Modulation method of three-phase inverter and three-phase inverter

Abstract

The invention relates to the field of inverters, in particular to a modulation method of a three-phase inverter and the three-phase inverter, and the method comprises the steps: obtaining a three-phase original modulation wave based on the voltage information and current information of the three-phase inverter; determining a modulation factor based on a power factor angle and the phase angle of the three-phase original modulation wave; obtaining a zero-sequence voltage based on the modulation factor and the three-phase original modulation wave; superposing the zero-sequence voltage and the three-phase original modulation wave to obtain a final modulation wave; and comparing the final modulated wave with a carrier wave to obtain a driving signal of the three-phase inverter. Smooth change of the zero-sequence voltage is achieved, and the problem of high-frequency resonance caused by zero-sequence voltage abrupt change is solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of inverters, in particular to a modulation method of a three-phase inverter and a three-phase inverter. BACKGROUND

[0002] With the increasingly serious energy crisis and environmental pollution, new energy power generation such as photovoltaic, wind power, energy storage and electric vehicles has become a research hotspot. As a power conversion device of new energy power generation, the performance of the three-phase inverter plays a crucial role in the efficient and reliable use of new energy, so it has attracted widespread attention in academia and industry.

[0003] In new energy application scenarios, the conversion efficiency, harmonic distortion rate, common-mode voltage / leakage current and other performance indicators of the three-phase inverter become the focus of attention, and the modulation strategy is closely related to the above key performance indicators. In the three-phase inverter, the modulation strategy can be mainly divided into two categories: continuous pulse width modulation strategy (CPWM) and discontinuous pulse width modulation strategy (DPWM). The most common continuous pulse width modulation strategies are sine pulse width modulation strategy and space vector pulse width modulation strategy. Compared with continuous pulse width modulation strategy, discontinuous pulse width modulation strategy is widely welcomed by the industry because it can reduce the switching loss of the three-phase inverter, but it has the problem of high-frequency resonance caused by the sudden change of zero sequence voltage. SUMMARY

[0004] Therefore, it is necessary to provide a modulation method of a three-phase inverter and a three-phase inverter to solve the above technical problems.

[0005] In a first aspect, an embodiment of the present application provides a modulation method of a three-phase inverter, the method comprising:

[0006] obtaining a three-phase original modulation wave based on voltage information and current information of the three-phase inverter;

[0007] determining a modulation factor based on a power factor angle and a phase angle of the three-phase original modulation wave;

[0008] obtaining a zero sequence voltage based on the modulation factor and the three-phase original modulation wave;

[0009] superimposing the zero sequence voltage and the three-phase original modulation wave to obtain a final modulation wave;

[0010] comparing the final modulation wave with a carrier wave to obtain a driving signal of the three-phase inverter.

[0011] In an embodiment, the method further comprises:

[0012] If the three-phase inverter adopts a continuous pulse width modulation strategy, the modulation factor is determined as a first preset value;

[0013] If the three-phase inverter adopts a non-continuous pulse width modulation strategy, the modulation factor is determined based on a power factor angle and a phase angle of the three-phase original modulation wave.

[0014] In an embodiment, if a modulation degree is less than or equal to a modulation degree threshold value, it is determined that the three-phase inverter adopts a continuous pulse width modulation strategy;

[0015] If the modulation degree is greater than the modulation degree threshold value, it is determined that the three-phase inverter adopts a non-continuous pulse width modulation strategy.

[0016] In an embodiment, the modulation degree is determined based on a direct current bus voltage and an amplitude of the three-phase original modulation wave.

[0017] In an embodiment, the determination of the modulation factor based on the power factor angle and the phase angle of the three-phase original modulation wave comprises:

[0018] determining an offset angle based on the power factor angle;

[0019] determining the modulation factor based on the phase angle, the offset angle, and a preset smoothing curve; the preset smoothing curve being a relationship curve of the modulation factor with respect to the phase angle and the offset angle.

