Control method of inverter and three-level inverter system

Abstract

The application discloses a control method of an inverter and a three-level inverter system. A midpoint voltage deviation value, an active power absolute value and a reactive power absolute value of the inverter are obtained, and a zero sequence voltage amplitude is obtained by proportionally integrating the midpoint voltage error value. In a high power factor condition, a first injection direction symbol is determined based on the signs of three-phase modulation wave original voltages and three-phase currents, and a direct current zero sequence voltage is determined based on the first injection direction symbol and the zero sequence voltage amplitude. In a low power factor condition, the sign of the reactive power is determined, a second injection direction symbol is determined based on the signs of the three-phase modulation wave original voltages and the reactive power, and a sixth power zero sequence voltage is determined based on the second injection direction symbol and the zero sequence voltage amplitude. The direct current zero sequence voltage or the sixth power zero sequence voltage is added to original three-phase SVPWM modulation waves to obtain modulation waves, and the modulation waves are converted into switch driving signals. The application aims to improve the midpoint current regulation capability and balance efficiency of the three-level inverter system.

Description

Technical Field

[0001] This invention relates to the field of inverter technology, and in particular to an inverter control method and a three-level inverter system. Background Technology

[0002] Three-level NPC (Neutral Point Clamped) inverters hold a significant position in industrial applications due to their topological advantages, but they suffer from a midpoint potential imbalance problem. Under existing SVPWM modulation frameworks, control strategies often rely on the allocation and regulation of redundant small vectors, such as adjusting the action time of small vectors by introducing regulation coefficients. However, such methods cannot maintain high performance across the entire power factor range. Their control logic depends on the detection of the load current direction, and its effectiveness depends on the inverter operating at a high power factor, i.e., a strong correspondence between the current direction and the voltage vector direction. When the inverter operates at a low power factor, the active current component approaches zero, severely blurring the mapping between the current direction and the vector regulation direction. This causes the control logic based on "current direction determining small vector allocation" to lose its judgment benchmark, leading to a significant decrease in midpoint potential regulation capability or even complete loss of control. Summary of the Invention

[0003] The main objective of this invention is to provide a control method for an inverter and a three-level inverter system, aiming to improve the midpoint current regulation capability and balance efficiency of the three-level inverter system.

[0004] To achieve the above objectives, the present invention proposes a control method for an inverter, the control method comprising: Obtain the inverter's midpoint voltage deviation, absolute value of active power, and absolute value of reactive power; The midpoint voltage deviation value is adjusted proportionally and integrally to obtain the required injected zero-sequence voltage amplitude; The current operating condition of the inverter is determined based on the absolute value of the active power and the absolute value of the reactive power, and the operating condition includes a high power factor condition and a low power factor condition. When the operating condition is a high power factor condition, the first injection direction sign of the zero-sequence voltage is determined based on the sign of the original voltage and the sign of the three-phase modulation wave and the three-phase current, and the required injected DC zero-sequence voltage is determined based on the first injection direction sign and the amplitude of the zero-sequence voltage. When the operating condition is a low power factor condition, the sign of the reactive power is determined, and the second injection direction sign of the zero-sequence voltage is determined based on the original voltage of the three-phase modulation wave and the sign of the reactive power. The sixth-power wave zero-sequence voltage to be injected is determined based on the second injection direction sign and the amplitude of the zero-sequence voltage. Based on the operating conditions, the DC zero-sequence voltage or the sixth-power wave zero-sequence voltage is added to the original three-phase SVPWM modulation wave to obtain the modulation wave after injecting the zero-sequence voltage. The modulated wave after injecting zero-sequence voltage is converted into a switching drive signal to control the midpoint voltage of the inverter.

[0005] In one embodiment, obtaining the inverter's midpoint voltage deviation, absolute active power, and absolute reactive power includes: Obtain the positive half-bus voltage and negative half-bus voltage on the DC side of the inverter, and use the difference between the positive half-bus voltage and the negative half-bus voltage as the midpoint voltage deviation value. Obtain the direct-axis current, quadrature-axis current, direct-axis voltage, and quadrature-axis voltage in a synchronous rotating coordinate system, and determine the active power and reactive power based on the direct-axis current, quadrature-axis current, direct-axis voltage, and quadrature-axis voltage, and determine the absolute value of the active power and the absolute value of the reactive power based on the active power.

[0006] In one embodiment, determining active power and reactive power based on the direct-axis current, the quadrature-axis current, the direct-axis voltage, and the quadrature-axis voltage includes determining reactive power based on a preset active power calculation formula, a preset reactive power calculation formula, the direct-axis current, the quadrature-axis current, the direct-axis voltage, and the quadrature-axis voltage. The preset active power calculation formula includes: ; The preset reactive power calculation formula includes: ; in, Active power Reactive power For direct-axis current, For quadrature axis current, It is the direct-axis voltage. It is the quadrature-axis voltage.

[0007] In one embodiment, determining the current operating condition of the inverter based on the absolute value of the active power and the absolute value of the reactive power includes: If the absolute value of the active power is greater than K times the absolute value of the reactive power, the current operating condition of the inverter is determined to be a high power factor condition. If the absolute value of the active power is less than or equal to K times the absolute value of the reactive power, the current operating condition of the inverter is determined to be a low power factor condition; where K is a natural number.

[0008] In one embodiment, the control method for the inverter further includes: Obtain the direct-axis voltage and quadrature-axis voltage in a synchronously rotating coordinate system; The direct-axis voltage and the quadrature-axis voltage are sequentially subjected to inverse PARK transform and inverse CLARK transform to obtain the original voltage of the three-phase modulated wave. Obtain the three-phase current output from the inverter and extract the sign of the three-phase current.

[0009] In one embodiment, determining the first injection direction sign of the zero-sequence voltage based on the sign of the original voltage and three-phase current of the three-phase modulated wave, and determining the desired injected DC zero-sequence voltage based on the first injection direction sign and the amplitude of the zero-sequence voltage, includes: Based on the magnitude of the original voltage of the three-phase modulated wave, the sector where the current voltage vector is located is determined; Based on the symbols of the sector and the three-phase current, a preset symbol mapping table is queried to obtain the first injection direction symbol; The product of the first injection direction sign and the zero-sequence voltage amplitude is taken as the required injected DC zero-sequence voltage.

[0010] In one embodiment, determining the second injection direction sign of the zero-sequence voltage based on the sign of the original three-phase modulated wave voltage and the reactive power, and determining the desired injected sixth-power wave zero-sequence voltage based on the second injection direction sign and the zero-sequence voltage amplitude, includes: Based on the magnitude of the original voltage of the three-phase modulated wave, the sector where the current voltage vector is located is determined; Based on the symbols of the sector, the original voltage of the three-phase modulation wave, and the reactive power, a preset symbol mapping table is consulted to obtain the symbol of the second injection direction. The product of the second injection direction sign and the zero-sequence voltage amplitude is taken as the sixth-power zero-sequence voltage to be injected.

[0011] In one embodiment, the preset addition formula used to add the DC zero-sequence voltage or the sixth-power zero-sequence voltage to the original three-phase SVPWM modulation wave includes: ; ; ; in, The injected DC zero-sequence voltage or the sixth-power zero-sequence voltage, , , The three-phase voltages corresponding to the original three-phase SVPWM modulation wave. , , The three-phase voltage corresponding to the modulation wave after the DC zero-sequence voltage or the sixth-power wave is injected.

