Design and control method of modular multilevel converter

Abstract

The invention discloses a design and control method of a modular multilevel converter, which belongs to the technical field of converters, and comprises the following steps: firstly, setting converter parameters, and further deducing a converter unified model under various conditions and a difference expression between a sub-module capacitor voltage and a sub-module output voltage under the condition of considering margin; under the condition that passive circulating current suppression is adopted, whether a frequency doubling voltage component control method is adopted or not needs to be further judged to further improve the capacitance value reduction effect. And recalculating parameters such as a direct-current component operation value and a sub-module capacitance value of the converter sub-module in the converter design according to a selection result, and designing the converter according to the recalculated parameters. According to the method, the voltage and current stress of a switching device is not influenced, the system efficiency and the active and reactive operation interval of the converter are not influenced, the capacitance value of a sub-module capacitor of the converter is remarkably reduced, and the overall size and cost of the converter are remarkably reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of converters and relates to a design and control method of a modular multi-level converter. Background Art

[0002] Modular multilevel converters are key equipment in the field of high-voltage direct current transmission due to their modularity, ability to achieve a large number of level outputs, and common DC busbars. According to the different circulating current suppression methods used, they can be further divided into active circulating current suppression methods based on control and passive circulating current suppression methods based on external bridge arm circulating current filters. Each bridge arm of the modular multilevel converter is composed of submodules connected in series. It is a single-phase structure and needs to withstand large power fluctuations of the base frequency and double frequency during operation, which in turn causes large fluctuations in the submodule capacitor voltage, endangering the safe operation of the switching device. Existing solutions often require the use of capacitors with large capacitance to reduce the amplitude of the capacitor voltage fluctuation to within the allowable range (generally 5% to 10% of the DC component of the submodule capacitor voltage). The cost of the capacitor in the existing submodule is equivalent to that of the switching device, and the volume of the capacitor generally accounts for 70% to 80% of the total volume of the submodule. The large submodule capacitor seriously affects the volume and cost of the converter. Therefore, reducing the capacitance value required for the submodule in the modular multilevel converter is very effective in reducing the overall converter cost and volume. Summary of the invention

[0003] In order to solve the problems existing in the prior art, the present invention proposes a design and control method for a modular multi-level converter. The method does not affect the voltage and current stress of the switching devices, does not affect the system efficiency and the reactive power operation range of the converter, and the sub-module capacitance of the converter can be significantly reduced, and the overall volume and cost of the converter can be significantly reduced.

[0004] The present invention adopts the following technical solutions to achieve the above-mentioned invention object:

[0005] In a first aspect, the present invention provides a design method for a modular multilevel converter, comprising:

[0006] S1, setting converter parameters, including: rated voltage of converter AC and DC sides, active and reactive operating range, actual value of connection reactance from converter to grid, and number of bridge arm submodules;

[0007] S2, based on the set converter parameters, derive the unified converter model under various conditions and the difference expression between the submodule capacitor voltage and the submodule output voltage under the condition of considering the margin;

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

[0009] S4, adopt active circulating current suppression, directly proceed to step S5, adopt passive circulating current suppression, determine whether it is necessary to adopt double frequency voltage component control to further increase the capacitance reduction effect; if not, directly proceed to step S5, if yes, add the corresponding double frequency voltage component control to the converter control, and proceed to step S6;

[0010] S5, calculating the DC component operating value of the converter submodule, the submodule capacitance value and other parameters in the converter design according to the selection result, and performing converter design 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, it is necessary to add to the converter control structure: detecting the double frequency voltage component output by the three-phase bridge arm submodule and controlling it to the target value through a control loop.

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

[0013] Among them, the bridge arm reactor is two reactors L 1 ,L 2 In the form of series connection, the midpoint of the bridge arm reactor appears in each bridge arm; a resonant capacitor C is connected across the midpoint of the bridge arm reactor of the upper and lower bridge arms of the phase unit. 0 ; The resonant capacitor C 0 Value and reactance value 2L 1 The design is about the double frequency resonance state of the AC side, so that each phase DC side contains a double frequency parallel resonance cavity; the resonance cavity bears the double frequency voltage of the AC side under any circumstances and blocks the flow of the double frequency circulating current in the converter.

[0014] As a further improvement of the present invention, the derivation of the unified converter model in various cases and the expression of the difference between the submodule capacitor voltage and the submodule output voltage in consideration of the margin include:

[0015] A unified converter model compatible with active circulating current suppression, without passive circulating current suppression, and with passive circulating current suppression is derived, and the difference expression between the submodule capacitor voltage and the submodule output voltage considering the margin is calculated.

[0016] As a further improvement of the present invention, the derivation of the unified converter model in various cases and the expression of the difference between the submodule capacitor voltage and the submodule output voltage in consideration of the margin specifically include:

[0017] According to the converter model, circuit parameters and ideal control results after applying control in steady state under different conditions, the submodule capacitor voltage minus the submodule output voltage is calculated, and then the result of the target margin setting value of the two is subtracted to obtain the final difference expression; then the substitution method is applied to simplify the difference expression into a form without trigonometric functions, and the formula components that appear only when passive circulating current suppression is adopted and the submodule output voltage double frequency component control is not adopted are multiplied by the coefficient λ N ; For the formula components that only appear when passive circulating current suppression and submodule output voltage double frequency component control are used, multiply them by the coefficient λ C .

