Independent value function model predictive control method and device for MMC

Abstract

The present invention discloses a predictive control method and device for an independent value function model for MMC, the method comprising: determining the submodule quantity set to be put into use by the upper bridge arm and the lower bridge arm at the next moment; establishing a first value function for the output current of the AC side of the converter at the next moment, rolling optimization of the submodule quantity set, and screening to obtain a first submodule quantity combination; increasing or decreasing the number of submodules put into use on each bridge arm to obtain a second submodule quantity combination; establishing a second value function for the AC component circulation of the bridge arm at the next moment, rolling optimization of the first submodule quantity combination and the second submodule quantity combination, and screening to obtain the optimal submodule quantity combination. The above technical solution can reduce the complexity and computational complexity of the control system, achieve good circulation suppression capability and dynamic response capability, and at the same time, two independent value functions are respectively rolled optimized to avoid the selection of weight factors.

Description

Technical Field

[0001] The present invention relates to the technical field of high-voltage and high-power power electronics, and in particular to an independent value function model predictive control method and device for MMC. Background Art

[0002] The two-level and three-level converters commonly used in the field of power electronics cannot meet the requirements of high voltage and high power. At the same time, power electronic devices have loss problems during use, which also limits its application in high voltage and high power occasions to a certain extent.

[0003] Compared with traditional two-level and three-level converters, modular multilevel converters (MMC) have great technical advantages. Specifically, MMC adopts a modular design and can be well applied to different voltage levels. The number of output levels is proportional to the number of sub-modules in each bridge arm of MMC. In addition, the output voltage of modular multilevel converters has less harmonic content and does not require bulky filters.

[0004] Due to the fluctuation of the submodule capacitor voltage, the MMC has a bridge arm circulation. In order to reduce the loss of the MMC during operation, the circulation needs to be suppressed. At the same time, in order to ensure the stability of the output voltage, the capacitor voltage of the submodule needs to be balanced and controlled. However, the problem is that the traditional proportional resonant controller and proportional integral controller cannot achieve simultaneous control of multiple targets of the MMC. Therefore, when the MMC control targets are multiple, the application of traditional controllers will lead to a very complex control system, and there are many controller parameters, the amount of calculation increases, the control efficiency decreases, the control result is not ideal, and it is difficult to achieve good circulation suppression ability and dynamic response ability. Summary of the invention

[0005] Purpose of the invention: The present invention provides an independent value function model predictive control method and device for MMC, aiming to introduce two independent value functions to perform rolling optimization on multiple sub-module quantity combinations, avoid the selection of weight factors, select the optimal sub-module quantity combination, reduce the complexity and computational complexity of the control system, and achieve good circulation suppression capability and dynamic response capability.

[0006] Technical solution: The present invention provides an independent value function model predictive control method for MMC, comprising: according to the number of sub-modules put into use in the upper bridge arm and the lower bridge arm at the current moment, on the basis of ensuring that the output level of the converter at the next moment is an adjacent level of the output level at the current moment, determining the number of sub-modules put into use in the upper bridge arm and the lower bridge arm at the next moment; the modular multi-level converter includes three phases, each phase includes an upper bridge arm and a lower bridge arm, and multiple sub-modules are arranged on the upper bridge arm and the lower bridge arm; according to the absolute value of the error between the reference value and the actual value of the output current on the AC side of the converter at the next moment, a first value function is established, and the sub-module number set is calculated using the first value function. Rolling optimization is performed to screen out the first submodule quantity combination; according to the sum of the number of submodules put into use on each phase in the first submodule quantity combination, the number of submodules to be increased or decreased on each bridge arm is determined to obtain the second submodule quantity combination; according to the absolute value of the error between the reference value and the actual value of the AC component circulating current of the bridge arm at the next moment, a second value function is established, and the first submodule quantity combination and the second submodule quantity combination are rolled optimized using the second value function to screen out the optimal submodule quantity combination; according to the bridge arm current at the current moment, the submodule with a larger or smaller capacitor voltage is judged to be put into use, and the number of submodules put into use is determined according to the optimal submodule quantity combination.

[0007] Specifically, the output level of the converter at the next moment is the adjacent level of the output level at the current moment, including: the number of sub-modules put into use in the upper bridge arm and the lower bridge arm at the k moment is n respectively. jp and n jn At time k+1, the number of submodules put into use in the upper bridge arm and the lower bridge arm are n jp +m and n jn +n, where j is a, b and c, a, b and c correspond to the three phases of the converter respectively, and the number of submodules put into use meets the following constraints: when m and n are positive integers, |mn|<2, |m|+|n|≤N+3-n jp -n jn , N is the total number of submodules on each bridge arm, 2N is the total number of submodules on each phase; when m and n are negative integers and zero, |mn|<2, |m|+|n|≤n jp +n jn -N+3.

