A predictive control method for a hybrid-level three-phase grid-connected inverter

Abstract

The present invention provides a predictive control method for a hybrid level three-phase grid-connected inverter, the method comprising: applying an initial switch state to the hybrid level three-phase grid-connected inverter; sampling three-phase voltage and current and transforming them to αβ coordinates; selecting a basic voltage vector in each cycle, and calculating its corresponding predicted current according to a prediction function; selecting the sum of the absolute values ​​of the reference current and the predicted current error as an evaluation function; judging whether to traverse all basic voltage vectors, if not traversing all basic voltage vectors, selecting other basic voltage vectors to continue the cycle; if all basic voltage vectors have been traversed, selecting the basic voltage vector that minimizes the evaluation function, and applying it to the inverter. The present invention adopts an MPC control strategy. By minimizing the cost function, finding the optimal input index, improving the stability, robustness and response speed of the system, the generated output voltage waveform not only reduces the common mode voltage but also reduces the dv / dt of the common mode voltage.

Description

Technical Field

[0001] The present invention relates to the technical field of inverter control, and in particular to a predictive control method for a hybrid-level three-phase grid-connected inverter. Background Art

[0002] Distributed power generation systems composed of renewable energy such as wind energy, solar energy, and tidal energy have played an important role in improving the economic efficiency of power grid operation, enhancing the safety of power systems, and building environmentally friendly power systems.

[0003] Three-phase voltage source inverters are widely used in variable frequency drives and solar photovoltaic power generation, where the most widely used inverter topology is the traditional six-switch two-level inverter. However, applying well-known PWM strategies such as sinusoidal pulse width modulation or space vector modulation to this topology may cause some problems or interference in the system due to the switching nature of the pole voltage. The presence of common-mode voltage between the power supply and the load neutral point is a possible problem. The high-frequency switching nature of the common-mode voltage can cause many problems such as electromagnetic interference and reduced electromagnetic compatibility. In the case of variable frequency drives, common-mode voltages may cause motor degradation and sometimes even winding insulation failure. In solar photovoltaic applications, common-mode inductance may cause safety issues in addition to ground leakage current, which reduces efficiency and causes grid current distortion.

[0004] Common control methods for three-phase inverters include PI control, PR control, and deadbeat control. PI control is widely used in engineering practice due to its advantages such as simple control and wide application scenarios. However, the response speed of traditional PI control strategies is relatively slow and cannot quickly adapt to changes in the dynamic characteristics of the system. At the same time, the parameter design of the PI controller is relatively complex and requires experienced engineers to adjust. The PI controller is sensitive to external interference and is easily affected by factors such as environmental changes and power supply fluctuations. Summary of the invention

[0005] In order to solve these problems, the present invention proposes a novel predictive control method for a hybrid-level three-phase grid-connected inverter, which adopts an MPC control strategy.

[0006] The present invention provides a predictive control method for a hybrid level three-phase grid-connected inverter, comprising:

[0007] The present invention uses a hybrid level three-phase grid-connected inverter. Compared with a traditional two-level inverter, the inverter can effectively reduce the output common mode voltage and the dv / dt of the common mode voltage.

[0008] The present invention uses a model predictive control algorithm. Model Predictive Control (MPC) is an advanced control method for dynamic system control that uses a mathematical model of the system to make multi-step predictions and then optimizes the control input at the current moment to minimize the performance indicators within the prediction period. MPC re-optimizes at each sampling moment, so that it can cope with system dynamic changes and uncertainties to better handle the continuous state and control input of the system.

[0009] Specifically, the present invention provides a predictive control method for a hybrid level three-phase grid-connected inverter, comprising:

[0010] applying an initial switching state to the hybrid-level three-phase grid-connected inverter;

[0011] Sample three-phase voltage and current and transform them to αβ coordinates;

[0012] In each cycle, a basic voltage vector is selected and its corresponding predicted current is calculated according to the prediction function;

[0013] The sum of the absolute values ​​of the reference current and the predicted current errors is selected as the evaluation function;

[0014] It is determined whether all basic voltage vectors have been traversed. If not, other basic voltage vectors are selected to continue the cycle. If all basic voltage vectors have been traversed, a basic voltage vector that minimizes the evaluation function is selected and applied to the inverter.

