Improved deadbeat control method for reducing switching loss

Abstract

The invention discloses an improved dead-beat control method for reducing switching loss, and the method specifically comprises the following steps: 1, carrying out the single-phase circuit modeling of an MMC, and building a single-phase upper bridge arm node current equation and a single-phase lower bridge arm node current equation; 2, discretizing the established equation, and establishing a dead-beat control system by taking the circular flow as a reference quantity; step 3, voltage acquisition and voltage dispersion discrimination are carried out on upper and lower bridge arm sub-modules of each phase of the MMC; 4, calculating the voltage of the upper and lower bridge arms output in the step 2 to obtain the number of sub-modules which should be conducted by the upper and lower bridge arms of each phase in the next control period of the MMC; and step 5, sorting the sub-module voltages of each phase processed in the step 3, and switching on or switching off the specific sub-modules according to the number of sub-modules which should be switched on by the upper and lower bridge arms of each phase in the next control period obtained in the step 4. The dynamic performance of the system can be improved, the quality of output voltage is improved, and the number of times of system sub-module conduction is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of flexible direct current power transmission methods, and in particular relates to an improved deadbeat control method for reducing switching losses. Background Art

[0002] Under the background of "dual carbon", achieving green, efficient and sustainable energy utilization has become a consensus among countries around the world. Modular Multilevel Converter (MMC) is a high-performance power electronic device with the advantages of high power density, low harmonic content and strong scalability through modular design. Therefore, it has a wide range of applications in power system energy conversion and utilization. Its main application areas include flexible DC transmission systems, power quality controllers, high-voltage and high-power special power supplies, electric vehicle charging stations, and distributed power generation grid connection. However, when running with a large number of sub-modules, MMC has the problems of complex control system and poor dynamic characteristics. At the same time, the long operation time of the controller and the frequent switching of sub-modules lead to large switching losses. Summary of the invention

[0003] The purpose of the present invention is to provide an improved deadbeat control method for reducing switching losses, which solves the problem in the prior art that when the MMC runs with a large number of sub-modules, the control system is complex and the dynamic characteristics are poor.

[0004] The technical solution adopted by the present invention is an improved deadbeat control method for reducing switching loss, which is specifically implemented in the following steps:

[0005] Step 1, single-phase circuit modeling of the MMC is performed, and the current equations of the single-phase upper and lower bridge arm nodes are established by considering the circulating current and the DC side current;

[0006] Step 2, discretize the equation established in step 1, and use the circulation flow as a reference quantity to establish a deadbeat control system;

[0007] Step 3, collect voltages from the upper and lower bridge arm submodules of each phase of the MMC and determine the voltage dispersion. If the set voltage dispersion condition is met, the voltage quantity of each half of the upper and lower bridge arm submodules of each phase is processed. If the set voltage dispersion condition is not met, no processing is performed;

[0008] Step 4, calculating the upper and lower bridge arm voltages output in step 2 to obtain the number of submodules that should be turned on in each phase of the upper and lower bridge arms in the next control cycle of the MMC;

[0009] Step 5, sorting the voltages of each submodule of each phase after processing in step 3, and turning on or off specific submodules according to the number of submodules that should be turned on in the upper and lower bridge arms of each phase in the next control cycle obtained in step 4.

[0010] The present invention is also characterized in that:

[0011] In step 1, the MMC three-phase structure is the same and has a total of 6 bridge arms. Each phase can be divided into two bridge arms, the upper and lower bridge arms. Each bridge arm is composed of n sub-modules with the same structure and the bridge arm inductor L in cascade. Among them, U dc is the DC output voltage; u Px with u Nx are 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 are the currents flowing through the upper and lower bridge arms of phase x respectively; u x is the output voltage of the MMC AC port; u sx with i x are the phase voltage and phase current of the AC side respectively; L s It is a filter inductor for energy interaction between AC power supply and MMC.

