A three-level converter neutral-point voltage balance control method and device

Abstract

The application discloses a three-level converter neutral-point voltage balance control method and device, and relates to the technical field of power electronics and electric energy conversion control, wherein the method comprises the following steps: collecting the DC bus capacitor voltage of the converter, measuring and calculating the neutral-point voltage average value and determining the deviation state, and determining the neutral-point current regulation direction in combination with the modulation ratio; a multi-constraint optimization model is built based on a half-wave symmetrical pulse form, and an initial switching angle sequence is obtained by solving; it is verified whether the current flow direction corresponding to the sequence matches the regulation and control demand, if yes, the sequence is directly adopted, otherwise, a target sequence is obtained by rearranging according to rules; bridge arm driving signals are output according to the target sequence, the output harmonic is optimized, the voltage deviation is corrected by means of the neutral-point current component, and the neutral-point voltage balance control is synchronously completed. The application relies on the neutral-point current and the modulation mode to optimize the pulse waveform, realizes harmonic suppression and potential balance, and simplifies the operation to adapt to the working conditions of high-power converters.

Description

Technical Field

[0001] This invention relates to the field of power electronics and power conversion control technology, and in particular to a method and apparatus for controlling the midpoint voltage balance of a three-level converter. Background Technology

[0002] Three-level neutral point clamped (NPC) converters are widely used in high-power energy conversion applications such as pumped storage excitation systems, wind power converters, and medium-voltage motor drives due to their low device voltage stress, high output voltage waveform quality, and low switching losses. In these systems, to reduce losses and improve energy conversion efficiency, the converter often operates at a low switching frequency, making optimized pulse pattern (OPP) modulation an important technical approach.

[0003] For three-level NPC converters, neutral point voltage balance is a critical issue that must be addressed. When the bridge arm outputs a zero level, the corresponding phase current flows into the neutral point, generating a neutral point current. This causes uneven charging and discharging of the upper and lower DC-side capacitors, resulting in neutral point voltage fluctuations and even deviations in the average value. If the neutral point voltage remains unbalanced for a long period, it will lead to uneven voltage distribution among components, deterioration of the output waveform, and decreased control performance. In severe cases, it may even affect the safe operation of the system.

[0004] There is a clear mathematical coupling between the midpoint current and the pulse-mode switching function. For a three-level pulse mode, the absolute value of the pulse-mode switching function can be used as a key quantity to describe the zero-level distribution characteristics. The DC component of the midpoint current is generated by multiplying the phase current by the absolute value of the pulse-mode switching function. Typically, the phase current mainly consists of the fundamental frequency and other odd-order harmonics. Therefore, to generate a DC component in the product, the absolute value of the pulse-mode switching function must include odd-order components corresponding to the phase current.

[0005] While traditional 1 / 4-cycle symmetrical pulse modes and traditional half-wave symmetrical pulse modes offer advantages such as fewer switching angles and better harmonic optimization performance, their symmetrical constraints mean that the absolute value of the pulse mode switching function does not contain odd-order components used to couple with the phase current to generate the DC component of the midpoint current. Therefore, they cannot provide effective degrees of freedom for midpoint voltage balance control. On the other hand, while traditional full-wave symmetrical pulse modes can introduce odd-order components of the absolute value of the pulse mode switching function by changing the pulse distribution throughout the entire cycle, thereby generating the DC component of the midpoint current after multiplying the phase current by the absolute value of the pulse mode switching function, the optimization problem of full-wave symmetrical pulse modes has high dimensionality, large computational load, large online lookup table storage space requirements, and complex controller implementation.

[0006] Therefore, there is an urgent need for a new pulse structure and a corresponding OPP modulation method that can generate an adjustable DC component of the midpoint current after coupling the phase current with the absolute value of the pulse mode switching function to achieve active balance control of the midpoint voltage, while maintaining a small number of independent switching angles to reduce the amount of optimization computation and storage requirements. Summary of the Invention

[0007] The main objective of this invention is to provide a method for controlling the neutral point voltage balance of a three-level converter.

[0008] Another objective of this invention is to provide a neutral point voltage balance control device for a three-level converter.

[0009] The third objective of this invention is to provide an electronic device.

[0010] A fourth objective of this invention is to provide a non-transitory computer-readable storage medium.

[0011] To achieve the above objectives, a first aspect of the present invention provides a method for controlling the neutral point voltage balance of a three-level converter, comprising:

[0012] The voltage of the upper and lower bus capacitors on the DC side of the converter is collected, the real-time midpoint voltage is calculated and the average value of the midpoint voltage is obtained. The voltage deviation status is determined by combining the preset voltage deviation threshold. At the same time, the converter modulation ratio command is obtained, and the adjustment direction of the required DC component of the midpoint current is determined according to the voltage deviation status. An optimization model containing output harmonic performance constraints and midpoint current DC component constraints is constructed based on a preset half-wave symmetrical pulse mode, and the initial switching angle sequence corresponding to the modulation ratio command is obtained by solving the model. The direction of the DC component of the midpoint current generated by the initial switching angle sequence is calculated and compared with the preset adjustment direction. If the direction is consistent, it is directly determined as the target switching angle sequence. Otherwise, the initial switching angle sequence is rearranged according to the preset rearrangement rules to generate the target switching angle sequence. The drive signal for the converter bridge arm is generated based on the target switching angle sequence to complete the converter output harmonic optimization and control. At the same time, the average deviation of the midpoint voltage is corrected based on the DC component of the midpoint current to achieve midpoint voltage balance control.

[0013] Optionally, the voltage of the upper and lower bus capacitors on the DC side of the converter is collected, the real-time midpoint voltage is calculated, and the average value of the midpoint voltage is obtained. The voltage deviation state is determined by combining this with a preset voltage deviation threshold. Simultaneously, the converter modulation ratio command is acquired, and the adjustment direction of the required DC component of the midpoint current is determined based on the voltage deviation state, including: The voltage of the upper bus capacitor and the voltage of the lower bus capacitor on the DC side of the three-level NPC converter are collected and substituted into the predetermined calculation formula to obtain the real-time neutral point voltage of the converter. The real-time midpoint voltage is smoothed by using low-pass filtering or periodic averaging to extract a stable average midpoint voltage. The average value of the midpoint voltage is compared with the preset midpoint voltage deviation threshold to distinguish three types of deviation states: midpoint voltage too high, midpoint voltage too low, and midpoint voltage in the equilibrium range. Synchronously acquire the modulation ratio command parameters corresponding to the converter's operating conditions, and determine the required control direction for the DC component of the neutral point current—positive adjustment, negative adjustment, or no adjustment—based on the completed neutral point voltage deviation state.

