Fault Tolerant Control Strategy of 3-phase AC-DC PWM Converter under Multiple Open-switch Faults Conditions

Won-Sang Im, Jong-Joo Moon, Jang-Mok Kim

Department of Electrical Engineering Pusan National University

Busan, KOREA {won42, moonjongjoo, jmok }@pusan.ac.kr

Dong-Choon Lee

Department of Electrical Engineering Yeungnam University Kyungsan, KOREA

dclee@yu.ac.kr

Kyo-Beum Lee

Department of Electronic Engineering Ajou University Suwon, KOREA kyl@ajou.ac.kr

Abstract—3-phase AC-DC PWM (Pulse Width Modulation) converter has a possibility of open-switch faults due to some trouble of the switching devices and gate drivers. In this case, the converter causes imbalance of the AC input currents and the pulsation of DC-link voltage. This paper proposes fault tolerant control strategy of 3-phase AC-DC PWM converter under multiple open-switch fault conditions. So a detection method of the open-switch faults and fault tolerance methods are suggested according to fault cases. In this paper, two or more switches fault cases are dealt with. Both computer simulation and experimental results verify the usefulness of the proposed fault-tolerant control algorithm.

I. INTRODUCTION The 3-phase pulse width-modulation (PWM) AC-DC

converter has been increasingly employed in recent years owing to its advanced features including sinusoidal input current with unity power factor and high-quality dc output voltage [1]. It is estimated that about 38% of the faults in voltage source conversion system are due to failures of power devices such as insulated gate bipolar transistors (IGBTs). IGBTs failures can be broadly categorized as short-circuit faults and open-switch faults [2].

If short-circuit fault is happened in the voltage source converter or inverter system, it causes system shutdown. So its source is blocked by protection circuits such as circuit breakers and fuses. Otherwise, the switching elements and peripheral devices which are the control board and gating circuit of the inverter are destroyed critically. Hence, protection circuits are most important in faulted circuit. On the other hand, open-switch fault of the system doesn’t cause system shutdown, but degrades its performance. In this case, the converter causes the imbalance of the AC input current and pulsation of DC-link voltage. If such an abnormal operation lasts continuously, accumulated fatigue with unstable operation may generate malfunctions of converter system. Accordingly, there is high possibility of secondary faults in the converter system, load, and grid. Therefore, the fault detection and the tolerant control technics are required

for open-switch faults to solve the above mentioned situation [3-6].

Some fault detection methods for open-switch fault of DC-AC inverter have been developed during the last decade [7-12]. However, inverter and converter have different current patterns each other under the same condition of IGBT open switch fault. Hence, new detection methods are required. In this paper, fault detection method of [13] is used for open-switch fault of PWM converter. In [13], a fault tolerant control method is also proposed under only one switch open fault condition. However, multiple faulty conditions are not considered. So, this paper proposes fault tolerant control strategy of 3-phase AC-DC PWM converter under multiple open switch fault condition. Two or more switches fault cases are dealt with. The usefulness of this paper is verified through the computer simulation and the experimentation, respectively.

II. OPEN-SWITCH FAULT OF CONVERTER Fig.1 shows 3-phase AC-DC PWM converter. The

converter can provide unity power factor and high-quality dc output voltage by using the PWM strategy of the converter. Generally, space vector PWM (SVPWM) can be used for the high performance of the converter. Fig.2 shows 3-phase currents and conducting devices of the normal operation of the PWM converter. One cycle of the currents can be divided by 12 regions as shown in Fig.2. The 3-phase currents have the same phase as grid 3-phase voltages due to unity power factor.

This work has been supported by KESRI (Korea Electrical Engineering and Science Research Institute) (2009T100100651), which is funded by MKE (Ministry of Knowledge Economy).

Fig.1 3-phase AC-DC PWM converter

978-1-4577-1216-6/12/$26.00 ©2012 IEEE 789

And, the output voltage of PI current regulators of the PWM converter is shown in Fig.3 as V*. The rotating direction is the counterwise.

