A three-level converter neutral point potential control method and system

Abstract

The application provides a three-level converter neutral point potential control method and system, relates to the technical field of intelligent control, samples positive bus voltage and negative bus voltage, calculates voltage deviation, and generates an additional control quantity according to the voltage deviation and the sign of alternating current, wherein the additional control quantity is obtained through sign function and proportional control processing; the additional control quantity is subjected to amplitude limiting processing to obtain an amplitude-limited additional control quantity; the amplitude-limited additional control quantity is superimposed on a positive modulation wave, and the amplitude-limited additional control quantity is inverted and superimposed on a negative modulation wave to obtain final positive and negative modulation waves; the final positive and negative modulation waves are compared with a carrier wave to generate a driving signal of a power device; and the driving signal is used to control the on-off of the switching device, so that corresponding levels are output to realize neutral point potential balance. The application can simultaneously realize low-frequency fluctuation and DC offset suppression of the neutral point voltage.

Description

Technical Field

[0001] This invention relates to the field of intelligent control technology, and in particular to a method for controlling the midpoint potential of a three-level converter. Background Technology

[0002] In the current mainstream power electronic converter topologies, three-level converters are widely used in many fields such as new energy power generation, industrial transmission, and transportation due to their advantages such as low output voltage harmonic content, high output voltage level, small output filter size, and low switching transistor voltage stress.

[0003] However, three-level converters are prone to low-frequency fluctuations in the neutral point voltage and DC offset during actual operation. Taking the most common Type I NPC three-level converter as an example, when the output is at zero level, the load current flows into and out of the DC side neutral point, causing a fluctuation of three times the fundamental frequency at the DC side neutral point potential. This fluctuation introduces low-frequency harmonics and reduces power quality. In addition, due to differences in actual capacitor parameters, load disturbances, and other factors, DC offset of the neutral point voltage is likely to occur. This will also cause the power switching devices to be subjected to different voltage stresses, accelerating device aging and damage, and affecting the normal operation of the converter.

[0004] Existing solutions can be broadly categorized into hardware and software solutions. Hardware solutions adjust the midpoint voltage by adding high-power components such as IGBTs, diodes, and inductors, but this not only increases initial costs but also places higher demands on system stability. In software solutions, conventional carrier-based zero-sequence voltage injection methods adjust the duty cycle of the zero state by superimposing unbalanced control components on the modulated wave, resulting in limited control effectiveness under high modulation intensities and low power factors. While some space vector-based modulation strategies, such as adjusting the duration of positive and negative small vectors and virtual space vectors, have some effect, they suffer from computational complexity and poor parameter robustness. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method for controlling the midpoint potential of a three-level converter, which can simultaneously achieve low-frequency fluctuations in the midpoint voltage and suppression of DC offset.

[0006] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows:

[0007] In a first aspect, a method for controlling the neutral point potential of a three-level converter, the method comprising the following steps:

[0008] Step 1: Inject a zero-sequence voltage into the three-phase modulation wave. The zero-sequence voltage is calculated based on the maximum, intermediate, and minimum values ​​of the three-phase modulation wave. The zero-sequence voltage is then superimposed onto the original three-phase modulation wave to form a new three-phase modulation wave.

[0009] Step 2: Decompose the new three-phase modulation wave into a positive modulation wave and a negative modulation wave, where the positive modulation wave and the negative modulation wave correspond to the maximum value, intermediate value and minimum value phase of the three-phase modulation wave, respectively;

[0010] Step 3: Sample the positive bus voltage and negative bus voltage, calculate the voltage deviation, and generate an additional control quantity based on the voltage deviation and the sign of the AC current. The additional control quantity is obtained through sign function and proportional control processing.

[0011] Step 4: Limit the additional control quantity to obtain the limited additional control quantity; superimpose the limited additional control quantity onto the positive modulation wave, and simultaneously invert the limited additional control quantity and superimpose it onto the negative modulation wave to obtain the final positive and negative modulation waves.

[0012] Step 5: Compare the final positive and negative modulated waves with the carrier wave to generate the drive signal for the power device;

[0013] Step 6: Control the switching device to turn on and off according to the drive signal, thereby outputting the corresponding level to achieve midpoint potential balance.

[0014] Furthermore, in step 1, the method for calculating the zero-sequence component is as follows:

[0015] Real-time acquisition of the standardized original three-phase modulated wave, and calculation of its maximum, median and minimum values;

[0016] Add the maximum and minimum values, invert the result, and then divide by two to obtain the zero-order component.

[0017] This zero-sequence component is simultaneously injected into the original three-phase modulation wave to form a new three-phase modulation wave in the center.

