Multi-level inverter hybrid modulation method and device

Abstract

The invention discloses a multi-level inverter hybrid modulation method and device, solves the current problems of limited number of cascade modules and unbalanced power of an asymmetric cascade multi-level inverter, and belongs to the technical field of cascade multi-level inverters. The invention is suitable for an asymmetric cascaded multilevel inverter topological structure. The topological structure comprises a single high-voltage module and (n-1) low-voltage modules which are cascaded together, the polarities of output voltages of all the cascade modules are non-different, and the high-voltage modules are modulated by adopting low-frequency step waves; and the low-voltage module adopts high-frequency carrier amplitude-shift modulation based on carrier displacement. According to the invention, the number of output levels of the inverter can be further increased, so that output voltage waveforms with higher quality can be obtained. When a low-voltage module follows a carrier replacement principle, all modules of the inverter can keep the ratio of output power to be k: 1: 1 in a full modulation range while the polarities of output voltages are non-different, so that the problems of power imbalance and power backward flow are well solved.

Description

Technical Field

[0001] The invention relates to a multi-level inverter hybrid modulation method and device, belonging to the technical field of cascade multi-level inverters. Background Art

[0002] At present, multilevel inverters based on pulse width modulation strategy are widely used in medium and high voltage power regulation systems such as high voltage direct current transmission systems, flexible alternating current transmission systems and energy storage systems, as well as high power drive occasions such as electric locomotives, industrial motors and electric vehicles due to their excellent harmonic performance, high efficiency and low dv / dt. Among them, asymmetric cascade multilevel inverters powered by unequal DC voltage sources can effectively reduce the number of switch tubes and improve the utilization efficiency of switch devices, opening up new ideas for the development of multilevel inverters. Compared with the cascade topology with symmetrical structure, asymmetric cascade multilevel inverters can output more levels with equal number of cascade modules, improve the output power quality, and its low cost, high efficiency and high performance have attracted more and more research and attention.

[0003] Despite its many advantages, asymmetric cascade multilevel inverters face safety hazards such as limited cascade number, power imbalance and power backflow under traditional multilevel modulation. Since each cascade module outputs active power independently, the average output power of different cascade modules needs to be controlled during the modulation process. For asymmetric cascade multilevel inverters powered by multi-winding phase-shifting transformers or battery packs, this is because power imbalance will cause inconsistent discharge characteristics of the DC side power supply, resulting in deviations in the power supply voltage, introducing a large number of low-order harmonics in the output voltage, and causing large differences in the service life of the power supply, increasing additional maintenance costs. In traditional modulation technology, only the carrier phase shift strategy can achieve power balance control between cascade modules, but there is a problem of poor harmonic characteristics of the inverter output voltage.

[0004] On the other hand, the power backflow problem caused by the active power output of the high-voltage module flowing to the low-voltage module also has an adverse effect on the inverter. First, power backflow will affect the voltage stability of the system, introduce additional voltage drop, cause the DC link voltage to fluctuate beyond the allowable range, and affect the overall performance and reliability of the inverter; secondly, power backflow will cause current distortion in the system, change the current distribution between sub-modules, introduce nonlinear current and harmonic distortion, increase losses, reduce efficiency, and may cause inverter components to overheat; in addition, power backflow may also introduce harmonics and distortion to the power grid or other connected devices, affecting voltage stability and frequency stability, increasing electromagnetic interference, and affecting the operational stability of the system. Finally, in order to reduce the impact of power backflow, complex control strategies need to be adopted, which increases the complexity and cost of the system. Summary of the invention

[0005] In view of the problems of limited number of cascade modules and unbalanced power currently faced by asymmetric cascade multi-level inverters, the present invention provides a multi-level inverter hybrid modulation method and device.