[0014] Optionally, an optimization model is constructed based on a preset half-wave symmetrical pulse mode, including output harmonic performance constraints and midpoint current DC component constraints, to obtain the initial switching angle sequence corresponding to the modulation ratio command, including: Two types of symmetrical pulse structures, namely novel half-wave symmetrical pulse mode A and novel half-wave symmetrical pulse mode B, are pre-defined. The switching function and the mathematical model of the absolute value of the switching function of the single-bridge arm pulse mode corresponding to the two types of pulse modes are established respectively. The harmonic component decomposition of the switching function and its absolute value is completed by Fourier series. Combined with the amplitude and phase parameters of each harmonic of the three-phase phase current, the coupling analytical relationship of the DC component of the midpoint current under two pulse modes is derived and solved. With optimal output harmonic performance as the optimization objective, a complete OPP optimization solution model is constructed by integrating fundamental amplitude constraints, switching angle sequence constraints, and midpoint current DC component balance constraints. The obtained modulation ratio command is imported into the optimization model to complete the numerical solution, and the output is the initial switching angle sequence adapted to the current operating conditions.

[0015] Optionally, the direction of the DC component of the midpoint current generated by the initial switching angle sequence is calculated and compared with the preset adjustment direction, including: By combining current sampling with harmonic observation algorithm, the amplitude and phase parameters of each harmonic of the three-phase phase current are collected in real time. By combining the harmonic decomposition coefficients of the pulse mode corresponding to the initial switching angle sequence and substituting them into the formula for calculating the DC component of the midpoint current, the actual value and actual flow direction of the DC component of the midpoint current generated by this set of initial switching angle sequences are calculated. The actual flow direction of the calculated DC component of the midpoint current is compared with the preset adjustment direction of the DC component of the midpoint current to complete the flow direction consistency verification operation.

[0016] Optionally, if the flow directions are consistent, the initial switching angle sequence is directly determined as the target switching angle sequence; otherwise, the initial switching angle sequence is rearranged according to a preset rearrangement rule to generate the target switching angle sequence, including: When the DC component of the midpoint current generated by the initial switching angle sequence flows in the same direction as the preset adjustment direction, the initial switching angle sequence is directly designated as the target switching angle sequence for final control. When the DC component of the midpoint current generated by the initial switching angle sequence deviates from the preset adjustment direction, the pre-set exclusive rearrangement rules are invoked to perform angle interval translation and recombination operation on half-wave symmetrical pulse mode A and angle inversion and recombination operation on half-wave symmetrical pulse mode B. The original initial switching angle sequence is completely rearranged to generate a new target switching angle sequence with opposite control polarity and unchanged harmonic operation characteristics.

[0017] Optionally, drive signals for the converter bridge arms are generated based on the target switching angle sequence to achieve converter output harmonic optimization and control. Simultaneously, the average deviation of the neutral point voltage is corrected based on the DC component of the neutral point current to achieve neutral point voltage balance control, including: The final determined target switching angle sequence is converted into timing drive control signals corresponding to each bridge arm of the three-level NPC converter. The output drive control signal controls the power switching devices of the converter to turn on and off in sequence. The optimized pulse mode suppresses the harmonic content on the output side of the converter, thus completing the output harmonic performance optimization and control. Simultaneously, by utilizing the inherent characteristic that the DC component of the midpoint current can be reversed by rearranging the switching angle sequence before and after, and by using the matched and adapted DC component of the midpoint current to correct the average deviation of the midpoint voltage, the dual control tasks of converter harmonic optimization operation and dynamic balance of DC side midpoint voltage are completed simultaneously.

[0018] To achieve the above objectives, a second aspect of the present invention provides a three-level converter midpoint voltage balance control device, comprising: The deviation determination module is used to collect the voltage of the upper and lower bus capacitors on the DC side of the converter, calculate the real-time midpoint voltage and obtain the average value of the midpoint voltage, determine the voltage deviation status in combination with the preset voltage deviation threshold, and at the same time obtain the converter modulation ratio command, and determine the adjustment direction of the required DC component of the midpoint current according to the voltage deviation status. An angle solving module is used to construct an optimization model based on a preset half-wave symmetrical pulse mode, which includes output harmonic performance constraints and midpoint current DC component constraints, and solve for the initial switching angle sequence corresponding to the modulation ratio command. The flow direction correction module is used to calculate the flow direction of the DC component of the midpoint current generated by the initial switching angle sequence and compare it with the preset adjustment direction. If the flow direction is consistent, it is directly determined as the target switching angle sequence; otherwise, the initial switching angle sequence is rearranged according to the preset rearrangement rules to generate the target switching angle sequence. The collaborative control module is used to generate drive signals for the converter bridge arm based on the target switching angle sequence, complete the converter output harmonic optimization control, and at the same time correct the average deviation of the midpoint voltage based on the DC component of the midpoint current to achieve midpoint voltage balance control.

[0019] Regarding the apparatus in the above embodiments, the specific manner in which each module performs its operation has been described in detail in the embodiments related to the method, and will not be elaborated upon here.

[0020] To achieve the above objectives, a third aspect of this application provides an electronic device, including a processor and a memory; wherein the processor reads executable program code stored in the memory to run a program corresponding to the executable program code, for implementing a three-level converter midpoint voltage balance control method as described in the first aspect embodiment.

[0021] To achieve the above objectives, a fourth aspect of this application provides a non-transitory computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements a three-level converter midpoint voltage balance control method as described in the first aspect embodiment.

[0022] The embodiments of the present invention have the following beneficial effects: 1. Based on the inherent relationship between midpoint current and pulse modulation mode, the symmetrical design of pulse waveform is optimized to induce the absolute value of the switching function to form odd components, effectively expanding the adjustment means of midpoint potential balance control and laying a theoretical foundation for precise control of midpoint voltage.

