Improved dead-beat control method for reducing switching losses

Abstract

The application discloses an improved deadbeat control method for reducing switching loss, and is implemented according to the following steps: step 1, modeling a single-phase circuit of an MMC, and meanwhile, establishing a single-phase upper and lower bridge arm node current equation; step 2, discretizing the established equation, and taking a ring current as a reference quantity to establish a deadbeat control system; step 3, collecting voltages of upper and lower bridge arms of each phase of the MMC and discriminating voltage discretization; step 4, operating the upper and lower bridge arm voltages output by step 2 to obtain the number of sub-modules that should be turned on for each phase of the upper and lower bridge arms of the MMC in the next control period; and step 5, sorting each sub-module voltage of each phase after being processed in step 3, and meanwhile, turning on or turning off specific sub-modules according to the number of sub-modules that should be turned on for each phase of the upper and lower bridge arms in the next control period obtained in step 4. The application can improve system dynamic performance, improve output voltage quality and reduce the number of times of turning on sub-modules of the system.

Description

Technical Field

[0001] This invention belongs to the technical field of flexible DC transmission methods, specifically relating to an improved deadbeat control method for reducing switching losses. Background Technology

[0002] Against the backdrop of a "dual carbon" environment, achieving green, efficient, and sustainable energy utilization has become a global consensus. Modular multilevel converters (MMCs), as high-performance power electronic devices, offer advantages such as high power density, low harmonic content, and strong scalability through modular design, making them widely used in power system energy conversion and utilization. Their main application areas include flexible DC transmission systems, power quality controllers, high-voltage, high-power special power supplies, electric vehicle charging stations, and distributed generation grid connection. However, when operating with a large number of submodules, MMCs suffer from complex control systems, poor dynamic characteristics, long controller operation times, and high switching losses due to frequent submodule switching. Summary of the Invention

[0003] The purpose of this invention is to provide an improved deadbeat control method to reduce switching losses, which solves the problems of complex control system and poor dynamic characteristics in the prior art when the number of sub-modules in MMC is large.

[0004] The technical solution adopted in this invention is an improved deadbeat control method for reducing switching losses, which is implemented according to the following steps:

[0005] Step 1: Model a single-phase circuit for the MMC, and establish the node current equations for the upper and lower bridge arms of the single phase, taking into account both circulating current and DC side current.

[0006] Step 2: Discretize the equations established in Step 1, and establish a deadbeat control system using the circulation flow as a reference quantity;

[0007] Step 3: Collect voltage data for each phase upper and lower bridge arm submodule of MMC and determine voltage dispersion. If the set voltage dispersion condition is met, process the voltage data for half of each phase upper and lower bridge arm submodule. If the set voltage dispersion condition is not met, no processing is performed.

[0008] Step 4: Calculate the number of sub-modules that should be turned on for each phase of the upper and lower bridge arms in the next control cycle of the MMC by calculating the voltage of the upper and lower bridge arms output in Step 2.

[0009] Step 5: Sort the voltage of each sub-module of each phase after processing in Step 3, and turn on or off specific sub-modules according to the number of sub-modules that should be turned on in the next control cycle of each phase upper and lower bridge arms obtained in Step 4.

[0010] The invention is further characterized by:

[0011] In step 1, the MMC three-phase structure is identical with a total of 6 bridge arms. Each phase can be divided into upper and lower bridge arms. Each bridge arm consists of n identical sub-modules cascaded with the bridge arm inductor L, where U dc This is the DC output voltage; u Px with u Nx These represent the output voltages of the upper and lower bridge arms of phase x, respectively; the number of phases x = a, b, c; i Px with i Nx These represent the currents flowing through the upper and lower arms of phase x, respectively; u x For the output voltage of the MMC AC port; u sx with i x These are the AC side phase voltage and phase current, respectively; L s A filter inductor for energy exchange between AC power supply and MMC.

