Automatic detection and balance control device for neutral point potential imbalance in three-level converter

Abstract

The application discloses a three-level converter neutral point potential imbalance automatic detection and balance control device, and belongs to the technical field of neutral point potential balance control of power electronic converters. The device comprises a DC support unit, a voltage detection unit, an imbalance comparison unit, a driving unit and an energy transfer and power control unit. The voltage detection unit detects the voltage of the first and second DC side voltage dividing capacitors and outputs a voltage deviation signal; the imbalance comparison unit generates a control signal corresponding to the deviation direction according to the voltage deviation signal; and the driving unit drives the energy transfer and power control unit according to the control signal, so that the energy of the voltage dividing capacitor with higher voltage is transferred to the voltage dividing capacitor with lower voltage. The application can inhibit neutral point potential deviation, reduce the risk of overvoltage of power devices and damage of capacitors, and improve the operation reliability of the converter.

Description

Technical Field

[0001] This invention belongs to the field of power electronic converter midpoint potential balance control technology, specifically relating to an automatic detection and balance control device for midpoint potential imbalance in a three-level converter. It is applicable to three-level reactive power compensation devices, harmonic suppression devices, three-phase four-wire power supplies, and other power electronic conversion systems that require balance control of the DC side midpoint potential. Background Technology

[0002] Three-level converters, due to their advantages such as multiple output voltage levels, low voltage withstand capability of switching devices, low harmonic content of output current, and relatively small filter size, have been widely used in power electronic systems such as reactive power compensation, active filtering, three-phase four-wire power supply, motor drive, and new energy grid connection. For diode-clamped, T-type, and other three-level topologies, the DC side typically consists of two voltage-dividing capacitors connected in series to form the DC bus midpoint. The normal operation of the converter depends on the voltages of the two voltage-dividing capacitors maintaining a relative balance.

[0003] However, in actual operation, due to factors such as grid voltage imbalance, three-phase load imbalance, neutral current fluctuation, differences in switching device parameters, DC-side capacitor capacity deviation, aging differences, and harmonic impacts, the charging and discharging states of the two voltage-dividing capacitors on the DC side are prone to inconsistency, leading to a shift or fluctuation in the DC bus midpoint potential. When the midpoint potential continues to shift, on the one hand, it will cause an imbalance in the voltage across the upper and lower bridge arm power devices, increasing the risk of overvoltage failure of some switching devices; on the other hand, it will cause distortion of the output voltage waveform, increase harmonic content, reduce the power quality of the converter output, and in severe cases, may even cause overvoltage or damage to the DC-side capacitors or system protection shutdown.

[0004] Existing neutral point potential balance control methods mainly include software control methods based on modulation algorithms and methods with additional hardware balancing circuits. Software control methods based on Space Vector Pulse Width Modulation (SVPWM) or Sinusoidal Pulse Width Modulation (SPWM) typically require three-phase coordinate transformation, sector determination, selection of redundant small vectors, or injection of zero-sequence components. Their control process relies on the main controller algorithm, resulting in high computational complexity. Under conditions of sudden load changes, rapid changes in neutral current, or high harmonic content, they suffer from limited response speed, lag in the adjustment process, and strong coupling between the control strategy and the main inverter control. While additional hardware balancing circuits offer advantages such as fast response speed and relatively independent control, existing solutions still suffer from high detection circuit costs, the need for an additional power supply, complex balancing branch structures, large device size, and frequent switching of the neutral point potential near the threshold.

[0005] Therefore, how to quickly detect the DC side midpoint potential imbalance without significantly increasing the complexity of the main control algorithm of the three-level converter, and actively transfer energy according to the direction of imbalance to suppress midpoint potential drift and reduce the risk of device overvoltage and capacitor damage, is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] The technical problem to be solved by this invention is to address the shortcomings of the prior art by providing an automatic detection and balancing control device for neutral point potential imbalance in a three-level converter. The device obtains the required balancing voltage for the DC link and the current neutral point potential of the DC support unit through a detection unit. These two voltages are then sent to an imbalance comparison unit, which generates a control signal after imbalance comparison. This signal is then used by a drive circuit to activate the switching transistor of the energy transfer circuit, enabling energy transfer through the charging and discharging of the DC support unit capacitors. This achieves self-balancing control of the potential. This invention addresses the technical problem of existing three-level converters where, under conditions of grid voltage imbalance, load imbalance, neutral current fluctuations, DC-side capacitor parameter differences, and harmonics, the voltages of the two voltage divider capacitors on the DC side are prone to imbalance, leading to neutral point potential shifts or fluctuations. This, in turn, causes uneven voltage stress on power devices, output voltage distortion, power quality degradation, and an increased risk of DC-side capacitor overvoltage damage.

[0007] Furthermore, this invention also addresses the problems of existing software-based midpoint potential balancing control methods based on SVPWM or SPWM, such as complex coordinate transformation and sector judgment processes, relatively slow adjustment response, and strong coupling between the control strategy and the main inverter control. It also addresses the problems of existing hardware balancing circuits, such as complex detection and power supply structures, high cost, frequent switching near the threshold, and large volume of the balancing branch.

