A model predictive control method for a three-phase three-level t-type grid-connected inverter

Abstract

A model predictive control method of a three-phase three-level T-type grid-connected inverter, comprising: sampling grid current and voltage, and performing alpha-beta coordinate transformation on the grid current and voltage through Clarke transformation; obtaining predicted current by combining Euler formula, and calculating cost function according to the predicted current and a reference value thereof; comparing which voltage vector in six sectors in a vector radiation range can make the value of the cost function minimum, and then selecting a first sector in which the corresponding voltage vector is located to perform next calculation; again calculating the predicted current, and then applying voltage vectors in all subsectors in the first sector to calculate the cost function, and comparing to obtain an optimal value. The method has the advantages that: the sector optimization selection and the application of combined voltage vectors can reduce total harmonic distortion of the current and promote midpoint potential balance; and the two-step judgment method can significantly reduce the calculation amount.

Description

Technical Field

[0001] This invention belongs to the field of power electronics technology, specifically relating to a model predictive control method for a three-phase three-level T-type grid-connected inverter. Background Technology

[0002] Grid-connected inverters, as devices that directly connect DC generators to the power grid, play a crucial role in the entire power grid system. Among them, the three-phase three-level T-type circuit topology has received widespread attention and application in recent years. This circuit structure is relatively simple and has high efficiency in low-to-medium power applications. Common inverter control methods include proportional-integral (PI) control, repetitive control, and proportional-resonant (PR) control. However, these linear controls use simplified system linearization models, which sometimes cannot accurately describe the characteristics of nonlinear systems. When used in power electronic inverters, this can lead to poor system anti-interference capabilities. If robustness is low, the intended performance requirements will not be met. Model predictive control, on the other hand, is well-suited to the characteristics of nonlinear systems. Furthermore, finite set model predictive control, due to its advantages such as fast transient response, simple implementation, and direct handling of nonlinear constraints, is now widely used in various power electronic converters. Finite set model predictive control consists of a predictive model, an objective function, and rolling optimization. It can perform cyclic calculations and predictions under finite switching states, and all switching states can be calculated online, exhibiting good adaptability and stability. Moreover, it does not require a PWM modulation module, making system control easier to implement.

[0003] Currently, traditional finite control set model predictive control strategies typically employ voltage vector enumeration, which results in a high computational load and increases the computational burden on the microprocessor. Secondly, the single-vector output method does not adequately consider current tracking and neutral point potential balance, easily leading to current ripple and neutral point potential deviation.

[0004] Currently, there are techniques to reduce computational load by decreasing the control set. This approach uses the voltage vector from the previous time step and its adjacent voltage vectors as the control set for the current time step, reducing the number of candidate vectors from 125 to 7. However, the optimal vector selected by this method may not be in the control set for the current time step, thus reducing the inverter's control performance. There are also other approaches that promote midpoint potential balance by controlling the duration of action of smaller vectors.

[0005] Traditional finite control set model predictive control strategies require enumerating and calculating 27 voltage vectors, resulting in a significant computational load. Single-vector output methods do not adequately consider current tracking and neutral point potential balance, easily leading to current ripple and neutral point potential deviation. Summary of the Invention

[0006] The objective of this invention is achieved through the following technical solutions.

[0007] A model predictive control method for a three-phase, three-level, T-type grid-connected inverter includes the following steps:

[0008] Sample the grid current and voltage, and perform an αβ coordinate transformation on them using the Clarke transform.

[0009] The predicted current is obtained by combining Euler's formula, and the cost function is calculated based on the predicted current and its reference value.

[0010] Compare which voltage vector in the six sectors of the vector radiation range minimizes the cost function value, and then select the first sector where the corresponding voltage vector is located for the next calculation.

[0011] The predicted current is calculated again, and the cost function is calculated by applying the voltage vectors in all sub-sectors of the first sector. The voltage vector that minimizes the cost function is compared to obtain the optimal voltage vector. The switching state corresponding to the optimal voltage vector is then applied to the switching devices of the inverter.

[0012] Furthermore, the three-phase three-level T-type grid-connected inverter includes three T-type NPC flat bridges.

[0013] Furthermore, according to Kirchhoff's voltage law, the output phase voltage of the inverter is expressed as:

[0014]

[0015] Where Rs is the line resistance, Ls is the filter inductance, and e a e b e c For the three-phase power grid voltage, i a i b i c For the three-phase grid-connected current flowing to the power grid through the filter inductor Ls and the line resistance Rs, u aN u bN u cN This provides the three-phase output voltage for the inverter.