[0008] Specifically, determining the set of sub-module numbers to be put into use in the upper bridge arm and the lower bridge arm at the next moment includes: obtaining the AC component circulation of the bridge arm at the current moment, if the value is greater than 0, increasing the number of sub-modules put into use on the corresponding bridge arm, and the total number of sub-modules put into use on the corresponding phase is between N and N+3; if the value is less than 0, reducing the number of sub-modules put into use on the corresponding bridge arm, and the total number of sub-modules put into use on the corresponding phase is between N-3 and N; calculating all combinations of sub-module numbers to obtain a set of sub-module numbers.

[0009] Specifically, the first value function is as follows:

[0010]

[0011] Among them, J1 is the first value function, i j * (k+1) is the reference value of the output current on the AC side of the converter at time k+1, i j (k+1) is the actual value of the output current on the AC side of the converter at time k+1;

[0012] The method of using the first value function to perform rolling optimization on the submodule quantity set and screening to obtain the first submodule quantity combination includes: taking the minimum output of the first value function as the optimal and screening to obtain the first submodule quantity combination.

[0013] Specifically, determining the number of sub-modules to be increased or decreased in use on each bridge arm includes: calculating the number of sub-modules put into use on each phase in the first sub-module number combination, if the numbers are N-3, N-2 and N+1 respectively, then the number of sub-modules put into use on the upper bridge arm and the lower bridge arm of each phase is increased by 1 respectively; if the numbers are N-1, N+2 and N+3 respectively, then the number of sub-modules put into use on the upper bridge arm and the lower bridge arm of each phase is reduced by 1 respectively; if the numbers are both N and the value of the AC component circulating current of the bridge arm at the current moment is greater than 0, then the number of sub-modules put into use on the upper bridge arm and the lower bridge arm of each phase is increased by 1 respectively; if the numbers are both N and the value of the AC component circulating current of the bridge arm at the current moment is less than 0, then the number of sub-modules put into use on the upper bridge arm and the lower bridge arm of each phase is reduced by 1 respectively.

[0014] Specifically, the second value function is as follows:

[0015] J2=|i jzo (k+1)|,

[0016] Among them, J2 is the second value function, i jzo (k+1) is the actual value of the AC component circulating current of the bridge arm at time k+1, and the reference value of the AC component circulating current of the bridge arm at time k+1 is 0;

[0017] The method uses the second value function to perform rolling optimization on the first submodule quantity combination and the second submodule quantity combination to screen out the optimal submodule quantity combination, including: taking the minimum output of the second value function as the optimal, and screening out the optimal submodule quantity combination.

[0018] Specifically, according to the bridge arm current at the current moment, it is determined whether to put into use a sub-module with a larger or smaller capacitor voltage, including: when the bridge arm current is greater than 0, the sub-modules are arranged in order from small to large capacitor voltages, and the sub-module with a smaller capacitor voltage is put into use; when the bridge arm current is less than 0, the sub-modules are arranged in order from large to small capacitor voltages, and the sub-module with a larger capacitor voltage is put into use.

[0019] The present invention also provides an independent value function model predictive control device for MMC, comprising: a submodule quantity set calculation unit, a first rolling optimization unit, a level compensation unit, a second rolling optimization unit and a control unit, wherein: the submodule quantity set calculation unit is used to determine the submodule quantity set put into use of the upper bridge arm and the lower bridge arm at the next moment based on the number of submodules put into use of the upper bridge arm and the lower bridge arm at the current moment, on the basis of ensuring that the output level of the converter at the next moment is an adjacent level of the output level at the current moment; the modular multi-level converter includes three phases, each phase includes an upper bridge arm and a lower bridge arm, and a plurality of submodules are arranged on the upper bridge arm and the lower bridge arm; the first rolling optimization unit is used to establish a first value function based on the absolute value of the error between the reference value and the actual value of the output current on the AC side of the converter at the next moment, The first value function is used to perform rolling optimization on the submodule quantity set, and the first submodule quantity combination is screened out; the level compensation unit is used to determine the number of submodules to be increased or decreased in each bridge arm according to the sum of the number of submodules put into use on each phase in the first submodule quantity combination, and obtain the second submodule quantity combination; the second rolling optimization unit is used to establish a second value function according to the absolute value of the error between the reference value and the actual value of the AC component circulating current of the bridge arm at the next moment, and use the second value function to perform rolling optimization on the first submodule quantity combination and the second submodule quantity combination, and obtain the optimal submodule quantity combination; the control unit is used to determine the submodule with a larger or smaller capacitor voltage to be put into use according to the bridge arm current at the current moment, and the number of submodules put into use is determined according to the optimal submodule quantity combination.

