Open switch fault diagnosis in three phase inverter using diagnostic variable method

IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
_______________________________________________________________________________________
Volume: 02 Issue: 12 | Dec-2013, Available @ http://www.ijret.org 636
OPEN SWITCH FAULT DIAGNOSIS IN THREE PHASE INVERTER
USING DIAGNOSTIC VARIABLE METHOD
Mala Ratan Ubale1
, R. B. Dhumale2
, S. D. Lokhande3
Department of Electronics and Telecommunication Engineering,
SCOE, Pune, Maharashtra, India
1
malaubale448@gmail.com, 2
rbd.scoe@gmail.com, 3
sdlokhande.scoe@sinhgad.edu
Abstract
The reliability of power electronics system such as three phase inverter is important in various applications. Different types of
faults occur in it, which may influence the operation of system. Such faults require unexpected maintenance, which increases the
cost of manufacturing. Therefore fault diagnosis of such devices plays vital role in industry. One possible fault that occurs in
inverter is an open switch fault. This paper provides a new technique based on diagnostic variable which detects single as well as
multiple open switch fault in three phase inverter. In this method, diagnostic variables are used to detect faulty phase. Along with
these diagnostic variables, an average current of three phase inverter is used for the detection of single as well as multiple faulty
switches.
Key Words: Power electronics, open switch fault, diagnostic variables.
--------------------------------------------------------------------***----------------------------------------------------------------------
1. INTRODUCTION
The voltage source inverter (VSI) is one of the most used
circuit configurations for speed control of three-phase
induction motors. The main objective of inverter is to
produce an ac output waveform from a dc power supply. A
fixed dc input voltage is given to the inverter and a
controlled ac output voltage is obtained by adjusting the on
and off periods of the inverter components. Three phase
inverter provides a three-phase voltage source, where the
amplitude, phase, and frequency of the voltages should
always be controllable. The Pulse Width Modulation
(PWM) is used to control the output voltage of inverter.
Three phase PWM inverter is used in many industrial
applications such as electric motor drives, UPS, active
power filters, etc. Different types of unexpected faults may
occurs in it. These AC drives systems are sensitive to
different types of fault occurring at the input rectifier, or at
the power inverter stage, or at the control sub system. In
general, when one of these faults occurs, system may
damage. The cost of these steps can be high. These inverter
faults may influence the operation of whole system. It is
necessary to avoid this harmful influence and to enhance the
reliability of the system. Therefore, fault detection and
diagnosis are needed.
It is estimated that among all types of faults in variable-
speed ac drives in industry, about 38% of the faults are due
to failures of power devices [1]. Most of these inverters use
insulated gate bipolar transistors (IGBTs) as the power
device because of their high voltage and current ratings and
ability to handle short-circuit currents for periods exceeding
10 μs. But they suffer failures due to excess electrical and
thermal stress that are experienced in many applications.
IGBT failures can be broadly classified as open-circuit
faults, short-circuit faults, and intermittent gate-misfiring
faults. There are several methods are available for detection
of inverter faults. In [2] and [3], the technique using Park’s
vector was proposed in which neural network is used for
diagnosis. But this technique requires very complex pattern
recognition algorithm. In [4] a fault detection using voltage
sensors was proposed. This method is fast but it requires
additional sensors. A method using a discrete Fourier
transform (DFT) was proposed in [5]. In this method Fourier
coefficients of current signal are used for fault diagnosis. In
[6] and [7], neural network is used. This neural network is
trained with the seven fault patterns which are calculated by
average current Concordia vector trajectory.
Most methods are used to find single switch fault and not
multiple switch faults. In this paper, open-circuit fault in
inverter is discussed. Presented method is able to detect
single as well as multiple open switch fault in three phase
inverter. The average absolute values of current are used to
calculate diagnostic variables. Some characteristics are
obtained with these diagnostic variables and average current
of inverter which are used to detect faulty phase and faulty
switches. This method is more robust to false alarm.
2. THREE PHASE INVERTER
An inverter is an electronic device which converts Direct
Current (DC) to Alternating Current (AC). This AC can be
of any required voltage and frequency. The Variable
Frequency Drive (VFD) mostly is of two types, Voltage
Source Inverter (VSI) and Current Source Inverter (CSI).