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

[0013]

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

[0015]

[0016] Step 2 is as follows: discretize equation (1) to obtain the output voltage equations of the upper and lower bridge arms of the MMC in the kth control cycle:

[0017]

[0018] Where: U dc (k) and u x (k) are the DC side voltage and AC output port voltage of the kth control period respectively; u Px (k) and u Nx (k) and i Px (k), i Nx (k) is the sum of the voltages of all submodule capacitors cascaded into the upper and lower bridge arms of the MMC main circuit and the current flowing through the upper and lower bridge arms in the kth control cycle; i * Px (k+1), i * Nx(k+1) is the upper and lower arm current at the start of the k+1 control cycle, that is, the upper and lower arm current prediction reference value. According to formula (2), it can be obtained:

[0019]

[0020] In the above formula, i x_ref is the AC current reference value; i cir_ref To achieve system circulating current suppression and reduce the interphase circulating current reference value of submodule capacitor voltage fluctuation; I d_ref is the DC side current reference value, and the submodule capacitor voltage reference value U c_ref With its average value U c_aver The difference between the two can be controlled by PI to give the interphase circulating current reference value i cir_ref :

[0021]

[0022] Where: k p With k i are proportional and integral coefficients respectively;

[0023] Substituting equation (4) into equation (3) and equation (5) together constructs a deadbeat current prediction control system; the MMC deadbeat current prediction control system is divided into three parts: voltage outer loop, current inner loop, nearest level approximation modulation and sorting algorithm. The voltage outer loop gives the phase-to-phase circulating current reference value i cir_ref With the AC current reference value i x_ref and I d_ref / 3 is substituted into formula (4) to obtain the output current reference value i of the upper and lower bridge arms of each phase Px_ref 、i Nx_ref , change i Px_ref 、i Nx_ref Substituting into equation (3) again, we can get the reference value u of the upper and lower bridge arm voltages of each phase in the kth control cycle: Px (k) and u Nx (k) The submodules that should be put into use in the upper and lower bridge arms of each phase are given through the nearest level approximation modulation and sorting algorithm, and the trigger pulses of the submodules of each phase are generated;

[0024] In order to compensate for the influence of time delay on the control effect, the output voltage of the AC port is predicted one beat in advance, and the upper and lower bridge arm currents are predicted two beats in advance. Therefore, the control cycle +1 processing can be performed on equation (3), and the result is:

[0025]

[0026] When the MMC is running stably, the DC side voltage fluctuation is small, so it can be considered that:

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

[0028] From equations (3) and (6), we can know that the AC port output voltage u x 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 cycle value is small, and it can be considered that in each control cycle u x The increments of u are equal, and the first-order forward difference method is used to calculate u x After prediction and downsampling, we can get:

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

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

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

[0032] First, substitute equation (9) into equation (6); then substitute i in equation (3) * Px (k+1), i * Nx (k+1) is replaced by i Px (k+1), i Nx (k+1), and use formula (3) again to get i Px (k+1) and i Nx (k+1) expression and substitute it into equation (6), and finally combine it with equation (7), we can get:

[0033]

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

[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, x Make predictions:

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

[0039] The t predicted by formula (11) k+1 Substituting the AC current value at the moment into formula (12), we can 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 ) is substituted into i in formula (4) x_ref , we can get:

[0042]

[0043] Among them, d_ref Doing the same processing, we can get:

[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, in order to ensure the consistency of the submodule capacitor voltage, a discreteness threshold criterion is set based on the complete sorting voltage-sharing strategy. Before the submodule capacitor voltage is sorted in each control cycle, the discreteness of each submodule capacitor voltage is judged. If the maximum discreteness of the submodule is within the set range, the improved sorting strategy is entered; if the maximum discreteness of the submodule is not within the set range, the complete sorting strategy is entered;

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

[0047]

[0048] When δ>δ ref When δ<δ ref When the charging and discharging status of each submodule in the current control cycle is combined, the improved sorting algorithm is entered.