[0020] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: reducing the complexity and computational complexity of the control system, achieving good circulation suppression capability and dynamic response capability, and at the same time, two independent value functions are respectively optimized in a rolling manner, which can avoid the selection of weight factors. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1A schematic diagram of the structure of a modular multi-level converter provided by the present invention;

[0022] Figure 2 A control schematic diagram of a modular multilevel converter provided by the present invention;

[0023] Figure 3 A schematic diagram of a flow chart of level compensation provided by the present invention;

[0024] Figure 4(a) , 4(b) , 4(c) and 4(d) are the three-phase voltage waveform, three-phase current waveform, a-phase capacitor voltage waveform and three-phase bridge arm circulating current waveform obtained by using the control method of “Design and Experimental Evaluation of Fast Model Predictive Control for Modular Multilevel Converters” published in “IEEE Transactions on Industrial Electronics”;

[0025] Figure 5(a) , 5(b) 5(c) and 5(d) are respectively a three-phase voltage waveform diagram, a three-phase current waveform diagram, a phase a capacitor voltage waveform diagram, and a three-phase bridge arm circulating current waveform diagram obtained by applying the control method provided by the present invention;

[0026] FIG6(a) is a THD waveform diagram of the MMC AC side output current obtained by using the control method of “Design and Experimental Evaluation of Fast Model Predictive Control for Modular Multilevel Converters” published in IEEE Transactions on Industrial Electronics; FIG6(b) is a THD waveform diagram of the MMC AC side output current obtained by using the control method provided by the present invention. DETAILED DESCRIPTION

[0027] The technical solution of the present invention is further described below in conjunction with the accompanying drawings.

[0028] See also Figure 1 , which is a schematic diagram of the structure of the modular multi-level converter provided by the present invention; Figure 2 , which is a control schematic diagram of the modular multilevel converter provided by the present invention.

[0029] In the embodiment of the present invention, the AC side of the modular multilevel converter is composed of three phases, each phase is composed of an upper bridge arm and a lower bridge arm, and the three phases have the same structure. Each bridge arm is composed of a half-bridge sub-module (SM) connected in series (points A and B are connected to the circuit), and each bridge arm is also connected in series with a branch inductor L m And the equivalent resistance R m Each half-bridge submodule includes two insulated gate bipolar transistors IGBT and a capacitor. The AC side output end of the modular multilevel converter is connected to the resistor R l and inductor L l Connected, V dc Indicates the DC side voltage, I dc Indicates the DC side current; i jp 、i jn (j=a, b, c) represent the current of the upper bridge arm and the lower bridge arm respectively; u jp 、u jn (j=a, b, c) represent the voltage of the upper bridge arm and the lower bridge arm respectively; i j (j=a,b,c) represents the output current on the AC side; u j (j=a, b, c) represents the output voltage on the AC side, and a, b and c represent each of the three phases respectively.

[0030] In the specific implementation, the control objectives of MMC mainly include the tracking of the output side AC current for a given reference current; ensuring that the AC component circulating current is as close to 0 as possible when the MMC is working; and ensuring that the submodule capacitor voltage fluctuates around the rated voltage in order to improve the stability of the MMC operation; for the control objectives, the output side AC current tracks the given reference current and ensures that the circulating current of the MMC is as close to 0 as possible when it is working, and the finite control set model predictive control strategy is used for control. In order to establish the control model of the independent value function model predictive control method, the expression of the AC component in the AC side output current and the circulating current of the MMC continuous domain is discretized using the Euler method. For the submodule capacitor voltage balance control, the sorting algorithm is used to ensure the balance of the submodule capacitor voltage.

[0031]

[0032]

[0033] Formula (1) and formula (2) are the expressions of the AC current and circulating current at the output side of the continuous domain respectively. The Euler method is used to discretize formula (1) and formula (2) to obtain the discrete mathematical model of the MMC as shown in formula (3) and formula (4):

[0034]

[0035]

[0036] The expressions for the voltage values ​​of the upper bridge arm and lower bridge arm capacitors at time k+1 are as follows:

[0037]

[0038]

[0039]

[0040]

[0041] Among them, i jzo (k), i jzo (k+1), i j (k), i j (k+1),u jp (k+1),u jn (k+1), V cjpi (k), V cjpi (k+1), V cjni (k), V cjni (k+1) respectively represents the AC component circulating current at moment k, the AC component circulating current at moment k+1, the output side AC current at moment k, the output side AC current at moment k+1, the sum of the capacitor voltages of the submodules put into use in the upper bridge arm at moment k+1, the sum of the capacitor voltages of the submodules put into use in the lower bridge arm at moment k+1, the capacitor voltage value of each submodule in the upper bridge arm at moment k, the capacitor voltage value of each submodule in the upper bridge arm at moment k+1, the capacitor voltage value of each submodule in the lower bridge arm at moment k, and the capacitor voltage value of each submodule in the lower bridge arm at moment k+1.

[0042] In an embodiment of the present invention, based on the number of sub-modules put into use in the upper bridge arm and the lower bridge arm at the current moment, and on the basis of ensuring that the output level of the converter at the next moment is an adjacent level of the output level at the current moment, a set of sub-module numbers put into use in the upper bridge arm and the lower bridge arm at the next moment is determined.