[0018] As a further improvement of the present invention, the derivation of the unified converter model in various cases and the difference expression between the submodule capacitor voltage and the submodule output voltage in consideration of the margin includes the following steps:

[0019] According to the rated voltage U SN With the AC side rated current I N Solve the per unit value X of the AC side connection reactance pu as follows

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

[0021] Where X eq is the actual value of the connection reactance between the converter and the grid; according to the rated voltage U SN With DC link voltage U dc , solve the rated modulation ratio m 0 as follows

[0022]

[0023] Further according to the AC side current standard value And the rated modulation ratio m 0 , calculate the coefficient as follows

[0024]

[0025] Definition coefficient K dc as follows

[0026]

[0027] where ε ul(NR) is the rated capacitance voltage fluctuation ratio of the converter, εul(HR) is the converter capacitor voltage fluctuation ratio;

[0028] The variable time t and the power factor angle Replace with variables x and y as follows:

[0029]

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

[0031]

[0032] 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;

[0033] According to the set submodule output voltage lower limit design value u min ,coefficient Rated modulation ratio m 0 And the variable y can represent the modulation ratio m of the double frequency component h as follows:

[0034]

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

[0036]

[0037] Among them, m h is the modulation ratio of the double frequency component, K dc and is the calculated coefficient, m 0 is the rated modulation ratio, X pu is the per unit value of the AC side connection reactance, x, y are variables; and then we can get a certain margin M diff In the case of , the difference between the submodule capacitor voltage and the submodule output voltage is expressed as The result is

[0038]

[0039]

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

[0041] Set the coefficient λ in the difference expression according to the selection result N With λ C ; For active circulation suppression, set the coefficient λ N =0 and λ C = 0, for the case of passive circulating current suppression and no double frequency voltage component control, set the coefficient λ N =1 and λ C = 0, for the case of passive circulating current suppression and double frequency voltage component control, set the coefficient λ N =0 and λ C =1; then, according to the difference expression, the DC component operating value and the submodule capacitance value of the submodule required for the corresponding design of the converter are calculated, and the converter is designed accordingly.

[0042] As a further improvement of the present invention, the S5 specifically includes:

[0043] In the converter design stage, the following processing is performed according to the selected situation to solve the difference expression for the minimum value of variables x and y, and further set its minimum value equal to 0 to calculate the converter capacitor voltage fluctuation ratio ε at this time ul(HR) ; For solving the difference expression The minimum point position in the range of x,y -1 to 1 and the converter capacitor voltage fluctuation ratio ε ul(HR) , obtained by the following method; first solve the equation

[0044]

[0045] Find the solution that satisfies the defined interval. If there is a solution, further check the second-order derivative to determine whether the extreme point is a maximum, minimum or saddle point, and only keep the solution of the minimum point;

[0046] At the same time, for the four boundary conditions of x=1, x=-1, y=1, y=-1 The linear function extreme value is found, and the extreme value points on the four boundaries are finally compared with the values ​​of the extreme value points inside the definition domain to determine the coordinates x of the minimum value of the surface. dmin ,y dimin Is it located on the boundary or inside the definition? Further, according to the location of the minimum point, the corresponding equations are combined with The minimum value is equal to 0, so solve it;

[0047] If the minimum value is inside the domain, then the solution to the system of equations is

[0048]

[0049] If the minimum value is on the boundary x = -1, then the solution to the system of equations is

[0050]

[0051] The same is true for the results of the minimum value on other boundaries; by solving the equations, we get Minimum point coordinate x dmin ,y dimin , and the converter capacitor voltage fluctuation ratio ε ul(HR) ;

[0052] Then according to the converter rated capacitance voltage fluctuation ratio ε ul(NR) The ratio of the converter capacitor voltage fluctuation ε ul(HR) , calculate the DC component operating value U of the submodule that the converter needs to design accordingly cap(HR) as follows:

[0053]

[0054] Among them, U cap(NR) is the rated value of the DC component of the submodule capacitor voltage before the proposed method is adopted, U cap(HR) is the calculated DC component operating value of the submodule; U cap(NR) The DC voltage U dc The number of converter bridge arm submodules N is calculated as follows:

[0055]

[0056] According to the rated capacitance voltage fluctuation ratio ε of the converter ul(NR) , capacitor voltage fluctuation ratio ε ul(HR) , and the maximum value of the capacitor voltage fluctuation function Calculate the submodule capacitance value C required for the converter design at this time sm(HR) as follows:

[0057]

[0058] Among them, I N is the rated current of the AC side, is the per unit value of the AC current, ω is the angular frequency of the AC side, U dc is the DC side voltage, and N is the number of bridge arm submodules.

[0059] As a further improvement of the present invention, specifically comprising:

[0060] Detect the AC side voltage and the AC side current of the converter, and then calculate the positive and negative zero sequence results of the AC side voltage and the AC side current of the converter by using the positive and negative sequence decoupling method;

[0061] The AC side current of the converter is converted to the positive sequence dq coordinate system and the rated current I NThe result after standardization is

[0062]

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

[0064] Then, according to the converter setting parameters, the submodule 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 double frequency component in the submodule output voltage is calculated;

[0065] In a second aspect, the present invention provides a control method for a modular multilevel converter, wherein the converter is designed using the design method for a modular multilevel converter, and the control method comprises:

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

[0067] As a further improvement of the present invention, the control method specifically includes:

[0068] The double frequency voltage component modulation ratio m h Expressed as

[0069]

[0070] Will and Bring in

[0071]

[0072] The current AC current relationship Substituting into, the above formula becomes

[0073]

[0074] The target value of the negative sequence double frequency voltage component to be injected is given by U dc / N per unit value It can be expressed as

[0075]

[0076] Convert it to the negative sequence double frequency dq coordinate system as follows

[0077]

[0078] in

[0079]

[0080] Therefore, the double frequency voltage target value in the final submodule output voltage is dc The result of the per unit value of / N in the dq coordinate system Derived as

[0081]

[0082] where u min is the lower limit design value of the submodule output voltage, m 0 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;

[0083] Then detect the converter resonant capacitor C 0 The voltage at both ends or the output voltage of the bridge arm is used to obtain the actual value of the double frequency component in the output voltage of the submodule; the actual value of the double frequency component in the output voltage of the submodule obtained by the detection is controlled to be the calculated target value of the double frequency component in the output voltage of the submodule through the feedback control link; the feedback control link outputs the corresponding modulation wave adjustment value;

[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 to complete the control.

