METHOD AND SYSTEM FOR DIAGNOSING OPEN CIRCUIT (OC) FAULT OF T-TYPE THREE-LEVEL (T23L) INVERTER UNDER MULTIPLE POWER FACTORS

Abstract

A method and a system for diagnosing an open circuit (OC) fault of an insulated gate bipolar transistor (IGBT) of a T-type three-level (T.sup.23L) inverter under multiple power factors based on instantaneous current distortion are provided. Similar characteristics of current distortion may be caused by an OC fault of a T.sup.23L inverter, making it is difficult to locate the fault. The method for diagnosing an OC fault of a grid-connected T.sup.23L inverter, can diagnose the OC fault hierarchically; four switch transistors in a phase can be divided into two groups according to the similarity analysis of current distortion under different power factors; group-based fault diagnosis is realized by half cycles in which a zero domain occurs; and then, a specific switching signal is injected to realize equipment-based OC fault diagnosis. The OC fault diagnosis of a T.sup.23L inverter is realized without additional hardware circuits.

Claims

1. A method for diagnosing an open circuit (OC) fault of an insulated gate bipolar transistor (IGBT) of a T-type three-level (T.sup.23L) inverter under multiple power factors based on instantaneous current distortion, comprising: collecting target parameters of a T.sup.23L inverter in real time, wherein the target parameters comprise an output current ix, a grid-connected current command i.sub.d.sub._ref of axis d, a grid-connected current command i.sub.q.sub._ref of axis q, a grid voltage ex, and a switching cycle T.sub.s, X represents a fault phase, X=A, B, C; calculating a theoretical zero crossing point based on the target parameters to obtain positive and negative half-cycle regions, determining positive and negative half cycles in which a zero domain occurs, performing group-based OC fault diagnosis, and outputting a group-based fault diagnosis signal; injecting a specific switching signal to a space vector pulse width modulation (SVPWM) or sinusoidal pulse width modulation (SPWM) module based on the group-based fault diagnosis signal; and determining, based on a current value ix under the specific switching signal, whether a current is in the zero domain, performing equipment-based OC fault diagnosis, and outputting an equipment-based fault diagnosis signal, wherein when a power factor pf is greater than or equal to 0, theoretical zero crossing points t.sub.p2z and t.sub.n2z are obtained according to: $\left\{\begin{array}{c}{t}_{n2z}=T_s-t_{pf}\\ t_{p2z}=T_{s_{/2}}-t_{pf}\end{array}\right);\text{and}$ when the power factor pf is less than 0, the theoretical zero crossing points t.sub.p2z and t.sub.n2z are obtained according to: $\left\{\begin{array}{c}{t}_{n2z}=t_{pf}\\ t_{p2z}=T_{\raisebox{1ex}{$s$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}+t_{pf}\end{array},\right)$ wherein t.sub.p2z, represents time when the current changes from a positive current to a zero current, t.sub.n2z represents time when the current changes from a negative current to the zero current, T.sub.s represents the switching cycle, and t.sub.pf represents leading or lagging time under different power factors; rules for group-based OC fault diagnosis are as follows: TABLE-US-00006 Group-based fault Range in which the zero domain occurs ZS.sub.n2z + ZS.sub.p2z pf F.sub.X1/4 (t.sub.n2z+ D.sub.th, T.sub.s- D.sub.th) ∪ (D.sub.th, t.sub.p2z- D.sub.th) 1 pf ≥ 0 (t.sub.n2Z+ D.sub.th, t.sub.p2z- D.sub.th) 1 pf< 0 F.sub.X2/3 (t.sub.p2z+ D.sub.th, t.sub.n2z- D.sub.th) 1 pf ≥ 0 (t.sub.p2z+ D.sub.th, T.sub.s- D.sub.th) ∪ (D.sub.th, t.sub.n2z- D.sub.th) 1 pf< 0 wherein, F.sub.X1/4 and F.sub.X2/3 represent group-based fault diagnosis signals, t.sub.n2z and t.sub.p2z, represent the theoretical zero crossing points, D.sub.th represents a first preset time threshold, ZS.sub.n2z represents a zero-domain mark from the negative current to the zero current, ZS.sub.p2z represents a zero-domain mark from the positive current to the zero current, pf represents the power factor, pf>0 indicates that the current leads a voltage, and pf<0 indicates the current lags behind the voltage; when F.sub.X1/4 is 1, it indicates that switch transistor X1 or switch transistor X4 is faulty, and when F.sub.X2/3 is 1, it indicates that switch transistor X2 or switch transistor X3 is faulty; and in the step of injecting a specific switching signal to an SVPWM or SPWM module based on the group-based fault diagnosis signal: when F.sub.X1/4 is 1, specific switching signals [S.sub.A1, S.sub.A3, S.sub.A2, S.sub.A4] are [1,0,0,0]; and when F.sub.X2/3 is 1, the specific switching signals [S.sub.A1, S.sub.A3, S.sub.A2, S.sub.A4] are [0,0,1,0]; and rules for equipment-based OC fault diagnosis are as follows: TABLE-US-00007 Equipment-based fault Group-based fault [S.sub.x1, S.sub.X3, S.sub.X2, S.sub.X4] |i.sub.x| N F.sub.X1 F.sub.X1/4 [1, 0, 0, 0] ≤I.sub.th2 ≥N.sub.th F.sub.X4 >Ith2 <Nth F.sub.X2 F.sub.X2/3 [0, 0, 1, 0] < I.sub.th2 ≥N.sub.th F.sub.X3 >Ith2 <Nth wherein, I.sub.th2 represents a preset current threshold, N represents duration in which the current is in the zero domain, N.sub.th represents a second preset time threshold, and F.sub.xy(Y = 1, 2, 3, 4) represents the equipment-based fault diagnosis signal, when F.sub.X1 is 1, it indicates that switch transistor X1 is faulty; when F.sub.X2 is 1, it indicates that switch transistor X2 is faulty; when F.sub.X3 is 1, it indicates that switch transistor X3 is faulty; and when Fx4 is 1, it indicates that switch transistor X4 is faulty.