[0006] A multi-level inverter hybrid modulation method of the present invention is applicable to an asymmetric cascade multi-level inverter topology structure, wherein the topology structure includes a single high-voltage module and n-1 low-voltage modules cascaded together;

[0007] The output voltage polarity of all cascaded modules is the same, and the high-voltage module adopts low-frequency step wave modulation;

[0008] The low voltage module uses high frequency carrier amplitude shift modulation based on carrier replacement:

[0009] T o is the modulation wave period, T o / 2 cycle is a modulation area. In the same modulation area, the carrier waveforms of each low-voltage module do not overlap.

[0010] The carrier replacement period of the low voltage module is (n-1)T o , before (n-1)T o The / 2 cycle includes n waveforms, and n-1 waveforms are distributed in different order in the front (n-1)T of each low-voltage module. o / 2 cycle;

[0011] During the carrier replacement period, each low voltage module carrier waveform is symmetrical about the center point, and the center point is t=(n-1)T o / 2 intersection with the coordinate axis;

[0012] According to the number of levels of the inverter, the carrier is divided into multiple domains in space. Each domain corresponds to a voltage level. The carrier controls the output voltage of the low-voltage module by adjusting the phase and amplitude within these domains. The carrier distribution domains of each low-voltage module in the first n modulation areas of the carrier replacement period are different.

[0013] Preferably, the high voltage module adopts a sinusoidal modulation wave:

[0014] v mH =(k+n-1)MEsin(ωt)

[0015] Among them, v mH It represents the voltage value of the sinusoidal modulation wave, M represents the modulation degree (0≤M≤1), the DC side voltage of the high voltage module is kE, the DC side voltage of each low voltage module is E, and k represents the ratio.

[0016] As a preference, the low-frequency step wave of the high-voltage module is:

[0017]

[0018] Among them, v cr1+ Indicates the positive step carrier voltage value, v cr1- represents the negative step carrier voltage value, N is the highest level of the multilevel inverter output voltage, and θ0 is the conduction angle of the high-voltage module.

[0019] Preferably, the conduction angle θ0 of the high voltage module is:

[0020]

[0021] Preferably, the average output power of the main module is:

[0022]

[0023] Among them, I o is the output current amplitude of the multilevel inverter, is the power factor angle.

[0024] The present invention also provides a multi-level inverter hybrid modulation device, which is suitable for an asymmetric cascade multi-level inverter topology structure, the topology structure comprising a single high-voltage module and n-1 low-voltage modules cascaded together; the modulation device comprises a main modulation module and an auxiliary modulation module;

[0025] A main modulation module, used for adopting low-frequency step wave modulation to the high-voltage module;

[0026] Auxiliary modulation module, used to adopt high-frequency carrier amplitude shift modulation based on carrier replacement for low-voltage modules:

[0027] T o is the modulation wave period, T o / 2 cycle is a modulation area. In the same modulation area, the carrier waveforms of each low-voltage module do not overlap.

[0028] The carrier replacement period of the low voltage module is n-1T o , first n-1T o The / 2 cycle includes n waveforms, and n-1 waveforms are distributed in different order in the first n-1T of each low-voltage module. o / 2 cycle;

[0029] During the carrier replacement period, each low voltage module carrier waveform is symmetrical about the center point, and the center point is t=(n-1)T o / 2 intersection with the coordinate axis;

[0030] According to the number of levels of the inverter, the carrier is divided into multiple domains in space. Each domain corresponds to a voltage level. The carrier controls the output voltage of the low-voltage module by adjusting the phase and amplitude within these domains. The carrier distribution domains of each low-voltage module in the first n modulation areas of the carrier replacement period are different.

[0031] The main modulation module and the auxiliary modulation module make the output voltage polarities of all cascaded modules non-different.