[0012] In one embodiment, after performing proportional-integral adjustment on the midpoint voltage deviation value to obtain the desired injected zero-sequence voltage amplitude, the inverter control method further includes: The zero-sequence voltage amplitude is limited to obtain a zero-sequence voltage amplitude not exceeding 0.4M, where M is the output voltage modulation ratio.

[0013] The present invention also proposes a three-level inverter system, which includes a control device and a three-level inverter. The three-level inverter includes a DC-side capacitor assembly and a three-phase inverter bridge arm. The DC-side capacitor assembly includes two capacitors connected in series, and the connection point of the two capacitors is the DC neutral point. The neutral point of the three-phase inverter bridge arm is connected to the DC neutral point. The control device is electrically connected to the three-phase inverter bridge arm. The control device is used to execute the control method of the inverter described in any of the above claims. The control device is also used to output a switch drive signal to control the on / off state of the three-phase inverter bridge arm.

[0014] In practical applications, this method effectively improves the problem of midpoint potential regulation failure in three-level NPC inverters under low power factor conditions by injecting a zero-sequence voltage with controllable amplitude and polarity within the SVPWM framework, maintaining high performance across the entire power factor range. Compared to traditional redundancy vector allocation schemes that rely on load current direction, the inverter control method of this invention eliminates the dependence on active current direction. Even when the active current approaches zero due to low power factor, it can still accurately establish a control reference for the midpoint current flow direction, eliminating the regulation blind zone of traditional schemes in the low power factor region. Simultaneously, under high power factor conditions, voltage and current are in phase, and the DC zero-sequence voltage can achieve midpoint charge compensation with relatively low computational cost, improving response speed. Under low power factor conditions, a sixth-power wave is used as the zero-sequence component. Leveraging its high-frequency polarity reversal characteristics, it mitigates the problem caused by phase difference, generating a greater midpoint current regulation capability than a sine wave injection under the same amplitude constraints, thus improving the midpoint potential balance efficiency of the three-level inverter system. Attached Figure Description

[0015] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 A flowchart illustrating an embodiment of the control method for the inverter of the present invention; Figure 2 A flowchart illustrating another embodiment of the control method for the inverter of the present invention; Figure 3 A flowchart illustrating another embodiment of the control method for the inverter of the present invention; Figure 4 A flowchart is provided for yet another embodiment of the control method for the inverter of the present invention; Figure 5 A flowchart illustrating another embodiment of the control method for the inverter of the present invention; Figure 6 A flowchart illustrating another embodiment of the control method for the inverter of the present invention; Figure 7 A specific circuit diagram is provided for an embodiment of the three-level inverter system of the present invention; Figure 8 An algorithm logic block diagram provided for an embodiment of the inverter control method of the present invention; Figure 9 This is a schematic diagram of the midpoint current waveform provided in another embodiment of the control method for the inverter of the present invention.

[0018] The objectives, features, and advantages of this invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0019] It should be understood that the specific embodiments described herein are merely illustrative of the technical solutions of the present invention and are not intended to limit the present invention.

[0020] To better understand the technical solution of the present invention, a detailed description will be provided below in conjunction with the accompanying drawings and specific embodiments.

[0021] Three-level NPC (Neutral Point Clamped) inverters hold a significant position in industrial applications due to their topological advantages, but they suffer from a midpoint potential imbalance problem. Under existing SVPWM modulation frameworks, control strategies often rely on the allocation and regulation of redundant small vectors, such as adjusting the action time of small vectors by introducing regulation coefficients. However, such methods cannot maintain high performance across the entire power factor range. Their control logic depends on the detection of the load current direction, and its effectiveness depends on the inverter operating at a high power factor, i.e., a strong correspondence between the current direction and the voltage vector direction. When the inverter operates at a low power factor, the active current component approaches zero, severely blurring the mapping between the current direction and the vector regulation direction. This causes the control logic based on "current direction determining small vector allocation" to lose its judgment benchmark, leading to a significant decrease in midpoint potential regulation capability or even complete loss of control.

[0022] Therefore, refer to Figure 1 This invention proposes a control method for an inverter, the control method comprising: Step S100: Obtain the inverter's midpoint voltage deviation, absolute value of active power, and absolute value of reactive power.

[0023] In this embodiment, the inverter control method of the present invention can be applied to the control device of the inverter system. The control device can be implemented by a main controller, such as MCU (Microcontroller Unit), DSP (Digital Signal Processor), FPGA (Field Programmable Gate Array), PLC (Programmable Logic Controller), SOC (System On Chip), etc.

[0024] In this embodiment, the midpoint voltage deviation value reflects the degree of imbalance of the current midpoint potential (i.e., the target quantity that needs to be adjusted), and serves as a feedback quantity for the midpoint voltage imbalance, thereby adjusting the zero-sequence voltage amplitude to form closed-loop control; the absolute values ​​of active power and reactive power are used as power factor criteria to evaluate the current power factor status of the inverter.

[0025] It should be noted that the reference Figure 7The inverter system includes a control device and an inverter. The inverter includes a DC-side capacitor assembly and a three-phase inverter bridge arm. The DC-side capacitor assembly includes two capacitors connected in series, and the connection point of the two capacitors is the DC neutral point. The neutral point of the three-phase inverter bridge arm is connected to the DC neutral point. The control device is electrically connected to the three-phase inverter bridge arm. In this embodiment, a voltage sensor can be set to collect the positive and negative half-bus voltages on the DC side, wherein the positive half-bus voltage is the voltage across capacitor C1. The negative half-bus voltage is the voltage across capacitor C2. The midpoint voltage deviation value is then... and The difference. At the same time, the three-phase output voltage can be collected by a voltage sensor and the three-phase output current can be collected by a current sensor. Then, through coordinate transformation, it is converted to a synchronous rotating coordinate system (dq axis). Then, the active power and reactive power are calculated using the preset active power calculation formula and the preset reactive power calculation formula, and the absolute values ​​of active power and reactive power are taken respectively.

[0026] Step S200: The midpoint voltage deviation value is proportionally and integrally adjusted to obtain the required injected zero-sequence voltage amplitude.

[0027] In this embodiment, the midpoint voltage deviation is converted into a zero-sequence voltage amplitude command. This involves closed-loop negative feedback control of the midpoint voltage deviation, outputting an amplitude command representing the compensation strength. The magnitude of the zero-sequence voltage amplitude reflects the zero-sequence voltage strength required to eliminate the midpoint potential deviation in real time. Since a proportional-integral (PI) regulator can eliminate steady-state errors, it ensures that the midpoint potential eventually returns precisely to zero. Therefore, by adjusting the midpoint voltage deviation value using proportional-integral regulation, low-frequency drift or oscillation of the midpoint potential can be eliminated. The PI regulator ensures that the midpoint voltage deviation is zero in steady state, dynamically adjusting the compensation strength according to the deviation magnitude. Therefore, the acquired midpoint voltage deviation value can be input into the PI regulator, which outputs the zero-sequence voltage amplitude based on the magnitude and accumulation of the deviation.

