Direct-current side neutral-point potential balancing structure of grid-connected converter

Abstract

The invention discloses a grid-connected converter direct-current side midpoint potential balance structure, and the structure comprises a direct-current circuit structure which comprises a direct-current positive electrode, a midpoint potential and a direct-current negative electrode; the multi-winding transformer is provided with a primary side winding and a secondary side winding, and the primary side winding is electrically connected to the direct current circuit structure between the direct current positive electrode and the midpoint potential and the direct current circuit structure between the midpoint potential and the direct current negative electrode through a primary side H-bridge inversion module; and the voltage source type inverter is electrically connected to the secondary side winding, so that an energy interaction channel is formed between the voltage source type inverter and the multi-winding transformer, and when the unbalanced magnetomotive force is generated, the secondary side winding generates a compensation current for the neutral point potential through the energy interaction channel. The primary side of the multi-winding transformer is connected with two capacitors on the direct current side, the secondary side of the multi-winding transformer is connected with the voltage source type inverter, current compensation is carried out on the offset neutral-point potential, the neutral-point potential can be automatically corrected, the development difficulty and delay of a balance algorithm are eliminated, and the adjusting speed and the balance capacity are improved.

Description

Technical Field

[0001] This invention belongs to the technical field of multilevel converter topology, specifically relating to the DC side midpoint potential balance structure of grid-connected converters. Background Technology

[0002] With the rapid increase in the proportion of new energy power generation, the performance and reliability of grid-connected converters, as the core unit of power conversion, are of paramount importance. Compared with traditional two-level converters, active neutral-point clamped (ANPC) multilevel converter topologies are widely used in medium- and high-power applications because they can output higher quality power waveforms, withstand higher voltage levels, and have lower voltage stress on each power switch. However, converters with neutral-point clamping structures such as ANPC suffer from fluctuations and imbalances in the DC-side neutral-point potential caused by various factors such as load and grid fluctuations. Neutral-point potential imbalance affects the converter's output waveform quality, DC capacitor lifespan, and potential power switch damage.

[0003] In related technologies, the solution to the midpoint potential balance problem is mostly based on software algorithms such as small vector allocation, injection of zero-sequence voltage, and model prediction. These methods have complex software algorithms that are difficult to debug, and the algorithms have a delay of several switching cycles. In converters with switching frequencies of tens or hundreds of kHz, the balance accuracy drops significantly. Summary of the Invention

[0004] The purpose of this invention is to create an energy channel by setting up a multi-winding transformer and a voltage source inverter through the magnetomotive force compensation principle of transformer flux balance, thereby avoiding algorithm delay and enhancing the regulation speed and balancing capability of the grid-connected converter DC side midpoint potential balance structure.

[0005] To achieve the above objectives, this invention proposes a DC-side midpoint potential balancing structure for a grid-connected converter, comprising: a DC circuit structure including a DC positive terminal, a midpoint potential, and a DC negative terminal; a multi-winding transformer with a primary winding and a secondary winding, wherein the primary winding is electrically connected to the DC circuit structure between the DC positive terminal and the midpoint potential, and between the midpoint potential and the DC negative terminal, via a primary-side H-bridge inverter module; and a voltage source inverter electrically connected to the secondary winding, thereby forming an energy interaction channel between the voltage source inverter and the multi-winding transformer, so that when an unbalanced magnetomotive force is generated, the secondary winding generates a compensation current for the midpoint potential through the energy interaction channel.

[0006] In one optional embodiment, the voltage source inverter includes: a secondary-side H-bridge inverter module having an output terminal; and a flying capacitor connected to the output terminal of the secondary-side H-bridge inverter module and the secondary-side winding, such that the flying capacitor and the multi-winding transformer form the energy interaction channel.

[0007] In one optional implementation, both the secondary-side H-bridge inverter module and the primary-side H-bridge inverter module are composed of four switching transistors in a full-bridge configuration, and use the same type of semiconductor switching devices with the same power rating, highly similar switching parameters, and thermal performance.

[0008] In one optional implementation, the process by which the secondary winding generates a compensating current for the midpoint potential through the energy interaction channel satisfies a balance constraint condition, the expression of which is: In the formula, This refers to the active power on the secondary side; For energy storage in flying capacitors; The voltage across the flying capacitor; This refers to the current in the secondary winding. The power factor represents the proportion of active power on the secondary side to the total power.

[0009] In one optional implementation, the expression for the compensation current is: In the formula, This refers to the current in the secondary winding. The voltage across the flying capacitor; The current flowing through the flying capacitor; The power factor represents the proportion of active power on the secondary side to the total power. This is the voltage corresponding to the midpoint potential.

[0010] In one optional embodiment, the DC circuit structure is connected to a first capacitor and a second capacitor, which creates a potential imbalance between the first capacitor and the second capacitor. To determine whether an unbalanced magnetomotive force is generated on the primary side, the degree of imbalance... The expression is: In the formula, The voltage across the first capacitor; The voltage across the second capacitor; This is the total voltage on the DC side; This is the deviation of the midpoint 0 from the ideal midpoint potential; if or Then an unbalanced magnetomotive force is generated on the primary side.

[0011] In one alternative embodiment, the primary winding comprises two windings with an equal number of turns. The secondary winding Three windings with equal number of turns are wound on the same closed magnetic core, with two primary windings and three secondary windings wound in parallel using a double-wire winding method, so that the inductance and resistance values ​​of the two primary windings are consistent.

[0012] In one optional embodiment, the two primary windings are connected in anti-phase series, so that when the DC midpoint deviates from the rated value, the magnetomotive force generated in the two primary windings is in opposite directions. The three secondary windings are connected in parallel in the same direction, so that the voltage potential induced in all secondary windings is the same. Any secondary winding and any primary winding are of the same polarity, so that the voltage change on the secondary side follows the voltage change on the primary side. The secondary windings and the primary windings have the same polarity at their corresponding terminals.