[0018] Furthermore, in step 2, the three-phase modulation wave after zero-sequence voltage injection is decomposed into positive modulation wave and negative modulation wave according to the maximum value phase, intermediate value phase and minimum value phase, respectively;

[0019] For the maximum value phase, its positive modulation wave is set to 1, and its negative modulation wave is set to the modulation wave of that phase minus 1;

[0020] For the intermediate phase, its positive modulation wave is set to half the difference between the modulation wave of the phase and the modulation wave of the maximum value phase plus 1, and its negative modulation wave is set to half the difference between the modulation wave of the phase and the modulation wave of the minimum value phase minus 1.

[0021] For the minimum phase, its positive modulation wave is set to the phase modulation wave plus 1, and its negative modulation wave is set to -1.

[0022] Furthermore, in step 3, the method for calculating the additional control quantity is as follows:

[0023] Collect the positive bus voltage and the negative bus voltage, and calculate their voltage deviation.

[0024] The product of the voltage deviation and the instantaneous value of the corresponding phase AC current is processed by the first sign function to obtain the first result;

[0025] The result of adding 1 to the negative modulation wave and subtracting the positive modulation wave is processed by the second symbol function to obtain the second result;

[0026] Based on the first result, the second result, the preset proportional control coefficient, and the voltage deviation, the additional control quantity for each phase is obtained.

[0027] Furthermore, in step 3, when the additional control quantity is subjected to amplitude limiting processing, it is determined whether the difference between the positive modulation wave and the negative modulation wave is greater than 1.

[0028] When the difference is greater than 1, the amplitude limit is taken as the smaller of the absolute value of the positive modulation wave and the absolute value of the opposite number of the negative modulation wave.

[0029] When the difference is less than 1, the limiting value is the smaller of 1 minus the difference of the positive modulation wave and 1 plus the sum of the negative modulation wave.

[0030] Furthermore, in step 4, the additional control quantity after amplitude limiting is superimposed onto the positive modulation wave, and the inverted additional control quantity is superimposed onto the negative modulation wave. Specifically:

[0031] The final positive modulated wave is obtained by adding the additional control quantity after amplitude limiting to the positive modulated wave, and the final negative modulated wave is obtained by subtracting the additional control quantity after amplitude limiting from the negative modulated wave.

[0032] Furthermore, in step 5, when a dual-carrier in-phase superimposed modulation method is adopted, the positive carrier range is set to 0 to 1, and the load wave range is -1 to 0; the final positive modulation wave is compared with the positive carrier, and the positive level switch signal is 1 when the positive modulation wave is greater than the positive carrier, otherwise it is 0; the final negative modulation wave is compared with the load wave, and the negative level switch signal is 1 when the negative modulation wave is less than the load wave, otherwise it is 0.

[0033] Furthermore, in step 5, when a single-carrier modulation method is used, the carrier range is set to 0 to 1;

[0034] The final positive modulated wave is compared with the carrier wave. When the positive modulated wave is greater than the carrier wave, the positive level switch signal is 1; otherwise, it is 0.

[0035] The final negative modulated wave is incremented by one and moved to the 0 to 1 interval. It is then compared with the carrier wave. When the shifted negative modulated wave is less than the carrier wave, the negative level switch signal is 1; otherwise, it is 0.

[0036] Furthermore, in step 6, the difference between the positive level switch signal and the negative level switch signal is calculated to obtain a three-level control signal;

[0037] When the difference is 1, a positive level is output, turning on the two upper switches of the corresponding phase;

[0038] When the difference is 0, the output is at zero level, turning on the two middle switches of the corresponding phase;

[0039] When the difference is negative 1, a negative level is output, turning on the next two switching devices of the corresponding phase.

[0040] A three-level converter neutral point potential control system, comprising:

[0041] One or more processors;

[0042] A storage device for storing one or more programs that, when executed by one or more processors, cause the one or more processors to implement the method.

[0043] The above-described solution of the present invention has at least the following beneficial effects:

[0044] By introducing closed-loop control, the DC component of the positive and negative bus voltage deviation is used as an error signal. After proportional control, it is used as an additional control quantity of the modulation wave. At the same time, the amplitude of this additional control quantity is limited. The DC side capacitor value and switching cycle are not required, which greatly simplifies the calculation process and makes it more practical. Attached Figure Description

[0045] Figure 1 This is a flowchart illustrating the present invention.

[0046] Figure 2 These are comparison waveforms before and after the three-phase modulated wave is injected with zero-sequence voltage.

[0047] Figure 3 It is a comparison waveform before and after the positive and negative decomposition of the modulated wave.

[0048] Figure 4 yes A schematic diagram of the modulation wave when using carrier stacking modulation in certain situations.

[0049] Figure 5 yes A schematic diagram of the modulation wave when using carrier stacking modulation in this case.

[0050] Figure 6 yes A schematic diagram of the modulation wave when using single-carrier modulation in this situation.