[0020] In an embodiment, the modulation factor satisfies the following formula:

[0021]

[0022] wherein k represents the modulation factor, δ represents the offset angle, θ represents the phase angle, f1(θ, θ d ) represents a first preset smoothing curve, f2(θ, θ d ) represents a second preset smoothing curve, and θ d represents a preset adjustment angle.

[0023] In an embodiment, the offset angle satisfies the following formula:

[0024]

[0025] or,

[0026]

[0027] wherein, represents the power factor angle.

[0028] In an embodiment, the first preset value is 0.5.

[0029] In an embodiment, when switching between the continuous pulse width modulation strategy and the discontinuous pulse width modulation strategy, during the transition period, the modulation factor is determined based on a preset switching curve, the first preset value, a first threshold value and a second threshold value, the first threshold value being the minimum value of the modulation factor, the second threshold value being the maximum value of the modulation factor, the second threshold value > the first preset value > the first threshold value.

[0030] In an embodiment, the zero sequence voltage satisfies the following formula:

[0031] u0 = -(1-k)max(u a * ,u b * ,u c * )-kmin(u a * ,u b * ,u c * )+1-2k

[0032] wherein u0 represents the zero sequence voltage, 0≤k≤1, u a *, u b *, u c * represents the three-phase original modulation wave.

[0033] In a second aspect, an embodiment of the present application provides a three-phase inverter, comprising an inverter circuit and a controller connected with the three-phase bridge arm circuit, wherein the controller executes the method according to the first aspect.

[0034] Based on the voltage information and the current information of the three-phase inverter, the three-phase original modulation wave is obtained, the modulation factor is determined based on the power factor angle and the phase angle of the three-phase original modulation wave, the zero sequence voltage is obtained based on the modulation factor and the three-phase original modulation wave, the final modulation wave is obtained by superimposing the zero sequence voltage on the three-phase original modulation wave, the driving signal of the three-phase inverter is obtained by comparing the final modulation wave with the carrier wave, and the zero sequence voltage smooth change is realized, thereby solving the high-frequency resonance problem caused by the sudden change of the zero sequence voltage. BRIEF DESCRIPTION OF DRAWINGS

[0035] Figure 1 a topology structure diagram of a three-phase half-bridge grid-connected inverter provided for some embodiments of the present application;

[0036] Figure 2 a flowchart of a modulation method of a three-phase inverter provided for some embodiments of the present application;

[0037] Figure 3 A flowchart illustrating a method for determining modulation factors provided in some embodiments of this application;

[0038] Figure 4 This is a schematic diagram illustrating the selection of modulation factor k when a discontinuous pulse width modulation strategy is used in some embodiments of this application for a three-phase inverter.

[0039] Figure 5 This is a schematic diagram of the waveforms of the original modulation wave, the final modulation wave, and the zero-sequence voltage of phase A when the three-phase inverter adopts a discontinuous pulse width modulation strategy in some embodiments of this application.

[0040] Figure 6(a) is a schematic diagram showing the changes in the final modulated wave, zero-sequence voltage, modulation factor, and grid-connected current when switching from a discontinuous pulse width modulation strategy to a continuous pulse width modulation strategy in some embodiments of this application.

[0041] Figure 6(b) is a schematic diagram showing the changes in the final modulated wave, zero-sequence voltage, modulation factor, and grid-connected current when switching from a continuous pulse width modulation strategy to a non-continuous pulse width modulation strategy in some embodiments of this application.

[0042] Figure 7(a) shows the modulation scheme M in some embodiments of this application. i Simulated waveforms of the final modulated wave, grid-connected current, and grid voltage when the modulation value is 0.56;

[0043] Figure 7(b) shows the modulation scheme M in some embodiments of this application. i Simulated waveforms of the final modulated wave, grid-connected current, and grid voltage when the modulation value is 0.89;

[0044] Figure 8(a) shows the simulation waveforms of the final modulation wave, zero-sequence voltage, grid-connected current, and grid voltage when the three-phase inverter adopts a discontinuous pulse width modulation strategy and the power factor is +0.8 in some embodiments of this application.