[0023] 2. The two novel half-wave symmetrical pulse modulation methods designed in this paper can stably maintain the harmonic optimization effect of the optimal pulse modulation at low switching frequency, and can also effectively generate midpoint DC current, thus achieving the dual control objectives of output harmonic suppression and midpoint potential balance in one integrated manner.

[0024] 3. Compared with the traditional full-range constrained pulse modulation, this scheme reduces the number of independently controllable switch angles, compresses the calculation scale of optimization, reduces data processing time, reduces the pressure on offline parameter storage of the system, and further improves the effectiveness of the modulation strategy.

[0025] 4. This modulation control method is suitable for the low switching frequency operation of high-power power conversion devices and fits the actual operation scenario of three-level neutral point clamping converters. The overall control scheme is stable and reliable, with a wide range of practical applications and excellent engineering application potential and widespread use value. Attached Figure Description

[0026] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the following description of the embodiments taken in conjunction with the accompanying drawings, wherein: Figure 1 A flowchart of a three-level converter midpoint voltage balance control method provided in an embodiment of the present invention; Figure 2 This is a logic diagram for generating the reference value of the DC component of the midpoint current provided in an embodiment of the present invention. Figure 3 This is a schematic diagram of a 1 / 4-cycle symmetrical pulse mode provided in an embodiment of the present invention; Figure 4 This is a schematic diagram of a traditional half-wave symmetrical pulse mode provided in an embodiment of the present invention; Figure 5 This is a schematic diagram of half-wave symmetrical pulse mode A provided in an embodiment of the present invention; Figure 6 This is a schematic diagram of half-wave symmetrical pulse mode B provided in an embodiment of the present invention; Figure 7 The control flowchart of the three-level NPC converter midpoint voltage balance control method provided in the embodiment of the present invention is shown below; Figure 8 This is a schematic diagram of the simulation results of the midpoint voltage balance control when the fundamental current phase is 0, provided in an embodiment of the present invention. Figure 9 A schematic diagram of the simulation results of the midpoint voltage balance control when the fundamental current phase is 90 degrees, provided in an embodiment of the present invention. Figure 10 This is a structural diagram of a three-level converter midpoint voltage balance control device provided in an embodiment of the present invention. Detailed Implementation

[0027] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0028] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0029] The following description, with reference to the accompanying drawings, describes a method and apparatus for controlling the midpoint voltage balance of a three-level converter according to an embodiment of the present invention.

[0030] Example 1 This invention provides a method for neutral point voltage balance control of a three-level converter. Figure 1 This is a flowchart illustrating a method for controlling the midpoint voltage balance of a three-level converter according to an embodiment of the present invention. Figure 1 As shown, the method includes the following steps: Step S1: Collect the voltage of the upper and lower bus capacitors on the DC side of the converter, calculate the real-time midpoint voltage and obtain the average value of the midpoint voltage, determine the voltage deviation state in combination with the preset voltage deviation threshold, and at the same time obtain the converter modulation ratio command. Based on the voltage deviation state, determine the adjustment direction of the required DC component of the midpoint current.

[0031] In this embodiment of the application, in order to accurately grasp the DC side midpoint potential state of the three-level NPC converter, a high-precision voltage sampling unit is first used to collect the DC side upper bus capacitor voltage in real time. With lower bus capacitor voltage The instantaneous neutral point voltage of the converter can be obtained through simple difference calculation. The calculation formula is as follows:

[0032] in, This refers to the real-time instantaneous midpoint voltage during converter operation, expressed in units of 1 / 2π. Consistent (usually V), when hour, This indicates that the midpoint voltage is in equilibrium; when hour, This indicates that the midpoint voltage is too high; when hour, This indicates that the midpoint voltage is too low.

[0033] Due to the real-time acquisition of midpoint voltage The voltage may contain high-frequency interference signals (mainly from the switching actions of the converter's power switching devices). If directly used for deviation determination, this could lead to misjudgments and affect control accuracy. Therefore, in this embodiment, mature data processing methods such as low-pass filtering or periodic averaging are used to analyze the real-time midpoint voltage. Smoothing operations are performed to filter out high-frequency interference components, thereby extracting a stable and reliable average midpoint voltage. The calculation process is expressed as follows:

[0034] in, This represents data smoothing processing logic such as low-pass filtering algorithm and periodic mean calculation algorithm. Through this processing, high-frequency interference can be effectively filtered out, ensuring the accuracy of midpoint voltage state determination.

[0035] To determine whether the neutral point voltage needs adjustment and in what direction, a neutral point voltage deviation threshold is preset based on the converter's rated operating conditions. The calculated average midpoint voltage is compared with a preset threshold to classify three different operating conditions. Based on the relationship between the average midpoint voltage and the threshold, a corresponding command value for the DC component of the midpoint current is set, such as... Figure 2 As shown, the specific correspondence is as follows: when When the midpoint voltage is determined to be too high, a negative DC component of the midpoint current needs to be introduced, and its reference value is denoted as... ， The negative DC component of the midpoint current consumes the charge of the lower bus capacitor and replenishes the charge of the upper bus capacitor, thereby reducing the average midpoint voltage. When the average value of the midpoint voltage When the voltage decreases to less than or equal to 0, it indicates that the midpoint voltage is close to equilibrium. At this point, the DC component of the midpoint current is no longer needed, and its reference value is denoted as [value missing]. Stop the midpoint voltage regulation operation.

[0036] when When the midpoint voltage is determined to be low, a positive DC component of the midpoint current needs to be introduced, and its reference value is denoted as... The positive DC component of the midpoint current consumes the charge of the upper bus capacitor and replenishes the charge of the lower bus capacitor, thereby increasing the average value of the midpoint voltage. When the average value of the midpoint voltage When the value increases to a value greater than or equal to 0, it indicates that the midpoint voltage is close to equilibrium. At this point, the DC component of the midpoint current is no longer needed, and its reference value is denoted as [value missing]. Stop the midpoint voltage regulation operation.

[0037] when When the midpoint voltage is determined to be within the allowable equilibrium range, no additional DC component of the midpoint current is required. The reference value for the DC component of the midpoint current is denoted as [reference value]. The converter can simply maintain its original operating state.

