Three-level inverter output current and neutral-point potential collaborative optimization method based on predictive control

Abstract

The invention discloses a three-level inverter output current and midpoint potential collaborative optimization method based on predictive control, which comprises the following steps: 1, establishing a prediction model of an NPC type three-level inverter in a discrete system for predicting midpoint potential offset and output voltage; 2, after a sector where an output voltage predicted value is located is determined, a double-vector modulation strategy is adopted, a double-vector switch switching sequence can be determined only through four times of rolling optimization calculation in each control period, it is ensured that the switching frequency is constant, the calculated amount is greatly reduced, and the dependency of traditional single-vector model prediction on the high sampling frequency is eliminated; and 3, the neutral-point potential equalization control is realized by directly adjusting the action time of the redundant small vector. According to the invention, through a double-vector modulation strategy, the switching frequency is constant, and the two control targets of output current harmonic and neutral-point potential equalization are uniformly optimized, so that the dynamic response speed and the steady-state control precision of the system are enhanced, and the overall performance can be improved.

Description

Technical Field

[0001] The invention relates to a method for collaboratively optimizing output current and midpoint potential of a three-level inverter based on predictive control, and belongs to the field of energy storage current conversion control technology and intelligent control. Background Art

[0002] With the development of control theory and software and hardware technology, the application of finite control set mode predictive control (FCS-MPC) in high-power grid-connected technology has received more and more attention. Traditional FCS-MPC includes the discrete characteristics and nonlinearity of the inverter, does not require a modulation unit, and realizes multi-control objective collaborative optimization control through weight factors. Its core is to calculate the predicted values ​​of the control variables corresponding to different switch states, and rely on the value function to accurately evaluate the error between the control variable command value and the predicted value. Finally, a set of switch states that minimizes the absolute value of the value function is selected as the output of the controller in one control cycle, so that the system has strong robustness and dynamic response capabilities. However, each control cycle requires the enumeration method to perform rolling optimization calculations on 27 sets of switch states to determine the optimal switch state, and the amount of calculation will increase significantly. In addition, in order to achieve multi-objective collaborative control, it is necessary to introduce weight factors in the value function. The weight factors are generally obtained by trial and error, but due to the coupling effect between different control objectives, the weight factors are difficult to adjust. In addition, each control cycle only selects the optimal switching state independently based on the current system state and using model prediction, ignoring the continuity and correlation of the switching state selection between different control cycles, resulting in the inverter switching frequency being not fixed, the current harmonic content increasing, and thus increasing the difficulty of filter design. In order to ensure the dynamic performance and stability of the system, a higher sampling frequency is usually required, but in high-power grid-connected inverters, the inverter efficiency is closely related to the switching loss. In single loading mode, the sampling frequency is usually equivalent to the switching frequency. Therefore, to improve the inverter operating efficiency, it is necessary to reduce the switching frequency to reduce energy loss, while to enhance the stability of the system, it is necessary to fix the switching frequency, which limits the application scope of FCS-MPC to a certain extent. In summary, although FCS-MPC has significant advantages in high-power grid-connected technology, the difficulties it faces still need further research and resolution. Summary of the invention

[0003] In view of the shortcomings of the prior art mentioned above, the present invention proposes a method for optimizing the output current and midpoint potential of a three-level inverter based on predictive control, in order to achieve a constant switching frequency through a dual vector modulation strategy, and uniformly optimize the two control objectives of output current harmonics and midpoint potential balance, so as to enhance the dynamic response speed and steady-state control accuracy of the system, thereby improving the overall performance.

[0004] In order to achieve the above-mentioned purpose, the present invention adopts the following technical scheme:

[0005] The invention discloses a method for coordinating and optimizing the output current and midpoint potential of a three-level inverter based on predictive control, which comprises the following steps:

[0006] Step 1: Establish a continuous state mathematical model of an NPC type three-level inverter in a two-phase stationary coordinate system, and after discretizing the continuous state mathematical model, obtain an expression for a predicted value of a midpoint potential offset and an expression for a predicted value of an inverter output voltage;

[0007] Step 2: Establish the corresponding relationship between the switch state and the basic composite voltage vector, determine the switch state corresponding to different types of basic composite voltage vectors, and then divide the 60° area between each two adjacent middle vectors into a sector, so as to divide the two-phase stationary coordinate system into AF sectors, and each sector has a total of 6 basic composite voltage vectors;

[0008] Step 3: Based on the predicted voltage output of the three-level inverter at time k The angle θ(k) between the positive half axis of α and the two-phase stationary coordinate system is determined by The sector in which it is located;

[0009] Step 4: The redundant small vector in each sector is used as the central small vector, and the remaining four vectors in each sector are used as candidate vectors. The optimal candidate vector in the candidate vectors of each sector is determined by a value function based on the voltage constraint variable, so as to form a dual vector combination of each sector with the central small vector of the corresponding sector.