[0028] Optionally, after step S200, the inverter control method further includes: The zero-sequence voltage amplitude is limited to obtain a zero-sequence voltage amplitude not exceeding 0.4M, where M is the output voltage modulation ratio.

[0029] In this embodiment, to prevent excessive zero-sequence voltage injection from causing the modulation wave to exceed the linear modulation region, thereby resulting in output voltage waveform distortion or over-modulation of the switching devices in the three-phase inverter bridge arm, a limiter can be set in the control device to set the upper limit of the zero-sequence voltage amplitude output by the PI regulator to 0.4M, where M is the current output voltage modulation ratio. Thus, by adopting a limiting strategy dynamically bound to the modulation ratio M, the effectiveness of the midpoint potential control is ensured, while also considering the linearity of the output voltage and power quality. The zero-sequence voltage amplitude after limiting is used as the zero-sequence voltage amplitude shared by the two subsequent operating conditions. Participating in negative feedback closed-loop control The midpoint voltage deviation changes dynamically.

[0030] Step S300: Determine the current operating condition of the inverter based on the absolute value of the active power and the absolute value of the reactive power. The operating condition includes a high power factor condition and a low power factor condition.

[0031] In this embodiment, the control device can divide the inverter's operating state into high power factor condition and low power factor condition according to the ratio of active power to reactive power, thereby realizing adaptive switching of control strategy based on the operating state. This step aims to automatically select the corresponding midpoint balancing algorithm according to the real-time operating conditions to ensure the control performance of the inverter system under all operating conditions.

[0032] In this embodiment, researchers can pre-determine a power factor judgment coefficient K based on a large amount of experimental data, comparing the absolute values ​​of active power and reactive power with the product of K. For example, if the absolute value of active power is greater than the product of the absolute value of reactive power and K, it is determined to be a high power factor operating condition; otherwise, it is determined to be a low power factor operating condition. In this way, the inverter's operating condition can be quickly classified based on instantaneous power characteristics, serving as a key step in achieving full-condition control.

[0033] It should be noted that K determines the switching sensitivity. This sensitivity can be preset by the developers and stored in the control device's memory, or it can be varied based on the power factor angle. For example, K can be set to... K is At that time, the corresponding power factor angle is 30°.

[0034] Step S400: When the operating condition is a high power factor condition, determine the first injection direction sign of the zero-sequence voltage based on the sign of the original voltage and the sign of the three-phase modulation wave and the three-phase current, and determine the required injected DC zero-sequence voltage based on the first injection direction sign and the amplitude of the zero-sequence voltage.

[0035] In this embodiment, under high power factor conditions, the polarity of the zero-sequence voltage is determined by the direction of the load current, thereby generating a DC zero-sequence voltage. By generating a midpoint current that deviates from the midpoint voltage, midpoint potential shift is effectively suppressed.

[0036] In this embodiment, the control device can determine the original voltage of the three-phase modulated wave ( , , The combination of positive and negative signs of the current signal () determines the sector where the current voltage vector resides. Then, combining the current sector and the sign of the corresponding phase current, a preset symbol mapping table is consulted to obtain the first injection direction sign. Finally, the first injection direction sign is multiplied by the zero-sequence voltage amplitude after amplitude limiting to obtain the DC zero-sequence voltage. The preset symbol mapping table is pre-set by the R&D personnel and stored in the control device's memory.

[0037] Understandably, under high power factor conditions, the load current and output voltage are almost in phase (i.e., the reactive component is minimal). The midpoint potential shift is caused by the midpoint current. Under high power factor conditions, when a phase voltage is in the positive half-cycle and the current is also positive, the phase current will continuously flow into or out of the midpoint. At this time, injecting a DC zero-sequence voltage of fixed polarity can precisely change the duty cycle of that phase when it is in the zero-level state, thereby generating an average midpoint current opposite to the direction of the midpoint voltage deviation. Since the voltage and current are in phase, the control logic is simple. It is only necessary to determine the current sector (determined by the modulation wave sign) and the current sign to directly obtain the injection direction by looking up a table, without the need for complex trigonometric function calculations. The dynamic response is fast, and the high voltage utilization of SVPWM can be maintained. In the high power factor region, the injection method is equivalent to the redistribution of the small vector action time in traditional SVPWM, using the load current direction to generate a midpoint current opposite to the zero-sequence voltage amplitude, thereby suppressing the midpoint potential shift. The entire process does not require calculation of the voltage vector angle, reducing the computational load of the control device.

[0038] Step S500: When the operating condition is a low power factor condition, determine the sign of the reactive power, and determine the second injection direction sign of the zero-sequence voltage based on the original voltage of the three-phase modulation wave and the sign of the reactive power, and determine the required injected sixth-power wave zero-sequence voltage based on the second injection direction sign and the amplitude of the zero-sequence voltage.

[0039] In this embodiment, under low power factor conditions, the polarity of the zero-sequence voltage is determined by the direction of reactive power and the magnitude of the original voltage of the three-phase modulation wave, generating a sixth-power wave zero-sequence voltage. This improves the control blind zone of the traditional small vector method under low power factor conditions. The sixth-power wave can generate a larger average midpoint current at the same amplitude, thereby quickly correcting the midpoint potential under low power factor conditions.

[0040] In this embodiment, it is necessary to determine the sign (positive or negative) of the reactive power Q. Then, combining the magnitude of the original voltage of the three-phase modulation wave (to determine the sector) and the sign of the reactive power, a preset sign mapping table is consulted to obtain the second injection direction sign. Finally, the second injection direction sign is multiplied by the zero-sequence voltage amplitude after amplitude limiting to obtain the sixth-power wave zero-sequence voltage. The preset sign mapping table is pre-set by the R&D personnel and stored in the memory of the control device.

[0041] It should be noted that when the inverter operates under low power factor conditions (i.e., a large proportion of reactive power and a significant phase difference between voltage and current), the DC zero-sequence injection method will have a control blind zone. Under low power factor conditions, the zero-crossing points of the current and voltage are severely misaligned. When the voltage is near its peak value (where the small vector has the longest duration), the current may be just near its zero-crossing point. This means that even if a DC zero-sequence voltage is injected to adjust the duty cycle of the switch drive signal, the average midpoint current generated is also small due to the small current flowing at this time, making it impossible to effectively correct the midpoint potential shift. However, the polarity of the sixth-power wave zero-sequence voltage will flip six times within one fundamental cycle. This high-frequency flipping characteristic allows the sixth-power wave to alternately squeeze or stretch the modulation wave during its positive and negative half-cycles, regardless of the current phase. This can generate a continuous and sufficient charge compensation current in the range of larger currents, thereby breaking the control blind zone under low power factor conditions. Under the same amplitude command, the contribution of the sixth-power zero-sequence voltage to the average midpoint current of the modulated wave is far greater than that of conventional sinusoidal or DC zero-sequence injection. This allows the control device to pull the midpoint potential back to the equilibrium point with extremely high efficiency even under extreme reactive power conditions. Furthermore, the sign of the second injection direction is directly obtained from a table looking up the sector and reactive power signs, eliminating the need for trigonometric function and angle calculations, thus ensuring rapid dynamic response under low power factor conditions.

