Performance of Diagnosis Methods for IGBT Open Circuit Faults in Three
Phase Voltage Source Inverters for AC Variable Speed Drives
Kai Rothenhagen, Friedrich W. Fuchs
Christian-Albrechts-University of Kiel
Kaiserstr.2, 24143 Kiel, Germany
Tel. 0049-431-880-6105
kro@tf.uni-kiel.de, fwf@tf.uni-kiel.de
Keywords
Diagnostics, Variable Speed Drive, Vector Control, Voltage Source Inverters.
Abstract
Variable speed drives have become industrial standard in many applications. Therefore fault diagnosis
of voltage source inverters is becoming more and more important. One possible fault within the
inverter is an open circuit transistor fault. An overview of the different strategies to detect this fault is
given, including the algorithms used to localize the open transistor. Previous work showed significant
differences among the available methods to detect such a fault for a mains side active rectifier. This
paper extends the performance evaluation for the inverter connected to the machine with variable
stator voltage and frequency. Simulation results are presented. They show the influence of the applied
standard field oriented control on the currents during a fault. An experimental setup in the laboratory
is used to validate simulation results. Typical detection results are presented including time-todetection measurements. Robust detection of open transistor faults has been found to be possible.
I. Introduction
Voltage source inverter (VSI) fed variable speed
drives have become the standard in industrial
applications. While safety critical applications
are equipped with high sophisticated fault
diagnostic systems, standard applications are
regularly only equipped with standard fault
detection. Increased converter costs are usually
the reason for this, but advanced diagnosis
features can on the other hand be economically
reasonable, considering operation costs of the
converter and the whole system. High costs due
to standstill and repair, as well as secondary
faults caused by unnoticed damages make
advanced diagnosis methods interesting.
Fig. 1: VSI feeding a Permanent Magnetized
Synchronous Machine, with fault in T4.
Within the variable speed drive, faults can occur in the motor, rectifier, or the inverter. While the
diagnosis of electrical machines is thoroughly investigated, with an overview given by Capolino [1],
diagnosis of the rectifier and inverter are not as well researched Fuchs [2]. Within the inverter,
semiconductors are one of the main causes of faults next to electrolytic capacitors. A classification of
thinkable faults in VSI has for example been published by Kastha [3].
The usual fault mode of semiconductors is a short circuit, but an open circuit fault can also occur.
While short circuit protection by means of detection via collector-emitter voltage has become a
standard feature of today's VSI, making inverters short circuit proof, the open circuit fault has not yet
received so much attention. Open circuit faults may for example be caused by the lifting of bonding
wires due to thermic cycling, by a driver failure, or by a short circuit fault induced rupture of the
IGBT. An open circuit fault will not necessarily cause the drive to be inoperable. It may therefore be
undetected for an extended period of time. Since an open transistor causes pulsating current and
torque, it may lead to secondary faults in other semiconductors, the inverter, the motor or the load.
Other researchers have developed several detection methods for open transistor faults [4, 5, 6]. These
methods have been subject to a performance comparison for a voltage source active rectifier [7]. This
research has shown major differences in the performance, tuning and computing effort, and false
alarms resistivity. As a result, two of the methods have been modified. This research shall herewith be
extended to a voltage source inverter feeding a permanent magnet synchronous machine. Here,
especially the influence of variable AC voltage and frequency has to be investigated.
The paper is organized as follows: At first, behaviour and different diagnosis techniques of transistor
open circuit faults will be presented, including the modifications as published by [7]. Then, simulation
results of these diagnosis methods will be given. Measurement results back up these results, and
typical detection sequences are shown. This work will be summed up in a conclusion.
