Inverter Fault Tolerance and Neutral Point Voltage Balance Control Method Based on Modulation Wave Decomposition

Abstract

The present invention provides an inverter fault-tolerant and neutral-point voltage balance control method based on modulation wave decomposition, which generates an initial modulation wave and injects a zero-sequence component into the initial modulation wave; analyzes the current path in the case of a switch tube fault, and divides a fundamental wave period into a healthy half-cycle and a fault half-cycle according to the fault-phase current direction; in the healthy half-cycle, decomposes the modulation wave of the middle phase and calculates the control quantity; in the fault half-cycle, performs carrier reconstruction, does not use non-existent switching states, selects the optimal decomposition phase according to the neutral-point voltage, the non-fault-phase current direction, and the control force, and calculates the control quantity, and decomposes the modulation wave according to the calculated control quantity. The present invention reconstructs the carrier according to the fault-phase current direction to achieve fault-tolerant control, and reasonably selects one of the three-phase modulation waves for decomposition to adjust the neutral-point current, realizes the decoupling of fault-tolerant control and neutral-point voltage balance control, and ensures the safe and reliable operation of the inverter system.

Description

Technical Field

[0001] The present invention belongs to the technical field of inverter control, and particularly relates to a fault-tolerant and neutral-point voltage balance control method for an inverter based on modulation wave decomposition. Background Art

[0002] The statements in this part only provide background technical information related to the present invention, and do not necessarily constitute prior art.

[0003] At present, with the vigorous development of energy equipment, inverters, as the hub connecting new energy and the power grid, have received extensive attention in the industry. It converts DC electrical energy into AC electrical energy and is widely used in uninterruptible power supplies, electric vehicles, wind power generation, solar power generation, power active filters and other fields. Compared with two-level inverters, three-level inverters have higher voltage levels and lower harmonic contents, so they are more advantageous. Among three-level inverters, T-type three-level inverters are widely used due to their advantages such as fewer switching devices, lower conduction losses and uniform power losses.

[0004] Safe and reliable operation is the premise for the application of inverters. The switching devices in the T-type three-level inverter system operate at a high frequency and have a large number of switching times. Coupled with complex operating conditions and their own aging, the possibility of system switching device failures increases significantly. When a switching device fails, the system output current is severely distorted, and the total harmonic distortion rate of the current will exceed the grid connection requirements, even endangering the system and personal safety.

[0005] The fault types of switching devices are divided into two cases: short circuit and open circuit. When a short-circuit fault occurs, the instantaneous large current usually threatens the entire inverter system, so the system must be shut down emergently. When an open-circuit fault occurs, the grid-connected current of the system is distorted, and it will further cause secondary damage to other components. Compared with short-circuit faults, open-circuit faults do not instantaneously cause devastating damage to power devices and even the entire system. The corresponding faults can be eliminated through fault-tolerant control methods to reduce the losses caused by fault shutdown.

[0006] In addition, the balance of the DC-side neutral-point voltage is also the key to ensuring the safe and stable operation of the T-type three-level inverter. Unbalanced neutral-point voltage will generate a large amount of harmonic pollution on the AC side, and cause overvoltage of DC capacitors and power devices, reducing the system stability. However, there is a coupling between fault-tolerant control and neutral-point voltage balance control, increasing the difficulty of controlling the neutral-point voltage balance and threatening the safe and reliable operation of the T-type three-level inverter. Summary of the Invention

[0007] To solve the above problems, the present invention proposes an inverter fault tolerance and neutral point voltage balance control method based on modulation wave decomposition. According to the fault phase current direction, the present invention reconstructs the carrier wave to achieve fault tolerance control, and reasonably selects one of the three-phase modulation waves for decomposition to adjust the neutral point current, realizing the decoupling of fault tolerance control and neutral point voltage balance control, achieving neutral point voltage balance control without affecting fault tolerance control, and ensuring the safe and reliable operation of the inverter system.