[0042] Step S600: Based on the operating conditions, add the DC zero-sequence voltage or the sixth-power wave zero-sequence voltage to the original three-phase SVPWM modulation wave to obtain the modulation wave after injecting the zero-sequence voltage.

[0043] In this embodiment, under high power factor conditions, the DC zero-sequence voltage is superimposed as a voltage component onto the fundamental modulation wave (the original three-phase SVPWM modulation wave); under low power factor conditions, the sixth-power zero-sequence voltage is superimposed as a voltage component onto the fundamental modulation wave. That is, based on the currently determined operating condition, the calculated DC zero-sequence voltage or the sixth-power zero-sequence voltage is superimposed onto the original three-phase SVPWM modulation wave. Thus, without changing the original SVPWM modulation architecture and high voltage utilization, the zero-sequence component is used to shift only the three-phase modulation wave, without changing the line voltage fundamental component (i.e., without affecting motor torque or grid-connected active power output), and the midpoint current is adjusted only by changing the common-mode component of the phase voltage. Compared to a sinusoidal voltage injection of the same amplitude, this can generate a larger duty cycle offset in the SVPWM modulation wave, thereby exciting a larger average midpoint current and significantly improving the midpoint regulation capability of the three-phase inverter system.

[0044] Step S700: Convert the modulated wave after injecting zero-sequence voltage into a switching drive signal to control the midpoint voltage of the inverter.

[0045] In this embodiment, the control device can convert the modulation wave into a switching drive signal to drive the switching devices of the three-phase inverter bridge arm, thereby adjusting the neutral point potential. Optionally, the control device can compare the updated modulation wave (i.e., the modulation wave after injecting zero-sequence voltage) with a preset high-frequency triangular carrier wave to generate a PWM (Pulse Width Modulation) signal. Since the injected zero-sequence voltage can change the amplitude of the modulation wave, the amplitude of the modulation wave will change the pulse width (duty cycle) of the PWM signal generated after comparison with the triangular carrier wave. The change in pulse width directly determines the dwell time of the zero-level state in the inverter. The dwell time determines the direction and magnitude of the neutral point current, thereby adjusting the neutral point potential and improving the neutral point potential imbalance problem. Thus, by changing the dwell time of the zero-level state, the neutral point current flows in the correct direction (into or out of the neutral point), thereby regulating the charging and discharging of the upper and lower capacitors of the DC-side capacitor assembly, ultimately achieving dynamic balance of the neutral point potential.

[0046] It should be noted that traditional SVPWM midpoint balancing strategies rely on the direction of active current to determine the impact of small vectors on the midpoint potential. However, under low power factor conditions, the power factor angle φ is approximately 90 degrees, at which point the active current ip = Imcosφ ≈ 0. This means that the traditional mapping relationship of "current direction → voltage vector selection" fails, causing conventional control strategies to be unable to generate an effective midpoint current to correct the potential deviation. This method introduces active power and reactive power as the discrimination criteria. Since reactive power has the largest value and a clear direction under low power factor conditions, this method utilizes the sign reversal characteristic of reactive power to re-establish the mapping relationship between the polarity of zero-sequence voltage and the direction of midpoint current flow. Thus, even under low power factor conditions where the active current approaches zero, it can still accurately determine whether a positive or negative zero-sequence voltage should be injected, thereby eliminating the control blind zone under low power factor conditions.

[0047] In this embodiment, under low power factor conditions, the present invention uses a sixth-order sine wave instead of a traditional sixth-order sine wave as the zero-sequence component injection modulation wave. From the perspective of Fourier series expansion, the fundamental component amplitude of the square wave is significantly higher than that of a sine wave of the same amplitude. When this sixth-order sine wave zero-sequence voltage is superimposed on the original three-phase SVPWM modulation wave, it modulates the midpoint current by changing the duty cycle of the three-phase inverter bridge arms when they are at zero level. Because the square wave has a steeper transition edge and a wider flat-top region, under the same amplitude constraint, the injection of the sixth-order sine wave can cause a more drastic change in duty cycle than the injection of the sixth-order sine wave, thereby inducing a larger average midpoint current on the AC side. Therefore, compared with the scheme of injecting the zero-sequence component of the sixth-order sine wave, this method has a higher regulation gain and can achieve the same potential balance effect with a smaller injection amount, thus maintaining the high voltage utilization of SVPWM while ensuring midpoint balance.

[0048] In this embodiment, a dynamic closed loop is constructed between the voltage deviation and the injected amplitude using a PI controller. When a deviation occurs in the midpoint voltage, the PI controller responds quickly and outputs the required sixth-power wave amplitude. A limiting circuit is introduced to ensure that the injected zero-sequence component is always within the linear modulation region (i.e., ensuring that the modulated wave does not overmodulate). Thus, the control device possesses dynamic adaptive capability: in steady state, the injected amount automatically decreases to reduce harmonic losses; during transient disturbances or low power factor startup, the injected amount rapidly increases to provide maximum midpoint current compensation. This, combined with the high voltage utilization advantage of SVPWM, achieves balanced control of the midpoint potential under low power factor conditions.

[0049] In practical applications, this method effectively improves the problem of midpoint potential regulation failure in three-level NPC inverters under low power factor conditions by injecting a zero-sequence voltage with controllable amplitude and polarity within the SVPWM framework, maintaining high performance across the entire power factor range. Compared to traditional redundancy vector allocation schemes that rely on load current direction, the inverter control method of this invention eliminates the dependence on active current direction. Even when the active current approaches zero due to low power factor, it can still accurately establish a control reference for the midpoint current flow direction, eliminating the regulation blind zone of traditional schemes in the low power factor region. Simultaneously, under high power factor conditions, voltage and current are in phase, and the DC zero-sequence voltage can achieve midpoint charge compensation with relatively low computational cost, improving response speed. Under low power factor conditions, a sixth-power wave is used as the zero-sequence component. Leveraging its high-frequency polarity reversal characteristics, it mitigates the problem caused by phase difference, generating a greater midpoint current regulation capability than a sine wave injection under the same amplitude constraints, thus improving the midpoint potential balance efficiency of the three-level inverter system.

[0050] In another embodiment, reference Figure 2 The acquisition of the inverter's midpoint voltage deviation, absolute active power, and absolute reactive power includes: Step S110: Obtain the positive half bus voltage and negative half bus voltage on the DC side of the inverter, and use the difference between the positive half bus voltage and the negative half bus voltage as the midpoint voltage deviation value. Step S120: Obtain the direct-axis current, quadrature-axis current, direct-axis voltage, and quadrature-axis voltage in the synchronous rotating coordinate system, and determine the active power and reactive power based on the direct-axis current, the quadrature-axis current, the direct-axis voltage, and the quadrature-axis voltage, and determine the absolute value of the active power based on the active power, and determine the absolute value of the reactive power based on the reactive power.

[0051] The determination of active power and reactive power based on the direct-axis current, the quadrature-axis current, the direct-axis voltage, and the quadrature-axis voltage includes determining reactive power based on a preset active power calculation formula, a preset reactive power calculation formula, the direct-axis current, the quadrature-axis current, the direct-axis voltage, and the quadrature-axis voltage. The preset active power calculation formula includes: ; The preset reactive power calculation formula includes: ; in, Active power Reactive power For direct-axis current, For quadrature axis current, It is the direct-axis voltage. It is the quadrature-axis voltage.

