A method and system for balancing the neutral point voltage of a three-level converter

Abstract

The application provides a midpoint voltage balance control method and system of a three-level converter, and the method comprises the following steps: selecting a certain phase in three-phase output current as a target injection phase; and performing differential mode voltage adjustment on the phase in a control period. The differential mode voltage adjustment comprises a first injection stage and a second injection stage, and the first differential mode voltage and the second differential mode voltage are respectively injected, the first differential mode voltage and the second differential mode voltage are opposite in sign, and the algebraic sum of the product of each differential mode voltage and the duration of each differential mode voltage is zero. Through the differential mode injection strategy in stages, the application artificially creates a current capable of repairing the midpoint potential deviation, while ensuring that the average value of the output voltage command is unchanged by following the volt-second balance principle, and solves the problem that the midpoint voltage is difficult to balance under a low modulation condition.

Description

Technical Field

[0001] This invention relates to the field of three-level converters, and more particularly to a method and system for controlling the midpoint voltage balance of a three-level converter. Background Technology

[0002] Three-level converters are widely used in motor drives and grid-connected power generation due to their superior output waveform quality. Their DC side is typically supported by two capacitors connected in series, and the balance of the midpoint potential is crucial for the stable operation of the system. If the midpoint voltage shifts, it will not only cause distortion of the AC output waveform and increase current harmonics, but in severe cases, it can also damage the DC bus capacitors or power switching devices due to overvoltage. Therefore, taking effective measures to maintain the balance of the midpoint potential is a key aspect of ensuring the safe and reliable operation of the three-level converter.

[0003] Existing technologies typically regulate the neutral point current to achieve voltage balance by injecting common-mode voltage into the three-phase modulated waveform. However, this control strategy based on common-mode voltage injection has significant limitations under low-voltage operating conditions. When the system is in a low-voltage state, the differential-mode components of the three-phase voltages are small, and injecting common-mode voltage often leads to a convergence of polarity among the three-phase voltages. At this time, the effects of the phase currents on the neutral point cancel each other out, making it impossible for the system to generate sufficient neutral point regulating current. This results in the conventional control strategy effectively losing its ability to control the neutral point voltage in the low-voltage region.

[0004] Therefore, in view of the problem that the current technology is insufficient in the midpoint balance control capability under low-voltage regime, there is an urgent need for a new control scheme that can provide effective and controllable midpoint voltage balance capability across the entire operating range, especially under low-voltage regime conditions. Summary of the Invention

[0005] In order to overcome the above-mentioned technical defects, the purpose of this invention is to provide a method and system for controlling the midpoint voltage balance of a three-level converter.

[0006] This invention discloses a method for controlling the midpoint voltage balance of a three-level converter, comprising: Obtain the midpoint voltage deviation and three-phase output current of the three-level converter, and select one phase of the three-phase output current as the target injection phase; During the control cycle, differential mode voltage regulation is performed on the voltage command of the target injection phase. The regulation process includes a first injection stage and a second injection stage. In the first injection stage, a first differential voltage is injected into the voltage command of the target injection phase; in the second injection stage, a second differential voltage is injected into the voltage command of the target injection phase. The first differential voltage and the second differential voltage have opposite signs, and the algebraic sum of the product of the first differential voltage and the first injection phase time and the product of the second differential voltage and the second injection phase time is zero.

[0007] Preferably, selecting one phase of the three-phase output current as the target injection phase includes: Compare the absolute values ​​of the currents in each of the three phases; determine the phase with the largest absolute value of the three-phase output current as the target injection phase.

[0008] Preferably, the direction of the first differential voltage is configured as follows: the desired direction of the midpoint current is determined according to the polarity of the midpoint voltage deviation value; the sign of the first differential voltage is determined according to the desired direction of the midpoint current and the current direction of the target injected phase, so that the midpoint current generated after injecting the first differential voltage can reduce the midpoint voltage deviation value.

[0009] Preferably, the amplitude of the first differential voltage is configured such that the midpoint voltage deviation value is multiplied by a preset scaling factor to obtain the amplitude of the first differential voltage.

[0010] Preferably, the amplitude of the first differential voltage is configured as follows: proportional-integral control is performed on the midpoint voltage deviation value to obtain the desired midpoint current command; the amplitude of the first differential voltage is calculated based on the desired midpoint current command and the current value of the target injected phase.

[0011] Preferably, when the voltage command of the target injection phase already includes a common-mode voltage component, the amplitude of the first differential-mode voltage is smaller than the amplitude of the common-mode voltage component.