[0008] The present invention adopts the following technical solution: The automatic detection and balance control device for neutral point potential imbalance of a three-level converter includes a DC support unit, a voltage detection unit, an imbalance comparison unit, a drive unit, and an energy transfer and power control unit. The DC support unit includes a first voltage divider capacitor and a second voltage divider capacitor connected in series, and the connection point of the first voltage divider capacitor and the second voltage divider capacitor forms the midpoint of the DC support unit. The voltage detection unit is connected to the DC support unit and is used to detect the voltage of the first voltage divider capacitor and the voltage of the second voltage divider capacitor, and output the voltage deviation signal between the voltage of the first voltage divider capacitor and the voltage of the second voltage divider capacitor. The imbalance comparison unit is connected to the voltage detection unit and is used to generate a first control signal corresponding to a first deviation direction or a second control signal corresponding to a second deviation direction based on the voltage deviation signal. The drive unit is connected to the imbalance comparison unit and is used to drive the energy transfer and power control unit according to the first control signal or the second control signal. The energy transfer and power control unit is connected to the DC support unit and is used to transfer the energy of the first voltage divider capacitor to the second voltage divider capacitor under the control state corresponding to the first control signal, and to transfer the energy of the second voltage divider capacitor to the first voltage divider capacitor under the control state corresponding to the second control signal, so that the voltage of the first voltage divider capacitor and the voltage of the second voltage divider capacitor tend to be balanced.

[0009] Preferably, the voltage detection unit is used to detect the positive DC bus voltage, the negative DC bus voltage, and the midpoint potential of the midpoint of the DC support unit, and to obtain the voltage deviation signal based on the positive DC bus voltage, the negative DC bus voltage, and the midpoint potential.

[0010] Preferably, the unbalanced comparison unit includes a first comparison circuit and a second comparison circuit. The first comparison circuit is used to output the first control signal when the voltage of the first voltage divider capacitor is higher than the voltage of the second voltage divider capacitor, and the second comparison circuit is used to output the second control signal when the voltage of the first voltage divider capacitor is lower than the voltage of the second voltage divider capacitor.

[0011] Preferably, both the first comparison circuit and the second comparison circuit are hysteresis comparison circuits, and the hysteresis comparison circuit has a hysteresis threshold voltage to suppress frequent switching of the first control signal or the second control signal when the voltage deviation signal is near the comparison threshold.

[0012] Preferably, the system further includes a self-powered comparator circuit, which includes a charging resistor, a reverse discharge protection diode, an energy storage voltage divider capacitor, and a Zener diode. The self-powered comparator circuit is used to draw power from the DC side and supply power to the first comparator circuit and the second comparator circuit.

[0013] Preferably, the energy transfer and power control unit includes a first switch, a second switch, an inductor, and an energy storage capacitor, and the driving unit is used to drive the first switch to conduct according to the first control signal, or to drive the second switch to conduct according to the second control signal.

[0014] Preferably, when the voltage of the first voltage divider capacitor is higher than the voltage of the second voltage divider capacitor, the first switch is turned on, so that the energy in the first voltage divider capacitor is transferred through the first switch, the inductor and the energy storage capacitor, and after the first switch is turned off, it is transferred to the second voltage divider capacitor through the freewheeling effect of the inductor.

[0015] Preferably, when the voltage of the first voltage divider capacitor is lower than the voltage of the second voltage divider capacitor, the second switch is turned on, so that the energy in the second voltage divider capacitor is transferred through the second switch, the inductor and the energy storage capacitor, and after the second switch is turned off, it is transferred to the first voltage divider capacitor through the freewheeling effect of the inductor.

[0016] Preferably, the energy storage capacitor is connected in parallel with the first voltage divider capacitor and cooperates with the second voltage divider capacitor to form an impedance voltage regulation and current shunt circuit when the first switch is turned on, or it is connected in parallel with the second voltage divider capacitor and cooperates with the first voltage divider capacitor to form an impedance voltage regulation and current shunt circuit when the second switch is turned on.

[0017] Preferably, the parameters of the inductor and the energy storage capacitor are determined based on the unbalanced energy transfer, the total energy transferred to the inductor and the energy storage capacitor, the maximum allowable voltage deviation of the system, the neutral current harmonic constraint, and the power circuit loss constraint. The automatic detection and balance control device for the neutral point potential imbalance of the three-level converter is in standby mode when the voltage of the first voltage divider capacitor is balanced with the voltage of the second voltage divider capacitor.

[0018] Compared with the prior art, the present invention has at least the following beneficial effects: The automatic detection and balancing control device for neutral point potential imbalance in a three-level converter defines the DC support unit, voltage detection unit, imbalance comparison unit, drive unit, and energy transfer and power control unit as a cooperating whole. It clearly defines the complete control link from voltage detection of the voltage divider capacitor, deviation direction judgment, drive control to bidirectional energy transfer. Through hardware detection and energy transfer branch, it directly acts on the DC side voltage divider capacitor, enabling energy to be transferred from the higher voltage side to the lower voltage side, thereby quickly suppressing the neutral point potential deviation. It can reduce the dependence on complex modulation algorithms such as SVPWM and SPWM, reduce the coupling between the device and the main inverter control strategy, and make the device more adaptable and portable. It can be used in various application scenarios such as three-level reactive power compensation, harmonic suppression, and three-phase four-wire power supply.

