Fault diagnosis method for open-winding dual three-level inverter based on common-mode voltage difference value

Abstract

The invention relates to the technical field of open-winding dual-inverter open-circuit faults, and provides a common-mode voltage-based open-winding dual three-level inverter open-circuit fault diagnosis method, which comprises the following steps of: constructing an MLD common-mode voltage model of a dual three-level inverter according to a switching function; obtaining a fault characteristic quantity according to an MLD common-mode voltage model of the dual three-level inverter; judging an initial phase of the fault characteristic quantity, and identifying a fault bridge arm pair according to the initial phase of the fault characteristic quantity; obtaining a fault characteristic value according to the fault bridge arm pair and the fault characteristic quantity; and judging a fault type according to the fault characteristic value and a threshold value. According to the invention, the difference between the ideal common-mode voltage and the actual common-mode voltage is used as the characteristic value to detect the fault, and the specific fault switch tube is positioned according to the specific phase, amplitude and positive and negative of the characteristic value, so that the fault diagnosis and positioning of the specific switch tube can be completed only by detecting the common-mode voltage.

Description

Technical Field

[0001] The invention relates to the technical field of open-circuit fault diagnosis of open-winding dual inverters, and in particular to a fault diagnosis method of open-winding dual three-level inverters based on common-mode voltage difference. Background Art

[0002] The dual converter cascade multi-level topology structure, the two-terminal converter can flexibly combine any existing converter. With its multi-level output characteristics, effective improvement of DC voltage utilization, low-cost fault redundancy and other advantages, it is widely used in new energy power generation fields such as wind power generation, photovoltaic power generation, and new energy electric vehicles. Statistics show that more than one-third of the converter system failures are caused by the failure of the switching device, which shows that the switching device is one of the most vulnerable parts of the entire system. Among these switching device failures, there are two different types of faults: short circuit and open circuit. Modern drive circuits have mature protection mechanisms for short circuit faults, so it is relatively easy to detect. After the open circuit fault occurs, it is difficult to detect the fault because the system can continue to run with the fault for a period of time. Therefore, it is very important to quickly and accurately detect and locate the faulty IGBT. In the open winding system, fault diagnosis methods can be roughly divided into two categories: traditional fault diagnosis methods based on models and new fault diagnosis methods based on artificial intelligence. The model-based method extracts fault features by building a mathematical model of the system, which is convenient for engineering application and has strong practicality and reliability; while the artificial intelligence-based method uses machine learning, neural network and other technologies for fault identification. Although it is powerful, it is more complicated to implement and requires more data support. However, most studies focus on single inverter topologies, and the fault diagnosis in open winding systems is not well explored. Although some studies have attempted to diagnose open circuit faults, they have failed to achieve accurate fault location. Summary of the invention

[0003] The present invention aims to solve at least one of the technical problems existing in the related art. To this end, the present invention provides an open-winding dual three-level inverter fault diagnosis method based on common-mode voltage difference, which completes the fault diagnosis and positioning of a specific switch tube by detecting the common-mode voltage.

[0004] The present invention provides a method for diagnosing faults of an open-winding dual three-level inverter based on a common-mode voltage difference, comprising: S1: Construct the MLD (Mixed Logical Dynamical) common mode voltage model of the dual three-level inverter based on the switching function; S2: Obtain fault characteristic quantity according to the MLD common mode voltage model of the dual three-level inverter; S3: determining the initial phase of the fault characteristic quantity, and identifying the faulty bridge arm pair according to the initial phase of the fault characteristic quantity; S4: Obtaining a fault characteristic value according to the fault bridge arm pair and the fault characteristic quantity; S5: Determine the fault type based on the fault characteristic value and threshold.

[0005] According to a common-mode voltage difference-based open-winding dual three-level inverter fault diagnosis method provided by the present invention, the method further includes inputting a switching signal into an MLD model to calculate the common-mode voltage value under an ideal normal working state in real time, thereby constructing an MLD common-mode voltage model of the dual three-level inverter. The MLD common-mode voltage model of the dual three-level inverter is: in, is the system common mode voltage under ideal conditions and normal working conditions calculated in MLD, is the switching function of phase A of inverter 1, is the switching function of phase B of inverter 1, The switching function of phase C of inverter 1, is the switching function of phase A of inverter 2, is the switching function of phase B of inverter 2, is the switching function of phase C of inverter 2, is a DC voltage.