[0012] In step 1, a single-phase model of the MMC is performed, ignoring the influence of the AC power supply and the AC-side filter inductor. Taking a single phase as an example, KVL equations are established for its upper and lower arms:

[0013]

[0014] The MMC three-phase structure is symmetrical, and the DC-side current is evenly distributed among the three phases. The phase unit submodule capacitor voltage fluctuates, resulting in voltage differences between phase units and forming a second-harmonic circulating current. Considering the DC-side current, the current equations for the single-phase upper and lower bridge arm nodes are established:

[0015]

[0016] Step 2 specifically involves discretizing equation (1) to obtain the output voltage equations for the upper and lower arms of the MMC in the k-th control cycle:

[0017]

[0018] In the formula: U dc (k) and u x (k) represent the DC-side voltage and AC output port voltage during the kth control cycle, respectively; u Px (k), u Nx (k) and i Px (k), i Nx (k) represents the sum of the voltages across all submodule capacitors in the upper and lower arms of the control cascaded MMC main circuit during the k-th control cycle, and the current flowing through the upper and lower arms, respectively; i * Px (k+1),i * Nx(k+1) represents the upper and lower arm currents at the start of the k+1 control cycle, i.e., the predicted reference values ​​of the upper and lower arm currents. According to equation (2), we can obtain:

[0019]

[0020] In the above formula, i x_ref This is the reference value for alternating current; i cir_ref To achieve system circulating current suppression and reduce interphase circulating current reference values ​​for submodule capacitor voltage fluctuations; I d_ref The reference value for the DC-side current is obtained by using the reference value U of the submodule capacitor voltage. c_ref Its average value U c_aver The difference can be used to provide a reference value for interphase circulating current i after PI control. cir_ref :

[0021]

[0022] In the formula: k p With k i These are the proportional and integral coefficients, respectively.

[0023] Substituting equation (4) into equation (3) and equation (5) together, a deadbeat current prediction control system is constructed. The MMC deadbeat current prediction control system consists of three parts: voltage outer loop, current inner loop, and nearest-level approximation modulation and sorting algorithm. The voltage outer loop provides the interphase circulating current reference value i. cir_ref With AC current reference value i x_ref and I d_ref Substituting / 3 into equation (4) gives the reference values ​​of the output current i for each phase's upper and lower bridge arms. Px_ref i Nx_ref , change i Px_ref i Nx_ref Substituting this into equation (3) gives the reference value u of the upper and lower bridge arm voltages of each phase in the kth control cycle. Px (k), u Nx (k) The sub-modules that should be put into each phase upper and lower bridge arm are given by the nearest level approximation modulation and sorting algorithm, and trigger pulses for each phase sub-module are generated;

[0024] To compensate for the impact of time delay on the control effect, the AC port output voltage is predicted one beat in advance, and the upper and lower bridge arm currents are predicted two beats in advance. Therefore, equation (3) can be processed by adding 1 to the control cycle, resulting in:

[0025]

[0026] When the MMC is operating stably, the DC-side voltage fluctuation is small, which can be considered as follows:

[0027] U dc (k)=U dc(k+1)=U dc_ref (7)

[0028] From both equations (3) and (6), we can see that the AC port output voltage u x With the predicted current i of the upper and lower bridge arms * Px i * Nx It is a linear relationship with a control period of +1, because T s The period value is relatively small, so it can be assumed that u is small in each control cycle. x The increments are equal, and the first-order forward difference method is used to apply the formula to u. x After performing prediction and price reduction processing, we can obtain:

[0029] u x (k+1)-u x (k)=u x (k)-u x (k-1) (8)

[0030] u x The predicted reference value for the k+1 control period is:

[0031] u x (k+1)=2u x (k)-u x (k-1) (9)

[0032] First, substitute equation (9) into equation (6); second, change the i in equation (3). * Px (k+1),i * Nx (k+1) is replaced by i Px (k+1),i Nx (k+1), and then using equation (3) again, we get i Px (k+1) and i Nx Substituting the expression (k+1) into equation (6), and finally combining it with equation (7), we get:

[0033]

[0034] The AC current prediction expression established using Newton's interpolation quadratic polynomial is as follows:

[0035] i x_ref (t k+1 ) = i x (t k-2 )+i x [t k-2 ,t k-1 ](t k+1 -t k-2 )+i x[t k-2 ,t k-1 , t k ](t k+1 -t k-2 )(t k+1 -t k-1 )

[0036] =i x (t k-2 )-3i x (t k-1 )+3i x (t k (11)

[0037] Using the first-order forward difference method for i x Make a prediction:

[0038] i x_ref (t k+2 )=2i x (t k+1 )-i x (t k (12)

[0039] The t predicted by equation (11) k+1 Substituting the value of the alternating current at any given time into equation (12), we get:

[0040] i x_ref (t k+2 )=2i x (t k-2 )-6i x (t k-1 )+5i x (t k (13)

[0041] will i x_ref (t k+2 Substitute i into equation (4) x_ref We can obtain:

[0042]

[0043] Among them, for I d_ref By performing the same procedure, we can obtain:

[0044] I d_ref =I d (t k+2 )=2I d (t k-2 )-6I d (t k-1 )+5I d (t k (15).

[0045] In step 3, to ensure the consistency of the capacitor voltage of the sub-modules, a dispersion threshold criterion is set based on the full sorting and equalization strategy. Before sorting the capacitor voltage of the sub-modules in each control cycle, the dispersion of the capacitor voltage of each sub-module is judged. If the maximum dispersion of the sub-module is within the set range, the improved sorting strategy is entered; if the maximum dispersion of the sub-module is not within the set range, the full sorting strategy is entered.

[0046] Let the maximum and minimum values ​​of the submodule capacitor voltage and the difference between them be u, respectively. cmax (t), u cmin (t), Δu cmax Therefore, the voltage dispersion δ of the submodule capacitor can be expressed as:

[0047]

[0048] When δ>δ ref When δ < δ, the submodule capacitor voltages are directly and completely sorted; when δ < δ ref At this time, it is necessary to combine the charging and discharging status of each sub-module in the current control cycle to enter the improved sorting algorithm.

[0049] In step 3, submodules that meet the dispersion threshold should maintain their original switching state in the next control cycle to the greatest extent possible. Therefore, submodules in the charging state are multiplied by a switching retention coefficient H1 less than 1, and submodules in the discharging state are multiplied by a switching retention coefficient H2 greater than 1. Specifically, the following steps are taken: the number N of submodules to be engaged in each bridge arm is calculated by the modulation strategy; the voltage and current of each phase bridge arm submodule are collected, and the dispersion threshold is judged for the voltage of each phase upper and lower bridge arm submodule. When the voltage of each phase bridge arm submodule is less than the set dispersion reference value, the next step is initiated; otherwise, the complete sorting algorithm is initiated; the switching state of the submodule is judged. If it is in the disconnected state, no processing is performed; if it is in the engaged state, the next step is initiated; half of the submodules in each phase bridge arm are selected to judge the charging and discharging state of the submodules to reduce the system's computational load. When the submodule is in the charging state, it is multiplied by the switching retention coefficient H1; when it is in the discharging state, it is multiplied by the switching retention coefficient H2.

[0050] In step 3, the voltage dispersion criterion δ is satisfied. ref When the percentage is 10%, introduce switching retention coefficients H1 and H2, setting H1 = 0.98 and H2 = 1.02.

[0051] In step 4, the number of submodules that should be turned on for each phase of the upper and lower bridge arms in the next control cycle of the MMC is calculated based on the voltage output from step 2. Therefore, the number of submodules that should be turned on for each phase of the upper and lower bridge arms in the next control cycle is:

[0052]

[0053] In the formula, u J U is the modulation amplitude value. C Submodule capacitor voltage rating.

[0054] The beneficial effects of this invention are

[0055] 1) Significantly improved dynamic performance: Through innovative control strategies, this invention effectively reduces the switching time of dynamic processes and improves the dynamic performance of the system. Experimental verification shows that the dynamic process can be reduced from 180ms to 120ms, thus improving the dynamic response capability of the equipment.