[0010] Step 5, calculating the respective action time of each vector in the double vector combination;

[0011] Step 6: When the predicted value of the midpoint potential offset is 0, construct the time allocation factor expression and calculate the time for the central small vector to act in each of the two switching states.

[0012] The method for coordinated optimization of output current and midpoint potential of a three-level inverter based on predictive control according to the present invention is also characterized in that step 1 comprises:

[0013] Using equation (1), the continuous-time state model of the three-level inverter in the two-phase stationary coordinate system is obtained:

[0014] (1)

[0015] In formula (1), is the three-phase output voltage of the inverter , , In the expression under the two-phase stationary coordinate system, L is the filter inductance, R is the parasitic resistance of the filter inductance, is the inductor current in the two-phase stationary coordinate system, is the grid voltage in the two-phase stationary coordinate system;

[0016] Using equation (2), the dynamic constraint equations of the upper and lower capacitor voltages and the neutral current on the DC side are obtained:

[0017] (2)

[0018] In formula (2), is the neutral current. When the neutral current flows out of the midpoint of the DC side, the direction of the neutral current is positive, otherwise it is negative. C represents the DC side capacitance. is the offset of the midpoint potential, is the voltage of the capacitor on the DC side, is the voltage of the capacitor on the DC side;

[0019] Using formula (3), the switching function of the three-phase bridge arm is obtained:

[0020] (3)

[0021] In formula (3): express The switching function of the phase bridge arm, , P represents The switching function of the phase bridge arm is in the first switch tube of the upper bridge arm. The second switch tube of the upper bridge arm is turned on. The switch state when conducting, O represents The switching function of the phase bridge arm is in the second switch tube of the upper bridge arm. The first switch tube of the lower bridge arm is turned on. The switch state when conducting, N represents The switching function of the phase bridge arm is in the first switch tube of the lower bridge arm. The second switch tube of the lower bridge arm is turned on. The switch state when conducting;

[0022] After discretizing equation (1) using forward Euler, the predicted value of the inductor current is obtained as shown in equation (4):

[0023] (4)

[0024] In formula (4), is the predicted value of the inductor current in the two-phase stationary coordinate system at time k+1, is the inductor current value in the two-phase stationary coordinate system at time k; To control the cycle; is the grid voltage value in the two-phase stationary coordinate system at time k;

[0025] After discretizing equation (2) using forward Euler, the expression of the predicted value of the midpoint potential offset is obtained as shown in equation (5):

[0026] (5)

[0027] In formula (5), , are respectively the predicted value of the midpoint potential shift at time k+1 and the shift value of the midpoint potential at time k; is the current value of the center line at time k; , are the voltage values ​​of the upper and lower capacitors on the DC side at time k+1 respectively; , are the voltage values ​​of the upper and lower capacitors on the DC side at time k respectively;

[0028] Using formula (6), we can obtain the reference prediction value of the inductor current at time k+1 in the two-phase stationary coordinate system: :

[0029] (6)

[0030] In formula (6), is the reference predicted value of the inductor current at time k in the two-phase stationary coordinate system, is the reference predicted value of the inductor current at time k-1 in the two-phase stationary coordinate system, is the reference predicted value of the inductor current at time k-2 in the two-phase stationary coordinate system;

[0031] After combining equations (4) and (6), we get the expression of the predicted value of the inverter output voltage as shown in equation (7):

[0032] (7)

[0033] In formula (7), is the predicted value of the inverter output voltage at time k.

[0034] Furthermore, in step 2, equation (8) is used to establish the corresponding relationship between the switch state and the basic synthetic voltage vector:

[0035] (8)

[0036] In formula (8), is the DC bus voltage, , , express The switching function of the phase bridge arm; , They are the projection components of the voltage vector corresponding to the switching state on the α-axis and β-axis respectively.

[0037] The types of basic composite voltage vectors include: zero vector, redundant small vector, medium vector and large vector; the three-phase switch states corresponding to different types of basic composite voltage vectors are determined according to formula (8):

[0038] The three-phase switch state corresponding to the zero vector is OOO;

[0039] The P-type three-phase switch states corresponding to the redundant small vector are POO, PPO, OPO, OPP, OOP, POP;

[0040] The N-type three-phase switch states corresponding to the redundant small vector are ONN, OON, NON, NOO, NNO, and ONO;

[0041] The three-phase switch states corresponding to the middle vector are PON, PNO, OPN, NPO, NOP, and PNP;

[0042] The three-phase switch states corresponding to the large vector are PNN, PPN, NPN, NPP, NNP, and PNP;

[0043] The three-phase switch states corresponding to the six basic synthetic voltage vectors in each sector are as follows:

[0044] The P-type and N-type three-phase switch states corresponding to the redundant small vectors in sector A are POO and ONN respectively; the three-phase switch states corresponding to the two middle vectors are PON and PNO respectively; the three-phase switch state corresponding to a zero vector is OOO; and the three-phase switch state corresponding to a large vector is PNN;