[0012] Preferably, the direction of the first differential voltage is configured to be determined by the sign of the common-mode voltage component, the polarity of the midpoint voltage deviation value, and the current direction of the target injected phase.

[0013] Preferably, the midpoint voltage balance control method further includes: Determine whether the modulation index of the three-level converter is lower than the preset modulation index threshold, or determine whether the differential voltage component corresponding to the three-phase output current is lower than the preset voltage threshold; if so, adjust the differential voltage of the voltage command of the target injected phase.

[0014] A second aspect of this application also provides a midpoint voltage balance control system for a three-level converter, used to implement the midpoint voltage balance control method as described in any of the foregoing claims; the midpoint voltage balance system includes: The acquisition module acquires the midpoint voltage deviation and three-phase output current of the three-level converter. The selection and adjustment module selects one phase of the three-phase output current as the target injection phase, and performs differential mode voltage adjustment on the voltage command of the target injection phase within the control cycle.

[0015] Compared with existing technologies, the above technical solution has the following advantages: 1. This invention solves the technical problem of unbalanced neutral point voltage in three-level converters under low-voltage operating conditions by employing a phased differential-mode voltage injection strategy. Under low-voltage conditions, due to the extremely close duty cycles of the three phases, conventional common-mode voltage injection methods cannot generate effective neutral point regulation current. This scheme actively injects differential-mode voltage into the target phase in the first stage, artificially creating a current capable of correcting the neutral point potential deviation; then, in the second stage, a reverse voltage is injected, strictly adhering to the volt-second balance principle, ensuring that the algebraic sum of the injected voltages within one control cycle is zero. This mechanism generates effective neutral point balance correction capability within a micro-cycle and macroscopically maintains the average value of the output voltage command unchanged, thus achieving effective regulation of the neutral point voltage without affecting the output fundamental voltage. 2. Optimized phase selection and sign determination logic ensures efficient and robust regulation. By selecting the phase with the largest absolute current value as the injection target, the maximum midpoint regulation capability can be achieved under the same injection amplitude, or the voltage amplitude requirement can be reduced under the same regulation demand. In terms of control logic, the algorithm comprehensively considers the polarity of the midpoint voltage deviation and the current flow direction to ensure effective regulation without affecting the normal operation of the load. Specifically for complex operating conditions that already include common-mode voltage components, the prediction model of the midpoint current is corrected by limiting the differential-mode voltage amplitude to be less than the common-mode component and introducing the common-mode sign as a third judgment dimension. This effectively prevents incorrect regulation direction or polarity reversal caused by common-mode voltage modulation, ensuring that the algorithm can accurately determine the regulation direction under various modulation strategies and maintain system stability. 3. This application also provides flexible amplitude calculation and operation strategies to adapt to different control accuracy requirements. The scheme supports both computationally simple proportional control to meet the need for fast response, and high-precision control based on proportional-integral (PI) and current back-calculation. The latter achieves accurate tracking of the midpoint voltage by decoupling the influence of load current. In addition, by setting the modulation index or differential mode component threshold, this invention implements an on-demand activation mechanism: the control method is activated only in the low-voltage or low-modulus "dead zone" where conventional methods fail, and the intervention is withdrawn in the normal region, thereby allowing and optimizing the converter's operation mode across the entire operating range. Attached Figure Description

[0016] Figure 1 A flowchart illustrating the midpoint voltage balance control method for the three-level converter provided in this application; Figure 2 A schematic diagram of the control method for the midpoint voltage balance control method of the three-level converter provided in this application when there is a common-mode voltage. Detailed Implementation

[0017] The advantages of the present invention will be further illustrated below with reference to the accompanying drawings and specific embodiments.

[0018] Exemplary embodiments will now be described in detail, examples of which are illustrated in the accompanying drawings. When the following description relates to the drawings, unless otherwise indicated, the same numerals in different drawings denote the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with this disclosure. Rather, they are merely examples of apparatuses and methods consistent with some aspects of this disclosure as detailed in the appended claims.

[0019] The terminology used in this disclosure is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. The singular forms “a,” “the,” and “the” as used in this disclosure and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the term “and / or” as used herein refers to and includes any and all possible combinations of one or more of the associated listed items.

[0020] It should be understood that although the terms first, second, third, etc., may be used in this disclosure to describe various information, such information should not be limited to these terms. These terms are used only to distinguish information of the same type from one another. For example, without departing from the scope of this disclosure, first information may also be referred to as second information, and similarly, second information may also be referred to as first information. Depending on the context, the word "if" as used herein may be interpreted as "when," "when," or "in response to determination."