2. The method according to claim 1, wherein the leading or lagging time t.sub.pf under different power factors is obtained according to: $t_{pf}=\frac{\theta }{2\pi f_s},$ wherein θ represents a power factor angle, and f.sub.s represents a switching frequency.

3. The method according to claim 2, wherein the power factor angle θ is obtained according to: $\cos \left(\theta \right)=\frac{i_{d\_ref}}{\sqrt{i_{d\_ref}^2+i_{q\_ref}^2}},$ wherein i.sub.d_.sub.ref and i.sub.q.sub._ref represent the grid-connected current commands of axis d and axis q respectively.

4. A system for diagnosing an OC fault of an IGBT of a T.sup.23L inverter under multiple power factors based on instantaneous current distortion, comprising: a parameter obtaining module configured to collect target parameters of a T.sup.23L inverter in real time, wherein the target parameters comprise an output current ix, a grid-connected current command i.sub.d.sub._ref of axis d, a grid-connected current command iq ref of axis q, a grid voltage ex, and a switching cycle T.sub.s, and X represents a fault phase, wherein X = A, B, C; a group-based fault diagnosis module configured to calculate a theoretical zero crossing point based on the target parameters to obtain positive and negative half-cycle regions, determine positive and negative half cycles in which a zero domain occurs, perform group-based OC fault diagnosis, and output a group-based fault diagnosis signal; a specific-switching signal injection module configured to inject a specific switching signal to an SVPWM or SPWM module based on the group-based fault diagnosis signal; and an equipment-based fault diagnosis module configured to determine, based on a current value ix under the specific switching signal, whether a current is in the zero domain, perform equipment-based OC fault diagnosis, and output an equipment-based fault diagnosis signal, wherein when a power factor pf is greater than or equal to 0, theoretical zero crossing points t.sub.p2z and t.sub.n2z are obtained according to: $\left\{\begin{array}{c}{t}_{n2z}=T_s-t_{pf}\\ t_{p2z}=T_{\raisebox{1ex}{$s$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}-t_{pf}\end{array}\right);\text{and}$ when the power factor pf is less than 0, the theoretical zero crossing points t.sub.p2z and t.sub.n2z are obtained according to: $\left\{\begin{array}{c}{t}_{n2z}=t_{pf}\\ t_{p2z}=T_{\raisebox{1ex}{$s$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}+t_{pf}\end{array},\right)$ wherein t.sub.p2z, represents time when the current changes from a positive current to a zero current, t.sub.n2z represents time when the current changes from a negative current to the zero current, T.sub.s represents the switching cycle, and t.sub.pf represents leading or lagging time under different power factors; rules for group-based OC fault diagnosis are as follows: TABLE-US-00008 Group-based fault Range in which the zero domain occurs ZS.sub.n2z + ZS.sub.p2z pf F.sub.X1/4 (t.sub.n2z+ D.sub.th, T.sub.s- D.sub.th) ∪ (D.sub.th, t.sub.p2z- D.sub.th) 1 pf ≥ 0 (t.sub.n2Z+ D.sub.th, t.sub.p2z- D.sub.th) 1 pf< 0 F.sub.X2/3 (t.sub.p2z+ D.sub.th, t.sub.n2z- D.sub.th) 1 pf ≥ 0 (t.sub.p2z+ D.sub.th, T.sub.s- D.sub.th) ∪ (D.sub.th, t.sub.n2z- D.sub.th) 1 pf< 0 wherein, F.sub.X1/4 and F.sub.X2/3 represent group-based fault diagnosis signals, t.sub.n2z and t.sub.p2z, represent the theoretical zero crossing points, D.sub.th represents a first preset time