[0043] In the embodiment of the present invention, the modular multilevel converter includes three phases, each phase includes an upper bridge arm and a lower bridge arm, and a plurality of submodules are arranged on the upper bridge arm and the lower bridge arm.

[0044] The output side AC current i is calculated by the MMC discrete mathematical model. j and the AC component in the circulation i jzo According to formula (3) and formula (4), the output side AC current i can be changed by changing the output voltage of the upper bridge arm and the lower bridge arm. j and the AC component in the circulation i jzoThe number of submodules that may be put into use in each bridge arm of the MMC at the next moment is determined by the adjacent level selection method.

[0045] In the embodiment of the present invention, the number of submodules put into use in the upper bridge arm and the lower bridge arm at time k (current time) is n respectively. jp and n jn At time k+1 (next time), the number of submodules put into use in the upper bridge arm and the lower bridge arm are n jp +m and n jn +n, where j is a, b and c, a, b and c correspond to the three phases of the converter respectively, and the number of submodules put into use meets the following constraints: when m and n are positive integers, |mn|<2, |m|+|n|≤N+3-n jp -n jn , N is the total number of submodules on each bridge arm (upper bridge arm or lower bridge arm), 2N is the total number of submodules on each phase; when m and n are negative integers and zero, |mn|<2, |m|+|n|≤n jp +n jn -N+3.

[0046] In a specific implementation, j takes values ​​of a, b and c (j=a, b, c). The present invention includes letter identifiers for j, and the value range of j is the same as this, all representing the corresponding three phases.

[0047] Rated reference voltage V of the MMC submodule cref With DC side voltage V dc The relationship between is as shown in formula (9):

[0048]

[0049] Assume that the submodule voltage is stable at the rated reference voltage V cref , without considering the fluctuation of capacitor voltage, idealizing formula (3) to formula (6) to obtain the following formula:

[0050]

[0051]

[0052] From formula (10), we can see that when i jzo (k) is greater than the reference value 0, at the next moment, i jzo (k+1) value can be reduced to increase the total number of sub-modules in the upper and lower bridge arms. jzo (k) is less than the reference value 0, at the next moment, i jzo Increasing the value of (k+1) can reduce the total number of submodules put into use in the upper bridge arm and the lower bridge arm. jzo(k) For the judgment of reference value 0, when i jzo (k) is greater than the reference value 0, in order to suppress the circulation, it is only necessary to put the sub-modules in each phase into a total of N to N + 3, which can reduce the amount of calculation in the first rolling optimization. Similarly, when i jzo When (k) is less than the reference value 0, in order to suppress the circulating current, it is only necessary to put the total number of sub-modules put into operation in each phase from N-3 to N.

[0053] In an embodiment of the present invention, the AC component circulation of the bridge arm at the current moment is obtained. If the value is greater than 0, the number of sub-modules put into use on the corresponding bridge arm is increased, and the total number of sub-modules put into use on the corresponding phase is between N and N+3; if the value is less than 0, the number of sub-modules put into use on the corresponding bridge arm is reduced, and the total number of sub-modules put into use on the corresponding phase is between N-3 and N; all combinations of sub-module quantities are calculated to obtain a sub-module quantity set.

[0054] In the specific implementation, the AC component circulating current i of the bridge arm at the next moment jzo The formula for (k+1) is as follows:

[0055]

[0056] Among them, T s is the sampling period, V cref Indicates the rated reference voltage of the submodule, i jzo (k) is the AC component circulating current of the bridge arm at the current moment. According to the above formula, when i jzo When (k) is greater than 0, at the next moment, i jzo (k+1) value can be reduced to increase the number of submodules used in the upper and lower bridge arms. jzo When (k) is less than 0, at the next moment, i jzo Increasing the value of (k+1) can reduce the number of submodules used in the upper bridge arm and the lower bridge arm.

[0057] In the specific implementation, the number of submodules put into use in each bridge arm of the MMC at the next moment can be determined by the adjacent level selection method. Under the premise of satisfying the constraint conditions, the total number of submodules put into use on each phase is between N and N+3 or between N-3 and N, and the number of submodules put into use on each phase is between N and N+3 or between N-3 and N. jzo The size of (k) determines the final number of multiple sub-modules combined to form a sub-module number set M jrl .

[0058] The design of the value function mainly reflects the tracking effect of the actual output of the control target at the next moment on the reference value. Therefore, the value function of the output side AC current and the bridge arm AC component circulation can be established in the form of absolute value of error.

[0059] In an embodiment of the present invention, a first value function is established based on the absolute value of the error between the reference value and the actual value of the output current on the AC side of the converter at the next moment, and the first value function is used to perform rolling optimization on the sub-module quantity set to screen out the first sub-module quantity combination.

[0060] In the embodiment of the present invention, the first value function is as follows:

[0061]

[0062] Among them, J1 is the first value function, i j * (k+1) is the reference value of the output current on the AC side of the converter at time k+1, i j (k+1) is the actual value of the output current on the AC side of the converter at time k+1.