[0051] Figure 7 yes A schematic diagram of the modulation wave when using single-carrier modulation in this situation. Detailed Implementation

[0052] Exemplary embodiments of the present disclosure will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0053] like Figures 1 to 7 As shown, an embodiment of the present invention proposes a method for controlling the neutral point potential of a three-level converter, the method comprising the following steps:

[0054] Step 1: Inject a zero-sequence voltage into the three-phase modulation wave. The zero-sequence voltage is calculated based on the maximum, intermediate, and minimum values ​​of the three-phase modulation wave. The zero-sequence voltage is then superimposed onto the original three-phase modulation wave to form a new three-phase modulation wave.

[0055] Step 2: Decompose the new three-phase modulation wave into a positive modulation wave and a negative modulation wave, where the positive modulation wave and the negative modulation wave correspond to the maximum value, intermediate value and minimum value phase of the three-phase modulation wave, respectively;

[0056] Step 3: Sample the positive bus voltage and negative bus voltage, calculate the voltage deviation, and generate an additional control quantity based on the voltage deviation and the sign of the AC current. The additional control quantity is obtained through sign function and proportional control processing.

[0057] Step 4: Limit the additional control quantity to obtain the limited additional control quantity; superimpose the limited additional control quantity onto the positive modulation wave, and simultaneously invert the limited additional control quantity and superimpose it onto the negative modulation wave to obtain the final positive and negative modulation waves.

[0058] Step 5: Compare the final positive and negative modulated waves with the carrier wave to generate the drive signal for the power device;

[0059] Step 6: Control the switching device to turn on and off according to the drive signal, thereby outputting the corresponding level to achieve midpoint potential balance.

[0060] In this embodiment of the invention, the zero-sequence voltage is calculated and injected based on the maximum, intermediate, and minimum values ​​of the three-phase modulation wave in step 1, achieving centering of the original modulation wave and effectively improving the utilization efficiency of the DC-side voltage. The positive and negative modulation wave decomposition in step 2, combined with the additional control in steps 3-4 (which does not rely on DC-side capacitor values ​​or switching cycles, simplifying calculations), can specifically suppress the three-fold fundamental frequency fluctuation of the midpoint voltage, reducing low-frequency harmonics to improve power quality. Furthermore, through the coordinated control of voltage deviation and current sign, it can eliminate DC offset of the midpoint potential caused by capacitor parameter differences and load disturbances. The additional control quantity is generated through a sign function and proportional control, without... This method avoids complex multi-variable calculations (such as capacitance value and switching cycle) required in existing technologies, thus avoiding the impact of variable deviations on control performance and significantly improving practicality and parameter robustness. By balancing the positive and negative bus voltages, it avoids uneven voltage stress on power switching devices, slows down device aging and damage, and extends service life. At the same time, it eliminates the need for additional hardware components (such as IGBTs and inductors), does not increase primary costs, and reduces additional requirements for system stability. Steps 5-6 generate drive signals by comparing the modulated wave with the carrier wave and control the switching on and off, ensuring precise switching of output positive, zero, and negative levels, stably achieving neutral point potential balance, and ensuring long-term reliable operation of the converter.

[0061] In a preferred embodiment of the present invention, the method for calculating the zero-sequence component in step 1 is as follows:

[0062] Real-time acquisition of the standardized original three-phase modulated wave, and calculation of its maximum, median and minimum values;

[0063] Add the maximum and minimum values, invert the result, and then divide by two to obtain the zero-order component.

[0064] This zero-sequence component is simultaneously injected into the original three-phase modulation wave to form a new three-phase modulation wave in the center.

[0065] In this embodiment, the zero-sequence component is obtained simply by inverting (maximum + minimum) and dividing by two the maximum and minimum values ​​of the real-time collected per-unit original three-phase modulation wave. This eliminates the need for complex variables such as DC-side capacitor values ​​and system switching cycles, significantly simplifying the calculation process, reducing controller computing power consumption, improving control real-time performance, and avoiding control delays or performance fluctuations caused by complex calculations. By injecting the zero-sequence component through this method, the original three-phase modulation wave can be accurately centered, making the new three-phase modulation wave more reasonably adapted to the DC-side voltage range. This avoids the problem of insufficient DC voltage utilization caused by modulation wave offset, and fully leverages the high output voltage level advantage of the three-level converter. The zero-sequence component calculation is based on the real-time acquisition of modulation wave extrema, which can dynamically respond to changes in the original three-phase modulation wave. Even under conditions where the control effect of existing technologies is limited, such as high modulation degree and low power factor, it can still stably generate a suitable zero-sequence component, ensuring that the new three-phase modulation wave always remains in a centered state, avoiding modulation wave anomalies caused by operating condition fluctuations, and improving the operating condition adaptability of the overall control scheme. The centered new three-phase modulation wave can make the waveforms of subsequent steps (such as positive and negative modulation wave decomposition) more regular and the range more controllable, reducing the decomposition error caused by the offset of the original modulation wave, thereby ensuring the accuracy of subsequent links such as the superposition of additional control quantities and the generation of drive signals, and indirectly improving the reliability of the midpoint potential balance control.

