A fault-tolerant control method and system for inverter under high and low voltage ride-through conditions

Abstract

The present invention provides a fault-tolerant control method and system for inverter high / low voltage ride-through conditions. The method performs high / low voltage ride-through of the inverter during a power grid fault. The method analyzes the current path in the event of an inverter switch failure to obtain a voltage vector that does not exist in the event of a switch failure. The method calculates the leading and lagging maximum power factor angles and analyzes the sectors in which the reference current may exist. Under different current polarities, the method discards nonexistent space vectors and selects usable vectors for synthesis to achieve fault-tolerant control. A small vector that affects the midpoint voltage in the opposite direction to the midpoint vector is selected, and the duty cycle is accurately calculated to suppress midpoint voltage oscillations. A compensation value is introduced into the duty cycle of the small vector to eliminate midpoint voltage deviations. The present invention simultaneously achieves fault-tolerant control and midpoint voltage balance control in the inverter high / low voltage ride-through conditions, ensuring safe and reliable operation of the inverter system.

Description

Technical Field

[0001] The present invention belongs to the technical field of inverter fault-tolerant control, and in particular relates to a fault-tolerant control method and system for an inverter under high and low voltage ride-through conditions. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] As the interface between the generator and the grid or load, the inverter is an important component for the safe, stable and reliable operation of renewable energy power generation systems.

[0004] Compared to two-level inverters, three-level inverters experience less switching stress on their switching devices and produce better output waveform quality. Among three-level inverters, those based on a T-type topology are widely used in photovoltaic power generation due to their advantages, including a small number of switches, low conduction losses, and uniform power loss.

[0005] On the one hand, as the penetration rate of photovoltaic systems increases, their impact on the power grid is increasing, leading to frequent grid failures. When the grid voltage drops or rises, the photovoltaic system must remain connected and operate continuously within specific time intervals and voltage sag zones, transmitting reactive power to the grid to support the grid until normal operation is restored. Inverters with this high and low voltage ride-through capability have become a prerequisite for grid connection.

[0006] On the other hand, during the inverter's high and low voltage ride-through period, overcurrent or overvoltage can easily occur, seriously threatening the normal operation of the inverter's switching transistors and significantly increasing the probability of switch failure. When a switch fails, the system's output current becomes severely distorted, and the total harmonic distortion of the current exceeds grid-connected requirements, potentially endangering system and personnel safety. Furthermore, if the inverter goes offline due to a switch failure, it will be unable to provide support to the grid. Therefore, fault-tolerant control during inverter high and low voltage ride-through conditions is particularly important.

[0007] Typically, switching tube failures are categorized as short circuit and open circuit. When a short circuit occurs, the instantaneous high current generated in the power circuit often burns out the power devices or even the entire converter system, necessitating an emergency shutdown to ensure the safety of the rest of the system. Open circuit failures distort the system's grid-connected current and can further damage other components. Unlike short circuits, open circuits do not instantly cause catastrophic damage to power devices or the entire system. These faults can be mitigated through control methods, reducing losses caused by downtime. This method of mitigating the effects of faults is known as fault-tolerant control.

[0008] Furthermore, maintaining a balanced DC midpoint voltage is crucial for ensuring safe and stable inverter operation. Unbalanced midpoint voltage can trigger the system's DC over / undervoltage protection, causing overvoltage on DC capacitors and power devices, and degrading system stability. Summary of the Invention

[0009] In order to solve the above problems, the present invention proposes a fault-tolerant control method and system for the inverter under high and low voltage ride-through conditions. High and low voltage ride-through is achieved by injecting the negative sequence component of the grid voltage and changing the active and reactive currents according to the ride-through requirements. The available vectors are selected and the action time is accurately calculated to achieve fault-tolerant control and midpoint voltage balance control at the same time, ensuring the safe and reliable operation of the inverter system.

[0010] According to some embodiments, the present invention adopts the following technical solutions:

[0011] A fault-tolerant control method for an inverter under high and low voltage ride-through conditions comprises the following steps:

[0012] Perform high / low voltage ride-through of the inverter during grid faults;

[0013] Analyze the current path in the case of an inverter switch failure and obtain the voltage vector that does not exist in the case of a switch failure;

[0014] Calculate the maximum leading and lagging power factor angles and analyze the sectors where the reference current may exist;

[0015] Under different current polarities, the non-existent space vectors are discarded and the available vectors are selected for synthesis to achieve fault-tolerant control.