[0015] Furthermore, according to the different switching states of the grid-connected inverter, corresponding to 12 voltage vectors, the output grid-connected current is different; the optimal voltage vector is selected from the different current prediction values ​​so that the output current i at time k+1 is α (k+1), i β The error between (k+1) and the reference current is the smallest.

[0016] Further, select the reference current i under the αβ coordinate * The sum of the absolute values ​​of the errors with the predicted current i is used as the evaluation function:

[0017]

[0018] Further, the calculating the corresponding predicted current according to the prediction function includes:

[0019]

[0020] Where: i α (k), i β(k) is the grid-connected current in αβ coordinates obtained by coordinate transformation in the kth sampling period; T s is the sampling period; U α (k) U β (k) is the voltage vector in αβ coordinates obtained by coordinate transformation in the kth sampling period; e α (k), e β (k) is the grid voltage vector in αβ coordinates obtained by coordinate transformation in the kth sampling period; i α (k+1), i β (k+1) is the predicted current in the k+1th sampling period under the αβ coordinates; R is the value of the filter resistor, and L is the value of the filter inductor.

[0021] Furthermore, the predicted current is calculated for all voltage vectors in each cycle to obtain a set of optimal voltage vectors that minimize the evaluation function, and the switch tubes of the mixed-level three-phase grid-connected inverter are controlled to make their states consistent with the switch tube states corresponding to the optimal voltage vectors.

[0022] Furthermore, the mixed-level three-phase grid-connected inverter includes: eight power electronic switching tubes, including three pairs of power electronic switching tubes connected in parallel between the positive and negative poles of a DC power supply, each pair including two power electronic switching tubes connected in series, and a fourth pair of power electronic switching tubes connected in series at the connection between the midpoint of the DC bus and the middle of the first pair of power electronic switching tubes.

[0023] Furthermore, phase A of the mixed-level three-phase grid-connected inverter is connected to the midpoint of the DC bus through the fourth pair of power electronic switch tubes, phase B is connected to the middle connection of the second pair of power electronic switch tubes, and phase C is connected to the middle connection of the third pair of power electronic switch tubes.

[0024] Furthermore, the A phase can output three levels, namely V dc , 0.5V dc and 0; Phase B and Phase C can only output two levels, namely V dc and 0, where V dc represents the voltage of the DC power supply.

[0025] The advantages of the present invention are as follows: the present invention proposes a novel predictive control method for a hybrid level three-phase grid-connected inverter, which adopts an MPC control strategy. By minimizing the cost function, the optimal input index is found, the stability, robustness and response speed of the system are improved, and an output voltage waveform with a certain quality is generated, which not only reduces the common mode voltage but also reduces the dv / dt of the common mode voltage. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Various other advantages and benefits will become apparent to those of ordinary skill in the art by reading the detailed description of the preferred embodiments below. The accompanying drawings are only for the purpose of illustrating the preferred embodiments and are not to be considered as limiting the present invention. Moreover, the same reference symbols are used throughout the accompanying drawings to represent the same components. In the accompanying drawings:

[0027] Figure 1 A circuit diagram of a hybrid-level three-phase grid-connected inverter according to an embodiment of the present invention is shown.

[0028] Figure 2 This is the voltage vector diagram of the hybrid level inverter proposed by the present invention.

[0029] Figure 3 The present invention is a flow chart of a predictive control method for a novel hybrid-level three-phase grid-connected inverter.

[0030] Figure 4 It is a schematic diagram of simulated three-phase current of the present invention.

[0031] Figure 5 Schematic diagram of output current THD of the present invention. DETAILED DESCRIPTION

[0032] The exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although the exemplary embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments described herein. On the contrary, these embodiments are provided in order to enable a more thorough understanding of the present disclosure and to fully convey the scope of the present disclosure to those skilled in the art.

[0033] Model Predictive Control (MPC) is an advanced control method for dynamic system control. It uses the mathematical model of the system to make multi-step predictions and then optimizes the control input at the current moment to minimize the performance index within the prediction period. MPC re-optimizes at each sampling moment, so that it can cope with system dynamic changes and uncertainties to better handle the continuous state and control input of the system.