[0052] In this embodiment, a voltage sensor (such as a Hall effect voltage sensor) installed on the DC side can be used to collect the voltage between the positive half bus and the DC negative terminal in real time. Voltage of the negative half bus to the DC negative terminal Subtracting the positive half-bus voltage from the negative half-bus voltage yields the midpoint voltage deviation value. ;refer to Figure 8 ,when A value greater than 0 indicates that the voltage of capacitor C1 is too high, and the midpoint potential is decreasing; conversely, a value less than 0 indicates that the voltage of capacitor C2 is too high, and the midpoint potential is increasing. This allows for real-time quantification of the imbalance in the DC-side midpoint potential. The midpoint voltage deviation is the direct input to the subsequent closed-loop control (PI regulator), reflecting the magnitude and direction of the error that the system needs to compensate for.

[0053] In this embodiment, reference Figure 8 The inverter obtains the real-time phase angle of the grid voltage through a phase-locked loop (PLL). Then, using this phase angle, the AC voltage and current in the three-phase stationary coordinate system are mapped to the synchronous rotating coordinate system (dq coordinate system) via Park transformation, yielding the direct-axis voltage, quadrature-axis voltage, direct-axis current, and quadrature-axis current. In the synchronous rotating coordinate system, algebraic operations are performed using preset active power calculation formulas and preset reactive power calculation formulas to obtain the corresponding active and reactive power. The absolute values ​​of active and reactive power are then used as power factor criteria to evaluate the current power factor status of the inverter. Thus, under high power factor conditions, the polarity of the zero-sequence voltage is determined by the direction of the load current, thereby generating a DC zero-sequence voltage. By generating a midpoint current opposite to the midpoint voltage deviation, midpoint potential shift is effectively suppressed. Under low power factor conditions, the active current approaches zero, and reactive power becomes the core variable that dominates the current flow direction of the system. Therefore, by extracting the current reactive power state of the inverter, the reactive power can be accurately obtained. The sign of the reactive power determines the polarity of the sixth-order wave injection (the sign of the second injection direction), which replaces the current direction information that fails under low power factor conditions, ensuring that the zero-sequence voltage injection method can generate the midpoint current in the correct direction.

[0054] Optionally, refer to Figure 3 The determination of the inverter's current operating condition based on the absolute value of active power and the absolute value of reactive power includes: Step S310: If the absolute value of the active power is greater than K times the absolute value of the reactive power, determine that the current operating condition of the inverter is a high power factor condition. Step S320: If the absolute value of the active power is less than or equal to K times the absolute value of the reactive power, determine that the current operating condition of the inverter is a low power factor condition; where K is a natural number.

[0055] Based on the above embodiments, in obtaining the absolute value of active power abs ( ) and the absolute value of reactive power abs ( After that, the absolute values ​​of active power (abs) can be compared. ) and the absolute value of reactive power abs ( Multiply by the calibrable coefficient K to determine the current operating condition of the inverter.

[0056] In this embodiment, if abs ( ) > K×abs ( If abs( )≤K×abs( If the value is 0, it is determined that the current operating condition is a low power factor condition.

[0057] Understandably, introducing a calibrable coefficient K as a judgment threshold is equivalent to setting a clear safety boundary between the two control strategies corresponding to low power factor and high power factor operating conditions. This prevents the control device from frequently switching between the two strategy algorithms due to power fluctuations near the critical power factor, ensuring smooth and stable control. When abs( ) > K×abs ( This means that active power dominates, and voltage and current are almost in phase. In this case, the DC zero-sequence injection method based on the current sign can accurately match the charge compensation requirements. When abs ( )≤K×abs( This means that reactive power accounts for a very large proportion, and voltage and current are severely misaligned. At this point, the current zero-crossing point coincides with the voltage peak value, necessitating a switch to the sixth-power wave injection method based on the reactive power sign. Therefore, comparing the absolute value of active power (abs) ) and the absolute value of reactive power abs ( Multiplying this by the calibrable coefficient K essentially calculates and assesses the severity of the phase difference in the inverter system in real time.

[0058] In one embodiment, reference Figure 4 The control method for the inverter further includes: Step S010: Obtain the direct-axis voltage and quadrature-axis voltage in the synchronous rotating coordinate system; Step S020: Perform inverse PARK transformation and inverse CLARK transformation on the direct-axis voltage and the quadrature-axis voltage in sequence to obtain the original voltage of the three-phase modulated wave; Step S030: Obtain the three-phase current output by the inverter and extract the sign of the three-phase current.

[0059] In this embodiment, reference Figure 8 The real-time phase angle of the grid voltage is obtained through a phase-locked loop (PLL). This phase angle is then used to map the AC voltage and current in the three-phase stationary coordinate system to a synchronous rotating coordinate system (dq coordinate system) using the Park transformation, yielding the direct-axis voltage Ud and quadrature-axis voltage Uq. Since the power switches (such as IGBTs or MOSFETs) of the three-phase inverter bridge arms cannot directly understand the voltage commands in the rotating coordinate system, an inverse transformation is used to map them back to the three-phase physical world. Therefore, the abstract voltage commands (Ud and Uq) in the rotating coordinate system need to be inversely restored to the physical voltage commands (Ua, Ub, and Uc) in the three-phase stationary coordinate system through coordinate transformation, obtaining the original voltages of the three-phase modulated wave. The inverse Park and inverse Clark transformations are existing technologies and will not be elaborated upon here.

[0060] In this embodiment, in a three-level inverter, the shift in the neutral point potential is caused by the neutral point current, and the magnitude and direction of the neutral point current directly depend on the flow direction of the phase currents. Extracting the signs of the three-phase currents is the core physical basis for subsequently determining whether to inject or extract charge into the neutral point. In this embodiment, the three-phase currents Ia, Ib, and Ic can be obtained using current sensors (such as Hall sensors or sampling resistors) combined with an analog-to-digital converter module. Extracting the signs of the three-phase currents is essentially determining the instantaneous flow direction of each phase current (a positive sign represents flow out of the inverter, and a negative sign represents flow into the inverter). For example, determining whether the values ​​of the three-phase currents Ia, Ib, and Ic are greater than 0. If greater than 0, the sign is recorded as 1 (or positive); if less than 0, the sign is recorded as 0 (or negative).

[0061] Optionally, refer to Figure 5 The step of determining the first injection direction sign of the zero-sequence voltage based on the sign of the original voltage and three-phase current of the three-phase modulated wave, and determining the required injected DC zero-sequence voltage based on the first injection direction sign and the amplitude of the zero-sequence voltage, includes: Step S410: Based on the magnitude of the original voltage of the three-phase modulated wave, determine the sector where the current voltage vector is located; Step S420: Based on the symbols of the sector and the three-phase current, query the preset symbol mapping table to obtain the first injection direction symbol; Step S430: The product of the first injection direction sign and the zero-sequence voltage amplitude is taken as the required injected DC zero-sequence voltage.