[0045] Figure 8(b) shows the simulation waveforms of the final modulation wave, zero-sequence voltage, grid-connected current, and grid voltage when the three-phase inverter adopts a discontinuous pulse width modulation strategy and the power factor is -0.8 in some embodiments of this application.

[0046] Figure 9 In some embodiments of this application, when a three-phase inverter operates under a discontinuous pulse width modulation strategy and this modulation method is used, the grid voltage, common-mode voltage, and leakage current i... leak Simulation waveform diagram;

[0047] Figure 10 When a three-phase inverter operates under a discontinuous pulse width modulation strategy, a traditional modulation method is used, considering the grid voltage, common-mode voltage, and leakage current i. leak The simulation waveform diagram. DETAILED DESCRIPTION

[0048] For the purpose of the present application, the technical solutions and advantages are more clearly and obviously understood, the present application is described and explained below in combination with the drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application, and are not used to limit the present application. Based on the examples provided in the present application, all other examples obtained by those of ordinary skill in the art without making creative efforts fall within the scope of the present application. In addition, it can be understood that although the efforts made in this development process can be complex and lengthy, some design, manufacture or production changes made on the basis of the technical content disclosed in the present application are only routine technical means for those of ordinary skill in the art related to the content disclosed in the present application, and should not be understood as insufficient disclosure of the present application.

[0049] In the present application, the term "embodiment" means that the specific features, structures or properties described in combination with the embodiment can be included in at least one embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment to other embodiments. Those of ordinary skill in the art explicitly and implicitly understand that the embodiments described in the present application can be combined with other embodiments without conflict.

[0050] Unless otherwise defined, the technical terms or scientific terms involved in the present application should be understood as the usual meaning understood by those of ordinary skill in the art to which the present application belongs. The terms "one", "a", "an", "the", and similar words involved in the present application do not represent quantity limitation, but can represent singular or plural. The terms "include", "contain", "have", and any variations thereof involved in the present application are intended to cover non-exclusive inclusion; for example, a process, method, system, product or device including a series of steps or modules (units) is not limited to the listed steps or units, but can also include steps or units not listed, or can also include other steps or units inherent to the process, method, product or device. The terms "connected", "connected", "coupled" and similar words involved in the present application are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. The term "multiple" in the present application means greater than or equal to two. The term "and / or" describes the association between the associated objects, which means that there can be three relationships, for example, "A and / or B" can mean that A exists alone, A and B exist together, and B exists alone. The terms "first", "second", "third" and the like in the present application are only to distinguish similar objects, and do not represent a specific order for the objects.

[0051] The modulation method of the three-phase inverter can be applied to a controller of the three-phase inverter, the controller is configured with a program corresponding to the modulation method, and the controller executes the corresponding program to implement the modulation method of the three-phase inverter.

[0052] The three-phase inverter can be a three-phase grid-connected inverter or a three-phase off-grid inverter.

[0053] As Figure 1 The three-phase inverter is a three-phase half-bridge grid-connected inverter, and the inverter includes an inverter circuit and a filter circuit. The inverter circuit is configured to convert a DC bus voltage u dc into an AC voltage, and includes a first bridge arm, a second bridge arm and a third bridge arm connected in parallel. The first bridge arm includes connected switching tubes Q1 and Q2, the second bridge arm includes connected switching tubes Q3 and Q4, and the third bridge arm includes connected switching tubes Q5 and Q6. The filter circuit is configured to filter the output of the inverter circuit to obtain a three-phase AC output, and includes a filter inductor L of each phase. Specifically, the midpoint (i.e., the midpoints a, b and c) of each bridge arm is connected to the corresponding filter inductor L. Further, the inverter further includes capacitors C1 and C2 connected to the input end of the inverter.

[0054] wherein i ga , i gb , i gc is a three-phase grid-connected current, u ga , u gb , u gc is a three-phase grid voltage.

[0055] In some embodiments, the three-phase inverter can be a multi-level inverter.

[0056] In some embodiments, the three-phase inverter can be used in a photovoltaic power generation system to convert DC power provided by a photovoltaic DC power supply PV into AC power and output to a power grid; it can also be used in energy storage systems, electric vehicles, etc.

