Capacitor voltage balance control method for flying capacitor three-level inverter based on MPC

Abstract

The invention discloses an MPC-based capacitor voltage balance control method for a flying capacitor three-level inverter, and the method comprises the following steps: 1), measuring the current Ix of each phase of the flying capacitor three-level inverter and the voltage UCx of a flying capacitor, and then building a prediction model for predicting the phase current and the voltage of the flying capacitor; 2) obtaining a working mode of the inverter in a prediction range through the prediction model, and constructing a cost function according to the working mode; and 3) substituting the obtained predicted values of the capacitor voltage and the output current into a cost function for judgment, and selecting the switch state represented by the capacitor voltage and the output current with the minimum cost function to operate the flying capacitor three-level inverter. According to the invention, based on characteristic analysis of flying capacitor topology, comprehensive modeling is carried out on flying capacitor voltage and output current, a cost function about capacitor voltage is established, and a state enabling the cost function to be minimum is selected as a switching state at the next moment, so that stable control on flying capacitor three-level capacitor voltage is realized.

Description

Technical Field

[0001] The present invention relates to a capacitor voltage balance control method of a transcapacitor three-level inverter based on model predictive control, which realizes the optimization control of flying capacitor voltage stability and belongs to the invention technology in the field of switching power supply design. Background Art

[0002] With the advent of high-voltage, high-current IGBT devices, three-level inverters have been widely used in many fields. The design of the three-level inverter not only effectively reduces the system's demand for the withstand voltage level of power devices, but also significantly reduces the harmonic content of the inverter output waveform. Among them, the flying capacitor three-level inverter can significantly improve the quality of the voltage waveform, and can significantly reduce the harmonic content without increasing the switching frequency, making the output voltage waveform smoother and more stable. The introduction of the flying capacitor not only reduces the voltage stress of the switching device, but also further improves the overall performance and stability of the converter. The significant advantage of the flying capacitor three-level topology is that it is easy to expand to multiple levels, which can be achieved by simply adding the corresponding clamping capacitor, and there is no problem of neutral point voltage imbalance. Its disadvantage is that the voltage balance of the flying capacitor needs to be controlled. Therefore, it is of great significance to the stable operation and performance of the flying capacitor three-level converter to maintain the voltage balance of the flying capacitor through the strategy of model predictive control. Summary of the invention

[0003] In order to solve the capacitor voltage imbalance problem encountered when using a flying capacitor three-level inverter, the present invention proposes a capacitor voltage balance control scheme based on model predictive control.

[0004] The present invention is achieved by the following technical means:

[0005] A flying capacitor three-level inverter capacitor voltage balance control method based on MPC, comprising the following steps:

[0006] 1) The current I of each phase of the flying capacitor three-level inverter x And the flying capacitor voltage U Cx Measurements are made and then a prediction model is established to predict the phase current and flying capacitor voltage;

[0007] 2) Obtain the working mode of the inverter within the prediction range through the prediction model, and construct a cost function based on the working mode;

[0008] 3) Substitute the obtained predicted values ​​of capacitor voltage and output current into the cost function for judgment, and select the switching state represented by the capacitor voltage and output current with the smallest cost function to operate the flying capacitor three-level inverter.

[0009] The prediction model is established by calculating the inverter phase current I at time k.x(k) The flying capacitor voltage U Cx(k) Take measurements, x=A, B, C, and then estimate the phase current and capacitor voltage at time (k+1) using the measured values ​​at time k.

[0010] The voltage U between the load end common point O and the DC side reference point N ON And the load voltage U xO The expressions are:

[0011]

[0012] U xO (k+1)=U xN (k+1)+U NO (k+1)

[0013] U AN , U BN , U CN is the voltage between points A, B, C on the inverter output side and the DC side reference point N, which satisfies

[0014]

[0015] Among them, S x1(k) With S x2(k) are the switch states at time k, U Cx(k) is the flying capacitor voltage at time k, and the DC voltage Udc is a known quantity.

[0016] The phase current expression is:

[0017]

[0018] L is the filter inductance value, R is the load resistance value, U xO (k+1) is the voltage generated by the inverter at time (k+1), T s is the switching cycle, I xO (k) and I xO (k+1) are the phase currents at time k and time (k+1) respectively.

[0019] The flying capacitor voltage at time (k+1) is:

[0020]

[0021] Where C is the capacitance of the flying capacitor, U Cx (k) and I Cx (k+1) are the flying capacitor voltages at time k and time (k+1) respectively.