[0013] In one alternative implementation, any one of the secondary windings number of turns With any one of the primary windings number of turns The following ratio relationship must be satisfied: ; In the formula, k and m This is the proportionality coefficient; This is the total voltage of the DC bus; This is the rated voltage of the flying capacitor.

[0014] In one optional embodiment, an insulation structure is provided between the primary winding and the secondary winding, and the insulation structure adopts a multi-layer insulation structure that combines functional insulation and reinforced insulation.

[0015] The beneficial effects of this invention are as follows: by connecting two capacitors on the DC side of the primary side of a multi-winding transformer and a voltage source inverter on the secondary side, current compensation is performed on the offset midpoint potential, realizing automatic correction of the midpoint potential, eliminating the development difficulty of the balancing algorithm, avoiding algorithm delays, and enhancing the adjustment speed and balancing capability. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the overall topology of the DC-side midpoint potential balance structure of the grid-connected converter provided in an embodiment of the present invention; Figure 2 A flowchart of the balance current of the DC side midpoint potential balance structure of a grid-connected converter provided in an embodiment of the present invention; Figure 3This is a schematic diagram of the rising midpoint O potential signal flow of the DC side midpoint potential balance structure of a grid-connected converter provided in an embodiment of the present invention; Figure 4 This is a schematic diagram of the signal flow direction for reducing the neutral point O potential of a grid-connected converter according to an embodiment of the present invention. Figure 5 This is a simulation waveform of the DC side midpoint potential without a midpoint potential balancing strategy provided in an embodiment of the present invention. Figure 6 This is a simulation waveform of the DC-side midpoint potential when a midpoint potential balancing strategy is added to the DC-side midpoint potential balancing structure of the grid-connected converter according to an embodiment of the present invention. Figure 7 An enlarged simulation waveform of the DC-side midpoint potential when a midpoint potential balancing strategy is added to the DC-side midpoint potential balancing structure of the grid-connected converter according to an embodiment of the present invention.

[0017] Figure descriptions: 10. Primary side H-bridge inverter module; 20. Multi-winding transformer; 21. Primary side winding; 22. Secondary side winding; 30. Secondary side H-bridge inverter module. Detailed Implementation

[0018] In existing technologies, regulation based on some software algorithms still relies on the selection of redundant vectors in the modulation strategy, resulting in high coupling between the control and modulation processes and increasing implementation complexity. The balance control of capacitor voltage has feedback delay and insufficient dynamic response, and is prone to significant voltage fluctuations when there are sudden load changes or grid disturbances. Frequent switching of redundant vectors can introduce high-frequency harmonic currents, increase losses and electromagnetic interference, and reduce system efficiency and output power quality. The midpoint current is affected by many factors such as load characteristics, power factor and modulation ratio, and the regulation effect of the control algorithm is limited under light load or low power factor conditions. If the potential is unbalanced for a long time, it may also lead to uneven voltage stress on the devices, accelerate the aging of DC bus capacitors, and shorten the system life.

[0019] Therefore, relying solely on algorithmic regulation is insufficient to meet the reliability requirements of high-power applications, necessitating hardware-based implementation. Hardware-implemented midpoint potential balancing eliminates the development difficulties of balancing algorithms, avoids algorithmic delays, and enhances regulation speed and balancing capability. Therefore, developing hardware-level active midpoint clamping converter midpoint potential balancing technology is of significant practical importance.

[0020] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0021] like Figure 1As shown, according to an embodiment of the present invention, in one aspect, a grid-connected converter DC side midpoint potential balancing structure is provided, comprising: a DC circuit structure including a DC positive terminal, a midpoint potential, and a DC negative terminal; a multi-winding transformer 20 having a primary winding 21 and a secondary winding 22, the primary winding 21 being electrically connected to the DC circuit structure between the DC positive terminal and the midpoint potential, and between the midpoint potential and the DC negative terminal; and a voltage source inverter electrically connected to the secondary winding 22, such that an energy interaction channel is formed between the voltage source inverter and the multi-winding transformer 20, so that when an unbalanced magnetomotive force is generated, the secondary winding 22 generates a compensation current for the midpoint potential through the energy interaction channel.

[0022] This invention proposes a DC-side midpoint potential balancing method based on magnetomotive force compensation. It introduces a novel hardware structure design, focusing on topology optimization. By combining the DC circuit structure with a multi-winding transformer 20 and a voltage source inverter, an energy transfer balancing structure is constructed. When the system generates an unbalanced magnetomotive force, the secondary winding 22 of the multi-winding transformer 20 can quickly generate a compensation current for the midpoint potential through an energy interaction channel formed with the voltage source inverter. This mechanism can accurately and efficiently regulate the midpoint potential, maintaining it in a stable equilibrium state and avoiding a series of problems caused by midpoint potential deviation, such as increased low-order harmonics in the grid-connected current and distortion of the output voltage waveform, thereby significantly improving the power quality of the grid-connected converter.

[0023] By utilizing the magnetomotive force compensation principle of a voltage source inverter, automatic and delay-free energy compensation of the neutral point potential is achieved. This fundamentally solves the problem of complex algorithm control and improves the real-time update of the neutral point potential balance. Through faster hardware balancing, the reliance on complex software algorithms is eliminated, accelerating the system's response speed, shortening the compensation time, suppressing distortion of the neutral point potential waveform, and ensuring the converter's operational performance.

[0024] Compared to traditional midpoint potential balancing schemes, this structure eliminates the need for complex additional control circuits and cumbersome control algorithms. It utilizes the characteristics of its own circuit structure to achieve midpoint potential balancing, reducing hardware costs and software programming complexity, lowering the system failure rate, improving system reliability and stability, and also reducing maintenance costs and difficulty.

[0025] Because it can balance the midpoint potential in a timely and effective manner, it reduces energy loss caused by potential imbalance, making the system more efficient in the energy conversion process, improving the overall performance and energy utilization of the grid-connected converter, and helping to improve the operating efficiency of the entire power system and achieve a more energy-efficient and environmentally friendly power supply.

