CN108303611B

CN108303611B - On-line diagnosis method for open-circuit fault of inverter power tube

Info

Publication number: CN108303611B
Application number: CN201810214277.2A
Authority: CN
Inventors: 成庶; 伍珣; 李凯迪; 罗屿; 田睿
Original assignee: Central South University
Current assignee: Central South University
Priority date: 2018-03-15
Filing date: 2018-03-15
Publication date: 2021-03-02
Anticipated expiration: 2038-03-15
Also published as: CN108303611A

Abstract

The invention discloses an on-line diagnosis method for open-circuit faults of inverter power tubes, which comprises the following steps: based on the inverter control principle and the signal relation between the modulation wave and the carrier wave, deriving and obtaining a switch control signal of each phase and a relation between the line voltage and the voltage at two ends of the power tube; analyzing possible values of a normal working condition and a fault working condition according to the obtained relation based on each line voltage to obtain a corresponding relation list among a switch control signal, phase current, a voltage value of the normal working condition of the line voltage and a voltage value of the fault working condition of the line voltage; deducing to obtain a fault characteristic relation table of each fault and two corresponding line voltages according to the same probability of the positive and negative values of the line voltages under the normal working condition; and determining the fault position information of the current inverter by acquiring the characteristic information of the corresponding line voltage of the current inverter and searching a fault characteristic relation table. The method and the device can quickly and effectively realize the on-line diagnosis of the open-circuit fault of the inverter power tube, and have the advantages of small calculated amount and high accuracy.

Description

On-line diagnosis method for open-circuit fault of inverter power tube

Technical Field

The invention relates to the technical field related to fault diagnosis of locomotive power tubes, in particular to an on-line diagnosis method for open-circuit faults of an inverter power tube.

Background

Inverters have the ability to convert direct current to alternating current, and have wide application in electric locomotives. Research shows that in the variable frequency speed control system of the motor, the failure of the inverter occupies a main component in all failures, and the failure of the inverter is generally caused by the failure of a power tube. The power tube faults comprise two types of open circuit faults and short circuit faults, and the short circuit faults of the power tubes can be converted into open circuit faults by connecting the fast fuses in the bridge arms in series. Therefore, most power tube faults typically occur in the form of open circuit faults. If a certain power tube fails, other power tubes are also affected by connection, so that the whole converter system is possibly broken down, and personal safety is threatened in some cases. Therefore, it is necessary to develop a real-time open fault diagnosis method.

At present, few documents are available for developing air conditioner inverter fault diagnosis research for 25T passenger cars common in China. The inverter open-circuit fault diagnosis method may be classified into three categories according to the types of the diagnosis variables: current-based diagnostics, voltage-based diagnostics, and diagnostics based on various signals such as voltage and current. Due to the different diagnostic variables, some diagnostic methods are not suitable for diagnosing open circuit faults of locomotive inverters, and the reasons include: 1) the system on the locomotive is not allowed to be modified randomly, control signals are not easy to obtain or additional sensors are not easy to add, and the method of taking the control signals or other measurement signals as diagnosis basis is not applicable any more; 2) the operating environment of the locomotive inverter is very complex, a large amount of calculation and complex diagnosis processes require sufficient data storage space and a high-performance processor, and the probability of errors in the calculation and processing processes is relatively high. Therefore, the problem of diagnosis efficiency and accuracy still exists in the current fault diagnosis of the locomotive inverter, and the fault diagnosis is difficult to be rapidly and effectively realized.

Disclosure of Invention

In view of the above, the present invention aims to provide an inverter power tube open-circuit fault online diagnosis method, which can quickly and effectively implement inverter power tube open-circuit fault online diagnosis, and has the advantages of small calculation amount and high accuracy.

