Method for balancing the neutral point potential of a three-level inverter with active leg based on ripple compensation

Abstract

The application discloses a three-level inverter neutral point potential balance method based on a ripple compensation active branch, which comprises the following steps: taking a full-bridge inverter as a ripple compensation active branch containing to-be-designed parameters, and introducing the ripple compensation active branch into a preset three-level inverter; connecting the AC side of the ripple compensation active branch in series between the bus capacitor of the three-level inverter and the negative bus; the AC side of the ripple compensation active branch is used for generating compensation ripples; a design strategy for balancing the neutral point potential of the three-level inverter is constructed to design the to-be-designed parameters in the ripple compensation active branch, so that the neutral point potential of the three-level inverter is balanced based on the ripple compensation active branch. The application solves the problem that the existing method causes the system cost to increase and the efficiency to decrease due to the introduction of additional hardware circuits under extreme working conditions such as high modulation ratio and low power factor.

Description

Technical Field

[0001] This invention relates to the field of power electronic converter technology, and in particular to a method for balancing the midpoint potential of a three-level inverter based on an active branch with ripple compensation. Background Technology

[0002] Three-level inverters have gained increasing attention and research in fields such as high-power high-voltage frequency conversion, photovoltaic power generation, wind power generation, and electric drive for new energy vehicles due to their advantages such as low voltage stress, high power quality, and high efficiency. In three-level inverters, neutral point potential balance is fundamental to maintaining the safe, stable, and efficient operation of the system. When the neutral point potential deviates, the voltage stress on the switching devices increases, which can lead to device breakdown and system failure in severe cases. It also results in reduced output current quality and a significantly shortened lifespan of the bus capacitors.

[0003] Current research categorizes midpoint potential balancing methods into two main types: software and hardware. Software solutions maintain midpoint potential balance by optimizing the modulation strategy through methods such as dynamic allocation of redundant small vectors and zero-sequence component injection. These solutions do not require additional hardware costs, but they lose their balance control capability under extreme conditions such as high modulation ratios and low power factors. Hardware solutions maintain midpoint potential balance by adding auxiliary branches such as buck-boost to create additional energy flow channels between the two bus capacitors. These solutions can still effectively balance the midpoint potential under extreme conditions such as high modulation ratios and low power factors, but the introduction of additional hardware circuitry leads to increased system costs and reduced efficiency. Summary of the Invention

[0004] This invention provides a method for balancing the midpoint potential of a three-level inverter based on an active branch with ripple compensation, in order to overcome the above-mentioned technical problems.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows: A method for balancing the neutral point potential of a three-level inverter based on an active branch with ripple compensation specifically includes the following steps: S1: Construct a ripple compensation active branch containing a full-bridge inverter with unknown design parameters, and introduce it into a pre-set three-level inverter; connect the AC side of the ripple compensation active branch in series between the bus capacitor and the negative bus of the three-level inverter; the AC side of the ripple compensation active branch is used to generate compensation ripple. The design parameters include the capacitance value of the bus capacitor connected in series with the AC side of the ripple compensation active branch, the capacitance value of the DC side capacitor of the full-bridge inverter, and the voltage stability value of the DC side capacitor of the full-bridge inverter. S2: Construct a design strategy for balancing the midpoint potential of a three-level inverter, in order to design the parameters to be designed in the ripple compensation active branch, and thus achieve the balance of the midpoint potential of the three-level inverter based on the ripple compensation active branch.