[0062] In this embodiment, the SVPWM algorithm divides the 360-degree space into 6 sectors. Determining the sectors is the basis of the lookup table method; it determines which two adjacent basic vectors should be used to synthesize the target voltage, and also determines the potential flow path of the midpoint current. Therefore, based on the instantaneous values ​​of Ua, Ub, and Uc, the position of the current synthesized voltage vector in the space vector plane is determined (6 sectors, each 60 degrees). In this embodiment, the control device directly determines the magnitude of Ua, Ub, and Uc, such as their signs, where positive and negative represent direction. A positive sign indicates the direction is consistent with the direction of the midpoint current flow, and a negative sign indicates the direction is opposite to the direction of the midpoint current flow. Table 1 is the preset sign mapping table. For example, when Ua > 0, Ub < 0, and Uc < 0, the sector where the current voltage vector is located is sector 1. At this time, it is necessary to extract the sign of the A-phase current Ia, where the preset sign function sgn(X) is used to calculate the sign of X. When the inverter is currently operating in a high power factor condition, the symbol of Ia can be determined as the first injection direction symbol S1 by consulting the preset symbol mapping table. Then, the product of the first injection direction symbol and the zero-sequence voltage amplitude is used as the required injected DC zero-sequence voltage. ,in, It is the DC zero-sequence voltage.

[0063] Table 1

[0064] In one embodiment, reference Figure 6 The step of determining the second injection direction sign of the zero-sequence voltage based on the original voltage of the three-phase modulated wave and the sign of the reactive power, and determining the required injected sixth-power wave zero-sequence voltage based on the second injection direction sign and the amplitude of the zero-sequence voltage, includes: Step S510: Based on the magnitude of the original voltage of the three-phase modulated wave, determine the sector where the current voltage vector is located; Step S520: Based on the sector, the original voltage of the three-phase modulation wave, and the symbol of the reactive power, query the preset symbol mapping table to obtain the second injection direction symbol; Step S530: The product of the second injection direction sign and the zero-sequence voltage amplitude is taken as the sixth-power wave zero-sequence voltage to be injected.

[0065] Based on the above embodiments, the position of the current synthesized voltage vector in the space vector plane is determined according to the instantaneous values ​​of Ua, Ub, and Uc (a total of 6 sectors, each 60 degrees). The core characteristic of the sixth-order zero-sequence voltage is that its polarity flips 6 times within one fundamental cycle. Determining the current sector is the spatial reference for deciding whether the sixth-order wave should be in the positive or negative half-cycle at the current moment. There is no need to calculate complex rotation angles; the signs of the original three-phase modulated wave voltages Ua, Ub, and Uc can be directly determined. For example, if Ua > 0, Ub < 0, and Uc > 0, then the current voltage vector is located in sector 6, and the reactive power symbol Sgn is extracted. The sign of the voltage difference between Ua and Uc is Sgn(Ua-Uc). Then, based on the sign of the reactive power and the sign of the voltage difference between Ua and Uc, the sign of the second injection direction is determined as S2. If either Sgn(Q) or Sgn(Ua-Uc) is negative, then S2 is negative; if both Sgn(Q) and Sgn(Ua-Uc) are negative, then S2 is positive; if both Sgn(Q) and Sgn(Ua-Uc) are positive, then S2 is positive. The control device uses the product of the second injection direction sign and the zero-sequence voltage amplitude as the desired injected sixth-power wave zero-sequence voltage. ,in, It is a sixth-order zero-sequence voltage.

[0066] It should be noted that Table 1 is essentially a linear mapping of sgn[sin(6θ)]×gn(-Q) across six sectors (θ is the voltage vector phase angle), which was pre-defined by the researchers based on sgn[sin(6θ)]×sgn(-Q). The data is compiled into Table 1 and stored in the memory of the control device. In this way, the control device does not need to calculate trigonometric functions, which reduces the computational burden and improves the response speed.

[0067] Optionally, the control method for the inverter further includes obtaining the voltage vector phase angle, wherein obtaining the voltage vector phase angle specifically includes: Step S10: Perform inverse PARK transformation on the direct-axis voltage and the quadrature-axis voltage to obtain the first voltage component and the second voltage component in the two-phase stationary coordinate system; Step S20: Determine the voltage vector phase angle based on the first voltage component and the second voltage component.

[0068] The step of determining the voltage vector phase angle based on the first voltage component and the second voltage component includes determining the voltage vector phase angle based on a preset arctangent formula, the first voltage component, and the second voltage component; The preset arctangent formula includes: ; in, The voltage vector phase angle, This is the first voltage component; This is the second voltage component.

[0069] Understandably, the inverse PARK transformation restores the DC variable in the synchronous rotating coordinate system (dq coordinate system) back to the AC variable in the two-phase stationary coordinate system (αβ coordinate system). In the current loop control of an inverter, to eliminate AC signal coupling, the three-phase AC power is usually converted into DC in the dq coordinate system for PI regulation. However, if it is necessary to observe the actual physical position of the voltage vector in space (i.e., spatial angle), the DC in the dq coordinate system cannot directly reflect this spatial information. Therefore, it is necessary to use the inverse PARK transformation to restore it back to the stationary αβ coordinate system, thereby obtaining the AC component (the first voltage component) that varies sinusoidally with time. Second voltage component ).

[0070] In this embodiment, the dq axis voltage ( and The voltage vector is restored to a first and second voltage component in a two-phase stationary coordinate system. These two components constitute an orthogonal projection of the voltage vector onto a two-dimensional plane, serving as the direct input for calculating the spatial phase angle. In this embodiment, the control device can utilize the four-quadrant arctangent function (preset arctangent formula) from the mathematical library to calculate the voltage vector phase angle based on the first and second voltage components. The preset arctangent formula can be pre-set and stored in the internal or external memory of the control device for future use.

[0071] The voltage vector phase angle is the actual electrical angle of the current voltage vector in space, serving as the reference beat for generating the "sixth-order wave" (because the frequency of the sixth-order wave is six times the fundamental frequency, requiring 6θ). Thus, obtaining the voltage vector phase angle ensures that the control device can track the spatial position of the voltage vector in real time and without error, providing a reliable phase reference for the subsequent generation of a sixth-order zero-sequence voltage synchronized with the fundamental frequency.

[0072] In another embodiment, reference Figure 4 The control method for the inverter further includes: Determining the sign of the second injection direction based on the voltage vector phase angle and the reactive power specifically includes: Step S30: Calculate the sixth harmonic sine value of the voltage vector phase angle and extract the sign of the sixth harmonic sine value; Step S40: Invert the reactive power to obtain the negative value of the reactive power, and extract the sign of the negative value of the reactive power. Step S50: Multiply the sign of the sixth harmonic sine value with the sign of the negative value of the reactive power to obtain the sign of the sixth power wave.

[0073] It is understandable that the midpoint current of a three-level inverter has a six-pulse characteristic within one fundamental cycle (i.e., a polarity reversal occurs once every 60 electrical degrees). Extracting the sign of the sixth harmonic sine wave essentially generates a reference square wave with a duty cycle of 50% and a period of 1 / 6 of the fundamental cycle, which determines the natural spatial alternation of the zero-sequence voltage.

[0074] Based on the above embodiments, refer to Figure 8 The control device can calculate the sixth harmonic sine value sin(6θ) based on the obtained voltage vector phase angle, and calculate the sign of the sixth harmonic sine value using a preset sign function sgn(X). For example, when sin(6θ) is greater than 0, the sign is +1, and when sin(6θ) is less than 0, the sign is -1. That is, the sign of the sixth harmonic sine value is extracted through sgn[sin(6θ)].