[0063] In an embodiment of the present invention, the rolling optimization of the sub-module quantity set using the first value function to screen out the first sub-module quantity combination includes: taking the minimum output of the first value function as the optimal, and screening out the first sub-module quantity combination.

[0064] In the specific implementation, Among them, i j * (k-1) and i j * (k) represent the reference value of the output current of the AC side of the converter at the previous moment and at present respectively.

[0065] In a specific implementation, the first value function is used to perform rolling optimization on the submodule quantity set, specifically, a plurality of submodule quantity combinations in the submodule quantity set are used one by one to control the number of submodules put into use on the three-phase converter, and the output of the corresponding first value function is calculated, and the minimum output is taken as the optimal (the rolling optimization of the second value function in the present invention is the same), and the submodule quantity combination with the minimum output is screened, and the number of submodules put into use on the upper bridge arm and the lower bridge arm are m respectively. jp 、m jn .

[0066] See also Figure 3 , which is a schematic diagram of the flow of level compensation provided by the present invention.

[0067] In the embodiment of the present invention, the number of submodules to be increased or decreased in use on each bridge arm is determined according to the sum of the number of submodules in use on each phase in the first submodule number combination to obtain the second submodule number combination.

[0068] In the embodiment of the present invention, the number of submodules (m) put into use on each phase in the first submodule number combination is calculated. jpand m jn If the number H is N-3, N-2 and N+1 respectively, then the upper arm m of each phase jp and lower bridge arm m jn The number of submodules put into use increases by 1 respectively; if the number H is N-1, N+2 and N+3 respectively, then the upper bridge arm m of each phase jp and lower bridge arm m jn The number of submodules put into use is reduced by 1 respectively; if the number H is N and the current value of the bridge arm AC component circulation is greater than 0, then the upper bridge arm m of each phase jp and lower bridge arm m jn The number of submodules put into use increases by 1 respectively; if the number H is N and the current value of the bridge arm AC component circulation is less than 0, then the upper bridge arm m of each phase jp and lower bridge arm m jn The number of submodules put into use is reduced by 1 respectively.

[0069] In the specific implementation, i j (k) is the actual value of the output current on the AC side of the converter at the current moment. According to the formula, increasing or decreasing the number of submodules in use on the upper bridge arm and the lower bridge arm by the same amount has no effect on the magnitude of the AC current on the output side at the next moment. Therefore, after the first rolling optimization, the number of submodules in use on the upper bridge arm and the lower bridge arm is m. jp and m jn , while increasing or decreasing the same number, thereby expanding the input range of the second rolling optimization, improving the circulating current suppression capability and dynamic response capability, while increasing the number of level outputs, and the output voltage has a smaller dv / dt (voltage change rate), improving the safety of people and equipment at the output end.

[0070] In an embodiment of the present invention, a second value function is established by using the absolute value of the error between the reference value and the actual value of the AC component circulating current of the bridge arm at the next moment, and the first sub-module quantity combination and the second sub-module quantity combination are rolled optimized using the second value function to screen out the optimal sub-module quantity combination.

[0071] In the embodiment of the present invention, the second value function is as follows:

[0072] J2=|i jzo (k+1)|,

[0073] Among them, J2 is the second value function, i jzo (k+1) is the actual value of the AC component circulating current of the bridge arm at time k+1, and the reference value of the AC component circulating current of the bridge arm at time k+1 is 0.

[0074] In the embodiment of the present invention, the use of the second value function to perform rolling optimization on the first submodule quantity combination and the second submodule quantity combination to screen out the optimal submodule quantity combination includes: taking the minimum output of the second value function as the optimal, screening out the optimal submodule quantity combination, the number of submodules used in the upper bridge arm and the lower bridge arm are n respectively jp 、n jn .

[0075] In the embodiment of the present invention, the capacitor voltage V is determined based on the bridge arm current at the current moment. c For larger or smaller submodules, the number of submodules put into use is determined according to the optimal submodule number combination (the submodules put into use on the upper bridge arm S jp , the submodule S of the lower bridge arm is put into use jn ).

[0076] In an embodiment of the present invention, when the bridge arm current is greater than 0, the sub-modules are arranged in order from small to large capacitor voltages, and the sub-modules with smaller capacitor voltages are put into use on the corresponding upper bridge arm and lower bridge arm; when the bridge arm current is less than 0, the sub-modules are arranged in order from large to small capacitor voltages, and the sub-modules with larger capacitor voltages are put into use on the corresponding upper bridge arm and lower bridge arm.

[0077] In the present invention, 2N sub-modules are arranged in each phase of the MMC, and the corresponding output level is 2N+1. Compared with the N+1 level AC side output current, the quality is higher, and the output voltage has a smaller dv / dt, which improves the safety factor of people and equipment at the output end, and has better circulating current suppression capability and dynamic response capability.