[0026] Furthermore, the primary winding 21 includes a first winding and a second winding with the same number of turns. The first winding is electrically connected to the DC circuit structure between the DC positive terminal and the midpoint potential, and the second winding is electrically connected to the DC circuit structure between the midpoint potential and the DC negative terminal.

[0027] Since the first and second windings have the same number of turns, they exhibit symmetrical effects on voltage distribution and current regulation between the DC positive terminal, the midpoint potential, and the midpoint potential and the DC negative terminal in a DC circuit. When a midpoint potential shift occurs on the DC side, the first and second windings, with the same number of turns, can generate symmetrical compensation effects in their respective connected circuit sections based on their symmetrical electromagnetic characteristics. By adjusting the current, they push the midpoint potential back to equilibrium, effectively ensuring the symmetrical balance of the DC side potential and avoiding subsequent power quality problems caused by potential asymmetry.

[0028] The symmetrical winding structure makes the electrical characteristics on both sides of the DC circuit more balanced, reducing the risks of unilateral overload and voltage fluctuations caused by differences in the number of winding turns. During system operation, this symmetrical design makes the circuit more capable of withstanding load changes and grid disturbances, reducing the probability of system failure and improving the overall stability and reliability of operation. It also provides a stable DC-side foundation for subsequent energy interaction with multi-winding transformer 20, voltage source inverter, etc.

[0029] The first and second windings, with the same number of turns, can achieve uniform energy distribution during the transfer of energy from the DC side to the primary side of the transformer. Whether it is power transmission during normal operation or compensating energy transfer during midpoint potential balance adjustment, the symmetrical winding structure enables more efficient and balanced energy transmission and distribution in the DC positive-midpoint potential and midpoint potential-DC negative potential ranges, reducing energy loss, improving the system's energy conversion efficiency, and providing strong support for the efficient operation of the entire grid-connected converter system.

[0030] The symmetrical winding structure makes the electrical characteristics of the DC-side circuit more regular, eliminating the need for complex differentiated control for windings with different numbers of turns when designing system control strategies. Control algorithms can be developed based on the symmetrical characteristics, reducing the complexity of control logic, the workload of software programming and debugging, and also facilitating the standardized design and production of hardware circuits, thereby reducing the design and manufacturing costs of the system.

[0031] The number of turns in the primary winding 21 and the secondary winding 22 are different.

[0032] Different turns ratios can achieve voltage transformation between the primary and secondary sides. The turns ratio can be flexibly adjusted according to the voltage level requirements of the DC and AC sides (voltage source inverter side) in the system, so that the voltages on both sides are better matched. No additional voltage transformation device is required, which simplifies the system structure and enables efficient energy transfer, thereby improving the overall voltage adaptability and energy transfer efficiency of the system.

[0033] The difference in turns ratio due to different numbers of turns allows for corresponding scaling and adjustment of electrical quantities such as current and voltage during energy interaction. During DC-side midpoint potential balance adjustment, it enables more precise control of the compensation current output from the secondary winding 22, making the compensation current better match the DC-side potential deviation, thereby improving the adjustment accuracy of the midpoint potential balance and more effectively suppressing midpoint potential fluctuations.

[0034] Different turns ratios provide more adjustment dimensions for system control. By changing the turns ratio and combining it with control strategies (such as the control of voltage source inverters), the power factor, reactive power and other parameters of the system can be adjusted more flexibly to meet the requirements of system operating characteristics under different operating conditions (such as grid connection requirements, load changes, etc.), and improve the system's adaptability and control flexibility under complex operating conditions.

[0035] Designing different turns ratios based on actual energy transmission and regulation requirements allows the capacity, current, and other parameters of the primary winding 21 and the secondary winding to better match their respective working tasks. This avoids situations where the windings are "oversized" or "overloaded" due to the same number of turns, fully utilizes the winding performance, improves equipment utilization efficiency, and reduces equipment design and manufacturing costs.

[0036] Furthermore, the voltage source inverter includes: a secondary-side H-bridge inverter module 30 with an output terminal; and a flying capacitor connected to the output terminal of the secondary-side H-bridge inverter module 30 and the secondary-side winding 22, so that an energy interaction channel is formed between the flying capacitor and the multi-winding transformer 20.

[0037] In this embodiment, the flying capacitor is connected through the secondary-side H-bridge inverter module 30, which is equivalent to a voltage source inverter.

[0038] According to the Ampere-turn balance principle The net magnetomotive force imbalance on the primary side must be compensated by the secondary side; otherwise, the transformer flux will saturate, forcing a compensation current to be generated on the secondary side. For example... Figure 3 and Figure 4As shown, when the imbalance of the voltage of the upper and lower capacitors on the primary side causes the midpoint potential to rise or fall, the voltage of the same polarity winding on the secondary side will also be pulled up or down accordingly, resulting in an increase or decrease in the output current of the H-bridge. The increase and decrease in current are due to the discharge and charging of the flying capacitor. The discharge of the flying capacitor means that energy needs to be absorbed from the DC side capacitor, and the charging of the flying capacitor means that energy needs to be released from the DC side capacitor.

[0039] The energy interaction channel formed by the flying capacitor and the multi-winding transformer 20 can flexibly store and release energy. When the DC side midpoint potential shifts, the flying capacitor can respond quickly, precisely adjusting the magnitude and direction of the compensation current through energy interaction with the secondary winding 22, further enhancing the ability to balance the midpoint potential. Compared with a single structure, it can achieve more stable and faster potential balance control, effectively suppressing midpoint potential fluctuations. The topology formed by the secondary H-bridge inverter module 30 and the flying capacitor utilizes the voltage division characteristics of the capacitor and the flexible switching modulation of the H-bridge to optimize the output voltage waveform. During energy interaction, it can effectively filter out some high-frequency harmonic components, reduce the harmonic distortion rate of the system output current and voltage, improve power quality, make the grid-connected converter more compliant with grid access standards, and reduce harmonic pollution to the grid. The flying capacitor plays a buffering and stabilizing role in energy interaction, reducing voltage stress on power devices, reducing voltage surge impacts on switching devices, and extending device lifespan. Meanwhile, this structure can enhance the fault tolerance of the system to a certain extent. When a small-scale anomaly occurs in the system, the flying capacitor works in conjunction with the secondary-side H-bridge inverter module 30 and the multi-winding transformer 20 to maintain the stable operation of the energy interaction channel, ensure the DC side midpoint potential balance, and improve the overall reliability of the system.