The invention provides an inverter power tube open-circuit fault online diagnosis method based on the above purpose, which comprises the following steps:

step 1: based on the basic principle of an inverter control circuit and the signal relation between a modulation wave and a carrier wave, deducing to obtain a switching control signal of each phase and a relational expression between line voltage and voltage at two ends of a power tube;

step 2: on the basis of each line voltage, respectively analyzing possible values of a normal working condition and a fault working condition according to the relation obtained in the step 1 to obtain a corresponding relation list among a switch control signal, a phase current, a voltage value of the normal working condition of the line voltage and a voltage value of the fault working condition of the line voltage;

and step 3: correspondingly deducing to obtain a fault characteristic relation table of each fault and two corresponding line voltages according to the same probability of the positive and negative values of the line voltages under the normal working condition;

and 4, step 4: and (3) determining the fault position information of the current inverter by acquiring the characteristic information of the line voltage corresponding to the current inverter and searching the fault characteristic relation table obtained in the step (3).

Optionally, the modulated wave signal is represented as:

wherein u is_ra(t)、u_rb(t)、u_rc(t) are three-phase modulated wave signals, respectively; m is the modulation depth; f. of_rIs the modulation frequency;

the carrier signal is represented as:

wherein k is a natural number; t is_cAnd f_cCarrier period and carrier frequency, respectively;

the switch control signal is expressed as:

wherein phase ═ a, b, c; when Trig_phaseWhen (t) is 1, the upper bridge arm is conducted, and the lower bridge arm is turned off; when Trig_phaseWhen the t is equal to 0, the lower bridge arm is conducted, and the upper bridge arm is turned off;

the relation between the line voltage and the voltage at two ends of the power tube is expressed as follows:

wherein u is_ab(t)、u_bc(t)、u_ca(t) is a line voltage, u_v1(t)、u_v2(t)、u_v3(t)、u_v4(t)、u_v5(t)、u_v6(t) are respectively corresponding to power tubes V₁、V₂、V₃、V₄、V₅、V₆The voltage across; u shape_dIs the input voltage.

Optionally, the step of obtaining a list of correspondence relationships between the switching control signal, the phase current, the voltage value of the normal working condition of the line voltage, and the voltage value of the fault working condition of the line voltage includes:

obtaining line voltage output according to circuit principle and containing + U _d0 and-U_dThree amplitudes; the level is regularly alternated in each cycle and its value is equal to the modulation wave u_raAnd u_rbAbout, divide into two kinds of cases: A) when u is_raU is greater than or equal to_rbWhen u is turned on_abIs equal to + U_dOr 0; B) when u is_raLess than u_rbWhen u is turned on_abIs equal to-U_d；

According to modulated wave u_raAnd u_rbThe frequency of occurrence of A and B in a period is equal to obtain the level + U_dThe number of occurrences is equal to-U_dThe number of occurrences;

assuming that the positive direction of the current is directed towards the load, V₁When an open circuit fault occurs, u_abCan be calculated from the relation in step 1, wherein at any time the current i_aAre all non-positive;

u in each switch state in class A_abThe values of (c) were calculated as follows:

when Trig_a＝1，Trig_bWhen 1, if i_aIs equal to 0, then u_ab＝-U _d2; if i_aLess than 0, then u_ab＝0；

When Trig_a＝1，Trig_bWhen equal to 0, if i_aIs equal to 0, then u_ab＝U _d2; if i_aLess than 0, then u_ab＝U_d；

When Trig_a＝0，Trig_bWhen equal to 0If i is_aIs equal to 0, then u_ab＝U _d2; if i_aLess than 0, then u_ab＝0；

U in each switch state in class B_abThe values of (c) were calculated as follows:

When Trig_a＝0，Trig_bWhen 1, if i_aIs equal to 0, then u_ab＝-U _d2; if i_aLess than 0, then u_ab＝-U_d；

When Trig_a＝0，Trig_bWhen equal to 0, if i_aIs equal to 0, then u_ab＝U _d2; if i_aLess than 0, then u_ab＝0；

Then obtaining V₁When open circuit fault occurs, listing the corresponding relation among the switch control signal, the phase current, the voltage value of the line voltage normal working condition and the voltage value of the line voltage fault working condition;

and similarly, a relation table of the open circuit faults of the other power tubes can be obtained through derivation.