[0016] Select a small vector that affects the midpoint voltage in the opposite direction to the direction of the neutral vector, and accurately calculate the duty cycle to suppress the midpoint voltage oscillation;

[0017] A compensation amount is introduced into the duty cycle of the small vector to eliminate the midpoint voltage deviation.

[0018] As an optional embodiment, the inverter is a T-type three-level inverter system, including A-phase, B-phase and C-phase bridge arms, each phase bridge arm includes four power switching tubes, consisting of two half-bridge switching tubes and two midpoint switching tubes, and the DC side includes two capacitors connected in series, and a neutral point is formed in the middle of the two capacitors.

[0019] As an optional implementation, each phase bridge arm of the inverter has three working states [P], [O], and [N], with the neutral point of the two capacitors on the DC side as the reference point.

[0020] As an optional implementation, the specific process of performing high / low voltage ride-through of the inverter during a grid fault includes, during a grid fault, changing the reference values ​​of active and reactive currents according to the degree of grid voltage drop or rise, and injecting the negative sequence component of the grid voltage into the initial modulation wave.

[0021] As an optional implementation method, the specific process of analyzing the current path in the event of an inverter switch failure and obtaining a voltage vector that does not exist in the event of a switch failure includes: when only considering the failure of the midpoint switch and a single switch failure, analyzing the current path of each phase, determining a state that cannot be generated, and obtaining a non-existent voltage vector.

[0022] As an optional implementation method, the specific process of calculating the maximum leading and lagging power factor angles and analyzing the sectors where the reference current may exist includes re-determining the position of the reference current based on the adjustment of the reference values ​​of the active and reactive currents, calculating the maximum leading power factor angle and the maximum lagging power factor angle respectively, and performing separate analysis in different sectors with reference voltage to determine the sector where the reference current may be located.

[0023] As an optional implementation, in different current polarity situations, the specific process of discarding non-existent space vectors and selecting usable vectors for synthesis to achieve fault-tolerant control includes determining the current polarity situation based on the possible sectors where the reference current is located, and performing separate control in each current polarity situation;

[0024] Determine whether the midpoint voltage of the center vector increases or decreases in this case. If the effect of the available small vector is opposite to that of the center vector, select the available small vector and calculate the corresponding duty cycle. If the effect of the available small vector is the same as that of the center vector, adopt a vector sequence that does not use the small vector and calculate the duty cycle.

[0025] As an optional implementation, if the initial value of the mid-point voltage is not zero, a compensation amount is introduced into the duty cycle of the small vector to balance the mid-point voltage deviation.

[0026] A fault-tolerant control system for an inverter under high and low voltage ride-through conditions, comprising:

[0027] A fault ride-through module is configured to perform high / low voltage ride-through of the inverter in the event of a grid fault;

[0028] a current path analysis module configured to analyze the current path in the event of a switch failure of the inverter and obtain a voltage vector that does not exist in the event of a switch failure;

[0029] a sector analysis module configured to calculate leading and lagging maximum power factor angles and analyze sectors where the reference current may exist;

[0030] A fault-tolerant control module is configured to discard non-existent space vectors and select usable vectors for synthesis to achieve fault-tolerant control under different current polarity conditions;

[0031] an oscillation suppression module configured to select a small vector that affects the midpoint voltage in a direction opposite to that of the midpoint vector, and accurately calculate a duty cycle to suppress midpoint voltage oscillation;

[0032] The compensation module is configured to introduce a compensation amount into the duty cycle of the small vector to eliminate the midpoint voltage deviation.

[0033] As an optional embodiment, the compensation amount is generated by a proportional controller.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] This invention implements fault-tolerant control of a T-type three-level inverter under high and low voltage ride-through conditions. In the event of grid and switch failures, this technology achieves inverter fault ride-through while reducing the total harmonic distortion of the three-phase output current, maintaining DC side midpoint voltage balance and suppressing midpoint voltage oscillations.