[0034] The present invention provides a novel hybrid level three-phase inverter circuit topology structure as shown in Figure 1 This topology is composed of 8 power electronic switch tubes, including three pairs of power electronic switch tubes S connected in parallel between the positive and negative poles of the DC power supply. A1 , S A2 , S B1 , S B2 , S C1 , S C2Each pair includes two power electronic switch tubes connected in series, and a power electronic switch tube S connected in series with the midpoint of the DC bus. A1 , S A2 The fourth pair of power electronic switch tubes S at the middle connection A3 .

[0035] Phase A of the hybrid level three-phase grid-connected inverter is connected to the power supply via the fourth pair of power electronic switch tubes S A3 It is connected to the midpoint of the DC bus, phase B is connected to the middle connection of the second pair of power electronic switch tubes, and phase C is connected to the middle connection of the third pair of power electronic switch tubes.

[0036] In the present invention, phase A is passed through S A3 It is connected to the midpoint of the DC bus, similar to the T-type three-level inverter topology, and the B phase and C phase are the same as the traditional two-level inverter. In the novel hybrid level inverter topology of the present invention, the power electronic switches all use insulated-gate bipolar transistors (IGBT). Figure 1 In the new hybrid level three-phase inverter topology, phase A can output three levels, namely V dc , 0.5V dc and 0. However, phase B and phase C can only output two levels, namely V dc and 0.

[0037] First, define the switching function S i (i=a,b,c,),S a There are three values: 1, -1 and 0, and S b and S c The only possible values ​​are 1 and -1.

[0038] Let the switch state vector S be

[0039]

[0040] Where a = e j2π / 3 , e represents the base of natural logarithm. The inverter output voltage vector is

[0041]

[0042] Where U aN , U bN and U cN is the inverter output to neutral point voltage. The relationship between voltage vector U and switch vector S is:

[0043] U=U dc S (3)

[0044] Considering all the switch state combinations, 12 voltage vectors are obtained, such as Figure 2 shown.

[0045] exist Figure 2 In the figure, the squares represent the eight voltage vectors in the traditional six-switch two-level inverter, and the circles represent the additional four voltage vectors of the hybrid-level inverter.

[0046] Assuming that the three-phase grid voltage is balanced, the grid current vector i and grid voltage vector e are expressed as

[0047]

[0048]

[0049] The grid voltage equation is:

[0050]

[0051] According to the formula from abc coordinates to αβ coordinates:

[0052]

[0053] The expression of formula (6) in αβ coordinates is:

[0054]

[0055] Discretize and simplify the above formula to get

[0056]

[0057] Where: i α (k), i β (k) is the grid-connected current in αβ coordinates obtained by coordinate transformation in the kth sampling period; T s is the sampling period; U α (k) U β (k) is the voltage vector in αβ coordinates obtained by coordinate transformation in the kth sampling period; e α (k), e β (k) is the grid voltage vector in αβ coordinates obtained by coordinate transformation in the kth sampling period; i α (k+1), i β (k+1) is the predicted current in the k+1th sampling period under the αβ coordinates; R is the value of the filter resistor, and L is the value of the filter inductor.

[0058] According to the different switching states of the grid-connected inverter, there are 12 corresponding voltage vectors, and the output grid-connected current is different. The present invention selects the optimal voltage vector from different predicted values ​​so that the output current i at time k+1 is α (k+1), i βThe error between (k+1) and the reference current is the smallest. Select the reference current i * The sum of the absolute values ​​of the errors with the predicted current i is used as the evaluation function:

[0059]

[0060] The control algorithm flow is as follows Figure 3 As shown, including:

[0061] applying an initial switching state to the hybrid-level three-phase grid-connected inverter;

[0062] Sample three-phase voltage and current and transform them to αβ coordinates;

[0063] In each cycle, a basic voltage vector is selected and its corresponding predicted current is calculated according to the prediction function;

[0064] The sum of the absolute values ​​of the reference current and the predicted current errors is selected as the evaluation function;

[0065] It is determined whether all basic voltage vectors have been traversed. If not, other basic voltage vectors are selected to continue the cycle. If all basic voltage vectors have been traversed, a basic voltage vector that minimizes the evaluation function is selected and applied to the inverter.

[0066] In this way, the present invention calculates the predicted current for all voltage vectors in each cycle, obtains the set of optimal voltage vectors that minimize the evaluation function, and controls the switch tubes of the mixed-level three-phase grid-connected inverter to make their states consistent with the switch tube states corresponding to the optimal voltage vectors.