[0019] Furthermore, this provides a clear and reliable voltage sampling basis for determining midpoint potential imbalance. By detecting the positive and negative DC bus voltages and the midpoint potential of the DC support unit, the voltage states of the first and second voltage-dividing capacitors can be obtained more accurately. This allows for the determination of whether the midpoint potential deviates from the equilibrium position. Compared to detecting only a single voltage quantity, this detection method can simultaneously reflect the overall DC bus voltage state and the relative deviation between the upper and lower voltage-dividing capacitors, avoiding misjudgments of midpoint offset due to fluctuations in the total bus voltage. By converting the above detection results into a voltage deviation signal and outputting it to the imbalance comparison unit, the subsequent comparison circuit can directly determine the degree and direction of imbalance based on the deviation signal, improving the specificity of the detection results and the accuracy of the control actions, and providing a reliable basis for energy transfer branch selection.

[0020] Furthermore, the midpoint potential imbalance state is divided into two cases: the voltage of the first voltage divider capacitor is too high and the voltage of the second voltage divider capacitor is too high. The first comparison circuit outputs a first control signal when the voltage of the first voltage divider capacitor is higher than the voltage of the second voltage divider capacitor, and the second comparison circuit outputs a second control signal when the voltage of the first voltage divider capacitor is lower than the voltage of the second voltage divider capacitor, thus directly converting the deviation direction into the corresponding control signal. The control logic is clear and the response path is short, which can avoid the two energy transfer branches from malfunctioning or acting simultaneously, thereby improving the directionality and safety of the midpoint potential balance control.

[0021] Furthermore, both the first and second comparator circuits are hysteresis comparator circuits, which solves the problem that the control signal is prone to frequent switching when the voltage deviation signal fluctuates near the comparison threshold. By setting the hysteresis threshold voltage, the trigger threshold and reset threshold of the comparator circuit form a certain difference, which enables the control signal to resist disturbances, reduces the jitter of the switching transistor in the critical state, reduces switching losses and device electrical stress, and improves the device's tolerance to noise and small voltage fluctuations, making the midpoint potential balance control process more stable and reliable.

[0022] Furthermore, to enable the unbalanced comparator unit to obtain operating power from the DC side, this self-powered circuit includes a charging resistor, a reverse discharge protection diode, an energy storage voltage divider capacitor, and a Zener diode. The charging resistor limits the current draw, the reverse discharge protection diode prevents reverse discharge from the energy storage capacitor, the energy storage voltage divider capacitor stores and distributes electrical energy, and the Zener diode provides a stable voltage for the comparator and operational amplifier. This reduces the need for external power supply interfaces and auxiliary power modules, lowering hardware costs and installation complexity. Simultaneously, the self-powered structure directly links the detection and comparator circuit to the DC side operating state, facilitating integration into existing three-level converter systems and reducing device size, thus improving on-site layout convenience.

[0023] Furthermore, the first and second switching transistors correspond to two opposite energy transfer directions, respectively. The inductor limits current surges and provides freewheeling energy after the switching transistors are turned off, while the energy storage capacitor temporarily stores and buffers unbalanced energy. This allows for bidirectional regulation with fewer power devices, enabling the device to selectively conduct corresponding branches according to different deviation directions, avoiding the slow response and weak regulation capabilities of passive voltage equalization methods. The inductor and energy storage capacitor together form a... LC The structure can also mitigate the current surge during energy transfer and suppress high-frequency components in the neutral current, thus balancing the neutral potential equilibrium speed, device safety, and system stability.

[0024] Furthermore, when U c1 Higher than U c2 At that time, the first voltage divider capacitor C Excessive energy storage on one side, if not adjusted promptly, will lead to uneven voltage distribution between the upper and lower bridge arm devices, potentially causing overvoltage on the first voltage divider capacitor side. By turning on the first switch, the energy in the first voltage divider capacitor is transferred through the first switch, inductor, and energy storage capacitor. After the first switch is turned off, the energy is released to the second voltage divider capacitor via the inductor's freewheeling current. This allows energy to be gradually released from the side with higher voltage, while the side with lower voltage is compensated. The energy transfer direction matches the deviation direction, enabling rapid reduction of voltage. U c1 and U c2 The difference between them prevents the midpoint potential offset from continuing to expand and improves the operational safety of the three-level converter under the condition that the voltage on the first voltage divider capacitor side is too high.

[0025] Furthermore, when U c1 Below U c2 At that time, the second voltage divider capacitor C Excessive energy storage on both sides, if left unadjusted, will cause the DC midpoint potential to shift in the opposite direction, subjecting the corresponding power devices and voltage divider capacitors to unbalanced voltage stress. By turning on the second switch, the energy in the second voltage divider capacitor is transferred through the second switch, inductor, and energy storage capacitor. After the second switch is turned off, the energy is released to the first voltage divider capacitor through the inductor's freewheeling current, achieving corresponding reverse compensation. The advantage of this setup is that it gives the device bidirectional active balancing capability; regardless of which direction the midpoint potential shifts, the corresponding branch can be selected for correction, thereby improving the device's adaptability to complex operating conditions such as load imbalance, neutral current variations, and capacitor parameter differences.