[0008] According to some embodiments, the present invention adopts the following technical solutions:

[0009] An inverter fault tolerance and neutral point voltage balance control method based on modulation wave decomposition, comprising the following steps:

[0010] Generate an initial modulation wave and inject a zero-sequence component into the initial modulation wave;

[0011] Analyze the current path in the case of a switch tube fault, and divide a fundamental wave period into a healthy half-period and a fault half-period according to the fault phase current direction;

[0012] In the healthy half-period, decompose the intermediate phase modulation wave and calculate the control quantity;

[0013] In the fault half-period, perform carrier wave reconstruction, do not use non-existent switch states, select the optimal decomposition phase according to the neutral point voltage, non-fault phase current direction and control force, calculate the control quantity, and decompose the modulation wave according to the calculated control quantity.

[0014] As an alternative implementation, the switch states of the switch tubes of the inverter include [P], [O], and [N].

[0015] As an alternative implementation, the initial modulation wave includes three-phase initial modulation waves and represents the maximum value, intermediate value, and minimum value of the three-phase initial modulation waves.

[0016] As an alternative implementation, the specific process of injecting a zero-sequence component into the initial modulation wave includes injecting the negative of the average value of the maximum value and the minimum value of the corresponding phase initial modulation wave into each phase initial modulation wave.

[0017] As an alternative embodiment, the specific process of dividing a fundamental wave period into a healthy half-period and a faulty half-period according to the faulty phase current direction includes, when a switching tube fails, analyzing the current path before and after the switching tube fails, obtaining the non-existent switching state according to the faulty phase current direction, and dividing a fundamental wave period into a healthy half-period and a faulty half-period; within the healthy half-period, two carriers with an amplitude of 1 are still used, and three levels of [P], [O], and [N] are still output; within the faulty half-period, a carrier with an amplitude of 2 is used, and only two levels of [P] and [N] are output, avoiding the use of the non-existent switching state [O], eliminating the distortion of the output current, and realizing fault-tolerant control.

[0018] As an alternative embodiment, according to the modulated wave after injecting the zero-sequence component, a neutral point current model is established. If the modulated wave is greater than 0, the modulated wave is decomposed into m x +y and -y. If the modulated wave is less than 0, the modulated wave is decomposed into m x -y and y, which are respectively compared with the positive load wave, and by adjusting the control quantity y, the duty ratio of the [O] state is adjusted.

[0019] As an alternative embodiment, in the healthy half-period, the middle phase is decomposed, and the control quantity accurately calculated according to the three-phase modulated wave, three-phase current, and neutral point voltage is expressed as

[0020]

[0021] Among them, C is the capacitance value of the DC-side capacitor, ΔV PN is the difference between the upper and lower capacitor voltages of the DC side, T s is the sampling period, m max 、m mid and m min are respectively the maximum value, intermediate value, and minimum value of the modulated wave, i max 、i mid and i min are respectively the corresponding phase currents;

[0022] The control quantity needs to satisfy:

[0023]

[0024] As an alternative embodiment, in the faulty half-period, when the difference between the upper and lower capacitor voltages of the DC side is greater than or equal to zero, the voltage of the lower capacitor is lower than that of the upper capacitor, and the corresponding phase is decomposed to reduce the neutral point current;

[0025] If the neutral point current increases in all phases, no decomposition is performed.

[0026] As a further step, when phase A is the faulty phase, the control quantity of the corresponding phase is:

[0027]

[0028] n is b or c, i a , i b , i c are the phase a, b, and c currents respectively, C is the capacitance value of the DC-side capacitor, and ΔV PN is the difference between the upper and lower capacitor voltages on the DC side, and T s is the sampling period.