[0040] Furthermore, the secondary winding 22 includes multiple windings with the same number of turns, and the number of phases of the flying capacitor is the same as the number of multiple windings with the same number of turns, so that the multiple windings with the same number of turns are electrically connected to the phases corresponding to the flying capacitor respectively.

[0041] By connecting multiple secondary windings 22 with the same number of turns to the corresponding phases of the flying capacitor, the magnetomotive force and compensation current generated by each phase winding are ensured to have high symmetry. When the DC side midpoint potential is unbalanced, each winding can generate an equal and coordinated compensation current injected into the corresponding phase flying capacitor based on the same number of turns characteristic, thereby achieving precise and symmetrical adjustment of the midpoint potential, avoiding new imbalance problems caused by asymmetrical compensation, and greatly improving the accuracy and stability of midpoint potential balance control.

[0042] Multiple windings are matched with the corresponding phases of the flying capacitor, forming multiple independent yet coordinated energy interaction channels. When sudden changes in system operating conditions cause fluctuations in the neutral point potential, each channel can respond simultaneously, quickly and collaboratively adjusting the charging and discharging process of the flying capacitor, accelerating the generation and transmission of compensation current, enabling the system to restore neutral point potential balance in a shorter time, and significantly enhancing the system's response speed and adaptability in the face of dynamic scenarios such as load changes and grid disturbances.

[0043] The design, where the number of windings matches the number of flying capacitor phases, simplifies circuit connections and control logic. During system design, this facilitates standardized selection and layout planning, reducing design errors caused by complex connections. During maintenance, the clear correspondence between windings and capacitor phases allows for more targeted troubleshooting and repair, enabling rapid problem location, reducing maintenance difficulty and time costs, and improving system operation and maintenance efficiency.

[0044] This structural design offers excellent scalability. When system capacity needs to be increased or performance optimized, it can be achieved by adding windings and flying capacitors with the same configuration, without requiring significant modifications to the overall architecture. Furthermore, when adapting to grid-connected converters of different specifications and power levels, system compatibility can be quickly achieved by flexibly adjusting the number of winding turns and capacitor parameters, effectively reducing product development costs and timelines, and enhancing product market competitiveness.

[0045] Among them, multiple windings with the same number of turns are divided into three, and the phase of the flying capacitor is three-phase.

[0046] The configuration of three-phase windings and three-phase flying capacitors is compatible with mainstream three-phase power grid architectures and can accurately compensate for the neutral point potential shift caused by factors such as unbalanced loads and grid fluctuations in three-phase systems. Through the energy interaction between the three windings and the corresponding phase flying capacitors, the three-phase output voltage imbalance can be effectively suppressed, the harmonic content of the three-phase current can be reduced, the grid-connected power quality can be significantly improved, and the stringent three-phase power grid access standards can be met.

[0047] The structure consisting of three windings and a three-phase flying capacitor supports independent neutral point potential regulation of the three-phase system. When a potential fluctuation occurs in a phase, the corresponding winding and flying capacitor can respond quickly and compensate independently. At the same time, the three phases can cooperate to achieve overall balance of the three-phase potential through magnetic coupling between the windings and energy exchange between the capacitors, effectively coping with complex and ever-changing three-phase imbalance conditions and improving the system's operational stability.

[0048] By employing three windings and a three-phase flying capacitor, mature technologies and components of three-phase systems can be directly reused, reducing customization requirements and lowering hardware costs and system size. At the same time, the control strategy under the three-phase structure is simpler and more efficient, which can reduce software development and debugging costs and improve the overall cost-effectiveness and market competitiveness of the solution.

[0049] The three-phase structure inherently possesses redundancy. If a fault occurs in one phase winding or flying capacitor, the remaining two phases can be adjusted through appropriate control strategies to maintain basic system operation and neutral point potential balance, preventing system shutdown due to a single point of failure. This redundancy design significantly enhances the system's fault tolerance, improves the reliability and continuous operation capability of the grid-connected converter under complex operating conditions, and reduces maintenance costs and power outage risks.

[0050] Both the secondary-side H-bridge inverter module and the primary-side H-bridge inverter module are composed of four switching transistors in a full-bridge configuration, and use the same type of semiconductor switching devices with the same power rating, as well as highly similar switching parameters and thermal performance.

[0051] Each of the five H-bridge inverter modules is composed of four switching transistors in a full-bridge configuration.

[0052] The identical switching parameters and thermal performance simplify the design and debugging process of the control system. When designing control algorithms and parameters, there is no need for complex parameter matching and optimization for the primary and secondary H-bridge inverter modules separately, reducing design difficulty and workload, and shortening the product development cycle. Furthermore, during system debugging and maintenance, the consistency of the modules makes it easier to identify the cause of faults, improving maintenance efficiency.

[0053] The primary winding consists of two windings with equal number of turns, and the secondary winding consists of three windings with equal number of turns. The two primary windings and the three secondary windings are wound on the same closed magnetic core and a double-wire parallel winding method is used to make the inductance and resistance values ​​of the two primary windings consistent.

