MODEL-BASED METHOD AND SYSTEM FOR DIAGNOSING OPEN-CIRCUIT FAULT OF POWER TRANSISTOR OF THREE-PHASE CONVERTER

Abstract

A model-based method and system for diagnosing an open-circuit fault of a power transistor of a three-phase converter are provided, which belong to the technical field of fault diagnosis of power electronic equipment and can implement fast and accurate diagnosis of the open-circuit fault of the power transistor of the three-phase converter without adding an additional hardware. The fault diagnosis method of the disclosure only needs current and voltage sampling signals and drive signals that already exist in a control system of the converter and has the advantage of simple implementation. A cycle accumulated value of a difference between a sampling current and an estimated current after the power transistor of the converter has the open-circuit fault is used as a diagnostic variable, which can quickly and accurately complete diagnosis of a faulty power transistor and has relatively strong practicability.

Claims

1. A model-based method for diagnosing an open-circuit fault of a power transistor of a three-phase converter, comprising: Step (1) of obtaining relevant signals for diagnosis from a control system of the converter, wherein the relevant signals comprise an alternating current side three-phase current sampling signal i.sub.X[k] of the converter, an alternating current side three-phase voltage sampling signal e.sub.X[k], a direct current side voltage sampling signal U.sub.dc[k], and drive signals s.sub.1[k] to s.sub.6[k] output by the control system, where a subscript X=A, B, or C and represents a present phase sequence, and k represents a sampling time; Step (2) of calculating an estimated change value Δi.sub.EX[k] of a three-phase current during each switching cycle T.sub.s through the alternating current side three-phase voltage sampling signal e.sub.X[k], the direct current side voltage sampling signal U.sub.dc[k], and the drive signals s.sub.1[k] to s.sub.6[k] output by the control system according to a converter model; Step (3) of calculating a residual δi.sub.X[k] of the three-phase current during each switching cycle T.sub.s according to a change value Δi.sub.X[k] of a three-phase current sampling signal during each switching cycle T.sub.s and the calculated estimated change value Δi.sub.EX[k] of the three-phase current; Step (4) of calculating an accumulated value δi.sub.TX[k] of a residual of the three-phase current during a previous basic cycle T.sub.0 of the switching cycle T.sub.s according to the residual δi.sub.X[k] of the three-phase current during each switching cycle T.sub.s; and Step (5) of determining the power transistor that has the open-circuit fault according to a comparison between the accumulated value δi.sub.TX[k] of the residual of the three-phase current during one basic cycle T.sub.0 and a threshold Th.

2. The method for diagnosing the open-circuit fault of the power transistor according to claim 1, wherein: in Step (2), the converter model refers to a mathematical model derived from Kirchhoff s voltage law and Kirchhoff s current law in combination with a topology of the converter.

3. The method for diagnosing the open-circuit fault of the power transistor according to claim 2, wherein: in Step (2), the switching cycle T.sub.s is a reciprocal of a switching frequency f.sub.s, and the estimated change value Δi.sub.EX[k] of the three-phase current during each switching cycle T.sub.s is implemented through a state observer and a mixed logic dynamic model according to the converter model.

4. The method for diagnosing the open-circuit fault of the power transistor according to claim 1, wherein: in Step (3), the change value Δi.sub.X[k] of the three-phase current sampling signal during each switching cycle T.sub.s calculated from Δi.sub.X[k]=i.sub.X[k]−i.sub.X[k−1] refers to a difference between three-phase current sampling signal i.sub.X[k] corresponding to a start and an end of each switching cycle T.sub.s.

5. The method for diagnosing the open-circuit fault of the power transistor according to claim 4, wherein: in Step (4), the basic cycle T.sub.0 refers to a reciprocal of a three-phase voltage frequency f.sub.0, and the accumulated value δi.sub.TX[k] of the residual of the three-phase current during the basic cycle T.sub.0 calculated from δi.sub.TX[k]=δi.sub.X[k−T.sub.0/T.sub.s+1]+δi.sub.X[k−T.sub.0/T.sub.s+2]+ . . . +δi.sub.X[k−1]+δi.sub.X[k] refers to a sum of the residual δi.sub.X[k] of the three-phase current during all the switching cycles during the previous basic cycle T.sub.0 of a current time.