[0038] While determining the midpoint voltage deviation status, the modulation ratio command parameter m corresponding to the converter's operating condition is simultaneously acquired, and its value range is [missing value]. The midpoint voltage deviation is determined by external control commands or the converter's internal control algorithm based on factors such as load demand and operating efficiency. It is one of the core parameters for subsequently building an optimization model and solving the switching angle sequence. Based on the above-distinguished midpoint voltage deviation states, the corresponding control direction of the required midpoint current DC component is determined: when the midpoint voltage is high, the control direction is negative (i.e., generating a negative midpoint current DC component); when the midpoint voltage is low, the control direction is positive (i.e., generating a positive midpoint current DC component); when the midpoint voltage is balanced, the control direction is no control (i.e., no midpoint current DC component is generated).

[0039] In addition, to provide data support for the accurate calculation of the DC component of the subsequent midpoint current, a current sampling device (such as a high-precision current sensor) synchronously collects the three-phase current of the three-level NPC converter. ,in These are the real-time current signals for phases A, B, and C, respectively. Considering that the actual output current contains multiple harmonic components, the three-phase output current is uniformly expressed in the form of harmonic superposition as follows:

[0040] Where k is the characteristic harmonic order of the output current. Based on the operating characteristics of the converter, the 1st, 5th and 7th harmonics are selected for calculation. These three types of harmonics have the most significant impact on the midpoint current. This represents the amplitude of the corresponding harmonic current. The initial phase of the corresponding harmonic current is represented by θ, which is the electrical angle measured in radians. The acquired three-phase current is analyzed by the built-in harmonic observer to accurately extract the amplitude and phase parameters corresponding to each harmonic current, providing fundamental data support for the subsequent derivation of the midpoint current coupling relationship.

[0041] To establish the correlation between the pulse mode and the midpoint current, this embodiment pre-defines the single-phase pulse mode switching function, setting the independent variable as the electrical angle. Construct a pulse mode switching function Based on the operating characteristics of the switching devices in a three-level converter, the range of values ​​for this function is limited to: It also has the characteristic of periodic operation, with a fixed operating cycle of... The periodic constraint is .

[0042] The switching function takes corresponding values ​​according to different electrical angle intervals within a single complete cycle, and its piecewise expression is as follows:

[0043] In the formula, to For the corresponding values ​​of the switching function within different angle intervals, all values ​​satisfy the following: Range of values; to There are a total of 4d independent switching angles, and all switching angles strictly follow the arrangement sequence constraint. This ensures the rationality and standardization of the switching functions.

[0044] At any switching angle At the specified position, the switching function will complete the numerical transition. Based on this, a switch position switching variable is defined to characterize the value change of the switching function in different angle intervals. Its expression is:

[0045] In practical applications, to standardize calculations and avoid parameter confusion, the initial interval values ​​are uniformly limited. By relying on all switching angles and switching variables, the shape of the pulse switching function can be fully characterized, laying the foundation for subsequent Fourier decomposition. Simultaneously, considering the operational requirements of converter unipolar modulation, unipolar modulation constraints are added, specifically: .

[0046] Since the pulse switching function is a periodic alternating signal, harmonic decomposition can be performed using Fourier series. Its standard Fourier expansion is as follows:

[0047] In the formula , The Fourier coefficients corresponding to the switching function are given by the integral formula for solving the coefficients:

[0048]

[0049] Similarly, the absolute value of the pulse switching function is also a periodic signal, and its Fourier expansion is:

[0050] In the formula , The Fourier coefficients corresponding to the absolute value of the switching function are given by the integral formula:

[0051] Based on the single-phase switching function and combined with the phase difference characteristics of the three-phase current, the expressions for the three-phase switching functions are further derived, corresponding to the pulse switching functions of phase A, phase B, and phase C, respectively. The specific expressions are as follows:

[0052] In the formula, , , These correspond to the pulse switching functions for phases A, B, and C in sequence.

[0053] By combining the three-phase switching function and the three-phase harmonic current, and substituting the Fourier decomposition result of the absolute value of the switching function into the calculation, the complete expression for the midpoint current is obtained:

[0054] in, This indicates that the three-level NPC converter is in electrical angle The instantaneous midpoint current at time t. For the reason The midpoint current generated by the secondary phase current has the following specific expression:

[0055] To analyze the impact of each harmonic on the midpoint current, the midpoint current component generated by a single k-th harmonic current is extracted separately. The calculation is performed according to the harmonic order intervals, and the DC component of the midpoint current corresponding to the single harmonic current is finally obtained. The calculation formula is as follows:

[0056] in, This represents the DC component of the midpoint current corresponding to the single harmonic current.

[0057] By summing the DC components corresponding to all characteristic subharmonic currents, the DC component of the total midpoint current of the system can be obtained. The calculation formula is as follows:

[0058] in, This represents the DC component of the total midpoint current.

[0059] In this embodiment, this step completes the acquisition of core electrical parameters of the AC / DC side of the converter, signal optimization processing, and harmonic decomposition. It determines the degree of deviation and adjustment direction of the midpoint potential, classifies characteristic harmonics with different patterns, completes the instantaneous midpoint current calculation formulas corresponding to the two types of harmonics, and establishes a complete mathematical calculation system between the switching function, harmonic current, and midpoint current. It also derives the general calculation formulas for various Fourier coefficients and midpoint DC components, laying a solid foundation of underlying data and theory for subsequent pulse modulation mode optimization design and accurate solution of switching angle.

[0060] Step S2: Based on the preset half-wave symmetrical pulse mode, construct an optimization model that includes output harmonic performance constraints and midpoint current DC component constraints, and solve for the initial switching angle sequence corresponding to the modulation ratio command.

[0061] In this embodiment, a comprehensive characteristic analysis of the traditional pulse modulation modes commonly used in the industry is first performed. Among them, Figure 3 This is a schematic diagram of a 1 / 4-cycle symmetrical pulse mode, whose waveform exhibits the following characteristics: The property of symmetry; Figure 4 This is a schematic diagram of a traditional half-wave symmetrical pulse mode, where the waveform satisfies... The half-wave symmetry constraint. Based on intuitive waveform structure combined with mathematical derivation, the operational advantages and practical application shortcomings of the two types of traditional modulation modes can be clearly identified.