[0006] Furthermore, the design strategy for balancing the midpoint potential in a three-level inverter, constructed in S2, specifically includes the following steps: S21: Model the midpoint current of the three-level inverter to obtain the midpoint current model; rewrite the midpoint current model by introducing harmonic amplitude to obtain a simplified midpoint current model; Based on the simplified midpoint current model, the bus capacitance flowing through the AC side of the ripple-compensated active branch is obtained. current ; S22: Based on the capacitance flowing through the busbar current Obtain the change in charge According to the change in charge Obtain bus capacitance Voltage change at both ends According to the voltage change Obtain the AC side output voltage of the ripple-compensated active branch. ; S23: Power balancing based on the full-bridge inverter, according to the AC side output voltage. DC-side capacitor of the ripple compensation active branch Current flowing through Obtain the DC side capacitor Voltage at both ends ; S24: Modulation ratio based on full-bridge inverter According to the AC side output voltage Voltage across the DC-side capacitor Cb Obtain the first constraint condition used to implement ripple compensation; S25: Derivation of bus capacitance based on S24 Capacitance value and DC-side capacitor of full-bridge inverter voltage stability value The relationship between them is used to obtain a second constraint for achieving ripple compensation; S26: Based on the first and second constraints, the parameters to be designed are selected; and the parameters to be designed include the bus capacitance. The capacitance value of the DC-side capacitor of the full-bridge inverter capacitance and capacitor Voltage stability value .

[0007] Furthermore, step S21 specifically includes the following steps: S211: Model the midpoint current of the three-level inverter, and obtain the midpoint current model as follows:

[0008]

[0009]

[0010] In the formula: This represents the midpoint current of a three-level inverter; This represents the modulation wave of the three phases a, b, and c of a three-level inverter; Indicates the three-phase output current; Indicates the modulation ratio; Indicates the output current amplitude; Indicates the power factor angle; Indicates the fundamental frequency angle and ; Indicates the fundamental frequency; Indicates time; S212: By introducing harmonic amplitude, the midpoint current model is rewritten with the third harmonic component as follows:

[0011] In the formula: Indicates the amplitude of the third harmonic; Indicates the third harmonic phase; The midpoint current model is simplified based on the third harmonic component, and the simplified midpoint current model is obtained as follows:

[0012] S213: Based on the simplified midpoint current model, obtain the bus capacitance connected in series with the AC side of the ripple compensation active branch. current for: .

[0013] Furthermore, step S22 specifically includes the following steps: S221: Based on the capacitance flowing through the busbar current Obtain the change in charge for:

[0014] In the formula: Represents electric charge The change in; This represents the initial charge at time t=0; S222: Based on the change in charge Obtain bus capacitance Voltage change at both ends for:

[0015] S223: Based on voltage change Obtain the AC side output voltage of the ripple-compensated active branch. : .

[0016] Furthermore, step S23 specifically includes the following steps: S231: The expression for obtaining the power balance of the full-bridge inverter is:

[0017] In the formula: Indicates AC side capacitance Voltage at both ends; This indicates the current flowing through the AC side capacitor; Indicates DC side capacitor Voltage at both ends; This indicates that current flows through the DC-side capacitor and ; S232: Power balancing based on the full-bridge inverter, according to the AC side output voltage. DC-side capacitor of the ripple compensation active branch Current flowing through The voltage across the DC-side capacitor Cb is obtained. The differential equation is:

[0018] The differential equation can be rewritten as:

[0019] Integrating both sides of the rewritten differential equation, we obtain the integrated equation:

[0020] The voltage across the DC-side capacitor Cb can be obtained from the integrated equation. The expression is:

[0021] In the formula: Denotes the constant to be solved, and when At that time, Defined as target stable value able to obtain ; and then obtain the DC-side capacitor Voltage at both ends for:

[0022] In the formula: express The target stable value of the voltage across the terminals.

[0023] Furthermore, step S24 specifically includes the following steps: S241: Based on the AC side output voltage Voltage across the DC-side capacitor Cb The modulation ratio of the full-bridge inverter is obtained. And let Its expression is:

[0024] S242: Solve the expression for S241 and obtain the first constraint condition for implementing ripple compensation as follows: .

[0025] Furthermore, step S25 specifically includes the following steps: S251: Derivation of bus capacitance based on S24 Capacitance value and DC-side capacitor of full-bridge inverter voltage stability value The relationship between them can be expressed as:

[0026] S252: Based on S251, the second constraint for achieving ripple compensation is further obtained as follows: .