[0026] Furthermore, the energy storage capacitor not only functions as a regular energy storage element, but also works in conjunction with the first voltage divider capacitor when the first switch is turned on, and with the second voltage divider capacitor when the second switch is turned on. This allows the energy on the side with the higher voltage to be gradually released through impedance regulation and shunt paths. Compared to direct hard switching or single-inductor energy transfer, the voltage regulation and shunt paths formed by the energy storage capacitor make the energy release process smoother, reducing current surges and voltage spikes, and lowering the transient stress on the switch, inductor, and DC-side capacitor. It also helps improve the smoothness of the midpoint potential recovery process, avoiding over-adjustment or oscillation, and improving the stability of the balance control process.

[0027] Furthermore, if the inductor and energy storage capacitor are too small, the energy transfer capability may be insufficient or the current fluctuation may be too large; if they are too large, the device size, cost, and losses will increase. Therefore, it is necessary to comprehensively consider the unbalanced energy transfer and the direction of energy transfer. LC The total energy transferred by the components, the maximum allowable voltage deviation of the system, the neutral current harmonic constraint, and the power circuit loss constraint are selected to allow the device to perform parameter matching according to different voltage levels, load conditions, and power quality requirements. This ensures the balancing speed while controlling harmonics, current surges, and power losses. Furthermore, the device is in standby mode when the voltage of the voltage divider capacitor is balanced, which reduces ineffective operations and additional losses without altering the normal operating characteristics of a traditional three-level converter.

[0028] In summary, this invention utilizes voltage detection, hysteresis comparison, drive control, and... LC The coordinated operation of the energy transfer branches enables automatic detection and bidirectional active balancing of the DC side neutral point potential of the three-level converter. This reduces reliance on complex modulation algorithms, lowers the risk of neutral point potential drift, device overvoltage, and capacitor damage, and improves the reliability and power quality of the converter under load changes, harmonics, and three-phase imbalance conditions.

[0029] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0030] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the following description of the relative embodiments will be briefly introduced. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0031] Figure 1 This is a block diagram of the midpoint potential imbalance control device system. Figure 2 For the circuit of the automatic midpoint potential balance detection and control device; Figure 3Circuit for the application of control devices in a three-phase four-wire converter power supply system; Figure 4 Three-level converter dq Block diagram of DC voltage PI control strategy in rotating coordinates; Figure 5 The simulation results are for the unbalanced control circuit. Detailed Implementation

[0032] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "one side," "one end," and "one side," etc., indicating the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, are only for the convenience of describing the 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, and therefore should not be construed as a limitation of the invention. 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 indicated technical features. Thus, a feature defined with "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0034] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0035] It should be understood that, when used in this specification and the appended claims, the terms "comprising" and "including" indicate the presence of the described features, integrals, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or collections thereof.

[0036] It should also be understood that the terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms unless the context clearly indicates otherwise.

[0037] It should also be further understood that the term "and / or" as used in this specification and the appended claims refers to any combination of one or more of the associated listed items and all possible combinations, and includes such combinations.

[0038] The accompanying drawings illustrate various structural schematic diagrams according to embodiments disclosed in this invention. These drawings are not to scale, and some details have been enlarged for clarity, and some details may have been omitted. The shapes of the various regions and layers shown in the drawings, as well as their relative sizes and positional relationships, are merely exemplary and may deviate from reality due to manufacturing tolerances or technical limitations. Furthermore, those skilled in the art can design regions / layers with different shapes, sizes, and relative positions as needed.

[0039] This invention provides an automatic detection and balancing control device for neutral point potential imbalance in a three-level converter. It relates to converters such as three-level reactive power compensation devices, harmonic suppression devices, and three-phase four-wire power supplies, as well as applications requiring neutral point potential balancing control. By sampling the DC bus neutral point voltage and positive and negative bus voltages in real time, a high-speed digital signal processor dynamically identifies voltage deviations and automatically determines the degree and direction of neutral point potential imbalance. When the detected deviation exceeds a set threshold, the device immediately activates its built-in neutral point potential adjustment module, achieving active control of the neutral point current under the regulation of the balancing circuit. This device features fault warning and protection output functions, effectively suppressing neutral point voltage drift, preventing overvoltage of switching devices and capacitor damage, and significantly improving the operational reliability and power quality of the three-level converter under load changes and harmonic conditions.