[0062] Based on the framework of the basic full-wave symmetric pulse mode, a half-wave symmetric constraint condition is added. This allows the formation of a traditional half-wave symmetrical pulse mode. This mode simplifies the number of switching angles through symmetrical constraints, retaining only 2d independent switching angles. Within the electrical angle range of 0 to π, the piecewise expression of the switching function is:

[0063] In this mode, all switch position switching variables uniformly follow... The switching function waveform in the π to 2π interval can be derived directly from the half-wave symmetry constraint relationship without additional settings, greatly simplifying the control logic. Fourier decomposition of the traditional half-wave symmetric pulse mode switching function yields the corresponding coefficient results:

[0064]

[0065]

[0066]

[0067]

[0068] This provides support for the subsequent calculation of the DC component of the midpoint current, and also derives the Fourier coefficients of the absolute value of the switching function in the traditional half-wave symmetrical pulse mode. The calculation formula is as follows:

[0069]

[0070]

[0071]

[0072]

[0073] On top of the traditional half-wave symmetric pulse mode, a 1 / 4-period symmetric constraint is further superimposed. This allows the formation of a quarter-cycle symmetrical pulse mode. This mode further simplifies the number of independent switching angles, retaining only d independent switching angles. Within the electrical angle range of 0 to 2π, the switching function expression is:

[0074] In this mode, the switch position switching variable satisfies The waveforms in the remaining electrical angle ranges are directly derived based on symmetry, further simplifying the control logic. Fourier decomposition of the switching function for this mode yields its Fourier coefficients:

[0075] The Fourier coefficients corresponding to the absolute value of the switching function are also derived and calculated, with the following results:

[0076] By comprehensively analyzing the spectral characteristics of the two sets of traditional pulse modes, the core defects of the two types of traditional modes can be clearly identified: the absolute values ​​of the switching functions of the traditional 1 / 4-cycle symmetrical pulse mode and the traditional half-wave symmetrical pulse mode do not contain odd-order harmonic components. Combined with the formula for calculating the DC component of the midpoint current derived in S1, it can be seen that these two types of modes cannot generate the DC component of the midpoint current for potential regulation and cannot achieve midpoint voltage balance control. Although the full-wave symmetrical pulse mode can generate the DC component of the midpoint current, it requires the configuration of 4d independent switching angles, which leads to a significant increase in computation and storage, resulting in poor practicality in actual engineering applications and difficulty in meeting real-time control requirements.

[0077] To address the aforementioned technical deficiencies, this application proposes a novel type of half-wave symmetric pulse mode—half-wave symmetric pulse mode A—as follows. Figure 5As shown, this mode achieves controllable generation of the DC component of the midpoint current while retaining the half-wave symmetry characteristics and simplifying the number of switching angles.

[0078] The piecewise expression of the full-cycle switching function of half-wave symmetric pulse mode A is as follows:

[0079] This pulse mode simultaneously satisfies With periodic loop constraints This ensures both waveform symmetry and operational stability. The rule for dividing the switching position variables is as follows:

[0080] Fourier decomposition is performed on half-wave symmetrical pulse mode A. The fundamental Fourier coefficients of its switching function are consistent with those of the traditional half-wave symmetrical pulse mode. The focus is on deriving its unique absolute value Fourier coefficients of the switching function, which are used for subsequent calculation of the DC component of the midpoint current. The specific formulas are as follows:

[0081] Based on the Fourier coefficients of this pulse mode, and combined with the general formula for the DC component of the midpoint current derived in S1, the formula for calculating the DC component of the midpoint current corresponding to the half-wave symmetrical pulse mode A is further derived:

[0082] To further enhance the flexibility and adaptability of midpoint voltage balance control, this application also proposes a second type of novel half-wave symmetrical pulse mode structure—half-wave symmetrical pulse mode B, such as... Figure 6 As shown, this mode adopts a different symmetric constraint than mode A, but it also achieves the controllable generation of the DC component of the midpoint current and is adapted to different operating conditions.

[0083] The piecewise expression for the full-cycle switching function of half-wave symmetrical pulse mode B is as follows:

[0084] This pulse mode satisfies symmetry constraints. In addition to periodic constraints, all switch position switching variables are unified as follows: This simplifies the variable setting and calculation process.

[0085] Fourier decomposition of the switching function of half-wave symmetric pulse mode B yields its Fourier coefficients:

[0086] Simultaneously, the Fourier coefficients corresponding to the absolute value of the switching function in this mode are derived for calculating the DC component of the midpoint current. The specific formula is as follows:

[0087] Combining the general derivation logic in S1, the formula for calculating the DC component of the midpoint current corresponding to the half-wave symmetrical pulse mode B is obtained:

[0088] After completing the mathematical modeling and component formula derivation for two novel pulse modes, an OPP optimization solution model with both harmonic optimization and midpoint voltage balance control capabilities is constructed to achieve coordinated control of harmonic optimization and midpoint voltage balance. First, a basic optimization model is established with the minimum output current harmonic distortion as the sole optimization objective, and its expression is:

[0089] The basic operating constraints are: In the formula, m represents the actual operating modulation ratio parameter of the converter. This constraint ensures that the fundamental amplitude of the converter output is stable and meets the load operation requirements.

[0090] To achieve active midpoint voltage regulation, a weighted penalty term for the DC component of the midpoint current is added to the basic objective function, thus constructing a comprehensive optimized objective function that balances the two major control requirements of harmonic suppression and midpoint balance.

[0091] In the formula, λ is the weighting factor of the DC component of the midpoint current. The sign of the weighting factor directly determines the output adjustment polarity of the DC component of the midpoint current, which can be flexibly set according to actual adjustment needs.

[0092] To address the angular distribution characteristics of the half-wave symmetrical pulse mode A, specific angular arrangement constraints are set to ensure the rationality and standardization of the switching angles. The specific constraints are as follows:

[0093] To address the angular distribution characteristics of the half-wave symmetrical pulse mode B, specific angular arrangement constraints are also set, as follows:

[0094] In practical applications, by inputting the modulation ratio parameters corresponding to the current operating condition of the converter, and using numerical optimization algorithms to solve the constrained optimization equations, the initial switching angle sequences corresponding to the two new pulse modes can be obtained. To improve real-time control efficiency, operators can pre-solve all switching angle sequences corresponding to different modulation ratio levels offline, compile and summarize them into a standardized data table, and easily access the table directly during online equipment operation, such as... Figure 7 As shown, the complete control logic from parameter acquisition and mode selection to drive signal generation is presented.