[0006] According to a common mode voltage difference based open winding dual three-level inverter fault diagnosis method provided by the present invention, the difference between the ideal common mode voltage and the actual common mode voltage is also used as a fault characteristic quantity, and the calculation expression of the fault characteristic quantity is: in, is the fault characteristic quantity, is the common mode voltage value actually collected in the circuit, It is the system common mode voltage under ideal conditions and normal working conditions calculated in the MLD common mode voltage model of the dual three-level inverter.

[0007] According to the present invention, a fault diagnosis method for an open-winding dual three-level inverter based on common-mode voltage difference is provided, which further includes step S3, dividing each power frequency cycle into N equal parts by a sector judgment module.

[0008] According to a common mode voltage difference based open winding dual three-level inverter fault diagnosis method provided by the present invention, when an upper bridge arm outer tube fails, the fault characteristic value of the upper bridge arm is: in, is the fault characteristic value of the upper bridge arm, For inverter 1 Phase ideal common mode voltage, is a DC voltage.

[0009] According to a common mode voltage difference based open winding dual three-level inverter fault diagnosis method provided by the present invention, when the outer tube of the lower bridge arm fails, the fault characteristic value of the lower bridge arm is: in, is the fault characteristic value of the lower bridge arm, For inverter 2 Phase ideal common mode voltage, is a DC voltage.

[0010] According to a common mode voltage difference based open winding dual three-level inverter fault diagnosis method provided by the present invention, when the internal tube of the upper bridge arm fails, both the upper and lower bridge arms are not conducting, and the fault characteristic value of the upper bridge arm is: in, is the fault characteristic value of the upper bridge arm, For inverter 1 Phase ideal common mode voltage, For inverter 2 The actual common-mode voltage of the phase.

[0011] According to a common mode voltage difference based open winding dual three-level inverter fault diagnosis method provided by the present invention, when the inner tube of the lower bridge arm fails, both the upper and lower bridge arms are not conducting, and the fault characteristic value of the lower bridge arm is: in, is the fault characteristic value of the lower bridge arm, For inverter 2 Phase ideal common mode voltage, For inverter 1 The actual common-mode voltage of the phase.

[0012] According to a common mode voltage difference-based open-winding dual three-level inverter fault diagnosis method provided by the present invention, the method also includes judging the faulty bridge arm pair by the positive and negative of the fault characteristic value. If the fault characteristic value is positive, the upper bridge arm of inverter 1 is faulty; if the fault characteristic value is negative, the lower bridge arm of inverter 2 is faulty.

[0013] According to a common-mode voltage difference-based open-winding dual three-level inverter fault diagnosis method provided by the present invention, the method also includes judging the inner and outer tube faults by comparing the fault characteristic value with the threshold, calculating the outer tube fault characteristic value and the inner tube fault characteristic value under ideal conditions, taking the value between the outer tube fault characteristic value and the inner tube fault characteristic value as the threshold, if the fault characteristic value is greater than or equal to the threshold, it is an inner tube fault, and if the fault characteristic value is less than the threshold, it is an outer tube fault.

[0014] The above one or more technical solutions in the embodiments of the present invention have at least one of the following technical effects: The present invention provides an open-winding dual three-level inverter fault diagnosis method based on common-mode voltage difference, which completes the fault diagnosis and positioning of a specific switch tube by detecting the common-mode voltage. The fault diagnosis method of the present invention can realize accurate fault positioning for a total of 24 switch tubes of the dual three-level inverter. It is not affected by the parameters of the motor body, and can ensure the obvious distinction of fault characteristics under any parameters. The present invention integrates the entire fault location process into one power frequency cycle, so it takes at most one power frequency cycle to complete the fault location. The present invention can cooperate with the traditional current mean method. After using the traditional mean to judge the switch pair, the difference in common-mode voltage is used to specifically locate the double-tube fault. The change in the fault characteristic quantity is still obvious, and there is no need to add active interference to the control strategy after the fault.