[0056] 2) Significant reduction in switching frequency: This invention optimizes the modulation strategy and processes the voltage of half of the upper and lower bridge arms of each phase that meet the set voltage dispersion conditions, effectively reducing the number of switching of the sub-modules. Simulation verification shows that the average number of switching can be reduced by 28%, which directly improves the conversion efficiency of the converter and makes energy utilization more efficient, which is of great significance for energy conservation and emission reduction.

[0057] 3) The implementation method is simple and easy to implement: The implementation process of this invention is simpler and clearer, and it is easy to apply in engineering. It not only reduces the complexity of technical implementation, but also shortens the product development cycle, which is conducive to the rapid promotion and industrialization of the technology.

[0058] 4) This invention significantly improves the dynamic performance of the system through an innovative deadbeat control strategy and greatly reduces the number of switching operations during the operation of sub-modules, thereby achieving remarkable results in improving the energy efficiency and dynamic performance of the MMC system and promoting the industrialization of the technology. Attached Figure Description

[0059] Figure 1 This is an overall structural diagram of the present invention;

[0060] Figure 2 This is a diagram of the MMC topology in this invention;

[0061] Figure 3 This is a single-phase modeling diagram of the present invention;

[0062] Figure 4 This is a flowchart of the discreteness threshold criterion of the present invention;

[0063] Figure 5 Here is a flowchart of the improved sorting algorithm of this invention:

[0064] Figure 6 This is a simulation diagram of the dynamic performance of the present invention (three-phase AC current);

[0065] Figure 7 This is a simulation detail diagram of the dynamic performance of the present invention (three-phase AC current);

[0066] Figure 8 This is a dynamic performance test diagram of the present invention. Detailed Implementation

[0067] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0068] Example 1

[0069] This invention provides an improved deadbeat control method for reducing switching losses, specifically implemented according to the following steps:

[0070] Step 1: Model a single-phase circuit for the MMC, and establish the node current equations for the upper and lower bridge arms of the single phase, taking into account both circulating current and DC side current.

[0071] Step 2: Discretize the equations established in Step 1, and establish a deadbeat control system using the circulation flow as a reference quantity;

[0072] Step 3: Collect voltage data for each phase upper and lower bridge arm submodule of MMC and determine voltage dispersion. If the set voltage dispersion condition is met, process the voltage data for half of each phase upper and lower bridge arm submodule. If the set voltage dispersion condition is not met, no processing is performed.

[0073] Step 4: Calculate the number of sub-modules that should be turned on for each phase of the upper and lower bridge arms in the next control cycle of the MMC by calculating the voltage of the upper and lower bridge arms output in Step 2.

[0074] Step 5: Sort the voltage of each sub-module of each phase after processing in Step 3, and turn on or off specific sub-modules according to the number of sub-modules that should be turned on in the next control cycle of each phase upper and lower bridge arms obtained in Step 4.

[0075] Example 2

[0076] An improved deadbeat control method to reduce switching losses, and the MMC main circuit topology as follows: Figure 2 As shown, in step 1, the MMC three-phase structure has 6 identical bridge arms. Each phase can be divided into upper and lower bridge arms. Each bridge arm consists of n identical sub-modules cascaded with the bridge arm inductor L, where U dc This is the DC output voltage; u Px with u Nx These represent the output voltages of the upper and lower bridge arms of phase x, respectively; the number of phases x = a, b, c; i Px with i Nx These represent the currents flowing through the upper and lower arms of phase x, respectively; u x For the output voltage of the MMC AC port; u sx with i x These are the AC side phase voltage and phase current, respectively; L s A filter inductor for energy exchange between AC power supply and MMC.