Optionally, the fault feature relation table of each fault and corresponding two line voltages adopts a ratio of the positive level rising times to the negative level falling times as a fault feature, and a fluctuation threshold is set, so that as long as the ratio is within a fluctuation threshold range, the voltage is normal, otherwise, the voltage is abnormal, and further the fault feature relation table of the fault and corresponding two line voltages is obtained.

Optionally, the step of obtaining the fault characteristic relationship table of each fault and corresponding to the two line voltages further includes:

firstly, defining the rising of a positive level as a positive rising edge and the falling of a negative level as a negative falling edge; wherein, the rising and falling amplitudes are both U_d/2；

Secondly, the ratio of the number of positive rising edges to the number of negative falling edges in one period is x; normally, x equals 1; considering the influence of noise and voltage fluctuation, giving a fluctuation range x epsilon (1-alpha, 1+ alpha), wherein alpha is a fluctuation threshold value smaller than 1, and the value of the fluctuation threshold value is adjusted according to the running state of the inverter; when the value of x is not in the normal range, indicating that the voltage is abnormal;

the state of the resulting line voltage is expressed as:

the fault can be found by detecting the level, and the fault characteristic u of each line voltage is calculated by counting one period of positive rising edge and negative falling edge^*(x) A value;

finally, according to fault characteristics [ u ]_ab*，u_bc*]And obtaining a fault characteristic relation table of each fault and corresponding two line voltages according to the corresponding fault relation.

From the above, the on-line diagnosis method for the open-circuit fault of the inverter power tube provided by the invention can determine the characteristic relation table between the fault and the corresponding line voltage by analyzing the relation of the corresponding signals in the locomotive inverter and the occurrence condition of the fault, and finally can realize the extraction diagnosis of the fault by looking up the table; the method does not need to change the locomotive, and can realize accurate positioning of the fault only by correspondingly detecting the change condition of the line voltage in one period, so that the detection efficiency can be improved, the calculated amount is smaller, and the same diagnosis of the existing equipment can be realized. Therefore, the inverter power tube open-circuit fault online diagnosis method can quickly and effectively realize the inverter power tube open-circuit fault online diagnosis, and is small in calculation amount and high in accuracy.

Drawings

Fig. 1 is a flowchart of an embodiment of an online diagnosis method for an open-circuit fault of an inverter power tube according to the present invention;

FIG. 2 is a schematic diagram of a circuit structure of a 25T-model bus air conditioner inverter in the prior art;

FIG. 3 is an equivalent circuit under the A-class condition provided by the present invention;

FIG. 4 is an equivalent circuit for the class B case provided by the present invention;

FIG. 5 is a schematic diagram of a verification test system according to the present invention;

FIG. 6 shows T provided by the present invention₁The diagnosis result after the fault happens at any moment;

FIG. 7 shows T provided by the present invention₂The diagnosis result after the fault happens at any moment;

FIG. 8 is a diagnostic result for a switching frequency of 2.5kHz as provided by the present invention;

fig. 9 is a diagnostic result after the load change provided by the present invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to specific embodiments and the accompanying drawings.

It should be noted that all expressions using "first" and "second" in the embodiments of the present invention are used for distinguishing two entities with the same name but different names or different parameters, and it should be noted that "first" and "second" are merely for convenience of description and should not be construed as limitations of the embodiments of the present invention, and they are not described in any more detail in the following embodiments.

The simple online diagnosis method for the open-circuit fault of the power tube is provided for the fault of the locomotive inverter, particularly the 25T-type bus air-conditioning inverter. The method has the advantages that the calculated amount is small, redundant sensors are not needed, and the faults can be accurately positioned by utilizing the change trend of the voltage of the existing sensor detection line. Meanwhile, test results show that the method can effectively diagnose the open circuit fault of the locomotive inverter, and the diagnosis result has better robustness on fault occurrence time, switching frequency and load change.