[0085] The present invention adopts the above technical solution and has the following beneficial effects:

[0086] The present invention is directed to a modular multi-level converter using different circulating current suppression methods. In the design stage, according to the converter steady-state model, the minimum difference between the submodule capacitor voltage and the submodule output voltage is accurately calculated, and according to the converter setting parameters, under the premise of maintaining a certain design margin, the maximum value of the submodule allowed capacitor voltage fluctuation can be obtained. Then, the DC component operating value of the new submodule capacitor voltage and the design value of the submodule capacitor value 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 method can be accurately designed according to the target voltage margin of the submodule output voltage and the submodule capacitor voltage, effectively reducing the generation of excessive margins and effectively reducing the required capacitor value of the converter submodule.

[0087] In the case of passive circulating current suppression, the present invention can be applied to improve the effect of reducing the capacitance of the converter by applying the control method of the present invention. The proposed control method is to detect the double frequency voltage component output by the three-phase bridge arm submodule, and control it to the target value through the control loop. This control method effectively realizes the reduction of the submodule output voltage peak value by controlling the double frequency voltage component in the submodule output voltage, increases the voltage range in which the submodule capacitor voltage is allowed to fluctuate, and thus allows the submodule capacitor voltage fluctuation to further increase, and the corresponding submodule capacitor capacitance can be significantly reduced. After applying the design and control method of the present invention, the submodule capacitor can operate under high capacitor voltage fluctuations, and the converter does not need to add additional hardware equipment, does not affect the voltage and current stress of the switching device, does not affect the system efficiency and the reactive operation range of the converter, and the submodule capacitor capacitance of the converter is significantly reduced, and the overall volume and cost of the converter are significantly reduced. At the same time, during operation, the submodule voltage of the converter naturally runs at a lower value, and there is no risk of capacitor voltage peak exceeding the limit in the transient process. BRIEF DESCRIPTION OF THE DRAWINGS

[0088] The drawings described herein are for explanation purposes only and are not intended to limit the scope of the present invention in any way. In addition, the shapes and proportional dimensions of the components in the drawings are only for illustration purposes to help understand the present invention, and are not intended to specifically limit the shapes and proportional dimensions of the components of the present invention. In the drawings:

[0089] Figure 1 is a schematic structural diagram of a modular multilevel converter involved in the method of the present invention;

[0090] Figure 2 It is a schematic diagram of the submodule capacitor voltage waveform of the method of the present invention and the submodule capacitor voltage waveform of the traditional method, as well as the submodule output voltage waveform in various cases;

[0091] Figure 3 It is a flow chart of a control and design method of a modular multi-level converter proposed by the present invention.

[0092] Figure 4 It is a simulation waveform diagram of the steady-state operation condition of the modular multilevel converter under the condition of active circulating current suppression in one embodiment of the present invention.

[0093] Figure 5 It is a simulation waveform diagram of the steady-state operation condition of the modular multilevel converter in one embodiment of the present invention when passive circulating current suppression is adopted and double frequency voltage component control is not adopted.

[0094] Figure 6 It is a simulation waveform diagram of the steady-state operation condition of the modular multilevel converter under the condition of adopting passive circulating current suppression and double frequency voltage component control in one embodiment of the present invention.

[0095] Specific implementation

[0096] In order to make the purpose and technical solution of the present invention clearer and easier to understand, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments.

[0097] In the description of the present invention, it should be understood that the terms "first" and "second" are only used for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, unless otherwise specified, the meaning of "multiple" is two or more. In the description of the present invention, it should be noted that, unless otherwise clearly specified and defined, the terms "installed", "connected" and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be directly connected, or indirectly connected through an intermediate medium, or it can be the internal connection of two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0098] The first purpose of the present invention is to propose a design method for a modular multi-level converter to achieve a reduction in the capacitance of the converter sub-modules and a reduction in the overall converter volume and weight. For modular multi-level converters using different circulating current suppression methods, in the design stage, based on the converter steady-state model, the minimum difference between the sub-module capacitor voltage and the sub-module output voltage is accurately calculated, and based on the converter setting parameters, the maximum value of the sub-module capacitor voltage fluctuation allowed at this time can be calculated while maintaining a certain design margin. Then, the DC component operating value of the new sub-module capacitor voltage and the design value of the sub-module capacitor capacitance can be calculated. The specific steps are described in sequence as follows:

[0099] Step 1: First, set the converter parameters, including: rated voltage of the converter AC and DC sides, active and reactive operating range, actual value of the connection reactance from the converter to the grid, and number of bridge arm submodules.

[0100] Step 2, derive a unified converter model that is compatible with active circulating current suppression, passive circulating current suppression without adding the double frequency voltage component control of the present invention, and passive circulating current suppression after adding the double frequency voltage component control of the present invention, and calculate the difference expression between the sub-module capacitor voltage and the sub-module output voltage while considering the margin.

[0101] Step 3: Determine whether to use active circulation suppression or passive circulation suppression.

[0102] Step 4: determine whether it is necessary to adopt the double frequency voltage component control of the present invention to further increase the capacitance reduction effect.

[0103] Step 5: Set the coefficient λ in the difference expression according to the selection result N With λ C For active circulation suppression, set the coefficient λ N =0 and λ C = 0, for the case of passive circulating current suppression and no double frequency voltage component control, set the coefficient λ N =1 and λ C = 0, for the case of passive circulating current suppression and double frequency voltage component control, set the coefficient λ N =0 and λ C = 1. Then, the DC component operating value and the submodule capacitance value of the submodule required to be designed by the converter are calculated according to the difference expression, and the converter is designed accordingly.