[0015] Additional aspects and advantages of the present invention will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0017] Figure 1 The present invention is a flowchart of a method for diagnosing faults of an open-winding dual three-level inverter based on common-mode voltage difference.

[0018] Figure 2 4 is a topology diagram of a dual three-level inverter with a common DC bus according to an embodiment of the present invention.

[0019] Figure 3 The k phase of the embodiment of the present invention is analyzed. Schematic diagram of the current flow path of phase k under normal operating conditions and P level.

[0020] Figure 4 The k phase of the embodiment of the present invention is analyzed. Schematic diagram of the k-phase current flow path when the external tube fails.

[0021] Figure 5 The k phase of the embodiment of the present invention is analyzed. Schematic diagram of the k-phase current flow path under normal operating conditions and O / N level.

[0022] Figure 6 The k phase of the embodiment of the present invention is analyzed. Schematic diagram of the current flow path of phase k under normal operating conditions and P / O level.

[0023] Figure 7 The k phase of the embodiment of the present invention is analyzed. Schematic diagram of the k-phase current flow path when the inner tube fails.

[0024] Figure 8 The k phase of the embodiment of the present invention is analyzed. Schematic diagram of the current flow path of phase k under normal operating conditions and N level. DETAILED DESCRIPTION

[0025] In order to make the purpose, technical scheme and advantages of the present invention clearer, the technical scheme in the present invention will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in the field without creative work are within the scope of protection of the present invention. The following embodiments are used to illustrate the present invention, but cannot be used to limit the scope of the present invention.

[0026] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the embodiment of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

[0027] Combine the following Figures 1 to 8 The present invention describes a fault diagnosis method for an open-winding dual three-level inverter based on a common-mode voltage difference.

[0028] like Figure 1 As shown, a fault diagnosis method for an open-winding dual three-level inverter based on a common-mode voltage difference comprises: S1: Construct the MLD common-mode voltage model of the dual three-level inverter based on the switching function; The topology diagram of the dual three-level inverter with a common DC bus is as follows: Figure 2 As shown, the capacitor and capacitor There are two equal DC input capacitors with voltage division and energy buffering functions. Each phase of inverter 1 (IN1) includes 4 IGBT switch tubes S K11 ~S K14 And two clamping diodes D k15 , D k16 Each phase of inverter 2 (IN2) includes 4 IGBT switches S K21 ~S K24 And two clamping diodes D k25 , D k26 , (k=a, b, c), OEWIM (Open-Winding Induction Motors) is an open-winding induction motor. When working normally, the dual three-level inverter can output " , There are five switch states: 0, 1 and 2. The corresponding IGBT switch states are shown in Table 1. Table 1 Relationship between switch state and output voltage of dual three-level inverter The following situations may occur in the case of a single-tube open-circuit fault: A single power device open-circuit fault may occur in the three-phase bridge arms a, b, and c, that is, S k11 , S k12 , S k13 , S k14 , S k21 , S k22 , S k23 , S k24 ( k =a, b, c) Any power device may have an open circuit failure, so there are a total of The corresponding relationship between the switch states of the power devices in the dual three-level inverter circuit under the typical open-circuit fault mode is shown in Table 2. Table 2 Switching states of power devices in dual three-level inverter circuit under typical open circuit fault mode When a single switch tube open circuit fault occurs in the dual inverter open winding system, the inverter will not be able to output the entire voltage vector. Considering the freewheeling effect of the voltage source inverter diode, theoretically, a single switch tube open circuit fault only affects the current and voltage waveforms of half a cycle. Switching tube S a11 The possible current flow paths after an open circuit fault occurs are shown in Figure 3 to Figure 5 As shown in the figure, the current The reference direction is indicated by an arrow; when the current is in the positive reference direction, the output voltage for any phase Output, by Figure 3As shown, the switch tube is in the normal conduction state. Figure 4 As shown in the figure, the P level will be forced to be clamped to the O level; Figure 5 As shown, it has no effect on the output O level and N level instructions; when the current is in the reference negative direction, it has no effect regardless of the normal conduction or freewheeling state; therefore, when the upper bridge arm external tube fails, the output P level will be forced to clamp to the O level only when the current is in the same direction as the reference direction, and there is no effect in other cases.