Referring to fig. 1, a flowchart of an embodiment of an online diagnosis method for an open-circuit fault of an inverter power tube according to the present invention is shown. The on-line diagnosis method for the open-circuit fault of the inverter power tube comprises the following steps:

optionally, the modulated wave signal is represented as:

the carrier signal is represented as:

the switch control signal is expressed as:

wherein u is_ab(t)、u_bc(t)、u_ca(t) is a line voltage, u_v1(t)、u_v2(t)、u_v3(t)、u_v4(t)、u_v5(t)、u_v6(t) respectively correspond to the power tubes V₁、V₂、V₃、V₄、V₅、V₆The voltage across; u shape_dIs the input voltage.

Further, the step of obtaining a fault characteristic relation table of each fault and corresponding two line voltages further includes: preferably, the positive level is defined as a positive rising edge and a negative levelFlat falling is a negative falling edge; wherein, the rising and falling amplitudes are both U_d/2；

the state of the resulting line voltage is expressed as:

According to the embodiment, the on-line diagnosis method for the open-circuit fault of the inverter power tube can determine the characteristic relation table between the fault and the corresponding line voltage by analyzing the relation of the corresponding signals in the locomotive inverter and the occurrence condition of the fault, and finally can realize the extraction diagnosis of the fault by looking up the table; the method does not need to change the locomotive, and can realize accurate positioning of the fault only by correspondingly detecting the change condition of the line voltage in one period, so that the detection efficiency can be improved, the calculated amount is smaller, and the same diagnosis of the existing equipment can be realized. Therefore, the inverter power tube open-circuit fault online diagnosis method can quickly and effectively realize the inverter power tube open-circuit fault online diagnosis, and is small in calculation amount and high in accuracy.

In some optional embodiments of the present application, a 25T model passenger car air conditioner inverter in our country is taken as an example for detailed description.

Referring to fig. 2, the circuit structure diagram of the 25T bus air conditioner inverter is shown. The input is 600V direct current voltage, and the output is three-phase 380V alternating current voltage. SPWM modulation is adopted, and the switching frequency is 3 kHz. KM1, L₁And R₅For charging a buffer circuit, C₁～C₄And R₁～R₄For the DC side power component, the voltage stabilizing function is achieved, KM2 and R₆Is a discharge circuit. V₁～V₆There are 6 IGBTs with anti-parallel diodes each. Modulated wave u_ra、u_rb、u_rcAnd carrier u_cThe control signal is generated by a modulation circuit. The sensors available at the output of the inverter include a three-phase current sensor for measuring the three-phase current and two line voltage sensors for measuring the line voltage u, respectively_abSum line voltage u_bc。

The three-phase SPWM-based sine wave with a phase difference of 120 ° is generally adopted as a modulation wave, and therefore, a modulation wave signal can be expressed as:

the carrier signal is represented as:

the phase is derived from the above two equations, and the switching control signal is expressed as:

wherein phase ═ a, b, c; when Trig_phaseWhen (t) is 1, the upper bridge arm is conducted, the lower bridge arm is turned off, and the voltage borne by the upper bridge arm is U_d(ii) a When Trig_phaseWhen (t) is equal to 0, the lower bridge arm is conducted, the upper bridge arm is turned off, and the voltage borne by the lower bridge arm is U_d；

According to kirchhoff's law, the relation between the line voltage and the voltage at two ends of the power tube is expressed as follows:

Further, the possible values of the line voltage under normal conditions and open circuit faults are analyzed. At line voltage u_abFor example, other line voltage analysis methods are similarly derived.

Under normal conditions, the line voltage output contains + U _d0 and-U_dThree amplitudes. The level is regularly alternated in each cycle and its value is equal to the modulation wave u_raAnd u_rbAbout, divide into two kinds of cases: A) when u is_raU is greater than or equal to_rbWhen u is turned on_abIs equal to + U_dOr 0; B) when u is_raLess than u_rbWhen u is turned on_abIs equal to-U_d。

According to modulated wave u_raAnd u_rbThe number of occurrences of A and B in a period is equal, i.e., level + U_dThe number of occurrences is equal to-U_dThe number of occurrences.