[0095] Step S3: Calculate the direction of the DC component of the midpoint current generated by the initial switching angle sequence and compare it with the preset adjustment direction. If the direction is consistent, it is directly determined as the target switching angle sequence. Otherwise, the initial switching angle sequence is rearranged according to the preset rearrangement rules to generate the target switching angle sequence.

[0096] In this embodiment of the application, the initial sequence is first clearly defined and quantitatively analyzed. The initial switching angle sequence of the half-wave symmetrical pulse mode A is denoted as A1, and its specific arrangement is as follows:

[0097] The Fourier coefficients of the pulse mode switching function corresponding to the initial sequence and the absolute value of the switching function Fourier coefficients They are respectively:

[0098]

[0099]

[0100]

[0101] The initial switching angle sequence of the half-wave symmetrical pulse mode B is denoted as B1, and its arrangement is consistent with that of A1, specifically:

[0102] It should be noted that although A1 and B1 have the same sequence form, their actual switching angle values ​​differ due to the different piecewise functions and symmetric constraints of the two types of modes, and they should not be used interchangeably.

[0103] Combined with the harmonic current amplitudes collected and analyzed in step S1 With phase parameter Substituting these values ​​into the two mode-specific DC component calculation formulas derived in S2, the actual DC components of the midpoint current that can be generated by the two initial sequences are accurately calculated and denoted as follows: and The specific calculation formula is as follows:

[0104]

[0105] To accelerate the correction rate of midpoint voltage deviation and improve control response speed, this embodiment adopts the principle of "selecting the best option," prioritizing the use of pulse modes with stronger adjustment capabilities. The specific judgment rule is to compare the absolute values ​​of the two sets of DC components. When the conditions are met... When the half-wave symmetrical pulse mode A has a stronger regulating capability, the selected sequence (A1) is chosen as the candidate control sequence; conversely, when The selected sequence (B1) is used as the candidate control sequence.

[0106] After completing the candidate sequence screening, it is also necessary to ensure that the adjustment direction of the candidate sequence is consistent with the system requirements. Therefore, the polarity of the DC component of the midpoint current actually generated by the candidate sequence is adjusted to match the preset adjustment command polarity determined in step S1. Perform a consistency comparison. If the two adjustment polarities are completely consistent, it means that the candidate sequence can be directly used for control without any adjustment, and the candidate switch angle sequence is directly determined as the target switch angle sequence for final use; if the two adjustment polarities are opposite, it means that the adjustment direction of the candidate sequence does not meet the system requirements. In this case, the initial switch angle sequence must be reorganized and arranged in an overall manner according to the preset rearrangement rules to achieve the reversal of adjustment polarity.

[0107] A specific angle rearrangement rule is formulated for the half-wave symmetric pulse mode A. After the initial sequence A1 is rearranged according to the predetermined rule, a brand new usable sequence is obtained. The specific arrangement is as follows:

[0108] Fourier coefficients of the pulse mode switching function corresponding to the rearranged new sequence and the absolute value of the switching function Fourier coefficients They are respectively:

[0109]

[0110]

[0111]

[0112] Comparing the Fourier coefficients before and after the rearrangement reveals that: the even-order cosine coefficients of the pulse-mode switching function are opposites, while the odd-order sine coefficients are the same; therefore, the fundamental amplitude is unaffected, and the total harmonic distortion of the output current is also unaffected. Furthermore, the even-order cosine coefficients of the absolute value of the pulse-mode switching function are the same, while the odd-order sine coefficients are opposites; therefore, the DC component of the midpoint current generated after interaction with the same odd-order phase current is also opposite. .

[0113] For the half-wave symmetric pulse mode B, a corresponding angle rearrangement rule is formulated. After the initial sequence B1 is rearranged according to the rule, a completely new sequence is generated. The specific arrangement is as follows:

[0114] Fourier coefficients of the pulse mode switching function corresponding to the rearranged new sequence and the absolute value of the switching function Fourier coefficients They are respectively:

[0115]

[0116]

[0117]

[0118] Comparing the Fourier coefficients before and after the rearrangement reveals that: the odd-order sine coefficients of the pulse-mode switching function are the same, while the even-order sine coefficients are opposites; therefore, the fundamental amplitude is unaffected, and the total harmonic distortion of the output current is also unaffected. Similarly, the even-order cosine coefficients of the absolute value of the pulse-mode switching function are the same, while the odd-order cosine coefficients are opposites; therefore, the DC component of the midpoint current generated after interaction with the same odd-order phase current is an opposite to it. .

[0119] It is important to note that the switching angle sequence after the above-mentioned regularized rearrangement only reverses the polarity of the DC component output of the midpoint current. It does not change the fundamental output amplitude corresponding to the pulse mode, nor does it affect the total harmonic distortion performance of the converter output current. Therefore, it eliminates the need for repeated iterative solutions to the optimization model, effectively reducing the real-time computational burden during equipment operation and ensuring the real-time performance and stability of the control. The actual control effects under different current phase conditions in the embodiments of this application can be referred to as follows: Figure 8 The simulation results of the midpoint voltage balance control when the fundamental current phase is 0° are shown in the figure. Figure 9 The simulation results of the midpoint voltage balance control when the fundamental current phase is 90° are shown to verify the effect and performance, and intuitively confirm the actual feasibility and stability of the sequence rearrangement strategy.

[0120] Step S4: Generate the drive signal of the converter bridge arm according to the target switching angle sequence to complete the converter output harmonic optimization and control. At the same time, correct the average deviation of the midpoint voltage based on the DC component of the midpoint current to achieve midpoint voltage balance control.

[0121] After completing the optimal selection and polarity calibration of the switching angle sequence through step S3, the actual drive control execution stage can begin. The core task of this stage is to convert the calibrated and optimized switching angle sequence into a drive level signal that can be recognized and executed by the power switching devices of the converter, so as to successfully implement the entire control strategy.