[0016] Furthermore, the αβ coordinate transformation performed by the Clarke transformation includes:

[0017] Applying the Clarke transformation to equation (1) above, we obtain the mathematical expression in the α-β coordinate system:

[0018]

[0019] i α i β Let u be the current in the α-β coordinate system. α u β Let e ​​be the voltage in the α-β coordinate system.α e β This is the output AC signal in the α-β coordinate system.

[0020] Furthermore, the method of obtaining the predicted current by combining Euler's formula includes:

[0021] Current i α i β The rate of change over a sampling period can be obtained by the Euler approximation:

[0022]

[0023] Where k is the number of samples, T S The sampling period;

[0024] Therefore, the predicted current value can be obtained:

[0025]

[0026] Furthermore, the cost function is expressed as:

[0027]

[0028] in, and Let i be the current reference value given in the αβ coordinate system at time k+1. α (k+1) and i β (k+1) is the predicted current value in the αβ coordinate system at time k+1.

[0029] Furthermore, the six sectors are divided based on the effective radiation range of the voltage vector, which is the region with the shortest linear distance from the reference vector to the given vector.

[0030] The advantages of this invention are: by optimizing sector selection and applying combined voltage vectors, it can reduce total harmonic distortion of the current and promote midpoint potential balance. The two-step judgment method can significantly reduce the computational load. Attached Figure Description

[0031] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings:

[0032] Figure 1 A circuit topology diagram of a three-phase three-level T-type grid-connected inverter according to an embodiment of the present invention is shown.

[0033] Figure 2A schematic diagram of the vector radiation range and sector division is shown.

[0034] Figure 3 A flowchart of the improved FCS-MPC algorithm is shown.

[0035] Figure 4 The simulation diagram of the output current under the traditional algorithm is shown.

[0036] Figure 5 The simulation diagram of phase a voltage and output current under the traditional algorithm is shown.

[0037] Figure 6 This shows i under the traditional algorithm a The total harmonic distortion simulation diagram.

[0038] Figure 7 The simulation diagram of the output current under the improved algorithm is shown.

[0039] Figure 8 Simulation diagrams of phase a voltage and output current under the improved algorithm are shown.

[0040] Figure 9 The improved algorithm is shown below. a The total harmonic distortion simulation diagram.

[0041] Figure 10 The simulation diagram of the output current under the improved algorithm without two-step judgment is shown.

[0042] Figure 11 This demonstrates an improved algorithm without two-step decision-making for i. a The total harmonic distortion simulation diagram.

[0043] Figure 12 The neutral point potential V under the traditional algorithm is shown. p V n and their difference.

[0044] Figure 13 The neutral point potential V under the improved algorithm is shown. p V n and their difference. Detailed Implementation

[0045] Exemplary embodiments of the present disclosure will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0046] Terminology Explanation:

[0047] Three-level inverter: The bridge arm of a three-level inverter has four power semiconductor devices. It can achieve a multi-level stepped wave output voltage by different combinations of voltage division and switching operations on the DC side, which can make the waveform closer to a sine wave.

[0048] Finite control set model predictive control: The basic idea of ​​finite control set model predictive control is to divide the system state into a finite number of sets, then predict the probability of future states based on information from the current and historical states, and implement control based on the prediction results. The advantage of this method is that it can handle nonlinear, time-varying, and uncertain systems, and can consider multiple objectives and constraints.

[0049] Total Harmonic Distortion (THD): Total harmonic distortion indicates that when a power amplifier is working, the second and third harmonics generated by unavoidable oscillations or other resonances in the circuit are superimposed on the actual input signal. As a result, the signal output at the output terminal is not simply a signal with the same components as the input signal, but a signal that includes harmonic components. The comparison between these extra harmonic components and the actual input signal, expressed as a percentage, is called total harmonic distortion.

[0050] Midpoint potential balance: In an inverter, an unbalanced DC-side midpoint potential will cause the switching devices to experience different voltages, which can damage the equipment in severe cases. In addition, harmonics will also be generated in this process, which will affect the inverter's output performance. Therefore, it is necessary to control the DC-side midpoint potential balance.

[0051] This invention applies sector optimization selection to the model predictive control of a three-phase, three-level T-type grid-connected inverter. Sectors are divided based on the effective radiation range of the voltage vector, and combined voltage vectors are used to replace the traditional small and medium voltage vectors that affect the neutral point potential. Based on this, a two-step determination method is applied to identify the sector based on the medium voltage vector.