Two switch faults can be classified as three cases. First case is two switches of any one phase, which is {S1, S4}, {S3, S6} and {S5, S2}. Second case is two of three top switches or two of three bottom switches, which is {S1, S3}, {S3, S5}, {S4, S6} and so on. Last case is one of three top switches and one of three bottom switches, which is {S1, S6}, {S5, S4} and so on. Typical three cases are shown in Fig.4.

Fig.4(a) shows an example of the first case. If {S1, S4} is faulted, all twelve regions of Fig.2 are affected. So, there are no normal controllable regions as shown in Fig.4(a). The distorted currents with an imbalance are appeared, and the a-phase current is similar to a current shape of well-known diode rectifier due to two switches fault of a-phase. In 3-phase AC-DC PWM converter, despite of open switches fault, the currents can flow like a diode rectifier because freewheeling diodes are conducted when one pole voltage is lower than the other poles. Fig.4(b) shows an example of the second case. In this case, two regions are controllable and ten regions are uncontrollable. Two regions among ten regions are affected by both switches. Fig.4(c) shows an example of the last case. This case has just ten regions because two uncontrollable regions are marked as box in Fig.4(c) are not appeared. Hence, ten regions can be divided as four controllable regions and six uncontrollable regions. Although there are four controllable

regions, the performance is very low. It is because any currents cannot flow around the disappeared regions.

If the converter has three or more switches open fault, the case has at least two fault phenomena simultaneously except a special case. Therefore, their current imbalances are poorer than two switches fault cases. However, in a case like three switches fault of top side or bottom side, although the currents are distorted and have many harmonics, there is no current imbalance as shown in Fig.5.

(a) {S1, S4} open switch fault case

(b) {S1, S3} open switch fault case

(c) {S1, S6} open switch fault case

Fig.4 Three cases of two switch open faults

Fig. 2 3-phase currents, conducting diodes and switches at normal operation

Fig.3 SVPWM hexagon

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The current imbalances cause DC-link voltage ripples and drops of efficiency and power factor. Hence, possibilities of secondary fault are increased. Therefore, fault detection is required. Also, a proper tolerant control strategy is needed according to the fault cases.

III. FAULT DETECTION METHOD[13] In order to detect an open switch fault, the fault detection

method uses a phenomenon of 2-phase conduction when the fault is happened. Fig.6 shows current angle of six cases with one switch fault. The detection regions are independent each other because each switch has each assigned region. Hence, it can use a superposition principle in the multi-fault conditions. Therefore, the detection method is also available under the multiple open switch faults condition.

For example, Fig.7 shows phase currents and the angle of the current vector before and after open switch fault of S1 and S3. At normal operation, balanced 3-phase currents and current angle of constant frequency are displayed. However, after two switches fault, distorted imbalance currents and current angle are appeared. The distorted current angle is represented as a sum of S1 and S3 switch fault cases. Therefore, the faulty switches can be found from the detection method.

As shown in Fig.6, in detection region ○1 , the starting current of faulty phase is zero and the current is not produced during π/6 due to associated switch fault. However, in detection region ○2 , the starting current is nonzero. So, the remaining current reduces into zero first of all. And then, the current maintains zero during the remaining interval.

Therefore, it is more difficult to detect the fault in detection region ○2 than in the detection region ○1 . Nevertheless, it is needed for fast fault detection time because the detection is possible under the low load conditions.

IV. FAULT TOLERANT CONTROL In normal operation of the converter without fault, voltage

output of SVPWM can be expressed as (1).

1 21 ,n n

S S

T TV V V

T Tα β α β+

⎛ ⎞= + = =⎜ ⎟

⎝ ⎠ (1)

where TS is sampling period of the PWM converter, T1 and T2 are two active vector times, and n is sector number of Fig.3. If any open switch fault is happened, the voltage output is distorted due to the faulty switches. One of twelve regions is related with three switches as shown in Fig.2. One fault of three switches is sorted by two cases. If first or last among three switches is fault, the faulted vector is distorted by only a zero vector like (2).