[0036] The present invention realizes high and low voltage ride-through of a T-type three-level inverter system by injecting a negative sequence component of the grid voltage and modifying the active and reactive current reference values, thereby providing support for grid recovery.

[0037] The present invention discards the non-existent space vectors caused by the switch tube failure and only uses the existing vectors, so that the T-type three-level inverter system can reduce the inverter output current distortion rate in the event of a switch tube failure, maintain the safe and stable operation of the system, and ensure the reliability of the system.

[0038] The present invention realizes balanced control and oscillation suppression of DC side midpoint voltage deviation by accurately calculating the duty cycle of the selected vector and introducing compensation into the small vector.

[0039] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, preferred embodiments are given below and described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The accompanying drawings, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0041] Figure 1 The T-type three-level inverter system provided in this embodiment;

[0042] Figure 2 A three-level space vector diagram provided in this embodiment;

[0043] Figure 3 The grid voltage fault type and required reactive current provided for this embodiment;

[0044] Figure 4 The switch tube S provided in this embodiment a2 Current path diagram of phase A before and after the fault occurs;

[0045] Figure 5 The possible reference current position when the reference voltage provided in this embodiment is in sector 1;

[0046] Figure 6 A system control block diagram provided for this embodiment;

[0047] Figure 7 The switch tube S provided in this embodiment a2 Output waveforms of different grid voltage fault types during faults. DETAILED DESCRIPTION

[0048] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0049] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present invention belongs.

[0050] 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 form is intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0051] This embodiment is directed to a T-type three-level inverter system. Figure 1 The topology diagram of a T-type three-level inverter system includes the A, B, and C phases. Each phase leg includes four power switches, consisting of two half-bridge (HB) switches and two neutral-point (NP) switches. The DC side includes two series-connected capacitors, with the neutral point formed between them.

[0052] Specifically, each phase arm of the T-type three-level inverter system has three operating states: [P], [O], and [N]. Taking the neutral point of the two capacitors on the DC side as the reference point, Table 1 shows the switching state, output voltage, and operating conditions of each power switch tube in each phase arm of the T-type three-level inverter system.

[0053] Table 1 Switching status, output voltage and switching tube working conditions of T-type three-level inverter

[0054]

[0055] Different switching states constitute 27 vectors, including 3 zero vectors, 12 small vectors, 6 medium vectors and 6 large vectors. The space vector diagram is as follows: Figure 2 shown.

[0056] Common grid voltage fault types include three-phase symmetrical fault (Type A fault) and single-phase asymmetrical fault (Type B fault). The fault type and the compensation curve of the inverter sending reactive current are as follows: Figure 3 As shown in Figure 1, the negative sequence component of the grid voltage is injected into the modulation wave, and the active and reactive current reference values ​​are obtained based on the relationship between the sag depth and the reactive compensation current, thus achieving low voltage ride-through of the T-type three-level inverter. Similarly, high voltage ride-through is achieved using the same method.

[0057] During the inverter fault ride-through period, the overcurrent or overvoltage generated can easily cause the inverter switch tube to fail. When the HB tube fails, the modulation index needs to be reduced to below 0.5 to achieve fault-tolerant control, which is not desirable for photovoltaic grid-connected inverter systems. Therefore, this invention only considers the case of NP tube failure. At the same time, multiple switch tubes rarely fail at the same time. This invention uses S a2 Take the fault as an example for analysis, the current path of phase A is as follows Figure 4 As shown. When i a >0, the [O] state of phase A cannot be generated, that is, the vector of phase A in the [O] state will not exist. At this time, if the non-existent vector is used to synthesize the reference vector, the output current will be distorted; when i a <0, the current path is consistent with the normal situation. a When >0, the vector with phase A in the [O] state is not used for synthesis, current distortion will be eliminated, and fault-tolerant control can be achieved.

[0058] As mentioned above, in order to achieve the inverter's fault ride-through, the reference values ​​of the active and reactive currents change according to the ride-through requirements, which in turn causes the power factor to change. Therefore, the position of the reference current needs to be re-determined. The maximum leading power factor angle and the maximum lagging power factor angle are:

[0059]

[0060] like Figure 5 As shown, we take the reference voltage in sector 1 as an example for analysis. At this time, the reference current may be in sectors 1, 2, 10, 11, and 12. This means there will be three possible three-phase current polarities: +--, ++-, and +-+. Therefore, we will discuss three cases:

[0061] Case 1: The polarity of the three-phase current is +--.