VSI has advantages like higher efficiencies, minimizing
installation timing, eliminating interconnect power cabling
costs, and reducing building floor space. Efficiencies are

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97% with high power factor through all load and speed
ranges. Fig. 1 shows basic structure of a three phase voltage
source inverter. In this model S1 to S6 are Insulated Gate
Bipolar Transistors (IGBTs) which can be on or off
Sinusoidal pulse width modulation (SPWM) is a common
technique for controlling the switches. The average value of
voltage fed to the load is controlled by turning the switch
between supply and load on and off at a high frequency.
Fig -1: Three Phase PWM Inverter
The simplest way to generate a PWM signal is the
intersective method, which requires a triangle waveform and
a comparator. When the value of the reference signal is
more than the modulation waveform, the PWM signal is in
the high state, otherwise it is in the low state.
In open circuit fault condition, the IGBT falls in the off state
and remains in this situation regardless of the gate voltage
value. Open-circuit fault occurs due to lifting of bonding
wires caused by thermic cycling. It may be caused by a
driver fault or a short-circuit-fault-induced IGBT rupture.
Open circuit faults generally do not cause system shutdown,
but degrade its performance. Therefore, these diagnostic
methods can be used in device-fault-tolerant systems.
3. FAULT DIAGNOSTIC METHOD:
3.1 Park’s vector approach:
The Park’s vector approach or dq transformation is
mathematical transformation which simplifies three phase
circuit. This method is used to transform three phase current
of voltage source inverter (𝑖 𝑎 ,𝑖 𝑏 ,𝑖 𝑐) into two phase current
(𝑖 𝑑 ,𝑖 𝑞 ). The Park’s vector components are given by [1],
𝑖 𝑑 =
2
3
𝑖 𝑎 −
1
6
𝑖 𝑏 −
1
6
𝑖 𝑐 (1)
iq =
1
2
ib − ic (2)
Using dq transformation the current patterns can be obtained
which indicates faulty situation of inverter current. It gives
different six patterns. A normal operation is represented by a
circle. If an open switch fault has occurred, there is a change
in the phase current value at the location of fault. Therefore,
a change in the circle shape represents the occurrence of an
open fault condition. When fault occurs, circle becomes
semicircle. Fig.2 shows dq transformation for normal and
faulty phase current of three phase inverter.
3.2 Normalization of currents:
The proposed method uses the dq transformation which
transforms three phase current into two phase. This method
requires only three phase currents as inputs and it does not
require any extra sensors.
(a) For normal operation
Fig -2: DQ transformation
Therefore, system complexity reduces. The normalization of
measured current is used to overcome the problem of
operating condition dependency and false diagnostics [8].
For normalization, dq transformation or Park’s vector
approach is used to calculate Park’s vector modulus, given
as
ǀ𝑖𝑠ǀ= 𝑖 𝑑
2
+ 𝑖 𝑞
2 (3)
Where 𝑖 𝑑 and 𝑖 𝑞 are the Park’s vector components. The
normalization is done by dividing three phase current by
Park’s vector modulus. The normalized three phase current
is given by,
𝑖 𝑛𝑁 =
𝑖 𝑛
ǀ𝑖 𝑠ǀ
(4)
Where n=a,b,c. Therefore, assuming that the motor is fed by
a healthy inverter generating a perfectly balanced three-
phase sinusoidal current system
𝑖 𝑛 =
𝑖 𝑎 = 𝐼 𝑚 sin 𝑤𝑠 𝑡 + 𝜙
𝑖 𝑏 = 𝐼 𝑚 sin 𝑤𝑠 𝑡 −
2𝛱
3
+ 𝜙
𝑖 𝑐 = 𝐼 𝑚 sin 𝑤𝑠 𝑡 +
2𝛱
3
+ 𝜙
(5)
where 𝐼 𝑚 is the currents maximum amplitude, 𝑤𝑠 is the
motor currents frequency, and 𝜑 is the initial phase angle.