threshold, ZS.sub.n2z represents a zero-domain mark from the negative current to the zero current, ZS.sub.p2z represents a zero-domain mark from the positive current to the zero current, pfrepresents the power factor, pf>0 indicates that the current leads a voltage, and pf<0 indicates the current lags behind the voltage; when F.sub.X1/4 is 1, it indicates that switch transistor X1 or switch transistor X4 is faulty, and when F.sub.X2/3 is 1, it indicates that switch transistor X2 or switch transistor X3 is faulty; in the step of injecting a specific switching signal to an SVPWM or SPWM module based on the group-based fault diagnosis signal: when F.sub.X1/4 is 1, specific switching signals [S.sub.A1, S.sub.A3, S.sub.A2, S.sub.A4] are [1,0,0,0]; and when F.sub.X2/3 is 1, the specific switching signals [S.sub.A1, S.sub.A3, S.sub.A2, S.sub.A4] are [0,0,1,0]; and rules for equipment-based OC fault diagnosis are as follows: TABLE-US-00009 Equipment-based fault Group-based fault [S.sub.X1, S.sub.X3, S.sub.X2, S.sub.X4] |i.sub.X| N F.sub.X1 F.sub.X1/4 [1, 0, 0, 0] < I.sub.th2 ≥N.sub.th F.sub.X4 >Ith2 <Nth F.sub.X2 F.sub.X2/3 [0, 0, 1, 0] < Ith2 ≥N.sub.th F.sub.X3 >Ith2 <Nth wherein, I.sub.th2 represents a preset current threshold, N represents duration in which the current is in the zero domain, N.sub.th represents a second preset time threshold, and F.sub.XY(Y = 1, 2, 3, 4) represents the equipment-based fault diagnosis signal, when F.sub.X1 is 1, it indicates that switch transistor X1 is faulty; when F.sub.X2 is 1, it indicates that switch transistor X2 is faulty; when F.sub.X3 is 1, it indicates that switch transistor X3 is faulty; and when Fx4 is 1, it indicates that switch transistor X4 is faulty.

5. A computer-readable storage medium, which stores a computer program, wherein the computer program is executed by a processor to implement the steps of the method according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] FIG. 1 is a schematic flowchart of a method for diagnosing an OC fault of an IGBT under multiple power factors based on instantaneous current distortion according to an embodiment of the present disclosure;

[0031] FIG. 2 is a schematic diagram showing a result of an OC fault diagnosis experiment under a unit power factor according to an embodiment of the present disclosure;

[0032] FIG. 3 is a schematic diagram showing a result of an OC fault diagnosis experiment under a leading power factor according to an embodiment of the present disclosure; and

[0033] FIG. 4 is a schematic diagram showing a result of an OC fault diagnosis experiment under a lagging power factor according to an embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0034] To make the objectives, technical solutions, and advantages of the present disclosure clearer, the present disclosure is further described below in detail with reference to the drawings and embodiments. Understandably, the specific embodiments described herein are merely intended to explain the present disclosure but not to limit the present disclosure. Further, the technical features involved in the various implementations of the present disclosure described below may be combined with each other as long as they do not constitute a conflict with each other.

[0035] In the embodiments of the present disclosure, the terms such as “first” and “second” are intended to distinguish between different objects, rather than describe a specific order or sequence.