II. Transistor Open Fault Behavior and Diagnosis Methods
Transistor Open Faults Behavior
The topology of the inverter as basis for the investigation is given in figure 1, with a fault in switch 4
indicated by an open gate connection. Figure 2 shows the inverter AC currents for a healthy inverter in
Park's vector reference frame. In case of an open circuit fault, the machine current in the faulty phase
can either be only negative or only positive, depending on which transistor is damaged. For a fault in
switch 1, this leads to currents as shown in figure 3, where the current of phase u can be only negative.
It is important to remark that all line currents contain a direct component, whereas currents in healthy
condition do not. Using the Park’s Vector transformation (1), (2) [5] on the currents yields to
trajectories as displayed in figure 4. The measurements are explained later.
Fig 2: Inverter AC Current trajectory in Park's
Vector depiction without fault (Measurement).
Iα = I a
Iβ =
Ib − Ic
3
Fig 3: Inverter AC currents in no fault and fault
condition (Measurement).
(1)
(2)
Fig. 4: Trajectory of inverter AC currents in Park’s Vector depiction (Measurement, 7 ARMS).
Previous Research on Open Transistor Fault Diagnosis
Park’s Vector Method
As can be seen from figure 4, an open transistor fault can be detected by considering the "centre of
gravity" of the park's vector trajectory, as suggested by Mendes [5].
The algorithm is based on averaging over one period (3) in order to calculate the direct component of
the line currents. Then a Park's Vector transform (1), (2) is applied to compute magnitude (4) and
angle (5) of the AC current in the complex plain. For a system without open transistor fault the current
space vector runs in a circle and the mean value is zero. If a fault occurs, the magnitude of the space
vector is not zero, will exceed the threshold and the actual faulty switch can be identified by
considering the argument, as shown in table I.
µν =
1
N
N
∑ Iν ( kτ )
(3)
k =1
µ = µ =
(4)
µ α2 + µ β2
 µβ 

arg{ µ } = arctan 
µ
 α 
1
= Nτ
f mains
(5)
(6)
ν ∈ [α , β ]
(7)
Table I: Localisation of Fault with Park's Vector Method
Transistor
T1
T2
T3
T4
T5
T6
Magnitude µ
Argument (deg)
Exceed
Threshold
150 to 210
210 to 270
270 to 330
330 to 30
30 to 90
90 to 150
Normalised DC Current Method
One drawback of the above mentioned method is the load dependence of the algorithm. The direct
component calculated by (3) will be larger the larger the AC current is. In order to make the scheme
independent from the load, Abramik [6] suggested to use a normalised direct component instead. To
achieve this, the first order harmonic coefficients of the inverter AC currents are computed by means
of a DFT (9), (10). The direct component as calculated by (3) is then divided by the absolute value of
the first harmonic (8). This is done for each of the three phases (13).
For identification of the faulty switch, the resulting residual γi is compared to thresholds (11), (12).
Using table II, the faulty switch can be identified. The threshold of 0.45 is reported to be a universal
value derived from experience.
γi =
µi
(8)
a12,i + b12,i
a1,i =
2 N
 2πk 
I i (kτ ) cos

∑
N k =1
 N 
(9)
b1,i =
2 N
 2πk 
I i (kτ )sin 

∑
N k =1
 N 
(10)
1 : γ > 0
d1,i =  i
0 : γ i ≤ 0
(11)
1 : γ i > 0.45
d 2,i = 
0 : γ i ≤ 0.45
i ∈ [ a , b, c ]
(12)
(13)
Table II: Localisation of fault with Normalised DC Current
Transistor
d1, a
d1,b
d1, c
d 2, a
d 2,b
d 2, c
T1
T2
T3
T4
T5
T6
1
0
0
0
1
1
0
1
0
1
0
1
0
0
1
1
1
0
1
0
0
1
0
0
0
1
0
0
1
0
0
0
1
0
0
1
Modified Normalised DC Current Method
As stated in [7], the Normalised DC Current Method has some drawbacks when implemented in a
closed loop control scheme. Therefore, its adaptation for better usability has been proposed.