[0066] In a preferred embodiment of the present invention, in step 2, the three-phase modulation wave after zero-sequence voltage injection is decomposed into a positive modulation wave and a negative modulation wave according to the maximum value phase, the intermediate value phase and the minimum value phase, respectively.

[0067] For the maximum value phase, its positive modulation wave is set to 1, and its negative modulation wave is set to the modulation wave of that phase minus 1;

[0068] For the intermediate phase, its positive modulation wave is set to half the difference between the modulation wave of the phase and the modulation wave of the maximum value phase plus 1, and its negative modulation wave is set to half the difference between the modulation wave of the phase and the modulation wave of the minimum value phase minus 1.

[0069] For the minimum phase, its positive modulation wave is set to the phase modulation wave plus 1, and its negative modulation wave is set to -1.

[0070] In this embodiment, a differentiated decomposition rule is adopted for the different attributes of the maximum, intermediate, and minimum phases of the three-phase modulation waves after zero-sequence injection. The positive modulation wave of the maximum phase is fixed at 1, and the negative modulation wave is associated with its own waveform. The minimum phase is the opposite. The intermediate value is calculated by combining the waveforms of the maximum / minimum phases. This allows the decomposed positive and negative modulation waves to accurately match the amplitude range of each phase waveform, avoiding waveform disorder after decomposition. The decomposition process relies only on the amplitude of the three-phase modulation waves after zero-sequence injection (maximum, intermediate, and minimum values), and is achieved through simple mathematical operations such as "halving the difference and adding or subtracting 1" and "direct waveform calculation". It does not require calling complex external variables such as DC-side capacitor values ​​and system switching cycles, which greatly simplifies the calculation process, reduces the computing power occupied by the controller, improves the real-time performance of decomposition, and avoids control lag problems caused by complex calculations. The amplitude range of the positive modulation wave (e.g., the maximum phase is fixed at 1) and the negative modulation wave (e.g., the minimum phase is fixed at -1) after decomposition is clear and controllable. When additional control quantities are superimposed in subsequent steps, the amplitude limit boundary (e.g., the maximum movement amplitude can be determined more accurately) can be judged. The decomposition rules rely solely on the real-time amplitude of the three-phase modulation waves, without depending on external parameters. Even under conditions where the control effect is limited by high modulation intensity and low power factor, as long as the three-phase modulation waves after zero-sequence injection can output effective maximum / medium / minimum values, the decomposition can be stably completed without being affected by external parameter deviations (such as capacitor parameter drift and load disturbances), thus improving the overall control scheme's adaptability and parameter robustness. The decomposed positive and negative modulation waves correspond to positive and negative level control logic, respectively (e.g., the maximum value phase positive modulation wave 1 is adapted to positive level output, and the minimum value phase negative modulation wave -1 is adapted to negative level output). When compared with the carrier wave later, the switching timing of the switching transistors can be controlled more precisely, ensuring that the current flow direction when each phase outputs positive, zero, and negative levels meets the requirements of midpoint current balance, indirectly helping to suppress midpoint voltage third harmonic fluctuations and DC offset.

[0071] In a preferred embodiment of the present invention, the method for calculating the additional control quantity in step 3 is as follows:

[0072] Collect the positive bus voltage and the negative bus voltage, and calculate their voltage deviation.

[0073] The product of the voltage deviation and the instantaneous value of the corresponding phase AC current is processed by the first sign function to obtain the first result;

[0074] The result of adding 1 to the negative modulation wave and subtracting the positive modulation wave is processed by the second symbol function to obtain the second result;

[0075] Based on the first result, the second result, the preset proportional control coefficient, and the voltage deviation, the additional control quantity for each phase is obtained.

[0076] In this embodiment, the calculation of additional control quantities relies only on the real-time collected positive / negative bus voltage deviation, the instantaneous value of the corresponding phase AC current, and the preset proportional control coefficient. It does not require complex external variables such as DC-side capacitor value and system switching cycle as in the prior art. The core calculation can be completed through simple logical judgment of two symbolic functions, which greatly simplifies the calculation process, reduces the computing power occupied by the controller, and avoids control performance fluctuations caused by external variable deviations (such as capacitor parameter drift and switching cycle fluctuations), thus improving practicality.