[0040] Please see Figure 1This invention discloses an automatic detection and balancing control device for neutral point potential imbalance in a three-level converter, comprising a DC support unit, a voltage detection unit, an imbalance comparison unit, a drive unit, and an energy transfer and power control unit. The voltage detection unit obtains the required balancing voltage for the DC link and the current neutral point potential of the DC support unit. These two voltages are then sent to the imbalance comparison unit, which compares them and generates a control signal. This signal, transmitted through the drive circuit, activates the switching transistor in the energy transfer circuit, causing the voltage divider capacitor in the DC support unit to charge and discharge, transferring energy and achieving self-balancing control of the neutral point potential. This achieves the goals of rapidly suppressing neutral point potential deviation, reducing overvoltage risk to power devices, protecting DC-side capacitors, and improving the operational reliability and output power quality of the three-level converter.

[0041] The DC support unit is used to form the DC bus of the three-level converter, including a first voltage divider capacitor connected in series. C 1 and second voltage divider capacitors C 2. First voltage divider capacitor C 1 and the second voltage divider capacitor C The connection point between points 2 and 3 forms the midpoint of the DC support unit. When the first voltage divider capacitor... C voltage of 1 U c1 With the second voltage divider capacitor C 2 voltage U c2 When the voltages are equal or within the allowable deviation range, the midpoint potential of the DC support unit remains balanced; when U c1 and U c2 When a significant difference occurs, the potential at the midpoint of the DC support unit shifts, requiring the activation of balance control.

[0042] The voltage detection unit is connected to the DC support unit and is used to collect the positive DC bus voltage, the negative DC bus voltage, and the midpoint potential of the DC support unit, and to obtain the first voltage-dividing capacitor. C voltage of 1 U c1 Second voltage divider capacitor C 2 voltage U c2 The voltage detection unit will U c1 and U c2 The difference between the voltage and the preset equilibrium voltage, or the deviation from the preset equilibrium voltage, is output as a voltage deviation signal to the imbalance comparison unit. Through this detection method, the device can simultaneously determine whether the midpoint potential has shifted and the direction of the shift, providing a basis for subsequently selecting the corresponding energy transfer path.

[0043] The imbalance comparator unit includes a first comparator circuit and a second comparator circuit. The first comparator circuit is used to... U c1 Higher than U c2 And when the threshold value is exceeded, a first control signal is output; the second comparison circuit is used to... U c1 Below U c2 When the voltage exceeds a preset comparison threshold, a second control signal is output. The first comparison circuit and the second comparison circuit are preferably hysteresis comparison circuits. By setting a hysteresis threshold voltage, the control signal will not frequently flip when the voltage deviation signal fluctuates slightly near the comparison threshold. This avoids the first switch T1 and the second switch T2 from repeatedly turning on or off in the critical state, thereby improving the control stability of the device and the reliability of the switching devices.

[0044] Please see Figure 2 The circuit of this invention belongs to the category of power electronic circuits in power supply technology. It provides a feasible circuit for suppressing voltage deviation and fluctuation in the DC midpoint of a three-level converter.

[0045] The drive unit is connected between the unbalanced comparator unit and the energy transfer and power control unit, and is used to convert the first control signal or the second control signal into a drive signal suitable for driving the switching transistor. When the first comparator circuit outputs the first control signal, the drive unit drives the first switching transistor T1 to conduct; when the second comparator circuit outputs the second control signal, the drive unit drives the second switching transistor T2 to conduct. Through the isolation, amplification, or shaping function of the drive unit, the driving capability of the comparator circuit output signal to the power switching transistor can be improved, and the risk of power-side interference to the control-side circuit can be reduced.

[0046] The energy transfer and power control unit includes a first switch T1, a second switch T2, and an inductor. L and energy storage capacitors C This unit is used to establish a corresponding energy transfer circuit based on the deviation direction determined by the unbalanced comparator unit, causing the voltage divider capacitor on the higher voltage side to release some energy, and then using an inductor... L Energy storage and freewheeling functions and energy storage capacitors C The buffering effect transfers energy to the voltage-dividing capacitor on the lower voltage side. Compared with simply relying on software modulation algorithms to adjust the midpoint potential, this hardware energy transfer structure has a shorter response link and a more direct adjustment process, enabling it to suppress DC-side midpoint potential shifts more quickly during load changes or neutral current fluctuations.

[0047] This invention uses a voltage detection unit to collect data from the first voltage divider capacitor. C voltage of 1 U c1Second voltage divider capacitor C 2 voltage U c2 and according to U c1 and U c2 The difference between them yields a voltage deviation signal; the unbalanced comparison unit determines the offset direction of the DC support unit's midpoint potential based on the voltage deviation signal, when... U c1 Higher than U c2 When the threshold value is exceeded, the first comparison circuit outputs a first control signal. U c1 Below U c2 When the voltage exceeds a preset comparison threshold, the second comparison circuit outputs a second control signal; the driving unit selectively drives the corresponding switching transistor to turn on according to the first or second control signal, so that the energy in the voltage divider capacitor on the higher voltage side passes through the inductor. L and energy storage capacitors C The voltage is transferred to the voltage divider capacitor on the lower voltage side, thereby achieving self-balancing control of the DC side midpoint potential. Both the first and second comparator circuits of this invention are designed as hysteresis comparator circuits, which frequently oscillate and switch near the comparison threshold by setting a hysteresis threshold voltage suppression circuit.