The Modified Normalised DC Current method uses the same algorithms as the Normalised DC
Current method, but employs a less restrictive way to localize the faulty switch, as displayed in table
III. Blanks in table III mean this state is not relevant. To prevent that more than one condition is
fulfilled, only the largest absolute value of γa, γb and γc as calculated in (8) is considered.
Table III: Localisation of fault with Normalised DC Current Method
Transistor
d1, a
T1
T2
T3
T4
T5
T6
1
d1,b
d1, c
d 2, a
d 2,b
d 2, c
1
1
1
1
1
0
1
0
1
0
1
The Slope Method
In another method suggested by Peuget [4], the slope of the trajectory in the complex plain can be
used for fault detection and identification. The trajectory is calculated by using the Park's Vector
Transform (1), (2) of the line currents. As one can see from figure 4, the trajectory comprises of a
semicircle and a linear section. The linear section has a characteristic slope, depending on the faulty
inverter bridge. By calculation of the slope according to (14), the faulty bridge can be identified using
table IV. It is reported that a quarter period of unchanged successive values are a valid indicator for a
fault.
σ =
∆iα
∆i β
(14)
In order to identify the faulty switch within a faulty bridge, it is necessary to detect whether the current
in the faulty phase is positive or negative. This can for instance be done by a Schmitt-trigger that
monitors the line current, as depicted in figure 5. This way, the faulty transistor can be identified.
Table IV: Localisation of fault with Slope
Method
Faulty
phase
Phase u
σ
0
Phase v
3
Phase w
− 3
Fig. 5: Hysteresis to detect current polarity for
Slope method
Simple Direct Current Method
A straight forward and easy to implement fault detection scheme is the Simple DC Current method, as
described in [7]. This method uses the direct component calculated by (3) for each of the three phases.
No transformations or normalisation is needed. For identification, the largest direct component of the
three phases is compared to a threshold according to table V. This way, the faulty switch can be
detected.
Table V: Localisation of fault with Simple Direct Current Method
Transistor
µa
T1
T2
T3
T4
T5
T6
>δ
µb
µc
>δ
>δ
<-δ
<-δ
<-δ
III. Analysis of Transistor Open Fault Diagnosis Methods by Simulation
Simulations concerning the behaviour of a voltage source inverter, connected to a permanent magnet
synchronous machine with field oriented control [8], with an open transistor have been carried out
using MATLAB-Simulink and the PLECS-Toolbox. The used control parameters and strucure
where the same for all examined diagnosis methods. The simulations were carried out for output
reference frequencies of 50 Hz, 25 Hz, and 10 Hz. The frequency is controlled by a speed controller
and therefore subject to fluctuation. Also, the internal torque of the PMSM will fluctuate because of
the faulty switch. The load was adjusted for a current of 6.8, 3.4, and 0.7 Ampere rms. The used
topology is displayed in figure 6. The very same topology is also used for the later experimental
analysis, except that a real inverter and machine is used instead of the models. Figures 7 and 13
compare simulation and experiment results and show good compliance.
Fig. 6. Topology of drive system and control used in simulation and experiment
Evaluation of Performance
The performance was evaluated and is summarised in table VI. An "X" means the fault detection was
successful, a "-" means the detection was not successful, whereas "(X)" means that the detection was
ambiguous. It can be said that most detection methods had problems with small currents, as already
mentioned in [7]. Only the Modified Normalised DC Current method is able to detect the faulty switch
in every simulated case. Therefore it was chosen for further testing by experiment. The other methods
that were realised in experiment are Simple Direct Current method and the Slope Method.
Table VI: Overview of Simulation Results for Open Transistor Detection
Methods
Park’s Vector
Slope
Normalized DC
Current
Simple DC
Current
Modified
Normalized DC
Current
Frequency
50 Hz
25 Hz
10 Hz
50 Hz
25 Hz
10 Hz
50 Hz
25 Hz
10 Hz
50 Hz
25 Hz
10 Hz
50 Hz
25 Hz
10 Hz
6.8
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Current [Arms]
3.4
X
X
X
(X)
X
X
X
X
X
X
X
X
X
0.7
X
X
X
X
X
Fig. 7: Inverter AC Currents in Park's Vector frame at various frequencies, (clockwise rotation).