[0029] An inverter fault-tolerant and neutral-point voltage balancing control system based on modulation wave decomposition, comprising:

[0030] A modulation range increasing module, configured to generate an initial modulation wave and inject a zero-sequence component into the initial modulation wave;

[0031] A fundamental wave period decomposition module, configured to analyze the current path in the case of a switch tube fault, and divide a fundamental wave period into a healthy half-period and a fault half-period according to the direction of the fault phase current;

[0032] A first control module, configured to decompose the modulation wave of the middle phase and calculate the control quantity in the healthy half-period;

[0033] A second control module, configured to perform carrier reconstruction in the fault half-period, not use the non-existent switch states, select the optimal decomposition phase according to the neutral-point voltage, the direction of the non-fault phase current, and the control force, calculate the control quantity, and decompose the modulation wave according to the calculated control quantity.

[0034] Compared with the prior art, the beneficial effects of the present invention are:

[0035] 1. The present invention simultaneously realizes the fault-tolerant control and the neutral-point voltage balance control of the T-type three-level inverter. In the case of a switch tube fault, the total harmonic distortion rate of the three-phase output current is reduced, the neutral-point voltage of the DC side is kept balanced, and the neutral-point voltage oscillation is suppressed.

[0036] 2. The method of the present invention avoids the use of non-existent switch states by reconstructing the carrier, enables the T-type three-level inverter system to significantly reduce the total harmonic distortion rate of the output current in the case of a switch tube fault, realizes the fault-tolerant control, and ensures the reliability of the system.

[0037] 3. The method of the present invention decomposes the modulation wave, increases a degree of freedom for controlling the neutral-point voltage, can control the neutral-point voltage balance without affecting the fault-tolerant control, and realizes the decoupling of the two.

[0038] 4. The method according to the present invention accurately calculates the required control quantities based on the three-phase modulation wave, three-phase current, and neutral point voltage, and reasonably selects the optimal phase for decomposition, realizing the balanced control and oscillation suppression of the DC-side neutral point voltage and maintaining the safe and stable operation of the system.

[0039] To make the above objects, features, and advantages of the present invention more obvious and understandable, the following specifically enumerates preferred embodiments and, in conjunction with the accompanying drawings, makes a detailed description as follows. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The specification drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention.

[0041] Figure 1 The T-type three-level inverter system provided by the embodiment of the present invention;

[0042] Figure 2 The switching tube S provided by the embodiment of the present invention a2 The current path diagram of phase A before and after the fault occurs;

[0043] Figure 3 The fault-tolerant control method based on carrier reconstruction provided by the embodiment of the present invention;

[0044] Figure 4 The schematic diagram of modulation wave decomposition in a fundamental wave period provided by the embodiment of the present invention;

[0045] Figure 5 The system control block diagram provided by the embodiment of the present invention;

[0046] Figure 6 The normal operating condition, S a2 The output waveform diagrams under open-circuit fault conditions and using the method proposed by the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0047] The present invention will be further described below in conjunction with the drawings and embodiments.

[0048] It should be noted that the following detailed descriptions are all illustrative and are intended to provide a further description of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs.

[0049] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular forms are also intended to include the plural forms. In addition, it should also be understood that when the terms "comprise" and / or "include" are used in this specification, they specify the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0050] The present invention is directed to a T-type three-level inverter system. Figure 1 As the topological structure diagram of the T-type three-level inverter system, it includes phase A, phase B, and phase C bridge arms. Each phase bridge arm includes four power switch tubes, which are composed of two half-bridge (HB) switch tubes and two neutral point (NP) switch tubes. The DC side includes two capacitors connected in series, and a neutral point is formed in the middle of the two capacitors.

[0051] Specifically, each phase bridge arm of the T-type three-level inverter system has three working states [P], [O], [N]. Taking the neutral point of the two capacitors on the DC side as the reference point, Table 1 shows the switching states, output voltages, and the working conditions of each power switch tube in each phase bridge arm of the T-type three-level inverter system.