[0048] The comparator circuit is self-powered by a charging resistor. R 0. Reverse discharge protection diode D0, energy storage voltage divider capacitor C 01 and C 02 and Zener diode V z The self-powered circuit draws power from the DC side and supplies it to the first comparator circuit, the second comparator circuit, and related operational amplifiers after current limiting, energy storage, voltage division, and voltage regulation. This structure eliminates the need for an independent auxiliary power supply for the comparator circuits, reducing external power supply interfaces, lowering device size and cost, and improving integration convenience within existing three-level converter systems.

[0049] Please see Figure 3 In a three-phase four-wire power supply system, the first voltage divider capacitor C The voltage of 1 is U c1 Second voltage divider capacitor C The voltage of 2 is U c2 DC side voltage is U dc Unbalanced loads, changes in neutral current, or harmonic current surges in a three-phase four-wire system can all cause [problems / issues]. U c1 andU c2 Deviation from equilibrium. The device of this invention detects this deviation independently. U c1 and U c2 The changes in voltage and the activation of the corresponding energy transfer branch after the deviation reaches the threshold can serve as a supplementary control link to the main DC voltage control strategy of a three-level converter.

[0050] Please see Figure 4 Three-level converters can still be based on d - q A DC voltage PI control strategy for rotating coordinates stabilizes the total DC bus voltage. U dc The device of this invention does not replace the main DC voltage control, but rather performs local balance control to address the uneven voltage distribution between the upper and lower voltage divider capacitors on the DC side. In other words, the main control strategy primarily maintains the stability of the total DC voltage, while the device of this invention primarily maintains the stability of the DC midpoint potential. The two control objectives are different and work together, which can reduce the dependence of the midpoint potential balance control on the main modulation algorithm.

[0051] The working principle of the device of the present invention under different working conditions is as follows: Operating Condition 1: U C1 = U C2 When the midpoint potential is balanced, the comparator circuits all output a low level, and both switching transistors T1 and T2 are in the off state.

[0052] Operating Condition 2: U C1 > U C2 When comparator circuit 1 outputs a high level and comparator circuit 2 outputs a low level, switch transistor T1 is turned on. C 1. T1 L , C Forming a loop C Energy is transferred from 1 to the inductor L and capacitor C In the middle, after T1 is turned off, the inductor current continues, and the inductor flows into the capacitor. C 2, ultimately making (1) In fact, after T1 is turned on, the energy storage capacitor in the circuit C First, connect with the DC side voltage divider capacitor. C 1. Connected in parallel, then with the DC-side voltage divider capacitor. C Two capacitors connected in series form an impedance regulating and shunt circuit, with the high-side capacitor... C 1. Voltage decreasing trend is (2) This process is also an unbalanced energy diversion process. After the voltage balances, T1 is turned off, stopping the regulation, causing the capacitor to... C The voltage of 1 is Voltage balance is crucial. In applications, the appropriate energy storage capacitor value needs to be selected and calculated based on the degree of system imbalance caused by actual operating conditions.

[0053] Operating Condition 3: U c1 < U c2 Comparator circuit 2 outputs a high level, comparator circuit 1 outputs a low level, and switch transistor T2 is turned on. L , C 2. C Forming a loop C Energy is transferred to the inductor in step 2. L and capacitor C In the middle, after T2 is turned off, the inductor current continues, and the inductor flows into the capacitor. C 1. Ultimately, this led to (3) Similarly, after T2 is turned on, the energy storage capacitor in the circuit... C First, connect with the DC side voltage divider capacitor. C 2 in parallel, then connected with the DC side voltage divider capacitor. C 1. A series connection forms an impedance regulating and shunt circuit, with a low-side capacitor. C 2. The voltage decreasing trend is as follows: (4) This process is also an unbalanced energy diversion process. After the voltage balances, T2 is turned off, stopping the regulation, causing the capacitor to... C The voltage of 2 is Voltage balance is crucial. In applications, the appropriate energy storage capacitor value needs to be selected and calculated based on the degree of system imbalance caused by actual operating conditions.

[0054] When in equilibrium, the circuit is in standby mode, does not participate in power regulation, and does not change the characteristics of the traditional circuit topology.

[0055] Calculation method for inductor and capacitor values ​​in power transfer circuit of unbalanced control device Unbalanced energy transfer: (5) The unbalanced energy transfer to the inductor and capacitor yields the expression for the total energy allowed to transfer to the inductor and capacitor: (6) The maximum allowable voltage deviation of the system caused by capacitance deviation due to factors such as temperature, lifespan, manufacturing, and operating conditions (which can be defined as DC-side voltage imbalance): (7) Neutral current harmonic constraint conditions: (8) Power circuit loss limitation: The capacitor loss of the power circuit should not exceed 10% of the DC capacitor loss.

[0056] Based on the maximum allowable voltage deviation, harmonic constraints, and loss constraints, the inductance and capacitance values ​​of the power transfer circuit can be calculated.