[0027] Beneficial Effects: This invention provides a method for balancing the midpoint potential of a three-level inverter based on an active branch with ripple compensation. The auxiliary branch, i.e., the active branch with ripple compensation, compensates for the ripple component of the bus capacitor voltage in the three-level inverter. The AC side output of the auxiliary branch full-bridge inverter is controlled to produce a ripple signal with the same amplitude but opposite direction to the ripple component of the bus capacitor voltage. This ensures that the voltage between the neutral point and the negative bus remains stable at half the bus voltage even when the bus capacitor value is small, achieving midpoint potential balance in the three-level inverter. This invention avoids high losses in switching devices caused by large compensation currents, reduces power losses in the introduced auxiliary circuit, and contributes to improving the overall system efficiency. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 This is a flowchart of the method for balancing the midpoint potential of a three-level inverter based on an active branch with ripple compensation, according to the present invention. Figure 2 This is a schematic diagram of the structure of a three-level inverter based on a ripple-compensated active auxiliary branch introduced in this embodiment; Figure 3 This is a schematic diagram of the auxiliary branch control loop in this embodiment; Figure 4 This is a waveform diagram of the midpoint voltage before the introduction of the auxiliary branch in this embodiment; Figure 5 This is a waveform diagram of the midpoint voltage after introducing an auxiliary branch in this embodiment; Figure 6 This is a waveform diagram of the DC-side capacitor voltage of the full-bridge inverter after the introduction of the auxiliary branch in this embodiment. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] This embodiment addresses the issue of reduced overall efficiency in hardware solutions for midpoint potential balancing by proposing a midpoint potential balancing method for three-level inverters based on an active branch with ripple compensation. In a three-level inverter, when the bus capacitor value is too small to maintain a stable voltage across its terminals, the midpoint potential fluctuates. At this time, the voltages across both the upper and lower capacitors pulsate at a low frequency at half the bus voltage. This method, based on the idea of ​​ripple compensation, introduces a full-bridge inverter as an auxiliary branch connected in series between the lower bus capacitor and the negative bus. This auxiliary branch generates a ripple with the same amplitude but opposite direction as the ripple of the lower bus capacitor, thereby stabilizing the lower branch voltage at half the bus voltage. Since the sum of the upper and lower branch voltages is a constant bus voltage, the upper branch voltage is also clamped at half the bus voltage, thus achieving midpoint potential balancing in the three-level inverter. Most existing hardware solutions connect the auxiliary branch in parallel with the bus capacitor branch, controlling the auxiliary branch to generate a compensation current in the opposite direction to the current in the bus capacitor branch to offset midpoint fluctuations. As the output current increases, the midpoint current also increases, significantly increasing the losses of the switching devices through which the required compensation current flows, thus further reducing the overall system efficiency. Hardware solutions based on current compensation generally suffer from this problem. This invention, based on a novel voltage compensation approach, avoids the high losses of switching devices caused by large compensation currents, reduces the power losses of the introduced auxiliary circuit, and contributes to improving the overall system efficiency.