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

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

[0106] Specifically, the principle of the method of the present invention is to first set the converter parameters, further derive the unified converter model under various conditions and the difference expression between the submodule capacitor voltage and the submodule output voltage under the condition of considering the margin, and then determine the converter circulation suppression method. In the case of passive circulation suppression, it is necessary to further determine whether to use the double frequency voltage component control method to further improve the capacitance reduction effect. According to the selection result, the DC component operating value of the converter submodule, the submodule capacitance value and other parameters are recalculated in the converter design, and the converter design is performed accordingly. If the double frequency voltage component control method is selected in the case of passive circulation suppression, the corresponding double frequency voltage component control needs to be further added to the converter control.

[0107] As an example, the present invention first sets the basic parameters of the converter, such as rated power, rated voltage, switching frequency, etc., according to the specific application scenarios and performance requirements of the converter. Then, based on these parameters, a unified mathematical model of the converter under various operating conditions is derived. This step is crucial to understanding the dynamic behavior and performance of the converter. Based on the margin between the submodule capacitor voltage and the submodule 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] Furthermore, the circulating current suppression method to be adopted is determined according to the specific design of the converter. Circulating current suppression is an important part of converter design, which is used to reduce the loss and heat caused by circulating current. If the passive circulating current suppression method is adopted, it is necessary to further determine whether to adopt the double frequency voltage component control method to further improve the effect of reducing the capacitance value.

[0109] Furthermore, according to the results of the circulating current suppression method and whether to use the double frequency voltage component control method, the DC component operating value of the converter submodule, the submodule capacitance value 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 the double frequency voltage component control method is chosen under passive circulating current suppression, the corresponding double frequency voltage component control logic needs to be added to the converter control system. This control logic detects the double frequency voltage component output by the three-phase bridge arm submodule and controls it to the target value through the control loop, thereby further reducing the capacitance and improving the performance of the converter.

[0111] Therefore, this method is precisely designed according to the target voltage margin of the submodule output voltage and the submodule capacitor voltage, effectively reducing the generation of excessive margin. This can not only reduce the demand for capacitor capacitance, but also improve 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. This method is suitable for different types of converter designs and application scenarios. By flexibly adjusting parameters and control strategies, the needs and performance requirements of different users can be met. The design and control logic of this method is relatively simple and clear, and it is easy to implement and maintain. This helps to reduce development costs and time while improving the reliability and maintainability of the system.

[0112] The second object of the present invention is to propose a control method for a modular multilevel converter. This control method effectively reduces the peak value of the submodule output voltage by controlling the double frequency voltage component in the submodule output voltage, increases the voltage range within which the submodule capacitor voltage is allowed to fluctuate, and thereby allows the submodule capacitor voltage fluctuation to further increase, and the corresponding submodule capacitor value to significantly decrease.

[0113] After applying the design and control method of the present invention, the submodule capacitor can operate under high capacitor voltage fluctuations, the converter does not need to add additional hardware equipment, 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 submodule capacitor value 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 submodule voltage of the converter naturally runs at a lower value, and there is no risk of capacitor voltage peak exceeding the limit during the transient process.

[0114] In summary, the present invention effectively reduces the required capacitance of the converter submodule and improves the performance of the converter by accurately designing the margin relationship between the submodule capacitor voltage and the output voltage and adopting a double frequency voltage component control method.

[0115] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0116] The present invention proposes a design and control method for a modular multilevel converter, wherein the structural schematic diagram of the modular multilevel converter involved is as follows: Figure 1 The modular multilevel converter adopting active circulating current suppression method is shown in Figure 1 As shown in (a), the converter is composed of three-phase units connected in parallel. Each phase contains two upper and lower bridge arms. Each bridge arm consists of N submodules and bridge arm reactors L. 0 The two bridge arms are connected in series to form a phase unit structure. After the series connection, the midpoint of the two bridge arms is the AC output point, and the upper and lower ends are the DC output points. The DC output point of the three-phase unit is connected in parallel to the DC bus, and the AC output point of the three-phase unit is connected to the AC grid.

[0117] Different from the traditional modular multilevel converter using active circulating current suppression, in the modular multilevel converter using passive circulating current suppression, the bridge arm reactor is two reactors L 1 ,L 2 In the form of series connection, the midpoint of the bridge arm reactor appears in each bridge arm. Further, a resonant capacitor C is connected across the midpoint of the bridge arm reactor of the upper and lower bridge arms of the phase unit. 0 ,like Figure 1 As shown in (b). 0 Value and reactance value 2L 1Designed to be in a double frequency resonance state on the AC side, each phase DC side can contain a double frequency parallel resonant cavity. This resonant cavity will bear the double frequency voltage on the AC side under any circumstances and block the flow of the double frequency circulating current in the converter. Therefore, in a modular multilevel converter with passive circulating current suppression, there is no need to add a double frequency circulating current controller, and its resonant cavity naturally makes the converter double frequency circulating current zero under any circumstances. Various forms of sub-modules can be used for modular multilevel converters. Only the most commonly used half-bridge sub-modules are shown here, such as Figure 1 As shown in (c).

[0118] The method of the present invention needs to recalculate the DC component operating value of the converter submodule, the submodule capacitance value and other parameters in the converter design, and optionally add the detection and control of the submodule output voltage double frequency component to the existing control structure of the converter. The steps of the method of the present invention are explained as follows:

[0119] Step 1: First, set the converter parameters, including: rated voltage of the converter AC and DC sides, active and reactive operating range, actual value of the connection reactance from the converter to the grid, and number of bridge arm submodules.