[0029] Switching tube Sa12 The possible current flow paths after an open circuit fault occurs are as follows Figures 6 to 8 As shown in the figure, the current The reference direction is indicated by an arrow. When the current is in the positive reference direction, the output voltage for any phase Output, by Figure 6 It can be seen that if the switch tube is in the normal conduction state, such as Figure 7 As shown in Figure 2, both the P level and O level output states will be forced to be clamped to the N level. Figure 8 As shown, for the N level, if the switch tube is in the freewheeling state, there is no effect and the N level continues to be output; when the current is in the reference negative direction, no matter it is in the normal conduction or freewheeling state, it has no effect on all switch states.

[0030] For inner tube failure Figure 3 Clamped to Figure 4 The circuit analysis is carried out when the fault occurs in the internal tube during the positive half cycle of the current; if the internal tube fault occurs during the negative half cycle of the current, there will be no follow-flow, and the current should have no current path in the positive half cycle.

[0031] Therefore, when the internal tube of the upper bridge arm fails, only when the current is in the same direction as the reference direction and the fault occurs in the positive half-cycle of the phase current, the output P and O levels will be forced to clamp to the N level; if the fault occurs in the negative half-cycle of the phase current, the positive half-cycle current will become 0 and there will be no voltage output; it has no effect on other situations.

[0032] The power switch failure causes the output voltage of a certain phase to changes have occurred, The change will cause the common mode voltage to deviate from that in normal working conditions. The present invention uses the difference between the ideal common mode voltage calculated by MLD and the actual common mode voltage as a fault diagnosis variable to detect the fault.

[0033] The changes in the common mode voltage difference under different fault conditions are shown in Table 3, where g k Corresponding to the various switch states in Table 1, S kxRepresenting different faulty switch tubes, (k=a, b, c; x=1~8), the four different changes in the common-mode voltage difference are represented by the letters D / E / F / G: Table 3 Changes in the common mode voltage difference under different fault conditions Combined with the rules shown in Table 3, it is possible to identify the faulty internal and external tubes and accurately locate the fault of a switch tube inside the "faulty switch pair".

[0034] In this embodiment, RT-LAB equipment and two sets of NPC three-level inverters are selected for experimental verification. The model of RT-LAB is OP5700, and the inverter is FS8003IA40D70C100. The two inverters are connected according to the common DC bus wiring method. The circuit structure is as follows: Figure 1 As shown in the figure, the common DC bus open winding dual three-level inverter topology consists of three phases and six bridge arms, each phase of which contains two upper and lower bridge arms, and each bridge wall is composed of two IGBTs in series. The midpoints of the two inverters are connected in parallel, and the DC side buses are connected in parallel, and the buses are powered by the same DC power supply.

[0035] The power switch failure causes the output voltage of a certain phase to will change, The change will cause the common mode voltage to deviate from the normal working condition. The difference between the ideal common mode voltage calculated by the mixed logic dynamic model MLD and the actual common mode voltage is used as the fault diagnosis feature to detect the fault. The calculation expression is: in, is the fault characteristic quantity, is the A-phase output voltage of inverter 1, is the B-phase output voltage of inverter 1, is the C-phase output voltage of inverter 1, is the A-phase output voltage of inverter 2, is the B-phase output voltage of inverter 2, is the C-phase output voltage of inverter 2.

