC component rated value of the sub-module capacitor voltage before the method is used, U cap(HR) is the calculated DC component operating value of the sub-module; U cap(NR) can be calculated as follows by the converter DC voltage U dc and the number of sub-modules of the converter bridge arm N:

[0055]

[0056] According to the converter rated capacitor voltage fluctuation proportion value ε ul(NR) , the capacitor voltage fluctuation proportion value ε ul(HR) , and the maximum value of the capacitor voltage fluctuation function , the sub-module capacitor value C sm(HR) required by the converter at this time is calculated as follows:

[0057]

[0058] Wherein, I N is the AC side current rated value, is the AC side current unit value, ω is the AC side angular frequency, U dc is the DC side voltage, and N is the number of bridge arm sub-modules.

[0059] As a further improvement of the present application, specifically includes:

[0060] The converter AC side voltage and AC side current are detected, and then the positive and negative sequence results of the converter AC side voltage and AC side current are calculated by the positive and negative sequence decoupling method;

[0061] The converter AC side current is converted to the positive sequence dq coordinate system, and the result after the rated current I N is normalized is

[0062]

[0063] Wherein, is the actual value of the AC side current of the converter converted to the positive sequence dq coordinate system, is the unit value of the AC side current of the converter converted to the positive sequence dq coordinate system, I N is the rated value of the AC side current, is the unit value of the AC side current;

[0064] Further, according to the converter setting parameters, the sub-module minimum output voltage setting value, and the positive and negative zero sequence results of the calculated AC side voltage and AC side current of the converter, the target value of the twice frequency component in the sub-module output voltage is calculated.

[0065] In a second aspect, the application provides a control method of a modular multilevel converter, which is designed by using the design method of the modular multilevel converter, and the control method comprises the following steps:

[0066] The twice frequency voltage component output by the three-phase bridge arm sub-module is detected, and is controlled to the target value through a control loop.

[0067] As a further improvement of the application, the control method specifically comprises the following steps:

[0068] The twice frequency voltage component modulation ratio m h is expressed as

[0069]

[0070] is and is obtained

[0071]

[0072] The current AC current relationship is brought in, and the above formula becomes

[0073]

[0074] The target value of the required injected negative sequence twice frequency voltage component is expressed as the unit value of U dc / N may be expressed as

[0075]

[0076] It is converted to the negative sequence twice frequency dq coordinate system as follows

[0077]

[0078] wherein

[0079]

[0080] Thus the target value of the double frequency voltage in the final submodule output voltage is for U dc The result in dq coordinate system The derivation is

[0081]

[0082] Where u min is the set submodule output voltage lower limit design value, m0 is the rated modulation ratio, X pu is the AC side connection reactance per unit value, is the q-axis per unit value of the converter AC side current converted to the positive sequence dq coordinate system;

[0083] Then the voltage across the resonant capacitor C0 of the converter or the bridge arm output voltage is detected to obtain the actual value of the double frequency component in the submodule output voltage; the actual value of the double frequency component in the submodule output voltage detected is controlled to the target value of the double frequency component in the submodule output voltage calculated through a feedback control link; and the corresponding modulation wave adjustment value is output by the feedback control link;

[0084] Finally, the modulation wave adjustment value is added to the modulation wave result of the converter, and finally sent to the modulator to generate the switching signal required by each switching device of the converter, and the control is completed.

[0085] The technical scheme of the present application has the following beneficial effects:

[0086] The present application is aimed at the modular multilevel converter under different circulating current suppression methods, and in the design stage, the minimum value of the difference between the submodule capacitor voltage and the submodule output voltage is accurately calculated according to the steady-state model of the converter, and the maximum value of the allowable capacitor voltage fluctuation of the submodule can be obtained under the premise of keeping a certain design margin according to the set parameters of the converter. Further, the new DC component operating value of the submodule capacitor voltage and the design value of the submodule capacitor capacity can be calculated. Compared with the traditional design method of setting the lower limit of the submodule capacitor voltage as a constant value, the design method of the present application can accurately design according to the target voltage margin of the submodule output voltage and the submodule capacitor voltage, effectively reduces the generation of excessive margin, and effectively reduces the required capacitor capacity of the converter submodule.

[0087] The application can be applied to the passive circulating current suppression, and the control method of the application can be selected to improve the reduction effect of the converter capacity, the control method detects the double frequency voltage component of the three-phase bridge arm sub-module output, and controls the double frequency voltage component to be a target value through a control loop. The control method controls the double frequency voltage component in the sub-module output voltage, effectively reduces the peak value of the sub-module output voltage, increases the voltage range of the sub-module capacitor voltage fluctuation, and further increases the sub-module capacitor voltage fluctuation, and correspondingly, the sub-module capacitor capacity can be significantly reduced. After the design and control method of the application is applied, the sub-module capacitor can operate under high capacitor voltage fluctuation, the converter does not need to increase additional hardware devices, does not affect the voltage and current stress of the switching device, does not affect the system efficiency and the reactive power operation range of the converter, the sub-module capacitor capacity of the converter can be significantly reduced, and the overall volume and cost of the converter can be significantly reduced. At the same time, the sub-module voltage of the converter naturally operates at a low value during operation, and there is no risk of exceeding the peak value of the capacitor voltage in the transient process. BRIEF DESCRIPTION OF DRAWINGS

[0088] The drawings described herein are for illustrative purposes only and are not intended to limit the scope of the present disclosure in any way. In addition, the shapes and proportions of the components in the drawings are only illustrative and are used to help understand the present application, and are not specific limitations on the shapes and proportions of the components. In the drawings:

[0089] Figure 1 is a structural schematic diagram of a modular multilevel converter related to the method of the application;

[0090] Figure 2 is a schematic diagram of the sub-module capacitor voltage waveform of the method of the application and the sub-module capacitor voltage waveform of the conventional method, and the sub-module output voltage waveform under various conditions;

[0091] Figure 3 is a flowchart of a control and design method of a modular multilevel converter proposed by the application.