[0062] At this time, the middle vector [PON] increases the midpoint voltage. However, all available small vectors [POO], [PPO], [NON], [NOO], [NNO], and [POP] also increase the midpoint voltage. Therefore, in order to prevent the midpoint voltage from further deteriorating, a five-segment vector sequence [PPP]-[PON]-[PNN]-[PON]-[PPP] is adopted without using small vectors. The duty cycle calculation at this time is as follows

[0063]

[0064] Where M is the modulation index and θ is the reference voltage phase angle.

[0065] Case 2: The three-phase current polarity is ++-.

[0066] At this time, the middle vector [PON] reduces the midpoint voltage. In order to control the midpoint voltage, the small vector [POO] that increases the midpoint voltage will be used. The vector sequence at this time is [PPP]-[POO]-[PON]-[PNN]-[PON]-[POO]-[PPP]. In order to suppress the midpoint voltage oscillation, the midpoint voltage reduced by the middle vector should be equal to the midpoint voltage increased by the small vector. The initial duty cycle of the small vector [POO] is

[0067]

[0068] However, if the initial value of the midpoint voltage is not 0, the duty cycle calculated by the above formula will not be able to balance the midpoint voltage. Therefore, it is necessary to introduce a compensation amount into the duty cycle of the small vector to balance the midpoint voltage deviation, which can be generated by a proportional controller. According to the volt-second balance principle, the final duty cycle is expressed as

[0069]

[0070] Where k is the compensation factor, Δv PN is the midpoint voltage deviation. At the same time, since the duty cycle cannot be negative, it is necessary to satisfy

[0071] d S ≤min{2d L0 ,2d Z0} (5)

[0072] Case 3: The polarity of the three-phase current is +-+.

[0073] At this time, the middle vector [PON] increases the midpoint voltage, and the small vector [PPO] that reduces the midpoint voltage is selected. The vector sequence at this time is [PPP]-[PPO]-[PON]-[PNN]-[PON]-[PPO]-[PPP], and the duty cycle is calculated as

[0074]

[0075] It should be noted that when the small vector increases the midpoint voltage, the sign of the compensation amount is positive, and when the small vector reduces the midpoint voltage, the sign of the compensation amount is negative. At the same time, the range of the duty cycle needs to meet

[0076] 0≤d S ≤min{d M0 ,2d Z0} (7)

[0077] The calculation method for other sectors can be implemented in the same way. Finally, the vector sequences of all sectors under different current polarities are summarized in Table 2.

[0078] Table 2 Vector sequences of all sectors under different current polarities

[0079]

[0080]

[0081]

[0082] It is worth noting that in sectors 3, 4, 9, and 10, the median vector is in i a It cannot be used when >0. In this case, the zero vector and the two large vectors adjacent to this sector are synthesized. Since the middle vector and the small vector are not used, there is no effect on the midpoint voltage.

[0083] The above method realizes fault-tolerant control and midpoint voltage balance control of T-type three-level inverter under high and low voltage ride-through conditions based on vector selection and accurate calculation of action time. Figure 6 Shown is the control block diagram of the system.

[0084] Figure 7 The output waveform of the T-type three-level inverter system when this method is used includes the three-phase grid voltage, three-phase output current, phase voltage of phase A, line voltage between phase A and phase B, and voltage of two capacitors on the DC side. It can be seen that no matter what kind of fault occurs in the three-phase grid, the three-phase current does not increase or decrease, proving that this method can achieve high and low voltage ride-through of the T-type three-level inverter. At the same time, in the switching tube S a2During a fault, the three-phase current THDi remained consistently low, meeting the inverter grid-connected requirements, demonstrating that this method simultaneously achieves fault-tolerant control. More importantly, the midpoint voltage remained balanced with minimal oscillation, demonstrating that this method simultaneously achieves DC side midpoint voltage balance control and oscillation suppression.