[0057] The modulation method of the three-phase inverter proposed in the present application is described by taking the three-phase inverter in Figure 1 as an example. Those skilled in the art can understand that the three-phase inverter shown in Figure 1 is only one application scenario of the technical solution of the present application, and does not constitute a limitation on the application scenarios of the technical solution of the present application.

[0058] As Figure 2 shown, the present application provides a modulation method of a three-phase inverter, including the following steps:

[0059] S100: Based on the voltage information and current information of the three-phase inverter, obtain three-phase original modulation waves.

[0060] The voltage information of the three-phase inverter includes three-phase grid voltages u ga , u gb , u gc , and the current information includes three-phase grid currents i ga , i gb , i gc . According to the sampled voltage information and current information, the three-phase original modulation waves u a * , u b * , u c * are obtained through closed-loop control.

[0061] S102: Based on the three-phase grid voltage, the phase angle of the grid voltage is obtained through a phase-locked loop.

[0062] S104: The three-phase grid voltage and the grid current are transformed to the d, q axes through coordinate transformation, and the output voltage u d , u q of the three-phase inverter in the d, q axes is obtained through closed-loop control.

[0063]

[0064] wherein G i (s) is the transfer function of the current loop, is the grid current reference value in the d axis, is the grid current reference value in the q axis, i d , i q are the grid currents in the d, q axes, u gd , u gq are the grid voltages in the d, q axes, ω0 is the grid angular frequency, and L represents the inductance value of the filter inductance L.

[0065] Specifically, for example, the three-phase grid voltages u ga , u gb , u gc and the grid currents i ga , i gb , i gc are transformed to the d, q axes through Park transformation to obtain the output voltage u d , u q of the three-phase inverter in the d, q axes.

[0066] S106: The output voltage of the three-phase inverter is subjected to coordinate inverse transformation to obtain the three-phase original modulation waves u a *、u b*, u c *.

[0067] wherein the three-phase original modulation wave is a value after normalization, i.e. its range is -1 to 1.

[0068] Specifically, for example, the output voltage u d , u q of the three-phase inverter is obtained by inverse Park transform (iPark transform) to obtain the three-phase original modulation wave u a *, u b *, u c *.

[0069] S200: determining a modulation factor based on the power factor angle and the phase angle of the three-phase original modulation wave.

[0070] In the case where the three-phase inverter solely adopts the discontinuous pulse width modulation strategy, the modulation factor can be determined based on the power factor angle and the phase angle of the three-phase original modulation wave.

[0071] In the case where the three-phase inverter adopts the discontinuous pulse width modulation strategy and the continuous pulse width modulation strategy, in the case where the three-phase inverter adopts the discontinuous pulse width modulation strategy, the modulation factor can also be determined based on the power factor angle and the phase angle of the three-phase original modulation wave.

[0072] The determination method of the modulation factor will be described in detail later.

[0073] S300: obtaining a zero sequence voltage based on the modulation factor and the three-phase original modulation wave.

[0074] In this step, the expression of the zero sequence voltage can be:

[0075] u0=-(1-k)max(u a * , u b * , u c * )-kmin(u a * , u b * , u c * )+1-2k (2)

[0076] wherein u0 represents the zero sequence voltage, k represents the modulation factor, and 0≤k≤1.

[0077] The obtained modulation factor k and the three-phase original modulation wave u a *, u b *, u c *are substituted into the above expression of the zero sequence voltage to obtain the zero sequence voltage u0.

[0078] S400: superimpose the zero sequence voltage and the three-phase original modulation wave to obtain a final modulation wave.

[0079] The expression of the final modulation wave is:

[0080]

[0081] wherein u a **, u b **, u c ** represents the final modulation wave of each phase.

[0082] S500: compare the final modulation wave with a carrier wave to obtain a driving signal of the three-phase inverter.

[0083] The carrier wave is, for example, a triangular carrier wave, but is not limited thereto.