Embodiment 1

[0036] The present disclosure summarizes characteristics of current distortion in the case of an OC fault, and realizes OC fault diagnosis based on the characteristics of the current distortion. As shown in FIG. 1, a method for diagnosing an OC fault of an IGBT under multiple power factors based on instantaneous current distortion according to the present disclosure includes the following steps: [0037] S1: Collect an output current i.sub.X(X = A, B, C), grid-connected current commands i.sub.d_.sub.ref andi.sub.q_.sub.ref, and a grid voltagee.sub.x of a T.sup.23L inverter in real time, obtain a system parameter, namely, a switching cycle T.sub.s, calculate a theoretical zero crossing point to obtain positive and negative half-cycle regions, determine positive and negative half cycles in which a zero domain occurs, perform group-based OC fault diagnosis, and output a group-based fault diagnosis signal F.sub.X1/4 or F.sub.X2/3. [0038] S2: Inject a specific switching signal to an SVPWM or SPWM module based on the group-based fault diagnosis signal. [0039] S3: Determine, based on a current value .sub.iX under the specific switching signal, whether a current is in the zero domain, perform equipment-based OC fault diagnosis, and output an equipment-based fault diagnosis signal F.sub.XY(Y= 1, 2, 3, 4).

[0040] When a power factor pf is greater than or equal to 0, theoretical zero crossing points t.sub.p2Z and t.sub.n2z are obtained according to:

[00005]$\left\{\begin{array}{c}{t}_{n2z}=T_s-t_{pf}\\ t_{p2z}=\raisebox{1ex}{$T_s$}\!\left/ \!\raisebox{-1ex}{$2$}\right.-t_{pf}\end{array}\right);$

and when the power factor pf is less than 0, the theoretical zero crossing points t.sub.p2,, and t.sub.n2z are obtained according to:

[00006]$\left\{\begin{array}{c}{t}_{n2z}=t_{pf}\\ t_{p2z}=\raisebox{1ex}{$T_s$}\!\left/ \!\raisebox{-1ex}{$2$}\right.+t_{pf}\end{array}\right),$

wherein t.sub.p2z represents time when the current changes from a positive current to a zero current, t.sub.n2z represents time when the current changes from a negative current to the zero current, T.sub.s represents the switching cycle, and t.sub.pf represents leading or lagging time under different power factors.

[0041] Specifically, t.sub.pf is obtained according to:

[00007]$t_{pf}=\frac{\theta }{2\pi \text{}{f}_s},$

wherein θ represents a power factor angle, and f.sub.s represents a switching frequency.

[0042] The power factor angle θ is calculated according to:

[00008]$\cos \left(\theta \right)=\frac{i_{d\_ref}}{\sqrt{i_{d\_ref}^2+i_{q\_ref}^2}},$

wherein i.sub.d_.sub.ref and i.sub.q_.sub.ref represent the grid-connected current commands of axis d and axis q respectively.

[0043] In step S1, rules for group-based OC fault diagnosis are as follows:

TABLE-US-00003 Rules for group-based OC fault diagnosis Group-based fault Range in which the zero domain occurs zs.sub.n2z + zs.sub.p2z pf F.sub.X1/4 (t.sub.n2z+ D.sub.th, T.sub.s- D.sub.th) ∪ (D.sub.th, t.sub.p2z- D.sub.th) 1 pf≥ 0 (t.sub.n2z+ D.sub.th, t.sub.p2z- D.sub.th) 1 pf< 0 F.sub.X2/3 (t.sub.p2z+ D.sub.th, t.sub.n2z- D.sub.th) 1 pf≥ 0 (t.sub.p2z+ Dth, T.sub.s- D.sub.th) ∪ (D.sub.th, t.sub.n2z- D.sub.th) 1 pf< 0

[0044] As described above, F.sub.X1/4 and F.sub.X2/3 represent group-based fault diagnosis signals, where the value 1 indicates that a faulty transistor is located in the group, and the value 0 indicates that the faulty transistor is not located in the group; t.sub.n2z and t.sub.p2z represent the theoretical zero crossing points; D.sub.th represents a first preset time threshold; zs.sub.n2z represents a zero-domain mark from the negative current to the zero current, where a value 1 indicates that the current is from a negative value to zero; zs.sub.p2z represents a zero-domain mark from the positive current to the zero current, where the value 1 indicates that the current is changed from the positive current to the zero current; pfrepresents the power factor; pf>0 indicates that the current leads a voltage; pf<0 indicates the current lags behind the voltage; and the zero domain indicates that the current is 0, namely, ix = 0.