6. The method for diagnosing the open-circuit fault of the power transistor according to claim 5, wherein: in Step (5), the threshold Th refers to a threshold value set to prevent misdiagnosis, after the accumulated value δi.sub.TX[k] of the residual of the three-phase current during the basic cycle T.sub.0 exceeds the threshold Th, it is determined that the power transistor has the open-circuit fault.

7. The method for diagnosing the open-circuit fault of the power transistor according to claim 6, wherein: in Step (5), for different faults of the power transistor, the accumulated value δi.sub.TX[k] of the residual of the three-phase current during the basic cycle T.sub.0 is different, so that a faulty power transistor can be located through the accumulated value δi.sub.TX[k].

8. The method for diagnosing the open-circuit fault of the power transistor according to claim 7, wherein: in Step (5), when a maximum value of the accumulated value δi.sub.TX[k] of the residual of the three-phase current during the basic cycle T.sub.0 is greater than the set threshold Th, it is determined that a fault has occurred.

9. A model-based system for diagnosing an open-circuit fault of a power transistor of a three-phase converter, comprising: a diagnosis signal obtaining module, configured to obtain relevant signals for diagnosis from a control system of the converter, wherein the relevant signals comprise an alternating current side three-phase current sampling signal i.sub.X[k] of the converter, an alternating current side three-phase voltage sampling signal e.sub.X[k], a direct current side voltage sampling signal U.sub.dc[k], and drive signals s.sub.1[k] to s.sub.6[k] output by the control system, where a subscript X (=A, B, or C) represents a present phase sequence and k represents a sampling time; a first calculation module, configured to calculate an estimated change value Δi.sub.EX[k] of a three-phase current during each switching cycle T.sub.s through the alternating current side three-phase voltage sampling signal e.sub.X[k], the direct current side voltage sampling signal U.sub.dc[k], and the drive signals s.sub.1[k] to s.sub.6[k] output by the control system according to a converter model; a second calculation module, configured to calculate a residual δi.sub.X[k] of the three-phase current during each switching cycle T.sub.s according to a change value Δi.sub.X[k] of a three-phase current sampling signal during each switching cycle T.sub.s and the estimated change value Δi.sub.EX[k] of the three-phase current; a third calculation module, configured to calculate an accumulated value δi.sub.TX[k] of a residual of the three-phase current during one basic cycle T.sub.0 according to the residual δi.sub.X[k] of the three-phase current during each switching cycle T.sub.s; and a fault diagnosis module, configured to determine the power transistor that has the open-circuit fault according to a comparison between the accumulated value δi.sub.TX[k] of the residual of the three-phase current during each basic cycle T.sub.0 and a threshold Th.

10. A computer-readable storage medium stored with a computer program, characterized in that when the computer program is executed by a processor, steps of the method according to claim 1 are implemented.

11. The method for diagnosing the open-circuit fault of the power transistor according to claim 2, wherein: in Step (3), the change value Δi.sub.X[k] of the three-phase current sampling signal during each switching cycle T.sub.s calculated from Δi.sub.X[k]=i.sub.X[k]−i.sub.X[k−1] refers to a difference between three-phase current sampling signal i.sub.X[k] corresponding to a start and an end of each switching cycle T.sub.s.

12. The method for diagnosing the open-circuit fault of the power transistor according to claim 11, wherein: in Step (4), the basic cycle T.sub.0 refers to a reciprocal of a three-phase voltage frequency f.sub.0, and the accumulated value δi.sub.TX[k] of the residual of the three-phase current during the basic cycle T.sub.0 calculated from δi.sub.TX[k]=δi.sub.X[k−T.sub.0/T.sub.s+1]+δi.sub.X[k−T.sub.0/T.sub.s+2]+ . . . +δi.sub.X[k−1]+δi.sub.X[k] refers to a sum of the residual δi.sub.X[k] of the three-phase current during all the switching cycles during the previous basic cycle T.sub.0 of a current time.

13. The method for diagnosing the open-circuit fault of the power transistor according to claim 12, wherein: in Step (5), the threshold Th refers to a threshold value set to prevent misdiagnosis, after the accumulated value δi.sub.TX[k] of the residual of the three-phase current during the basic cycle T.sub.0 exceeds the threshold Th, it is determined that the power transistor has the open-circuit fault.

14. The method for diagnosing the open-circuit fault of the power transistor according to claim 13, wherein: in Step (5), for different faults of the power transistor, the accumulated value δi.sub.TX[k] of the residual of the three-phase current during the basic cycle T.sub.0 is different, so that a faulty power transistor can be located through the accumulated value δi.sub.TX[k].