Assuming that the positive direction of the current is directed towards the load, V₁When an open circuit fault occurs, u_abCan be calculated from the relationship between the line voltage and the voltage across the power tube, wherein at any time i_aAre all non-positive. Specifically, the equivalent circuit in the case of class a is shown in fig. 3. U in each switch state_abThe values of (c) were calculated as follows:

The equivalent circuit for the class B case is shown in fig. 4. U in each switch state_abThe values of (c) were calculated as follows:

When Trig_a＝0，Trig_bWhen equal to 0, if i_aIs equal to 0, then u_ab＝U _d2; if i_aLess than 0, then u_ab＝0。

V₁Line voltage u at open circuit fault_abAll possible values are shown in tables 1 and 2. Under normal conditions and partial fault conditions, i_aThe case of zero exists only at the instant of zero crossing, which is negligible in the table. It can be seen that an open fault results in a new level being generated. When Trig_a＝1，Trig_bWhen 1, u is present in both class A and class B_ab＝-U_d/2. When Trig_a＝1，Trig_bWhen 0, u is present in the case of class A_ab＝U_d/2. When Trig_a＝0，Trig_bThe condition of class B when 1 is present as u_ab＝-U_d/2. The number of occurrences of negative levels increases significantly and is greater than the number of occurrences of positive levels during a cycle.

TABLE 1 possible values in the case of class A

TABLE 2 possible values in the case of class B

Depending on the relationship between the line voltages, the fault of any phase can be determined from any two line voltages, e.g. line voltage u_abAnd u_bc. They may represent fault signatures, such as [ u ]_ab u_bc]＝[0 1]And [ u ]_ab u_bc]＝[1 0]Where a zero state represents normal and a non-zero state represents abnormal. There are 6 cases in total for the open circuit fault of a single power tube of the inverter, therefore, at least 3 states are required for each line voltage to represent all fault characteristics. These 3 states are obtained from the above analysis.

A positive rising edge is defined as a positive rising edge and a negative falling edge is defined as a negative falling edge. Wherein, the rising and falling amplitudes are both U_d/2. The ratio of the number of positive rising edges to negative falling edges in a cycle is x. Normally, x is close to 1. Considering the influence of noise and voltage fluctuation, consider x ∈ (1- α, 1 +)α) is normal, wherein α is a positive number smaller than 1, and is adjusted according to the operation state of the inverter. When the value of x is not within the normal range, it is considered that the voltage is abnormal. The state of the line voltage can be expressed as:

and after the inverter is put into operation, carrying out uninterrupted detection on the line voltage level. When a fault occurs, the line voltage develops an abnormal level. By detecting the level, it is possible to detect faults immediately and count a cycle for positive and negative rising edges, and after x is calculated, u for each line voltage is determined by the above equation^*(x) The value is obtained. Finally, from the fault signature [ u ]_ab*u_bc*]And positioning the fault. For example, V₁Open circuit fault causes line voltage u_abGenerating abnormal level, since the number of negative falling edges is greater than that of positive rising edges, the value of x is less than normal range, then

u

_ab1, and line voltage u_bcWithout being significantly affected, x is close to 1, with u_bc*＝0。

Thus, a unique distinction V can be obtained₁Fault signature of open circuit fault u_ab*u_bc*]＝[-1 0]. Table 3 lists the open circuit fault characteristics of all power tubes.

TABLE 3 open-circuit Fault characteristics of Power tubes

Open circuit fault	u_ab*	u_bc*
			V₁	-1	0
V₂	0	-1
			V₃	1	-1
V ₄	1	0
			V ₅	0	1
V₆	-1	1

The method provided by the application is not influenced by the fault occurrence time. Line voltage level monitoring is performed at all times while the inverter is operating, and a fault occurrence can be detected as soon as a line voltage level sends an abnormality. Since a complete cycle is always detected before the fault is located, the value of x does not change substantially and the resulting diagnostic result is not affected.