[0092] Figure 4 is a simulation waveform diagram of the steady-state operating condition of the modular multilevel converter in the active circulating current suppression case in an embodiment of the application.

[0093] Figure 5 is a simulation waveform diagram of the steady-state operating condition of the modular multilevel converter in the passive circulating current suppression case without double frequency voltage component control in an embodiment of the application.

[0094] Figure 6 is a simulation waveform diagram of the steady-state operating condition of the modular multilevel converter in the passive circulating current suppression case with double frequency voltage component control in an embodiment of the application.

[0095] DETAILED DESCRIPTION

[0096] In order to make the purpose and technical scheme of the present application more clear and convenient to understand, the present application is further described in detail below in combination with the drawings and examples.

[0097] In the description of the present application, it should be understood that the terms "first", "second" are only for the purpose of description, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined with "first", "second" can explicitly or implicitly include one or more of the features. In the description of the present application, unless otherwise specified, the meaning of "a plurality of" is two or more. In the description of the present application, it should be noted that, unless otherwise specified and limited, the terms "mounting", "connection", "connection" should be understood in a broad sense, for example, it can be fixed connection, or detachable connection, or integral connection; it can be mechanical connection, or electrical connection; it can be directly connected, or indirectly connected through an intermediate medium, or the communication inside two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0098] The first object of the present application is to provide a design method of modular multilevel converter, to realize the reduction of sub-module capacitor value and the reduction of overall converter volume and weight. For the modular multilevel converter using different circulating current suppression methods, in the design stage, according to the steady-state model of the converter, the minimum value of the difference between the sub-module capacitor voltage and the sub-module output voltage is accurately calculated, and according to the set parameters of the converter, the maximum value of the sub-module capacitor voltage fluctuation is calculated under the premise of keeping a certain design margin. Further, the new DC component operating value of the sub-module capacitor voltage and the design value of the sub-module capacitor value can be calculated. The specific steps are described as follows:

[0099] Step 1, first set the converter parameters, including: converter AC / DC side rated voltage, active and reactive operation range, actual value of converter to grid connection reactance, and bridge arm sub-module number.

[0100] Step 2, derive the unified model of the converter with active circulating current suppression, passive circulating current suppression, without adding the two-frequency voltage component control of the present application, and passive circulating current suppression with the two-frequency voltage component control of the present application, to calculate the difference expression between the sub-module capacitor voltage and the sub-module output voltage considering the margin.

[0101] Step 3, determine whether to use active circulating current suppression or passive circulating current suppression.

[0102] Step 4, judging whether the double frequency voltage component control of the application needs to be used to further increase the capacitance reduction effect.

[0103] Step 5, setting the coefficient λ in the difference value expression according to the selection result N and λ C . For the case of using active circulating current suppression, set the coefficient λ N = 0 and λ C = 0, for the case of using passive circulating current suppression and not using double frequency voltage component control, set the coefficient λ N = 1 and λ C = 0, for the case of using passive circulating current suppression and using double frequency voltage component control, set the coefficient λ N = 0 and λ C = 1. Further, the direct current component operating value of the corresponding designed sub-module of the converter and the sub-module capacitance value at this time are calculated according to the difference value expression, and the converter is designed based on this.

[0104] Step 6, if the double frequency voltage component control is selected to further improve the capacitance reduction effect in step 4, the following link needs to be added in the converter control structure: detecting the double frequency voltage component output by the three-phase bridge arm sub-module, and controlling it to the target value through the control loop.

[0105] Compared with the traditional design method of setting the lower limit of the sub-module capacitance voltage as a constant value, the design method of the present application can be accurately designed according to the target voltage margin of the sub-module output voltage and the sub-module capacitance voltage, effectively reducing the generation of excessive margin and effectively reducing the required capacitance value of the converter sub-module. Further, for the case of using passive circulating current suppression, the control method of the present application can be selected to improve the capacitance reduction effect of the converter, and the control method is to detect the double frequency voltage component output by the three-phase bridge arm sub-module, and control it to the target value through the control loop.

[0106] Specifically, the principle of the method of the present application is to first set the converter parameters, further deduce the unified model of the converter under various conditions and the difference value expression of the sub-module capacitance voltage and the sub-module output voltage considering the margin, and then judge the circulating current suppression method of the converter. For the case of using passive circulating current suppression, it is further necessary to judge whether the double frequency voltage component control method is used to further improve the capacitance reduction effect. According to the selection result, the direct current component operating value of the converter sub-module, the sub-module capacitance value and other parameters are recalculated in the converter design, and the converter is designed based on this. If the double frequency voltage component control method is selected in the case of passive circulating current suppression, the corresponding double frequency voltage component control needs to be further added in the converter control.

[0107] As an example, the present application first sets the basic parameters of the converter, such as rated power, rated voltage, switching frequency, etc., according to the specific application scenario and performance requirements of the converter. Then, based on these parameters, the unified mathematical model of the converter under various operating conditions is derived. This step is crucial for understanding the dynamic behavior and performance of the converter. Based on considering the margin between the sub-module capacitor voltage and the sub-module output voltage, a difference expression is derived. This difference expression reflects the dynamic relationship between the capacitor voltage and the output voltage, and is the basis for subsequent design and control.