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

[0121] According to the converter model, circuit parameters, and ideal control results after applying control in different steady-state conditions, the submodule capacitor voltage minus the submodule output voltage can be calculated, and then the result of the target margin setting value of the two can be subtracted to obtain the final difference expression. Then, by applying a certain substitution method, the difference expression can be simplified to a form without trigonometric functions. For example, let x = cos(ωt), The time variable t and the power factor Replace with variables x and y to facilitate subsequent solutions. Due to the difference in the difference in the output voltage double frequency component control of the submodule using this method, the difference expression obtained by solution is different. In order to facilitate the distinction between different methods, the coefficient λ is added to the formula N With λ C For the formula components that only appear in the formula when passive circulating current suppression is used and the submodule output voltage double frequency component control is not used, multiply them by the coefficient λ N For the formula components that only appear when passive circulating current suppression and submodule output voltage double frequency component control are used, multiply them by the coefficient λ C The specific process is explained as follows:

[0122] First, according to the rated voltage U SN With the AC side rated current I N Solve 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 link voltage U dc , solve the rated modulation ratio m 0 as follows

[0125]

[0126] Further according to the AC side current standard value And the rated modulation ratio m 0 , calculate the coefficient as follows

[0127]

[0128] Definition coefficient K dc as follows

[0129]

[0130] where ε ul(NR) is the rated capacitance voltage fluctuation ratio of the converter, ε ul(HR) is the converter capacitor voltage fluctuation ratio after the 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 Rated modulation ratio m 0 And the variable y can represent the modulation ratio m of the double frequency component h as follows

[0137]

[0138] Then the capacitor voltage fluctuation function is calculated The maximum value of under any x and y conditions is the per unit value of the AC side current, f rS (x,y) is the system fluctuation function, which is expressed as follows

[0139]

[0140] Among them, m h is the modulation ratio of the double frequency component, K dc and is the calculated coefficient, m 0 is the rated modulation ratio, X pu is the per unit value of the AC side connection reactance, x, y are variables; and then we can get a certain margin M diff In the case of , the difference between the submodule capacitor voltage and the submodule output voltage is expressed as The result is

[0141]

[0142] Step 3: Determine whether to use active circulation suppression or passive circulation suppression.

[0143] Step 4: determine whether it is necessary to adopt the double frequency voltage component control of the present invention to further increase the capacitance reduction effect.

[0144] Step 5: Set the coefficient λ in the difference expression according to the selection result N With λ C For active circulation suppression, set the coefficient λ N =0 and λ C = 0, for the case of passive circulating current suppression and no double frequency voltage component control, set the coefficient λ N =1 and λ C = 0, for the case of passive circulating current suppression and double frequency voltage component control, set the coefficient λ N =0 and λ C =1. Then, the DC component operating value and the submodule capacitance value of the submodule required for the converter are calculated based on the difference expression, and the converter is designed accordingly. The specific process is explained as follows:

[0145] In the converter design stage, the following processing is performed according to the selected situation to solve the minimum value of the difference expression about the variables x and y, and further set the minimum value equal to 0, and calculate the capacitor voltage fluctuation ratio value ε of the converter designed by this method at this time ul(HR) . To solve the difference expression The minimum point position in the range of x,y -1 to 1 and the converter capacitor voltage fluctuation ratio ε after the design using this method ul(HR) , can be obtained by the following method. First solve the equation:

[0146]

[0147] Find a solution that satisfies the defined interval. If there is a solution, further check the second-order derivative to determine whether the extreme point is a maximum, minimum or saddle point, and only keep the solution of the minimum point.

[0148] At the same time, for the four boundary conditions of x=1, x=-1, y=1, y=-1 The linear function extreme value is found, and the extreme value points on the four boundaries are finally compared with the values ​​of the extreme value points inside the definition domain to determine the coordinates x of the minimum value of the surface. dmin ,y dimin Is it located on the boundary or inside the definition? Further, according to the location of the minimum point, the corresponding equations are combined with The minimum value is equal to 0, and the solution is:

[0149] If the minimum value is inside the domain, the solution to the system of equations is:

[0150]

[0151] If the minimum value is on the boundary x = -1, then the solution to the system of equations is:

[0152]

[0153] Similarly, we can get the minimum value on other boundaries. By solving the equations, we can get Minimum point coordinate x dmin ,y dimin , and the converter capacitor voltage fluctuation ratio ε after the design using this method ul(HR) .

[0154] Then according to the converter rated capacitance voltage fluctuation ratio ε ul(NR) The converter capacitor voltage fluctuation ratio ε after the design using this method ul(HR) , the DC component operating value U of the submodule required by the converter can be calculated cap(HR) as follows:

[0155]

[0156] Among them U cap(NR) is the rated value of the DC component of the submodule capacitor voltage before the proposed method is adopted, U cap(HR) It is the calculated DC component operation value of the submodule after adopting the design method of the present invention. cap(NR) The DC voltage U dc The number of converter bridge arm submodules N is calculated as follows:

[0157]

[0158] According to the rated capacitance voltage fluctuation ratio ε of the converter ul(NR) , the converter capacitor voltage fluctuation ratio value ε after the design using this method ul(HR) , and the maximum value of the capacitor voltage fluctuation function The submodule capacitance value C required for the converter design can be calculated. sm(HR) as follows:

[0159]

[0160] Among them I N is the rated current of the AC side, is the per unit value of the AC current, ω is the angular frequency of the AC side, U dc is the DC side voltage, and N is the number of bridge arm submodules.

[0161] Step 6: If the double frequency voltage component control is selected in step 4 to further improve the capacitance reduction effect, the following steps need to be added to the converter control structure: detect the double frequency voltage component output by the three-phase bridge arm submodule, and control it to the target value through the control loop. The specific process is explained as follows:

[0162] The AC side voltage and the AC side current of the converter are detected, and then the positive and negative zero sequence results of the AC side voltage and the AC side current of the converter can be calculated by 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 rated current I N The result after standardization is:

[0164]

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

[0166] Then, according to the converter setting parameters, the submodule 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 double frequency component in the submodule output voltage is calculated.