[0077] The first symbolic function processes the product of voltage deviation and instantaneous AC current, accurately determining the direction of midpoint potential imbalance (e.g., positive bus voltage higher / lower than negative bus voltage) and the synergistic relationship between current flow direction (inflow / outflow from converter). The second symbolic function processes "1 + negative modulation wave - positive modulation wave," combining the modulation wave's own state after zero-sequence injection. The combination of these two functions dynamically locates different imbalance conditions (e.g., voltage deviation and current flow direction combination scenarios), enabling the additional control quantity to be specifically matched to the imbalance type, avoiding blind adjustment and improving the accuracy of midpoint potential balance control. The preset proportional control coefficient can be flexibly adjusted according to the power level, capacitor specifications, load characteristics, and other actual scenarios of the three-level converter, adapting to different application scenarios such as new energy power generation and industrial transmission without modifying the core calculation logic. At the same time, because it does not rely on external parameters that are susceptible to environmental (temperature, aging) influences, it can still stably output the appropriate additional control quantity even under complex conditions such as load disturbances and high-frequency regulation, enhancing the system's adaptability and parameter robustness. Both the voltage deviation and the instantaneous value of the AC current are real-time acquired data. The additional control quantity can be dynamically updated according to the neutral point potential imbalance state (voltage deviation change) and current flow direction. It can quickly respond to small deviations or fluctuations in the neutral point potential, avoid the accumulation and expansion of the imbalance state, promptly suppress DC deviation and low-frequency fluctuations of the neutral point voltage, reduce the impact on power quality, and at the same time avoid the accelerated aging of power switching devices due to long-term uneven voltage stress, ensuring the stable operation of the converter.

[0078] In a preferred embodiment of the present invention, in step 3, when the additional control quantity is subjected to amplitude limiting processing, it is determined whether the difference between the positive modulation wave and the negative modulation wave is greater than 1.

[0079] When the difference is greater than 1, the amplitude limit is taken as the smaller of the absolute value of the positive modulation wave and the absolute value of the opposite number of the negative modulation wave.

[0080] When the difference is less than 1, the limiting value is the smaller of 1 minus the difference of the positive modulation wave and 1 plus the sum of the negative modulation wave.

[0081] In this embodiment, by determining the difference between the positive and negative modulating waves and limiting the amplitude accordingly, the final modulated wave after the superposition of additional control quantities can be strictly controlled to always be within a reasonable range (such as the [0,1] or [-1,0] interval adapted to the subsequent carrier comparison). This avoids the generation of incorrect power device drive signals when comparing with the carrier due to the modulated wave exceeding the range, thereby preventing output level disorder (such as the inability to output positive, zero, and negative levels normally), and providing a basic guarantee for midpoint potential balance control. For the two modulation wave amplitude relationships with a difference greater than 1 and a difference less than 1, the smaller value of the absolute value of the positive and negative modulating waves, and the smaller value of the calculation result of 1 and the positive and negative modulating waves are used as the limiting value, rather than a fixed limiting value. This can accurately match the real-time state of the current modulated wave, avoiding both excessive limiting leading to insufficient effect of the additional control quantity (such as the inability to correct the midpoint potential deviation in time) and insufficient limiting causing modulation wave distortion. This ensures that the additional control quantity can fully and safely play its regulatory role and improve the accuracy of midpoint potential balance. The limiting judgment relies solely on the difference between the positive and negative modulating waves. The limiting value calculation only requires the smaller value of the two parameters, eliminating the need for complex external variables such as DC-side capacitor values ​​and system switching cycles. This simplifies the calculation process, reduces controller computational power consumption, avoids control delays caused by complex calculations, and ensures that additional control quantities can respond in real time to midpoint potential imbalances, promptly suppressing DC voltage deviation and low-frequency fluctuations. Enhanced control robustness and adaptability to various operating conditions: The limiting processing is based solely on the current modulation wave's own parameters (positive and negative modulation wave values), without relying on external parameters susceptible to environmental (temperature, device aging) or operating conditions (load disturbances, modulation index changes). Even under complex operating conditions such as high modulation index, low power factor, or sudden load changes, it can still accurately determine and generate appropriate limiting values, preventing limiting failure due to operating condition fluctuations and indirectly improving the overall robustness of midpoint potential control.

[0082] In a preferred embodiment of the present invention, in step 4, the additional control quantity after amplitude limiting is superimposed on the positive modulation wave, and the additional control quantity is inverted and superimposed on the negative modulation wave, specifically as follows:

[0083] The final positive modulated wave is obtained by adding the limited additional control quantity to the positive modulated wave, and the final negative modulated wave is obtained by subtracting the limited additional control quantity from the negative modulated wave. In step 5, when a dual-carrier in-phase superimposed modulation method is used, the positive carrier range is set to 0 to 1, and the load wave range is set to -1 to 0. The final positive modulated wave is compared with the positive carrier. When the positive modulated wave is greater than the positive carrier, the positive level switch signal is 1; otherwise, it is 0. The final negative modulated wave is compared with the load wave. When the negative modulated wave is less than the load wave, the negative level switch signal is 1; otherwise, it is 0. In step 5, when a single-carrier modulation method is used, the carrier range is set to 0 to 1.