[0036] The common mode voltage value under ideal normal working condition is calculated in real time in the MLD model by inputting the switching signal; First, the working state of the single-phase bridge arm is analyzed and the model of the single-phase bridge arm is established. Taking the inverter 1 (IN1) side as an example, the relationship between the switch tube signal and the terminal voltage of the k phase (k=a, b, c) is: in, For inverter 1 The switching signal of the first switching tube of the phase, For inverter 1 The switching signal of the second switching tube is For inverter 1 The switching signal of the third switching tube, For inverter 1 The switching signal of the fourth switching tube of the phase, For Take the conjugate, For Take the conjugate, For Take the conjugate, For Taking the conjugate, the value of the switch signal is 0 or 1.

[0037] Similarly, the expressions of the switching signal and terminal voltage on the inverter 2 (IN2) side can be obtained: in, For inverter 2 The switching signal of the first switching tube of the phase, For inverter 2 The switching signal of the second switching tube is For inverter 2 The switching signal of the third switching tube, For inverter 2 The switching signal of the fourth switching tube of the phase, For Take the conjugate, For Take the conjugate, For Take the conjugate, For Take the conjugate, Define the variable ε as the switch function, and the calculation expression is: in, is the first switching function, is the second switching function, The MLD common-mode voltage model of the dual three-level inverter is: in, is the system common mode voltage under ideal conditions and normal working conditions calculated in MLD, is the switching function of phase A of inverter 1, is the switching function of phase B of inverter 1, The switching function of phase C of inverter 1, is the switching function of phase A of inverter 2, is the switching function of phase B of inverter 2, is the switching function of phase C of inverter 2, is a DC voltage.

[0038] S2: Obtain fault characteristic quantity according to the MLD common mode voltage model of the dual three-level inverter; The difference between the ideal common mode voltage and the actual common mode voltage is taken as the fault characteristic quantity, and the calculation expression of the fault characteristic quantity is: in, is the fault characteristic quantity, is the common mode voltage value actually collected in the circuit, It is the system common mode voltage under ideal conditions and normal working conditions calculated in MLD.

[0039] S3: determining the initial phase of the fault characteristic quantity, and identifying the faulty bridge arm pair according to the initial phase of the fault characteristic quantity; Each power frequency cycle is divided into N equal parts through the sector judgment module. Since the switching signals of phases a, b, and c differ from each other by 120 degrees, and the switching signals of the upper and lower bridge arms of each phase differ from each other by 180 degrees, each power frequency cycle 2π can be divided into 6 parts, each of which represents a group of complementary bridge arm pairs. The faulty bridge arm pair can be identified by judging the initial phase of the fault characteristic quantity.

[0040] The sector judgment module can ensure that each 2π is divided into 6 even parts, and the change timing is consistent with the phase interval of the fault characteristic value showing the fault characteristic when the fault occurs in different phases or different bridge arms. According to the output value of the sector judgment module at the initial moment when the fault characteristic occurs, the faulty bridge arm pair can be accurately identified. The specific correspondence between the fault characteristic occurrence phase and the sector number is shown in Table 4.

[0041] When the fault characteristic value displays the fault characteristic, the sector output by the sector judgment module at this time is "sector 6", then the fault bridge arm range can be determined as one of the upper bridge arm of phase a of inverter 1 or the lower bridge arm of phase a of inverter 2, and the same applies to other sectors.

[0042] S4: Obtaining a fault characteristic value according to the fault bridge arm pair and the fault characteristic quantity; Take the upper bridge arm of phase k (k=a, b, c) of inverter 1 (IN1) and the lower bridge arm of phase k of inverter 2 (IN2) as an example for analysis. k11Fault, the voltage output command of IN1 is P and At this time The output level should be P, but it is clamped to O. The fault characteristic value is no longer equal to 0, but becomes in, is the fault characteristic value of the upper bridge arm, For inverter 1 Phase ideal common mode voltage, is a DC voltage.

[0043] When S k24 Fault, the voltage output command of IN2 is N and hour, The output level should be N, but it is clamped to O, and the fault characteristic value becomes: in, is the fault characteristic value of the lower bridge arm, For inverter 2 Phase ideal common mode voltage, is a DC voltage.