[0078] In the present invention, an independent value function is used to avoid the selection of weight factors, and two independent value functions are used for rolling optimization of the MMC AC side output current and the bridge arm circulating current. The first rolling optimization is performed for the control target AC output current, and the optimal switching state of the AC side output current is screened out. The level compensation method is used to perform level compensation on the optimal state after the first rolling optimization, and the second rolling optimization is performed for the control target bridge arm circulating current, and a set of optimal switching states are screened out to act on the modular multi-level converter.

[0079] In the present invention, calculation is performed 8 times at most in each sampling period, which significantly reduces the amount of calculation and is suitable for application occasions with a relatively large number of sub-modules.

[0080] The present invention also provides an independent value function model predictive control device for MMC, comprising: a submodule quantity set calculation unit, a first rolling optimization unit, a level compensation unit, a second rolling optimization unit and a control unit, wherein: the submodule quantity set calculation unit is used to determine the submodule quantity set put into use of the upper bridge arm and the lower bridge arm at the next moment based on the number of submodules put into use of the upper bridge arm and the lower bridge arm at the current moment, on the basis of ensuring that the output level of the converter at the next moment is an adjacent level of the output level at the current moment; the modular multi-level converter includes three phases, each phase includes an upper bridge arm and a lower bridge arm, and a plurality of submodules are arranged on the upper bridge arm and the lower bridge arm; the first rolling optimization unit is used to establish a first value function based on the absolute value of the error between the reference value and the actual value of the output current on the AC side of the converter at the next moment, The first value function is used to perform rolling optimization on the submodule quantity set, and the first submodule quantity combination is screened out; the level compensation unit is used to determine the number of submodules to be increased or decreased in each bridge arm according to the sum of the number of submodules put into use on each phase in the first submodule quantity combination, and obtain the second submodule quantity combination; the second rolling optimization unit is used to establish a second value function according to the absolute value of the error between the reference value and the actual value of the AC component circulating current of the bridge arm at the next moment, and use the second value function to perform rolling optimization on the first submodule quantity combination and the second submodule quantity combination, and obtain the optimal submodule quantity combination; the control unit is used to determine the submodule with a larger or smaller capacitor voltage to be put into use according to the bridge arm current at the current moment, and the number of submodules put into use is determined according to the optimal submodule quantity combination.

[0081] In the embodiment of the present invention, the submodule quantity set calculation unit is used for the number of submodules put into use in the upper bridge arm and the lower bridge arm at time k, respectively. jp and n jn At time k+1, the number of submodules put into use in the upper bridge arm and the lower bridge arm are n jp +m and n jn +n, where j is a, b and c, a, b and c correspond to the three phases of the converter respectively, and the number of submodules put into use meets the following constraints: when m and n are positive integers, |mn|<2, |m|+|n|≤N+3-n jp -n jn , N is the total number of submodules on each bridge arm, 2N is the total number of submodules on each phase; when m and n are negative integers and zero, |mn|<2, |m|+|n|≤n jp +n jn -N+3.

[0082] In an embodiment of the present invention, the sub-module quantity set calculation unit is used to obtain the AC component circulation of the bridge arm at the current moment. If the value is greater than 0, the number of sub-modules put into use on the corresponding bridge arm is increased, and the total number of sub-modules put into use on the corresponding phase is between N and N+3; if the value is less than 0, the number of sub-modules put into use on the corresponding bridge arm is reduced, and the total number of sub-modules put into use on the corresponding phase is between N-3 and N; all sub-module quantity combinations are calculated to obtain a sub-module quantity set.

[0083] In the embodiment of the present invention, the first rolling optimization unit is used to establish the first value function as follows:

[0084]

[0085] Among them, J1 is the first value function, i j * (k+1) is the reference value of the output current on the AC side of the converter at time k+1, i j (k+1) is the actual value of the output current on the AC side of the converter at time k+1;

[0086] The method of using the first value function to perform rolling optimization on the submodule quantity set and screening to obtain the first submodule quantity combination includes: taking the minimum output of the first value function as the optimal and screening to obtain the first submodule quantity combination.

[0087] In an embodiment of the present invention, the level compensation unit is used to calculate the number of sub-modules put into use on each phase in the first sub-module quantity combination. If the numbers are N-3, N-2 and N+1 respectively, the number of sub-modules put into use in the upper bridge arm and the lower bridge arm of each phase is increased by 1 respectively; if the numbers are N-1, N+2 and N+3 respectively, the number of sub-modules put into use in the upper bridge arm and the lower bridge arm of each phase is reduced by 1 respectively; if the numbers are both N and the value of the AC component circulating current of the bridge arm at the current moment is greater than 0, the number of sub-modules put into use in the upper bridge arm and the lower bridge arm of each phase is increased by 1 respectively; if the numbers are both N and the value of the AC component circulating current of the bridge arm at the current moment is less than 0, the number of sub-modules put into use in the upper bridge arm and the lower bridge arm of each phase is reduced by 1 respectively.

[0088] In the embodiment of the present invention, the second rolling optimization unit is used to establish the second value function as follows:

[0089] J2=|i jzo (k+1)|,

[0090] Among them, J2 is the second value function, i jzo (k+1) is the actual value of the AC component circulating current of the bridge arm at time k+1, and the reference value of the AC component circulating current of the bridge arm at time k+1 is 0;

[0091] The method uses the second value function to perform rolling optimization on the first submodule quantity combination and the second submodule quantity combination to screen out the optimal submodule quantity combination, including: taking the minimum output of the second value function as the optimal, and screening out the optimal submodule quantity combination.

[0092] In an embodiment of the present invention, the control unit is used to arrange the submodules in order of capacitor voltage from small to large after obtaining the optimal submodule quantity combination, and put the submodules with smaller capacitor voltage into use when the bridge arm current is greater than 0; when the bridge arm current is less than 0, arrange the submodules in order of capacitor voltage from large to small, and put the submodules with larger capacitor voltage into use.

[0093] See also Figure 4(a) , 4(b) , 4(c) and 4(d) are waveform diagrams obtained by using the control method of "Design and Experimental Evaluation of Fast Model Predictive Control for Modular Multilevel Converters" published in "IEEE Transactions on Industrial Electronics". It can be seen from the waveform diagram that the three-phase output voltage has a large dv / dt, which poses a certain safety hazard to people and equipment at the output end. The current amplitude in the three-phase bridge arm circulating current waveform is large, and the circulating current suppression ability of this method is poor.

[0094] See also Figure 5(a) , 5(b) , 5(c) and 5(d) are waveform diagrams obtained by applying the control method provided by the present invention, respectively. It can be clearly seen from the waveform diagrams that the three-phase output voltage dv / dt is small, the output side AC current waveform is good, the a-phase capacitor voltage is balanced, and the three-phase bridge arm circulating current is well suppressed, indicating that the control method provided by the present invention has excellent control performance.

[0095] Refer to Figure 6(a), which is a THD waveform of the MMC AC side output current obtained by using the control method of "Design and Experimental Evaluation of Fast Model Predictive Control for Modular Multilevel Converters" published in "IEEE Transactions on Industrial Electronics". Refer to Figure 6(b), which is a THD waveform of the MMC AC side output current obtained by using the control method provided by the present invention. From the comparison of the THD of the AC side output current under the two control methods, it is found that the quality of the AC side output current under the control method provided by the present invention is higher.

Claims

1. An independent value function model predictive control method for MMC, characterized in that: include: According to the number of submodules put into use in the upper bridge arm and the lower bridge arm at the current moment, on the basis of ensuring that the output level of the converter at the next moment is an adjacent level of the output level at the current moment, determine the number of submodules put into use in the upper bridge arm and the lower bridge arm at the next moment; the modular multilevel converter includes three phases, each phase includes an upper bridge arm and a lower bridge arm, and multiple submodules are arranged on the upper bridge arm and the lower bridge arm; The first value function is established based on the absolute value of the error between the reference value and the actual value of the output current on the AC side of the converter at the next moment. The first value function is used to perform rolling optimization on the submodule quantity set to screen out the first submodule quantity combination. The first value function is as follows: Among them, J1 is the first value function, i j * (k+1) is the reference value of the output current on the AC side of the converter at time k+1, i j (k+1) is the actual value of the output current on the AC side of the converter at time k+1; According to the sum of the number of submodules put into use on each phase in the first submodule number combination, determine the number of submodules put into use on each bridge arm to be increased or decreased, and obtain a second submodule number combination; The second value function is established based on the absolute value of the error between the reference value and the actual value of the AC component circulation of the bridge arm at the next moment. The second value function is used to perform rolling optimization on the first submodule quantity combination and the second submodule quantity combination to screen out the optimal submodule quantity combination. The second value function is as follows: J2=|i jzo (k+1)|, Among them, J2 is the second value function, i jzo (k+1) is the actual value of the AC component circulating current of the bridge arm at time k+1, and the reference value of the AC component circulating current of the bridge arm at time k+1 is 0; According to the bridge arm current at the current moment, the submodule with a larger or smaller capacitor voltage is judged to be put into use, and the number of submodules put into use is determined according to the optimal submodule number combination.

2. The independent value function model predictive control method for MMC according to claim 1, characterized in that: The output level of the converter at the next moment is an adjacent level of the output level at the current moment, including: At time k, the number of submodules put into use in the upper bridge arm and the lower bridge arm is n jp and n jn At time k+1, the number of submodules put into use in the upper bridge arm and the lower bridge arm are n jp +m and n jn +n, where j is a, b and c, a, b and c correspond to the three phases of the converter respectively, and the number of submodules put into use meets the following constraints: when m and n are positive integers, |mn|<2, |m|+|n|≤N+3-n jp -n jn , N is the total number of submodules on each bridge arm, 2N is the total number of submodules on each phase; when m and n are negative integers and zero, |mn|<2, |m|+|n|≤n jp +n jn -N+3.