[0084] In at least some embodiments of the present application, based on voltage information and current information of the three-phase inverter, the three-phase original modulation wave is obtained, based on the power factor angle and the phase angle of the three-phase original modulation wave, the modulation factor is determined, based on the modulation factor and the three-phase original modulation wave, the zero sequence voltage is obtained, the zero sequence voltage is superimposed with the three-phase original modulation wave to obtain the final modulation wave, the final modulation wave is compared with the carrier wave to obtain the driving signal of the three-phase inverter, the smooth change of the zero sequence voltage is realized, and the high-frequency resonance problem caused by the sudden change of the zero sequence voltage is solved.

[0085] The determination method of the modulation factor in step S200 will be described in detail below, as shown in FIG. 2, which specifically includes the following steps: Figure 3

[0086] S202: determine the offset angle based on the power factor angle.

[0087] In some embodiments, the offset angle δ is constructed as:

[0088]

[0089] wherein, is the power factor angle.

[0090] In some alternative embodiments, the offset angle δ is constructed as:

[0091]

[0092] According to the grid-connected current i d and i q under the d, q axes obtained in step S104, the power factor angle θ can be obtained by formula (6):

[0093] ​

[0094] S204: Determine the modulation factor based on the phase angle, offset angle, and preset smoothing curve.

[0095] The preset smooth curve is the relationship curve between the modulation factor and the phase angle θ and the offset angle δ.

[0096] In some embodiments, based on the phase angle θ and the power factor angle The preset angular relationship between the values ​​is used to select a modulation factor k. The selection of the modulation factor k satisfies the following formula:

[0097]

[0098] Where θ represents the phase angle, 0≤θ≤2π, f1(θ,θ) d f2(θ,θ) represents the first preset smooth curve. d ) represents the second preset smooth curve, θ d Indicates the preset adjustment angle, θ d >0 represents the angle of the phase interval corresponding to the first and second preset smooth curves. This can be achieved by adjusting θ. d The magnitude of the value can change the smoothness of the zero-sequence voltage.

[0099] In obtaining the power factor angle Substituting the phase angle θ of the three-phase original modulated wave into formula (7), the modulation factor k can be obtained.

[0100] As can be seen from formula (7), when the modulation factor k changes from 0 to 1 or from 1 to 0, the first preset smoothing curve f1(θ,θ) d ) and the second preset smooth curve f2(θ,θ) d The curve can be any preset smooth curve. In some embodiments, for simplicity, a first preset smooth curve f1(θ,θ) is set. d ) and the second preset smooth curve f2(θ,θ) d Since ) is a straight line, formula (7) can be rewritten as:

[0101]

[0102] In obtaining the power factor angle Substituting the phase angle θ of the three-phase original modulated wave into formula (8), the modulation factor k can be obtained.

[0103] Based on the three-phase original modulation wave obtained in step S100, the phase angle θ of the three-phase original modulation wave can be obtained by methods such as inverse trigonometric function calculation or phase-locked loop.

[0104] In the above embodiments, while solving the high-frequency resonance problem caused by zero-sequence voltage, the switching loss can be minimized and the system efficiency can be improved by constructing an offset angle δ under different power factor angles (-π / 2 to π / 2).

[0105] To facilitate a more intuitive understanding of how the modulation factor k is assigned in formula (8) and how the final modulated wave is clamped when a three-phase inverter employs a discontinuous pulse width modulation strategy, this paper focuses on how the modulation factor k is assigned and how the final modulated wave is clamped. Figure 4 A schematic diagram is given showing the selection of modulation factor k when a three-phase inverter adopts a discontinuous pulse width modulation strategy. Figure 4 middle Let be the space vectors of the original modulated wave voltage and the grid-connected current, respectively, and their expressions are as follows:

[0106]

[0107] according to Figure 4 It can be concluded that the clamping of the final modulated wave of different phases can be achieved by assigning a modulation factor k in different sectors.

[0108] Figure 5 The original modulation waveform u of phase A is given when a three-phase inverter employs a discontinuous pulse width modulation strategy and operates at unity power factor, with smooth transformation of modulation factor k. a * Phase A final modulation wave u a **Schematic diagram of the change in zero-sequence voltage u0.** As can be seen from the diagram, due to the smooth change of the modulation factor k between 0 and 1, the zero-sequence voltage u0 and the final modulation wave u of phase A... a The changes are also smooth.