The line voltage fluctuates due to the change of the load, the number of positive rising edges and negative falling edges changes, the value of x also changes, but the change trend is still the same, and therefore the diagnosis result is not influenced.

In addition, in theory, the change of the carrier frequency can cause the positive rising edge and the negative falling edge to change according to the same proportion, the value of x is basically unchanged, and the diagnosis result is not influenced. If the carrier frequency is so small that the modulated waveform is irregular, this will only lead to a misdiagnosis of the method. In addition, in the actual train operation process, if the carrier frequency is too high, the difficulty that each pulse can be distinguished by the AD sampling is increased, and at this time, a high-performance AD converter may need to be additionally installed. Based on the analysis, the scheme provided by the application has good stability.

In order to verify the feasibility of the method, a three-phase inverter model is also built in the application and is shown in fig. 5. The experimental device mainly comprises a rectifier, a voltage source inverter and a dSPACE system. The control signal is generated by the dSPACE system and transmitted to the inverter via the I/O interface. The relevant sensor signals are transmitted by the a/D interface. The diagnosis module monitors the line voltage and sends data to the computer for processing. The main experimental parameters are shown in table 4, and the method is mainly verified in the experiment by taking the open circuit fault of V1 as an example.

TABLE 4 Main Experimental parameters

Respectively at T₁～T₆Set open circuit fault at all times, where T₁～T₆The diagnosis results are shown in table 5, corresponding to different times in one cycle. After a fault, the line voltage u_abIncreased number of negative falling edges, x_abThe value of (A) is mainly distributed in the vicinity of 0.68 to 0.69, x_bcIs close to 1. FIGS. 7 and 8 show T₁And T₂The diagnosis result after the fault happens at any moment is that the figure has 4 channels u in total_abAnd u_bcRespectively representing two line voltages, and 1000V/div represents that each cell is 1000V; u. of_abA and u_bcAnd the abnormal states of the two line voltages are respectively represented, and 5V/div represents that each grid is 5V. It can be seen that in the event of failureAfter t₁And (3) detecting a fault at any moment, and positioning the fault through fault characteristics after a period, wherein the whole process takes about one line voltage period.

TABLE 5 diagnosis results of different failure occurrence times

Therefore, the method provided by the application can rapidly detect the fault under most conditions, and can complete fault location about one line voltage period after the fault is detected. Only in certain special cases may a short transition be required from the occurrence of a fault to the detection of the fault, such as when current freewheels from the anti-parallel diode of the faulty power transistor or the power transistor fails during the time period that the control is turned off. The line voltage level is still characterized as normal for this brief period of time, and the fault has little or no effect on the system. When the line voltage is distorted, the detection method immediately reacts and takes further diagnostic measures.

The effect of the switching frequency on the method is given in fig. 8 and 9. With the change of the switching frequency, the positive rising edge and the negative falling edge change in the same proportion, x_abAnd x_bcThe value of (a) is substantially unchanged and the diagnostic result is not affected. Table 6 and fig. 9 show the diagnostic results for different load situations. It can be seen that different load cases result in x_abThe value of (c) is different. When the load is doubled, x_abIs increased from 0.69 to 0.84, still below the normal range, x_bcThe value of (c) was not affected and the diagnostic results were consistent in both cases. Table 7 gives the diagnostic results for all power tubes. Each open circuit fault has a unique fault signature from which the fault can be detected and localized.