[0052] Table 1 Switching States, Output Voltages, and Switching Tube Working Conditions of the T-Type Three-Level Inverter

[0053]

[0054]

[0055] Let the three-phase initial modulation waves be

[0056]

[0057] where m is the modulation index. The maximum value, middle value, and minimum value of the three-phase initial modulation waves are respectively expressed as

[0058]

[0059] In order to increase the modulation range, the following zero-sequence components are usually injected into the initial modulation waves to expand the modulation range to 1.15.

[0060]

[0061] where x = a, b, c. The modulation waves after injecting the zero-sequence components are compared with the carrier wave to generate PWM waves to drive the switch tubes to operate. i max 、i mid and i min are respectively the phase currents corresponding to the maximum value, middle value, and minimum value of the modulation waves.

[0062] However, due to the high operating frequency and large number of switching times of the switching tubes in the T-type three-level inverter, combined with complex operating conditions and its own aging, etc., the switching tubes of the inverter are extremely prone to failure. When the HB tube fails, the modulation degree needs to be reduced to less than 0.5 to achieve fault-tolerant control, which is not advisable for inverters applied in fields such as photovoltaic grid connection or motor drive. Therefore, the present invention only considers the case where the NP tube fails. At the same time, it is extremely rare for multiple switching tubes to fail simultaneously. The present invention takes the S a2 fault as an example for analysis, and the current paths before and after the fault of phase A are as Figure 2 shown.

[0063] When i a > 0, the [O] state of phase A cannot be generated. At this time, if the non-existent switching state is used to synthesize the reference vector, the output current will be distorted; when i a < 0, the current path is the same as the normal situation. Therefore, according to the direction of the fault-phase current, a fundamental wave period is divided into a fault half-cycle (i a > 0) and a healthy half-cycle (i a < 0).

[0064] In the fault half-cycle, the [O] state of phase A is not used for synthesis, and the current distortion will be eliminated. Therefore, carrier reconstruction is performed in the fault phase, and the carrier as shown in Figure 3 is used. Among them, two carriers with an amplitude of 1 are still used in the healthy half-cycle, while in the fault half-cycle, a carrier with an amplitude of 2 is used instead. At this time, only the [P] state and the [N] state are used in the fault half-cycle, avoiding the use of the non-existent [O] state, and the fault-tolerant control is realized.

[0065] At the same time, the balance of the midpoint voltage on the DC side is also the key to ensuring the safe and stable operation of the T-type three-level inverter. However, there is a coupling between the traditional fault-tolerant control and the midpoint voltage balance control, and the decoupling control of the two needs to be studied urgently. Define the current flowing out of the midpoint of the DC side as positive. According to Kirchhoff's current law, the midpoint current is

[0066]

[0067] where C is the capacitance value of the DC-side capacitor, ΔV PN is the difference between the upper and lower capacitor voltages on the DC side, and T s is the sampling period. It can be seen that the key to controlling the midpoint voltage balance is to obtain the desired midpoint current. Since the midpoint current is only affected by the duty ratio of the [O] state of each phase, the midpoint current models in the healthy half-cycle and the fault half-cycle are different, and a classification discussion is carried out:

[0068] Case 1: In the healthy half-cycle, the midpoint current model is expressed as

[0069] i NPh=(1 - |m a |)·i a +(1 - |m b |)·i b +(1 - |m c |)·i c (5)

[0070] Since the balanced three-phase currents have the following relationship

[0071] i a + i b + i c = 0 (6)

[0072] Then equation (5) can be rewritten as

[0073] i NPh = -|m a |·i a -|m b |·i b -|m c |·i c (7)

[0074] To obtain the desired neutral point current, one of the three phases is decomposed to obtain a degree of freedom for controlling the neutral point current. If the modulation wave is greater than 0, the modulation wave is decomposed into m x + y and -y, which are respectively compared with the positive load wave, and by adjusting the control quantity y, the duty ratio of the [O] state is adjusted. As Figure 4 shown, the duration of the [O] state before decomposition is reduced by 2y and respectively superimposed on the [N] state and the [P] state, thus becoming a switching sequence from the [N] state to the [O] state and then to the [P] state. Since the increased [P] and [N] state times are equal, the sum of the three-phase duty ratios is ensured to be constant. Therefore, no matter how y is changed, the line voltage relationship of the three phases will not be damaged. Similarly, if the modulation wave is less than 0, the modulation wave is decomposed into m x - y and y, which will not be elaborated here