[0109] When a three-phase inverter employs both discontinuous pulse width modulation (PWM) and continuous pulse width modulation (PWM) strategies, if the three-phase inverter uses a PWM strategy, the modulation factor is determined to be a first preset value; if the three-phase inverter uses a discontinuous pulse width modulation (PWM) strategy, the modulation factor is determined based on the power factor angle and the phase angle of the original three-phase modulation wave.

[0110] The first preset value is, for example, 0.5.

[0111] In some embodiments, the three-phase inverter may employ a continuous pulse width modulation (PWM) strategy or a discontinuous pulse width modulation (DPWM) strategy, depending on the modulation index.

[0112] Specifically, based on the DC bus voltage u dc The amplitude u of the three-phase original modulated wave m Determine the adjustment system M i Adjustment system M i The calculation formula is as follows:

[0113]

[0114] When M i ≤ h, it is determined that the three-phase inverter adopts the continuous pulse width modulation strategy; and when M i > h, it is determined that the three-phase inverter adopts the non-continuous pulse width modulation strategy. Wherein h is a constant of 0 < h < 1, representing a modulation degree threshold.

[0115] In the case where the three-phase inverter adopts a hybrid modulation strategy, i.e. hybridly adopts the non-continuous pulse width modulation strategy and the continuous pulse width modulation strategy, in some embodiments, when switching between the continuous pulse width modulation strategy and the non-continuous pulse width modulation strategy, during the transition period, the modulation factor is determined based on a preset switching curve, a first preset value, a first threshold value and a second threshold value, the first threshold value being the minimum value of the modulation factor, and the second threshold value being the maximum value of the modulation factor, the second threshold value > the first preset value > the first threshold value.

[0116] When switching between the continuous pulse width modulation strategy and the non-continuous pulse width modulation strategy, during the transition period, the modulation factor varies according to the preset switching curve between the first preset value and the first threshold value or the second threshold value.

[0117] Specifically, when the three-phase inverter switches from the continuous pulse width modulation strategy to the non-continuous pulse width modulation strategy, during the transition period, the modulation factor is determined based on a first preset switching curve, a first preset value, a first threshold value and a second threshold value, the modulation factor switching from the first preset value to the first threshold value or the second threshold value according to the first preset switching curve; when the three-phase inverter switches from the non-continuous pulse width modulation strategy to the continuous pulse width modulation strategy, during the transition period, the modulation factor is determined based on a second preset switching curve, the first preset value, the first threshold value and the second threshold value, the modulation factor switching from the first threshold value or the second threshold value to the first preset value according to the second preset switching curve.

[0118] The first preset switching curve and the second preset switching curve can be any smooth curve such as a straight line or a parabola.

[0119] In an example embodiment, taking the first threshold value as 0, the second threshold value as 1, the first preset value as 0.5, and the preset switching curve as a straight line as an example, when the three-phase inverter switches from the continuous pulse width modulation strategy to the non-continuous pulse width modulation strategy, during the transition period, the modulation factor k can be set as:

[0120]

[0121] wherein the first preset switching curve is k=0.5+nt, n represents a slope of the first preset switching curve, and t represents time. At the switching moment t is 0, the modulation factor k is 0.5, and the modulation factor k changes smoothly with the increase of time t. When the value of the modulation factor k becomes 0 or 1, the non-continuous pulse width modulation strategy is adopted smoothly, and after the transition is completed, the modulation factor k is determined according to formula (7).

[0122] When the three-phase inverter switches from the non-continuous pulse width modulation strategy to the continuous pulse width modulation strategy, during the transition, the modulation factor k can be set as:

[0123]

[0124] wherein the second preset switching curve is k=0 / 1+mt, m represents a slope of the second preset switching curve, at the switching moment t is 0, the modulation factor k is 0 or 1, and the modulation factor k changes smoothly with the increase of time t. When the modulation factor k becomes 0.5, the continuous pulse width modulation strategy is adopted smoothly.