[0057] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0058] Please see Figure 5 The diagram illustrates the adjustment process of the unbalanced control circuit of the present invention after the midpoint potential changes from a balanced state to an unbalanced state. Circuit effect simulation is also provided. During the initial stage of the simulation, from 0s to 0.5s, the first voltage divider capacitor... C voltage of 1 U c1 With the second voltage divider capacitor C 2 voltage U c2 The basic structure is the same. The potential at the midpoint of the DC support unit remains balanced. Neither the first comparator circuit nor the second comparator circuit triggers a valid control signal. The first switch T1 and the second switch T2 are in the off state. The energy transfer and power control unit is in the standby state.

[0059] At time 0.5s, a midpoint potential imbalance condition is introduced into the simulation, making... U c1 and U c2 A voltage deviation occurs between the two voltage divider capacitors on the DC side. After the voltage detection unit detects the inconsistency in voltage between the two capacitors, it sends the voltage deviation signal to the unbalanced comparison unit. When the voltage deviation signal exceeds a preset comparison threshold, the comparison circuit corresponding to the deviation direction is triggered and outputs a control signal. The drive unit drives the corresponding switch to conduct according to this control signal, allowing the energy in the voltage divider capacitor on the higher voltage side to flow through the inductor...L and energy storage capacitors C The transfer is performed, and the inductor is used after the switching transistor is turned off. L The freewheeling effect continues to release current to the voltage divider capacitor on the lower voltage side.

[0060] Depend on Figure 5 It can be seen that, within the period of 0.5s to 0.6s, the midpoint potential imbalance is rapidly detected and begins to be regulated. U c1 and U c2 The voltage difference between them gradually decreases; during the period from 0.6s to 1s, as the energy transfer process continues, the voltages of the upper and lower voltage divider capacitors on the DC side gradually return to near parity, and the midpoint potential of the DC support unit tends to rebalance. The simulation results show that the device of the present invention can promptly activate the hardware balancing branch after the midpoint potential shift occurs, suppress the midpoint voltage drift through directional energy transfer, and avoid long-term accumulation of deviation.

[0061] Furthermore, Figure 5 Simulation results also demonstrate that the device of this invention does not participate in power regulation when the midpoint potential is balanced, but only activates the corresponding energy transfer branch according to the direction of deviation when imbalance occurs, thus exhibiting the characteristic of on-demand operation. This operating mode reduces additional losses in balanced states and can intervene promptly when imbalances are caused by sudden load changes, DC-side capacitance differences, or neutral current fluctuations. This helps reduce the overvoltage risk of power devices, protects the DC-side voltage divider capacitors, and improves the output power quality of the three-level converter under complex operating conditions.

[0062] The device's hardware circuitry can detect midpoint potential imbalance in real time. The detection circuit has a fast detection speed, and compared to voltage detection sensors, the detection unit circuit of this invention has a lower cost. The device's comparator circuit does not require an external power supply. This invention proposes a self-powered circuit utilizing DC-side voltage, which has application value in comparator and operational amplifier power supply systems, reducing device size and circuit cost; through inductance... L and energy storage capacitors C The energy transfer branch forms a circuit that transfers unbalanced energy, thereby suppressing voltage imbalance in the DC-side voltage divider capacitors under unbalanced operating conditions. This allows the device to achieve balanced potential control at the midpoint of the three-level converter, improving its operational reliability and safety. The energy transfer circuit uses... LC The structure stabilizes the midpoint potential and filters out high-frequency harmonics of the neutral current, improving the stability of the neutral system. The energy transfer and power control circuits employ... LC The structure can reduce the inductance of a single inductor and the size of the device.

[0063] In summary, this invention provides an automatic detection and balancing control device for midpoint potential imbalance in a three-level converter. Addressing the midpoint potential drift problem caused by inconsistent voltages in the DC-side voltage divider capacitors of a three-level converter, it establishes a hardware balancing control link consisting of voltage detection, imbalance comparison, drive control, and energy transfer. It can acquire the voltage states of the upper and lower DC-side voltage divider capacitors in real time and output corresponding control signals based on the direction of voltage deviation, driving the corresponding switches to conduct. This allows energy transfer from the higher-voltage side capacitor to the lower-voltage side through inductors and energy storage capacitors, gradually balancing the voltages of the first and second voltage divider capacitors. The hysteresis comparison circuit avoids frequent control signal switching when the voltage deviation fluctuates near the threshold, improving the stability of the switching action. The self-powered structure of the comparison circuit reduces the need for additional auxiliary power supplies, lowering the device's size and cost. LC The energy transfer structure can mitigate current surges during energy transfer and improve the stability of the neutral system. Simulation results show that after an imbalance occurs in the neutral point potential, the device of this invention can promptly detect and activate the balancing branch, allowing the neutral point potential to stabilize again. Therefore, this invention can improve the operational reliability, safety, and power quality of a three-level converter under conditions of load imbalance, harmonic surges, and differences in DC-side capacitor parameters.

[0064] The above content is only for illustrating the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solution based on the technical concept proposed in this invention shall fall within the scope of protection of the claims of this invention.