[0084] The final positive modulated wave is compared with the carrier wave. When the positive modulated wave is greater than the carrier wave, the positive level switch signal is 1; otherwise, it is 0.

[0085] The final negative modulation wave is incremented by one and moved to the 0 to 1 interval, then compared with the carrier wave. When the shifted negative modulation wave is less than the carrier wave, the negative level switch signal is 1, otherwise it is 0. In step 6, the positive level switch signal and the negative level switch signal are subtracted to obtain the three-level control signal.

[0086] When the difference is 1, a positive level is output, turning on the two upper switches of the corresponding phase;

[0087] When the difference is 0, the output is at zero level, turning on the two middle switches of the corresponding phase;

[0088] When the difference is negative 1, a negative level is output, turning on the next two switching devices of the corresponding phase.

[0089] In this embodiment, the method of superimposing the positive modulation wave and inverting the negative modulation wave in step 4 with the amplitude-limiting additional control quantity can specifically correct the amplitude of the positive and negative modulation waves. For example, when the positive bus voltage is too high, the additional control quantity directly reduces the zero-level action time by shifting the positive modulation wave down and the negative modulation wave up to suppress abnormal flow of midpoint current. It can dynamically offset the midpoint potential imbalance caused by voltage deviation and load disturbance, significantly improve the suppression effect of DC offset and third harmonic fluctuation of midpoint voltage, and because the additional control quantity has been limited, it can prevent the modulation wave from exceeding the effective range (such as the positive modulation wave exceeding 1 and the negative modulation wave being below -1), ensuring the accuracy of subsequent carrier comparison and laying the core foundation for midpoint balance.

[0090] Dual-carrier (positive carrier 0-1, load wave -1-0) directly compares the positive modulating wave with the positive carrier and the negative modulating wave with the load wave, without the need for additional waveform conversion. The logic is intuitive and suitable for scenarios with high real-time control requirements (such as industrial drives). Single-carrier (negative modulating wave +1 shifted to the 0-1 interval) only requires one carrier, simplifying the hardware carrier generation circuit or software operation logic, reducing system design and implementation costs. Moreover, neither method relies on complex external parameters and can still stably generate switching signals under conditions such as high modulation and low power factor, improving the scenario adaptability of the solution.

[0091] In practical applications, the above steps can be implemented through the following steps:

[0092] Step 1: Inject zero-sequence voltage to center the original modulated wave.

[0093] To improve DC voltage utilization, zero-sequence voltage is injected into the three-phase modulation wave. The specific process is as follows:

[0094] Acquire the original three-phase modulated wave after standardization Real-time calculation of the maximum value in this group of modulated waves median and minimum value ;

[0095] Define zero-order components Its calculation formula is ;

[0096] zero-sequence component Simultaneously superimposed on the original three-phase modulated wave, a new three-phase modulated wave is obtained. It satisfies the following system of equations:

[0097] ;

[0098] A comparison of the modulation waveforms before and after zero-sequence voltage injection is attached. Figure 2 As shown.

[0099] Step 2: Decompose the three-phase modulated wave after zero-sequence injection to obtain the positive and negative modulated waves:

[0100] Based on satisfying the voltage equivalence principle and the midpoint current balance principle within each switching cycle, the zero-sequence voltage obtained in step 1 is injected into the three-phase modulation wave, and the peak value phase, intermediate value phase, and minimum value phase (corresponding to respectively) are selected. Decomposed into positive modulation waves respectively and negative modulation wave The specific decomposition method is as follows:

[0101] Maximum phase: The positive modulation wave is fixed at... The negative modulation wave is ;

[0102] Intermediate phase: Positive modulation wave is The negative modulation wave is ;

[0103] Minimum phase: positive modulation wave is The negative modulation wave is fixed as .

[0104] A comparison of the waveforms of the single modulated wave after zero-sequence injection and the positive and negative modulated waves after decomposition is shown in the appendix. Figure 3 As shown.

[0105] Step 3: Calculate the additional control quantity and adjust it to match the DC imbalance of the midpoint potential.

[0106] To address the DC imbalance in the midpoint potential caused by differences in actual circuit parameters and load disturbances, additional control quantities are generated through the following steps:

[0107] Sample DC side positive bus voltage and negative bus voltage Calculate the voltage deviation between the two. ;

[0108] Define the sign function The logic is as follows: ;

[0109] in For input variables;

[0110] Acquire per-unit instantaneous values ​​of three-phase AC current ( (corresponding to the current of the phase with the maximum value, the middle value, and the minimum value), combined with the voltage deviation. Symbolic functions and preset proportional control coefficient Calculate the additional control quantities for each phase. The formula is:

[0111] ;

[0112] against Typical scenarios (corresponding positive and negative modulation waveforms are shown in the attached figure) Figure 4 As shown), further clarify the adjustment logic of the additional control quantity:

[0113] like and This indicates that the positive bus voltage is higher than the negative bus voltage, and AC current flows out of the converter. Therefore, the zero-level duration needs to be reduced (reducing the charging time of the positive bus capacitor and the discharging time of the negative bus capacitor). At this time, additional control quantities are required. A negative value shifts the positive modulation wave downwards and the negative modulation wave upwards, with a maximum shift amplitude of [value missing]. .