[0032] To achieve the above objectives, this embodiment provides a method for balancing the midpoint potential of a three-level inverter based on an active branch with ripple compensation, such as... Figure 1 As shown, the specific steps include: S1: Construct a ripple compensation active branch containing a full-bridge inverter with unknown design parameters, and introduce it into a pre-set three-level inverter; connect the AC side of the ripple compensation active branch in series between the bus capacitor and the negative bus of the three-level inverter; the AC side of the ripple compensation active branch is used to generate compensation ripple. The design parameters include the capacitance value of the bus capacitor connected in series with the AC side of the ripple compensation active branch, the capacitance value of the DC side capacitor of the full-bridge inverter, and the voltage stability value of the DC side capacitor of the full-bridge inverter. Specifically, such as Figure 2 As shown, based on the concept of ripple compensation, this embodiment introduces a full-bridge inverter as an auxiliary branch, and connects the AC side of the full-bridge inverter in series with the lower bus capacitor. Between the upper and lower busbars; where the capacitances of the upper and lower busbars are respectively The AC side of the full-bridge inverter is used to generate ripple of a specific amplitude. In this embodiment, a small capacitor, Cab, is selected. The DC side of the full-bridge inverter needs to maintain a stable capacitor voltage, so a capacitor with a certain capacitance value is selected. ; S2: Construct a design strategy for balancing the midpoint potential of a three-level inverter, in order to design the parameters to be designed in the ripple compensation active branch, and thus achieve the balance of the midpoint potential of the three-level inverter based on the ripple compensation active branch. Specifically, in this embodiment, in order to ensure that the auxiliary branch can generate a ripple signal with the same amplitude and opposite direction as the voltage ripple across the lower bus capacitor C2, the capacitance value of C2 needs to be adjusted. Cb capacitance and the stable voltage across Cb Parameter design includes the following steps: S21: Model the midpoint current of the three-level inverter to obtain the midpoint current model; rewrite the midpoint current model by introducing harmonic amplitudes to obtain a simplified midpoint current model; based on the simplified midpoint current model, obtain the bus capacitor connected in series with the AC side of the ripple compensation active branch. current The specific steps include: S211: Model the midpoint current of the three-level inverter, and obtain the midpoint current model as follows: (1) (2) (3) In the formula: This represents the midpoint current of a three-level inverter; This represents the modulation wave of the three phases a, b, and c of a three-level inverter; Indicates the three-phase output current; Indicates the modulation ratio; Indicates the output current amplitude; Indicates the power factor angle; Indicates the fundamental frequency angle and ; This indicates the fundamental frequency, which is usually taken as 50Hz or 60Hz. Indicates time; S212: Harmonic analysis of the midpoint current of a three-level inverter reveals that the third harmonic content is the largest and far exceeds that of other harmonics. Based on the sign of the modulation waves in each phase of different sectors, the midpoint current model can be accurately characterized using a 6-segment piecewise function. After Fourier expansion of the piecewise function under different power factor angles, the constant coefficients, as well as the sine and cosine coefficients of the fundamental, second, and third harmonics, are obtained. Furthermore, based on the sine coefficients of each harmonic... Sum of cosine coefficients The amplitude of each harmonic was calculated. for: (4) It was found that the second harmonic amplitude varies under different power factor angles. The constant coefficient and fundamental amplitude are both zero. Approximately 0, while the third harmonic amplitude The power factor angle initially decreases and then increases with increasing angle, reaching a maximum at ±90°, which is below 1. The Fourier expansion results are consistent with the harmonic analysis results. To facilitate further analysis, the midpoint current is... The approximate third harmonic component is: (5) In the formula: Indicates the amplitude of the third harmonic; The third harmonic phase is determined by the power factor angle; The midpoint current model is simplified based on the third harmonic component, and the simplified midpoint current model is obtained as follows: (6) S213: Based on the simplified midpoint current model, obtain the bus capacitance connected in series with the AC side of the ripple compensation active branch. current That is, half of the midpoint current, its expression is: (7) S22: Based on the capacitance flowing through the busbar current Obtain the change in charge According to the change in charge Obtain bus capacitance Voltage change at both ends Based on voltage change Obtain the AC side output voltage of the ripple-compensated