Left: Simulation at 6.8 Arms. Right: Measurement at 7 Arms. Colour code: Red: 10 Hz
(top), Blue: 20 Hz, Green: 25 Hz, Cyan: 30 Hz, Yellow: 40 Hz, Black: 50 Hz (bottom)
Influence of the control loops
Since a closed loop field oriented control is used, the control loops will influence the behaviour of the
variable speed drive during a fault. The faulty switch will not conduct current, causing a control
deviation upon which the current control will react with a modified voltage set value. Also, depending
on the load, the torque ripple may cause a deviation in speed upon which the speed controller reacts.
It is assumed and confirmed by simulation and measurement, that this influence is larger at a slower
output frequency. The control effect on the currents can be seen in figure 7, where the trajectory with
the lowest frequency is most "stretched" and "shifted", while the trajectory at 50 Hz almost remains a
semicircle, with zero as centre. Especially in measurement, further effects are to be seen. This
behaviour will most likely have an influence on the detection, since all detection algorithms employ
current measurements. Simulations show, however, that a change in frequency is much more tolerable
than small currents, as can be seen in table VI.
IV. Analysis of Transistor Open Fault Diagnosis by Experiment
The setup used for experimental analysis of the above mentioned fault diagnosis schemes includes, as
already introduced in figure 1, a current measurement, a microcontroller to execute the introduced
algorithms, and a small display. The variable speed drive is a permanent magnet synchronous machine
fed by a voltage source inverter. Any of the six switches of the inverter may be disabled by switching
the respective PWM signal to zero permanently. A torque controlled DC drive is used to load the
PMSM. Field oriented control is implemented by means of a dSpace-Controlsystem [8].
Fig. 8: Measurement of step response of
rotational speed
Fig. 9: Measurement of step response of
Torque-building current Iq
Figures 8 and 9 show the dynamic performance of the drive. Figure 8 shows the step response of the
rotational speed. The reference steps up from 500 rpm to 800 rpm. To characterise the current control
loop, the speed control loop was disabled. Figure 9 shows the step response of the quadrature current,
which is responsible for the torque, when the reference is switched from 1 Ampere to 5 Ampere.
Figure 10 shows the phase currents for various frequencies. The time is scaled per cycle duration, in
order to compare the controller influence. As can be seen, the magnitude of all currents is kept the
same for all frequencies. The currents fit well onto each other. Figure 10 also shows the current
waveforms for a fault in switch 1, located in phase u. Because of the fault, the current in phase u
cannot become positive. The currents of phases v and w are also shown. Each frequency is plotted in a
different colour. The full colour codes are listed beneath figure 7. As can be seen, the currents differ
for different frequencies. Generally, the reaction of the 10 Hz currents is largest among all currents
towards the end of the period, during which the faulty switch was supposed to conduct current. This is
reasonable, because this period lasts longest for the 10 Hz reference.
Fig. 10: Measurement of inverter AC currents at various frequencies. Time axis is scaled per unit.
Left: Healthy condition. Right: Fault in switch 1. Red: 10 Hz, Black: 50 Hz
Performance of the Detection Algorithms
Three algorithms were tested in experiment: The Simple Direct Current method, the Modified
Normalised DC Current method and the Slope Method. As can be seen from table VII, the Slope
Method showed poor performance.
Table VII: Time to Detection Measurements for Open Transistor Detection
Time to Detection Measurements Frequency
Slope
50 Hz
50 Hz
Modified Normalized DC Current 25 Hz
10 Hz
50 Hz
Simple DC
25 Hz
10 Hz
Current
7 ARMS
Average
NA
15.90 ms
25.85 ms
61.56 ms
10.2 ms
20.13 ms
49.17 ms
Standard Deviation
NA
4.28 ms
9.84 ms
23.67 ms
4.17 ms
8.27 ms
22.50 ms
Min.