15. The method for diagnosing the open-circuit fault of the power transistor according to claim 14, wherein: in Step (5), when a maximum value of the accumulated value δi.sub.TX[k] of the residual of the three-phase current during the basic cycle T.sub.0 is greater than the set threshold Th, it is determined that a fault has occurred.

16. The method for diagnosing the open-circuit fault of the power transistor according to claim 3, wherein: in Step (3), the change value Δi.sub.X[k] of the three-phase current sampling signal during each switching cycle T.sub.s calculated from Δi.sub.X[k]=i.sub.X[k]−i.sub.X[k−1] refers to a difference between three-phase current sampling signal i.sub.X[k] corresponding to a start and an end of each switching cycle T.sub.s.

17. The method for diagnosing the open-circuit fault of the power transistor according to claim 16, wherein: in Step (4), the basic cycle T.sub.0 refers to a reciprocal of a three-phase voltage frequency f.sub.0, and the accumulated value δi.sub.TX[k] of the residual of the three-phase current during the basic cycle T.sub.0 calculated from δi.sub.TX[k]=δi.sub.X[k−T.sub.0/T.sub.s+1]+δi.sub.X[k−T.sub.0/T.sub.s+2]+ . . . +δi.sub.X[k−1]+δi.sub.X[k] refers to a sum of the residual δi.sub.X[k] of the three-phase current during all the switching cycles during the previous basic cycle T.sub.0 of a current time.

18. The method for diagnosing the open-circuit fault of the power transistor according to claim 17, wherein: in Step (5), the threshold Th refers to a threshold value set to prevent misdiagnosis, after the accumulated value δi.sub.TX[k] of the residual of the three-phase current during the basic cycle T.sub.0 exceeds the threshold Th, it is determined that the power transistor has the open-circuit fault.

19. The method for diagnosing the open-circuit fault of the power transistor according to claim 18, wherein: in Step (5), for different faults of the power transistor, the accumulated value δi.sub.TX[k] of the residual of the three-phase current during the basic cycle T.sub.0 is different, so that a faulty power transistor can be located through the accumulated value δi.sub.TX[k].

20. The method for diagnosing the open-circuit fault of the power transistor according to claim 19, wherein: in Step (5), when a maximum value of the accumulated value δi.sub.TX[k] of the residual of the three-phase current during the basic cycle T.sub.0 is greater than the set threshold Th, it is determined that a fault has occurred.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] FIG. 1 is a schematic diagram of a main circuit and a control system of a three-phase two-level converter (rectifier mode) system according to an embodiment of the disclosure,

[0034] FIG. 2 is a schematic flowchart of a model-based method for diagnosing an open-circuit fault of a power transistor according to an embodiment of the disclosure.

[0035] FIG. 3 is a schematic diagram of an experimental result of adopting a method of the disclosure after a power transistor has an open-circuit fault according to an embodiment of the disclosure.

DETAILED DESCRIPTION OF DISCLOSED EMBODIMENTS

[0036] In order for the objectives, technical solutions, and advantages of the disclosure to be clearer, the following further describes the disclosure in detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the disclosure, but not to limit the disclosure. In addition, the technical features involved in the various embodiments of the disclosure described below may be combined with each other as long as there is no conflict therebetween.

[0037] In the example of the disclosure, “first”, “second”, etc. are used to distinguish different objects and are not necessarily used to describe a specific order or sequence.

[0038] As shown in FIG. 1, a typical three-phase two-level converter includes six power transistors S.sub.1 to S.sub.6 and six matching diodes D.sub.1 to D.sub.6. The power transistors, the diodes, a filter inductor L, and a filter capacitor C jointly form a main circuit portion of the converter. A control system of the converter obtains a three-phase alternating current voltage e.sub.X[k], a three-phase alternating current sampling signal i.sub.X[k], and a direct current voltage value U.sub.dc[k] of the main circuit through a sensor and an analog-to-digital converter (ADC), and calculates and outputs drive signals s.sub.1 to s.sub.6 to control the work of each power transistor.

[0039] The schematic diagram of the diagnosis method of the disclosure is shown in FIG. 2. The experimental result of the embodiment is shown in FIG. 3, where (a) of FIG. 3 represents the three-phase current.