[0052] The circuit topology of the three-phase three-level T-type grid-connected inverter used in this invention is as follows: Figure 1 As shown.

[0053] The T-type inverter topology includes three T-type NPC flat bridges, of which S ax ,S bx ,S cx (X = 1~4) represents the four switches in bridge arms a, b, and c. Taking bridge A as an example, when switch S... a1 ,S a2 On, switch S a3 ,S a4 When turned off, the output voltage is U. dc / 2, the output voltage is independent of the current direction, and the output state is defined as "P". When switch S a2 ,S a3 On, switch Sa1 ,S a4 When switched off, the output voltage is 0, and the output voltage is independent of the current direction; the output state is defined as "O". When switch S... a3 ,S a4 On, switch S a1 ,S a2 When turned off, the output voltage is -U dc / 2, the output voltage of the bridge is independent of the current direction, and the output state is defined as "N". The output states of the three-phase three-level T-type inverter are shown in Table 1.

[0054] Table 1 Output Status of Three-Phase Three-Level T-Type Grid-Connected Inverter

[0055]

[0056]

[0057] The mathematical model of the system used in this invention is shown in the following formula.

[0058] According to Kirchhoff's voltage law, the output phase voltage of the inverter can be expressed as:

[0059]

[0060] Where Rs is the line resistance, Ls is the filter inductance, and e a e b e c For the three-phase power grid voltage, i a i b i c For the three-phase grid-connected current flowing to the power grid through the filter inductor Ls and the line resistance Rs, u aN u bN u cN This provides the three-phase output voltage for the inverter.

[0061] Applying the Clarke transformation to the above equation, we obtain the mathematical expression in the α-β coordinate system:

[0062]

[0063] Current i α i β The rate of change over a sampling period can be obtained by the Euler approximation:

[0064]

[0065] Therefore, the predicted current value can be obtained:

[0066]

[0067] Then, the cost function is used to determine the predicted current value, thereby selecting the optimal combination of switching states to control the switching transistor. The cost function can be expressed as:

[0068]

[0069] In this expression, and Let i be the current reference value given in the αβ coordinate system at time k+1. α (k+1) and i β (k+1) is the predicted current value in the αβ coordinate system at time k+1.

[0070] This invention utilizes the effective radiation range based on voltage vectors to divide sectors. The effective radiation range of a voltage vector is the region with the shortest linear distance from a reference vector to a given vector. As follows... Figure 2 As shown, the effective radiation range is each polygonal region enclosed by the dashed line, dividing the entire large sector into six sectors.

[0071] In the traditional voltage vector selection process, both small and medium voltage vectors affect the neutral point voltage. Therefore, these voltage vectors are replaced by vector combinations. Taking sector I as an example, the combined vectors are shown in Table 2.

[0072] Table 2 Vector Combinations in Sector I

[0073] A <![CDATA[V 26 ]]> B <![CDATA[1 / 2V 13 +1 / 2V 14 ]]> C <![CDATA[1 / 2V 15 +1 / 2V 16 ]]> D <![CDATA[V1]]> E <![CDATA[1 / 2V1+1 / 2V2]]> F <![CDATA[V2]]>

[0074] To reduce computational load, a method based on medium voltage vectors is proposed, followed by voltage vector optimization within a selected large sector. This achieves the same performance as traditional methods with only one-third of the computational cost. For example, if the selected target voltage vector V7 yields the minimum cost function value, then sector I should be selected, and each voltage vector within that sector should be calculated and optimized.

[0075] as follows Figure 3 The diagram shows the flowchart of the improved finite control set model predictive control.

[0076] First, the grid current and voltage are sampled, and an αβ coordinate transformation is performed using the Clarke transform. Then, the Euler formula is used to obtain the predicted current. The cost function is calculated based on the predicted current and its reference value. The voltage vector in each of the six sectors that minimizes the cost function is compared, and the sector containing that voltage vector is selected for the next calculation. The predicted current is calculated again, and the cost function is calculated using the voltage vectors in all sub-sectors of the first sector. The voltage vector that minimizes the cost function is compared to determine the optimal voltage vector. The switching state corresponding to the optimal voltage vector is then applied to the inverter's switching devices. Assuming sector I is selected, the predicted current is calculated similarly, and the cost function is calculated using the voltage vectors in all sub-sectors of sector I. The optimal value is compared, and the process is repeated after waiting for the next sampling time. Similarly, the algorithm remains consistent for other sectors. (V1~V6 are defined as large voltage vectors, V7~V...) 12 Defined as the medium voltage vector, V 13 ~V 24 Defined as a small voltage vector, V 25 ~V 27 (Defined as a zero voltage vector). Specific implementation examples:

[0078] The simulation verification is performed below, and the specific simulation parameters are shown in Table 3.