[0049] In step 3, the submodules that meet the discreteness threshold should ensure to maintain the original switching state in the next control cycle to the greatest extent, so the submodules in the charging state are multiplied by a switching retention coefficient H1 less than 1, and the submodules in the discharging state are multiplied by a switching retention coefficient H2 greater than 1. The specific implementation is: the number of submodules N that should be put into each bridge arm is calculated by the modulation strategy; the voltage and current of each phase bridge arm submodule are collected, and the discreteness threshold is judged for the voltage of the upper and lower bridge arm submodules of each phase. When the voltage of each phase bridge arm submodule is less than the set discreteness reference value, enter the next step, otherwise enter the complete sorting algorithm; judge the switching state of the submodule, if it is in the cut-off state, do not do any processing to it; if it is in the put-in state, enter the next step; select half of the submodules in each phase bridge arm to judge the charging and discharging state of the submodule to reduce the system operation load, when it is in the charging state, multiply it by the switching retention coefficient H1; if it is in the discharging state, multiply it by the switching retention coefficient H2.

[0050] In step 3, the voltage dispersion criterion δ is satisfied. ref When =10%, the switching holding coefficients H1 and H2 are introduced, and H1=0.98 and H2=1.02 are set.

[0051] 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 in each phase of the upper and lower bridge arms in the next control cycle of the MMC. Then, the number of sub-modules that should be turned on in each phase of the upper and lower bridge arms in the next control cycle is:

[0052]

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

[0054] The beneficial effects of the present invention are

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

[0056] 2) Significant reduction in switching times: The present invention optimizes the modulation strategy and processes the voltage of half of the submodules in the upper and lower bridge arms of each phase that meet the set voltage dispersion conditions, effectively reducing the number of submodule switches. The simulation results show that the average number of switches can be reduced by 28%, which directly improves the conversion efficiency of the converter, making energy utilization more efficient, and is of great significance for energy conservation and emission reduction.

[0057] 3) The implementation method is simple and easy: The implementation process of the present invention is more concise and clear, and is easy to apply in engineering, which 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 technology;

[0058] 4) The present invention significantly improves the dynamic performance of the system through an innovative deadbeat control strategy and greatly reduces the number of switching times during submodule operation, thereby achieving remarkable results in improving the energy efficiency and dynamic performance of the MMC system and promoting the industrialization of technology. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0060] Figure 2 It is the MMC topological structure diagram in the present invention;

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

[0062] Figure 4 is a discreteness threshold criterion flow chart of the present invention;

[0063] Figure 5 It is the flow chart of the improved sorting algorithm of the present invention:

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

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

[0066] Figure 8 It is a dynamic performance experimental diagram of the present invention. DETAILED DESCRIPTION

[0067] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0068] Example 1

[0069] The present invention provides an improved deadbeat control method for reducing switching loss, which is specifically implemented in the following steps:

[0070] Step 1, single-phase circuit modeling of the MMC is performed, and the current equations of the single-phase upper and lower bridge arm nodes are established by considering the circulating current and the DC side current;

[0071] Step 2, discretize the equation established in step 1, and use the circulation flow as a reference quantity to establish a deadbeat control system;

[0072] Step 3, collect voltages from the upper and lower bridge arm submodules of each phase of the MMC and determine the voltage dispersion. If the set voltage dispersion condition is met, the voltage quantity of each half of the upper and lower bridge arm submodules of each phase is processed. If the set voltage dispersion condition is not met, no processing is performed;

[0073] Step 4, calculating the upper and lower bridge arm voltages output in step 2 to obtain the number of submodules that should be turned on in each phase of the upper and lower bridge arms in the next control cycle of the MMC;

[0074] Step 5, sorting the voltages of each submodule of each phase after processing in step 3, and turning on or off specific submodules according to the number of submodules that should be turned on in the upper and lower bridge arms of each phase in the next control cycle obtained in step 4.