[0045] In step S2, the specific switching signal is injected based on the group-based fault diagnosis signal. When F.sub.X1/4 is 1, specific switching signals [S.sub.A1, S.sub.A3, S.sub.A2, S.sub.A4] are [1,0,0,0]; and when F.sub.X2/3 is 1, the specific switching signals [S.sub.A1, S.sub.A3, S.sub.A2, S.sub.A4] are [0,0,1,0].

[0046] In step S3, rules for equipment-based OC fault diagnosis are as follows:

TABLE-US-00004 Rules for equipment-based OC fault diagnosis Equipment-based fault Group-based fault [S.sub.X1, S.sub.X3, S.sub.X2, S.sub.x4] |i.sub.x| N F.sub.X1 F.sub.X1/4 [1, 0, 0, 0] ≤ I.sub.th2 ≥ N.sub.th F.sub.X4 >I.sub.th2 <N.sub.th F.sub.X2 F.sub.X2/3 [0, 0, 1, 0] ≤ I.sub.th2 ≥ N.sub.th F.sub.X3 >I.sub.th2 <N.sub.th

[0047] As described above, I.sub.th2 represents a current threshold, N represents duration in which the current is in the zero domain, and N.sub.th represents a second preset time threshold. When F.sub.X1 is 1, it indicates that switch transistor X1 is faulty. Similarly, when F.sub.X2 is 1, it indicates that switch transistor X2 is faulty; when F.sub.X3 is 1, it indicates that switch transistor X3 is faulty; when Fx4 is 1, it indicates that switch transistor X4 is faulty, where X = A, B, C.

[0048] The method summarizes characteristics of an OC fault of an IGBT under different power factors. Characteristics of OC fault diagnosis of S.sub.X1 under different power factors are as follows: [0049] a) Under a leading power factor: t.sub.1 = t.sub.2 = t.sub.n2z, t4 = t.sub.p2z, zs.sub.p2z = 1. [0050] b) Under a unit power factor: t.sub.1 = t.sub.n2z, t.sub.4 = t.sub.p2z, zs.sub.n2z = 1. [0051] c) Under a lagging power factor: t.sub.1= t.sub.n2z, t.sub.3 = t.sub.4 = t.sub.p2z, zs.sub.n2z = 1.

[0052] As described above, t.sub.1 represents actual time from the negative current to the zero current, t.sub.2 represents actual time from the zero current to the positive current, t.sub.3 represents actual time from the positive current to the zero current, and t.sub.4 represents actual time from the zero current to the negative current.

[0053] Characteristics of OC fault diagnosis of S.sub.X4 under different power factors are as follows:

[00009]$t_1=t_{n2z},t_4=t_{p2z},Z{S}_{n2z}=1,Z{S}_{p2z}=1$

[0054] Characteristics of OC fault diagnosis of S.sub.X2 under different power factors are as follows: [0055] a) Under the leading power factor: t.sub.2 = t.sub.n2z, t.sub.3 = t.sub.4 = t.sub.p2z, zs.sub.n2z = 1. [0056] b) Under the unit power factor: t.sub.2 = t.sub.n2z, t.sub.3 = t.sub.p2z, zs.sub.p2z = 1. [0057] c) Under the lagging power factor: t.sub.1 = t.sub.2 = t.sub.n2z, t.sub.3 = t.sub.p2z, zs.sub.p2z = 1.

[0058] Characteristics of OC fault diagnosis of S.sub.X3 under different power factors are as follows:

[00010]$t_2=t_{n2z},t_3=t_{p2z},z{s}_{n2z}=1,z{s}_{p2z}=1$

[0059] This embodiment provides a simple method for diagnosing an OC fault of a grid-connected T.sup.23L inverter based on instantaneous current distortion. Firstly, characteristics of an OC fault of an output current under various power factors are analyzed and summarized in detail to provide a theoretical basis for the proposed diagnosis method. Secondly, a hierarchical diagnosis scheme is proposed to identify group-based and equipment-based faults. Finally, effectiveness and superiority of this method are verified by a large number of experiments. Compared with existing fault diagnosis methods, this method is applicable to various power factors of a photovoltaic grid-connected system and other systems. No additional sensor or sampling circuit is required. A sampling frequency is equal to the switching frequency. Time of group-based and equipment-based OC fault diagnosis is about half of a basic cycle. The diagnosis method is applicable to different power factors (including the unit power factor, the leading power factor, and the lagging power factor). Simple calculation and logical judgment are achieved.