[0040] (1) Through three-phase current sampling signal i.sub.X[k] corresponding to a start and an end of each switching cycle T.sub.s, a change value Δi.sub.X[k] of the three-phase current sampling signal during each switching cycle T.sub.s is calculated, that is:

[00001]$\Delta i_X\left[k\right]=i_X\left[k\right]-i_X\left[k-1\right]$

[0041] Taking A-phase as an example, a calculation result Δi.sub.A[k] is shown in (b) of FIG. 3.

[0042] (2) It is necessary to combine a converter model, an estimated change value Δi.sub.EX[k] of a three-phase current during each switching cycle T.sub.s is calculated through an alternating current side three-phase current sampling signal i.sub.X[k] of the converter, an alternating current side three-phase voltage sampling signal e.sub.X[k], a direct current side voltage sampling signal U.sub.dc[k], and drive signals s.sub.1[k] to s.sub.6[k] output by the control system.

[0043] In the converter, since the power transistors with the same phase cannot be turned on at the same time, a three-phase state S.sub.X is commonly used to represent a working state of a three-phase power transistor. When S.sub.X=1, it represents that upper bridge arm of an X-phase is turned on. When S.sub.X=0, it represents that lower bridge arm of the X-phase is turned on. A relationship between the three-phase state S.sub.X and the drive signals s.sub.1 to s.sub.6 and a direction p.sub.X[k] of the three-phase current sampling signal i.sub.X[k] is:

[00002]$\{\begin{array}{c}{S}_A={\stackrel{\_}{p}}_A.Math.s_1+p_A.Math.s_4\\ S_B={\stackrel{\_}{p}}_B.Math.s_3+p_B.Math.s_6\\ S_C={\stackrel{\_}{p}}_C.Math.s_5+p_C.Math.s_2\end{array}\hspace{1em}$

[0044] where p.sub.X[k] refers to the direction of the three-phase current sampling signal i.sub.X[k], which is a 0-1 variable. When the three-phase current sampling signal i.sub.X[k]>0, p.sub.X[k]=1, p.sub.X[k]=0, and conversely, p.sub.X[k]=0, p.sub.X[k]=1.

[0045] In addition, a basic vector V.sub.n may also be used to represent the state of the three-phase power transistor, where n=0, 1, 2, 3, 4, 5, 6, 7, and the relationship with the three-phase state S.sub.X is shown in Table 1.

TABLE-US-00001 TABLE 1 Basic vector V.sub.n A-phase state S.sub.A B-phase state S.sub.B C-phase state S.sub.C V.sub.0 0 0 0 V.sub.1 1 0 0 V.sub.2 1 1 0 V.sub.3 0 1 0 V.sub.4 0 1 1 V.sub.5 0 0 1 V.sub.6 1 0 1 V.sub.7 1 1 1

[0046] In the embodiment of the disclosure, the control system adopts double closed-loop control and seven-segment space vector pulse width modulation (SVPWM). Each different basic vector V.sub.n (phase state S.sub.X) corresponds to a respective action time t.sub.n.

[0047] In the embodiment of the disclosure, a mixed logic dynamic model is used to calculate the estimated change value Δi.sub.EX[k] of the three-phase current during each switching cycle T.sub.s. The mixed logic dynamic model regards a current change in a switching cycle as a piecewise function, and calculates the estimated change value Δi.sub.EX[k] of the three-phase current during each switching cycle T.sub.s through calculating a slope K.sub.n.sup.X of each segment and the corresponding time t.sub.n. In the three-phase two-level converter, the converter model may be obtained according to Kirchhoff s voltage law:

[00003]$L\frac{{\mathrm{di}}_X}{\mathrm{dt}}+{\mathrm{Ri}}_X=e_X-U_{dc}{S}_X-U_{\mathrm{MN}}$

[0048] According to Kirchhoff's voltage and current laws, it can be known that i.sub.A[k]+i.sub.B[k]+i.sub.C[k]=0, e.sub.A[k]+e.sub.B[k]+e.sub.C[k]=0, so it can be obtained that:

[00004]$U_{\mathrm{MN}}=-\frac{S_A+S_B+S_C}{3}{U}_{dc}$

[0049] According to the above equation, the calculation equation of the slope K.sub.n.sup.X may be obtained:

[00005]$\left[\begin{array}{c}{K}_n^A\\ K_n^B\\ K_n^C\end{array}\right]=\frac{d}{\mathrm{dt}}\left[\begin{array}{c}{i}_A\\ i_B\\ i_C\end{array}\right]=-\frac{R}{L}\left[\begin{array}{c}{i}_A\\ i_B\\ i_C\end{array}\right]+\frac{1}{L}\left[\begin{array}{c}{e}_A\\ e_B\\ e_C\end{array}\right]-\frac{U_{dc}}{3L}\left[\begin{array}{ccc}2& -1& -1\\ -1& 2& -1\\ -1& -1& 2\end{array}\right]\left[\begin{array}{c}{S}_A\\ S_B\\ S_C\end{array}\right]$

[0050] Therefore, the basic vector V.sub.n and the corresponding time t.sub.n may be used to calculate the estimated change value Δi.sub.EX[k] of the three-phase current during each switching cycle T.sub.s:

[00006]$\Delta i_{\mathrm{EX}}\left[k\right]=\underset{n=0}{\overset{7}{.Math.}}{K}_n^X{t}_n$

[0051] Taking A-phase as an example, a calculation result Δi.sub.EA[k] is shown in (c) of FIG. 3.

[0052] (3) A residual δi.sub.X[k] of the three-phase current during each switching cycle is calculated through the above the calculation result Δi.sub.EX[k] and the change value Δi.sub.X[k], that is:

[00007]$\delta i_X\left[k\right]=\Delta i_X\left[k\right]-\Delta i_{\mathrm{EX}}\left[k\right]$

[0053] Taking A-phase as an example, a calculation result δi.sub.A[k] is shown in (d) of FIG. 3.

[0054] (4) The residual δi.sub.X[k] of the three-phase current during all the switching cycles during a previous basic cycle T.sub.0 of a current time is accumulated to obtain an accumulated value δi.sub.TX[k] of the residual of the three-phase current during the basic cycle T.sub.0, that is:

[00008]$\delta i_{\mathrm{TX}}\left[k\right]=\delta i_X\left[k-T_0/T_s+1\right]+\delta i_X\left[k-T_0/T_s+2\right]+\mathrm{.Math.}+\delta i_X\left[k-1\right]+\delta i_X\left[k\right]$

[0055] The results of accumulated values δi.sub.TA[k], δi.sub.TB[k], and δi.sub.TC[k] of the residual of the three-phase current during the basic cycle T.sub.0 are respectively shown in (e), (f), and (g) of FIG. 3.

[0056] (5) According to a comparison between the accumulated value δi.sub.TX[k] of the residual of the three-phase current during one basic cycle T.sub.0 and a threshold Th, the power transistor that has the open-circuit fault is determined.

[0057] When the six power transistors are all normal, the estimated change value Δi.sub.EX[k] of the three-phase current during each switching cycle T.sub.s is basically the same as the change value Δi.sub.X[k] of the three-phase current sampling signal during each switching cycle T.sub.s. Therefore, the accumulated value δi.sub.TX[k] of the residual of the three-phase current during the basic cycle T.sub.0 is very small, as shown in (e), (f), and (g) of FIG. 3, before the power transistor S.sub.1 is faulty, the accumulated value δi.sub.TX[k] of the residual of the three-phase current during the basic cycle T.sub.0 is very small. When the power transistor has the open-circuit fault, the accumulated value δi.sub.TX[k] of the residual of the three-phase current during the basic cycle T.sub.0 increases, as shown in (e), (f), and (g) of FIG. 3, after the power transistor S.sub.1 is faulty, the accumulated value δi.sub.TX[k] of the residual of the three-phase current during the basic cycle T.sub.0 changes rapidly. Therefore, the same may be used as a judgment basis for the fault diagnosis. Due to the influence of factors such as sampling error, dead zone, and inductance error, as shown in (d) of FIG. 3, before the power transistor S.sub.1 is faulty, an A-phase current residual δi.sub.A[k] is not zero during each switching cycle T.sub.s. Therefore, the accumulated value δi.sub.TA[k] of a residual of an A-phase current during the basic cycle T.sub.0 is also not zero. In order to prevent misdiagnosis, only when a maximum value of the accumulated value δi.sub.TX[k] of the residual of the three-phase current during the basic cycle T.sub.0 is greater than the set threshold Th, it is determined that a fault has occurred. The disclosure has low requirements for the setting of the threshold Th, and 80% to 200% of a rated current value may be selected as the threshold Th according to the requirements for the diagnosis speed and the reliability of the diagnosis. In the embodiment of the disclosure, 150% of the rated current (12 A) is selected as the threshold Th.