active branch. The specific steps include: S221: Based on the capacitance flowing through the busbar current Obtain the change in charge for: (8) In the formula: Represents electric charge The change in; This represents the initial charge at time t=0. The voltage across the terminals is half the bus voltage, that is... ;therefore, Can be represented as ; S222: Based on the change in charge Obtain bus capacitance Voltage change at both ends for: (9) S223: Based on voltage change Obtain the AC side output voltage of the ripple-compensated active branch. In this embodiment, the AC output voltage of the auxiliary branch full-bridge inverter The change in voltage across C2 Equivalent reverse ripple compensation is performed to maintain midpoint potential balance, and its expression is: (10) S23: Power balancing based on the full-bridge inverter, according to the AC side output voltage. DC-side capacitor of the ripple compensation active branch Current flowing through Obtain the DC side capacitor Voltage at both ends The specific steps include: S231: The expression for obtaining the power balance of the full-bridge inverter is: (11) In the formula: Indicates AC side capacitance Voltage at both ends; This indicates the current flowing through the AC side capacitor; Indicates DC side capacitor Voltage at both ends; This indicates that current flows through the DC-side capacitor and ; S232: Power balancing based on the full-bridge inverter, according to the AC side output voltage. DC-side capacitor of the ripple compensation active branch Current flowing through The voltage across the DC-side capacitor Cb is obtained. The differential equation is: (12) The differential equation can be rewritten as: (13) Integrating both sides of the rewritten differential equation, we obtain the integrated equation: (14) The voltage across the DC-side capacitor Cb can be obtained from the integrated equation. The expression is: (15) In the formula: Denotes the constant to be solved, and when At that time, Defined as target stable value able to obtain ; and then obtain the DC-side capacitor Voltage at both ends for: (16) In the formula: express The target stable value of the voltage across the terminals; S24: Modulation ratio based on full-bridge inverter According to the AC side output voltage Voltage across the DC-side capacitor Cb To obtain the first constraint for implementing ripple compensation, the specific steps include: S241: Based on the AC side output voltage Voltage across the DC-side capacitor Cb The modulation ratio of the full-bridge inverter is obtained. And let Its expression is: (17) S242: Solve the expression for S241 and obtain the first constraint condition for implementing ripple compensation as follows: (18) S25: Derivation of bus capacitance based on S24 Capacitance value and DC-side capacitor of full-bridge inverter voltage stability value The relationship between these factors is used to derive a second constraint for achieving ripple compensation, specifically including the following steps: S251: Derivation of bus capacitance based on S24 Capacitance value and DC-side capacitor of full-bridge inverter voltage stability value The relationship between them can be expressed as: (19) S252: Based on S251, the second constraint for achieving ripple compensation is further obtained as follows: (20) S26: Based on the first and second constraints, the parameters to be designed are selected; and the parameters to be designed include the bus capacitance. The capacitance value of the DC-side capacitor of the full-bridge inverter capacitance and capacitor Voltage stability value In this embodiment, the auxiliary branch is constructed by selecting the capacitance values ​​of the lower bus capacitor, the DC-side capacitor of the full-bridge inverter, and the stable capacitor voltage value under two constraints. To achieve midpoint potential balance, the proposed auxiliary branch adjusts the modulation ratio of the full-bridge inverter. The goal is to achieve two control objectives: ripple compensation and DC-side capacitor voltage stability of the full-bridge inverter. In the control loop, a high-pass filter is used to obtain the lower bus capacitor value. The ripple component of the voltage across the terminals is used as the control variable for the ripple compensation branch. Simultaneously, a low-pass filter is used to obtain the DC-side capacitance of the full-bridge inverter. The DC component of the voltage across the terminals is used as the input to a preset PI controller, and the difference between it and the target value is used as the input. The control quantity obtained after PI control is multiplied by the differential term of the ripple compensation branch to obtain the control quantity of the voltage stabilization branch. The sum of the two control quantities is used as the final modulation ratio of the full-bridge inverter. Then, bipolar pulse width modulation is used to obtain the switching drive signal for the full-bridge inverter, such as... Figure 2 As shown, terminals g1, g2, g3, and g4 are the drive signal terminals for the switching devices. The PWM signals of the four switching devices generated by the control are connected to these four terminals to control the switching devices to turn on and off.