[0022] The cost function is:

[0023]

[0024] Udc / 2 is the expected value of the flying capacitor voltage.

[0025] The judgment process includes the following steps:

[0026] 4-1) According to the phase current expression and cost function at time (k+1), the predicted values ​​of capacitor voltage and output current at time k+1 are obtained, and the predicted result of flying capacitor voltage value at time (k+2) is obtained:

[0027]

[0028] Among them, S'x1(k) and S'x2(k) have two redundant states

[10] and

[01] . The predicted values ​​of the flying capacitor voltage at time k+2 in these two states are calculated respectively, and they are respectively brought into the cost function for judgment. The cost function is as follows:

[0029]

[0030] 4-2) Compare the obtained cost functions and select the one with the lowest cost function as the switch state at time k+1 to act on the switch tube.

[0031] A computer device comprises a memory and a processor, wherein the memory stores a computer program, and is characterized in that the memory stores the computer program, and when the processor executes the computer program, the flying capacitor three-level inverter capacitor voltage balance control method based on MPC described in any of the above items is implemented.

[0032] 9. A computer-readable storage medium having a computer program stored thereon, characterized in that the storage medium includes a stored program, wherein when the program is running, the device where the storage medium is located is controlled to execute any of the above-mentioned MPC-based flying capacitor three-level inverter capacitor voltage balance control methods.

[0033] The present invention is based on the characteristic analysis of the flying capacitor topology, comprehensively models the flying capacitor voltage and output current, and establishes a cost function for the capacitor voltage. By selecting the state that minimizes the cost function as the switching state at the next moment, stable control of the three-level capacitor voltage of the flying capacitor is achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a schematic diagram of a three-level flying capacitor topology structure of the present invention;

[0035] Figure 2 This is a schematic diagram of the working state of the flying capacitor inverter of the present invention;

[0036] Figure 3 It is a flow chart of the present invention. DETAILED DESCRIPTION

[0037] The present invention is further described in detail below in conjunction with the accompanying drawings:

[0038] The present invention relates to a flying capacitor three-level inverter capacitor voltage balance control method based on MPC, comprising the following steps:

[0039] 1) The current I of each phase of the flying capacitor three-level inverter x And the flying capacitor voltage U Cx Measurements are made and then a prediction model is established to predict the phase current and flying capacitor voltage;

[0040] 2) Obtain the working mode of the inverter within the prediction range through the prediction model, and construct a cost function based on the working mode;

[0041] 3) Substitute the obtained predicted values ​​of capacitor voltage and output current into the cost function for judgment, and select the switching state represented by the capacitor voltage and output current with the smallest cost function to operate the flying capacitor three-level inverter.

[0042] The prediction model establishment comprises the following steps:

[0043] 2-1) The inverter phase current I at time k x(k) The flying capacitor voltage U Cx(k) Measure, x = A, B, C, and then estimate the phase current and capacitor voltage at time (k+1) through the measured value at time k. Assume that the flying capacitor voltage changes negligibly within a measurement time, and the switch state does not change at this moment. At this time, the voltage U between the inverter output side point x and the DC side reference point N is xN The expression is:

[0044]

[0045] Among them, S x1(k) With S x2(k) are the switch states at time k, U Cx(k) is the flying capacitor voltage at time k, and the DC voltage Udc is a known quantity;

[0046] Then the voltage U between the load end common point O and the DC side reference point N is ON And the load voltage U xO The expressions are:

[0047]

[0048] U xO (k+1)=UxN (k+1)+U NO (k+1)

[0049] 2-2) Applying Kirchhoff's law to the topology circuit of the FC three-level inverter, the FC three-level inverter system satisfies the following state equation:

[0050]

[0051] The reference potential of the three-phase output voltage of the inverter is the common point O at the load end, the filter inductor and capacitor are L and Cf respectively, the load resistance is R, the voltage generated by the inverter is U, and the load current is I;

[0052] By using the load current at discrete times to predict future values, the rate of change of the current within a sampling period Ts can be approximately expressed as:

[0053]

[0054] Where Ts is the switching period, Ixo(k) and Ixo(k+1) are the phase currents at time k and (k+1) respectively. The two formulas are combined to obtain the expression of the phase current Ixo(k+1):

[0055]

[0056] 2-3) The relationship between the flying capacitor voltage and the phase current is:

[0057]

[0058] Where C is the capacitance of the flying capacitor;

[0059] Transforming the capacitance-voltage relationship in the continuous domain into the discrete domain, we have:

[0060]

[0061] Among them, UCx(k) and ICx(k+1) are the flying capacitor voltages at time k and time (k+1) respectively. Combining the two formulas, the voltage UCx(k+1) at time (k+1) is obtained as follows:

[0062]

[0063] The cost function is:

[0064]

[0065] Udc / 2 is the expected value of the flying capacitor voltage.