[0085] In summary, the present invention achieves high and low voltage ride-through by injecting the negative sequence component of the grid voltage and modifying the active and reactive currents. By selecting the vector, the use of the space vector in the event of a switch failure is avoided, achieving fault-tolerant control. By accurately calculating the duty cycle and introducing a compensation factor, midpoint voltage balance control and oscillation suppression are achieved.

[0086] It will be understood by those skilled in the art that embodiments of the present invention may be provided as methods, systems, or computer program products. Thus, the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment combining software and hardware. Furthermore, the present invention may take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to magnetic disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0087] The present invention is described with reference to flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to embodiments of the present invention. It should be understood that each process and / or block in the flowcharts and / or block diagrams, as well as combinations of processes 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 a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowcharts and / or block diagrams. Figure 1 a process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0088] These computer program instructions may also be stored in a computer readable memory that can direct a computer or other programmable data processing device to work in a specific manner, so that the instructions stored in the computer readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 a process or multiple processes and / or boxes Figure 1 The function specified in one or more boxes.

[0089] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operational steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing the instructions executed on the computer or other programmable device for implementing the process. Figure 1 a process or multiple processes and / or boxes Figure 1 A step that specifies a function in one or more boxes.

[0090] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Various modifications and variations are readily apparent to those skilled in the art. Any modifications, equivalent substitutions, or improvements made by those skilled in the art that fall within the spirit and principles of the present invention and do not require creative effort are intended to be within the scope of protection of the present invention.

Claims

1. A fault-tolerant control method for an inverter under high and low voltage ride-through conditions, characterized in that: The following steps are involved: Perform high / low voltage ride-through of the inverter during grid faults; Analyze the current path in the case of an inverter switch failure and obtain the voltage vector that does not exist in the case of a switch failure; Calculate the maximum leading and lagging power factor angles and analyze the sectors where the reference current may exist; Under different current polarities, the non-existent space vectors are discarded and the available vectors are selected for synthesis to achieve fault-tolerant control. Select a small vector that affects the midpoint voltage in the opposite direction to the direction of the neutral vector, and accurately calculate the duty cycle to suppress the midpoint voltage oscillation; A compensation amount is introduced into the duty cycle of the small vector to eliminate the midpoint voltage deviation; When the reference voltage is in sector 1, the reference current may be in sectors 1, 2, 10, 11, or 12. There are three possible three-phase current polarities: 、 and , specifically: Case 1: The polarity of the three-phase current is : At this time, the middle vector [PON] increases the midpoint voltage; all available small vectors [POO], [PPO], [NON], [NOO], [NNO], and [POP] increase the midpoint voltage; a five-segment vector sequence [PPP]-[PON]-[PNN]-[PON]-[PPP] without using small vectors is adopted. The duty cycle at this time is calculated as: (2) in M To adjust the system, θ is the reference voltage phase angle; Case 2: The polarity of the three-phase current is : At this time, the middle vector [PON] reduces the midpoint voltage. To control the midpoint voltage, the small vector [POO] that increases the midpoint voltage is used. The vector sequence is [PPP]-[POO]-[PON]-[PNN]-[PON]-[POO]-[PPP]. To suppress midpoint voltage oscillation, the midpoint voltage reduced by the middle vector should be equal to the midpoint voltage increased by the small vector. The initial duty cycle of the small vector [POO] is: (3) If the initial value of the midpoint voltage is not 0, the duty cycle calculated by the above formula will not be able to balance the midpoint voltage. Therefore, a compensation amount needs to be introduced into the duty cycle of the small vector to balance the midpoint voltage deviation. This can be generated by a proportional controller. According to the volt-second balance principle, the final duty cycle is expressed as: (4) in k is the compensation factor, Δ v PN is the midpoint voltage deviation; at the same time, since the duty cycle cannot be negative, it needs to meet the following requirements: (5) Case 3: The polarity of the three-phase current is : At this time, the middle vector [PON] increases the midpoint voltage, and the small vector [PPO] that reduces the midpoint voltage is selected. The vector sequence at this time is [PPP]-[PPO]-[PON]-[PNN]-[PON]-[PPO]-[PPP], and the duty cycle is calculated as: (6) When the small vector increases the midpoint voltage, the sign of the compensation amount is positive, and when the small vector reduces the midpoint voltage, the sign of the compensation amount is negative. At the same time, the duty cycle range needs to meet the following requirements: (7)。 2. The fault-tolerant control method for an inverter under high and low voltage ride-through conditions according to claim 1, wherein: The inverter is a T-type three-level inverter system, including A-phase, B-phase and C-phase bridge arms. Each phase bridge arm includes four power switching tubes, consisting of two half-bridge switching tubes and two midpoint switching tubes. The DC side includes two capacitors connected in series, and the middle of the two capacitors forms a neutral point. Each phase bridge arm has three working states: [P], [O], and [N], with the neutral point of the two capacitors on the DC side as the reference point.