[0033] Compared with the prior art, the beneficial effects of the method described in this embodiment are as follows: The method described in this embodiment effectively suppresses midpoint potential fluctuations by controlling the auxiliary branch to generate a ripple signal with the same magnitude but opposite direction to the voltage fluctuation of the lower bus capacitor, thereby providing real-time dynamic compensation for the lower bus capacitor voltage. Compared to existing hardware solutions based on current compensation, the auxiliary branch switching transistor in this embodiment experiences less current and voltage stress, reducing the difficulty of device selection. Furthermore, the auxiliary branch power consumption is significantly reduced, improving the overall system efficiency and reliability. Compared to existing software solutions, the method described in this embodiment can maintain midpoint potential stability under all operating conditions.

[0034] This embodiment also includes the following application examples: S1: Construct a three-level neutral point clamping inverter. This inverter includes two parallel bus capacitor branches and three-phase output branches (a, b, and c). Refer to... Figure 2 As shown, the AC side capacitor Cab of the full-bridge inverter is connected in series with the lower bus capacitor. Between the negative bus and the auxiliary branch full-bridge inverter; by adjusting the modulation ratio of the auxiliary branch inverter. Control the output of the full-bridge inverter and the lower bus capacitor. The ripple signals with the same amplitude but opposite direction of the voltage ripple components at both ends are used to stabilize the lower branch voltage (i.e., the voltage between the neutral point and the negative bus) at half of the bus voltage through voltage ripple compensation. Then, based on the fact that the sum of the upper and lower branch voltages is a constant bus voltage, the upper branch voltage (i.e., the voltage between the positive bus and the neutral point) is stabilized at half of the bus voltage, thereby achieving neutral point voltage balance. S2: In this embodiment, to ensure that the AC side output of the full-bridge inverter can supply the lower bus capacitor... Compensation for the ripple component of the voltage across the terminals requires adjustment of the lower bus capacitor. Capacitance value, DC-side capacitor of full-bridge inverter Capacity and The DC component of the voltage at both ends is designed, and the midpoint current of the three-level inverter is modeled with reference to formula (1); S3: By expanding the Fourier function of formula (1), it is found that the midpoint current can be approximated as its third harmonic component. In order to facilitate the subsequent solution of the constraint conditions, the existing mathematical model of midpoint current is approximated by scaling as formula (2). S4: Based on the AC side output of the full-bridge inverter and the bus capacitor The equal and reverse voltage changes at both ends, along with the power balance between the DC and AC sides of the full-bridge inverter, allow for the calculation of the relevant lower bus capacitor. Capacitance value, DC-side capacitor of full-bridge inverter Capacity and The design constraints for the three parameters, namely the DC component of the voltage at both ends, are given by formulas (18) and (20); in the method described in this embodiment, the constant bus voltage of the three-level inverter is set to 400V, the output load R is 5Ω, the output filter inductor L is 0.001H, the inverter fundamental frequency is 50Hz, and the switching frequency is 20kHz; based on the simulated waveform of the output current of the three-level inverter without the auxiliary branch, the output current amplitude is... Set to 40A, three-level inverter modulation ratio Set to 1; first determine according to formula (20). Capacity and These two parameters are the DC component of the voltage across the terminals. The capacitance value is set to 300uF. DC component of the voltage across terminals Take 80V, and connect the bus capacitor. The capacitance value is related to The capacitance value is consistent, also 300uF; determined according to formula (18) and the two pre-determined parameters. The range of values ​​for capacitance. The capacitance value is 560uF; in the full-bridge inverter, the AC side is used to generate a ripple signal of a specific amplitude, which can be achieved with a small capacitor. In the method described in this embodiment, the capacitance value of the AC side capacitor Cab of the full-bridge inverter is 3uF, and the value of the AC side filter inductor Lb of the full-bridge inverter is 100uH; above, the hardware topology construction and the selection of specific device parameters have been completed. S5: For the modulation of the main circuit inverter, the method described in this embodiment selects carrier modulation; for the modulation of the auxiliary circuit full-bridge inverter, the method described in this embodiment selects bipolar pulse width modulation. To achieve ripple compensation of the auxiliary branch for the bus capacitor C2, a control loop needs to be established to modulate the full-bridge inverter's modulation ratio. Real-time dynamic control is performed, and the control loop is referenced. Figure 3 As shown, it includes two branches: ripple compensation and voltage stabilization, to achieve two major control objectives: specific ripple signal output on the AC side of the full-bridge inverter and DC side capacitor voltage stabilization. This includes the bus capacitor... The voltage ripple component obtained by high-pass filtering of the voltage across both ends is used as the control quantity for the ripple compensation branch. In the voltage stabilization branch, the DC-side capacitor voltage of the full-bridge inverter is used as the input quantity. After low-pass filtering, the DC component of the DC-side capacitor voltage is obtained. The difference between this DC component and the set stable value of the DC-side capacitor voltage is then calculated. The difference is used as the control quantity after PI control. The product of the rate of change of the ripple components of the voltage across the two terminals is used as the control quantity for the voltage stabilization branch. The control quantities of the two branches are superimposed to obtain the final control signal, which is the modulation ratio of the full-bridge inverter. Modulate the control signal ratio The switching signals of the four switching transistors of the full-bridge inverter are generated by bipolar pulse width modulation. The switching signals of the three-phase switching transistors of the main circuit inverter are obtained by comparing the carrier wave with the three-phase modulated wave. Under the same modulation strategy and operating conditions, the neutral point voltage waveforms without auxiliary branches and with auxiliary branches introduced in this embodiment are referenced. Figures 4 to 6As shown in the figure, the hardware solution proposed by the method described in this embodiment can stabilize the neutral point voltage between 200±6V, with a neutral point voltage fluctuation of only 3%, demonstrating excellent neutral point voltage stabilization. Simultaneously, the DC-side capacitor voltage of the full-bridge inverter is stabilized between 80±10V, ensuring the safe and stable operation of the full-bridge inverter. The voltage stress borne by the four switches in the auxiliary branch proposed by the method in this embodiment is equal to the DC-side capacitor voltage, less than 100V, which is less than 1 / 4 of the bus voltage. Compared to the voltage stress that the auxiliary branch switches in existing hardware solutions need to withstand, which is half or even higher than the bus voltage, the voltage stress of the auxiliary branch switches in the method described in this embodiment is significantly reduced. Furthermore, the current flowing through the auxiliary branch switches in the method described in this embodiment is less than the compensation current, resulting in even lower current stress. Therefore, the three-level inverter neutral point voltage balancing method based on ripple-compensated active auxiliary branch proposed in this embodiment has the characteristics of excellent neutral point voltage balancing, low auxiliary branch power loss, and higher overall system efficiency.