[0167] As mentioned above, the double frequency voltage component modulation ratio m h It can be expressed as

[0168]

[0169] Will and Bring in

[0170]

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

[0172]

[0173] The target value of the negative sequence double frequency voltage component to be injected is given by U dc / N per unit value It can be expressed as

[0174]

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

[0176]

[0177] in

[0178]

[0179] Therefore, the double frequency voltage target value in the final submodule output voltage is dc The result of the per unit value of / N in the dq coordinate system It can be deduced as

[0180]

[0181] where u min is the lower limit design value of the submodule output voltage, m 0 is the rated modulation ratio, X pu is the per unit value of the AC side connection reactance, It is the q-axis per unit value of the converter AC side current converted to the positive sequence dq coordinate system.

[0182] Then detect the converter resonant capacitor C 0The voltage at both ends or the output voltage of the bridge arm is used to obtain the actual value of the double frequency component in the output voltage of the submodule. The actual value of the double frequency component in the output voltage of the submodule obtained by the detection is controlled to the calculated target value of the double frequency component in the output voltage of the submodule through the feedback control link. The feedback control link outputs the corresponding modulation wave adjustment value.

[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 to complete the control.

[0184] like Figure 3 As shown, the design method of a modular multi-level converter disclosed in the present invention is mainly divided into the following steps:

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

[0186] Then, a unified converter model compatible with active circulating current suppression, passive circulating current suppression without the addition of the double frequency voltage component control of the present invention, and passive circulating current suppression with the addition of the double frequency voltage component control of the present invention is derived, and the difference expression between the sub-module capacitor voltage and the sub-module output voltage considering the margin is calculated.

[0187] Then determine whether the converter adopts active circulating current suppression or passive circulating current suppression.

[0188] It is further determined whether to adopt a submodule output double frequency voltage component control method to further improve the capacitance reduction effect.

[0189] Then set the coefficient λ in the difference expression according to the selection result N With λ C For active circulation suppression, set the coefficient λ N =0 and λ C = 0, for the case of passive circulating current suppression and no double frequency voltage component control, set the coefficient λ N =1 and λ C = 0, for the case of passive circulating current suppression and double frequency voltage component control, set the coefficient λ N =0 and λ C = 1. Then, the DC component operating value and the submodule capacitance value of the submodule required to be designed by the converter are calculated according to the difference expression, and the converter is designed accordingly.

[0190] Finally, if you choose to use passive circulating current suppression and choose to use the double frequency voltage component control in this method, you need to add the following link to the converter control structure: detect the double frequency voltage component output by the three-phase bridge arm submodule, and control it to the target value through the control loop.

[0191] Figure 4 : is a simulation waveform diagram of the steady-state operation condition of a modular multi-level converter in the case of active circulating current suppression after using the method of the present invention in one embodiment of the present invention. Figure 4 (a) is the steady-state operating waveform when the converter outputs rated capacitive reactive power. Figure 4 Figure (b) is a steady-state operating waveform diagram when the converter outputs rated active power, and Figure 4 (c) 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 in various cases. It can be seen that in all operating conditions, there are only DC and fundamental frequency components in the submodule output voltage. After applying the design method of the present invention, the margin between the submodule voltage and the submodule 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 submodule remains unchanged, so that the capacitor value required by the submodule can be greatly reduced, and then the size and cost of the converter can be significantly reduced. Under the traditional design, the rated value of the DC side component of the submodule capacitor voltage is 2kV, and the submodule capacitor voltage fluctuation limit is 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 4 It can be seen that for the modular multilevel converter using active circulating current suppression after using the method of the present invention, the submodule capacitor voltage peak value remains unchanged at 2.2 kV, and the valley value can be reduced to 1.71 kV, so the rated value of the DC side component of the submodule capacitor voltage is reduced to 1.954 kV, and the corresponding submodule capacitor voltage fluctuation increases to 12.56%, so the submodule capacitor value can be reduced to 83.15%.

[0192] Figure 5 It is a simulation waveform diagram of a steady-state operation condition of a modular multilevel converter using passive circulating current suppression after using the method of the present invention in one embodiment of the present invention, and not using the double frequency component control of the submodule output voltage. Figure 5 (a) is the steady-state operating waveform when the converter outputs rated capacitive reactive power. Figure 5 Figure (b) is the steady-state operating waveform of the converter when it outputs rated active power. Figure 5Figure (c) 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 required capacitance value of the submodule can be greatly reduced, and the size 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 limit is 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 under the condition of passive circulating current suppression and does not adopt the control of 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, so the rated value of the DC side component of the submodule capacitor voltage is reduced to 1.925kV, and the corresponding submodule capacitor voltage fluctuation rises to 14.34%, so the submodule capacitor value can be reduced to 75%.

[0193] Figure 6 It is a simulation waveform diagram of a steady-state operating condition of a modular multi-level converter using passive circulating current suppression after using the method of the present invention in one embodiment of the present invention, using a sub-module output voltage double frequency component control. 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 operation 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, there is a double frequency voltage component in the submodule output voltage that is controlled by the control loop in the proposed method. 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 allowable fluctuation space of the submodule capacitor voltage is further expanded, the submodule capacitor value, and the volume and cost of the converter can be further reduced. In traditional design cases, the rated value of the DC side component of the submodule capacitor voltage is 2kV, and the fluctuation limit of the submodule capacitor voltage is 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 adopting the passive circulating current suppression method of the present invention is used and the submodule output voltage double frequency component is controlled, the submodule capacitor voltage peak value remains unchanged at 2.2kV, and the valley value can be reduced to 1.55kV, so the rated value of the DC side component of the submodule capacitor voltage is reduced to 1.88kV, and the corresponding submodule capacitor voltage fluctuation rises to 16.7%, so the submodule capacitor value can be reduced to 66%. And the minimum value of the submodule output voltage in each case is controlled at the design value U dc u min / N, which is consistent with the design target.