Embodiment 2

[0060] To describe this embodiment more clearly, FIG. 2, FIG. 3 and FIG. 4 show experimental results under this embodiment. Main parameters used in the experimental results are shown in Table 3.

TABLE-US-00005 Main parameters used in the experimental results Parameter Symbol Value Grid-side voltage e.sub.X 50 Vrms Grid-side frequency f.sub.ac 50 Hz Direct current (DC)-side voltage U.sub.dc 200 V Switching/sampling frequency f.sub.s 10 kHz Dead time T.sub.D 0.1 .Math.s Filter inductance L.sub.f 4 mH DC-side capacitance C.sub.high/low 480 .Math.F Load power P 500 W

[0061] FIG. 2 shows a result of OC fault diagnosis of S.sub.A4 under a unit power factor. After an OC fault occurs on S.sub.A4, the number of times that zc.sub.n2z + zc.sub.p2z reaches a high level increases. A zero current lasts for a certain time and then becomes a zero domain, and zs.sub.n2z + zs.sub.p2z reaches the high level. It is known, by determining positive and negative half cycles in which the zero domain occurs, that S.sub.A1 or S.sub.A4 may be faulty, namely, F.sub.A1/4 is 1. A switching signal injection module sets switching signals [S.sub.A1, S.sub.A3, S.sub.A2, S.sub.A4] to [1,0,0,0]. If i.sub.A is not zero, it indicates that S.sub.A4 is faulty.

[0062] FIG. 3 shows a result of OC fault diagnosis of S.sub.A1 under a leading power factor. FIG. 4 shows a result of OC fault diagnosis of S.sub.A2 under a lagging power factor. When an OC fault occurs on an outer transistor (S.sub.Xi or S.sub.A2), a current of a fault phase is always zero after a specific signal is injected.

[0063] This embodiment is described for a specific OC fault, and an analysis result of another OC fault is the same as that in this embodiment.

Embodiment 3

[0064] This embodiment provides a system for diagnosing an OC fault of an IGBT under multiple power factors based on instantaneous current distortion, including: [0065] a parameter obtaining module configured to collect an output current i.sub.X(X = A, B, C), grid-connected current commands i.sub.d_.sub.ref and i.sub.q_ref, and a grid voltage e.sub.x of a T.sup.23L inverter in real time, obtain a system parameter, namely, a switching cycle T.sub.s, and calculate a theoretical zero crossing point to obtain positive and negative half-cycle regions; [0066] a group-based fault diagnosis module configured to calculate the theoretical zero crossing point to obtain the positive and negative half-cycle regions, determine positive and negative half cycles in which a zero domain occurs, perform group-based OC fault diagnosis, output a group-based fault diagnosis signal F.sub.X1/4or F.sub.X2/3; [0067] a specific-switching signal injection module configured to inject a specific switching signal based on a group-based fault diagnosis result, where when F.sub.X1/4 is 1, specific switching signals [S.sub.A1, S.sub.A3, S.sub.A2, S.sub.A4] are [1,0,0,0]; and when F.sub.X2/3 is 1, the specific switching signals [S.sub.A1, S.sub.A3, S.sub.A2, S.sub.A4] are [0,0,1,0]; and [0068] an equipment-based fault diagnosis module configured to start to detect a state of a current of a fault phase when the specific signal is injected, where if the specific switching signals [S.sub.A1, S.sub.A3, S.sub.A2, S.sub.A4] are [1,0,0,0] and the current is always zero, S.sub.A1 is faulty; or if the current is non-zero within specific time, S.sub.A4 is faulty.

[0069] It should be pointed out that, based on needs of implementation, each step/component described in the present disclosure can be divided into more steps/components, or two or more steps/components or some operations of the steps/components can be combined into a new step/component to achieve the objective of the present disclosure.

[0070] It is easy for those skilled in the art to understand that the above-mentioned contents are merely the preferred embodiments of the present disclosure, and are not intended to limit the present disclosure. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present disclosure should fall within the protection scope of the present disclosure.