[0035] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for balancing the neutral point potential of a three-level inverter based on an active branch with ripple compensation, characterized in that, Specifically, the following steps are included: S1: Construct a ripple compensation active branch containing a full-bridge inverter with unknown design parameters, and introduce it into a pre-set three-level inverter; connect the AC side of the ripple compensation active branch in series between the bus capacitor and the negative bus of the three-level inverter; the AC side of the ripple compensation active branch is used to generate compensation ripple. The design parameters include the capacitance value of the bus capacitor connected in series with the AC side of the ripple compensation active branch, the capacitance value of the DC side capacitor of the full-bridge inverter, and the voltage stability value of the DC side capacitor of the full-bridge inverter. S2: Construct a design strategy for balancing the midpoint potential of a three-level inverter, in order to design the parameters to be designed in the ripple compensation active branch, and thus achieve the balance of the midpoint potential of the three-level inverter based on the ripple compensation active branch.

2. The method for balancing the neutral point potential of a three-level inverter based on a ripple-compensated active branch according to claim 1, characterized in that, The design strategy for midpoint potential balance in a three-level inverter, constructed in S2, includes the following steps: S21: Model the midpoint current of the three-level inverter to obtain the midpoint current model; rewrite the midpoint current model by introducing harmonic amplitude to obtain a simplified midpoint current model; Based on the simplified midpoint current model, the bus capacitance flowing through the AC side of the ripple-compensated active branch is obtained. current ; S22: Based on the capacitance flowing through the busbar current Obtain the change in charge According to the change in charge Obtain bus capacitance Voltage change at both ends According to the voltage change Obtain the AC side output voltage of the ripple-compensated active branch. ; S23: Power balancing based on the full-bridge inverter, according to the AC side output voltage. DC-side capacitor of the ripple compensation active branch Current flowing through Obtain the DC side capacitor Voltage at both ends ; S24: Modulation ratio based on full-bridge inverter According to the AC side output voltage Voltage across the DC-side capacitor Cb Obtain the first constraint condition used to implement ripple compensation; S25: Derivation of bus capacitance based on S24 Capacitance value and DC-side capacitor of full-bridge inverter voltage stability value The relationship between them is used to obtain a second constraint for achieving ripple compensation; S26: Based on the first and second constraints, the parameters to be designed are selected; and the parameters to be designed include the bus capacitance. The capacitance value of the DC-side capacitor of the full-bridge inverter capacitance and capacitor Voltage stability value .