[0108] Further, according to the specific design of the converter, the adopted circulating current suppression method is determined. Circulating current suppression is an important part of converter design, which is used to reduce the loss and heating caused by circulating current. If passive circulating current suppression method is adopted, it is further determined whether to adopt double-frequency voltage component control method to further improve the reduction effect of capacitor capacitance.

[0109] Further, according to the judgment results of the circulating current suppression method and whether to adopt the double-frequency voltage component control method, the DC component operating value of the converter sub-module, the sub-module capacitance and other parameters are recalculated. Based on these recalculated parameters, the detailed design of the converter is carried out, including circuit topology, control strategy, protection strategy, etc.

[0110] If passive circulating current suppression method is selected and double-frequency voltage component control method is adopted, corresponding double-frequency voltage component control logic needs to be added in the converter control system. This control logic detects the double-frequency voltage component output by the three-phase bridge arm sub-module, and controls it to the target value through the control loop, thereby further reducing the capacitor capacitance and improving the performance of the converter.

[0111] Therefore, the present method accurately designs according to the target voltage margin between the sub-module output voltage and the sub-module capacitor voltage, effectively reducing the generation of excessive margin. This not only reduces the demand for capacitor capacitance, but also improves the efficiency and reliability of the converter. By adopting the double-frequency voltage component control method, the capacitor capacitance can be further reduced and the performance of the converter can be improved. This helps to reduce the cost and volume of the converter, while improving its operating efficiency and stability. The present method is suitable for different types of converter design and application scenarios. By flexibly adjusting parameters and control strategies, it can meet the needs and performance requirements of different users. The design and control logic of the present method is relatively simple and clear, easy to implement and maintain. This helps to reduce development cost and time, while improving the reliability and maintainability of the system.

[0112] The second object of the present application is to provide a control method of the modular multilevel converter, which effectively reduces the peak value of the output voltage of the sub-module by controlling the double-frequency voltage component in the output voltage of the sub-module, increases the voltage range of the allowable fluctuation of the capacitor voltage of the sub-module, and further makes the capacitor voltage fluctuation of the sub-module further rise, and accordingly the capacitor value of the sub-module can be significantly reduced.

[0113] After the design and control method of the present application is applied, the sub-module capacitor can operate under high capacitor voltage fluctuation, the converter does not need to increase additional hardware devices, does not affect the voltage and current stress of the switching device, does not affect the system efficiency and the reactive power operation range of the converter, the capacitor value of the sub-module of the converter can be significantly reduced, and the overall volume and cost of the converter can be significantly reduced. At the same time, during operation, the sub-module voltage of the converter naturally operates at a lower value, and there is no risk of exceeding the peak value of the capacitor voltage in the transient process.

[0114] In summary, the present application accurately designs the margin relationship between the capacitor voltage and the output voltage of the sub-module, and adopts the double-frequency voltage component control method, which effectively reduces the required capacitor value of the sub-module of the converter and improves the performance of the converter.

[0115] The present application will be described in detail below in combination with the drawings and specific embodiments.

[0116] The present application provides a design and control method of a modular multilevel converter, wherein the structure diagram of the modular multilevel converter involved is as shown in Figure 1 The modular multilevel converter using the active circulating current suppression method is as shown in Figure 1 Each bridge arm is composed of N sub-modules and a bridge arm reactor L0 in series, and the two bridge arms in series form a phase unit structure, and the midpoint of the two bridge arms after series connection is an AC output point, and the end points at the upper and lower ends are DC output points. The DC output points of the three-phase phase units are connected in parallel to a DC bus, and the AC output points of the three-phase phase units are connected to an AC power grid.

[0117] Unlike the conventional modular multilevel converter using active circulating current suppression, in the modular multilevel converter using passive circulating current suppression, the bridge arm reactor is in the form of two reactors L1, L2 in series, and the bridge arm reactor midpoints appear in each bridge arm. Further, a resonance capacitor C0 is connected across the bridge arm reactor midpoints of the upper and lower bridge arms of the phase unit, as shown in Figure 1The resonant capacitor Co value and the reactance value 2L1 are designed to be a double frequency resonance state with respect to the AC side, so that the double frequency parallel resonant cavity is contained in each phase DC side. The resonant cavity will withstand the double frequency voltage of the AC side in any case, and block the double frequency circulating current in the converter. Thus, in the passive circulating current suppression modular multilevel converter, without adding the double frequency circulating current controller, the resonant cavity naturally makes the double frequency circulating current of the converter zero in any case. It can be used for various forms of sub-modules of the modular multilevel converter, and here only the most commonly used half-bridge sub-module is shown, as shown in (a). Figure 1 As shown in (c).

[0118] The method of the present application needs to recalculate the DC component operating value of the sub-modules of the converter, the sub-module capacitor value and other parameters in the converter design, and optionally add detection and control of the double frequency component of the sub-module output voltage in the existing control structure of the converter. The steps of the method of the present application are as follows:

[0119] Step 1, first set the converter parameters, including: converter AC / DC side rated voltage, active and reactive operating range, actual value of converter to grid connection reactance, number of bridge arm sub-modules.

[0120] Step 2, derive the unified model of the converter compatible with active circulating current suppression, passive circulating current suppression without adding the double frequency voltage component control of the present application, and passive circulating current suppression with the double frequency voltage component control of the present application, and calculate the difference expression of the sub-module capacitor voltage and the sub-module output voltage considering the margin.