[0054] The primary winding of a multi-winding transformer T , The number of turns is equal, and the secondary winding is... , , The number of turns is equal. Winding the two primary windings and the three secondary windings together on the same closed magnetic core fully utilizes the core's permeability, optimizing electromagnetic coupling between the windings. Magnetic lines of force within the core can pass through each winding more efficiently, reducing magnetic leakage and thus improving the transformer's energy transmission efficiency, reducing electromagnetic interference, minimizing electromagnetic impact on surrounding electronic components, and enhancing the overall electromagnetic compatibility of the system. The parallel winding ensures consistent inductance and resistance values ​​for the two primary windings, effectively avoiding uneven current distribution caused by differences in winding parameters, resulting in uniform current distribution and reduced circulating current losses. The three secondary windings with equal turns are also wound similarly, ensuring balanced secondary output voltage, improving the quality and stability of the entire transformer's output power, and reducing additional losses and failure risks caused by parameter imbalances.

[0055] The parameter-balanced winding design enables the transformer to operate more stably when facing load changes and voltage fluctuations. The balanced electrical characteristics among the windings effectively reduce the risk of system oscillation, ensure the stability of the transformer output, provide a more stable power input for downstream electrical equipment, and improve the reliability and stability of the entire power system.

[0056] The two primary windings are connected in series in opposite phases, so that when the DC midpoint deviates from the rated value, the magnetomotive force generated in the two primary windings is in opposite directions. The three secondary windings are connected in parallel in the same direction, so that the voltage potential induced in all secondary windings is the same. Any secondary winding is of the same polarity as any primary winding, so that the voltage change on the secondary side follows the voltage change on the primary side. The secondary windings and the primary windings have the same polarity at their corresponding terminals.

[0057] , By adopting the "anti-phase series" connection method, when the DC midpoint deviates from the rated value, the magnetomotive force generated in the two primary windings is in opposite directions; when the DC midpoint deviates from the rated value, the magnetomotive force generated in the winding is in opposite directions. This can effectively cancel the bias magnetization phenomenon generated by the DC component in the magnetic core, avoid the magnetic saturation problem caused by DC bias magnetization, prevent the increase of magnetic core loss, the reduction of efficiency and the aggravation of heat generation, extend the service life of the transformer, and improve the reliability and stability of the system operation.

[0058] Secondary winding , , By employing a "parallel connection in the same direction" method, the induced voltage potential of all secondary windings is ensured to be the same, achieving a stable and balanced output voltage. No circulating current will occur between windings due to voltage differences, ensuring that the load current is evenly distributed among the secondary windings. This improves the transformer's load-carrying capacity and reliability, reduces the risk of localized overheating of the windings, and is particularly suitable for applications requiring high output voltage stability, such as precision instrument power supplies and communication power supplies.

[0059] any one of the secondary windings With primary winding The secondary winding and primary winding are of the same polarity, ensuring that changes in the secondary voltage follow changes in the primary voltage. The corresponding terminals of the secondary and primary windings are of the same polarity, meaning the primary and secondary windings are in phase (0°) and change synchronously. This guarantees that changes in the secondary voltage can follow changes in the primary voltage in real time. Under conditions such as input voltage fluctuations and load changes, the secondary output voltage can quickly respond to changes in the primary voltage, adjusting the output in a timely manner to achieve efficient and stable power transmission, improve system dynamic performance, reduce voltage adjustment time and overshoot, and ensure stable operation of electrical equipment.

[0060] Any winding on the secondary side number of turns With any one of the primary windings number of turns The following ratio relationship must be satisfied: ; In the formula, k and m This is the proportionality coefficient; This is the total voltage of the DC bus; This is the rated voltage of the flying capacitor.

[0061] By limiting the turns ratio of the primary and secondary windings, the voltage of the flying capacitor can be effectively controlled. Maintaining a stable flying capacitor voltage within a suitable range ensures its stable function in the circuit, preventing damage due to excessive voltage or disruption of normal circuit operation due to insufficient voltage, thus improving the reliability and safety of the circuit. A reasonable turns ratio range also facilitates more efficient energy transfer between the primary and secondary sides. This reduces losses such as copper and iron losses during energy transfer between windings, improving the overall energy conversion efficiency of the power conversion device and reducing energy waste.

[0062] An insulation structure is provided between the primary winding and the secondary winding. The insulation structure adopts a multi-layer insulation structure that combines functional insulation and reinforced insulation.

[0063] This insulation structure ensures that the AC voltage amplitudes on the primary and secondary sides are basically matched under rated conditions, thereby achieving smooth power transmission and preventing the transformer from generating excessive circulating current or saturation due to voltage mismatch. k and m Ideally, the value should be 1. However, considering factors such as line impedance and switch voltage drop, the range of values ​​is given as 0.8 ≤ k ≤ m ≤ 1.2.

[0064] Functional insulation meets the insulation requirements during normal operation, ensuring no electrical breakdown occurs between the primary and secondary windings under normal operating conditions. Reinforced insulation further enhances insulation capacity, effectively addressing abnormal conditions such as overvoltage and short circuits. It reduces the risk of high voltage from the primary side entering the secondary side, ensuring the safety of equipment and personnel. Especially in high-voltage power conversion scenarios, it significantly improves system safety and reliability. The multiple insulation structure, through the synergistic effect of different insulation methods, disperses insulation stress, reducing problems such as aging and damage that may occur with single insulation structures. It maintains good insulation performance during long-term operation, extending equipment lifespan, reducing the failure rate caused by insulation failure, and improving equipment reliability and stability.

[0065] Furthermore, the process by which the secondary winding 22 generates a compensating current for the midpoint potential through the energy interaction channel satisfies the balance constraint condition, the expression of which is: ; In the formula, This refers to the active power on the secondary side; For energy storage in flying capacitors; The voltage across the flying capacitor; This refers to the current in the secondary winding 22; The power factor represents the proportion of active power on the secondary side to the total power.

[0066] This formula clearly and accurately quantifies the energy balance relationship between secondary-side active power, flying capacitor voltage, secondary-side winding 22 current, power factor, and the rate of change of energy storage in the flying capacitor. This allows for precise calculation and control of energy input and output, as well as the energy storage change in the flying capacitor, during the process of generating a compensation current in the secondary-side winding 22 to balance the midpoint potential. This provides a clear energy quantification basis for midpoint potential balance control and improves control accuracy.