[0125] Based on the above embodiments, the non-continuous pulse width modulation strategy or the continuous pulse width modulation strategy can be adaptively selected according to the modulation degree, the efficiency and the power quality are taken into account, meanwhile, the modulation factor k can change smoothly between 0.5 and 0 / 1 or 0 / 1 and 0.5 according to the preset switching curve, so as to realize the smooth switching of the two modulation strategies, and the smooth change of the zero sequence voltage, thereby the high-frequency resonance problem and the waveform distortion problem caused by the sudden change of the zero sequence voltage during the transition can be solved.

[0126] Therefore, in at least one embodiment of the present application, the switching loss in the wide power factor range can be kept to be minimum, meanwhile, the harmonic distortion of the grid-connected current is taken into account and the high-frequency resonance problem caused by the sudden change of the zero sequence voltage is solved, and the application range is wide.

[0127] Fig. 6(a) and Fig. 6(b) respectively show the simulation waveform diagrams of the final modulation wave u a **, b **, c **, the zero sequence voltage u0, the modulation factor k, and the grid-connected current i ga , gb , gc from which it can be seen that when the non-continuous pulse width modulation strategy is adopted, the modulation factor k changes smoothly, and the zero sequence voltage u0 also changes smoothly and no sudden change occurs. In addition, during the transition of the strategy switching, the grid-connected current i ga , gb , gcHighly sinusoidal and without impact.

[0128] Table 1 System Parameters

[0129]

[0130] To verify the effectiveness of the proposed modulation method, Figure 1 The simulation verification platform for the three-phase inverter shown is constructed, and the system parameters are shown in Table 1. The modulation threshold h is set to 0.65 when switching between continuous pulse width modulation (PWM) and discontinuous pulse width modulation (DPWM) strategies. Figure 7(a) shows the modulation threshold M. i When the value is 0.56, the final modulated wave u a **、u b **、u c **, Grid-connected current i ga i gb i gc Grid voltage u ga u gb u gc The waveform diagram is shown. At this time, the three-phase inverter adopts a continuous pulse width modulation strategy. As can be seen from Figure 7(a), the grid-connected current i ga i gb i gc It exhibits a highly sinusoidal modulation scheme with a total harmonic distortion (THD) of only 1.35%. Figure 7(b) shows the modulation scheme M. i When the value is 0.89, the final modulated wave u a **、u b **、u c **, Grid-connected current i ga i gb i gc Grid voltage u ga u gb u gc The waveform diagram is shown below. At this time, the inverter adopts a discontinuous pulse width modulation strategy, and the total harmonic distortion (THD) of the grid-connected current is 1.02%. From the above results, it can be seen that the modulation method of the above embodiment can achieve good power quality under different modulation intensities.

[0131] Figures 8(a) and 8(b) show the final modulation waveform u of a three-phase inverter using a discontinuous pulse width modulation strategy with a power factor of ±0.8. a **、u b **、u c **, Zero-sequence voltage u0, Grid-connected current i ga i gb i gc Grid voltage u ga u gb u gcThe simulation waveform diagrams are shown in Figures 8(a) and 8(b). It can be seen from Figures 8(a) and 8(b) that the maximum amplitude of the grid-connected current coincides with the clamping region of the final modulated wave. Therefore, under non-unity power factor, the modulation method of the above embodiment can still minimize the system switching loss.

[0132] Figure 9 When a discontinuous pulse width modulation strategy is adopted for a three-phase inverter, the modulation method described in the above embodiment is used, and the grid voltage u ga u gb u gc Common-mode voltage u com and leakage current i leak The simulation waveform diagram, from Figure 9 As can be seen from the above, due to the modulation method adopted in the above embodiment, there is no sudden change in the zero-sequence voltage and the switching is smooth, so there is no sudden change in the leakage current. Figure 10 To employ traditional modulation methods under a discontinuous pulse width modulation strategy, the grid voltage u ga u gb u gc Common-mode voltage u com and leakage current i leak The simulation waveform diagram is from Figure 10 It is known that there is a large leakage current oscillation at the moment of zero-sequence voltage change, which can easily cause the leakage current to exceed the standard requirements.