[0032] The beneficial effects of the present invention are as follows: the hybrid modulation method of the asymmetric cascade multi-level inverter proposed in the present invention can be applied to an asymmetric cascade multi-level inverter with multiple cascade modules, which is conducive to further increasing the number of inverter output levels to obtain a higher quality output voltage waveform. When the low-voltage module follows the carrier replacement principle, all modules of the inverter can maintain an output power ratio of k:1:···:1 in the full modulation range while the output voltage polarity is non-different, properly solving the power imbalance and power backflow problems. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is an asymmetric cascade multi-level inverter topology;

[0034] Figure 2 It is the negative half cycle modulation wave and carrier processing scheme;

[0035] Figure 3 The hybrid modulation method principle of a five-module seventeen-level asymmetric cascade inverter is provided;

[0036] Figure 4 Comparative analysis of the number of switching times under different modulation strategies;

[0037] Figure 5 The instantaneous output voltage, output current and output power waveforms of each cascade module under the power matching modulation strategy. DETAILED DESCRIPTION

[0038] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0039] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0040] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments, but they are not intended to limit the present invention.

[0041] In the multi-level inverter hybrid modulation method of this embodiment, the inverter topology is as follows Figure 1 As shown, it is composed of a high-voltage module H1 with a DC side voltage of kE and n-1 low-voltage modules with a DC side voltage of E in cascade. H1 、v Hi 、v Hn Respectively represent the AC side output voltage of H1, H2, ..., Hn modules, v o is the inverter output voltage, and its expression is:

[0042]

[0043] Where: S i is the switch function of the ith module. When the values ​​are 1, 0, and -1, they represent that the ith module outputs positive level, zero level, and negative level, respectively. kS1E is the output voltage of the high-voltage module. is the output voltage of the low voltage module, so it can be known that the maximum level N of the inverter output voltage is:

[0044] N=k+n-1

[0045] i o is the inverter output current, which can be expressed as:

[0046]

[0047] Where: I o is the inverter output current amplitude; is the power factor angle.

[0048] Therefore, for Figure 1 For the asymmetric cascade multilevel inverter topology shown in the figure, the average output power of each cascade module in a single modulation wave cycle can be expressed as:

[0049]

[0050] Where: T o is the modulation wave period, V Hi(1) is the output voltage fundamental amplitude of the corresponding module.

[0051] Due to the difference between the amplitudes of the DC side voltage sources, after permutation and combination, the k:1:···:1 type asymmetric cascade multi-level inverter has a total of (2k+2n-1) output levels. The output level synthesis method of the inverter is shown in Table 1. Since the inverter outputs negative levels in a similar way to positive levels, only the synthesis methods of outputting positive levels and zero levels are listed in Table 1. The existence of multiple low-voltage modules makes the output level of the inverter correspond to a large number of switch state combinations, among which a large number of redundant switch state combinations contain switch state combinations with opposite output voltage polarities of the high-voltage module and the low-voltage module, which will cause the output energy of the high-voltage module to be fed back to the low-voltage module, resulting in a power backflow problem. In order to ensure the safe operation of the inverter and improve the quality of the output voltage waveform, it should be required that the output voltage polarity of all modules is the same as the inverter output voltage, or output zero level.

[0052] Table 1 Inverter output level synthesis method

[0053] Output Level High voltage module Low voltage module k+n-1 1 n-1 ··· 1 ··· k 1 0 ··· 0 ··· 1 0 1 0 0 0

[0054] To solve the problem of uneven distribution of output power between modules, this embodiment proposes an asymmetric cascade multi-level inverter hybrid modulation strategy that can be applied to any number of modules. The high-voltage module adopts low-frequency step wave modulation to control the output power of the high-voltage module by adjusting the conduction angle. Figure 1 The asymmetric cascade multilevel inverter shown in the figure has a sinusoidal modulation wave of the high-voltage module:

[0055] v mH =(k+n-1)MEsin(ωt)

[0056] Where: M represents the modulation degree (0≤M≤1).