3. The fault-tolerant control method for an inverter under high and low voltage ride-through conditions as claimed in claim 1, characterized in that: The specific process of performing high / low voltage ride-through of the inverter during a grid fault includes changing the reference values ​​of active and reactive currents according to the degree of grid voltage drop or rise during a grid fault, and injecting the negative sequence component of the grid voltage into the initial modulation wave.

4. The fault-tolerant control method for an inverter under high and low voltage ride-through conditions as claimed in claim 1, wherein: The specific process of analyzing the current path under the condition of an inverter switch failure and obtaining a voltage vector that does not exist under the condition of the switch failure includes: considering only the failure of a single midpoint switch, analyzing the current path of the faulty phase, determining a state that cannot be generated, and obtaining a non-existent voltage vector.

5. The fault-tolerant control method for an inverter under high and low voltage ride-through conditions as claimed in claim 1, characterized in that: The specific process of calculating the maximum leading and lagging power factor angles and analyzing the sectors where the reference current may exist includes re-determining the position of the reference current based on the adjustment of the active and reactive current reference values, respectively calculating the maximum leading power factor angle and the maximum lagging power factor angle, and performing separate analysis in different sectors with reference voltage to determine the sector where the reference current may be located.

6. The fault-tolerant control method for an inverter under high and low voltage ride-through conditions as claimed in claim 1, wherein Under different current polarity conditions, the specific process of discarding non-existent space vectors and selecting usable vectors for synthesis to achieve fault-tolerant control includes determining the current polarity according to the possible sectors where the reference current is located, and performing separate control under each current polarity condition; Determine whether the midpoint voltage of the center vector increases or decreases in this case. If the effect of the available small vector is opposite to that of the center vector, select the available small vector and calculate the corresponding duty cycle. If the effect of the available small vector is the same as that of the center vector, adopt a vector sequence that does not use the small vector and calculate the duty cycle.

7. The fault-tolerant control method for an inverter under high and low voltage ride-through conditions as claimed in claim 6, characterized in that: If the initial value of the midpoint voltage is not zero, a compensation amount is introduced into the duty cycle of the small vector to balance the midpoint voltage deviation.

8. The fault-tolerant control method for an inverter under high and low voltage ride-through conditions as claimed in claim 7, characterized in that: The compensation amount is generated by a proportional controller.

9. The fault-tolerant control method for an inverter under high and low voltage ride-through conditions as claimed in claim 6, wherein: The calculated duty cycle is limited to avoid a negative duty cycle.

10. A fault-tolerant control system for an inverter under high and low voltage ride-through conditions, based on a fault-tolerant control method for an inverter under high and low voltage ride-through conditions as claimed in any one of claims 1 to 9, characterized in that: include: A fault ride-through module is configured to perform high / low voltage ride-through of the inverter in the event of a grid fault; a current path analysis module configured to analyze the current path in the event of a switch failure of the inverter and obtain a voltage vector that does not exist in the event of a switch failure; a sector analysis module configured to calculate leading and lagging maximum power factor angles and analyze sectors where the reference current may exist; A fault-tolerant control module is configured to discard non-existent space vectors and select usable vectors for synthesis to achieve fault-tolerant control under different current polarity conditions; an oscillation suppression module configured to select a small vector that affects the midpoint voltage in a direction opposite to that of the midpoint vector, and accurately calculate a duty cycle to suppress midpoint voltage oscillation; The compensation module is configured to introduce a compensation amount into the duty cycle of the small vector to eliminate the midpoint voltage deviation.

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