[0133] This application provides a three-phase inverter, including an inverter circuit and a controller connected to the inverter circuit, the controller executing the modulation method described in the above embodiment.

[0134] The controller is used to control the inverter circuit to convert direct current (DC) into alternating current (AC). Specifically, it can be any of the following: a microcontroller unit (MCU), a central processing unit (CPU), a field-programmable gate array (FPGA), or a digital signal processor (DSP). Of course, the specific form of the controller is not limited to the examples above.

[0135] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0136] The above embodiments only express several implementation ways of the present application, and the description is specific and detailed, but it should not be understood as a limitation to the patent scope of the application. It should be pointed out that for ordinary skilled in the art, without departing from the concept of the present application, several modifications and improvements can be made, which are all within the protection scope of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. A modulation method for a three-phase inverter, characterized in that, The method includes: Based on the voltage and current information of the three-phase inverter, the original three-phase modulation wave is obtained; The modulation factor is determined based on the power factor angle and the phase angle of the original three-phase modulation wave. Based on the modulation factor and the three-phase original modulation wave, the zero-sequence voltage is obtained; The zero-sequence voltage is superimposed on the original three-phase modulation wave to obtain the final modulation wave; The final modulated wave is compared with the carrier wave to obtain the drive signal of the three-phase inverter.

2. The method according to claim 1, characterized in that, The method further includes: If the three-phase inverter adopts a continuous pulse width modulation strategy, then the modulation factor is determined to be a first preset value; If the three-phase inverter adopts a discontinuous pulse width modulation strategy, the modulation factor is determined based on the power factor angle and the phase angle of the original three-phase modulation wave.

3. The method according to claim 2, characterized in that, If the modulation degree is less than or equal to the modulation degree threshold, then the three-phase inverter is determined to adopt a continuous pulse width modulation strategy. If the modulation degree is greater than the modulation degree threshold, then the three-phase inverter is determined to adopt a discontinuous pulse width modulation strategy.

4. The method according to claim 3, characterized in that, The modulation index is determined based on the DC bus voltage and the amplitude of the original three-phase modulation wave.

5. The method according to claim 1 or 2, characterized in that, Determining the modulation factor based on the power factor angle and the phase angle of the three-phase original modulation wave includes: Determine the offset angle based on the power factor angle; The modulation factor is determined based on the phase angle, the offset angle, and the preset smoothing curve; the preset smoothing curve is the relationship curve of the modulation factor with respect to the phase angle and the offset angle.

6. The method according to claim 5, characterized in that, The modulation factor satisfies the following equation: Where k represents the modulation factor, δ represents the offset angle, θ represents the phase angle, and f1(θ,θ) d f2(θ,θ) represents the first preset smooth curve. d ) represents the second preset smooth curve, θ d This indicates the preset adjustment angle.

7. The method according to claim 5, characterized in that, The offset angle satisfies the following formula: or, in, It represents the power factor angle.

8. The method according to claim 2, characterized in that, The first preset value is 0.

5.

9. The method according to claim 2, characterized in that, When switching between continuous pulse width modulation (PWM) and non-continuous pulse width modulation (NPWM), during the transition period, the modulation factor is determined based on a preset switching curve, the first preset value, the first threshold, and the second threshold. The first threshold is the minimum value of the modulation factor, and the second threshold is the maximum value of the modulation factor. The second threshold > the first preset value > the first threshold.

10. The method according to claim 1, characterized in that, The zero-sequence voltage satisfies the following equation: u0⼝-(1-k)max(u a * ,the b * ,the c * )-kmin(u a * ,the b * ,the c * )+1-2k Where u0 represents the zero-sequence voltage, 0≤k≤1, u a *、u b *、u c * indicates the original three-phase modulation wave.

11. A three-phase inverter, characterized in that, The method includes an inverter circuit and a controller connected to the inverter circuit, the controller performing the method as described in any one of claims 1 to 10.

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