[0057] In order to achieve balanced output power of all modules, the value of the high-voltage module step carrier should depend on the proportion of its output level to the maximum output level of the inverter, that is:

[0058]

[0059] In the formula, v cr1+ Indicates the positive step carrier voltage value, v cr1- Represents the negative step carrier voltage value. The high-voltage module conduction angle θ0 is the angle corresponding to the first intersection of the sinusoidal modulation wave with the carrier in the positive half cycle, which can be expressed as:

[0060]

[0061] Therefore, the average output power of the high-voltage module is:

[0062]

[0063] It can be seen from the above formula that the conduction angle θ0 of the high-voltage module during power balancing is independent of the output voltage level k of the high-voltage module and the number n of modules in the asymmetric cascaded multi-level inverter. Therefore, the hybrid modulation method can achieve the purpose of fixing the proportion of the high-voltage module output power to the total output power of the inverter by constraining the conduction angle of the high-voltage module.

[0064] The high-voltage module does not have the problem of power backflow, so only the modulation scheme of the low-voltage module needs to be improved. The low-voltage module adopts high-frequency carrier amplitude shift modulation based on carrier replacement. Taking the four-module thirteen-level asymmetric cascade inverter as an example, Figure 2 Shown is the modulation wave and carrier processing scheme in the negative half cycle. Figure 2 (a) is the original solution without processing. In order to reduce the number of carriers, the modulation wave and carrier control freedom are used. Figure 2 In (b), first remove the negative carrier in the positive half cycle. Figure 2 In (c), the modulated wave and the carrier wave are inverted. Figure 2 In (d), the low voltage module carrier is flipped. Figure 2 The processed low-voltage module carrier replaces the carrier distribution domain every half output cycle on the basis of the original spatial distribution, so that the low-voltage module carrier is no longer fixedly distributed in a specific domain, but cyclically distributed between different carrier distribution domains over time. The module output voltage waveform changes after carrier replacement, but the inverter output voltage v o There is no change from the traditional carrier amplitude shift modulation strategy.

[0065] through Figure 2 The hybrid modulation method obtained by the method shown in the figure also has the following beneficial effects:

[0066] (1) There is no longer a carrier in the low-voltage module with a different output voltage polarity from that of the high-voltage module, which can completely eliminate the power backflow problem between the high-voltage module and the low-voltage module;

[0067] (2) The number of low-voltage module carriers is reduced to half of the original number, and the modulation complexity is effectively reduced;

[0068] (3) In the carrier replacement period 3T o / 2, the output power of all low voltage modules is This achieves a balanced distribution of the output power of all cascaded modules of the inverter at k:1:···:1.

[0069] However, the above hybrid modulation method is only applicable to the asymmetric cascade inverter with four modules and thirteen levels, and it should be extended to the asymmetric cascade multi-level inverter with any number of modules. Since the high-voltage module modulation scheme can be applied to the asymmetric cascade multi-level inverter with any number of modules, only the modulation scheme of the low-voltage module needs to be expanded. Figure 1 For the multi-level inverter shown in the figure, the carrier distribution of its n-1 low-voltage modules must meet the following requirements:

[0070] 1) T o / 2 cycle is a modulation area. In the same modulation area, the carrier waveforms of each low-voltage module do not overlap.

[0071] 2) Low voltage module i carrier v cri The permutation period is (n-1)T o , before (n-1)T o The / 2 cycle includes n waveforms, and n-1 waveforms are distributed in different order in the front (n-1)T of each low-voltage module. o / 2 cycle; during the carrier replacement cycle, each low-voltage module carrier waveform is symmetrical about the center point, and the center point is t = (n-1)T o / 2 intersects the coordinate axes.

[0072] 3) For low voltage module i, define Δ i(j) Indicates that low voltage module i is in modulation area A j The carrier distribution layer domains of each low-voltage module in the first n modulation areas of the carrier replacement period are different.

[0073] The distribution layer domain refers to the multiple regions or layers that divide the spatial distribution of the triangular carrier according to the number of levels of the converter (i.e., different discrete levels of output voltage) in the carrier amplitude shift modulation strategy of the multilevel converter. Each layer domain corresponds to one or more specific voltage levels, and the triangular carrier in each layer domain has specific amplitude and phase characteristics to ensure smooth transition of the converter between different voltage levels.