[0045] The P-type and N-type three-phase switch states corresponding to the redundant small vectors in sector B are PPO and OON respectively; the three-phase switch states corresponding to the two middle vectors are OPN and PON respectively; the three-phase switch state corresponding to a zero vector is OOO; and the three-phase switch state corresponding to a large vector is PPN;

[0046] The P-type and N-type three-phase switch states corresponding to the redundant small vectors in sector C are OPO and NON respectively; the three-phase switch states corresponding to the two middle vectors are NPO and OPN respectively; the three-phase switch state corresponding to a zero vector is OOO; and the three-phase switch state corresponding to a large vector is NPN;

[0047] The P-type and N-type three-phase switch states corresponding to the redundant small vectors in sector D are OPP and NOO respectively; the three-phase switch states corresponding to the two middle vectors are NOP and NPO respectively; the three-phase switch state corresponding to a zero vector is OOO; and the three-phase switch state corresponding to a large vector is NPP;

[0048] The P-type and N-type three-phase switch states corresponding to the redundant small vectors in sector E are OOP and NNO respectively; the three-phase switch states corresponding to the two middle vectors are ONP and NOP respectively; the three-phase switch state corresponding to a zero vector is OOO; and the three-phase switch state corresponding to a large vector is NNP;

[0049] The P-type and N-type three-phase switch states corresponding to the redundant small vectors in sector F are POP and ONO respectively; the three-phase switch states corresponding to the two middle vectors are PNO and ONP respectively; the three-phase switch state corresponding to a zero vector is OOO; and the three-phase switch state corresponding to a large vector is NPN.

[0050] Furthermore, the step 3 comprises:

[0051] when hour, Located in Sector A;

[0052] when hour, Located in sector B;

[0053] when hour, Located in sector C;

[0054] when ∪ hour, Located in sector D;

[0055] when hour, Located in Sector E;

[0056] when hour, Located in sector F.

[0057] Further, the step 4 comprises:

[0058] Step 4.1: Determine the optimal candidate vector using formula (9):

[0059] (9)

[0060] In formula (9), is the value function of the optimal candidate vector under the voltage constraint variable. When i=1, it means that The zero vector with the three-phase switch state of OOO in the sector is selected as the candidate vector and participates in the determination of the optimal candidate vector. When i=2, it means that A vector in the sector is selected as a candidate vector and participates in the determination of the optimal candidate vector. When i=3, it means that The remaining middle vector in the sector is used as a candidate vector and participates in the determination of the optimal candidate vector. When i=4, it means that The large vector in the sector is used as a candidate vector and participates in the determination of the optimal candidate vector;

[0061] Step 4.2: In each control cycle, calculate the value functions of the four candidate vectors in turn, select the candidate vector that minimizes the value function as the optimal candidate vector, and pair it with the central small vector to form a dual vector combination in each control cycle.

[0062] Further, the step 5 comprises:

[0063] Step 5.1: When n=1, use formula (10) to determine The value of the small center vector of the sector :

[0064] (10)

[0065] Step 5.2: Use formula (11) to obtain the action time of each of the two vectors:

[0066] (11)

[0067] In formula (11), for The action time of the central small vector of the sector in the control cycle, is the action time of the optimal candidate vector in the corresponding control period.

[0068] Further, the step 6 comprises:

[0069] The relationship between the neutral current and the three-phase output current is established using equation (12):

[0070] (12)

[0071] In formula (12): is the neutral current value at time k; is the output current of the bridge arm of phase A at time k; is the output current of the B-phase bridge arm at time k; is the output current of the C phase bridge arm at time k;

[0072] Using formula (13), we can get the expression of the average value of the neutral current:

[0073] (13)

[0074] In formula (13): is the average value of the neutral line current at time k; is the neutral current corresponding to the central small vector acting at time k; is the neutral current corresponding to the optimal candidate vector at time k; is the time allocation factor of the center small vector;

[0075] When the predicted value of the midpoint potential offset is 0, the central small vector time allocation factor is established using formula (14): The expression is:

[0076] (14)

[0077] Using formula (15), we can get the central small vector P-type three-phase switch state: The action time and the action time of N-type three-phase switch state :

[0078] (15).

[0079] An electronic device of the present invention includes a memory and a processor, wherein the memory is used to store a program that supports the processor to execute the method for co-optimizing the output current and midpoint potential of a three-level inverter based on predictive control, and the processor is configured to execute the program stored in the memory.

[0080] The present invention provides a computer-readable storage medium, and the computer-readable storage medium stores a computer program, wherein when the computer program is executed by a processor, the steps of the method for coordinated optimization of output current and midpoint potential of a three-level inverter based on predictive control are executed.

[0081] Compared with the prior art, the present invention has the following beneficial effects:

[0082] 1. The present invention adopts a dual-vector modulation strategy to evaluate and select a dual-vector combination in each control cycle, thereby achieving constant switching frequency control, overcoming the limitation of traditional single-vector model prediction that relies on high sampling frequency, thereby reducing output current harmonics and reducing the difficulty of filter design.