[0077] Example 3

[0078] An improved deadbeat control method to reduce switching losses, wherein step 1 involves single-phase modeling of the MMC, and the single-phase equivalent circuit of the MMC is as follows: Figure 3 As shown, ignoring the effects of the AC power supply and the AC-side filter inductor, taking a single phase as an example, the KVL equations for its upper and lower bridge arms are established:

[0079]

[0080] The MMC three-phase structure is symmetrical, and the DC-side current is evenly distributed among the three phases. The phase unit submodule capacitor voltage fluctuates, resulting in voltage differences between phase units and forming a second-harmonic circulating current. Considering the DC-side current, the current equations for the single-phase upper and lower bridge arm nodes are established:

[0081]

[0082] Example 4

[0083] An improved deadbeat control method to reduce switching losses, wherein in step 2, equation (1) is discretized to obtain the output voltage equations of the upper and lower arms of the MMC in the k-th control cycle:

[0084]

[0085] In the formula: U dc (k) and u x (k) represent the DC-side voltage and AC output port voltage during the kth control cycle, respectively; u Px (k), u Nx (k) and i Px (k), i Nx (k) represents the sum of the voltages across all submodule capacitors in the upper and lower arms of the control cascaded MMC main circuit during the k-th control cycle, and the current flowing through the upper and lower arms, respectively; i * Px (k+1),i * Nx (k+1) represents the upper and lower arm currents at the start of the k+1 control cycle, i.e., the predicted reference values ​​of the upper and lower arm currents. According to equation (2), we can obtain:

[0086]

[0087] In the above formula, i x_ref This is the reference value for alternating current; i cir_ref To achieve system circulating current suppression and reduce interphase circulating current reference values ​​for submodule capacitor voltage fluctuations; I d_ref The reference value for the DC-side current is obtained by using the reference value U of the submodule capacitor voltage. c_ref Its average value Uc_aver The difference can be used to provide a reference value for interphase circulating current i after PI control. cir_ref :

[0088]

[0089] In the formula: k p With k i These are the proportional and integral coefficients, respectively, with k as the coefficient. p 400, k i It is 60;

[0090] Substituting equation (4) into equation (3) and equation (5) together, we construct a deadbeat current prediction control system, such as... Figure 1 As shown. The MMC deadbeat current predictive control system consists of three parts: an outer voltage loop (circulating current suppression), an inner current loop (deadbeat current predictive control), and a nearest-level approximation modulation and sorting algorithm. The outer voltage loop provides the interphase circulating current reference value i. cir_ref With AC current reference value i x_ref and I d_ref Substituting / 3 into equation (4) gives the reference values ​​of the output current i for each phase's upper and lower bridge arms. Px_ref i Nx_ref , change i Px_ref i Nx_ref Substituting this into equation (3) gives the reference value u of the upper and lower bridge arm voltages of each phase in the kth control cycle. Px (k), u Nx (k) The sub-modules that should be put into each phase upper and lower bridge arm are given by the nearest level approximation modulation and sorting algorithm, and trigger pulses for each phase sub-module are generated;

[0091] To compensate for the impact of time delay on the control effect, the AC port output voltage is predicted one beat in advance, and the upper and lower bridge arm currents are predicted two beats in advance. Therefore, equation (3) can be processed by adding 1 to the control cycle, resulting in:

[0092]

[0093] When the MMC is operating stably, the DC-side voltage fluctuation is small, which can be considered as follows:

[0094] U dc (k)=U dc (k+1)=U dc_ref (7)

[0095] From both equations (3) and (6), we can see that the AC port output voltage u x With the predicted current i of the upper and lower bridge arms * Px i * Nx It is a linear relationship with a control period of +1, because Ts The period value is relatively small, so it can be assumed that u is small in each control cycle. x The increments are equal, and the first-order forward difference method is used to apply the formula to u. x After performing prediction and price reduction processing, we can obtain:

[0096] u x (k+1)-u x (k)=u x (k)-u x (k-1) (8)

[0097] u x The predicted reference value for the k+1 control period is:

[0098] u x (k+1)=2u x (k)-u x (k-1) (9)

[0099] First, substitute equation (9) into equation (6); second, change the i in equation (3). * Px (k+1),i * Nx (k+1) is replaced by i Px (k+1),i Nx (k+1), and then using equation (3) again, we get i Px (k+1) and i Nx Substituting the expression (k+1) into equation (6), and finally combining it with equation (7), we get:

[0100]

[0101] The AC current prediction expression established using Newton's interpolation quadratic polynomial is as follows:

[0102]

[0103] Using the first-order forward difference method for i x Make a prediction:

[0104] i x_ref (t k+2 )=2i x (t k+1 )-i x (t k (12)

[0105] The t predicted by equation (11) k+1 Substituting the value of the alternating current at any given time into equation (12), we get:

[0106] i x_ref (t k+2 )=2ix (t k-2 )-6i x (t k-1 )+5i x (t k (13)

[0107] will i x_ref (t k+2 Substitute i into equation (4) x_ref We can obtain:

[0108]

[0109] Among them, for I d_ref By performing the same procedure, we can obtain:

[0110] I d_ref =I d (t k+2 )=2I d (t k-2 )-6I d (t k-1 )+5I d (t k (15).

[0111] Example 5

[0112] An improved deadbeat control method for reducing switching losses includes a step 3 method where, to ensure the consistency of submodule capacitor voltages, a dispersion threshold criterion is set based on a fully sorted voltage equalization strategy. Before sorting the submodule capacitor voltages in each control cycle, the dispersion of each submodule capacitor voltage is determined. If the maximum dispersion of a submodule is within the set range, the improved sorting strategy is implemented; otherwise, the fully sorted strategy is implemented. The dispersion threshold criterion is as follows: Figure 4 As shown.

[0113] Let the maximum and minimum values ​​of the submodule capacitor voltage and the difference between them be u, respectively. cmax (t), u cmin (t), Δu cmax Therefore, the voltage dispersion δ of the submodule capacitor can be expressed as:

[0114]

[0115] When δ > δ ref When δ < δ, the submodule capacitor voltages are directly and completely sorted; when δ < δ ref At this time, it is necessary to combine the charging and discharging status of each sub-module in the current control cycle to enter the improved sorting algorithm.

[0116] Figure 5To improve the sorting algorithm flowchart, when the voltage dispersion criterion is met, a switching hold coefficient H1(0) is introduced.

[0117] 1) The number N of submodules to be deployed in each bridge arm is calculated from the modulation strategy;

[0118] 2) Collect the voltage and current of each phase bridge arm submodule, and perform a dispersion threshold judgment on the voltage of each phase upper and lower bridge arm submodule. When the voltage of each phase bridge arm submodule is less than the set dispersion reference value (the present invention sets δ), ref If the percentage is 10%, proceed to the next step; otherwise, proceed to the complete sorting algorithm.

[0119] 3) Determine the switching status of the submodule. If it is in the disconnected state, do not process it; if it is in the connected state, proceed to the next step.

[0120] 4) Select half of the submodules in each phase arm to determine the charging and discharging status of the submodules, thereby reducing the system's computational load. When the submodule is in a charging state, multiply it by the switching and holding coefficient H1; if it is in a discharging state, multiply it by the switching and holding coefficient H2.

[0121] Example 6

[0122] An improved deadbeat control method for reducing switching losses is proposed. In step 4, the upper and lower bridge arm voltages output in step 2 are calculated to obtain the number of sub-modules that should be turned on for each phase of the MMC in the next control cycle. Taking N = 10 (N being the number of sub-modules in each phase of the MMC upper and lower bridge arm), the number of sub-modules that should be turned on for each phase of the MMC in the next control cycle is:

[0123]

[0124] In the formula, u J U is the modulation amplitude value. C Submodule capacitor voltage rating.

[0125] This invention provides an improved deadbeat control method to reduce switching losses, effectively reducing the switching time of dynamic processes. For example... Figure 6-7 As shown, simulation verification of the three-phase eleven-level circuit shows that the dynamic process can be reduced from 1.074ms to 400.45μs (a disturbance is applied to the AC side current at 2s); Figure 8 ​As shown, the dynamic process can be reduced from 180ms to 120ms through single-phase five-level test, which improves the dynamic response capability of the equipment; it effectively reduces the number of sub-module switching, and the simulation verification shows that it can be reduced by an average of 28%, which directly improves the conversion efficiency of the converter and makes energy utilization more efficient.

Claims

1. An improved dead-beat control method to reduce switching losses, characterized in that, The method is implemented according to the following steps: Step 1, single-phase circuit modeling is performed on the MMC, and the circulating current and the DC side current are considered to establish the single-phase upper and lower bridge arm node current equation; Step 2, the equation established in step 1 is discretized, and the circulating current is taken as a reference quantity to establish a deadbeat control system; Step 3, the voltage of each phase upper and lower bridge arm sub-module of the MMC is collected and the voltage dispersion is distinguished, if the set voltage dispersion condition is met, the voltage of each half sub-module of each phase upper and lower bridge arm is processed, otherwise, no processing is performed; Step 4, the upper and lower bridge arm voltages output in step 2 are operated to obtain the number of sub-modules that should be turned on for each phase upper and lower bridge arm in the next control period of the MMC; Step 5, the sub-module voltages of each phase processed in step 3 are sorted, and the specific sub-modules are turned on or turned off according to the number of sub-modules that should be turned on for each phase upper and lower bridge arm in the next control period obtained in step 4; The MMC three-phase structure in step 1 is same and has 6 bridge arms, each phase can be divided into upper and lower bridge arms, each bridge arm is composed of n 6 sub-modules with same structure and bridge arm inductance L cascaded, wherein, U dc is a direct current output voltage; u Px and u Nx are respectively x phase upper and lower bridge arm output voltages; the number of phases x = a, b, c ; i Px and i Nx are respectively x phase upper and lower bridge arm currents; u x is an MMC alternating current port output voltage; u sx and i x are respectively alternating current side phase voltage and phase current; L s is a filter inductance for energy interaction between alternating current power supply and MMC; In step 1, single-phase modeling is performed on the MMC, and the influence of the AC power supply and the AC side filter inductance is ignored, and KVL equations are established for the upper and lower bridge arms in a single phase: （1） The three-phase structure of the MMC is symmetrical, and the DC side current is evenly distributed among the three phases; the capacitor voltage of each phase unit sub-module fluctuates, resulting in a voltage difference between the phase units, forming a double-frequency circulating current; the DC side current is also considered to establish the single-phase upper and lower bridge arm node current equation: （2） In step 2, formula (1) is discretized to obtain the output voltage equation of the MMC upper and lower bridge arms in the kth control period: （3） In the formula: U dc k u x k k u Px k u Nx k i Px k i Nx k k i Px k+ i Nx k+ k+ 1) is the upper and lower bridge arm current at the beginning of the 1 control cycle, that is, the upper and lower bridge arm current prediction reference value, which can be obtained according to formula (2):​​​​​​​​​​​​​​​​​​ （4） In the above formula, i x_ref is the AC current reference value; i cir_ref is the phase-to-phase circulating current reference value for realizing system circulating current suppression and reducing the inter-phase circulating current of the sub-module capacitor voltage fluctuation; I d_ref is the DC side current reference value, which is obtained by using the sub-module capacitor voltage reference value U c_ref and the average value thereof U c_aver The difference between the average value and the average value thereof i cir_ref is the phase-to-phase circulating current reference value （5） In the formula: k p and k i are the proportional and integral coefficients, respectively; The formula (4) is substituted into the formula (3) and the formula (5) to build a dead-beat current prediction control system; the MMC dead-beat current prediction control system is divided into three parts: a voltage outer ring, a current inner ring, a nearest level approximation modulation and sequencing algorithm, and the inter-phase circulating current reference value is given by the voltage outer ring i cir_ref The AC current reference value i x_ref and I d_ref are substituted into the formula (4) to give the upper and lower bridge arm output current reference values of each phase i Px_ref , i Nx_ref , i Px_ref , i Nx_ref The formula (3) is substituted again to give the sum of the voltages of all sub-module capacitors of the upper and lower bridge arms of the cascade MMC main circuit controlled by the upper and lower bridge arm voltages of each phase in the first control period k u Px k ) and u Nx k The nearest level approximation modulation and sequencing algorithm is used to give the sub-modules to be turned on of the upper and lower bridge arms of each phase and to generate the sub-module trigger pulses of each phase.​​​ In order to compensate for the influence of time delay on the control effect, the AC port output voltage is predicted one beat in advance, and the upper and lower bridge arm currents are predicted two beats in advance, so formula (3) can be processed in the control period +1 to obtain: (6) When the MMC is in stable operation, the DC side voltage fluctuation is small, and it can be considered that: (7) From equation (3) and equation (6), the AC port output voltage The upper and lower bridge arms predict current i Px , i Nx is a linear, control cycle +1 relationship, because T s The cycle value is small, and it can be considered that the increment is equal in each control cycle u x The first-order forward difference method is used to predict and downbeat the increment u x , and the following equation can be obtained: (8) u x In The predicted reference value for the control period is: (9) First, substitute equation (9) into equation (6); second, substitute equation (3) into equation (8) i Px ( k+ 1)、 i Nx ( k+ 1) into equation (8) i Px ( k+ 1)、 i Nx ( k + 1), and again use equation (3) to obtain i Px ( k+ 1) and i Nx ( k+ 1) and substitute into equation (6), and finally combine equation (7) to obtain: (10) The Newton interpolation quadratic polynomial is used to establish the AC current prediction expression as: (11) Using a first-order forward difference method to predict i x Prediction: (12) Substituting the predicted value of formula (11) into formula (12), we have: t k+1 Substituting the predicted value of formula (11) into formula (12), we have: (13) Substituting i x_ref ( t k+2 ) for i x_ref , we obtain: (14) wherein, for I d_ref The same procedure gives: (15)。 2. The improved dead-beat control method of reducing switching loss according to claim 1, characterized in that, In step 3, in order to ensure the consistency of the sub-module capacitor voltage, the complete sorting and voltage equalization strategy is set, and the dispersion threshold criterion is set, before the sub-module capacitor voltage is sorted in each control period, the dispersion of each sub-module capacitor voltage is judged, if the maximum dispersion of the sub-module is within the set range, the improved sorting strategy is entered; If the maximum dispersion of the sub-module is not within the set range, the complete sorting strategy is entered; Let the maximum and minimum values of the submodule capacitor voltage and the difference between the two be , , , so the submodule capacitor voltage dispersion can be expressed as: (16) When the sub-module capacitor voltage is directly fully sorted; when the charge and discharge states of each sub-module in the current control period are combined, the improved sorting algorithm is entered.