[0075] Since the duty ratio of the decomposed [O] state cannot be negative, it is necessary to satisfy

[0076]

[0077] To prevent excessive switching losses, only one phase is decomposed within one switching period. Since the duty ratio of the [O] state of the middle phase is the highest and the adjustment range of y is larger, only the middle phase is decomposed. At this time, the neutral point current in the healthy half cycle is rewritten as

[0078] i′ NPh = -|m max |·i max-m mid |·i mid -m min |·i min -2y·i mid (9)

[0079] Combining equations (4) and (9), the control quantity at this time can be accurately calculated as

[0080]

[0081] Case 2: In the fault half-cycle, since there is no [O] state in phase A after fault-tolerant control and it has no influence on the neutral point current, the neutral point current model at this time is expressed as

[0082] i NPf =-i a -|m b |·i b -|m c |·i c (11)

[0083] Since the decomposition of phase A has no influence on the neutral point current at this time, only the decomposition of phases B and C can control the neutral point voltage, and the control quantity at this time is calculated as

[0084]

[0085] At the same time, the range of the control quantity needs to satisfy

[0086]

[0087] However, if the decomposed phase is incorrect, it will cause the neutral point voltage not to be controlled in the optimal state and even exacerbate the neutral point voltage offset. Therefore, an optimal decomposed phase selection is required. When ΔV PN ≥0, the voltage of the lower capacitor is lower than that of the upper capacitor, and the neutral point current should be negative or a positive number as small as possible. In short, a control quantity should be added to reduce the neutral point current. At this time, a classification discussion is carried out according to the current polarities of phases B and C: If i b >0 and i c <0, only decomposing phase B can reduce the neutral point current; if i b <0 and i c >0, only decomposing phase C can reduce the neutral point current; if i b >0 and i c >0, both phases B and C can reduce the neutral point current. At this time, the phase with stronger control force should be selected for decomposition to obtain the optimal neutral point control effect. Therefore, the concept of control force is introduced. From equations (8) and (12), by comparing the control forces y b i b and y c i cin magnitude, decompose the phase with a greater control force; if i b <0 and i c <0, at this time, when decomposing, both the B-phase and the C-phase increase the neutral point current, which will exacerbate the imbalance of the neutral point voltage. Therefore, decomposition is not performed in this case. ΔV PN When <0, it can be analyzed by the same method, and the judgment method of the decomposed phase is summarized in Table 2.

[0088] Table 2 Judgment method of the decomposed phase in the fault half cycle

[0089]

[0090] The above method for decomposing the modulation wave realizes the decoupling of fault tolerance control and neutral point voltage balance control, reasonably selects the optimal phase to be decomposed and accurately calculates the desired control quantity, compares the decomposed modulation wave with the reconstructed carrier wave, and simultaneously realizes the fault tolerance control and neutral point voltage balance control of the T-type three-level inverter. Figure 5 The control block diagram of the system is shown as follows.

[0091] Figure 6 For the normal condition, S a2 open-circuit fault condition and the output waveform diagrams of the T-type three-level inverter system when the method proposed in the present invention is adopted, including the three-phase output current, the phase voltage of phase A, the line voltage between phase A and phase B, and the voltages of the two DC-side capacitors. It can be seen that after the switch tube S a2 fails, the three-phase current is significantly distorted, the THDi increases significantly and exceeds the grid connection requirement, and the neutral point voltage shifts, seriously threatening the safe and stable operation of the system. After adopting the proposed method, the three-phase current distortion is eliminated, the THDi is reduced to a lower value, and the neutral point voltage is controlled to be balanced, and there is a small neutral point voltage oscillation, proving that this method realizes both fault tolerance control and neutral point voltage balance control.