0 / 2ffault zero

S

TV V V

Tγ γ

⎛ ⎞= + =⎜ ⎟

⎝ ⎠ (2)

where T0 is zero vector time. Because symmetric PWM is used, a half of the T0 is distorted. Table I shows faulted zero vectors according to the switching elements and the direction of the phase current. Or, if the middle switch is fault, the faulted vector is distorted by not only a zero vector but also active vectors like (3).

( ) ffault zeroV V Vγ′= + (3)

So, the voltage vector by distorted active vector is represented as one available nearest active vector like (4).

( ) ( ) activeV Vα β′ = + (4)

In previous {S1, S4} fault case, above equations can be applied because only one switch is faulted per each region.

Fig. 6 Current angle of six cases with one open switch fault

Fig. 5 Switch fault case of {S1, S3, S5}

Fig.7 Phase currents and current angle before and after open switch fault of

S1 and S3

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However, (5) should be used for two switches fault including the middle switch.

( )fault activeV Vα β γ= + + (5)

Hence, in {S1, S3} fault case, the voltage vector can use only active vector V5 during two regions with two switches fault. And the duration of V5 is sum of α, β and γ.

In case of first and last switch fault, zero vectors cannot be produced due to the faulty two switches. Instead of the zero vectors, duration of remaining active vectors is increased as shown in (6).

( ) ( ) 1fault n nV V Vα γ β γ += + + + (6)

So, the stored current of the input reactor is just consumed during these regions because there is no zero vector time to store the currents. Therefore, in {S1, S6} fault case, the two switches fault regions are disappeared and all currents go toward into zero as shown in Fig.4(c).

In order to compensate the faulted voltage vectors, they need to recover or approach toward the original voltage vector. Discontinuous PWM (DPWM) technic [14-16] can be used for the compensation. Hence, (2) can change as (7) using DPWM.

compenV V= (7)

Because the voltage vector is recovered perfectly as original vector, normal control becomes possible during the region despite of the open switch fault. Fig.8 shows an example of above compensation. As shown in Table I, zero vector V7 acts like V4 by S1 switch fault. Hence, if V7 is removed and V0 is expanded as shown in Fig.8, the active vector can be recovered.

The faulted vector in (3) can be compensated into nearest

available voltage vector. The distortion by zero vector is compensated through the DPWM as well. In addition, the duration of available active vector is revised to approach the original voltage vector like (8).

( )/ 2compen activeV Vα β= + (8)

Also, (5) should be compensated like (8) because it is a nearest voltage vector. However, the compensation of (6) is impossible because both zero vector V0 and V7 cannot be used.

Fig.9 shows a compensation result of Fig.4(b) as an example for comprehension. The uncontrollable regions are reduced from ten to four. In six perfect compensation regions, (2) can change into (7) through DPWM. And, in two shaded intervals of Fig.9, first interval is related to (3) by {S1} switch fault and second interval is related to (5) by {S1, S3} switches fault. Both (3) and (5) can be compensated into (8). The remaining four regions are uncontrollable 2-phase conduction mode.

Fig.9 Compensation result of {S1, S3} fault case

V. SIMULATION RESULTS Computer simulation is performed by using MATLAB

Simulink. Fig.10 shows 3-phase currents and DC-link voltage about three cases. The simulation parameters and test conditions are shown in Table II.

Fig.10(a) is about two switches fault case of one phase. Aforementioned, when {S1, S4} is faulted, the currents are distorted due to faulted voltage vectors according to the region. And, DC-link voltage is vibrated as two times of grid frequency because of the distorted imbalance currents. Twelve regions are defective altogether. If tolerance control is applied after 0.25[s], the current imbalance is improved largely. Thus, the DC-link voltage ripple is also reduced. Eight regions change into controllable region through DPWM except four

Fig. 8 Compensation by DWPM in SecIII of SVPWM

TABLE I FAULTED ZERO VECTORS ACCORDING TO ONE SWITCH

FAULT SWITCH

PRE- CONDITION

ZERO VECTOR

Before(Vzero) After( fzeroV )

S1(A Top) if (Ia<0) V7(111) V4(011)