[0122] The control system relies on the final determined target switching angle sequence and combines it with the real-time operating electrical angle of the converter to complete the interval determination. It outputs corresponding control level signals according to the preset pulse timing sequence, precisely controlling all power switching devices to strictly complete the conduction and turn-off actions according to the preset timing sequence. Based on a novel pulse waveform structure tuned by an optimized algorithm, it can suppress the generation and outward propagation of various harmonic components from the pulse generation source, effectively optimizing the power quality of the converter output.

[0123] Meanwhile, the DC component of the midpoint current, after precise polarity calibration and matching, is continuously used for regulation. This controllable component continuously offsets the voltage difference between the upper and lower voltage divider capacitors on the DC side, forming a stable negative feedback regulation mechanism. Under various complex operating scenarios such as load changes, grid fluctuations, and operating condition switching, it can continuously and dynamically correct the deviation of the midpoint voltage and always keep the DC side midpoint voltage stable within the balanced operating range.

[0124] This overall control scheme has the ability to switch between flexible and autonomous operating modes. When the daily operating conditions are stable and there is no significant deviation in the midpoint potential, the system can automatically switch to the traditional symmetrical pulse modulation mode, which is simple in structure and convenient in operation, thus simplifying the overall control logic and reducing the power consumption and computational load of the equipment. Once an imbalance or deviation in the midpoint potential is detected, the system can quickly switch to two new types of half-wave symmetrical pulse modulation modes and immediately start the dynamic potential balance adjustment process.

[0125] The entire control scheme is implemented based on the full-process system of parameter acquisition, harmonic analysis, mathematical modeling, model optimization, and sequence calibration mentioned above. It follows the established operation logic and control rules throughout the process without the need for additional complex control links. It can stably and synchronously complete the two core control objectives of output power harmonic optimization and maintenance of DC side midpoint voltage dynamic balance. The overall control logic is a complete closed loop, with a wide range of operating conditions and strong practical engineering application value.

[0126] Example 2 This invention provides a neutral point voltage balance control device for a three-level converter. Figure 10 This is a schematic flowchart of a three-level converter midpoint voltage balance control device provided in an embodiment of the present invention. Figure 10 As shown, the device includes: The deviation determination module 100 is used to collect the voltage of the upper and lower bus capacitors on the DC side of the converter, calculate the real-time midpoint voltage and obtain the average value of the midpoint voltage, determine the voltage deviation state in combination with the preset voltage deviation threshold, and at the same time obtain the converter modulation ratio command and determine the adjustment direction of the required DC component of the midpoint current according to the voltage deviation state. Angle solving module 200 is used to construct an optimization model based on a preset half-wave symmetrical pulse mode, which includes output harmonic performance constraints and midpoint current DC component constraints, and solve for the initial switching angle sequence corresponding to the modulation ratio command. The flow direction correction module 300 is used to calculate the flow direction of the DC component of the midpoint current generated by the initial switching angle sequence and compare it with the preset adjustment direction. If the flow direction is consistent, it is directly determined as the target switching angle sequence; otherwise, the initial switching angle sequence is rearranged according to the preset rearrangement rules to generate the target switching angle sequence. The collaborative control module 400 is used to generate drive signals for the converter bridge arm according to the target switching angle sequence, complete the converter output harmonic optimization control, and at the same time correct the average deviation of the midpoint voltage based on the DC component of the midpoint current to achieve midpoint voltage balance control.

[0127] Regarding the apparatus in the above embodiments, the specific manner in which each module performs its operation has been described in detail in the embodiments related to the method, and will not be elaborated upon here.

[0128] Example 3 To implement the methods of the above embodiments, the present invention also provides an electronic device, which includes a memory and a processor; wherein the processor reads executable program code stored in the memory to run a program corresponding to the executable program code, so as to implement the various steps of the methods described above.

[0129] Example 4 To implement the above embodiments, this application also proposes a non-transitory computer-readable storage medium storing a computer program thereon, which, when executed by a processor, implements the method described in the foregoing embodiments.

[0130] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

[0131] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0132] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

Claims

1. A method for controlling the neutral point voltage balance of a three-level converter, characterized in that, include: The voltage of the upper and lower bus capacitors on the DC side of the converter is collected, the real-time midpoint voltage is calculated and the average value of the midpoint voltage is obtained. The voltage deviation status is determined by combining the preset voltage deviation threshold. At the same time, the converter modulation ratio command is obtained, and the adjustment direction of the required DC component of the midpoint current is determined according to the voltage deviation status. An optimization model containing output harmonic performance constraints and midpoint current DC component constraints is constructed based on a preset half-wave symmetrical pulse mode, and the initial switching angle sequence corresponding to the modulation ratio command is obtained by solving the model. The direction of the DC component of the midpoint current generated by the initial switching angle sequence is calculated and compared with the preset adjustment direction. If the direction is consistent, it is directly determined as the target switching angle sequence. Otherwise, the initial switching angle sequence is rearranged according to the preset rearrangement rules to generate the target switching angle sequence. The drive signal for the converter bridge arm is generated based on the target switching angle sequence to complete the converter output harmonic optimization and control. At the same time, the average deviation of the midpoint voltage is corrected based on the DC component of the midpoint current to achieve midpoint voltage balance control.

2. The method according to claim 1, characterized in that, The converter's DC-side upper and lower bus capacitor voltages are collected, the real-time midpoint voltage is calculated, and the average midpoint voltage is obtained. The voltage deviation state is determined based on a preset voltage deviation threshold. Simultaneously, the converter's modulation ratio command is acquired, and the required adjustment direction of the DC component of the midpoint current is determined according to the voltage deviation state, including: The voltage of the upper bus capacitor and the voltage of the lower bus capacitor on the DC side of the three-level NPC converter are collected and substituted into the predetermined calculation formula to obtain the real-time neutral point voltage of the converter. The real-time midpoint voltage is smoothed by using low-pass filtering or periodic averaging to extract a stable average midpoint voltage. The average value of the midpoint voltage is compared with the preset midpoint voltage deviation threshold to distinguish three types of deviation states: midpoint voltage too high, midpoint voltage too low, and midpoint voltage in the equilibrium range. Synchronously acquire the modulation ratio command parameters corresponding to the converter's operating conditions, and determine the required control direction for the DC component of the neutral point current to be positive, negative, or no, based on the completed neutral point voltage deviation state.