[0044] For a group of internal tube fault switch pair S k12 and S k23 When S k12 Fault, the voltage output command of IN1 is P or O and When , it is assumed that the freewheeling phase has ended, and there is no current flowing through phase k. Since the upper and lower bridge arms at point k1 are not in the on state, the potential of point k1 and point k2 are the same. At this time, the fault characteristic value of the upper bridge arm is no longer equal to 0, but becomes: in, is the fault characteristic value of the upper bridge arm, For inverter 1 Phase ideal common mode voltage, For inverter 2 The actual common-mode voltage of the phase.

[0045] When S k23 Fault, the voltage output command of IN2 is N or O and When , it is assumed that the freewheeling phase has ended, and there is no current flowing through phase k. Since the upper and lower bridge arms at point k2 are not in the on state, the potential of point k2 is the same as that of point k1. At this time, the fault characteristic value of the lower bridge arm is no longer equal to 0, but becomes: in, is the fault characteristic value of the lower bridge arm, For inverter 2 Phase ideal common mode voltage, For inverter 1 The actual common-mode voltage of the phase.

[0046] It can be seen that the change in the characteristic value when the inner tube fails is about twice that of the outer tube, and the change in the characteristic value when the upper bridge arm of inverter 1 fails is positive, while the change in the characteristic value when the lower bridge arm of inverter 2 fails is negative. Therefore, the faulty bridge arm pair can be judged by the positive and negative signs of the characteristic value change, and the inner and outer tubes can be distinguished by setting different thresholds.

[0047] Although the impact on the output voltage and current of each phase when an open circuit fault occurs in each faulty switch pair is exactly the same, the impact of the two switch tube failures in the group on the common mode voltage is completely opposite. Therefore, the fault diagnosis and location of the specific switch tube can be completed only by detecting the common mode voltage.

[0048] S5: Determine the fault type based on the fault characteristic value and threshold; The faulty bridge arm pair is determined by the positive or negative value of the fault characteristic value. If the fault characteristic value is positive, the upper bridge arm of inverter 1 is faulty. If the fault characteristic value is negative, the lower bridge arm of inverter 2 is faulty.

[0049] The inner and outer tube faults are judged by comparing the fault characteristic value with the threshold, the outer tube fault characteristic value and the inner tube fault characteristic value under ideal conditions are calculated, and the value between the outer tube fault characteristic value and the inner tube fault characteristic value is taken as the threshold. If it is greater than the threshold, it is an inner tube fault, and if it is less than the threshold, it is an outer tube fault.

[0050] The existing open-winding topology fault diagnosis strategy is proposed for dual two-level inverters. The fault diagnosis method of the present invention can accurately locate the faults of a total of 24 switch tubes in the dual three-level inverter.

[0051] The existing current diagnosis method will be affected by the motor body parameters. When the back electromotive force is too small, the current change is not obvious and it is difficult to distinguish the fault characteristics. The method proposed in the present invention is not affected by the motor body parameters and can ensure the obvious distinction of fault characteristics under any parameters.

[0052] The existing current diagnosis method is divided into the detection of faulty switch pairs and the distinction of specific switch tubes. Under the most unfavorable working conditions, two cycles are required to detect the fault. The present invention integrates the entire fault location process into one power frequency cycle, so the fault location can be completed in at most one power frequency cycle.

[0053] The existing current diagnosis method needs to add disturbance control after one phase is disconnected to complete fault diagnosis, which is not conducive to the control of the motor and cannot be extended to the fault diagnosis of the double-tube open circuit condition. The present invention can cooperate with the traditional current average method. After using the traditional average to determine the switch pair, the double-tube fault is specifically located by the difference in common mode voltage. The change of the fault characteristic quantity is still obvious, and there is no need to add active interference to the control strategy after the fault.

[0054] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A fault diagnosis method for an open-winding dual three-level inverter based on common-mode voltage difference, characterized in that: include: S1: Construct the MLD common-mode voltage model of the dual three-level inverter based on the switching function; S2: Obtain fault characteristic quantity according to the MLD common mode voltage model of the dual three-level inverter; S3: determining the initial phase of the fault characteristic quantity, and identifying the faulty bridge arm pair according to the initial phase of the fault characteristic quantity; S4: Obtaining a fault characteristic value according to the fault bridge arm pair and the fault characteristic quantity; S5: Determine the fault type based on the fault characteristic value and threshold.