[0194] The modular multilevel converter provided by the present invention 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 network transmission occasions, as well as in offshore and onshore wind power, large-scale photovoltaic and other new energy centralized transmission occasions. It can also be used as a medium-voltage AC / DC converter for medium-voltage motor drive, medium-voltage direct current grid, and all-electric ships and other occasions. The proposed method can significantly reduce the amount of capacitors required in the modular multilevel converter, thereby reducing the volume and cost of the overall converter. In particular, the volume of the overall converter can be greatly reduced, and a more compact converter can be achieved. A new idea is proposed to reduce the cost and volume of modular multilevel converters.

[0195] The above are only preferred embodiments of the present invention and do not limit the present invention in any way. Any simple modifications, changes and equivalent structural changes made to the above embodiments based on the technical essence of the present invention are still within the protection scope of the technical solution of the present invention.

[0196] The above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the same. Although the present invention has been described in detail with reference to the above embodiments, a person skilled in the art can still modify or make equivalent substitutions to the specific implementation schemes of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention are within the scope of protection of the claims of the present invention.

[0197] The above content is a further detailed description of the present invention. It cannot be determined that the specific implementation methods of the present invention are limited to these. For ordinary technicians in the technical field to which the present invention belongs, without departing from the concept of the present invention, they can also make several simple deductions or substitutions, which should be regarded as belonging to the protection scope of the present invention determined by the submitted claims.

Claims

1. A design method for a modular multilevel converter, characterized in that: include: S1, setting converter parameters, including: rated voltage of converter AC and DC sides, active and reactive operating range, actual value of connection reactance from converter to grid, and number of bridge arm submodules; S2, based on the set converter parameters, derive the unified converter model under various conditions and the difference expression between the submodule capacitor voltage and the submodule output voltage under the condition of considering the margin; S3, determining the converter circulating current suppression method: using active circulating current suppression or passive circulating current suppression; S4, adopt active circulating current suppression, directly proceed to step S5, adopt passive circulating current suppression, determine whether it is necessary to adopt double frequency voltage component control to further increase the capacitance reduction effect; if not, directly proceed to step S5, if yes, add the corresponding double frequency voltage component control to the converter control, and proceed to step S6; S5, calculating the DC component operating value of the converter submodule, the submodule capacitance value and other parameters in the converter design according to the selection result, and performing converter design 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, it is necessary to add to the converter control structure: detecting the double frequency voltage component output by the three-phase bridge arm submodule and controlling it to the target value through a control loop.

2. A modular multilevel converter design method according to claim 1, characterized in that: The converter is formed by connecting three-phase units in parallel, each phase contains two upper and lower bridge arms, each bridge arm is composed of N submodules and a bridge arm reactor in series, and two bridge arms are connected in series to form a phase unit structure. After the series connection, the midpoint of the two bridge arms is the AC output point, and the upper and lower endpoints are the DC output points; the DC output point of the three-phase unit is connected in parallel to the DC bus, and the AC output point of the three-phase unit is connected to the AC power grid; Among them, the bridge arm reactor is in the form of two reactors L1 and L2 connected in series, and the midpoint of the bridge arm reactor appears in each bridge arm; a resonant capacitor C0 is connected across the midpoint of the bridge arm reactor of the upper and lower bridge arms of the phase unit; the resonant capacitor C0 value and the reactance value 2L1 are designed to be in a double frequency resonant state about the AC side, so that the DC side of each phase contains a double frequency parallel resonant cavity; the resonant cavity bears the double frequency voltage on the AC side under any circumstances and blocks the flow of the double frequency circulating current in the converter.

3. The design method of a modular multilevel converter according to claim 1, characterized in that: The derivation of the unified converter model in various situations and the difference expression between the submodule capacitor voltage and the submodule output voltage in consideration of the margin include: A unified converter model compatible with active circulating current suppression, without passive circulating current suppression, and with passive circulating current suppression is derived, and the difference expression between the submodule capacitor voltage and the submodule output voltage considering the margin is calculated.

4. The design method of a modular multilevel converter according to claim 3, characterized in that: The derivation of the unified converter model in various situations and the difference expression between the submodule capacitor voltage and the submodule output voltage when the margin is considered specifically includes: According to the converter model, circuit parameters and ideal control results after applying control in steady state under different conditions, the submodule capacitor voltage minus the submodule output voltage is calculated, and then the result of the target margin setting value of the two is subtracted to obtain the final difference expression; then the substitution method is applied to simplify the difference expression into a form without trigonometric functions, and the formula components that appear only when passive circulating current suppression is adopted and the submodule output voltage double frequency component control is not adopted are multiplied by the coefficient λ N ; For the formula components that only appear when passive circulating current suppression and submodule output voltage double frequency component control are used, multiply them by the coefficient λ C .