3. The method according to claim 2, characterized in that, An optimization model is constructed based on a preset half-wave symmetrical pulse mode, including output harmonic performance constraints and midpoint current DC component constraints. The initial switching angle sequence corresponding to the modulation ratio command is obtained by solving the model, including: Two types of symmetrical pulse structures, namely novel half-wave symmetrical pulse mode A and novel half-wave symmetrical pulse mode B, are pre-defined. The switching function and the absolute value mathematical model of the single-bridge arm pulse mode corresponding to the two types of pulse modes are established respectively. The harmonic component decomposition of the switching function and its absolute value is completed by Fourier series. Combined with the amplitude and phase parameters of each harmonic of the three-phase phase current, the coupling analytical relationship of the DC component of the midpoint current under two pulse modes is derived and solved. With optimal output harmonic performance as the optimization objective, a complete OPP optimization solution model is constructed by integrating fundamental amplitude constraints, switching angle sequence constraints, and midpoint current DC component balance constraints. The obtained modulation ratio command is imported into the optimization model to complete the numerical solution, and the output is the initial switching angle sequence adapted to the current operating conditions.

4. The method according to claim 3, characterized in that, Calculate the direction of the DC component of the midpoint current generated by the initial switching angle sequence and compare it with the preset adjustment direction, including: By combining current sampling with harmonic observation algorithm, the amplitude and phase parameters of each harmonic of the three-phase phase current are collected in real time. By combining the harmonic decomposition coefficients of the pulse mode corresponding to the initial switching angle sequence and substituting them into the formula for calculating the DC component of the midpoint current, the actual value and actual flow direction of the DC component of the midpoint current generated by this set of initial switching angle sequences are calculated. The actual flow direction of the calculated DC component of the midpoint current is compared with the preset adjustment direction of the DC component of the midpoint current to complete the flow direction consistency verification operation.

5. The method according to claim 4, characterized in that, If the flow direction is consistent, the initial switching angle sequence is directly determined as the target switching angle sequence; otherwise, the initial switching angle sequence is rearranged according to a preset rearrangement rule to generate the target switching angle sequence, including: When the DC component of the midpoint current generated by the initial switching angle sequence flows in the same direction as the preset adjustment direction, the initial switching angle sequence is directly designated as the target switching angle sequence for final control. When the DC component of the midpoint current generated by the initial switching angle sequence deviates from the preset adjustment direction, the pre-set exclusive rearrangement rules are invoked to perform angle interval translation and recombination operation on half-wave symmetrical pulse mode A and angle inversion and recombination operation on half-wave symmetrical pulse mode B. The original initial switching angle sequence is completely rearranged to generate a new target switching angle sequence with opposite control polarity and unchanged harmonic operation characteristics.

6. The method according to claim 5, characterized in that, The drive signal for the converter bridge arm is generated based on the target switching angle sequence to complete the converter output harmonic optimization and control. Simultaneously, the average deviation of the neutral point voltage is corrected based on the DC component of the neutral point current to achieve neutral point voltage balance control, including: The final determined target switching angle sequence is converted into timing drive control signals corresponding to each bridge arm of the three-level NPC converter. The output drive control signal controls the power switching devices of the converter to turn on and off in sequence. The optimized pulse mode suppresses the harmonic content on the output side of the converter, thus completing the output harmonic performance optimization and control. Simultaneously, by utilizing the inherent characteristic that the DC component of the midpoint current can be reversed by rearranging the switching angle sequence before and after, and by using the matched and adapted DC component of the midpoint current to correct the average deviation of the midpoint voltage, the dual control tasks of converter harmonic optimization operation and dynamic balance of DC side midpoint voltage are completed simultaneously.

7. A three-level converter neutral point voltage balance control device, characterized in that, include: The deviation determination module is used to collect the voltage of the upper and lower bus capacitors on the DC side of the converter, calculate the real-time midpoint voltage and obtain the average value of the midpoint voltage, determine the voltage deviation status in combination with the preset voltage deviation threshold, and at the same time obtain the converter modulation ratio command, and determine the adjustment direction of the required DC component of the midpoint current according to the voltage deviation status. An angle solving module is used to construct an optimization model based on a preset half-wave symmetrical pulse mode, which includes output harmonic performance constraints and midpoint current DC component constraints, and solve for the initial switching angle sequence corresponding to the modulation ratio command. The flow direction correction module is used to calculate the flow direction of the DC component of the midpoint current generated by the initial switching angle sequence and compare it with the preset adjustment direction. If the flow direction is consistent, it is directly determined as the target switching angle sequence; otherwise, the initial switching angle sequence is rearranged according to the preset rearrangement rules to generate the target switching angle sequence. The collaborative control module is used to generate drive signals for the converter bridge arm based on the target switching angle sequence, complete the converter output harmonic optimization control, and at the same time correct the average deviation of the midpoint voltage based on the DC component of the midpoint current to achieve midpoint voltage balance control.

8. The apparatus according to claim 7, characterized in that, The deviation determination module is also used for: The voltage of the upper bus capacitor and the voltage of the lower bus capacitor on the DC side of the three-level NPC converter are collected and substituted into the predetermined calculation formula to obtain the real-time neutral point voltage of the converter. The real-time midpoint voltage is smoothed by using low-pass filtering or periodic averaging to extract a stable average midpoint voltage. The average value of the midpoint voltage is compared with the preset midpoint voltage deviation threshold to distinguish three types of deviation states: midpoint voltage too high, midpoint voltage too low, and midpoint voltage in the equilibrium range. Synchronously acquire the modulation ratio command parameters corresponding to the converter's operating conditions, and determine the required control direction for the DC component of the neutral point current to be positive, negative, or no, based on the completed neutral point voltage deviation state.

9. An electronic device, characterized in that, Including processor and memory; The processor reads executable program code stored in the memory to run a program corresponding to the executable program code, so as to implement the method as described in any one of claims 1-6.

10. A non-transitory computer-readable storage medium having a computer program stored thereon, characterized in that, When the program is executed by the processor, it implements the method as described in any one of claims 1-6.

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