[0074] Specifically, the division of the distribution layer domain is closely related to the number of levels of the multilevel converter. For example, for a nine-level converter, eight triangular carriers are usually used, each of which is modulated at a specific level of the output voltage. By performing different phase shifts (i.e., amplitude shifts) on the carriers, their spatial distribution can be divided into eight different layer domains to achieve precise control of the output voltage waveform.

[0075] The triangular carriers in these distribution layer domains are compared with the modulation signal (such as a sine wave) through appropriate combinations to determine the on and off of the switching device, ultimately affecting the waveform characteristics of the output voltage. Therefore, the distribution layer domain plays a vital role in the carrier amplitude shift modulation strategy, which effectively separates the modulation areas of different voltage levels, allowing the converter to generate a high-quality output voltage waveform.

[0076] The modulation method of this embodiment can also be implemented in the form of a modulation device, using a main modulation module to apply low-frequency step wave modulation to the high-voltage module, and using an auxiliary modulation module to apply high-frequency carrier amplitude shift modulation based on carrier replacement to the low-voltage module.

[0077] Based on the above principles, power backflow can be eliminated while achieving power balance for all cascaded modules. Taking a five-module, seventeen-level asymmetric cascaded inverter as an example, Figure 3 The principle of the hybrid modulation method is given. Figure 3 As shown, the carrier replacement period of the five-module seventeen-level asymmetric cascade inverter is 4T o . Δ i(j) for Figure 3 The layer domain A where the medium and low voltage modules i and the carrier are distributed j , in the first 2T o The carrier distribution of low voltage modules 1-4 in different areas is: 2(1) Δ 2(2) Δ 2(3) Δ 2(4) =1234,Δ 3(1) Δ 3(2) Δ 3(3) Δ 3(4) =2143, Δ 4(1) Δ 4(2) Δ 4(3) Δ 4(4) =3421, Δ 5(1) Δ 5(2) Δ 5(3) Δ 5(4) =4312, 2T later o The carrier waveform within the first 2T o The carrier waveform in the figure is about the straight line t = 2T o Symmetrical about the center of the intersection of the coordinate axes.

[0078] Figure 4 The graph of the output voltage THD value of the asymmetric cascade multi-level inverter with different numbers of modules changes with the modulation index M. Figure 4 It can be seen that the asymmetric cascaded multilevel inverter under the hybrid modulation method shows excellent harmonic performance, and as the number of modules increases, the inverter output voltage THD is further reduced.

[0079] In order to verify the feasibility of the hybrid modulation method proposed in this implementation, a four-module thirteen-level asymmetric cascade inverter experimental platform was established for physical experimental verification. The experiment used a resistive inductive load (20Ω-4mH), the power supply voltages of the high-voltage module and the low-voltage module were 36V and 12V respectively, and the carrier frequency was 8kHz.

[0080] When the four-module thirteen-level asymmetric cascade inverter adopts a hybrid modulation method, the output voltage of each module when the modulation index is 0.95 is as follows Figure 5 As shown, Figure 5 (a) is the output voltage waveform of the high voltage module H1. Figure 5 (b) is the output voltage waveform of the low voltage module H2. Figure 5 (c) is the output voltage waveform of the low voltage module H3. Figure 5 (d) is the output voltage waveform of the low voltage module H4. Under different modulation degrees, the output voltage waveforms of each module are about the vertical line t=3T o / 2 is symmetrical and uses carrier to replace the period, that is, the first 3T o The output voltage waveform in the module is used as the basic unit, and the output is repeated in a cycle. In the positive and negative half cycles, the output voltage polarity of each module is the same, and power backflow is effectively avoided under different modulation degrees. The experimental results prove the feasibility and effectiveness of the hybrid modulation method proposed in the present invention.