[0021] In the description of this invention, it should be understood that the terms "longitudinal", "lateral", "up", "down", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0022] In the description of this invention, unless otherwise specified and limited, it should be noted that the terms "installation", "connection" and "linking" should be interpreted broadly. For example, they can refer to mechanical or electrical connections, or internal connections between two components. They can be direct connections or indirect connections through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms according to the specific circumstances.

[0023] In the following description, suffixes such as "module," "part," or "unit" used to denote elements are used only for the convenience of the description of the invention and have no specific meaning in themselves. Therefore, "module" and "part" can be used interchangeably.

[0024] Please see Figure 1 , Figure 1 A flowchart illustrating the midpoint voltage balance control method for the three-level converter provided in this application.

[0025] like Figure 1 As shown, this invention discloses a method and system for controlling the midpoint voltage balance of a three-level converter.

[0026] This invention discloses a method for controlling the midpoint voltage balance of a three-level converter, comprising: Obtain the midpoint voltage deviation and three-phase output current of the three-level converter, and select one phase of the three-phase output current as the target injection phase; During the control cycle, differential mode voltage regulation is performed on the voltage command of the target injection phase. The regulation process includes a first injection stage and a second injection stage. In the first injection stage, a first differential voltage is injected into the voltage command of the target injection phase; in the second injection stage, a second differential voltage is injected into the voltage command of the target injection phase. The first differential voltage and the second differential voltage have opposite signs, and the algebraic sum of the product of the first differential voltage and the first injection phase time and the product of the second differential voltage and the second injection phase time is zero.

[0027] First, let's explain the principle: From the working principle of a three-level topology, we know that when the converter operates in zero-level output mode, current flows through the transverse tube, and the midpoint current ( This will directly cause fluctuations in the DC bus midpoint potential. Specifically, if the current flowing through the horizontal tube is positive, the voltage of the upper bus capacitor will rise, and the voltage of the lower bus capacitor will fall; and vice versa.

[0028] Therefore, if we define the three-phase output voltages (based on 0.5Vdc) as Ua, Ub, and Uc respectively, and the output currents as Ia, Ib, and Ic respectively, then, without considering additional injection, the contribution of the three-phase outputs to the neutral point current can be expressed by the following formula: In order to maintain midpoint balance (i.e., to achieve the desired voltage difference between the upper and lower bus capacitors) (If the value is 0), the conventional control method is to inject common-mode voltage. That is, to simultaneously superimpose the same bias value onto the three-phase voltage. However, under a low-mode control, this control method will be completely ineffective because: a low-mode control refers to a small differential-mode voltage component in the three phases. In this case, if a large common-mode voltage is injected to balance the neutral point, the control will fail. This will cause the final command voltages Ua, Ub, and Uc of the three phases to have the same positive and negative polarities. In this case, the above formula becomes: This means that under low-voltage conditions such as low-speed motor start-up and stall, the traditional common-mode injection method cannot generate an effective midpoint regulating current, resulting in the midpoint voltage losing its control capability.

[0029] Therefore, the control method provided in this application selects a phase as the target injection phase based on the adjustment requirements of the midpoint voltage deviation during the first injection stage (disturbance stage), and superimposes a first differential voltage onto its voltage command. In the second injection phase (compensation phase), the influence of the injection voltage in the first phase on the average value of the output waveform is eliminated, and a reversed second differential voltage is injected within the same cycle. Assuming the selected phase is phase a, and given the very small modulation density, the aforementioned formula can be simplified to: This allows, on the one hand, the first differential voltage to be used. Differential mode control enables precise (controllable and effective) adjustment of the midpoint current. This achieves voltage balance at the midpoint. On the other hand, based on the volt-second balance condition, the algebraic sum of the product of the first differential voltage and the duration of the first stage, plus the product of the second differential voltage and the duration of the second stage, can be controlled to be zero. This allows the output voltage command to remain constant despite the differential mode command changing in a very short time, thanks to compensation in the second injection stage. This solves the midpoint balance problem under low-voltage operating conditions and does not adversely affect subsequent load control.

[0030] The above is an explanation of the basic concept and principle of this application. The specific implementation methods of each step in this method will be explained below.

[0031] First, when implementing the above strategy, in order to maximize the adjustment efficiency, it is necessary to reasonably select the target injection phase.