[0079] Table 3 Simulation Parameters

[0080] <![CDATA[U dc ]]> DC bus voltage 800V <![CDATA[i ref ]]> Reference current 20～30A <![CDATA[C1,C2]]> DC side capacitor 50μF <![CDATA[L S ]]> AC side inductor 10mH <![CDATA[R S ]]> Inductor internal resistance 0.05Ω <![CDATA[T S ]]> Sampling time 50μs

[0081] First, simulations were performed to verify the traditional FCS-MPC algorithm and the improved FCS-MPC algorithm with sector optimization and two-step judgment. The reference current was changed from 20A to 30A at 0.05s. The steady-state performance and dynamic performance of the two algorithms were compared at the same time, and the total harmonic distortion of the two algorithms was analyzed and compared.

[0082] Secondly, the simulation experiments before and after the addition of the two-step judgment method should be compared to prove that it will not affect the performance of the algorithm, thus ensuring the rigor and accuracy of the experiment.

[0083] Finally, simulation analysis was used to compare the impact of the improved algorithm on the midpoint potential balance.

[0084] Experimental results of the present invention

[0085] Figure 4 and Figure 6 The simulation results show the output current and total harmonic distortion under the traditional FCS-MPC strategy. Figure 7 and Figure 9 The simulation results show the output current and total harmonic distortion under the proposed FCS-MPC strategy. Figure 5 and Figure 8 The grid phase a voltage and output current are shown under two control strategies. Under both strategies, the grid phase voltage a and the output current have the same phase angle, verifying the effectiveness of the simulation.

[0086] Simulation results show that when the reference current changes from 20A to 30A, the output current under both strategies maintains good tracking performance and a short response time. The output current stabilizes within 0.001s after abrupt changes in either strategy, demonstrating good dynamic performance. However, the figures also show that, compared to the traditional strategy, the strategy of this invention exhibits lower total harmonic distortion and better current quality.

[0087] Furthermore, to ensure that the secondary judgment of sectors in the improved FCS-MPC algorithm does not affect the optimization of voltage vectors and the quality of output current in the entire system, simulation analysis was also performed on the algorithm that does not use sectors, as shown above. Figure 10 and Figure 11 As shown. Since the total harmonic distortion value is the same, it can be inferred that the secondary sector judgment does not affect the output current performance.

[0088] Figure 12 and Figure 13 The neutral point potential V under the traditional algorithm and the improved algorithm are respectively. p V n And its difference. It can be seen that although conventional algorithms can achieve neutral point potential balance, this invention, through sector optimization and voltage vector combination, can make V... p and V n The values ​​are closer, thus making the neutral point potential balance more stable.

[0089] It should be noted that:

[0090] The algorithms and displays provided herein are not inherently related to any particular computer, virtual system, or other device. Various general-purpose systems can also be used in conjunction with the teachings herein. The required structure for constructing such systems is apparent from the above description. Furthermore, this application is not directed to any particular programming language. It should be understood that the content of this application described herein can be implemented using various programming languages, and the above description of specific languages ​​is for the purpose of disclosing the best mode of implementation of this application.

[0091] Numerous specific details are set forth in the specification provided herein. However, it will be understood that embodiments of this application may be practiced without these specific details. In some instances, well-known methods, structures, and techniques have not been shown in detail so as not to obscure the understanding of this specification.

[0092] Similarly, it should be understood that, in order to simplify this application and aid in understanding one or more of the various inventive aspects, in the above description of exemplary embodiments of this application, various features of this application are sometimes grouped together into a single embodiment, figure, or description thereof. However, this method of disclosure should not be construed as reflecting an intention that the claimed application requires more features than are expressly recited in each claim. Rather, as reflected in the following claims, inventive aspects lie in fewer than all features of a single foregoing disclosed embodiment. Therefore, the claims following the detailed description are hereby expressly incorporated into that detailed description, wherein each claim itself is a separate embodiment of this application.