S2(C Bot) if (Ic>0) V0(000) V5(001)

S3(B Top) if (Ib<0) V7(111) V6(101)

S4(A Bot) if (Ia>0) V0(000) V1(100)

S5(C Top) if (Ic<0) V7(111) V2(110)

S6(B Bot) if (Ib>0) V0(000) V3(010)

TABLE II SIMULATION PARAMETERS AND TEST CONDITIONS

Input voltage 3Φ 220[V] Grid frequency 60[Hz]

Reactor 5[mH] DC capacitor 2350[uF]

Reference voltage 400[V] DC load resistor 150[ohm]

Switching frequency

5[kHz] Control format SVPWM

Fault start time 0.2[s] Tolerant control start time

0.25[s]

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uncontrollable regions of 2-phase conduction. Fig.10(b) shows {S1, S3} fault case. The DC-link voltage has ripple of grid frequency in faulty condition. But, after tolerance control, the ripple is reduced largely because of the improvement of the

current imbalance as explained in Fig.9. In Fig.10(c) of {S1, S6} fault case, DC-link voltage ripple is largest among three cases because it has the point where every current is zero. Despite of tolerance control, the point does not disappear. However, two regions and one region are recovered perfectly and partially, respectively. So, current imbalance is reduced slightly. Thus, the DC-link voltage ripple is also reduced about half.

Fig.11 shows simulation waveforms under three switches fault conditions. As Fig.11(a) is {S1, S3, S4} fault case, the characteristics of Fig.10 are appeared together. After tolerance control, the performance is improved somewhat. Fig.11(b) shows a special case of {S1, S3, S5} fault case. Although the faulty currents aren’t sine waveform, there is no current imbalance as mentioned earlier. So, the DC-link voltage ripple is small. Also, although tolerance control is applied, the improvement of the performance is minimal.

VI. EXPERIMENTAL RESULTS Fig.12 shows the a-phase current, the current angle, the

angle variation and the fault detection signal for the open switch fault detection. The faulty switches can be found by the distorted current angle thought Fig.6 as mentioned in Fig.7.

(a) {S1, S4} fault case

(b) {S1, S3} fault case

(c) {S1, S6} fault case

Fig. 10 3-phase currents and DC-link voltage about three cases of two switches fault

(a) {S1, S3, S4} fault case

(b) {S1, S3, S5} fault case

Fig. 11 3-phase currents and DC-link voltage about three cases of three switches fault

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Fig.13 shows the experimental waveforms of the tolerance control. The experimental conditions are the same as test conditions of previous simulation. 3-phase currents, DC-link voltage and FFT result of DC-link voltage are shown in Fig.13. Fig.13(a) shows waveforms in normal operation of 3-phase AC-DC PWM converter. After open switches fault of {S1, S3}, the waveforms change into Fig.13(b). Distorted imbalance currents and vibrating DC-link voltage are shown like a previous simulation. If tolerance control is applied, waveforms of Fig.13(b) change into Fig.13(c). The current imbalance is dramatically reduced and the DC-link voltage ripple also is reduced. FFT result of DC-link voltage in Fig.13(b) is also improved like in Fig.13(c).

VII. CONCLUSIONS This paper proposed fault tolerant control strategy of 3-

phase AC-DC PWM converter to minimize the current imbalance and DC-link voltage ripple under multiple open switch fault condition. In various multiple switch fault cases, fault phenomena and tolerance control methods were analyzed, respectively. Hence, the results were represented before and after the tolerance control under multiple open switches fault conditions. This method is easily implementated by revising the switching pattern of SVPWM. The feasibility of the fault tolerant control strategy was proved by simulation and experimentation results.

ACKNOWLEDGMENT This work has been supported by KESRI (Korea

Electrical Engineering and Science Research Institute) (2009T100100651), which is funded by MKE (Ministry of Knowledge Economy).

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(a) Normal operation

(b) Open fault operation of {S1, S3}

(c) Tolerance control operation

Fig. 13 3-phase currents, DC-link voltage and FFT of the voltage

Fig. 12 Experimental waveforms for fault detection

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