[0066] The judgment process includes the following steps:

[0067] 4-1) According to the phase current expression and cost function at time (k+1), the predicted values ​​of capacitor voltage and output current at time k+1 are obtained, and the predicted result of flying capacitor voltage value at time (k+2) is obtained:

[0068]

[0069] Among them, S'x1(k) and S'x2(k) have two redundant states

[10] and

[01] . The predicted values ​​of the flying capacitor voltage at time k+2 in these two states are calculated respectively, and they are respectively brought into the cost function for judgment. The cost function is as follows:

[0070]

[0071] 4-2) Compare the obtained cost functions and select the one with the lowest cost function as the switch state at time k+1 to act on the switch tube.

[0072] The following is a detailed example:

[0073] The flying capacitor three-level inverter topology used is shown in the attached figure. Figure 1 As shown in the figure. For the problem of flying capacitor voltage balance control, it is found through analysis that the main factor affecting the capacitor voltage balance is the zero level state in the three-level state, where there are two redundant states in the zero state. The allocation of the redundant state affects the charging and discharging of the flying capacitor, thereby affecting the balance of the capacitor voltage. This paper designs the following Figure 3 The model prediction control process shown in the figure predicts the voltage and output current of the flying capacitor at time k+1 according to the switch state, flying capacitor voltage and output current at the current time k. In order to determine the selection of the zero redundant state at time k+1, the flying capacitor voltage at time k+2 is calculated, and the cost function is established according to the voltage at time k+2. The switching state of the two redundant states that minimizes the cost function is selected as the switching state at time k+1. This achieves stable control of the flying capacitor voltage.

[0074] Step 1: Establishment of the flying capacitor voltage and output current prediction model

[0075] Knowing the switch state at time k, the inverter phase current I x(k) The flying capacitor voltage U Cx(k) Measure, x = A, B, C, and then estimate the phase current and capacitor voltage at time (k+1) through the measured value at time k. Assume that the flying capacitor voltage changes negligibly within a measurement time, and the switch state does not change at this moment. At this time, the voltage U between the inverter output side point x and the DC side reference point N is xN The expression is:

[0076]

[0077] Among them, S x1(k) With S x2(k) are the switch states at time k, U Cx(k) is the flying capacitor voltage at time k, and the DC voltage Udc is a known quantity. Then the voltage U between the load end common point O and the DC side reference point N is ON And the load voltage U xO The expressions are:

[0078]

[0079] U xO (k+1)=U xN (k+1)+U NO (k+1) (3)

[0080] The reference potential of the three-phase output voltage of the inverter is the common point O at the load end. The filter inductor and capacitor are L and Cf respectively, the load resistance is R, the voltage generated by the inverter is U, and the load current is I. Applying Kirchhoff's law to the topological circuit of the FC three-level inverter, the FC three-level inverter system satisfies the following state equation:

[0081]

[0082] By using the load current at discrete times to predict future values, the rate of change of the current within a sampling period Ts can be approximately expressed as:

[0083]

[0084] Where Ts is the switching period, Ixo(k) and Ixo(k+1) are the phase currents at time k and time (k+1) respectively. Substituting equation (5) into equation (4), the expression of the phase current Ixo(k+1) at time (k+1) in the discrete domain is obtained:

[0085]

[0086] The relationship between the flying capacitor voltage and the phase current is:

[0087]

[0088] Where C is the capacitance of the flying capacitor. Converting the capacitance-voltage relationship in the continuous domain to the discrete domain, we have:

[0089]

[0090] Among them, UCx(k) and ICx(k+1) are the flying capacitor voltages at time k and time (k+1), respectively. Substituting equation (8) into equation (7), the voltage UCx(k+1) at time (k+1) is:

[0091]

[0092] Step 2: Construct the cost function

[0093] Through the prediction step, the working mode of the inverter within the prediction range is obtained, and the most appropriate switching state can be selected through the cost function g(k). The cost function is the deviation between the state variable and the expected value. In the three-level inverter, the expected value of the flying capacitor voltage is U dc / 2, the specific equation is as follows:

[0094]

[0095] Step 3: Principles for selecting the optimal switch state at the next moment