[0067] Based on this balance constraint, the system can monitor the active power on the secondary side and related parameters of the flying capacitor in real time, ensuring that energy conservation and efficient utilization are always followed during the interaction between the secondary winding 22 and the flying capacitor. This avoids unnecessary energy loss, improves the transmission and utilization efficiency of energy during the generation of compensation current, and thus enhances the energy conversion efficiency of the entire grid-connected converter system, contributing to the goal of energy conservation and consumption reduction.

[0068] Because the balance constraints clearly define the relationship between key electrical quantities and the energy storage changes of the flying capacitor, the system can adjust the operating state of the secondary winding 22 (such as current magnitude and phase) and the charging and discharging process of the flying capacitor in a timely manner based on these conditions. When the system is subjected to external disturbances (such as sudden load changes or grid fluctuations) that cause a shift in the neutral point potential, energy regulation can be quickly performed according to these constraints to match the energy storage changes of the flying capacitor with the active power on the secondary side, rapidly generating a suitable compensation current, restoring the neutral point potential balance, enhancing the system's ability to cope with disturbances, and improving operational stability and reliability.

[0069] According to the transformer ampere-turn balance principle, the system always satisfies the constraint equations: ; In the formula, The number of turns of the primary winding 21, i.e. the number of turns of the primary winding of the multi-winding transformer 20 connected to the DC circuit structure, determines the electromagnetic characteristics of the primary winding 21 and the turns ratio relationship with the secondary winding 22. The number of turns in the secondary winding 22 is the number of turns in the winding connected to the transformer and voltage source inverter, etc. These factors collectively affect the transformer's performance, including its voltage transformation and electromagnetic balance. The current in the first capacitor; This is the current in the second capacitor; For flying capacitor A The current of the phase corresponds to the current in the three phases of the flying capacitor. A This phase reflects the current change of the flying capacitor in that phase, and is related to the flying capacitor. A It is related to the charging and discharging, energy interaction and other processes of the phase; For flying capacitor B The phase current corresponds to the flying capacitor. B The phase reflects the current characteristics of the flying capacitor in that phase, involving B Energy exchange and other behaviors across capacitors; For flying capacitor C The phase current corresponds to the flying capacitor. C Phase, reflection C The change in current across the capacitor, and C The energy interaction between phases and capacitors is closely related to the process.

[0070] When the DC side midpoint potential shifts At that time, the input voltage difference of the primary winding 21 and This causes the upper and lower capacitors to... and current and An imbalance and disruption of the ampere-turn balance result in a net magnetomotive force. Unbalanced magnetomotive force This forces the secondary winding 22 to generate a compensation current. Rate of change of voltage across the capacitor The described energy transfer process achieves passive autonomous compensation of the midpoint potential. The generation of compensation current can be categorized based on the difference in the magnitude of the upper and lower capacitor currents: ; in, At this time, secondary side flying capacitor discharge compensation is required. The current decreases; among which At this time, secondary side flying capacitor discharge compensation is required. The current decreases, and compensation is performed. The expression for the compensation current is: ; In the formula, This refers to the current in the secondary winding 22; The voltage across the flying capacitor; The current flowing through the flying capacitor; The power factor represents the proportion of active power on the secondary side to the total power. This is the voltage corresponding to the midpoint potential.

[0071] Compensation current With midpoint potential recovery rate Satisfying the relation ,in The first and second capacitors are the DC bus capacitors, and the multi-winding coupling characteristics of the transformer are utilized.

[0072] Based on the traditional ANPC five-level topology, a multi-winding transformer 20 is added as the core component for energy coupling and balance control. The primary side of the transformer contains two windings, each with a maximum number of turns. winding and Each of these components is connected via an independent H-bridge inverter circuit between the DC positive terminal P and the midpoint O, and between the midpoint O and the DC negative terminal N, to sense the imbalance state of the DC bus voltage. (Where Vdc is the total DC bus voltage), and the voltage deviation is... Input voltage asymmetry converted to primary winding 21: ; In the formula, The voltage across the first capacitor; The voltage across the second capacitor; This is the total voltage on the DC side; This is the deviation of the midpoint 0 from the ideal midpoint potential.

[0073] The secondary side of the transformer contains multiple turns. winding , , These are respectively connected to the three-phase flying capacitors A, B, and C. , , The output of the H-bridge inverter circuit forms an energy interaction channel between the flying capacitor and the transformer.

[0074] Furthermore, the DC circuit structure is connected with a first capacitor and a second capacitor, which creates a potential imbalance between the first capacitor and the second capacitor. To determine whether an unbalanced magnetomotive force is generated on the primary side, the degree of imbalance. The expression is: ; if or If this occurs, an unbalanced magnetomotive force is generated on the primary side, and the primary side current becomes unbalanced.

[0075] Combination Figure 2 As shown, when the primary current is unbalanced, the secondary side flying capacitor charges and discharges to replenish it. Then, the DC side capacitor C1 exchanges energy with the flying capacitor, and finally the midpoint potential is restored.

[0076] Through imbalance It can quickly determine that an unbalanced magnetomotive force has been generated on the primary side, and then promptly trigger the mechanism of generating a compensation current in the secondary winding 22 through the energy interaction channel. This timely determination and triggering greatly shortens the time from the occurrence of magnetomotive force imbalance to the start of compensation action, improves the system's response speed to midpoint potential balance adjustment, and effectively suppresses midpoint potential fluctuations.

[0077] By accurately identifying and promptly compensating for the imbalance of the primary magnetomotive force, the continuous shift of the DC side midpoint potential caused by the long-term existence of the magnetomotive force imbalance can be effectively avoided. This prevents problems such as increased output current and voltage harmonics and waveform distortion of the grid-connected converter, ensuring the stability of system operation and enabling the grid-connected converter to deliver power to the grid more stably.