Claims

1. An automatic detection and balancing control device for neutral point potential imbalance in a three-level converter, characterized in that, It includes a DC support unit, a voltage detection unit, an imbalance comparison unit, a drive unit, and an energy transfer and power control unit; The DC support unit includes a first voltage divider capacitor and a second voltage divider capacitor connected in series, and the connection point of the first voltage divider capacitor and the second voltage divider capacitor forms the midpoint of the DC support unit. The voltage detection unit is connected to the DC support unit and is used to detect the voltage of the first voltage divider capacitor and the voltage of the second voltage divider capacitor, and output the voltage deviation signal between the voltage of the first voltage divider capacitor and the voltage of the second voltage divider capacitor. The imbalance comparison unit is connected to the voltage detection unit and is used to generate a first control signal corresponding to a first deviation direction or a second control signal corresponding to a second deviation direction based on the voltage deviation signal. The drive unit is connected to the imbalance comparison unit and is used to drive the energy transfer and power control unit according to the first control signal or the second control signal. The energy transfer and power control unit is connected to the DC support unit and is used to transfer the energy of the first voltage divider capacitor to the second voltage divider capacitor under the control state corresponding to the first control signal, and to transfer the energy of the second voltage divider capacitor to the first voltage divider capacitor under the control state corresponding to the second control signal, so that the voltage of the first voltage divider capacitor and the voltage of the second voltage divider capacitor tend to be balanced.

2. The automatic detection and balancing control device for neutral point potential imbalance in a three-level converter according to claim 1, characterized in that, The voltage detection unit is used to detect the positive DC bus voltage, the negative DC bus voltage, and the midpoint potential of the DC support unit, and to obtain the voltage deviation signal based on the positive DC bus voltage, the negative DC bus voltage, and the midpoint potential.

3. The automatic detection and balancing control device for neutral point potential imbalance in a three-level converter according to claim 1, characterized in that, The unbalanced comparison unit includes a first comparison circuit and a second comparison circuit. The first comparison circuit is used to output the first control signal when the voltage of the first voltage divider capacitor is higher than the voltage of the second voltage divider capacitor. The second comparison circuit is used to output the second control signal when the voltage of the first voltage divider capacitor is lower than the voltage of the second voltage divider capacitor.

4. The automatic detection and balancing control device for neutral point potential imbalance in a three-level converter according to claim 3, characterized in that, Both the first comparison circuit and the second comparison circuit are hysteresis comparison circuits. The hysteresis comparison circuit has a hysteresis threshold voltage to suppress frequent switching of the first control signal or the second control signal when the voltage deviation signal is near the comparison threshold.

5. The automatic detection and balancing control device for neutral point potential imbalance in a three-level converter according to claim 3, characterized in that, It also includes a comparator circuit self-powered circuit, which includes a charging resistor, a reverse discharge protection diode, an energy storage voltage divider capacitor, and a Zener diode. The comparator circuit self-powered circuit is used to draw power from the DC side and supply power to the first comparator circuit and the second comparator circuit.

6. The automatic detection and balancing control device for neutral point potential imbalance in a three-level converter according to claim 3, characterized in that, The energy transfer and power control unit includes a first switch, a second switch, an inductor, and an energy storage capacitor. The driving unit is used to drive the first switch to turn on according to the first control signal, or to drive the second switch to turn on according to the second control signal.

7. The automatic detection and balancing control device for neutral point potential imbalance in a three-level converter according to claim 6, characterized in that, When the voltage of the first voltage divider capacitor is higher than the voltage of the second voltage divider capacitor, the first switch is turned on, causing the energy in the first voltage divider capacitor to be transferred through the first switch, the inductor, and the energy storage capacitor, and then transferred to the second voltage divider capacitor through the freewheeling effect of the inductor after the first switch is turned off.

8. The automatic detection and balancing control device for neutral point potential imbalance in a three-level converter according to claim 6, characterized in that, When the voltage of the first voltage divider capacitor is lower than the voltage of the second voltage divider capacitor, the second switch is turned on, causing the energy in the second voltage divider capacitor to be transferred through the second switch, the inductor, and the energy storage capacitor, and then transferred to the first voltage divider capacitor through the freewheeling effect of the inductor after the second switch is turned off.

9. The automatic detection and balancing control device for neutral point potential imbalance in a three-level converter according to claim 6, characterized in that, When the first switch is turned on, the energy storage capacitor is connected in parallel with the first voltage divider capacitor and cooperates with the second voltage divider capacitor to form an impedance voltage regulation and current shunt circuit; or when the second switch is turned on, the energy storage capacitor is connected in parallel with the second voltage divider capacitor and cooperates with the first voltage divider capacitor to form an impedance voltage regulation and current shunt circuit.

10. The automatic detection and balancing control device for neutral point potential imbalance in a three-level converter according to claim 6, characterized in that, The parameters of the inductor and the energy storage capacitor are determined based on the unbalanced energy transfer, the total energy transferred to the inductor and the energy storage capacitor, the maximum allowable voltage deviation of the system, the neutral current harmonic constraint, and the power circuit loss constraint. The automatic detection and balance control device for the neutral point potential imbalance of the three-level converter is in standby mode when the voltage of the first voltage divider capacitor is balanced with the voltage of the second voltage divider capacitor.

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