[0067] Figure 3 In the figure, S(n) is the target switching state, m is the minimum value of the cost function, j ranges from 0 to 11 and corresponds to the 12 output voltage vectors of the hybrid level grid-connected inverter, and g(j) is the value of the evaluation function at the jth switching state.

[0068] Example

[0069] The present invention establishes a simulation model to verify the correctness of the theory. The study found that the strategy proposed by the present invention can output a current waveform with a certain quality, and the common mode voltage for Compared with the traditional six-switch two-level inverter The common mode voltage should be small. Figure 4 To simulate the current waveform, Figure 5 is the THD value of the output current.

[0070] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall be based on the protection scope of the claims.

Claims

1. A predictive control method for a hybrid level three-phase grid-connected inverter, characterized in that: include: applying an initial switching state to the hybrid-level three-phase grid-connected inverter; Sample three-phase voltage and current and transform them to αβ coordinates; In each cycle, a basic voltage vector is selected and its corresponding predicted current is calculated according to the prediction function; The sum of the absolute values ​​of the reference current and the predicted current errors is selected as the evaluation function; It is determined whether all basic voltage vectors have been traversed. If not, other basic voltage vectors are selected to continue the cycle. If all basic voltage vectors have been traversed, a basic voltage vector that minimizes the evaluation function is selected and applied to the inverter.

2. The predictive control method for a hybrid level three-phase grid-connected inverter according to claim 1, characterized in that: According to the different switching states of the grid-connected inverter, there are 12 voltage vectors corresponding to the output grid-connected current. Select the optimal voltage vector from different current prediction values ​​so that the output current i at time k+1 is α (k+1), i β The error between (k+1) and the reference current is the smallest.

3. The predictive control method for a hybrid level three-phase grid-connected inverter according to claim 1, characterized in that: Select the reference current i under αβ coordinates * The sum of the absolute values ​​of the errors with the predicted current i is used as the evaluation function:

4. The predictive control method for a hybrid level three-phase grid-connected inverter according to claim 3, characterized in that: The step of calculating the corresponding predicted current according to the prediction function includes: Where: i α (k), i β (k) is the grid-connected current in αβ coordinates obtained by coordinate transformation in the kth sampling period; T s is the sampling period; U α (k) U β (k) is the voltage vector in αβ coordinates obtained by coordinate transformation in the kth sampling period; e α (k), e β (k) is the grid voltage vector in αβ coordinates obtained by coordinate transformation in the kth sampling period; i α (k+1), i β (k+1) is the predicted current in the k+1th sampling period under the αβ coordinates; R is the value of the filter resistor, and L is the value of the filter inductor.

5. The predictive control method for a hybrid level three-phase grid-connected inverter according to claim 1, characterized in that: The predicted current is calculated for all voltage vectors in each cycle to obtain a set of optimal voltage vectors that minimize the evaluation function, and the switch tubes of the mixed-level three-phase grid-connected inverter are controlled to make their states consistent with the switch tube states corresponding to the optimal voltage vectors.

6. A predictive control method for a hybrid level three-phase grid-connected inverter according to any one of claims 1 to 5, characterized in that: The hybrid level three-phase grid-connected inverter includes: eight power electronic switch tubes, including three pairs of power electronic switch tubes connected in parallel between the positive and negative poles of a DC power supply, each pair including two power electronic switch tubes connected in series, and a fourth pair of power electronic switch tubes connected in series at the connection between the midpoint of the DC bus and the middle of the first pair of power electronic switch tubes.

7. The predictive control method for a hybrid level three-phase grid-connected inverter according to claim 6, characterized in that: Phase A of the mixed-level three-phase grid-connected inverter is connected to the midpoint of the DC bus through the fourth pair of power electronic switch tubes, phase B is connected to the middle connection of the second pair of power electronic switch tubes, and phase C is connected to the middle connection of the third pair of power electronic switch tubes.

8. The predictive control method for a hybrid level three-phase grid-connected inverter according to claim 7, characterized in that: The A phase can output three levels, namely V dc , 0.5V dc and 0; Phase B and Phase C can only output two levels, namely V dc and 0, where V dc represents the voltage of the DC power supply.

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