TABLE 6 diagnosis results under different loads

TABLE 7 open-Circuit Fault diagnosis results for all Power tubes

Open circuit fault	x_ab	x_bc	[u_ab ^u_bc ^]
				Is normal	1.02	1.01	[0 0]
V₁	0.68	1.02	[-1 0]
				V₂	1.03	0.63	[0 -1]
V₃	1.65	0.61	[1 -1]
				V₄	1.83	0.98	[1 0]
V₅	1.02	1.18	[0 1]
				V₆	0.66	1.59	[-1 1]

In summary, the present application provides a method for diagnosing an open-circuit fault of an inverter power tube based on line voltage pulse edge detection. Taking a 25T bus air conditioner inverter as an example, all possible values of the line voltage are analyzed. And defining a positive rising edge and a negative falling edge according to the change characteristic of the level, and obtaining the open-circuit fault characteristic of each power tube according to the ratio of the positive rising edge and the negative falling edge, thereby diagnosing and positioning the fault. Compared with the existing diagnosis method, the method has the advantages of less calculation amount, no need of additionally arranging an additional sensor, no need of changing the existing structure of the locomotive, simplicity, easiness in implementation and capability of reducing the fault detection cost to a certain extent. Meanwhile, the diagnosis result is not influenced by the fault occurrence time and the load condition, and has certain robustness on different switching frequencies.

Those of ordinary skill in the art will understand that: the discussion of any embodiment above is meant to be exemplary only, and is not intended to intimate that the scope of the disclosure, including the claims, is limited to these examples; within the idea of the invention, also features in the above embodiments or in different embodiments may be combined, steps may be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

The embodiments of the invention are intended to embrace all such alternatives, modifications and variances that fall within the broad scope of the appended claims. Therefore, any omissions, modifications, substitutions, improvements and the like that may be made without departing from the spirit and principles of the invention are intended to be included within the scope of the invention.

Claims

1. An inverter power tube open-circuit fault online diagnosis method is characterized by comprising the following steps:

step 1: based on the basic principle of an inverter control circuit and the signal relation between a modulation wave signal and a carrier signal, deducing to obtain a switching control signal of each phase and a relational expression between line voltage and voltage at two ends of a power tube;

step 2: on the basis of each line voltage, analyzing the possible values of the normal working condition and the fault working condition according to the relation obtained in the step 1 to obtain a corresponding relation list among the switch control signal, the phase current, the voltage value of the normal working condition of the line voltage and the voltage value of the fault working condition of the line voltage, wherein the list comprises the following steps:

obtaining line voltage output according to circuit principle and containing + U_d0 and-U_dThree amplitudes; the level is regularly alternated in each cycle and its value is equal to the modulation wave u_raAnd u_rbAbout, divide into two kinds of cases: A) when u is_raU is greater than or equal to_rbWhen u is turned on_abIs equal to + U_dOr 0; B) when u is_raLess than u_rbWhen u is turned on_abIs equal to-U_d；

when Trig_a＝1，Trig_bWhen 1, if i_aIs equal to 0, then u_ab＝-U_d2; if i_aLess than 0, then u_ab＝0；

When Trig_a＝1，Trig_bWhen equal to 0, if i_aIs equal to 0, then u_ab＝U_d2; if i_aLess than 0, then u_ab＝U_d；

When Trig_a＝0，Trig_bWhen equal to 0, if i_aIs equal to 0, then u_ab＝U_d2; if i_aLess than 0, then u_ab＝0；

When Trig_a＝0，Trig_bWhen 1, if i_aIs equal to 0, then u_ab＝-U_d2; if i_aLess than 0, then u_ab＝-U_d；

similarly, a relation table of the open circuit faults of the other power tubes can be obtained through deduction;

2. The method according to claim 1, wherein the modulated wave signal is expressed as:

the carrier signal is represented as:

the switch control signal is expressed as:

3. The method of claim 1, wherein the fault characteristic relationship table for each fault and for each of the two line voltages uses a ratio of a positive level rise time to a negative level fall time as the fault characteristic, and a fluctuation threshold is set such that the voltage is normal as long as the ratio is within the fluctuation threshold range, and otherwise the voltage is abnormal, thereby obtaining the fault characteristic relationship table for the fault and for each of the two line voltages.

4. The method of claim 3 wherein said step of deriving a fault signature relationship table for each fault and for two line voltages further comprises:

the state of the resulting line voltage is expressed as: