Control method and control device of neutral point clamped inverter

Abstract

The invention discloses a control method and a control device for a neutral point clamped inverter, and the control method comprises the steps: constructing a current prediction model, and carrying out the prediction of an output current based on an input voltage vector; constructing a first cost function, minimizing the first cost function, selecting a voltage vector, and judging the type of the selected voltage vector; constructing a second cost function configured to: select to output a switching state in which neutral point potential balancing is facilitated based on the type of the selected voltage vector; and in response to the fact that the type of the voltage vector is a large vector or a medium vector, if it is judged that the current midpoint potential deviation exceeds a preset midpoint potential deviation threshold value and the currently selected voltage vector is not beneficial to capacitor voltage balance, reselecting the voltage vector based on a first cost function, and otherwise, outputting the switching state corresponding to the selected voltage vector. According to the invention, the output current of the inverter can be predicted, and neutral-point voltage balance of the inverter can be realized.

Description

Technical Field

[0001] The present application relates to the technical field of inverters, and in particular to a control method and a control device for a mid-point clamped inverter. Background Art

[0002] As the topology with the longest development time and the most widespread application, the midpoint clamped three-level inverter is favored by many researchers for its simple structure, low control difficulty, low loss, and relatively balanced loss distribution of each power module. In terms of control strategy, the finite set model predictive control is widely used in the control of power converters due to its fast dynamic response, simple implementation, and multi-variable processing capabilities. However, the existing control methods need to predict inductance parameters such as weight factors and capacitance values, resulting in the existing control methods having a strong dependence on inductance parameters such as weight factors and capacitance values. Summary of the invention

[0003] In order to solve the deficiencies of the prior art, this application adopts the following technical solutions: In a first aspect, the present application provides a control method for a neutral point clamped inverter, the control method comprising the following steps: Constructing a current prediction model, wherein the current prediction model predicts the output current based on the input voltage vector to obtain a current prediction value; Constructing a first cost function, wherein the first cost function is composed of a current reference value and a current prediction value; With the goal of minimizing the first cost function, selecting a voltage vector and determining a type of the selected voltage vector; constructing a second cost function, wherein the second cost function is configured to: based on the type of the selected voltage vector, select and output a switch state that is conducive to promoting midpoint potential balance; Among them, in response to the type of the selected voltage vector being a large vector or a medium vector, if it is determined that the current midpoint potential deviation exceeds a preset midpoint potential offset threshold, and the currently selected voltage vector is not conducive to capacitor voltage balance, the voltage vector is reselected based on the first cost function, otherwise, the switch state corresponding to the selected voltage vector is output.

[0004] In summary, the present application provides a control method for a mid-point clamped inverter, which predicts the inverter output current by constructing a current prediction model, and screens the optimal voltage vector by minimizing the cost function without requiring the capacitance information and weight factor of the capacitor, thereby achieving control of the mid-point clamped inverter, and further selecting the switching state of the optimal voltage vector, thereby promoting the mid-point voltage balance of the mid-point clamped inverter without increasing the complexity of the control method.

[0005] Furthermore, the method further comprises: The midpoint voltage continuous time model of the midpoint clamped inverter is expressed as a midpoint voltage discrete prediction model, and the midpoint voltage discrete prediction model is expressed by the following formula: ; In the formula, v n ( k+ 1) indicates the midpoint voltage at the next moment, v n ( k ) represents the midpoint voltage at this moment, T s Indicates the sampling frequency or control frequency of the controller, C dc represents the bus capacitance on the DC side, i n Indicates the current flowing through the midpoint; The second cost function is designed based on the midpoint voltage discrete prediction model, and the switching state of the selected voltage vector is evaluated using the second cost function, and the switching state that can promote the midpoint voltage is selected. v n ( k+ 1) The switching state tends to 0 to achieve mid-point voltage balance.

[0006] Furthermore, the second cost function includes a cost function for evaluating a large vector and a medium vector, and the cost function for evaluating a large vector and a medium vector is expressed by the following formula: ; In the formula, sign(*) represents the sign function, v n ( k ) represents the midpoint voltage under the influence of the selected voltage vector, i n ( k ) represents the current flowing through the midpoint under the influence of the selected voltage vector, b v represents the preset midpoint potential offset threshold; In which, in response to the cost function J b =2, then reselect the voltage vector based on the first cost function; otherwise, output the switch state corresponding to the selected voltage vector.

[0007] Furthermore, the second cost function includes a cost function for evaluating a small vector, and the cost function for evaluating a small vector is expressed by the following formula: ; In the formula, sign(*) represents the sign function,v n ( k ) represents the midpoint voltage under the influence of the selected voltage vector, i n ( k ) represents the current flowing through the midpoint under the influence of the selected voltage vector; In response to the type of the selected voltage vector being a small vector, based on the cost function J v , in the two switch states corresponding to the small vector, the output is selected so that the cost function J v Minimum switching state.

[0008] Furthermore, the second cost function includes a cost function for evaluating a zero vector, and the cost function for evaluating a zero vector is expressed by the following formula: ; In the formula, S abc ( k ) represents the switch state at time k, represents the two-norm, The role of is to minimize the amount of switch state change between two control cycles; In response to the type of the selected voltage vector being a zero vector, based on the cost function J s , in the three switching states corresponding to the zero vector, the output is selected so that the cost function J s Minimum switching state.

[0009] Furthermore, the first cost function is expressed by the following formula: ; In the formula, represents the current reference value, represents the predicted current value.

[0010] Furthermore, the current prediction value is obtained by the following formula: ; In the formula, R o Indicates the load resistance value of the inverter, L o Indicates the load inductance value of the inverter, T s represents the sampling / control frequency of the controller, represents the inverter output current in a stationary orthogonal coordinate system, represents the output voltage of the inverter in a stationary orthogonal coordinate system, where the inverter output current Determined by the selected voltage vector with the objective of minimizing the first cost function.

[0011] Furthermore, the current reference value is obtained by the following formula: ; In the formula, i * ( k )represent k The current reference value at the moment.

[0012] Furthermore, current prediction is performed in any of the following ways: a current prediction structure based on an extended Kalman filter, or a current prediction structure based on a data-driven neural network predictor, or a current prediction structure based on an adaptive predictive control.

[0013] In a second aspect, the present application further provides a control device for a mid-point clamped inverter, wherein the control device applies the above-mentioned control method to control the mid-point clamped inverter. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 A flowchart of a method for controlling a midpoint clamped inverter according to an embodiment of the present application; Figure 2 A schematic diagram of a circuit topology structure in which a control method for a mid-point clamped inverter provided in an embodiment of the present application is applied to a three-level mid-point clamped inverter; Figure 3 A schematic diagram of a space voltage vector generated by a control method for a midpoint clamped inverter provided in an embodiment of the present application; Figure 4 An algorithm flow chart for achieving midpoint voltage balance in a control method for a midpoint clamped inverter provided in an embodiment of the present application; Figure 5 A diagram showing the effect of capacitor voltage balance of a control method for a mid-point clamped inverter provided in an embodiment of the present application. DETAILED DESCRIPTION

[0015] The present application will be described in detail below in conjunction with the specific implementation modes shown in the accompanying drawings, but these implementation modes do not limit the present application. Structural, methodological, or functional changes made by ordinary technicians in the field based on these implementation modes are included in the protection scope of the present application.

[0016] In order to solve the deficiencies of the prior art, in a first aspect, the present application provides a control method for a midpoint clamped inverter, such as Figure 1As shown, the control method includes the following steps: Step S11, constructing a current prediction model, the current prediction model predicts the output current based on the input voltage vector to obtain a current prediction value; Step S12, constructing a first cost function, the first cost function consisting of a current reference value and a current prediction value; Step S13, with the goal of minimizing the first cost function, selecting a voltage vector and determining the type of the selected voltage vector; Step S14, constructing a second cost function, where the second cost function is configured to: based on the type of the voltage vector, select and output a switch state that is beneficial to promoting midpoint potential balance; Among them, in response to the type of the selected voltage vector being a large vector or a medium vector, if it is determined that the current midpoint potential deviation exceeds a preset midpoint potential offset threshold, and the currently selected voltage vector is not conducive to capacitor voltage balance, the voltage vector is reselected based on the first cost function, otherwise, the switch state corresponding to the selected voltage vector is output.

[0017] The circuit topology of the three-level neutral point clamped inverter is as follows: Figure 2 As shown, the inverter is equipped with a resistive inductive load. o Indicates the resistance value of the load, L o Indicates the inductance of the load, C1 indicates the upper capacitor on the DC side, and C2 indicates the lower capacitor on the DC side. The inverter includes three bridge arms, each of which can produce three switching states. ,in The output voltage of each phase relative to the midpoint O It can be expressed as: (1); In the formula, Indicates the DC bus voltage, Indicates the bridge arm switch status.

[0018] The space voltage vector generated by the three-level neutral point clamped inverter is as follows: Figure 3 As shown in the figure, the voltage levels "P", "O" and "N" represent , 0, and The converter is capable of generating a total of There are 19 different switching states, resulting in 19 unique voltage vectors. According to their size, these voltage vectors can be divided into the following four categories: zero vector, small vector, medium vector and large vector.

[0019] Among them, the zero vector corresponds to three switching states: (OOO, PPP, NNN); the inner circle is six small vectors, each of which corresponds to the following switching states: (OOP, NNO), (OPP, NOO), (OPO, NON), (PPO, OON), (POO, ONN), (ONO, POP); the outer circle is 12 medium vectors / large vectors, each of which corresponds to one of the following switching states: (OPN), (PON), (PNO), (ONP), (NOP), (NPO); each large vector corresponds to one of the following switching states: (NPN), (PPN), (PNN), (PNP), (NNP), (NPP).

[0020] DC bus voltage It can be expressed as the sum of the upper capacitor voltage and the lower capacitor voltage on the DC side, that is, ,in Represents the capacitor voltage on the DC side, Represents the capacitor voltage on the DC side. Further, the voltage at the midpoint o of the three-level midpoint clamped inverter can be expressed as follows: (2); In the formula, represents the midpoint voltage of the three-level midpoint clamped inverter, Represents the capacitor voltage on the DC side, Indicates the capacitor voltage on the DC side.

[0021] Through the current prediction model, the output current at the next moment is predicted based on the input voltage vector, so as to obtain the current prediction value. After obtaining the current prediction model of the inverter control system, a first cost function is constructed to evaluate all possible voltage vectors in the three-level midpoint clamped inverter. The first cost function includes a current reference value and a current prediction value.

[0022] A voltage vector is input into the current prediction model, all possible voltage vectors are traversed to minimize the first cost function, a voltage vector that minimizes the function value of the first cost function is identified, and the vector type of the voltage vector is determined. Based on the vector type of the voltage vector, the balance of the midpoint voltage of the three-level midpoint clamped inverter is achieved through the second cost function.

[0023] In a three-level midpoint clamped inverter, the output voltage vector can be divided into six large vectors, six medium vectors, six small vectors and one zero vector according to its amplitude, for a total of 19 voltage vectors. There are 27 switching states in a three-level midpoint clamped inverter, some of which will produce the same voltage vector. The above-mentioned same voltage vectors are called redundant vectors, and the redundant vectors are mainly small vectors and zero vectors. The amplitude and phase of each group of redundant vectors are equal, but each group of redundant vectors has different effects on the midpoint voltage. When the ideal output voltage vector of the three-level midpoint clamped inverter belongs to a small vector or a zero vector, the balance of the capacitor voltage can be promoted by selecting the switching state.

[0024] Based on the optimal voltage vector obtained by the first cost function, the type of the optimal voltage vector is determined, and the second cost function selects and outputs a switch state that is conducive to the balance of the midpoint potential. In response to the type of the voltage vector being a large vector or a medium vector, if the current midpoint potential deviation exceeds the preset midpoint potential offset threshold, and the currently selected voltage vector is not conducive to the capacitor voltage balance, the first cost function is returned to reselect the voltage vector (e.g., the second smallest suboptimal voltage vector of the first cost function); if the above conditions are not met, the switch state corresponding to the selected voltage vector is directly output.

[0025] According to the above description, the present application provides a control method for a mid-point clamped inverter, which predicts the inverter output current by constructing a current prediction model, obtains the optimal voltage vector by minimizing the first cost function without requiring the capacitance information and weight factor of the capacitor, determines the optimal voltage vector type, and further selects the switching state corresponding to the optimal voltage vector through the second cost function to promote the mid-point voltage balance of the mid-point clamped inverter.

[0026] As an implementation method, the first cost function is expressed by the following formula: (3); In the formula, Indicates the current reference value, Indicates the predicted current value.

[0027] Through the first cost function, all possible voltage vectors of the three-level midpoint clamped inverter are traversed, the function value corresponding to each voltage vector is calculated, and the voltage vector with the smallest function value is selected as the initial optimal solution to ensure that the output current of the three-level midpoint clamped inverter is as close to the reference value as possible. Through the first cost function, the optimal voltage vector is screened to provide a high-precision candidate solution for the subsequent inverter midpoint potential balance.

[0028] Further, as an implementation method, the current prediction value of the first cost function is obtained by the following formula: (4); In the formula, R o Indicates the load resistance value of the inverter, L o Indicates the load inductance value of the inverter, T s represents the sampling / control frequency of the controller, represents the inverter output current in a stationary orthogonal coordinate system, Represents the output voltage of the inverter in a stationary orthogonal coordinate system.

[0029] Among them, the inverter output current The inverter output current under the influence of 19 voltage vectors is determined by the selected voltage vector with the goal of minimizing the first cost function. and output voltage Substitute into formula (4) for calculation to obtain different current prediction values. Substitute different current prediction values ​​into the first cost function for calculation, and select the voltage vector that minimizes the first cost function as the optimal voltage vector.

[0030] Further, as an implementation manner, the current reference value of the first cost function is obtained by the following formula: (5); In the formula, i * ( k )represent k The current reference value at that moment.

[0031] The reference current at the future moment is obtained by Lagrangian extrapolation. The continuous-time model of the output current and the midpoint voltage is discretized by the discretization method to realize the prediction of the driving current corresponding to the voltage vector. Combined with the first cost function, the global optimization selection of the voltage vector is realized.

[0032] After 19 different voltage vectors are brought into the prediction model and calculated through the first cost function, a voltage vector that minimizes the first cost function is obtained. Then, through the second cost function, the switching state of the voltage vector is carefully selected to promote the balance of the inverter midpoint voltage.

[0033] As an optional implementation, using first-order Euler discretization, the continuous-time model of the midpoint voltage of the inverter can be expressed as a discrete prediction model. The discrete prediction model of the midpoint voltage can be expressed as: (6); In the formula, v n ( k+ 1) indicates the midpoint voltage at the next moment,v n ( k ) represents the midpoint voltage at this moment, T s Indicates the sampling frequency or control frequency of the controller, C dc represents the bus capacitance on the DC side, i n Indicates the current flowing through the midpoint.

[0034] According to the parameter description of formula (6), the current coefficient is positive, making the midpoint voltage v n ( k ) and current i n The signs of the two are different, so that the midpoint voltage at the next moment v n ( k+ 1) tends to 0 to achieve inverter midpoint voltage balance.

[0035] Based on the discrete prediction model of the midpoint voltage, a second cost function can be designed to evaluate the switching state of the optimal voltage vector and select the one that can promote the midpoint voltage v n ( k+ 1) The switching state tends to 0 to achieve mid-point voltage balance.

[0036] As an implementation manner, the second cost function includes a cost function for evaluating a large vector and a medium vector, and the cost function for evaluating a large vector and a medium vector is expressed by the following formula: (7); In the formula, sign(*) represents the sign function, v n ( k ) represents the midpoint voltage under the influence of the selected voltage vector, i n ( k ) represents the current flowing through the midpoint under the influence of the selected voltage vector, b v Indicates the preset midpoint potential deviation threshold.

[0037] Among them, in response to the cost function J b =2, the voltage vector is reselected based on the first cost function, otherwise, the switch state corresponding to the selected voltage vector is output.

[0038] Specifically, if the cost function J b is equal to 2, which means that the sign function sign (| v n ( k )|- b v ) = 1 (indicates that the current voltage deviation of the inverter midpoint exceeds the preset midpoint potential deviation threshold b v ), and the symbolic function sign ( v n ( k ))* sign ( i n ( k ))=1 (indicating that the midpoint voltage and current signs are consistent under the influence of the currently selected voltage vector). In this case, the midpoint voltage offset tends to be serious, and the currently selected voltage vector is not conducive to the capacitor voltage balance of the inverter. At this time, the voltage vector is reselected based on the first cost function, and the type of the newly selected voltage vector is determined to avoid further offset of the inverter midpoint voltage.

[0039] If the cost function J b If it is not equal to 2, it means that the midpoint potential offset of the inverter does not exceed the preset midpoint potential offset threshold (i.e., it is within the allowable range of the midpoint potential offset), or the midpoint voltage and current under the influence of the currently selected voltage vector have different signs (i.e., the midpoint potential offset can be suppressed), and the switch state corresponding to the selected voltage vector is directly output.

[0040] As an implementation manner, the second cost function includes a cost function for evaluating a small vector, and the cost function for evaluating a small vector is expressed by the following formula: (8); In the formula, sign(*) represents the sign function, v n ( k ) represents the midpoint voltage under the influence of the selected voltage vector, i n ( k ) represents the current flowing through the midpoint under the influence of the selected voltage vector.

[0041] In response to the type of the selected voltage vector being a small vector, based on the cost function J v , in the two switch states corresponding to the small vector, the output is selected so that the cost function J v Minimum switching state.

[0042] Depend on Figure 3It can be seen that each small vector corresponds to two switching states, through the cost function J v The midpoint voltage and current under the influence of the two switching states corresponding to the voltage vector are multiplied by the sign. Combining formula (6) and formula (8), in the two switching states corresponding to the small vector, when the cost function J v The calculation result is negative, indicating that the midpoint voltage and current under the influence of the voltage vector have opposite signs, so the output makes the cost function J v The calculation result is a negative switching state, which achieves the balance of the inverter midpoint voltage.

[0043] As an implementation manner, the second cost function includes a cost function for evaluating a zero vector, and the cost function for evaluating a zero vector is expressed by the following formula: (9); In the formula, S abc ( k ) represents the switch state at time k, represents the two-norm, The role of is to minimize the amount of switch state change between two control cycles; In response to the type of the selected voltage vector being a zero vector, based on the cost function J s , among the three switch states corresponding to the zero vector, the output is selected so that the cost function J s Minimum switching state, thereby minimizing the switching frequency of the inverter.

[0044] In summary, if Figure 4 As shown, the midpoint voltage balancing strategy described above does not require the use of the capacitance information of the inverter DC side during implementation, and does not require the intervention of weight factors during the balancing process, which reduces the dependence of the control method of a midpoint clamped inverter provided in the present application on the inductance parameters and improves the applicability of the control method of the present application.

[0045] As an implementation method, current prediction is performed using any of the following methods: a current prediction structure based on an extended Kalman filter, or a data-driven neural network predictor, or a current prediction structure based on an adaptive predictive control.

[0046] Specifically, the extended Kalman filter is a state estimation algorithm suitable for nonlinear systems. It recursively updates the state estimation value by combining the system dynamic model and real-time measurement data. The current prediction structure based on the extended Kalman filter can use the state estimation and input voltage vector of the previous moment to predict the current value, and can adjust the estimation value by combining the Kalman gain through the difference between the real-time measured current and the predicted value, thereby suppressing the influence of noise and improving the prediction accuracy.

[0047] Alternatively, a data-driven neural network predictor is used to predict current. By collecting historical data of the inverter under different operating conditions, the neural network predictor learns the input-output mapping relationship of the inverter through offline training. It does not rely on accurate physical models and does not require accurate circuit equations or parameter calibration. It is suitable for processing predicted current under complex working conditions.

[0048] Alternatively, current prediction is performed based on a current prediction structure of adaptive predictive control, which dynamically adjusts the prediction model to cope with load or environmental changes by identifying system parameters online. The current prediction structure based on adaptive predictive control uses recursive least squares or gradient descent to estimate load parameters in real time, substitutes the identification results into the discrete state equation, updates the current prediction model, calculates the predicted current based on the updated model, and selects the optimal voltage vector through the cost function, further improving the robustness and real-time performance of the control system.

[0049] As another implementation method, the output voltage value can be directly calculated so that the inverter can reach the desired voltage reference value within one sampling period, without the need for stepwise approximation. By using the discrete state equation of the system, the output voltage at the next moment is predicted, and the switch state is adjusted instantly to achieve the target voltage.

[0050] To further illustrate a control method for a midpoint clamped inverter provided in the present application, an experimental platform is established below to verify the effectiveness of the method proposed in the present application. The experimental object is a three-level midpoint clamped inverter. The detailed parameters of the experimental platform are shown in Table 1:

[0051] The experimental results are as follows Figure 5As shown, the control method of a mid-point clamped inverter provided by the present application can achieve accurate and effective DC side capacitor voltage balance. The output current waveform of the three-level mid-point clamped inverter is also tested in the experiment. The amplitude, frequency and waveform quality of the output current are not disturbed or affected. The control method of a mid-point clamped inverter provided by the present application has high adaptability to the current control target. In the case of load changes or external interference, the control method of a mid-point clamped inverter provided by the present application can maintain a good voltage balance effect and maintain the stability of the output current at the same time.

[0052] According to the above description, the control method of a mid-point clamped inverter provided by the present application predicts the inverter output current by constructing a current prediction model, obtains the optimal voltage vector by minimizing the first cost function without requiring the capacitance information and weight factor of the capacitor, determines the type of the optimal voltage vector, and selects the switching state corresponding to the optimal voltage vector through the second cost function, thereby achieving the mid-point voltage balance of the mid-point clamped inverter without increasing the complexity of the control method; and through the above verification experiments, the control method of a mid-point clamped inverter provided by the present application can adapt to inverters of different capacitor types and maintain stable performance.

[0053] In a second aspect, the present application also provides a control device for a mid-point clamped inverter, which controls the mid-point clamped inverter using the control method described above, predicts the inverter output current, and achieves inverter mid-point voltage balance.

[0054] It will be appreciated that the word "exemplary" as used herein means "serving as an example, instance, or illustration". Any embodiment described as "exemplary" is not necessarily preferred or superior to other embodiments and / or does not exclude the combination of features of other embodiments. It will be appreciated that certain features of the present application described in the context of separate embodiments for the sake of clarity may also be provided in a single embodiment by combination. Conversely, various features of the present application described in the context of a single embodiment for the sake of clarity may also be provided individually or in any suitable combination or as any other described embodiment of the present application.

[0055] The above disclosure is only the preferred embodiment of the present application, but it is not intended to limit the scope of rights of the present application. A person of ordinary skill in the art can understand that without departing from the spirit and scope of the present application and the appended claims, changes, modifications, substitutions, combinations, and simplifications should all be equivalent replacement methods and still fall within the scope of the invention.

Claims

1. A control method for a midpoint clamped inverter, characterized in that: The control method comprises the following steps: Constructing a current prediction model, wherein the current prediction model predicts the output current based on the input voltage vector to obtain a current prediction value; Constructing a first cost function, wherein the first cost function is composed of a current reference value and a current prediction value; With the goal of minimizing the first cost function, selecting a voltage vector and determining a type of the selected voltage vector; constructing a second cost function, wherein the second cost function is configured to: based on the type of the selected voltage vector, select and output a switch state that is conducive to promoting midpoint potential balance; Among them, in response to the type of the selected voltage vector being a large vector or a medium vector, if it is determined that the current midpoint potential deviation exceeds a preset midpoint potential offset threshold, and the currently selected voltage vector is not conducive to capacitor voltage balance, the voltage vector is reselected based on the first cost function, otherwise, the switch state corresponding to the selected voltage vector is output.

2. The control method of the neutral point clamped inverter according to claim 1, characterized in that: The method further comprises: The midpoint voltage continuous time model of the midpoint clamped inverter is expressed as a midpoint voltage discrete prediction model, and the midpoint voltage discrete prediction model is expressed by the following formula: ； In the formula, v n ( k+ 1) indicates the midpoint voltage at the next moment, v n ( k ) represents the midpoint voltage at this moment, T s Indicates the sampling frequency or control frequency of the controller, C dc represents the bus capacitance on the DC side, i n Indicates the current flowing through the midpoint; The second cost function is designed based on the midpoint voltage discrete prediction model, and the switching state of the selected voltage vector is evaluated using the second cost function, and the switching state that can promote the midpoint voltage is selected. v n ( k+ 1) The switching state tends to 0 to achieve mid-point voltage balance.

3. The control method of the neutral point clamped inverter according to claim 2, characterized in that: The second cost function includes a cost function for evaluating a large vector and a medium vector, and the cost function for evaluating a large vector and a medium vector is expressed by the following formula: In the formula, sign(*) represents the sign function, v n ( k ) represents the midpoint voltage under the influence of the selected voltage vector, i n ( k ) represents the current flowing through the midpoint under the influence of the selected voltage vector, b v represents the preset midpoint potential offset threshold; In which, in response to the cost function J b =2, then reselect the voltage vector based on the first cost function; otherwise, output the switch state corresponding to the selected voltage vector.

4. The control method of the neutral point clamped inverter according to claim 2, characterized in that: The second cost function includes a cost function for evaluating a small vector, and the cost function for evaluating a small vector is expressed by the following formula: ; In the formula, sign(*) represents the sign function, v n ( k ) represents the midpoint voltage under the influence of the selected voltage vector, i n ( k ) represents the current flowing through the midpoint under the influence of the selected voltage vector; In response to the type of the selected voltage vector being a small vector, based on the cost function J v , in the two switch states corresponding to the small vector, the output is selected so that the cost function J v Minimum switching state.

5. The control method of the neutral point clamped inverter according to claim 2, characterized in that: The second cost function includes a cost function for evaluating a zero vector, and the cost function for evaluating a zero vector is expressed by the following formula: ; In the formula, S abc ( k ) represents the switch state at time k, represents the two-norm, The role of is to minimize the amount of switch state change between two control cycles; In response to the type of the selected voltage vector being a zero vector, based on the cost function J s , in the three switching states corresponding to the zero vector, the output is selected so that the cost function J s Minimum switching state.

6. The control method of the neutral point clamped inverter according to claim 1, characterized in that: The first cost function is expressed by the following formula: ; In the formula, represents the current reference value, represents the predicted current value.

7. The control method of the neutral point clamped inverter according to claim 6, characterized in that: The current prediction value is obtained by the following formula: In the formula, R o Indicates the load resistance value of the inverter, L o Indicates the load inductance value of the inverter, T s represents the sampling / control frequency of the controller, represents the inverter output current in a stationary orthogonal coordinate system, represents the output voltage of the inverter in a stationary orthogonal coordinate system, where the inverter output current Determined by the selected voltage vector with the objective of minimizing the first cost function.

8. The control method of the neutral point clamped inverter according to claim 6, characterized in that: The current reference value is obtained by the following formula: ; In the formula, i * ( k )represent k The current reference value at the moment.

9. The control method of the neutral point clamped inverter according to claim 1, characterized in that: The current prediction is performed by any of the following methods: a current prediction structure based on an extended Kalman filter, or a current prediction structure based on a data-driven neural network predictor, or a current prediction structure based on an adaptive predictive control.

10. A control device for a midpoint clamped inverter, characterized in that: The control device controls the neutral point clamped inverter using the control method according to any one of claims 1 to 9.

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