3. The independent value function model predictive control method for MMC according to claim 2, characterized in that: The step of determining the number of submodules of the upper bridge arm and the lower bridge arm to be put into use at the next moment includes: Get the AC component circulation of the bridge arm at the current moment. If the value is greater than 0, increase the number of submodules put into use on the corresponding bridge arm, and the total number of submodules put into use on the corresponding phase is between N and N+3; if the value is less than 0, reduce the number of submodules put into use on the corresponding bridge arm, and the total number of submodules put into use on the corresponding phase is between N-3 and N; Calculate all sub-module quantity combinations to obtain a sub-module quantity set.

4. The independent value function model predictive control method for MMC according to claim 3, characterized in that: The step of using the first value function to perform rolling optimization on the submodule quantity set and screening to obtain the first submodule quantity combination includes: According to the minimum output of the first value function as the optimal, the first sub-module quantity combination is screened and obtained.

5. The independent value function model predictive control method for MMC according to claim 4, characterized in that: The determining of increasing or decreasing the number of submodules put into use on each bridge arm includes: Calculate the number of submodules put into use on each phase in the first submodule quantity combination. If the numbers are N-3, N-2 and N+1 respectively, the number of submodules put into use in the upper bridge arm and the lower bridge arm of each phase will increase by 1 respectively; if the numbers are N-1, N+2 and N+3 respectively, the number of submodules put into use in the upper bridge arm and the lower bridge arm of each phase will decrease by 1 respectively; if the numbers are all N and the AC component circulating current value of the bridge arm at the current moment is greater than 0, the number of submodules put into use in the upper bridge arm and the lower bridge arm of each phase will increase by 1 respectively; if the numbers are all N and the AC component circulating current value of the bridge arm at the current moment is less than 0, the number of submodules put into use in the upper bridge arm and the lower bridge arm of each phase will decrease by 1 respectively.

6. The independent value function model predictive control method for MMC according to claim 5, characterized in that: The method of using the second value function to perform rolling optimization on the first submodule quantity combination and the second submodule quantity combination to screen out the optimal submodule quantity combination includes: The minimum output of the second value function is taken as the optimal one, and the optimal combination of sub-module numbers is screened out.

7. The independent value function model predictive control method for MMC according to claim 6, characterized in that: The method of judging the submodule with a larger or smaller capacitor voltage to be put into use according to the bridge arm current at the current moment includes: When the bridge arm current is greater than 0, the submodules are arranged in order of capacitor voltage from small to large, and the submodules with smaller capacitor voltage are put into use; when the bridge arm current is less than 0, the submodules are arranged in order of capacitor voltage from large to small, and the submodules with larger capacitor voltage are put into use.

8. An independent value function model predictive control device for MMC, characterized in that: include: A submodule quantity set calculation unit, a first rolling optimization unit, a level compensation unit, a second rolling optimization unit and a control unit, wherein: The submodule quantity set calculation unit is used to determine the submodule quantity set put into use of the upper bridge arm and the lower bridge arm at the next moment based on the number of submodules put into use of the upper bridge arm and the lower bridge arm at the current moment, on the basis of ensuring that the output level of the converter at the next moment is an adjacent level of the output level at the current moment; the modular multilevel converter includes three phases, each phase includes an upper bridge arm and a lower bridge arm, and a plurality of submodules are arranged on the upper bridge arm and the lower bridge arm; The first rolling optimization unit is used to establish a first value function based on the absolute value of the error between the reference value and the actual value of the output current on the AC side of the converter at the next moment, and use the first value function to perform rolling optimization on the submodule quantity set to screen out the first submodule quantity combination; the first value function is as follows: Among them, J1 is the first value function, i j * (k+1) is the reference value of the output current on the AC side of the converter at time k+1, i j (k+1) is the actual value of the output current on the AC side of the converter at time k+1; The level compensation unit is used to determine the number of submodules to be added or reduced in use on each bridge arm according to the sum of the number of submodules put into use on each phase in the first submodule number combination, so as to obtain the second submodule number combination; The second rolling optimization unit is used to establish a second value function based on the absolute value of the error between the reference value and the actual value of the bridge arm AC component circulation at the next moment, and use the second value function to perform rolling optimization on the first submodule quantity combination and the second submodule quantity combination to screen out the optimal submodule quantity combination; the second value function is as follows: J2=|i jzo (k+1)|, Among them, J2 is the second value function, i jzo (k+1) is the actual value of the AC component circulating current of the bridge arm at time k+1, and the reference value of the AC component circulating current of the bridge arm at time k+1 is 0; The control unit is used to determine the submodule with a larger or smaller capacitor voltage to be put into use according to the bridge arm current at the current moment, and the number of submodules put into use is determined according to the optimal submodule number combination.

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