[0114] like and This indicates that the positive bus voltage is higher than the negative bus voltage, and AC current flows into the converter. Therefore, the zero-level operating time needs to be increased (increasing the discharge time of the positive bus capacitor and the charging time of the negative bus capacitor). At this time, additional control quantities are required. A positive value indicates that the function is to shift the positive modulation wave upwards and the negative modulation wave downwards, with a maximum shift amplitude of [value missing]. .

[0115] for and , and In the same scenario, following the logic above, the core is to adjust the zero-level duration by adding control quantities to balance the positive and negative bus voltages.

[0116] Step 4: Limit the additional control input to generate the final modulated wave:

[0117] Based on the maximum movement amplitude specified in step 3 (e.g.) or ), for additional control quantities Perform amplitude limiting to obtain the additional control quantity after amplitude limiting. ;

[0118] The additional control quantity after amplitude limiting is superimposed on the positive modulation wave, and then multiplied by -1 and superimposed on the negative modulation wave to obtain the final modulation wave.

[0119] ;

[0120] in, This represents the final positive modulation wave. This represents the final negative modulated wave.

[0121] Step 5: Compare the modulated wave with the carrier wave to generate the power device drive signal:

[0122] Depending on the actual application requirements, either dual-carrier in-phase cascaded modulation or single-carrier modulation can be selected to generate the driving signal:

[0123] Method 1, Dual-carrier in-phase cascade modulation:

[0124] Set the positive carrier range to [0, 1] and the load wave (load wave) range to [-1, 0];

[0125] The final positive modulation wave Compared with a positive carrier, if Greater than the positive carrier wave, positive level switching signal ,otherwise ;

[0126] The final negative modulated wave Compared with the load wave, if Less than the load waveform, negative level switching signal ,otherwise A schematic diagram of this modulation method is attached. Figure 4 and attached Figure 5 As shown.

[0127] Method 2, a modulation method involving a single carrier and upward shifting of the negative modulated wave:

[0128] Set the carrier range to [0, 1];

[0129] The final positive modulation wave Compared with the carrier wave: If Greater than the carrier wave, positive level switching signal ,otherwise ;

[0130] The final negative modulated wave Add 1 to shift it to the [0, 1] interval (resulting in the upward-shifted negative modulation wave). );

[0131] The upward-shifted negative modulation wave Compared with the carrier wave: If Less than carrier, negative level switching signal ,otherwise A schematic diagram of this modulation method is attached. Figure 6 and attached Figure 7 As shown.

[0132] Step 6: Control the switch to turn on and off according to the drive signal, and output the target level:

[0133] Calculate the difference between the positive-level switching signal and the negative-level switching signal for each phase. ;

[0134] Based on the difference Control the corresponding phase power switching devices (taking the A-phase bridge arm of the Type I NPC three-level converter as an example). (For example) On / off states, output corresponding level:

[0135] like Output positive level ,control Open, Turn off;

[0136] like Output zero level, control Open, Turn off;

[0137] like Output negative level ,control Open, Turn off.

[0138] Other modulation methods can achieve similar results by analogy with the above logic, and will not be described in detail in this embodiment. Through the above steps, the neutral point potential of the three-level converter is balanced, suppressing low-frequency fluctuations and DC offset of the neutral point voltage.

[0139] A three-level converter neutral point potential control system, comprising:

[0140] One or more processors;

[0141] A storage device for storing one or more programs that, when executed by one or more processors, cause the one or more processors to implement the method.

[0142] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for controlling the neutral point potential of a three-level converter, characterized in that, The method includes the following steps: Step 1: Inject a zero-sequence voltage into the three-phase modulation wave. The zero-sequence voltage is calculated based on the maximum, intermediate, and minimum values ​​of the three-phase modulation wave. The zero-sequence voltage is then superimposed onto the original three-phase modulation wave to form a new three-phase modulation wave. Step 2: Decompose the new three-phase modulation wave into a positive modulation wave and a negative modulation wave, where the positive modulation wave and the negative modulation wave correspond to the maximum value, intermediate value and minimum value phase of the three-phase modulation wave, respectively; Step 3: Sample the positive and negative bus voltages, calculate the voltage deviation, and generate additional control quantities based on the voltage deviation and the sign of the AC current. These additional control quantities are obtained through a sign function and proportional control processing. The calculation method for the additional control quantities is as follows: Sample the positive and negative bus voltages and calculate their voltage deviation; multiply the voltage deviation by the instantaneous value of the corresponding phase AC current using a first sign function to obtain a first result; add 1 to the negative modulation wave and subtract the positive modulation wave, then process the result using a second sign function to obtain a second result; based on the first result, the second result, the preset proportional control coefficient, and the voltage deviation, obtain the additional control quantities for each phase. Step 4: Limit the additional control quantity to obtain the limited additional control quantity; The additional control quantity after limiting is superimposed on the positive modulation wave, and the additional control quantity after limiting is inverted and superimposed on the negative modulation wave to obtain the final positive and negative modulation waves. Step 5: Compare the final positive and negative modulated waves with the carrier wave to generate the drive signal for the power device; Step 6: Control the switching device to turn on and off according to the drive signal, thereby outputting the corresponding level to achieve midpoint potential balance.