[0096] According to the switch state at the current time k, by sampling the flying capacitor voltage and output current, the predicted values ​​of the capacitor voltage and output current at time k+1 are obtained by equations (6) and (9). In order to obtain the optimal switch state at time k+1, the flying capacitor voltage value at time k+2 needs to be predicted, and the prediction results are as follows:

[0097]

[0098] Where S' x1(k) With S' x2(k) There are two redundant states

[10] and

[01] . The predicted values ​​of the flying capacitor voltage at time k+2 under these two states are calculated respectively, and they are respectively substituted into the cost function for judgment. The one that minimizes the cost function is selected as the switch state at time k+1 and acts on the switch tube.

[0099]

Claims

1. A flying capacitor three-level inverter capacitor voltage balance control method based on MPC, characterized in that: The steps include: 1) The current I of each phase of the flying capacitor three-level inverter x And the flying capacitor voltage U Cx Measurements are made and then a prediction model is established to predict the phase current and flying capacitor voltage; 2) Obtain the working mode of the inverter within the prediction range through the prediction model, and construct a cost function based on the working mode; 3) Substitute the obtained predicted values ​​of capacitor voltage and output current into the cost function for judgment, and select the switching state represented by the capacitor voltage and output current with the smallest cost function to operate the flying capacitor three-level inverter.

2. The method for controlling capacitor voltage balance of a flying capacitor three-level inverter based on MPC according to claim 1, characterized in that: The prediction model is established by calculating the inverter phase current I at time k. x(k) The flying capacitor voltage U Cx(k) Take measurements, x=A, B, C, and then estimate the phase current and capacitor voltage at time (k+1) using the measured values ​​at time k.

3. The method for controlling capacitor voltage balance of a flying capacitor three-level inverter based on MPC according to claim 2, characterized in that: The voltage U between the load end common point O and the DC side reference point N ON And the load voltage U xO The expressions are: IN xO (k+1)=U xN (k+1)+U NO (k+1) U AN , U BN , U CN is the voltage between points A, B, C on the inverter output side and the DC side reference point N, which satisfies Among them, S x1 (k) and S x2 (k) are the switch states at time k, U Cx (k) is the flying capacitor voltage at time k, and the DC voltage Udc is a known quantity.

4. The method for controlling capacitor voltage balance of a flying capacitor three-level inverter based on MPC according to claim 2, characterized in that: The phase current expression is: L is the filter inductance value, R is the load resistance value, U xO (k+1) is the voltage generated by the inverter at time (k+1), T s is the switching cycle, I xO (k) and I xO (k+1) are the phase currents at time k and time (k+1) respectively.

5. The method for controlling capacitor voltage balance of a flying capacitor three-level inverter based on MPC according to claim 2, characterized in that: The flying capacitor voltage at time (k+1) is: Where C is the capacitance of the flying capacitor, U Cx (k) and I Cx (k+1) are the flying capacitor voltages at time k and time (k+1) respectively.

6. The method for controlling capacitor voltage balance of a flying capacitor three-level inverter based on MPC according to claim 1, characterized in that: The cost function is: Udc / 2 is the expected value of the flying capacitor voltage.

7. The method for controlling capacitor voltage balance of a flying capacitor three-level inverter based on MPC according to claim 1, characterized in that: The judgment process includes the following steps: 4-1) According to the phase current expression and cost function at time (k+1), the predicted values ​​of capacitor voltage and output current at time k+1 are obtained, and the predicted result of flying capacitor voltage value at time (k+2) is obtained: Among them, S'x1(k) and S'x2(k) have two redundant states [1 0] and [0 1]. The predicted values ​​of the flying capacitor voltage at time k+2 in these two states are calculated respectively, and they are respectively brought into the cost function for judgment. The cost function is as follows: 4-2) Compare the obtained cost functions and select the one with the lowest cost function as the switch state at time k+1 to act on the switch tube.

8. A computer device, characterized in that: The invention comprises a memory and a processor, wherein the memory stores a computer program, and is characterized in that the memory stores the computer program, and when the processor executes the computer program, the flying capacitor three-level inverter capacitor voltage balance control method based on MPC described in any one of claims 1 to 7 is implemented.

9. A computer-readable storage medium having a computer program stored thereon, characterized in that: The storage medium includes a stored program, wherein when the program is running, the device where the storage medium is located is controlled to execute the MPC-based flying capacitor three-level inverter capacitor voltage balance control method according to any one of claims 1 to 7.

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