NA
6.6 ms
9 ms
17 ms
3.4 ms
7.5 ms
20 sm
Max.
NA
25 ms
42 ms
111 ms
17.6 ms
36.5 ms
86 ms
Measurements were ambiguous and did not show steady results. If a fault was detected, the detection
time was usually long. Typical performance can be seen in figures 11 and 12. Figure 11 shows a nofault state. It can be seen that the slope is a typical arctan-function. Figure 12 shows the behaviour for
a fault in phase w, were the linear portion of the trajectory has a slope of − 3 . As can be seen, the
calculated slope shows a higher portion of values within proximity of the expected slope. But no clear,
steady value can be measured. The fault is detected after 35 ms, but the drawbacks of this method
become clear. Detecting a fault by the slope method is more difficult, because the slope of a healthy
trajectory, e.g. the tangent function, already has some values within the tolerance for a faulty state.
Tuning of the thresholds for − 3, 3 and zero and tuning of how many detected values are required
for a fault flag is necessary. The Modified Normalised DC Current Method (figure 13) and Simple
Direct Current method (figure 14) show much better and more reliable performance at all tested
frequencies. The good compliance of experiment and simulation is also demonstrated by figure 13.
Time to detection measurements for all methods are listed in table VII. Figure 15 shows the diagnosis
setup.
Fig 11:Typical no fault condition for detection by Fig 12: Detection of a Fault in phase w, by Slope
Slope Method. (50 Hz, 7 A)
Method. (50 Hz, 7 A)
Fig 13: Typical Detection of faulty switch by Modified Normalized DC Current scheme. Left:
Measurement, (50 Hz, 7 ARMS) Right: Simulation, (50 Hz, 6.8 ARMS).
Fig. 14: Typical detection of faulty switch by Fig. 15: Setup including PMSM,
measurement, and microcontroller C167.
Simple DC Current Method (25 Hz, 7 ARMS ).
current
V. Conclusion
Fault Detection and Identification is becoming more and more important for industrial applications.
Since Variable Speed Drives play a key role in automation, improving their fault diagnosis capabilities
is a necessary task. In this paper, the up to now less investigated diagnosis of open circuit faults in
three phase voltage source inverters connected to AC machines with field oriented control is analysed.
Here, a permanent magnet synchronous machine is used.
Several detection methods for open transistors are evaluated and compared for closed loop control.
The influence of the control loops is illustrated by means of current waveforms shown for various
current frequencies. Even though amplitude and frequency of the inverter AC currents are variable due
to machine operation and the control loop changes the waveforms of the currents, two of the compared
diagnosis algorithms are robust enough to achieve a stable detection for all tested frequencies. The
detection time is relatively long and depending on the current frequency, the computing effort is low
and tuning is easy. No additional hardware is needed, if current control loops are used. Executing the
algorithms can be performed by the inherent processor of the drive.
Including this diagnosis scheme will close a gap in today's diagnosis methods, because an open
transistor fault will not necessarily lead to a tripped fuse or an overcurrent fault detection by means of
collector-emitter voltage. The fault may therefore remain undetected and could cause secondary faults
due to torque pulsation or increased current in the healthy transistors.
Appendix
Table VIII: Variable Speed Drive Data
Type
Nominal Frequency
Nominal Power
Stator Resistance
Stator Inductance
Inertia
Inverter Nominal Power
Inverter DC Voltage
Inverter DC Capacitor
Inverter Pulse Frequency
Permanent Magnet Synchronous
50 Hz
15.5 kW
0.08 Ω
3.1 mH
0.0740 kg m²
22 kW
800 V
2200µF
3000 Hz
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