[0075] Example 2

[0076] Improved deadbeat control method to reduce switching loss, MMC main circuit topology such as Figure 2 As shown, in step 1, the MMC three-phase structure is the same with a total of 6 bridge arms, each phase can be divided into two bridge arms, upper and lower, each bridge arm is composed of n sub-modules with the same structure and a bridge arm inductor L cascaded, where U dc is the DC output voltage; u Px with u Nx are 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 are the currents flowing through the upper and lower bridge arms of phase x respectively; u x is the output voltage of the MMC AC port; u sx with i x are the phase voltage and phase current of the AC side respectively; L s It is a filter inductor for energy interaction between AC power supply and MMC.

[0077] Example 3

[0078] An improved deadbeat control method for reducing switching losses, wherein in step 1, a single-phase model of the MMC is constructed, and the single-phase equivalent circuit of the MMC is as follows: Figure 3 As shown, ignoring the influence of AC power supply and AC side filter inductance, taking single phase as an example, the KVL equations for the upper and lower bridge arms are established:

[0079]

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

[0081]

[0082] Example 4

[0083] The improved deadbeat control method for reducing switching loss, wherein in step 2, the equation (1) is discretized to obtain the output voltage equations of the upper and lower bridge arms of the MMC in the kth control cycle:

[0084]

[0085] Where: U dc (k) and u x (k) are the DC side voltage and AC output port voltage of the kth control period respectively; u Px (k) and u Nx (k) and i Px (k), i Nx (k) is the sum of the voltages of all submodule capacitors cascaded into the upper and lower bridge arms of the MMC main circuit and the current flowing through the upper and lower bridge arms in the kth control cycle; i * Px (k+1), i * Nx (k+1) is the upper and lower arm current at the start of the k+1 control cycle, that is, the upper and lower arm current prediction reference value. According to formula (2), it can be obtained:

[0086]

[0087] In the above formula, i x_ref is the AC current reference value; i cir_ref To achieve system circulating current suppression and reduce the interphase circulating current reference value of submodule capacitor voltage fluctuation; I d_ref is the DC side current reference value, and the submodule capacitor voltage reference value U c_ref With its average value Uc_aver The difference between the two can be controlled by PI to give the interphase circulating current reference value i cir_ref :

[0088]

[0089] Where: k p With k i are proportional and integral coefficients respectively, and k is taken p is 400, k i is 60;

[0090] Substituting equation (4) into equation (3) and equation (5) together constructs a deadbeat current prediction control system, as Figure 1 The MMC deadbeat current prediction control system consists of three parts: voltage outer loop (circulation current suppression), current inner loop (deadbeat current prediction control), nearest level approximation modulation and sorting algorithm. The voltage outer loop gives the interphase circulating current reference value i cir_ref With the AC current reference value i x_ref and I d_ref / 3 is substituted into formula (4) to obtain the output current reference value i of the upper and lower bridge arms of each phase Px_ref 、i Nx_ref , change i Px_ref 、i Nx_ref Substituting into equation (3) again, we can get the reference value u of the upper and lower bridge arm voltages of each phase in the kth control cycle: Px (k) and u Nx (k) The submodules that should be put into use in the upper and lower bridge arms of each phase are given through the nearest level approximation modulation and sorting algorithm, and the trigger pulses of the submodules of each phase are generated;

[0091] In order to compensate for the influence of time delay on the control effect, the output voltage of the AC port is predicted one beat in advance, and the upper and lower bridge arm currents are predicted two beats in advance. Therefore, the control cycle +1 processing can be performed on equation (3), and the result is:

[0092]

[0093] When the MMC is running stably, the DC side voltage fluctuation is small, so it can be considered that:

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

[0095] From equations (3) and (6), we can know that the AC port output voltage u x 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 cycle value is small, and it can be considered that in each control cycle u x The increments of u are equal, and the first-order forward difference method is used to calculate u x After prediction and downsampling, we can get:

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

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

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

[0099] First, substitute equation (9) into equation (6); then substitute i in equation (3) * Px (k+1), i * Nx (k+1) is replaced by i Px (k+1), i Nx (k+1), and use formula (3) again to get i Px (k+1) and i Nx (k+1) expression and substitute it into equation (6), and finally combine it with equation (7), we can get:

[0100]

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

[0102]