[0078] Through the commutation method and switching state of the ANPC, the flying capacitor draws forward or reverse current from the DC midpoint; the increase in the midpoint O current enhances... Potential, and conversely, it can be reduced. The potential of the DC side capacitors is shown in Table 1. The imbalance of the voltage between the upper and lower capacitors on the DC side corresponds to the degree of imbalance of the midpoint potential.

[0079] Table 1 Imbalance Status

[0080] The first capacitor and the second capacitor are respectively connected to the primary winding 21 through the primary H-bridge inverter module 10.

[0081] The primary-side H-bridge inverter module 10 has multiple switching state combinations, which can flexibly adjust the charging and discharging process of the first and second capacitors by controlling the switching action of the H-bridge. When the DC side midpoint potential deviates, it can accurately control the magnitude and direction of the capacitor charging or discharging current, quickly compensate for the midpoint potential, and restore the midpoint potential to a balanced state more quickly, thus improving the dynamic response speed of midpoint potential balance.

[0082] The H-bridge provides an efficient channel for energy exchange between the capacitor and the DC circuit structure. Through proper switching control, energy loss during the exchange process can be reduced, improving energy transfer efficiency. Simultaneously, it better enables bidirectional energy flow between the capacitor and the DC circuit, meeting the energy regulation needs under different operating conditions (such as load changes and grid fluctuations), and ensuring the high efficiency and stability of the system's energy exchange.

[0083] By utilizing the control of the primary-side H-bridge inverter module 10, excessive voltage surges can be avoided by directly subjecting the capacitors to them. During system operation, the voltage regulation and buffering effect of the H-bridge can control the voltage changes across the capacitors within a reasonable range, reducing voltage stress on the capacitors, extending their service life, and improving the overall reliability and durability of the system.

[0084] If one of the capacitors or the corresponding H-bridge section experiences a minor fault, the other capacitor and the H-bridge can adjust their own operating state to continue to maintain the regulation of the DC side midpoint potential to a certain extent, thereby enhancing the fault tolerance of the system and enabling the system to maintain basic operating functions even in the event of a local fault, thus improving the stability and continuity of system operation.

[0085] like Figure 1 The diagram shows the overall structure of the DC-side midpoint potential balancing method based on magnetomotive force compensation. The diagram uses an ANPC five-level converter as an example. In the diagram, H bridge represents the H-bridge inverter circuit, whose function is to balance the DC-side capacitor... and and three-phase flying capacitors , , The DC current is converted into AC current for easy connection to the multi-winding transformer 20; point O in the diagram is the midpoint potential that needs to be controlled; the multi-winding transformer 20 is used for energy exchange, and the number of turns on the primary and secondary windings are respectively... , The primary side includes windings and The secondary side includes windings , , Based on the fundamental principles of ANPC five-level converter commutation, it can be known that the DC midpoint potential in steady state is... The voltage across the three-phase flying capacitor is The flying capacitor can be charged and discharged through surrounding switches. In steady state, the primary side H-bridge inverter module 10 and the secondary side H-bridge inverter module 30 satisfy the ampere-turn balance. At this time, there is no net magnetomotive force, the transformer has no compensation current, and the system is stable. When factors such as unbalanced load cause the midpoint O potential to drop, That is, capacitor As voltage increases, the capacitor The voltage drop affects the input voltage of the primary-side H-bridge inverter module 10, and the capacitor... Transformer connected via H-bridge As the current increases, the capacitor Transformer connected via H-bridge The increased current leads to an imbalance of ampere-turns on the primary side of the transformer, resulting in a decrease in the net ampere-turns on the primary side. This generates a positive net magnetomotive force, which is due to the potential asymmetry created by the two capacitors with unbalanced midpoint potential. This is used to determine the net magnetomotive force imbalance on the primary side. When factors such as load imbalance cause the potential at the midpoint O to rise, ,correspond Similarly, the net magnetomotive force generated in the opposite direction on the primary side can be obtained.

[0086] This invention achieves midpoint potential balance, eliminates the development difficulty of the balance algorithm, avoids algorithm delays, and enhances the adjustment speed and balancing ability, which has strong practical significance.

[0087] This invention introduces the physical characteristics of ampere-turn balance of a multi-winding transformer 20 into an active neutral-point clamping multilevel converter, constructing a DC-side neutral-point potential balancing mechanism without complex control algorithms. The primary side of the transformer is connected to the capacitor section of the DC bus via an H-bridge inverter unit to sense the neutral-point potential deviation. Simultaneously, the secondary side is connected to the H-bridge inverter units of each phase's flying capacitors, thus upgrading the flying capacitors from simple level-generating elements into active actuators with both energy buffering and dynamic balancing functions. This achieves hardware resource reuse and system architecture integration. Any deviation in the neutral-point potential will cause voltage asymmetry in the primary winding 21, thereby disrupting the ampere-turn balance and forcing a compensation current on the secondary side. This current automatically adjusts its charging and discharging state through the flying capacitor H-bridge circuit, ultimately accurately correcting the neutral-point potential through energy transfer. The entire process is rapid and requires no software intervention, greatly reducing the complexity and computational burden of the control system. Furthermore, due to the multi-winding coupling structure of the transformer, this invention can simultaneously balance the voltage between each phase's flying capacitors while balancing the DC bus, improving the overall stability and reliability of the system. Figure 5 The diagram shows a traditional software modulation or external auxiliary circuit scheme. Compared to the traditional software modulation or external auxiliary circuit scheme, such as... Figure 6 and Figure 7 As shown, this invention combines the advantages of strong hardware balance and low software cost, making it particularly suitable for medium- and high-voltage high-power energy conversion scenarios with high requirements for reliability, dynamic performance, and power density.

[0088] This invention relates to the midpoint potential balancing of the DC-side clamping capacitor in a three-phase ANPC five-level grid-connected converter (including but not limited to ANPC, T-NPC, three-level, five-level, etc., and converters with similar topologies derived from NPC). Two DC-side capacitors are connected to the primary side of a multi-winding transformer 20, and three-phase flying capacitors are connected to the secondary side. This invention compensates for the midpoint potential of a deviated clamping capacitor by utilizing the magnetomotive force compensation principle of transformer flux balance. Through energy transfer, this method achieves automatic midpoint potential correction, eliminating the development difficulties of balancing algorithms, avoiding algorithmic delays, and enhancing the adjustment speed and balancing capability.