2. The open-winding dual three-level inverter fault diagnosis method based on common-mode voltage difference according to claim 1, characterized in that: By inputting the switching signal into the MLD model to calculate the common-mode voltage value under the ideal normal working state in real time, the MLD common-mode voltage model of the dual three-level inverter is constructed. The MLD common-mode voltage model of the dual three-level inverter is: in, is the system common mode voltage under ideal conditions and normal working conditions calculated in MLD, is the switching function of phase A of inverter 1, is the switching function of phase B of inverter 1, The switching function of phase C of inverter 1, is the switching function of phase A of inverter 2, is the switching function of phase B of inverter 2, is the switching function of phase C of inverter 2, is a DC voltage.

3. The open-winding dual three-level inverter fault diagnosis method based on common-mode voltage difference according to claim 1, characterized in that: The difference between the ideal common mode voltage and the actual common mode voltage is taken as the fault characteristic quantity, and the calculation expression of the fault characteristic quantity is: in, is the fault characteristic quantity, is the common mode voltage value actually collected in the circuit, It is the system common mode voltage under ideal conditions and normal working conditions calculated in the MLD common mode voltage model of the dual three-level inverter.

4. The open-winding dual three-level inverter fault diagnosis method based on common-mode voltage difference according to claim 1, characterized in that: In step S3, each power frequency cycle is divided into N equal parts by the sector determination module.

5. The open-winding dual three-level inverter fault diagnosis method based on common-mode voltage difference according to claim 1, characterized in that: When the upper arm outer tube fails, the fault characteristic value of the upper arm is: in, is the fault characteristic value of the upper bridge arm, For inverter 1 Phase ideal common mode voltage, is a DC voltage.

6. The open-winding dual three-level inverter fault diagnosis method based on common-mode voltage difference according to claim 1, characterized in that: When the outer tube of the lower bridge arm fails, the fault characteristic value of the lower bridge arm is: in, is the fault characteristic value of the lower bridge arm, For inverter 2 Phase ideal common mode voltage, is a DC voltage.

7. The open-winding dual three-level inverter fault diagnosis method based on common-mode voltage difference according to claim 1, characterized in that: When the internal tube of the upper bridge arm fails, both the upper and lower bridge arms are not conducting, and the fault characteristic value of the upper bridge arm is: in, is the fault characteristic value of the upper bridge arm, For inverter 1 Phase ideal common mode voltage, For inverter 2 The actual common-mode voltage of the phase.

8. The open-winding dual three-level inverter fault diagnosis method based on common-mode voltage difference according to claim 1, characterized in that: When the inner tube of the lower bridge arm fails, both the upper and lower bridge arms are not conducting, and the fault characteristic value of the lower bridge arm is: in, is the fault characteristic value of the lower bridge arm, For inverter 2 Phase ideal common mode voltage, For inverter 1 The actual common-mode voltage of the phase.

9. The open-winding dual three-level inverter fault diagnosis method based on common-mode voltage difference according to claim 1, characterized in that: The faulty bridge arm pair is determined by the positive or negative value of the fault characteristic value. If the fault characteristic value is positive, the upper bridge arm of inverter 1 is faulty. If the fault characteristic value is negative, the lower bridge arm of inverter 2 is faulty.

10. The open-winding dual three-level inverter fault diagnosis method based on common-mode voltage difference according to claim 1, characterized in that: The inner and outer tube faults are judged by comparing the fault characteristic value with the threshold, the outer tube fault characteristic value and the inner tube fault characteristic value under ideal conditions are calculated, and the value between the outer tube fault characteristic value and the inner tube fault characteristic value is taken as the threshold. If the fault characteristic value is greater than or equal to the threshold, it is an inner tube fault. If the fault characteristic value is less than the threshold, it is an outer tube fault.

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