[0121] According to the converter model, circuit parameters and ideal control results after applying control under different steady state conditions, the result of the sub-module capacitor voltage minus the sub-module output voltage minus the target margin setting value of the two is calculated to obtain the final difference expression. Further, by applying a certain substitution method, the difference expression can be simplified to a form without trigonometric functions, for example, let x = cos(ωt), Replace the time variable t and the power factor With variables x, y to facilitate subsequent solving. Since the difference expressions obtained by solving are different for different methods of whether to adopt the double frequency component control of the sub-module output voltage, a coefficient λ N and λ C are added in the formula to distinguish between different methods. For the formula component that only appears in the formula in the case of passive circulating current suppression without adopting the double frequency component control of the sub-module output voltage, multiply it by the coefficient λ N . And for the formula component that only appears in the formula in the case of passive circulating current suppression with the double frequency component control of the sub-module output voltage, multiply it by the coefficient λ CThe specific process is explained as follows:

[0122] First, according to the AC side rated voltage U SN With the AC side rated current I N Calculate the per-unit value X of the AC side connection reactance pu as follows

[0123] X pu =X eq / (U SN / I N )

[0124] where X eq It is the actual value of the connection reactance between the converter and the grid. SN With DC side voltage U dc , solve the rated modulation ratio m0 as follows

[0125]

[0126] Further according to the per unit value of the AC side current And the rated modulation ratio m0, calculation coefficient as follows

[0127]

[0128] Definition coefficient K dc as follows

[0129]

[0130] where ε ul(NR) is the set converter rated capacitance voltage fluctuation ratio, ε ul(HR) is the converter capacitor voltage fluctuation ratio after design using this method

[0131] Further, the variable time t and power factor angle Replace with variables x, y as follows

[0132]

[0133] sign(x)={1,ωt∈(0,π];-1,ωt∈[-π,0]}

[0134]

[0135] Where ω is the angular frequency of the AC side, it can be seen that for any t and The corresponding x and y values ​​are between -1 and 1.

[0136] Then, according to the set submodule output voltage lower limit design value u min ,coefficient The rated modulation ratio m0 and the variable y can represent the double-frequency component modulation ratio m h As follows

[0137]

[0138] Further calculate the capacitance voltage fluctuation function The maximum value of any x and y The AC side current unit value, f rS (x, y) is the system fluctuation function, which is expressed as follows

[0139]

[0140] Wherein, m h is the double-frequency component modulation ratio, K dc and is the calculated coefficient, m0 is the rated modulation ratio, X pu is the AC side connection reactance unit value, x, y is the variable; Further, the difference expression of the sub-module capacitance voltage and the sub-module output voltage considering a certain margin M diff The result is

[0141]

[0142] Step 3, determine whether to use active circulating current suppression or passive circulating current suppression.

[0143] Step 4, determine whether to use the double-frequency voltage component control of the application to further increase the capacitance value reduction effect.

[0144] Step 5, set the coefficients λ N and λ C in the difference expression according to the selection result. For the case of using active circulating current suppression, set the coefficients λ N = 0 and λ C = 0, for the case of using passive circulating current suppression and not using double-frequency voltage component control, set the coefficients λ N = 1 and λ C = 0, for the case of using passive circulating current suppression and using double-frequency voltage component control, set the coefficients λ N = 0 and λ C = 1. Further, according to the difference expression, calculate the DC component operating value of the corresponding designed sub-module required by the converter at this time and the sub-module capacitance value, and use it for converter design. The specific process is explained as follows:

[0145] ​In the design stage of the converter, according to the selected case, the following processing is performed, the minimum value of the difference expression with respect to the variables x, y is solved, and further and the minimum value is equal to 0, the capacitor voltage fluctuation proportion value ε of the converter designed by the method at this time is calculated ul(HR) . For solving the difference expression The minimum value point position of x, y belonging to the range of -1~1 and the capacitor voltage fluctuation proportion value ε of the converter designed by the method ul(HR) Can be obtained by the following method. First, solve the equation:

[0146]

[0147] Solve the solution that satisfies the definition interval, and if there is a solution, further determine whether the extreme value point is a maximum value, a minimum value or a saddle point by checking the second derivative, and only keep the solution of the minimum value point.

[0148] At the same time, for solving the first function extreme value of Under the four boundary conditions of x=1, x=-1, y=1, y=-1, find the extreme value points on the four boundaries, and finally compare the values of the boundary extreme value points and the extreme value points inside the definition domain to determine whether the coordinates x dmin , y dimin of the minimum value point are located on the boundary or inside the definition. Further, according to the minimum value point position, the corresponding equation set and The minimum value is equal to 0, and the equation set is solved:

[0149] If the minimum value is inside the definition domain, the equation set is solved as:

[0150]

[0151] If the minimum value is on the boundary x=-1, the equation set is solved as:

[0152]

[0153] Similarly, the results of the minimum value on other boundaries can be obtained. By solving the equation set, the coordinates x dmin , y dimin of the minimum value point and the capacitor voltage fluctuation proportion value ε of the converter designed by the method ul(HR) Can be obtained.

[0154] Further, according to the rated capacitor voltage fluctuation proportion value ε of the converter ul(NR) And the capacitor voltage fluctuation proportion value ε of the converter designed by the method ul(HR) , the direct current component operating value U cap(HR) of the corresponding designed sub-module of the converter at this time can be calculated as follows:

[0155] ​

[0156] wherein U cap(NR) is the rated value of the DC component of the capacitor voltage of the sub-module before the method is used, U cap(HR) is the calculated operating value of the DC component of the sub-module after the method is used. cap(NR) The DC voltage of the converter U dc is calculated as follows:

[0157]

[0158] According to the rated capacitor voltage fluctuation ratio value ε ul(NR) of the converter, the capacitor voltage fluctuation ratio value ε ul(HR) of the converter after the method is used, and the maximum value of the capacitor voltage fluctuation function the capacitor value C sm(HR) of the sub-module corresponding to the design required by the converter at this time can be calculated as follows:

[0159]

[0160] wherein I N is the rated value of the AC side current, is the unit value of the AC side current, and ω is the angular frequency of the AC side, U dc is the DC side voltage, and N is the number of bridge arm sub-modules.

[0161] Step 6, if the use of the double-frequency voltage component control is selected in step 4 to further improve the capacitor value reduction effect, the following link needs to be added to the converter control structure: detecting the double-frequency voltage component output by the three-phase bridge arm sub-module, and controlling it to the target value through the control loop. The specific process is explained as follows:

[0162] The AC side voltage and AC side current of the converter are detected, and the positive and negative sequence results of the AC side voltage and AC side current of the converter can be calculated through the positive and negative sequence decoupling method.

[0163] The AC side current of the converter is converted to the positive sequence dq coordinate system, and the result after the rated current I N is normalized is as follows:

[0164]

[0165] wherein, is the actual value of the AC side current of the converter converted to the positive sequence dq coordinate system, is the unit value of the AC side current of the converter converted to the positive sequence dq coordinate system, I N is the rated value of the AC side current, is the unit value of the AC side current;

[0166] Further, according to the converter setting parameters, the sub-module minimum output voltage setting value and the calculated positive and negative zero sequence results of the converter AC side voltage and AC side current, the target value of the twice frequency component in the sub-module output voltage is calculated.

[0167] The twice frequency voltage component modulation ratio m h Can be expressed as

[0168]

[0169] The And Substituted into, we get

[0170]

[0171] The current AC current relationship Substituted into, the above formula becomes

[0172]

[0173] The target value of the negative sequence twice frequency voltage component required to be injected is expressed as the per unit value of U dc / N Can be expressed as

[0174]

[0175] Convert it to the negative sequence twice frequency dq coordinate system as follows

[0176]

[0177] Where

[0178]

[0179] Thus, the per unit value of U dc / N of the final target value of the twice frequency voltage in the sub-module output voltage in the dq coordinate system is Can be derived as

[0180]

[0181] Where u min Is the set sub-module output voltage lower limit design value, m0 is the rated modulation ratio, X pu Is the per unit value of the AC side connection reactance, Is the q-axis per unit value of the converter AC side current converted to the positive sequence dq coordinate system.

[0182] Then the voltage across the resonant capacitor C0 of the converter or the bridge arm output voltage is detected to obtain the actual value of the second harmonic component in the submodule output voltage.

[0183] Finally, the modulation wave adjustment value is added to the modulation wave result of the existing control of the converter, and finally sent to the modulator to generate the switching signal required by each switching device of the converter, thereby completing the control.

[0184] As shown in Figure 3 , the design method of the modular multilevel converter disclosed by the application mainly includes the following steps:

[0185] First, the parameters of the converter are set, including: the rated voltage of the AC side and the DC side of the converter, the active and reactive operation range, the actual value of the connection reactance from the converter to the power grid, and the number of bridge arm submodules.

[0186] Further, the compatible active circulating current suppression, the passive circulating current suppression without adding the second harmonic voltage component control of the application, and the unified model of the converter after adding the second harmonic voltage component control of the application are derived, and the difference expression of the submodule capacitor voltage and the submodule output voltage considering the margin is calculated.

[0187] Then, it is determined whether the converter adopts active circulating current suppression or passive circulating current suppression.

[0188] It is further determined whether the submodule output second harmonic voltage component control method is adopted to further improve the capacitance reduction effect.

[0189] Then, the values of the coefficients λ N and λ C in the difference expression are set according to the selection result. For the case of adopting active circulating current suppression, the coefficients λ N = 0 and λ C = 0 are set, for the case of adopting passive circulating current suppression and not adopting the second harmonic voltage component control, the coefficients λ N = 1 and λ C = 0 are set, and for the case of adopting passive circulating current suppression and adopting the second harmonic voltage component control, the coefficients λ N = 0 and λ C = 1 are set. Further, the DC component operation value of the submodule required by the converter and the submodule capacitance value are calculated according to the difference expression, and the converter is designed based on the values.

[0190] Finally, if passive circulating current suppression is selected and the double frequency voltage component control in the present method is selected, the following link needs to be added in the converter control structure: detecting the double frequency voltage component output by the three-phase bridge arm sub-module, and controlling it to be the target value through the control loop.

[0191] Figure 4 is the simulation waveform diagram of the steady-state operating condition of the modular multilevel converter in the active circulating current suppression case after using the method of the present application in an embodiment of the present application. Among them Figure 4 (a) in FIG. 1 is a steady-state operating waveform diagram under the condition that the converter outputs the rated capacitive reactive power, Figure 4 (b) in FIG. 1 is a steady-state operating waveform diagram under the condition that the converter outputs the rated active power, and (c) in FIG. 1 is a steady-state operating waveform diagram under the condition that the converter outputs the rated inductive reactive power. The waveform diagrams of the sub-module output voltage and the sub-module capacitor voltage under each condition are shown. It can be seen that in all operating conditions, only the DC and fundamental frequency components exist in the sub-module output voltage. After applying the design method of the present application, the margin between the sub-module voltage and the sub-module output voltage can be further utilized. The capacitor voltage fluctuation can be increased while ensuring that the peak value of the capacitor voltage of the sub-module remains unchanged, so that the required capacitor capacity of the sub-module can be greatly reduced, and thus the size and cost of the converter can be significantly reduced. In the traditional design, the DC side component rating of the sub-module capacitor voltage is 2kV, and the sub-module capacitor voltage fluctuation limit is ±10%, that is, the peak value of the sub-module capacitor voltage is 2.2kV and the valley value is 1.8kV. While Figure 4 It can be seen that for the modular multilevel converter in the active circulating current suppression case after using the method of the present application, the peak value of the sub-module capacitor voltage remains unchanged at 2.2kV, and the valley value can be reduced to 1.71kV, so that the DC side component rating of the sub-module capacitor voltage is reduced to 1.954kV. At this time, the sub-module capacitor voltage fluctuation increases to 12.56%, so that the sub-module capacitor capacity can be reduced to 83.15%.

[0192] Figure 5 is the simulation waveform diagram of the steady-state operating condition of the modular multilevel converter in the passive circulating current suppression case after using the method of the present application in an embodiment of the present application, and without using the double frequency component control of the sub-module output voltage. Among them Figure 5 (a) in FIG. 1 is a steady-state operating waveform diagram under the condition that the converter outputs the rated capacitive reactive power, Figure 5 (b) in FIG. 1 is a steady-state operating waveform diagram under the condition that the converter outputs the rated active power, Figure 5Figure (c) in the middle is a steady-state operating waveform diagram when the converter outputs rated inductive reactive power. The figure shows the waveforms of the submodule output voltage and the submodule capacitor voltage under various conditions. It can be seen that in all operating conditions, there is a naturally fluctuating double-frequency voltage component in the submodule output voltage. This component changes the shape of the submodule output voltage and affects its peak value, which increases the margin of the submodule output voltage relative to the submodule capacitor voltage. The capacitor voltage fluctuation can be increased while ensuring that the peak value of the capacitor voltage of the submodule remains unchanged. Therefore, the capacitance value required by the submodule can be greatly reduced, and the volume and cost of the converter can be significantly reduced. Under traditional design conditions, the rated value of the DC side component of the submodule capacitor voltage is 2kV, and the submodule capacitor voltage fluctuation is limited to plus or minus 10%, that is, the submodule capacitor voltage peak value is 2.2kV and the valley value is 1.8kV. However, due to Figure 5 It can be seen that when the modular multilevel converter adopts the method of the present invention with passive circulating current suppression and does not control the double frequency component of the submodule output voltage, the peak value of the submodule capacitor voltage remains unchanged at 2.2kV, and the valley value can be reduced to 1.65kV. Therefore, the rated value of the DC side component of the submodule capacitor voltage is reduced to 1.925kV. Correspondingly, the submodule capacitor voltage fluctuation increases to 14.34%, so the submodule capacitor value can be reduced to 75%.

[0193] Figure 6 This is a simulation waveform diagram of a steady-state operating condition of a modular multi-level converter using passive circulating current suppression and controlled by a double frequency component of the sub-module output voltage after using the method of the present invention in one embodiment of the present invention. Figure 6 (a) is the steady-state operating waveform when the converter outputs rated capacitive reactive power. Figure 6 Figure (b) is the steady-state operating waveform of the converter when it outputs rated active power. Figure 6 Figure (c) shows the steady-state operating waveform of the converter outputting rated inductive reactive power. The figure shows the waveforms of the submodule output voltage and submodule capacitor voltage under various conditions. It can be seen that in all operating conditions, the submodule output voltage contains a double frequency voltage component controlled by the control loop in the proposed method, which is relatively Figure 6 The natural double frequency voltage component in the circuit, at this time the controlled double frequency voltage component has a larger amplitude, so its influence on the shape and peak value of the submodule output voltage is further increased, so that the margin of the submodule output voltage relative to the submodule capacitor voltage is further improved. Therefore, in this case, the submodule capacitor voltage is allowed to fluctuate further, the submodule capacitor value, and the volume and cost of the converter can be further reduced. Under traditional design conditions, the rated value of the DC side component of the submodule capacitor voltage is 2kV, and the submodule capacitor voltage fluctuation is limited to plus or minus 10%, that is, the submodule capacitor voltage peak value is 2.2kV, and the valley value is 1.8kV. However, due to Figure 6It can be seen that when the modular multilevel converter with passive circulating current suppression and the sub-module output voltage double frequency component control are adopted, the peak value of the sub-module capacitor voltage remains unchanged at 2.2 kV, the valley value can be reduced to 1.55 kV, and thus the sub-module capacitor voltage DC side component rating is reduced to 1.88 kV, and the corresponding sub-module capacitor voltage fluctuation is increased to 16.7%, and thus the sub-module capacitor capacity can be reduced to 66%. The lowest value of the sub-module output voltage in each case is controlled to be close to the design value U dc u min / N, which is consistent with the design target.

[0194] The modular multilevel converter provided by the application can be used as an AC / DC converter of a high-voltage direct-current transmission system, and can be used in high-voltage direct-current point-to-point or multi-terminal networking transmission occasions, and offshore and onshore wind power, large-scale photovoltaic and other new energy centralized transmission occasions. The modular multilevel converter can also be used as a medium-voltage AC / DC converter, and can be used in medium-voltage motor driving, medium-voltage DC power grids, and all-electric ships and other occasions. The method can significantly reduce the required capacitor capacity of the modular multilevel converter, and further reduce the size and cost of the overall converter. In particular, the size of the overall converter can be greatly reduced, and a more compact converter can be realized. A new idea is provided for reducing the cost and size of the modular multilevel converter.

[0195] The above is only a preferred embodiment of the application, and does not limit the application in any way. Any simple modification, change and equivalent structural change made to the above embodiment according to the technical essence of the application still falls within the protection scope of the technical solution of the application.

[0196] The above embodiments are only used to illustrate the technical solutions of the application and not to limit the application. Although the application has been described in detail with reference to the above embodiments, ordinary skilled in the art can still modify or equivalently replace the specific implementation of the application without departing from the spirit and scope of the application, and any modification or equivalent replacement that does not depart from the spirit and scope of the application is within the protection scope of the claims of the application.

[0197] The above is a further detailed description of the application, and the specific implementation of the application should not be limited to this. For ordinary skilled in the art to which the application belongs, some simple deductions or replacements can be made without departing from the concept of the application, and all of them should be regarded as falling within the protection scope of the application determined by the claims submitted.

Claims

1. A method of designing a modular multilevel converter, characterized in that, Comprise: S1, set the converter parameters, including: converter AC side voltage, active and reactive operation range, the actual value of the connection reactance of the converter to the grid, the number of bridge arm sub-modules; S2, on the basis of the set converter parameters, derive the unified model of the converter under various conditions and the difference expression of the sub-module capacitor voltage and the sub-module output voltage considering the margin; S3, determine the converter circulating current suppression method: active circulating current suppression or passive circulating current suppression; S4, using active circulating current suppression, directly proceed to step S5, using passive circulating current suppression, determine whether to use double-frequency voltage component control to further increase the capacitance reduction effect; if not, directly proceed to step S5, if yes, add corresponding double-frequency voltage component control to the converter control, proceed to step S6; S5, according to the selection result, calculate the DC component operating value of the converter sub-module, the sub-module capacitance and other parameters in the converter design, and design the converter according to the calculation result; S6, if the double-frequency voltage component control is selected in step S4 to further improve the capacitance reduction effect, the double-frequency voltage component output by the three-phase bridge arm sub-modules needs to be detected in the converter control structure, and the control loop is controlled to the target value.

2. The method of designing a modular multilevel converter according to claim 1, characterized in that, The converter is composed of three-phase units in parallel, each phase contains two bridge arms, and each bridge arm is composed of N sub-modules and a bridge arm reactor in series, and the two bridge arms are connected in series to form a phase unit structure. The midpoint of the two bridge arms after series connection is the AC output point, and the endpoints of the upper and lower ends are the DC output points; the DC output points of the three-phase units are connected in parallel to the DC bus, and the AC output points of the three-phase units are connected to the AC power grid; Wherein, the bridge arm reactor is in the form of two reactors L1, L2 in series, and the bridge arm reactor midpoint appears in each bridge arm; a resonance capacitor C0 is connected across the bridge arm reactor midpoint of the upper and lower bridge arms of the phase unit; the resonance capacitor C0 value and the reactance value 2L1 are designed as a double-frequency resonance state about the AC side, which can make each phase DC side contain a double-frequency parallel resonance cavity; the resonance cavity withstands the double-frequency voltage of the AC side in any case, and blocks the flow of double-frequency circulating current in the converter.

3. The method of designing a modular multilevel converter according to claim 1, wherein, The derivation of the unified model of the converter under various conditions and the difference expression of the sub-module capacitor voltage and the sub-module output voltage considering the margin, comprises: Derive the unified model of the converter compatible with active circulating current suppression, passive circulating current suppression without adding, and passive circulating current suppression after adding, and calculate the difference expression of the sub-module capacitor voltage and the sub-module output voltage considering the margin.

4. The method of designing a modular multilevel converter according to claim 3, wherein, The derivation of the unified model of the converter under various conditions and the difference expression of the sub-module capacitor voltage and the sub-module output voltage considering the margin, specifically comprises: According to the sub-module capacitor voltage minus the sub-module output voltage, minus the target margin setting value of the two, the final difference expression is obtained; and then the difference expression is simplified to a form without trigonometric function by using substitution method, for the formula component which only appears in the passive circulating current suppression without the sub-module output voltage double frequency component control, multiply it by coefficient λ N ; for the formula component which only appears in the passive circulating current suppression with the sub-module output voltage double frequency component control, multiply it by coefficient λ C .

5. The method of designing a modular multilevel converter according to claim 4, wherein, The derivation of the unified model of the converter under various conditions and the difference expression of the sub-module capacitor voltage and the sub-module output voltage considering the margin, comprises the following steps: According to the AC side rated voltage U SN and the AC side rated current I N solving for the AC side connected reactance in per unit X pu As follows X pu = X eq (U SN / I N ) where X eq is the actual value of the connection reactance of the converter to the grid; according to the AC-side rated voltage U SN and the DC-side voltage U dc , the rated modulation ratio m0 is solved as follows Further in accordance with the AC side current magnitude and the rated modulation ratio m0, the coefficient is calculated as follows Definition of the coefficient K dc As follows where ε ul(NR) is a set converter rated capacitor voltage fluctuation ratio value, ε ul(HR) is a converter capacitor voltage fluctuation ratio value; Replace the variable time t with the power factor angle x, y as follows: sign(x)={1,ωt∈(0,π];-1,ωt∈[-π,0]} where ω is the alternating side angular frequency, for any t and corresponding to x, y values between -1 and 1; According to the set sub-module output voltage lower limit design value u min , coefficient Rated modulation ratio m0 and variable y, indicating the modulation ratio m of the double frequency component h As follows: The capacitance voltage fluctuation function is calculated as The maximum value of the function at any x and y is the AC side current nominal value, f rS (x, y) is the system fluctuation function, which is expressed as where m h is the double frequency component modulation ratio, K dc and is the calculated coefficient, m0 is the rated modulation ratio, X pu is the AC side connection reactance per unit, x, y are variables; and the difference expression between the sub-module capacitor voltage and the sub-module output voltage is obtained under the condition of considering a certain margin M diff the result is​ 6. The method of designing a modular multilevel converter according to claim 1, wherein, The S5 comprises: According to the selection result, set the coefficient λ in the difference expression N With λ C ; for the case of adopting active circulating current suppression, set the coefficient λ N = 0 and λ C = 0, for the case of adopting passive circulating current suppression and not adopting the control of the double-frequency voltage component, set the coefficient λ N = 1 and λ C = 0, for the case of adopting passive circulating current suppression and adopting the control of the double-frequency voltage component, set the coefficient λ N = 0 and λ C = 1; further, calculate the direct-current component operating value of the corresponding designed sub-module required by the converter at this time and the sub-module capacitance value according to the difference expression, and design the converter based on the same.

7. The method of designing a modular multilevel converter according to claim 6, wherein, The S5, specifically comprises: At the stage of designing the inverter, the following process is performed according to the selected case, the minimum value of the difference expression with respect to the variables x, y is solved, and further, the minimum value is made equal to 0, and the inverter capacitor voltage fluctuation ratio value ε at this time is calculated ul(HR) ; for solving the difference expression At the minimum value point position where x, y belong to the range of -1 to 1, and the inverter capacitor voltage fluctuation ratio value ε ul(HR) , is obtained by the following method; first, the equation Solve the solution that meets the definition interval, if there is a solution, further through the check of second derivative to judge extreme point is maximum, minimum or saddle point, only keep the solution of minimum point; At the same time, for the extreme value of the first function of , find the extreme point on the four boundaries, and finally compare the value of the extreme point on the boundary with the value of the extreme point in the domain to determine the coordinates x dmin , y dimin of the minimum value of the surface, whether it is on the boundary or in the domain; further, according to the position of the minimum value point, solve the corresponding equation group and the minimum value is equal to 0. If the minimum value is in the domain, solve the equation set If the minimum value is on the boundary x = -1, solve the equation set The same reasoning applies to the minimum on the other boundaries; by solving the system of equations The minimum point coordinates x dmin , y dimin , and the inverter capacitance voltage fluctuation proportion value ε ul(HR) ; Further, according to the rated capacitance voltage fluctuation proportion value ε of the converter ul(NR) and the capacitance voltage fluctuation proportion value ε of the converter ul(HR) , the DC component operation value U of the corresponding designed sub-module required by the converter at this time is calculated as follows: cap(HR) U = U0 + ε · U0 Wherein, U cap(NR) is the rated value of the DC component of the capacitor voltage of the sub-module before using the proposed method, U cap(HR) is the calculated operating value of the DC component of the sub-module; U cap(NR) The DC voltage U dc of the converter is calculated as follows: According to the rated capacitor voltage fluctuation proportion value ε of the converter ul(NR) , the capacitor voltage fluctuation proportion value ε ul(HR) , and the capacitor voltage fluctuation function maximum value , the corresponding designed sub-module capacitor value C required by the converter at this time is calculated as follows: sm(HR) ​ wherein I N is the AC-side current rating, is the AC-side current unit value, ω is the AC-side angular frequency, U dc is the DC-side voltage, N is the number of bridge arm submodules.

8. The method of designing a modular multilevel converter according to claim 1, wherein, Specifically, the method comprises the steps of: Detecting the AC side voltage and the AC side current of the converter, and then calculating the positive and negative sequence results of the AC side voltage and the AC side current of the converter through a positive and negative sequence decoupling method; The current on the AC side of the converter is converted to the positive sequence dq coordinate system and scaled to the rated current I N The result in per unit is wherein, is the actual value of the converter AC side current converted to the positive sequence dq frame, is the per unit value of the converter AC side current converted to the positive sequence dq frame, I N is the AC side current rated value, is the AC side current per unit value; Then, according to the set parameters of the converter, the set value of the minimum output voltage of the sub-module and the positive and negative sequence results of the AC side voltage and the AC side current of the converter, the target value of the second frequency component in the output voltage of the sub-module is calculated.

9. A control method of a modular multilevel converter, characterized by, The control method comprises: Detecting the second frequency voltage component output by the three-phase bridge arm sub-module, and controlling the second frequency voltage component to be the target value through a control loop.

10. The control method of a modular multilevel converter according to claim 9, characterized in that, The control method specifically comprises: The second harmonic voltage component modulation ratio m h is represented as Bringing with in gives The current AC current relationship With this, the above equation becomes The target value of the negative sequence double frequency voltage component to be injected is expressed as dc in per unit of U is expressed as The second frequency voltage component is converted to a negative sequence second frequency dq coordinate system as follows Wherein Thus the target value of the double frequency voltage in the final sub-module output voltage is for U dc The result in dq coordinate system The derivation is wherein u min is a set sub-module output voltage lower limit design value, m0 is a rated modulation ratio, X pu is an AC side connection reactance per unit, is a q-axis per unit of the converter AC side current converted to the positive sequence dq coordinate system; Then, the voltage across the resonant capacitor C0 of the converter or the bridge arm output voltage is detected to obtain the actual value of the second frequency component in the output voltage of the sub-module; the actual value of the second frequency component in the output voltage of the sub-module detected through the feedback control link is controlled to be the target value of the second frequency component in the output voltage of the sub-module calculated; and the corresponding modulation wave adjustment value output by the feedback control link is obtained. Finally, the modulation wave adjustment value is added to the modulation wave result of the converter, and the switching signal required by each switching device of the converter is finally sent into a modulator to generate, so as to complete the control.

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