[0032] In one possible implementation, selecting one phase of the three-phase output current as the target injection phase includes: Compare the absolute values ​​of the currents in each of the three phases; determine the phase with the largest absolute value of the three-phase output current as the target injection phase.

[0033] As can be seen from the aforementioned formula, the midpoint current is proportional to the phase current amplitude. Therefore, by comparing the absolute values ​​of the three-phase output currents in real time and determining the phase with the largest absolute value (e.g., phase A) as the target injection phase, the midpoint repair current that can be generated by this phase can be maximized when injecting the same differential-mode voltage amplitude; or, under the same regulation requirements, a smaller voltage injection amplitude can be used, thereby reducing disturbances to the system.

[0034] Secondly, the direction of the first differential voltage is also unrestricted.

[0035] In one possible implementation, when the first differential voltage is used as the main regulation means, the direction of the first differential voltage is configured as follows: the desired direction of the midpoint current is determined according to the polarity of the midpoint voltage deviation value; the sign of the first differential voltage is determined according to the desired direction of the midpoint current and the current direction of the target injected phase, so that the midpoint current generated after injecting the first differential voltage can reduce the midpoint voltage deviation value.

[0036] This can be understood as: when a positive midpoint current is required ( When the selected phase current is positive (>0), the injected voltage should increase the contribution of that phase; conversely, the negative voltage should also increase the contribution. This allows for the regulation of the midpoint voltage.

[0037] Furthermore, the method of obtaining the amplitude of the first differential voltage is also unrestricted.

[0038] In one possible implementation, the amplitude of the first differential voltage is configured to be obtained by multiplying the midpoint voltage deviation value by a preset scaling factor.

[0039] The injected voltage amplitude is calculated directly by multiplying the voltage deviation by a scaling factor. This method has a simple calculation logic, does not require complex iteration or integration calculations, and can quickly adjust the injected voltage amplitude according to the magnitude of the deviation. It is suitable for applications that are sensitive to computational resources or have certain requirements for dynamic response.

[0040] In another possible implementation, the amplitude of the first differential voltage is configured as follows: proportional-integral control is performed on the midpoint voltage deviation value to obtain the desired midpoint current command; the amplitude of the first differential voltage is calculated based on the desired midpoint current command and the current value of the target injected phase.

[0041] The desired midpoint current is obtained through proportional-integral (PI) control, and then the required injection voltage amplitude is calculated by combining it with the current phase current. Compared with simple proportional control, the introduction of an integral element helps to eliminate the steady-state error of the midpoint voltage. At the same time, the influence of the current load current on the regulation effect is considered through back-calculation, so that the system can generate a more accurate desired midpoint current under different load current conditions, thus improving the control accuracy and consistency.

[0042] Finally, in certain special cases (such as when using SVPWM modulation), the converter's voltage command already includes a common-mode voltage component for vector synthesis. In this case, blindly injecting differential-mode voltage may disrupt the original modulation logic.

[0043] Please see Figure 2 , Figure 2 A schematic diagram of the control method for the midpoint voltage balance control method of the three-level converter provided in this application when there is a common-mode voltage.

[0044] like Figure 2 As shown, in one possible implementation, when the voltage command of the target injected phase already contains a common-mode voltage component, the amplitude of the first differential-mode voltage is smaller than the amplitude of the common-mode voltage component. Thus, by limiting the differential-mode voltage amplitude to be smaller than the common-mode voltage component, it is ensured that after superimposed disturbances, the overall sign characteristic of the voltage command is still dominated by the original common-mode component. This guarantees that while introducing midpoint balancing capability, the original modulation logic and output characteristics of the system are not interfered with or destroyed, achieving harmonious coexistence of the two control dimensions.

[0045] Furthermore, the direction of the first differential voltage is configured to be determined by the sign of the common-mode voltage component, the polarity of the midpoint voltage deviation value, and the current direction of the target injected phase.

[0046] With common-mode voltage command Then, the aforementioned formula is transformed into: Therefore, when determining the sign of the first differential-mode voltage, the sign of the common-mode voltage must be introduced as a third judgment dimension. Specifically, if the common-mode voltage is positive, the control logic is similar to the normal mode; if the common-mode voltage is negative, the injection logic may need to be inverted to counteract the effect of the common-mode sign.

[0047] The above is a complete description of the midpoint voltage balance control method of this application. Those skilled in the art will understand that this midpoint voltage balance control method is primarily aimed at scenarios with low modulation levels.