3. The method for balancing the neutral point potential of a three-level inverter based on a ripple-compensated active branch according to claim 2, characterized in that, S21 specifically includes the following steps: S211: Model the midpoint current of the three-level inverter, and obtain the midpoint current model as follows: In the formula: This represents the midpoint current of a three-level inverter; This represents the modulation wave of the three phases a, b, and c of a three-level inverter; Indicates the three-phase output current; Indicates the modulation ratio; Indicates the output current amplitude; Indicates the power factor angle; Indicates the fundamental frequency angle and ; Indicates the fundamental frequency; Indicates time; S212: By introducing harmonic amplitude, the midpoint current model is rewritten with the third harmonic component as follows: In the formula: Indicates the amplitude of the third harmonic; Indicates the third harmonic phase; The midpoint current model is simplified based on the third harmonic component, and the simplified midpoint current model is obtained as follows: S213: Based on the simplified midpoint current model, obtain the bus capacitance connected in series with the AC side of the ripple compensation active branch. current for: 。 4. The method for balancing the neutral point potential of a three-level inverter based on a ripple-compensated active branch according to claim 3, characterized in that, S22 specifically includes the following steps: S221: Based on the capacitance flowing through the busbar current Obtain the change in charge for: In the formula: Represents electric charge The change in; This represents the initial charge at time t=0; S222: Based on the change in charge Obtain bus capacitance Voltage change at both ends for: S223: Based on voltage change Obtain the AC side output voltage of the ripple-compensated active branch. : 。 5. The method for balancing the neutral point potential of a three-level inverter based on a ripple-compensated active branch according to claim 4, characterized in that, S23 specifically includes the following steps: S231: The expression for obtaining the power balance of the full-bridge inverter is: In the formula: Indicates AC side capacitance Voltage at both ends; This indicates the current flowing through the AC side capacitor; Indicates DC side capacitor Voltage at both ends; This indicates that current flows through the DC-side capacitor and ; S232: Power balancing based on the full-bridge inverter, according to the AC side output voltage. DC-side capacitor of the ripple compensation active branch Current flowing through The voltage across the DC-side capacitor Cb is obtained. The differential equation is: The differential equation can be rewritten as: Integrating both sides of the rewritten differential equation, we obtain the integrated equation: The voltage across the DC-side capacitor Cb can be obtained from the integrated equation. The expression is: In the formula: Denotes the constant to be solved, and when At that time, Defined as target stable value able to obtain ; and then obtain the DC-side capacitor Voltage at both ends for: In the formula: express The target stable value of the voltage across the terminals.

6. The method for balancing the neutral point potential of a three-level inverter based on a ripple-compensated active branch according to claim 5, characterized in that, S24 specifically includes the following steps: S241: Based on the AC side output voltage With DC side capacitor Voltage at both ends The modulation ratio of the full-bridge inverter is obtained. And let Its expression is: S242: Solve the expression for S241 and obtain the first constraint condition for implementing ripple compensation as follows: 。 7. A method for balancing the neutral point potential of a three-level inverter based on a ripple-compensated active branch according to claim 6, characterized in that, S25 specifically includes the following steps: S251: Derivation of bus capacitance based on S24 Capacitance value and DC-side capacitor of full-bridge inverter voltage stability value The relationship between them can be expressed as: S252: Based on S251, the second constraint for achieving ripple compensation is further obtained as follows: 。

Images