[0103] Using the first-order forward difference method, x Make predictions:

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

[0105] The t predicted by formula (11) k+1 Substituting the AC current value at the moment into formula (12), we can 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 ) is substituted into i in formula (4) x_ref , we can get:

[0108]

[0109] Among them, d_ref Doing the same processing, we can get:

[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, wherein in step 3, in order to ensure the consistency of the submodule capacitor voltage, a discreteness threshold criterion is set based on the complete sorting voltage balancing strategy. Before the submodule capacitor voltage is sorted in each control cycle, the discreteness of each submodule capacitor voltage is judged. If the maximum discreteness of the submodule is within the set range, the improved sorting strategy is entered; if the maximum discreteness of the submodule is not within the set range, the complete sorting strategy is entered. The discreteness threshold criterion is as follows: Figure 4 shown.

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

[0114]

[0115] When δ>δ ref When δ<δ ref When the charging and discharging status of each submodule in the current control cycle is combined, the improved sorting algorithm is entered.

[0116] Figure 5In order to improve the sorting algorithm flow chart, when the voltage dispersion criterion is met, the switching holding coefficient H1(0

[0117] 1) The number of sub-modules N that should be put into each bridge arm is calculated by the modulation strategy;

[0118] 2) Collect the voltage and current of each phase bridge arm submodule, and perform discrete 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 discrete reference value (the present invention sets δ ref =10%), go to the next step, otherwise go to the complete sorting algorithm;

[0119] 3) Determine the switching state of the submodule. If it is in the cut-off state, do nothing to it; if it is in the switched-on state, proceed to the next step;

[0120] 4) Select half of the sub-modules in each phase bridge arm to determine the charging and discharging status of the sub-modules to reduce the system computing load. When in the charging state, multiply it by the switching holding coefficient H1; if in the discharging state, multiply it by the switching holding coefficient H2.

[0121] Example 6

[0122] An improved deadbeat control method for reducing switching losses, wherein in step 4, the upper and lower bridge arm voltages output in step 2 are calculated to obtain the number of submodules that should be turned on in each phase of the upper and lower bridge arms of the MMC in the next control cycle, and N=10 (N is the number of upper and lower bridge arm submodules per phase of the MMC), then the number of submodules that should be turned on in each phase of the upper and lower bridge arms in the next control cycle is:

[0123]

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

[0125] The improved deadbeat control method for reducing switching loss of the present invention effectively reduces the switching time of the dynamic process. Figure 6-7 As shown in FIG. 1 , the dynamic process can be reduced from 1.074 ms to 400.45 μs (a disturbance is applied to the AC side current at 2 s) through three-phase eleven-level simulation verification. Figure 8 ​As shown in the figure, the single-phase five-level test verifies that the dynamic process can be reduced from 180ms to 120ms, which improves the dynamic response capability of the equipment. The sub-module switching times are effectively reduced, and the simulation verifies that the average reduction can be 28%, which directly improves the conversion efficiency of the converter and makes energy utilization more efficient.

Claims

1. An improved deadbeat control method for reducing switching losses, characterized in that: Please follow the steps below to implement: Step 1, single-phase circuit modeling of the MMC is performed, and the current equations of the single-phase upper and lower bridge arm nodes are established by considering the circulating current and the DC side current; Step 2, discretize the equation established in step 1, and use the circulation flow as a reference quantity to establish a deadbeat control system; Step 3, collect voltages from the upper and lower bridge arm submodules of each phase of the MMC and determine the voltage dispersion. If the set voltage dispersion condition is met, the voltage quantity of each half of the upper and lower bridge arm submodules of each phase is processed. If the set voltage dispersion condition is not met, no processing is performed; Step 4, calculating the upper and lower bridge arm voltages output in step 2 to obtain the number of submodules that should be turned on in each phase of the upper and lower bridge arms in the next control cycle of the MMC; Step 5, sorting the voltages of each submodule of each phase after processing in step 3, and turning on or off specific submodules according to the number of submodules that should be turned on in the upper and lower bridge arms of each phase in the next control cycle obtained in step 4.

2. The improved deadbeat control method for reducing switching loss according to claim 1, characterized in that: In step 1, the MMC three-phase structure is the same and has a total of 6 bridge arms. Each phase can be divided into two bridge arms, an upper bridge arm and a lower bridge arm. Each bridge arm is composed of n sub-modules with the same structure and a bridge arm inductor L in cascade connection. dc is the DC output voltage; u Px with u Nx are 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 are the currents flowing through the upper and lower bridge arms of phase x respectively; u x is the output voltage of the MMC AC port; u sx with i x are the phase voltage and phase current of the AC side respectively; L s It is a filter inductor for energy interaction between AC power supply and MMC.

3. The improved deadbeat control method for reducing switching loss according to claim 2, characterized in that: In step 1, a single-phase model is built for the MMC, and the influence of the AC power supply and the AC side filter inductance is ignored. Taking the single-phase as an example, the KVL equations for the upper and lower bridge arms are established: The three-phase structure of MMC is symmetrical, and the DC side current is evenly distributed among the three phases. The capacitor voltage of the phase unit submodule fluctuates, resulting in a voltage difference between the phase units, forming a double frequency circulating current. At the same time, considering the DC side current, the single-phase upper and lower bridge arm node current equations are established:

4. The improved deadbeat control method for reducing switching loss according to claim 1, characterized in that: The step 2 is specifically as follows: discretizing equation (1) to obtain the output voltage equations of the upper and lower bridge arms of the MMC in the kth control cycle: Where: U dc (k) and u x (k) are the DC side voltage and AC output port voltage of the kth control period respectively; u Px (k) and u Nx (k) and i Px (k), i Nx (k) is the sum of the voltages of all submodule capacitors cascaded into the upper and lower bridge arms of the MMC main circuit and the current flowing through the upper and lower bridge arms in the kth control cycle; i * Px (k+1), i * Nx (k+1) is the upper and lower arm current at the start of the k+1 control cycle, that is, the upper and lower arm current prediction reference value. According to formula (2), it can be obtained: In the above formula, i x_ref is the AC current reference value; i cir_ref Phase-to-phase circulating current reference value to achieve system circulating current suppression and reduce submodule capacitor voltage fluctuation; I d_ref is the DC side current reference value, and the submodule capacitor voltage reference value U c_ref With its average value U c_aver The difference between the two can be controlled by PI to give the interphase circulating current reference value i cir_ref : Where: k p With k i are proportional and integral coefficients respectively; Substituting equation (4) into equation (3) and equation (5) together constructs a deadbeat current prediction control system; the MMC deadbeat current prediction control system is divided into three parts: voltage outer loop, current inner loop, nearest level approximation modulation and sorting algorithm. The voltage outer loop gives the phase-to-phase circulating current reference value i cir_ref With the AC current reference value i x_ref and I d_ref / 3 is substituted into formula (4) to obtain the output current reference value i of the upper and lower bridge arms of each phase Px_ref 、i Nx_ref , change i Px_ref 、i Nx_ref Substituting into equation (3) again, we can get the reference value u of the upper and lower bridge arm voltages of each phase in the kth control cycle: Px (k) and u Nx (k) The submodules that should be put into use in the upper and lower bridge arms of each phase are given through the nearest level approximation modulation and sorting algorithm, and the trigger pulses of the submodules of each phase are generated; In order to compensate for the influence of time delay on the control effect, the output voltage of the AC port is predicted one beat in advance, and the upper and lower bridge arm currents are predicted two beats in advance. Therefore, the control cycle +1 processing can be performed on equation (3), and the result is: When the MMC is running stably, the DC side voltage fluctuation is small, so it can be considered that: IN dc (k)=U dc (k+1)=U dc_ref (7) From equations (3) and (6), we can know that the AC port output voltage u x 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 cycle value is small, and it can be considered that in each control cycle u x The increments of u are equal, and the first-order forward difference method is used to calculate u x After prediction and downsampling, we can get: in x (k+1)-th x (k)=u x (k)-u x (k-1) (8) u x The predicted reference value in the k+1 control cycle is: u x (k+1)=2u x (k)-u x (k-1) (9) First, substitute equation (9) into equation (6); then substitute i in equation (3) * Px (k+1), i * Nx (k+1) is replaced by i Px (k+1), i Nx (k+1), and use formula (3) again to get i Px (k+1) and i Nx (k+1) expression and substitute it into equation (6), and finally combine it with equation (7), we can get: The AC current prediction expression is established using Newton interpolation quadratic polynomial: 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 )=i x (t k-2 )-3i x (t k-1 )+3i x (t k ) (11) Using the first-order forward difference method, x Make predictions: and x-ref (t k+2 )=2i x (t k+1 )-and x (t k ) (12) The t predicted by formula (11) k+1 Substitute the AC current value at that moment into formula (12), We can get: i x_ref (t k+2 )=2i x (t k-2 )-6i x (t k-1 )+5i x (t k ) (13) will i x_ref (t k+2 ) is substituted into i in formula (4) x_ref , we can get: Among them, d_ref Doing the same processing, we can get: I d_ref =I d (t k+2 )=2I d (t k-2 )-6I d (t k-1 )+5I d (t k ) (15)。 5. The improved deadbeat control method for reducing switching loss according to claim 1, characterized in that: In step 3, in order to ensure the consistency of the submodule capacitor voltage, a discreteness threshold criterion is set based on the complete sorting voltage-sharing strategy. Before the submodule capacitor voltage is sorted in each control cycle, the discreteness of the capacitor voltage of each submodule is judged. If the maximum discreteness of the submodule is within the set range, the improved sorting strategy is entered; if the maximum discreteness of the submodule is not within the set range, the complete sorting strategy is entered; The maximum and minimum values ​​of the submodule capacitor voltage and the difference between them are u cmax (t),u c min (t), Δu c max , so the submodule capacitor voltage dispersion δ can be expressed as: When δ>δ ref When δ<δ ref When the charging and discharging status of each submodule in the current control cycle is combined, the improved sorting algorithm is entered.

6. The improved deadbeat control method for reducing switching loss according to claim 5, characterized in that: In step 3, the submodules that meet the discreteness threshold should ensure to maintain the original switching state in the next control cycle to the greatest extent, so the submodules in the charging state are multiplied by a switching retention coefficient H1 less than 1, and the submodules in the discharging state are multiplied by a switching retention coefficient H2 greater than 1. The specific implementation is: the number of submodules N that should be put into each bridge arm is calculated by the modulation strategy; the voltage and current of each phase bridge arm submodule are collected, and the discreteness threshold is judged for the voltage of the upper and lower bridge arm submodules of each phase. When the voltage of each phase bridge arm submodule is less than the set discreteness reference value, enter the next step, otherwise enter the complete sorting algorithm; judge the switching state of the submodule, if it is in the cut-off state, do not do any processing to it; if it is in the put-in state, enter the next step; select half of the submodules in each phase bridge arm to judge the charging and discharging state of the submodule to reduce the system operation load, when it is in the charging state, multiply it by the switching retention coefficient H1; if it is in the discharging state, multiply it by the switching retention coefficient H2.

7. The improved deadbeat control method for reducing switching loss according to claim 5, characterized in that: In step 3, the voltage dispersion criterion δ is satisfied. ref When =10%, the switching holding coefficients H1 and H2 are introduced, and H1=0.98 and H2=1.02 are set.

8. The improved deadbeat control method for reducing switching loss according to claim 1, characterized in that: In step 4, the upper and lower bridge arm voltages output in step 2 are calculated to obtain the number of submodules that should be turned on in each phase of the upper and lower bridge arms in the next control cycle of the MMC. Then, the number of submodules that should be turned on in each phase of the upper and lower bridge arms in the next control cycle is: In the formula, u J is the modulation wave amplitude, U C Submodule capacitor voltage rating.

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