[0083] 2. The present invention effectively reduces the switching loss of the inverter by lowering the sampling frequency, thereby improving the inverter operation efficiency and the controller processing efficiency.

[0084] 3. The present invention makes full use of the redundant characteristics of small vectors, which is beneficial to reducing the coupling effect between the two control objectives of grid-connected current and capacitor voltage balance control, so that the value function only needs to evaluate the control objective of current command tracking, and unified optimization control of multiple control objectives can be achieved without introducing weight factors, thereby improving the accuracy and efficiency of prediction and facilitating practical engineering applications.

[0085] 4. By optimizing the algorithm structure, the present invention can determine the dual-vector combination through four rolling optimization calculations of the value function in each control cycle, effectively avoiding repeated and invalid calculations, reducing the complexity of calculations, and helping to improve the dynamic response performance and steady-state control accuracy of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0086] Figure 1 This is a topological diagram of an NPC type three-level grid-connected inverter of the present invention;

[0087] Figure 2 A sector diagram of a basic voltage vector of the present invention;

[0088] Figure 3 The switching sequence and vector action time diagram of sector A

[0089] Figure 4 The switching sequence diagram of 24 groups of dual-vector combination switches of the present invention;

[0090] Figure 5 The control framework diagram of the three-level grid-connected inverter based on predictive control of the present invention;

[0091] Figure 6 The DC side midpoint potential offset voltage waveform diagram of the present invention;

[0092] Figure 7 Predict the DC side midpoint potential offset voltage waveform for the traditional single vector model;

[0093] Figure 8 It is a three-phase inductor current output waveform diagram of the present invention;

[0094] Fig. 9 The three-phase inductor current output waveform predicted by the traditional single-vector model. DETAILED DESCRIPTION

[0095] In this embodiment, in order to solve the limitations of the traditional finite set model prediction technology (FCS-MPC) applied to high-power grid-connected inverters, a method for coordinated optimization of the output current and midpoint potential of a three-level grid-connected inverter based on predictive control is proposed. First, a continuous state mathematical model of an NPC type three-level grid-connected inverter in a two-phase stationary coordinate system is established, and then the control system is discretized to obtain the expression of the predicted value of the midpoint potential offset and the inverter output voltage. Secondly, a dual-vector modulation strategy is adopted to eliminate the limitation of the traditional single-vector model prediction relying on high sampling frequency. After determining the sector where the reference voltage is located, each control cycle only needs to undergo 4 rolling optimization calculations to determine the switching sequence of the selected dual vector, ensuring the constant switching frequency and greatly reducing the amount of calculation. Finally, after the action time of the central small vector and the optimal candidate vector in each control cycle is obtained by the modulation model prediction, the action time of the redundant small vector is adjusted by using the calculated time allocation factor formula to achieve balanced coordinated optimization control of the output current and midpoint potential, avoiding the introduction of weight factors to increase the complexity of setting. Specifically, the following steps are included:

[0096] Step 1: NPC type three-level grid-connected inverter topology Figure 1 As shown, first, the continuous time state mathematical model in the two-phase stationary coordinate system is shown in formula (1):

[0097] (1)

[0098] In formula (1), The inverter output voltage , , In the expression under the two-phase stationary coordinate system, L is the filter inductance, R is the parasitic resistance of the filter inductance, is the inductor current in the two-phase stationary coordinate system, is the grid voltage in the two-phase stationary coordinate system.

[0099] Step 2: Define the midpoint potential offset as shown in formula (2):

[0100] (2)

[0101] In formula (2) is the midpoint potential offset, is the capacitor voltage on the DC side, is the capacitor voltage on the DC side.

[0102] Step 3: Construct the dynamic constraint equation of the DC side upper and lower capacitor voltages and the neutral line current as shown in equation (3):

[0103] (3)

[0104] In formula (3), The neutral current is defined as positive when the neutral current flows out of the midpoint of the DC side, and negative when it does not. is the capacitance on the DC side; is the DC side lower capacitance; for the convenience of analysis, .

[0105] Step 4: Construct the three-phase bridge arm switching function as shown in formula (4):

[0106] (4)

[0107] In formula (3): express Phase switching function, , P represents Phase switching function in the upper arm first switch tube , the second switch tube of the upper bridge arm The state when conducting, O means Phase switching function in the second switch tube of the upper bridge arm , the first switch tube of the lower bridge arm The state when conducting, N represents Phase switch function lower bridge arm first switch tube , the second switch tube of the lower bridge arm The state when conducting;

[0108] Step 5: Use equation (4) to establish the relationship between the neutral current and the three-phase output current as shown in equation (5):

[0109] (5)

[0110] in, is the inductor current, .