[0092] Those skilled in the art should understand that the embodiments of the present invention can be provided as a method, a system, or a computer program product. Therefore, the present invention can adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present invention can adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk memories, CD-ROMs, optical memories, etc.) containing computer-usable program codes.

[0093] The present invention is described with reference to the flowcharts and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It should be understood that each flow and / or block in the flowcharts and / or block diagrams, and combinations of flows and / or blocks in the flowcharts and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to the processors of general-purpose computers, special-purpose computers, embedded processors, or other programmable data processing devices to produce a machine, such that the instructions executed by the processors of the computer or other programmable data processing devices produce means for implementing the functions specified in one or more of the flows Figure 1 one or more of the flows and / or blocks Figure 1 or means for implementing the functions specified in one or more of the blocks.

[0094] These computer program instructions can also be stored in a computer-readable memory that can direct a computer or other programmable data processing device to operate in a particular manner, such that the instructions stored in the computer-readable memory produce a manufacture including instruction means for implementing the functions specified in one or more of the flows Figure 1 one or more of the flows and / or blocks Figure 1 or means for implementing the functions specified in one or more of the blocks.

[0095] These computer program instructions can also be loaded onto a computer or other programmable data processing device, such that a series of operational steps are performed on the computer or other programmable device to produce a computer-implemented process, and thus the instructions executed on the computer or other programmable device provide steps for implementing the functions specified in one or more of the flows Figure 1 one or more of the flows and / or blocks Figure 1 or means for implementing the functions specified in one or more of the blocks.

[0096] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, various modifications and variations can be made to the present invention. Any modifications, equivalent replacements, improvements, etc. made by those skilled in the art without creative efforts within the spirit and principles of the present invention shall be included in the protection scope of the present invention.

Claims

1. A fault-tolerant and neutral point voltage balance control method for an inverter based on modulation wave decomposition, characterized in that, It includes the following steps: Generate an initial modulation wave and inject a zero-sequence component into the initial modulation wave; Analyze the current path in the case of a switch tube fault, and divide a fundamental wave period into a healthy half-period and a fault half-period according to the fault phase current direction; In the healthy half-period, decompose the modulation wave of the middle phase and calculate the control quantity; In the fault half-period, perform carrier reconstruction, do not use the non-existent switch state, and select the optimal decomposed phase according to the midpoint voltage, non-fault phase current direction and control force, where the control force refers to the control ability of the midpoint voltage, and calculate the control quantity. Decompose the modulation wave according to the calculated control quantity, specifically: Adopt two carriers with an amplitude of 1 in the healthy half-period, and still output three levels of [P], [O], and [N]; adopt a carrier with an amplitude of 2 in the fault half-period, and only output two levels of [P] and [N], avoid using the non-existent switch state [O], eliminate the distortion of the output current, and achieve fault-tolerant control; Based on the modulated wave after injecting the zero-sequence component, a neutral point current model is established. If the modulated wave is greater than 0, the modulated wave is decomposed into m x + y and − y . If the modulated wave is less than 0, the modulated wave is decomposed into m x − y and y . Among them, is the modulated wave, x represents the three phases a, b, and c, and they are respectively compared with the positive load wave. By adjusting the control quantity y , the duty cycle of the [O] state is adjusted.

2. The inverter fault tolerance and neutral point voltage balance control method based on modulation wave decomposition according to claim 1, characterized in that, The switch states of the switch tubes of the inverter include three types: [P], [O], and [N].

3. A fault-tolerant and neutral point voltage balance control method for an inverter based on modulation wave decomposition according to claim 1, characterized in that, The initial modulation wave includes three-phase initial modulation waves and represents the maximum value, intermediate value, and minimum value of the three-phase initial modulation waves.