[0075] In this embodiment, reactive power can be... Invert the reactive power. Multiplying by -1 gives the negative value of reactive power. Extract the sign of the negative value of reactive power, i.e., sgn( For example, when When it is greater than 0, the sign is +1; when... When the value is less than 0, the sign is -1. Thus, a "correction factor" can be obtained that reflects the true current flow direction under low power factor conditions. Under low power factor conditions, active current approaches zero, and reactive current dominates. Regarding reactive power... Perform inversion (i.e.) This is to align the physical relationship between "reactive current flow" and "midpoint potential adjustment requirements" in mathematical logic, ensuring that the injected zero-sequence voltage can generate a midpoint current in the opposite direction to the midpoint voltage deviation.

[0076] In this embodiment, the control device can multiply the sign of the sixth harmonic sine wave with the sign of the negative value of the instantaneous reactive power to obtain the sign of the sixth power wave sgn[sin(6θ)] and the sign of the negative value of the instantaneous reactive power sgn( Multiply by , to obtain the symbol of the sixth-power wave S6 = sgn[sin(6θ)] × gn( Since the sign of the sixth-power wave and the sign of the negative value of instantaneous reactive power can only be +1 or -1, their product S6 can also only be +1 or -1. S6 is a sixth-power wave sign signal with a period of 1 / 6 of the fundamental wave and whose polarity is corrected in real time by reactive power. It can be used as a multiplier factor to multiply with the sixth-power wave amplitude to synthesize the final zero-sequence voltage injection command. In this way, the sign of the sixth-power wave can reliably indicate the direction of zero-sequence voltage injection under low power factor, making up for the defect of the traditional small vector method that fails due to reliance on the direction of active current.

[0077] In one embodiment, the preset addition formula used to add the DC zero-sequence voltage or the sixth-power zero-sequence voltage to the original three-phase SVPWM modulation wave includes: ; ; ; in, The injected DC zero-sequence voltage or the sixth-power wave zero-sequence voltage, , , The three-phase voltages corresponding to the original three-phase SVPWM modulation wave. , , The three-phase voltage corresponding to the modulation wave after the DC zero-sequence voltage or the sixth-power wave is injected.

[0078] In this embodiment, when the inverter is in a low power factor condition, the sixth-order zero-sequence voltage is... The updated three-phase modulation wave is generated by superimposing the original three-phase SVPWM modulation wave output from the current loop. Since the zero-sequence voltage is a common-mode component, shifting the three-phase voltages simultaneously does not alter the fundamental line voltage of the inverter output. This means that this operation improves the neutral point potential imbalance while remaining compatible with the native SVPWM modulation architecture, maintaining both the high voltage utilization advantage of SVPWM and its excellent harmonic characteristics. When the inverter is in a high power factor operating condition, the DC zero-sequence voltage... The updated three-phase modulation wave is generated by superimposing it onto the original three-phase SVPWM modulation wave output from the current loop.

[0079] In this embodiment, under low power factor conditions, the SVPWM modulation wave superimposed with a 6th power wave zero-sequence voltage generates a larger average midpoint current than the injection of a 6th power sinusoidal zero-sequence voltage under the SPWM framework.

[0080] The average midpoint current generated by the superposition of the SVPWM modulation wave and the 6th power wave in each electrical cycle is: ; The average midpoint current generated by the superposition of 6 sine waves on the SVPWM modulation wave in each electrical cycle is: ; Among them, the non-overmodulation region <0.75M; 1≤ ≤π / 6, for The solution; The output voltage modulation ratio. The amplitude is the sixth power. This represents the amplitude of the fundamental current.

[0081] Under low power factor conditions, the average midpoint current waveform generated by the SVPWM+6th power wave injected using the inverter control method proposed in this invention and the average midpoint current waveform generated by the lower SVPWM+6th power sine wave injected by the traditional scheme are as follows: Figure 9 As shown; Table 2 provides examples of some data for comparative analysis.

[0082] Table 2

[0083] From Table 2 and Figure 9 It is known that the inverter control method proposed in this application injects a sixth-order zero-sequence voltage into the original three-phase SVPWM modulation wave under low power factor conditions, which can generate a larger midpoint current than the traditional method of injecting a sixth-order sinusoidal zero-sequence voltage into SPWM. At the same time, it can give full play to the advantages of SVPWM in terms of voltage utilization and achieve midpoint balance under low power factor conditions.

[0084] This application also proposes a three-level inverter system, which includes a control device and a three-level inverter. The three-level inverter includes a DC-side capacitor assembly and three-phase inverter arms. The DC-side capacitor assembly includes two capacitors connected in series, and the connection point of the two capacitors is the DC neutral point. The neutral point of each three-phase inverter arm is connected to the DC neutral point. The control device is electrically connected to the three-phase inverter arms. The control device is used to execute the inverter control method described above. The control device is also used to output a switch drive signal to control the on / off state of the three-phase inverter arms.

[0085] Optionally, the three-level inverter includes a three-level NPC (Neutral Point Clamped) inverter.

[0086] In this embodiment, the control device is used to convert the modulated wave after injecting DC zero-sequence voltage or the sixth-power zero-sequence voltage into a switching drive signal to control the on / off state of the three-phase inverter bridge arm.

[0087] refer to Figure 7 , Figure 7 This is a three-level neutral-point clamped (NPC) inverter topology. Its core feature is the construction of a neutral point using two series-connected DC-side capacitors, C1 and C2, and the clamping diodes in the three-phase inverter bridge arms clamping the output level to positive, zero, or negative states. This topology is widely used in medium- and high-voltage, high-power applications due to its advantages such as low output voltage harmonics and low voltage stress on switching devices. In this topology, the DC side consists of two capacitors, C1 and C2, connected in series, with their connection point O being the DC neutral point and the target for controlling the overall system potential balance. Each phase of the three-phase inverter bridge arm consists of four switching transistors (typically IGBTs or MOSFETs) and two clamping diodes. By controlling different combinations of switching transistors, each phase output can generate one of three voltage levels relative to the neutral point O: +Udc / 2, 0, or -Udc / 2.

[0088] In this embodiment, the control device may further include a modulation module, which outputs a modulated wave after injecting a DC zero-sequence voltage or the sixth-power zero-sequence voltage to the modulation module, so that the modulation module outputs a corresponding switching drive signal, such as a PWM signal. The PWM signal directly controls the on / off timing of the four switching transistors in each phase bridge arm. Especially under low power factor conditions, the injection of the sixth-power wave changes the duty cycle distribution of each phase in the positive, zero, and negative level states, thereby affecting the average value of the current flowing through the neutral point O. When the current is greater than 0, current flows out from point O, charging C2 and discharging C1; conversely, the opposite occurs. Thus, through precise control... By determining the amplitude and polarity, the midpoint potential can be controlled. and The dynamic equilibrium.