[0111] Step 6: Use forward Euler to discretize equations (3) and (1) to obtain the predicted value of the midpoint potential offset and the predicted value of the inductor current as shown in equations (6) and (7):

[0112] (6)

[0113] (7)

[0114] in, , They are respectively the predicted value of the midpoint potential shift at time k+1 and the midpoint potential shift value at time k; is the neutral current value at time k; , They are the voltage values ​​of the upper and lower capacitors on the DC side at time k+1 respectively; , They are the voltage values ​​of the upper and lower capacitors on the DC side at time k respectively; is the predicted value of the inductor current in the two-phase stationary coordinate system at time k+1, is the inductor current value in the two-phase stationary coordinate system at time k; is the control system sampling period;

[0115] is the grid voltage value in the two-phase stationary coordinate system at time k.

[0116] Step 7: Obtain the inductor current reference prediction value shown in formula (8) by Lagrangian trend extrapolation method:

[0117] (8)

[0118] In formula (8), is the reference predicted value of the inductor current at time k+1 in the two-phase stationary coordinate system, is the inductor current reference value at time k in the two-phase stationary coordinate system, is the inductor current reference value at time k-1 in the two-phase stationary coordinate system, It is the reference value of the inductor current at time k-2 in the two-phase stationary coordinate system.

[0119] Step 8: By combining equations (7) and (8) through deadbeat control, the prediction equation based on the voltage constraint variable at time k can be obtained as shown in equation (9):

[0120] (9)

[0121] In formula (9), is the predicted value of the inverter output voltage at time k.

[0122] Step 9: According to formula (4), the NPC three-level topology has 27 different switching states. Each switching state corresponds to a basic synthetic voltage vector. The projection calculation method of the voltage vector corresponding to each switching state on the αβ axis is shown in formula (10):

[0123] (10)

[0124] In formula (10), is the DC bus voltage.

[0125] The basic synthetic vectors are divided into four categories according to the size of the modulus: zero vector, small vector, medium vector, and large vector; among them, the small vectors with the same output voltage polarity and modulus have redundant states, and are divided into P-type switching states and N-type switching states according to different switching states. The neutral current values ​​they generate are the same but in opposite directions. The switching states and neutral currents generated by different types of basic synthetic vectors are shown in Table 1:

[0126] Table 1 The switching states corresponding to different types of vectors and the resulting neutral current

[0127]

[0128] Step 10: The present invention divides the 60° area between each two adjacent vectors into a sector, so that The coordinate system is divided into six sectors, and the inverter output voltage prediction value The angle θ with the positive semi-axis of α determines the output voltage prediction value The sector is divided into the following sectors: Figure 2 shown.

[0129] The present invention adopts a dual-vector modulation strategy. In order to reduce the complexity of zero vector selection and avoid the influence of switch states PPN and NNN on the common mode voltage, the present invention only selects the zero vector with a switch state of OOO. In order to achieve constant switching frequency control and fully utilize the redundancy characteristics of small vectors to suppress the midpoint potential offset, Taking the first sector as an example, a pair of redundant small vectors in the sector is used as the central small vector, and the remaining one zero vector (OOO), two middle vectors and one large vector in the sector are used as candidate vectors. The present invention selects only the central small vector and the optimal candidate vector to form dual vector modulation in each control cycle, and adopts a five-stage switching sequence. Then, the four groups of dual vector combination switch switching sequences that can be selected by the Ath sector in each control cycle are as follows:

[0130] The first set of dual-vector combination switch switching sequence: ONN→OOO→POO→OOO→ONN;

[0131] The second set of dual-vector combination switch switching sequence: ONN→PON→POO→PON→ONN;

[0132] The third set of dual-vector combination switch switching sequence: ONN→PNO→POO→PNO→ONN;

[0133] The fourth group of dual-vector combination switch switching sequence: ONN→PNN→POO→PNN→ONN;

[0134] Step 11: Determine the optimal candidate vector as shown in formula (11) based on the value function based on the voltage constraint variable:

[0135] (11)

[0136] In formula (11), is the value function based on the voltage constraint variable. In the Ath sector, i=1 means that the voltage vector with the switch state of OOO is selected to participate in the evaluation, i=2 means that the voltage vector with the switch state of PON is selected to participate in the evaluation, i=3 means that the voltage vector with the switch state of PNO is selected to participate in the evaluation, and i=4 means that the voltage vector with the switch state of PNN is selected to participate in the evaluation.

[0137] Step 12: In each control cycle, the value functions of the four candidate vectors are evaluated in turn, and the candidate vector with the smallest value function is selected as the optimal candidate vector, and paired with the central small vector to form a dual vector combination.

[0138] The present invention aims to achieve the coordinated optimization control of the two control objectives of reducing the output current harmonics and balancing the midpoint potential on the basis of a fixed switching frequency. The above-mentioned dual-vector modulation strategy helps to determine the sector where the reference voltage is located. After that, the dual-vector combination can be selected by only four rolling optimization calculations in each control cycle, which optimizes the algorithm structure, greatly reduces the complexity of the calculation, improves the dynamic response performance and steady-state control accuracy of the system, and ensures the constant control of the switching frequency, reduces the output current harmonics, and reduces the difficulty of filter design. At the same time, the present invention makes full use of the redundant characteristics of small vectors, and directly realizes the balanced control of the DC side midpoint potential by adjusting the action time of the redundant small vectors, thereby avoiding the introduction of weight factors in the value function to reflect the balanced control of the midpoint potential, resulting in complex weight factor optimization problems.