[0048] Therefore, in one possible implementation, the midpoint voltage balance control method also includes: The system determines whether the modulation index of the three-level converter is lower than a preset modulation index threshold, or whether the differential-mode voltage component corresponding to the three-phase output current is lower than a preset voltage threshold. If so, differential-mode voltage adjustment is performed on the voltage command of the target injected phase. This ensures that the differential-mode voltage adjustment step is automatically activated when the differential-mode voltage component corresponding to the three-phase output current is lower than the preset voltage threshold; otherwise, it exits or uses conventional methods to further improve the neutral point voltage balance effect and speed.

[0049] A second aspect of this application also provides a midpoint voltage balance control system for a three-level converter, used to implement the midpoint voltage balance control method as described in any of the foregoing embodiments; the midpoint voltage balance system includes: The acquisition module acquires the midpoint voltage deviation and three-phase output current of the three-level converter. The selection and adjustment module selects one phase of the three-phase output current as the target injection phase, and performs differential mode voltage adjustment on the voltage command of the target injection phase within the control cycle.

[0050] This allows the aforementioned control method to be solidified into a specific industrial control device, ensuring the safe and stable operation of the three-level converter.

[0051] It should be noted that the embodiments of the present invention have better implementability and are not intended to limit the present invention in any way. Any person skilled in the art may use the above-disclosed technical content to change or modify it into equivalent effective embodiments. However, any modifications or equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A method for controlling the midpoint voltage balance of a three-level converter, characterized in that, The method includes: Obtain the midpoint voltage deviation value and the three-phase output current of the three-level converter, and select one phase of the three-phase output current as the target injection phase; During the control cycle, differential mode voltage regulation is performed on the voltage command of the target injection phase. The regulation process includes a first injection stage and a second injection stage. In the first injection phase, a first differential voltage is injected into the voltage command of the target injection phase; in the second injection phase, a second differential voltage is injected into the voltage command of the target injection phase. Wherein, the first differential mode voltage and the second differential mode voltage have opposite signs, and the algebraic sum of the product of the first differential mode voltage and the first injection phase time and the product of the second differential mode voltage and the second injection phase time is zero.

2. The midpoint voltage balance control method as described in claim 1, characterized in that, Selecting one phase of the three-phase output current as the target injection phase includes: Compare the absolute values ​​of the currents in each of the three phases; determine the phase with the largest absolute value of the three phase output current as the target injection phase.

3. The midpoint voltage balance control method as described in claim 1, characterized in that, The direction of the first differential voltage is configured to: determine the desired midpoint current direction based on the polarity of the midpoint voltage deviation value; and determine the sign of the first differential voltage based on the desired midpoint current direction and the current direction of the target injection phase, so that the midpoint current generated after injecting the first differential voltage can reduce the midpoint voltage deviation value.

4. The midpoint voltage balance control method as described in claim 1, characterized in that, The amplitude of the first differential voltage is configured to be obtained by multiplying the midpoint voltage deviation value by a preset scaling factor.

5. The midpoint voltage balance control method as described in claim 1, characterized in that, The amplitude of the first differential voltage is configured as follows: proportional-integral control is performed on the midpoint voltage deviation value to obtain the desired midpoint current command; the amplitude of the first differential voltage is calculated based on the desired midpoint current command and the current value of the target injected phase.

6. The midpoint voltage balance control method as described in claim 1, characterized in that, When the voltage command of the target injection phase already includes a common-mode voltage component, the amplitude of the first differential-mode voltage is less than the amplitude of the common-mode voltage component.

7. The midpoint voltage balance control method as described in claim 6, characterized in that, The direction of the first differential voltage is configured to be determined by the sign of the common-mode voltage component, the polarity of the midpoint voltage deviation value, and the current direction of the target injection phase.

8. The midpoint voltage balance control method as described in claim 1, characterized in that, The midpoint voltage balance control method further includes: Determine whether the modulation degree of the three-level converter is lower than a preset modulation degree threshold, or determine whether the differential mode voltage component corresponding to the three-phase output current is less than a preset voltage threshold; if so, adjust the differential mode voltage for the voltage command of the target injection phase.

9. A midpoint voltage balance control system for a three-level converter, characterized in that, The midpoint voltage balance control system is used to implement the midpoint voltage balance control method as described in any one of claims 1-8; the midpoint voltage balance system includes: The acquisition module acquires the midpoint voltage deviation value and the three-phase output current of the three-level converter; The selection and adjustment module selects one phase of the three-phase output current as the target injection phase, and performs differential mode voltage adjustment on the voltage command of the target injection phase within the control cycle.

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