5. The design method of a modular multilevel converter according to claim 4, characterized in that: The derivation of the unified converter model in various situations and the difference expression between the submodule capacitor voltage and the submodule output voltage in consideration of the margin includes the following steps: According to the rated voltage U SN With the AC side rated current I N Solve the per unit value X of the AC side connection reactance pu as follows X pu =X eq / (U SN / I N ) Where X eq is the actual value of the connection reactance between the converter and the grid; according to the rated voltage U SN With DC link voltage U dc , solve the rated modulation ratio m0 as follows Further according to the AC side current standard value And the rated modulation ratio m0, calculation coefficient as follows Definition coefficient K dc as follows where ε ul(NR) is the rated capacitance voltage fluctuation ratio of the converter, ε ul(HR) is the converter capacitor voltage fluctuation ratio; The variable time t and the power factor angle Replace with variables x and y as follows: sign(x)={1,ωt∈(0,π];-1,ωt∈[-π,0]} Where ω is the angular frequency of the AC side, for any t and The corresponding x and y values ​​are between -1 and 1; According to the set submodule output voltage lower limit design value u min ,coefficient The rated modulation ratio m0 and the variable y represent the modulation ratio m of the double frequency component. h as follows: Then the capacitor voltage fluctuation function is calculated The maximum value of under any x and y conditions is the per unit value of the AC side current, f rS (x,y) is the system fluctuation function, which is expressed as follows Among them, m h is the modulation ratio of the double frequency component, K dc and is the calculated coefficient, m0 is the rated modulation ratio, X pu is the per unit value of the AC side connection reactance, x, y are variables; and then we can get a certain margin M diff In the case of , the difference between the submodule capacitor voltage and the submodule output voltage is expressed as The result is 6. The design method of a modular multilevel converter according to claim 1, characterized in that: The S5 includes: Set the coefficient λ in the difference expression according to the selection result N With λ C ; For active circulation suppression, set the coefficient λ N =0 and λ C = 0, for the case of passive circulating current suppression and no double frequency voltage component control, set the coefficient λ N =1 and λ C = 0, for the case of passive circulating current suppression and double frequency voltage component control, set the coefficient λ N =0 and λ C =1; then, according to the difference expression, the DC component operating value and the submodule capacitance value of the submodule required for the corresponding design of the converter are calculated, and the converter is designed accordingly.

7. The design method of a modular multilevel converter according to claim 6, characterized in that: The S5 specifically includes: In the converter design stage, the following processing is performed according to the selected situation to solve the minimum value of the difference expression about the variables x and y, and further set its minimum value equal to 0 to calculate the converter capacitor voltage fluctuation ratio ε at this time ul(HR) ; For solving the difference expression The minimum point position in the range of x,y -1 to 1 and the converter capacitor voltage fluctuation ratio ε ul(HR) , obtained by the following method; first solve the equation Find the solution that satisfies the defined interval. If there is a solution, further check the second-order derivative to determine whether the extreme point is a maximum, minimum or saddle point, and only keep the solution of the minimum point; At the same time, for the four boundary conditions of x=1, x=-1, y=1, y=-1 The linear function extreme value is found, and the extreme value points on the four boundaries are finally compared with the values ​​of the extreme value points inside the definition domain to determine the coordinates x of the minimum value of the surface. dmin ,y dimin Is it located on the boundary or inside the definition? Further, according to the location of the minimum point, the corresponding equations are combined with The minimum value is equal to 0, so solve it; If the minimum value is inside the domain, then the solution to the system of equations is If the minimum value is on the boundary x = -1, then the solution to the system of equations is The same is true for the results of the minimum value on other boundaries; by solving the equations, we get Minimum point coordinate x dmin ,y dimin , and the converter capacitor voltage fluctuation ratio ε ul(HR) ; Then according to the converter rated capacitance voltage fluctuation ratio ε ul(NR) The ratio of the converter capacitor voltage fluctuation ε ul(HR) , calculate the DC component operating value U of the submodule that the converter needs to design accordingly cap(HR) as follows: Among them, U cap(NR) is the rated value of the DC component of the submodule capacitor voltage before the proposed method is adopted, U cap(HR) is the calculated DC component operating value of the submodule; U cap(NR) Through the converter DC voltage U dc The number of converter bridge arm submodules N is calculated as follows: According to the rated capacitance voltage fluctuation ratio ε of the converter ul(NR) , capacitor voltage fluctuation ratio ε ul(HR) , and the maximum value of the capacitor voltage fluctuation function Calculate the submodule capacitance value C required for the converter design at this time sm(HR) as follows: Among them, I N is the rated current of the AC side, is the per unit value of the AC current, ω is the angular frequency of the AC side, U dc is the DC side voltage, and N is the number of bridge arm submodules.

8. The design method of a modular multilevel converter according to claim 1, characterized in that: Specifically include: Detect the AC side voltage and the AC side current of the converter, and then calculate the positive and negative zero sequence results of the AC side voltage and the AC side current of the converter by using the positive and negative sequence decoupling method; The AC side current of the converter is converted to the positive sequence dq coordinate system and the rated current I N The result after standardization is in, is the actual value of the converter AC side current converted to the positive sequence dq coordinate system, is the per-unit value of the converter AC side current converted to the positive sequence dq coordinate system, I N is the rated current of the AC side, is the per unit value of the AC side current; Then, according to the converter setting parameters, the submodule 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 double frequency component in the submodule output voltage is calculated.

9. A control method for a modular multilevel converter, characterized in that: A converter designed by using a modular multilevel converter design method according to any one of claims 1 to 8, wherein the control method comprises: The double frequency voltage component output by the three-phase bridge arm submodule is detected and controlled to a 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 includes: The double frequency voltage component modulation ratio m h Expressed as Will and Bring in The current AC current relationship Substituting into, the above formula becomes The target value of the negative sequence double frequency voltage component to be injected is given by U dc / N per unit value Expressed as Convert it to the negative sequence double frequency dq coordinate system as follows in Therefore, the double frequency voltage target value in the final submodule output voltage is dc The result of the per unit value of / N in the dq coordinate system Derived as where u min is the lower limit design value of the submodule output voltage, 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; Then, the voltage across the converter resonant capacitor C0 or the output voltage of the bridge arm 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 obtained by the detection is controlled to be the calculated target value of the double frequency component in the submodule output voltage through the feedback control link; the feedback control link outputs the corresponding modulation wave adjustment value; 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 to complete the control.

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