[0139] Step 13, also taking the Ath sector as an example, after the central small vector and the optimal candidate vector are determined in each control cycle, the vector action time is obtained through the modulation model predictive control, that is, the vector action time is calculated according to the inverse of the value function. The larger the value function, the shorter the vector action time in the corresponding control cycle, and vice versa. The value function of the central small vector (POO, ONN) based on the voltage constraint variable is shown in formula (12). The central small vector and the optimal candidate vector time shown in formula (13) are obtained by connecting formula (11) and formula (12) in parallel:

[0140] (12)

[0141] (13)

[0142] in, is the value function of the small vector at the center of the Ath sector based on the voltage constraint variable; is the action time of the central small vector in the I sector control cycle, is the action time of the optimal candidate vector in the corresponding control period.

[0143] Step 14: Figure 3 The double vector time allocation diagram of sector A is shown in the figure. The present invention makes the midpoint potential offset the predicted value , the time allocation factor is used to adjust the action time of the two switching states of the central small vector to achieve the DC side midpoint potential balance control, and the relationship between the neutral line current and the three-phase output current is established using formula (14):

[0144] (14)

[0145] In formula (14): is the neutral current value at time k; is the output current of the bridge arm of phase A at time k; is the output current of the B-phase bridge arm at time k; is the output current of the C phase bridge arm at time k.

[0146] Step 15: Use formula (15) to get the neutral line current value expression:

[0147] (15)

[0148] In formula (15): is the average value of the neutral line current at time k; is the neutral current corresponding to the central small vector acting at time k; is the neutral current corresponding to the optimal candidate vector at time k; Time distribution factor for the center vector.

[0149] Step 16: Use formula (16) to establish the central small vector time allocation factor The expression is:

[0150] (16)

[0151] Step 17, using formula (17) to obtain the action time of the central small vector P-type switch state and N-type switch state:

[0152] (17)

[0153] In formula (17): is the action time of the central small vector P-type switch state; is the action time of the center small vector N-type switch state.

[0154] Similarly, 20 sets of dual-vector combinations can be obtained for the BF sector. The dual-vector combination switch sequence is as follows: Figure 4As shown in the figure, the time allocation factor is obtained by the same method. After determining the sector where the output voltage is located, the central small vector of each sector can be directly determined. In each control cycle, only four rolling optimization calculations are required to determine the optimal candidate vector, and then determine the dual vector combination participating in each control cycle. The dual vector action time is predicted by the modulation model, and the redundant small vector action time is adjusted by the time allocation factor to achieve capacitor voltage balancing control. The control framework of the three-level grid-connected inverter based on predictive control is shown in Figure 1. Figure 5 As shown in the figure.

[0155] The following are the main parameters in this embodiment:

[0156]

[0157] The control frequency adopted by the present invention is 3kHz, which reduces switching losses and improves the operating efficiency of the inverter on the one hand, and verifies that the present invention can still maintain the steady-state control accuracy of the system at a low control frequency on the other hand. Figure 6 The midpoint potential shift waveform diagram using the modulation strategy of the present invention is shown in FIG. Figure 7 The waveform diagram of the midpoint potential offset predicted by the traditional single vector model under the same circuit parameter conditions shows that the maximum deviation of the midpoint potential using the method proposed in the present invention is about 0.5%, while the maximum deviation of the midpoint potential using the traditional method is about 2.3%, indicating that the present invention can achieve balanced control of the DC side capacitor voltage by adjusting the action time of the redundant small vector by the time allocation factor. Figure 8 The inductor current waveform diagram using the modulation strategy of the present invention has an inductor current amplitude of about 2063A and a THD of about 2.83%. Fig. 9 The inductor current waveform of the traditional control strategy is about 2049A, and the THD is about 5.21%, which shows that the method used in the present invention makes the inverter output current harmonic content smaller. Figure 4 It can be inferred that the switching frequency of the inverter is fixed, so the present invention can achieve coordinated optimization control of the two control objectives of small output current harmonics and balanced control of DC side capacitor voltage on the basis of fixed switching frequency.