[0081] Although the present invention is described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the present invention. It should therefore be understood that many modifications may be made to the exemplary embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims. It should be understood that the various dependent claims and features described herein may be combined in a manner different from that described in the original claims. It should also be understood that features described in conjunction with individual embodiments may be used in other described embodiments.

Claims

1. A multi-level inverter hybrid modulation method, characterized in that: The method is applicable to an asymmetric cascade multi-level inverter topology, which includes a single high-voltage module and n-1 low-voltage modules cascaded together; The output voltage polarity of all cascaded modules is the same, and the high-voltage module adopts low-frequency step wave modulation; The low voltage module uses high frequency carrier amplitude shift modulation based on carrier replacement: T o is the modulation wave period, T o / 2 cycle is a modulation area. In the same modulation area, the carrier waveforms of each low-voltage module do not overlap. The carrier replacement period of the low voltage module is (n-1)T o , before (n-1)T o The / 2 cycle includes n waveforms, and n-1 waveforms are distributed in different order in the front (n-1)T of each low-voltage module. o / 2 cycle; During the carrier replacement period, each low voltage module carrier waveform is symmetrical about the center point, and the center point is t=(n-1)T o / 2 intersection with the coordinate axis; According to the number of levels of the inverter, the carrier is divided into multiple domains in space. Each domain corresponds to a voltage level. The carrier controls the output voltage of the low-voltage module by adjusting the phase and amplitude within these domains. The carrier distribution domains of each low-voltage module in the first n modulation areas of the carrier replacement period are different.

2. The multi-level inverter hybrid modulation method according to claim 1, characterized in that: The high voltage module adopts sinusoidal modulation wave: v mH =(k+n-1)MEsin(ωt) Among them, v mH It represents the voltage value of the sinusoidal modulation wave, M represents the modulation degree (0≤M≤1), the DC side voltage of the high voltage module is kE, the DC side voltage of each low voltage module is E, and k represents the ratio.

3. The multi-level inverter hybrid modulation method according to claim 2, characterized in that: The low-frequency step wave of the high-voltage module is: Among them, v cr1+ Indicates the positive step carrier voltage value, v cr1- represents the negative step carrier voltage value, N is the highest level of the multilevel inverter output voltage, and θ0 is the conduction angle of the high-voltage module.

4. The multi-level inverter hybrid modulation method according to claim 3, characterized in that: The conduction angle θ0 of the high voltage module is:

5. The multi-level inverter hybrid modulation method according to claim 4, characterized in that: The average output power of the main module is: Among them, I o is the output current amplitude of the multilevel inverter, is the power factor angle.

6. A multi-level inverter hybrid modulation device, characterized in that: The device is suitable for an asymmetric cascade multi-level inverter topology structure, which includes a single high-voltage module and n-1 low-voltage modules cascaded together; the modulation device includes a main modulation module and an auxiliary modulation module; A main modulation module, used for adopting low-frequency step wave modulation to the high-voltage module; Auxiliary modulation module, used to adopt high-frequency carrier amplitude shift modulation based on carrier replacement for low-voltage modules: T o is the modulation wave period, T o / 2 cycle is a modulation area. In the same modulation area, the carrier waveforms of each low-voltage module do not overlap. The carrier replacement period of the low voltage module is n-1T o , first n-1T o The / 2 cycle includes n waveforms, and n-1 waveforms are distributed in different order in the first n-1T of each low-voltage module. o / 2 cycle; During the carrier replacement period, each low voltage module carrier waveform is symmetrical about the center point, and the center point is t=(n-1)T o / 2 intersection with the coordinate axis; According to the number of levels of the inverter, the carrier is divided into multiple domains in space. Each domain corresponds to a voltage level. The carrier controls the output voltage of the low-voltage module by adjusting the phase and amplitude within these domains. The carrier distribution domains of each low-voltage module in the first n modulation areas of the carrier replacement period are different. The main modulation module and the auxiliary modulation module make the output voltage polarities of all cascaded modules non-different.

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