4. A fault-tolerant and neutral point voltage balance control method for an inverter based on modulation wave decomposition according to claim 3, characterized in that, The specific process of injecting the zero-sequence component into the initial modulation wave includes injecting the negative of the average value of the maximum value and the minimum value of the corresponding phase initial modulation wave into each phase initial modulation wave.

5. A fault-tolerant and neutral point voltage balance control method for an inverter based on modulation wave decomposition as claimed in claim 1, characterized in that, The specific process of dividing a fundamental wave period into a healthy half-period and a fault half-period according to the fault phase current direction includes, when a switch tube fails, analyzing the current path before and after the switch tube fails, obtaining the non-existent switch state according to the fault phase current direction, and dividing a fundamental wave period into a healthy half-period and a fault half-period.

6. The inverter fault tolerance and neutral point voltage balance control method based on modulation wave decomposition according to claim 1 is characterized in that, in In the healthy half-period, decompose the middle phase, and the control quantity accurately calculated according to the three-phase modulation wave, three-phase current, and midpoint voltage is expressed as Among them, C is the capacitance value of the DC-side capacitor, ΔV PN is the difference between the upper and lower capacitor voltages on the DC side, T s is the sampling period, m max 、 m mid and m min are the maximum value, intermediate value, and minimum value of the modulation wave respectively, i max 、 i mid and i min are the corresponding phase currents respectively; The control quantity needs to satisfy: 。 7. A fault-tolerant and neutral point voltage balance control method for an inverter based on modulation wave decomposition according to claim 1, characterized in that, In the fault half-period, when the voltage difference between the upper and lower capacitors on the DC side is greater than or equal to zero, the voltage of the lower capacitor is lower than that of the upper capacitor, and decompose the corresponding phase to reduce the midpoint current; If the midpoint current increases in all phases, no decomposition is performed.

8. A fault-tolerant and neutral point voltage balance control method for an inverter based on modulation wave decomposition according to claim 7, characterized in that, When a is the fault phase, the control quantity of the corresponding phase is: n is b or c , i a , i b , i c are respectively a 、 b 、 c phase currents, C is the capacitance value of the DC-side capacitor, ΔV PN is the difference between the upper and lower capacitor voltages on the DC side, T s is the sampling period, is the modulation wave of phase b, is the modulation wave of phase c.

9. A fault-tolerant and neutral point voltage balancing control system for an inverter based on modulation wave decomposition, characterized in that, It includes: A modulation range increasing module configured to generate an initial modulation wave and inject a zero-sequence component into the initial modulation wave; A fundamental wave period decomposition module configured to analyze the current path in the case of a switch tube fault and divide a fundamental wave period into a healthy half-period and a fault half-period according to the fault phase current direction; A first control module configured to decompose the modulation wave of the middle phase and calculate the control quantity in the healthy half-period; A second control module configured to perform carrier reconstruction in the fault half-period, not use the non-existent switch state, select the optimal decomposed phase according to the midpoint voltage, non-fault phase current direction and control force, where the control force refers to the control ability of the midpoint voltage, and calculate the control quantity. Decompose the modulation wave according to the calculated control quantity, specifically: During the healthy half-cycle, two carriers with an amplitude of 1 are used, and three levels of [P], [O], and [N] are still output; during the fault half-cycle, a carrier with an amplitude of 2 is used, and only two levels of [P] and [N] are output, avoiding the use of the non-existent switch state [O], eliminating the distortion of the output current, and realizing fault-tolerant control; Based on the modulation wave after injecting the zero-sequence component, a neutral-point current model is established. If the modulation wave is greater than 0, the modulation wave is decomposed into m x + y and − y . If the modulation wave is less than 0, the modulation wave is decomposed into m x − y and y . Among them, is the modulation wave, x represents the three phases a, b, and c, which are respectively compared with the positive load wave, and the duty ratio of the [O] state is adjusted by adjusting the control quantity y .

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