Claims

1. A method for collaboratively optimizing output current and midpoint potential of a three-level inverter based on predictive control, characterized in that: The following steps are involved: Step 1: Establish a continuous state mathematical model of an NPC type three-level inverter in a two-phase stationary coordinate system, and after discretizing the continuous state mathematical model, obtain an expression for a predicted value of a midpoint potential offset and an expression for a predicted value of an inverter output voltage; Step 2: Establish the corresponding relationship between the switch state and the basic composite voltage vector, determine the switch state corresponding to different types of basic composite voltage vectors, and then divide the 60° area between each two adjacent middle vectors into a sector, so as to divide the two-phase stationary coordinate system into AF sectors, and each sector has a total of 6 basic composite voltage vectors; Step 3: Based on the predicted voltage output of the three-level inverter at time k The angle θ(k) between the positive half axis of α and the two-phase stationary coordinate system is determined by The sector in which it is located; Step 4: The redundant small vector in each sector is used as the central small vector, and the remaining four vectors in each sector are used as candidate vectors. The optimal candidate vector in the candidate vectors of each sector is determined by a value function based on the voltage constraint variable, so as to form a dual vector combination of each sector with the central small vector of the corresponding sector. Step 5, calculating the respective action time of each vector in the double vector combination; Step 6: When the predicted value of the midpoint potential offset is 0, construct the time allocation factor expression and calculate the time for the central small vector to act in each of the two switching states.

2. The method for coordinated optimization of output current and midpoint potential of a three-level inverter based on predictive control according to claim 1, characterized in that: The step 1 comprises: Using equation (1), the continuous-time state model of the three-level inverter in the two-phase stationary coordinate system is obtained: (1) In formula (1), is the three-phase output voltage of the inverter , , In the expression under the two-phase stationary coordinate system, L is the filter inductance, R is the parasitic resistance of the filter inductance, is the inductor current in the two-phase stationary coordinate system, is the grid voltage in the two-phase stationary coordinate system; Using equation (2), the dynamic constraint equations of the upper and lower capacitor voltages and the neutral current on the DC side are obtained: (2) In formula (2), is the neutral current. When the neutral current flows out of the midpoint of the DC side, the direction of the neutral current is positive, otherwise it is negative. C represents the DC side capacitance. is the offset of the midpoint potential, is the voltage of the capacitor on the DC side, is the voltage of the capacitor on the DC side; Using formula (3), the switching function of the three-phase bridge arm is obtained: (3) In formula (3): express The switching function of the phase bridge arm, , P represents The switching function of the phase bridge arm is in the first switch tube of the upper bridge arm. The second switch tube of the upper bridge arm is turned on. The switch state when conducting, O represents The switching function of the phase bridge arm is in the second switch tube of the upper bridge arm. The first switch tube of the lower bridge arm is turned on. The switch state when conducting, N represents The switching function of the phase bridge arm is in the first switch tube of the lower bridge arm. The second switch tube of the lower bridge arm is turned on. The switch state when conducting; After discretizing equation (1) using forward Euler, the predicted value of the inductor current is obtained as shown in equation (4): (4) In formula (4), is the predicted value of the inductor current in the two-phase stationary coordinate system at time k+1, is the inductor current value in the two-phase stationary coordinate system at time k; To control the cycle; is the grid voltage value in the two-phase stationary coordinate system at time k; After discretizing equation (2) using forward Euler, the expression of the predicted value of the midpoint potential offset is obtained as shown in equation (5): (5) In formula (5), , are respectively the predicted value of the midpoint potential shift at time k+1 and the shift value of the midpoint potential at time k; is the current value of the center line at time k; , are the voltage values ​​of the upper and lower capacitors on the DC side at time k+1 respectively; , are the voltage values ​​of the upper and lower capacitors on the DC side at time k respectively; Using formula (6), we can obtain the reference prediction value of the inductor current at time k+1 in the two-phase stationary coordinate system: : (6) In formula (6), is the reference predicted value of the inductor current at time k in the two-phase stationary coordinate system, is the reference predicted value of the inductor current at time k-1 in the two-phase stationary coordinate system, is the reference predicted value of the inductor current at time k-2 in the two-phase stationary coordinate system; After combining equations (4) and (6), we get the expression of the predicted value of the inverter output voltage as shown in equation (7): (7) In formula (7), is the predicted value of the inverter output voltage at time k.