[0089] The three-level inverter system of this invention is suitable for both low power factor and high power factor operating conditions. It can reliably guide the flow of the midpoint current and maintain capacitor voltage balance, thus achieving midpoint balance across the entire power factor region of the NPC three-level inverter. The entire range is based on the SVPWM framework and implemented by injecting zero-sequence voltage. Furthermore, the computational load is small (no need to obtain voltage and current angles). The two injection methods corresponding to low power factor and high power factor operating conditions can share the same voltage sector judgment logic, making it easy to implement.

[0090] The three-level inverter system provided by this invention is based on the inverter control method described above. Compared with the prior art, the beneficial effects of the three-level inverter system provided by this invention are the same as those of the inverter control method provided in the above embodiments, and other technical features of the three-level inverter system are the same as those disclosed in the methods of the above embodiments, and will not be repeated here.

[0091] The above description is only a part of the embodiments of the present invention and does not limit the patent scope of the present invention. All equivalent structural transformations made under the technical concept of the present invention using the contents of the present invention specification and drawings, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. A control method for an inverter, characterized in that, The control method for the inverter includes: Obtain the inverter's midpoint voltage deviation, absolute value of active power, and absolute value of reactive power; The midpoint voltage deviation value is adjusted proportionally and integrally to obtain the required injected zero-sequence voltage amplitude; The current operating condition of the inverter is determined based on the absolute value of the active power and the absolute value of the reactive power, and the operating condition includes a high power factor condition and a low power factor condition. When the operating condition is a high power factor condition, the first injection direction sign of the zero-sequence voltage is determined based on the sign of the original voltage and the sign of the three-phase modulation wave and the three-phase current, and the required injected DC zero-sequence voltage is determined based on the first injection direction sign and the amplitude of the zero-sequence voltage. When the operating condition is a low power factor condition, the sign of the reactive power is determined, and the second injection direction sign of the zero-sequence voltage is determined based on the original voltage of the three-phase modulation wave and the sign of the reactive power. The sixth-power wave zero-sequence voltage to be injected is determined based on the second injection direction sign and the amplitude of the zero-sequence voltage. Based on the operating conditions, the DC zero-sequence voltage or the sixth-power wave zero-sequence voltage is added to the original three-phase SVPWM modulation wave to obtain the modulation wave after injecting the zero-sequence voltage. The modulated wave after injecting zero-sequence voltage is converted into a switching drive signal to control the midpoint voltage of the inverter.

2. The inverter control method as described in claim 1, characterized in that, The acquisition of the inverter's midpoint voltage deviation, absolute active power, and absolute reactive power includes: Obtain the positive half-bus voltage and negative half-bus voltage on the DC side of the inverter, and use the difference between the positive half-bus voltage and the negative half-bus voltage as the midpoint voltage deviation value. Obtain the direct-axis current, quadrature-axis current, direct-axis voltage, and quadrature-axis voltage in a synchronous rotating coordinate system, and determine the active power and reactive power based on the direct-axis current, quadrature-axis current, direct-axis voltage, and quadrature-axis voltage, and determine the absolute value of the active power and the absolute value of the reactive power based on the active power.

3. The inverter control method as described in claim 2, characterized in that, The determination of active power and reactive power based on the direct-axis current, the quadrature-axis current, the direct-axis voltage, and the quadrature-axis voltage includes determining reactive power based on a preset active power calculation formula, a preset reactive power calculation formula, the direct-axis current, the quadrature-axis current, the direct-axis voltage, and the quadrature-axis voltage. The preset active power calculation formula include: ； The preset reactive power calculation formula includes: ； in, Active power Reactive power For direct-axis current, For quadrature axis current, It is the direct-axis voltage. It is the quadrature-axis voltage.

4. The inverter control method as described in claim 1, characterized in that, The determination of the inverter's current operating condition based on the absolute value of active power and the absolute value of reactive power includes: If the absolute value of the active power is greater than K times the absolute value of the reactive power, the current operating condition of the inverter is determined to be a high power factor condition. If the absolute value of the active power is less than or equal to K times the absolute value of the reactive power, the current operating condition of the inverter is determined to be a low power factor condition; where K is a natural number.

5. The control method for the inverter as described in any one of claims 1 to 4, characterized in that, The control method for the inverter also includes: Obtain the direct-axis voltage and quadrature-axis voltage in a synchronously rotating coordinate system; The direct-axis voltage and the quadrature-axis voltage are sequentially subjected to inverse PARK transform and inverse CLARK transform to obtain the original voltage of the three-phase modulated wave. Obtain the three-phase current output from the inverter and extract the sign of the three-phase current.

6. The inverter control method as described in claim 5, characterized in that, The process of determining the first injection direction sign of the zero-sequence voltage based on the sign of the original voltage and three-phase current of the three-phase modulated wave, and determining the required injected DC zero-sequence voltage based on the first injection direction sign and the amplitude of the zero-sequence voltage, includes: Based on the magnitude of the original voltage of the three-phase modulated wave, the sector where the current voltage vector is located is determined; Based on the symbols of the sector and the three-phase current, a preset symbol mapping table is queried to obtain the first injection direction symbol; The product of the first injection direction sign and the zero-sequence voltage amplitude is taken as the required injected DC zero-sequence voltage.

7. The control method for the inverter as described in claim 1, characterized in that, The process of determining the second injection direction sign of the zero-sequence voltage based on the original voltage of the three-phase modulated wave and the sign of the reactive power, and determining the required injected sixth-power wave zero-sequence voltage based on the second injection direction sign and the amplitude of the zero-sequence voltage, includes: Based on the magnitude of the original voltage of the three-phase modulated wave, the sector where the current voltage vector is located is determined; Based on the symbols of the sector, the original voltage of the three-phase modulation wave, and the reactive power, a preset symbol mapping table is consulted to obtain the symbol of the second injection direction. The product of the second injection direction sign and the zero-sequence voltage amplitude is taken as the sixth-power zero-sequence voltage to be injected.

8. The control method for the inverter as described in claim 1, characterized in that, The preset addition formula used to add the DC zero-sequence voltage or the sixth-power zero-sequence voltage to the original three-phase SVPWM modulation wave includes: ； ； ； in, The injected DC zero-sequence voltage or the sixth-power zero-sequence voltage, , , The three-phase voltages corresponding to the original three-phase SVPWM modulation wave. , , The three-phase voltage corresponding to the modulation wave after the DC zero-sequence voltage or the sixth-power wave is injected.

9. The control method for the inverter as described in any one of claims 1 to 4, characterized in that, After performing proportional-integral adjustment on the midpoint voltage deviation value to obtain the desired injected zero-sequence voltage amplitude, the inverter control method further includes: The zero-sequence voltage amplitude is limited to obtain a zero-sequence voltage amplitude not exceeding 0.4M, where M is the output voltage modulation ratio.

10. A three-level inverter system, characterized in that, The three-level inverter system includes a control device and a three-level inverter. The three-level inverter includes a DC-side capacitor assembly and a three-phase inverter bridge arm. The DC-side capacitor assembly includes two capacitors connected in series, and the connection point of the two capacitors is the DC neutral point. The neutral point of the three-phase inverter bridge arm is connected to the DC neutral point. The control device is electrically connected to the three-phase inverter bridge arm. The control device is used to execute the control method of the inverter as described in any one of claims 1 to 9. The control device is also used to output a switch drive signal to control the on / off state of the three-phase inverter bridge arm.

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