[0089] This invention can also reduce costs. Considering the current flowing through the switching transistor, according to the capacitor charge formula... Midpoint potential imbalance will lead to and charge difference The total voltage on the DC side + DC bus midpoint voltage imbalance state When the DC side midpoint potential shifts At that time, the input voltage difference of the primary winding and Therefore, the charge difference Since the change in charge is equal to the integral of the capacitor current, the midpoint current... Taking a 1MW / 1000V system ANPC five-level inverter connected to the 10kV grid as an example, the allowable DC side voltage fluctuation is 2%, and the output current is: Calculation of the midpoint current of the foundation: This current is a high-frequency pulsating current, with its effective value much smaller than its peak value. 10A indicates the short-time overcurrent limit that the device needs to withstand. Only a standardized, low-cost H-bridge module needs to be monitored or replaced, reducing the overall lifecycle maintenance cost. The power rating of the H-bridge inverter module used... It accounts for only 0.5% of the apparent power of a 1MW system. The bulk purchase cost of small-power H-bridge inverter modules ranges from a few hundred yuan, and its proportion in the entire system is negligible. It also saves on software development and later maintenance costs.

[0090] The above embodiments are merely illustrative examples and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A grid-connected converter DC side midpoint potential balancing structure, characterized in that, The application relates to a direct-current circuit structure, a multi-winding transformer, and a voltage source inverter. The direct-current circuit structure comprises a direct-current positive pole, a midpoint potential and a direct-current negative pole. The multi-winding transformer is provided with a primary winding and a secondary winding, the primary winding is electrically connected to the direct-current circuit structure between the direct-current positive pole and the midpoint potential and between the midpoint potential and the direct-current negative pole through a primary-side H-bridge inverter module, and the voltage source inverter is electrically connected to the secondary winding, so that an energy interaction channel is formed between the voltage source inverter and the multi-winding transformer, and the secondary winding generates a compensation current for the midpoint potential through the energy interaction channel when an unbalanced magnetic motive force is generated. The voltage source inverter comprises a secondary-side H-bridge inverter module with an output end and a flying capacitor connected to the output end of the secondary-side H-bridge inverter module and the secondary winding, so that the flying capacitor and the multi-winding transformer form the energy interaction channel.

2. The grid-connected converter DC-side neutral-point potential balancing structure according to claim 1, characterized in that, The secondary-side H-bridge inverter module and the primary-side H-bridge inverter module are both formed by four switch tubes in a full-bridge mode, adopt the same type of semiconductor switch device, have the same power level, and have the same switching parameters and thermal performance. The process that the secondary winding generates the compensation current for the midpoint potential through the energy interaction channel satisfies a balance constraint condition, and the expression of the balance constraint condition is: The expression of the compensation current is:

3. The grid-connected converter DC-side neutral-point potential balancing structure according to claim 2, characterized in that, The primary winding comprises two windings with equal numbers of turns, the secondary winding comprises three windings with equal numbers of turns, the primary two windings and the secondary three windings are wound on the same closed magnetic core, and a double-wire parallel winding method is adopted, so that the inductance and resistance values of the primary two windings are consistent.

4. The grid-connected converter DC-side neutral-point potential balancing structure according to claim 2, characterized in that, The primary two windings are connected in anti-phase series, so that the magnetic motive forces generated in the primary two windings are opposite when the direct-current midpoint deviates from the rated value, the secondary three windings are connected in same-phase parallel, so that the voltage potentials induced by all the secondary windings are the same, any secondary winding and any primary winding have the same polarity, so that the voltage change of the secondary winding follows the voltage change of the primary winding, and the secondary winding and the primary winding have the same polarity corresponding to the same name end. ； wherein is the active power at the secondary side; is the energy stored in the flying capacitor; is the voltage of the flying capacitor; is the current of the secondary winding; is the power factor, representing the proportion of the active power at the secondary side to the total power.

5. The grid-connected converter DC-side neutral-point potential balancing structure according to claim 2, characterized in that, An insulation structure is arranged between the primary winding and the secondary winding, and the insulation structure adopts a multi-layer insulation structure combining functional insulation and reinforced insulation. ； In the formula, is the current of the secondary winding; is the voltage of the flying capacitor; is the current flowing through the flying capacitor; is the power factor, indicating the proportion of active power on the secondary side to the total power; is the voltage corresponding to the midpoint potential.

6. The grid-connected converter DC-side neutral-point potential balancing structure according to claim 5, characterized in that, The direct current circuit structure is connected with a first capacitor and a second capacitor, and the imbalance generated by the first capacitor and the second capacitor in potential is used to judge whether the primary side generates an unbalanced magnetic motive force The expression of the imbalance degree is: ； wherein is the voltage of the first capacitor; is the voltage of the second capacitor; is the total voltage of the DC side; is the deviation of the midpoint 0 from the ideal midpoint potential; If or a primary-side unbalanced magnetic motive force is generated.

7. The grid-connected converter DC-side neutral-point potential balancing structure according to any one of claims 1 to 6, characterized in that, ​ 8. The grid-connected converter DC-side neutral-point potential balancing structure according to claim 7, characterized in that, ​ 9. The grid-connected converter DC-side neutral-point potential balancing structure according to claim 7, characterized in that, the number of turns of any one winding on the secondary side the number of turns of any one winding on the primary side the number of turns of any one winding on the secondary side the number of turns of any one winding on the primary side satisfy the following transformation ratio relationship: ； wherein k and m is a proportionality factor; is the total DC bus voltage; is the flying capacitor rated voltage.

10. The grid-connected converter DC-side neutral-point potential balancing structure according to any one of claims 1 to 6, characterized in that, ​

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