3. The method for coordinated optimization of output current and midpoint potential of a three-level inverter based on predictive control according to claim 1, characterized in that: In step 2, equation (8) is used to establish the corresponding relationship between the switch state and the basic synthetic voltage vector: (8) In formula (8), is the DC bus voltage, , , express The switching function of the phase bridge arm; , are the projection components of the voltage vector corresponding to the switching state on the α-axis and β-axis respectively; The types of basic synthetic voltage vectors include: zero vector, redundant small vector, medium vector and large vector; According to formula (8), the three-phase switch states corresponding to different types of basic synthetic voltage vectors are determined: The three-phase switch state corresponding to the zero vector is OOO; The P-type three-phase switch states corresponding to the redundant small vector are POO, PPO, OPO, OPP, OOP, POP; The N-type three-phase switch states corresponding to the redundant small vector are ONN, OON, NON, NOO, NNO, and ONO; The three-phase switch states corresponding to the middle vector are PON, PNO, OPN, NPO, NOP, and PNP; The three-phase switch states corresponding to the large vector are PNN, PPN, NPN, NPP, NNP, and PNP; The three-phase switch states corresponding to the six basic synthetic voltage vectors in each sector are as follows: The P-type and N-type three-phase switch states corresponding to the redundant small vectors in sector A are POO and ONN respectively; the three-phase switch states corresponding to the two middle vectors are PON and PNO respectively; the three-phase switch state corresponding to a zero vector is OOO; and the three-phase switch state corresponding to a large vector is PNN; The P-type and N-type three-phase switch states corresponding to the redundant small vectors in sector B are PPO and OON respectively; the three-phase switch states corresponding to the two middle vectors are OPN and PON respectively; the three-phase switch state corresponding to a zero vector is OOO; and the three-phase switch state corresponding to a large vector is PPN; The P-type and N-type three-phase switch states corresponding to the redundant small vectors in sector C are OPO and NON respectively; the three-phase switch states corresponding to the two middle vectors are NPO and OPN respectively; the three-phase switch state corresponding to a zero vector is OOO; and the three-phase switch state corresponding to a large vector is NPN; The P-type and N-type three-phase switch states corresponding to the redundant small vectors in sector D are OPP and NOO respectively; the three-phase switch states corresponding to the two middle vectors are NOP and NPO respectively; the three-phase switch state corresponding to a zero vector is OOO; and the three-phase switch state corresponding to a large vector is NPP; The P-type and N-type three-phase switch states corresponding to the redundant small vectors in sector E are OOP and NNO respectively; the three-phase switch states corresponding to the two middle vectors are ONP and NOP respectively; the three-phase switch state corresponding to a zero vector is OOO; and the three-phase switch state corresponding to a large vector is NNP; The P-type and N-type three-phase switch states corresponding to the redundant small vectors in sector F are POP and ONO respectively; the three-phase switch states corresponding to the two middle vectors are PNO and ONP respectively; the three-phase switch state corresponding to a zero vector is OOO; and the three-phase switch state corresponding to a large vector is NPN.

4. The method for coordinated optimization of output current and midpoint potential of a three-level inverter based on predictive control according to claim 1, characterized in that: The step 3 comprises: when hour, Located in Sector A; when hour, Located in sector B; when hour, Located in sector C; when ∪ hour, Located in sector D; when hour, Located in Sector E; when hour, Located in sector F.

5. The method for coordinated optimization of output current and midpoint potential of a three-level inverter based on predictive control according to claim 3, characterized in that: The step 4 comprises: Step 4.1: Determine the optimal candidate vector using formula (9): (9) In formula (9), is the value function of the optimal candidate vector under the voltage constraint variable. When i=1, it means that The zero vector with the three-phase switch state of OOO in the sector is selected as the candidate vector and participates in the determination of the optimal candidate vector. When i=2, it means that A vector in the sector is selected as a candidate vector and participates in the determination of the optimal candidate vector. When i=3, it means that The remaining middle vector in the sector is used as a candidate vector and participates in the determination of the optimal candidate vector. When i=4, it means that The large vector in the sector is used as a candidate vector and participates in the determination of the optimal candidate vector; Step 4.2: In each control cycle, calculate the value functions of the four candidate vectors in turn, select the candidate vector that minimizes the value function as the optimal candidate vector, and pair it with the central small vector to form a dual vector combination in each control cycle.

6. The method for coordinated optimization of output current and midpoint potential of a three-level inverter based on predictive control according to claim 4, characterized in that: The step 5 comprises: Step 5.1: When n=1, use formula (10) to determine The value of the small vector at the center of the sector : (10) Step 5.2: Use formula (11) to obtain the action time of each of the two vectors: (11) In formula (11), for The action time of the central small vector of the sector in the control cycle, is the action time of the optimal candidate vector in the corresponding control period.

7. The method for coordinated optimization of output current and midpoint potential of a three-level inverter based on predictive control according to claim 6, characterized in that: The step 6 comprises: The relationship between the neutral current and the three-phase output current is established using equation (12): (12) In formula (12): is the neutral current value at time k; is the output current of the bridge arm of phase A at time k; is the output current of the B-phase bridge arm at time k; is the output current of the C phase bridge arm at time k; Using formula (13), we can get the expression of the average value of the neutral current: (13) In formula (13): is the average value of the neutral line current at time k; is the neutral current corresponding to the central small vector acting at time k; is the neutral current corresponding to the optimal candidate vector at time k; is the time allocation factor of the center small vector; When the predicted value of the midpoint potential offset is 0, the central small vector time allocation factor is established using formula (14): The expression is: (14) Using formula (15), we can get the central small vector P-type three-phase switch state: The action time and the action time of N-type three-phase switch state : (15)。 8. An electronic device, comprising a memory and a processor, characterized in that: The memory is used to store a program that supports the processor to execute the method for collaboratively optimizing the output current and midpoint potential of a three-level inverter based on predictive control as described in any one of claims 1-7, and the processor is configured to execute the program stored in the memory.

9. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of the method for coordinated optimization of output current and midpoint potential of a three-level inverter based on predictive control described in any one of claims 1 to 7 are executed.

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