[0058] When the maximum value of the accumulated value δi.sub.TX[k] of the residual of the three-phase current during the basic cycle T.sub.0 exceeds the threshold Th, the faulty power transistor is located according to Table 2, and corresponding fault signals F.sub.n (n=1, 2, 3, 4, 5, 6) of the power transistor change from 0 to 1, where F.sub.1 represents that the power switching transistor S.sub.1 is faulty, F.sub.2 represents that the power switching transistor S.sub.2 is faulty, F.sub.3 represents that the power switching transistor S.sub.3 is faulty, F.sub.4 represents that the power switching transistor S.sub.4 is faulty, F.sub.5 represents that the power switching transistor S.sub.5 is faulty, and F.sub.6 represents that the power switching transistor S.sub.6 is faulty.

TABLE-US-00002 TABLE 2 Faulty power transistor Phase δi.sub.TA δi.sub.TB δi.sub.TC None None −Th < −Th < −Th < δi.sub.TA < Th δi.sub.TB < Th δi.sub.TC < Th S.sub.1 A  >Th <0 <0 S.sub.4 A  <−Th >0 >0 S.sub.3 B <0  >Th <0 S.sub.6 B >0  <−Th >0 S.sub.5 C <0 <0  >Th S.sub.2 C >0 >0  <−Th

[0059] As shown in (e), (f), and (g) of FIG. 3, after the power transistor S.sub.1 is faulty, the accumulated value δi.sub.TA[k] rapidly exceeds the threshold Th, and at this time, the accumulated values δi.sub.TB[k] and δi.sub.TC[k] are both less than 0, which complies with Table 2. As shown in (h) of FIG. 3, 0.9 ms after the power transistor S.sub.1 is faulty, F.sub.1 rapidly changes from 0 to 1, and F.sub.2 to F.sub.6 remain at 0, that is, the model-based fault diagnosis algorithm of the disclosure can quickly and accurately complete the diagnosis of the open-circuit fault of the three-phase two-level power transistor.

[0060] The disclosure also provides a model-based system for diagnosing an open-circuit fault of a power transistor of a three-phase converter, which includes the following.

[0061] A diagnosis signal obtaining module is configured to obtain relevant signals for diagnosis from a control system of the converter. The relevant signals include an alternating current side three-phase current sampling signal i.sub.X[k] of the converter, an alternating current side three-phase voltage sampling signal e.sub.X[k], where the subscript X (=A, B, or C) represents a present phase sequence and k represents a sampling time, a direct current side voltage sampling signal U.sub.dc[k], and drive signals s.sub.1[k] to s.sub.6[k] output by the control system.

[0062] A first calculation module is configured to calculate an estimated change value Δi.sub.EX[k] of a three-phase current during each switching cycle T.sub.s through the alternating current side three-phase voltage sampling signal e.sub.X[k], the direct current side voltage sampling signal U.sub.dc[k], and the drive signals s.sub.1[k] to s.sub.6[k] output by the control system according to a converter model.

[0063] A second calculation module is configured to calculate a residual δi.sub.X[k] of the three-phase current during each switching cycle T.sub.s according to a change value Δi.sub.X[k] of a three-phase current sampling signal during each switching cycle T.sub.s and the estimated change value Δi.sub.EX[k] of the three-phase current.

[0064] A third calculation module is configured to calculate an accumulated value δi.sub.TX[k] of a residual of the three-phase current during one basic cycle T.sub.0 according to the residual δi.sub.X[k] of the three-phase current during each switching cycle T.sub.s.

[0065] A fault diagnosis module is configured to determine the power transistor that has the open-circuit fault according to a comparison between the accumulated value δi.sub.TX[k] of the residual of the three-phase current during each basic cycle T.sub.0 and a threshold Th.

[0066] For the specific implementation of each module, reference may be made to the description of the foregoing embodiment of the method, which will not be repeated in the embodiment of the disclosure.

[0067] It should be noted that according to implementation requirements, each step/component described in the disclosure may be split into more steps/components or two or more steps/components or partial operation of a step/component may be combined into a new step/component to implement the objective of the disclosure.

[0068] Persons skilled in the art may easily understand that the above are only preferred embodiments of the disclosure and are not intended to limit the disclosure. Any modification, equivalent replacement, improvement, etc., made within the spirit and principle of the disclosure should be included in the protection scope of the disclosure.