[0093] Those skilled in the art will understand that modules in the device of the embodiments can be adaptively changed and placed in one or more devices different from that embodiment. Modules, units, or components in the embodiments can be combined into a single module, unit, or component, and further, they can be divided into multiple sub-modules, sub-units, or sub-components. Except where at least some of such features and / or processes or units are mutually exclusive, any combination can be used to combine all features disclosed in this specification (including the accompanying claims, abstract, and drawings) and all processes or units of any method or device so disclosed. Unless expressly stated otherwise, each feature disclosed in this specification (including the accompanying claims, abstract, and drawings) may be replaced by an alternative feature that serves the same, equivalent, or similar purpose.

[0094] Furthermore, those skilled in the art will understand that although some embodiments described herein include certain features but not others included in other embodiments, combinations of features from different embodiments are intended to be within the scope of this application and form different embodiments. For example, in the following claims, any of the claimed embodiments can be used in any combination.

[0095] The various component embodiments of this application can be implemented in hardware, or as software modules running on one or more processors, or a combination thereof. Those skilled in the art will understand that microprocessors or digital signal processors (DSPs) can be used in practice to implement some or all of the functions of some or all of the components in the virtual machine creation system according to the embodiments of this application. This application can also be implemented as a device or system program (e.g., a computer program and computer program product) for performing part or all of the methods described herein. Such an implementation of this application can be stored on a computer-readable medium, or can be in the form of one or more signals. Such signals can be downloaded from an Internet website, provided on a carrier signal, or provided in any other form.

[0096] It should be noted that the above embodiments are illustrative of this application and not restrictive, and that those skilled in the art can devise alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses should not be construed as limiting the claims. The word "comprising" does not exclude the presence of elements or steps not listed in the claims. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. This application can be implemented by means of hardware comprising several different elements and by means of a suitably programmed computer. In the unit claims enumerating several systems, several of these systems may be embodied by the same item of hardware. The use of the words first, second, and third, etc., does not indicate any order. These words can be interpreted as names.

[0097] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A model predictive control method for a three-phase three-level T-type grid-connected inverter, characterized in that, Includes the following steps: Sample the grid current and voltage, and perform an αβ coordinate transformation on them using the Clarke transform. The predicted current is obtained by combining Euler's formula, and the cost function is calculated based on the predicted current and its reference value. Compare which voltage vector in the six sectors of the vector radiation range minimizes the cost function value, and then select the first sector where the corresponding voltage vector is located for the next calculation. The predicted current is calculated again, and the cost function is calculated by applying the voltage vectors in all sub-sectors of the first sector. The voltage vector that minimizes the cost function is compared to obtain the optimal voltage vector. The switching state corresponding to the optimal voltage vector is then applied to the switching devices of the inverter.

2. The model predictive control method for a three-phase three-level T-type grid-connected inverter according to claim 1, characterized in that, The three-phase three-level T-type grid-connected inverter includes three T-type NPC flat bridges.

3. The model predictive control method for a three-phase three-level T-type grid-connected inverter according to claim 1 or 2, characterized in that, According to Kirchhoff's voltage law, the output phase voltage of the inverter is expressed as: Where Rs is the line resistance, Ls is the filter inductance, and e a e b e c For the three-phase power grid voltage, i a i b i c For the three-phase grid-connected current flowing to the power grid through the filter inductor Ls and the line resistance Rs, u aN u bN u cN This provides the three-phase output voltage for the inverter.

4. The model predictive control method for a three-phase three-level T-type grid-connected inverter according to claim 3, characterized in that, The αβ coordinate transformation performed by Clarke transformation includes: Applying the Clarke transformation to equation (1) above, we obtain the mathematical expression in the α-β coordinate system: i α i β Let u be the current in the α-β coordinate system. α u β Let e ​​be the voltage in the α-β coordinate system. α e β This is the output AC signal in the α-β coordinate system.

5. The model predictive control method for a three-phase three-level T-type grid-connected inverter according to claim 4, characterized in that, The method of obtaining the predicted current by combining Euler's formula includes: Current i α i β The rate of change over a sampling period can be obtained by the Euler approximation: Where k is the number of samples, T S The sampling period; Therefore, the predicted current value can be obtained:

6. The model predictive control method for a three-phase three-level T-type grid-connected inverter according to claim 5, characterized in that, The cost function is expressed as: in, and Let i be the current reference value given in the αβ coordinate system at time k+1. α (k+1) and i β (k+1) is the predicted current value in the αβ coordinate system at time k+1.

7. The model predictive control method for a three-phase three-level T-type grid-connected inverter according to claim 1, characterized in that, The six sectors are divided based on the effective radiation range of the voltage vector, which is the region with the shortest linear distance from the reference vector to the given vector.

Images