2. The method for controlling the neutral point potential of a three-level converter according to claim 1, characterized in that, In step 1, the zero-sequence component is calculated as follows: Real-time acquisition of the standardized original three-phase modulated wave, and calculation of its maximum, median and minimum values; Add the maximum and minimum values, invert the result, and then divide by two to obtain the zero-order component. This zero-sequence component is simultaneously injected into the original three-phase modulation wave to form a new three-phase modulation wave in the center.

3. The method for controlling the neutral point potential of a three-level converter according to claim 2, characterized in that, In step 2, the three-phase modulation wave after zero-sequence voltage injection is decomposed into positive modulation wave and negative modulation wave according to the maximum value phase, intermediate value phase and minimum value phase, respectively. For the maximum value phase, its positive modulation wave is set to 1, and its negative modulation wave is set to the modulation wave of that phase minus 1; For the intermediate phase, its positive modulation wave is set to half the difference between the modulation wave of the phase and the modulation wave of the maximum value phase plus 1, and its negative modulation wave is set to half the difference between the modulation wave of the phase and the modulation wave of the minimum value phase minus 1. For the minimum phase, its positive modulation wave is set to the phase modulation wave plus 1, and its negative modulation wave is set to -1.

4. The method for controlling the neutral point potential of a three-level converter according to claim 3, characterized in that, In step 3, when the additional control quantity is subjected to amplitude limiting processing, it is determined whether the difference between the positive modulation wave and the negative modulation wave is greater than 1. When the difference is greater than 1, the amplitude limit is taken as the smaller of the absolute value of the positive modulation wave and the absolute value of the opposite number of the negative modulation wave. When the difference is less than 1, the limiting value is the smaller of 1 minus the difference of the positive modulation wave and 1 plus the sum of the negative modulation wave.

5. The method for controlling the neutral point potential of a three-level converter according to claim 4, characterized in that, In step 4, the additional control quantity after amplitude limiting is superimposed on the positive modulation wave, and the additional control quantity is inverted and superimposed on the negative modulation wave. Specifically: The final positive modulated wave is obtained by adding the additional control quantity after amplitude limiting to the positive modulated wave, and the final negative modulated wave is obtained by subtracting the additional control quantity after amplitude limiting from the negative modulated wave.

6. The method for controlling the neutral point potential of a three-level converter according to claim 5, characterized in that, In step 5, when a dual-carrier in-phase superimposed modulation method is adopted, the positive carrier range is set to 0 to 1, and the load wave range is -1 to 0; the final positive modulation wave is compared with the positive carrier. When the positive modulation wave is greater than the positive carrier, the positive level switch signal is 1, otherwise it is 0; the final negative modulation wave is compared with the load wave. When the negative modulation wave is less than the load wave, the negative level switch signal is 1, otherwise it is 0.

7. The method for controlling the neutral point potential of a three-level converter according to claim 6, characterized in that, In step 5, when a single-carrier modulation method is used, the carrier range is set to 0 to 1; The final positive modulated wave is compared with the carrier wave. When the positive modulated wave is greater than the carrier wave, the positive level switch signal is 1; otherwise, it is 0. The final negative modulated wave is incremented by one and moved to the 0 to 1 interval. It is then compared with the carrier wave. When the shifted negative modulated wave is less than the carrier wave, the negative level switch signal is 1; otherwise, it is 0.

8. The method for controlling the neutral point potential of a three-level converter according to claim 7, characterized in that, In step 6, the difference between the positive level switch signal and the negative level switch signal is calculated to obtain a three-level control signal; When the difference is 1, a positive level is output, turning on the two upper switches of the corresponding phase; When the difference is 0, the output is at zero level, turning on the two middle switches of the corresponding phase; When the difference is negative 1, a negative level is output, turning on the next two switching devices of the corresponding phase.

9. A neutral point potential control system for a three-level converter, characterized in that, include: One or more processors; A storage device for storing one or more programs, which, when executed by one or more processors, cause the one or more processors to implement the three-level converter midpoint potential control method as described in any one of claims 1 to 8.

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