3. The improved dead-beat control method with reduced switching loss according to claim 2, characterized in that, In step 3, the sub-modules satisfying the dispersion threshold should guarantee the original switching state to the greatest extent in the next control cycle, so the sub-modules in the charging state are multiplied by a switching maintenance coefficient H1 less than 1, and the sub-modules in the discharging state are multiplied by a switching maintenance coefficient H2 greater than 1, and the specific implementation is: the number of sub-modules to be switched on of each bridge arm is calculated according to the modulation strategy; the sub-module voltage and current of each phase bridge arm are collected, and the voltage dispersion threshold of the upper and lower bridge arms of each phase is judged; when the voltage of each phase bridge arm sub-module is less than the set dispersion reference value, the next step is entered, otherwise the complete sorting algorithm is entered; the switching state of the sub-module is judged, if it is in the cut-off state, no processing is performed; if it is in the switching-on state, the next step is entered; half of the sub-modules of each phase bridge arm are selected to judge the charging and discharging state of the sub-modules, and the system operation load is reduced; when in the charging state, the switching maintenance coefficient H1 is multiplied; when in the discharging state, the switching maintenance coefficient H2 is multiplied.

4. The improved dead-beat control method with reduced switching loss according to claim 2, characterized in that, The voltage dispersion criterion is satisfied in step 3 When, the switching retention coefficients H1 and H2 are introduced, H1 = 0.98 and H2 = 1.02 are set.

5. The improved zero-error control method for reducing switching loss according to claim 1, wherein, In step 4, the upper and lower bridge arm voltages output by step 2 are operated to obtain the number of sub-modules that should be turned on for each phase upper and lower bridge arm in the next control cycle of MMC, and then the number of sub-modules that should be turned on for each phase upper and lower bridge arm in the next control cycle of MMC is: (17) In the formula, u J To modulate the amplitude value, U C Sub-module capacitor voltage rating.

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