Because of this normalization process, the normalized motor
phase currents will always take values within the range of
-5 -4 -3 -2 -1 0 1 2 3 4 5
-5
-4
-3
-2
-1
0
1
2
3
4
5
DQ Transformation

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±
2
3
, which is independent of the measured currents
amplitude,
𝑖 𝑛𝑁 =
𝑖 𝑎𝑁 =
2
3
sin⁡(𝑤𝑠 𝑡 + 𝜙)
𝑖 𝑏𝑁 =
2
3
sin⁡(𝑤𝑠 𝑡 −
2𝛱
3
+ 𝜙)
𝑖 𝑐𝑁 =
2
3
sin⁡(𝑤𝑠 𝑡 +
2𝛱
3
+ 𝜙)
(6)
3.3 Average absolute value of currents:
Average absolute values of three phase normalized currents
< ǀ𝑖 𝑛𝑁 ǀ > are given by,
𝑤𝑠
2𝛱
ǀ𝑖 𝑛𝑁 ǀ𝑑𝑡
2𝛱
𝑤 𝑠
0
=
1
𝛱
8
3
(7)
3.4 Diagnostic variables:
The three diagnostic variables 𝑒 𝑛 (where n=a,b,c) are
obtained from the errors of the normalized current’s average
absolute values, given by
𝑒 𝑛 = 𝜉 − ǀ𝑖 𝑛𝑁 ǀ (8)
Where 𝜉 is a constant value equivalent to the average
absolute value of the normalized motor phase currents under
normal operating conditions given by (7), that is
𝜉 =
1
𝛱
8
3
≈ 0.5198 (9)
The three diagnostic variables used for fault diagnosis
because they have specific characteristics [9]. For normal
operation, all the diagnostic variables will take values near
to zero. If an inverter open-circuit fault is introduced, at
least one of the diagnostic variables will takes positive
value. But these variables only give the information about
the faulty phase and not about faulty switches. Hence, along
with diagnostic variables, current’s average value is used to
detect faulty switches.
𝐸𝑛 =
𝑁 𝑖𝑓 𝑒 𝑛 < 0
0 𝑖𝑓 0 ≤ 𝑒 𝑛 < 𝑘𝑓
𝑃 𝑖𝑓 𝑘𝑓 ≤ 𝑒 𝑛 < 𝑘 𝑑
𝐷 𝑖𝑓 𝑒 𝑛 ≥ 𝑘 𝑑
(9)
𝑀𝑛 =
𝐿 𝑖𝑓 𝑖 𝑛𝑁 < 0
𝐻 𝑖𝑓 𝑖 𝑛𝑁 > 0
(10)
The values of 𝐸𝑛 and 𝑀𝑛 are used to generate a distinct fault
signature. The threshold value 𝑘𝑓 is directly related to any
fault detection, while 𝑘 𝑑 performs an important role in case
of a double failure in the same inverter phase. Since the
method is normalized, it is not required to adjust these
values for each load and speed conditions. Table 1 gives
different diagnostic signatures. Here, value of 𝑘𝑓 and 𝑘 𝑑 are
considered as 0.00006 and 0.1996 respectively.
4. ARCHITECTURE OF FAULT DIAGNOSIS
First step of fault diagnosis of three phase PWM inverter is
to measure the three phase current and voltage. Measuring
of current and voltage is done with different conditions.
These measurements of current and signals are used for
further processing.
Table -1: Diagnostic signatures for faulty switch detection
Faulty
Switches
Ea Eb Ec Ma Mb Mc
S1 P N N L - -
S2 P N N H - -
S3 N P N - L -
S4 N P N - H -
S5 N N P - - L
S6 N N P - - H
S1,S2 D - - - - -
S3,S4 - D - - - -
S5,S6 - - D - - -
S1,S3 P P N L L H
S2,S4 P P N H H L
S3,S5 N P P H L L
S4,S6 N P P L H H
S1,S5 P N P L H L
S2,S6 P N P H L H
Data acquisition board is used to collect these signals and
transfer them from analog to digital signals. Because signal
processing can be done in digital domain. These signals then
import in MATLAB. The diagnostic variable method can be
applied on these signals to find whether inverter signals are
faulty or not. If so, then it will calculate location of faulty
switch. Fig. 3 shows fault diagnostic system for real time
application.
Fig -3: Fault detection system for real time application.
In this model, three phase inverter uses sinusoidal pulse
width modulation technique. Input for inverter is 1 ph 40 V,
50 Hz. Output voltage of inverter is 0 to 25 AC. Load is

_______________________________________________________________________________________
resistive in star or delta. Output frequency is 0 to 50 Hz.
Speed is controlled by frequency control. Data acquisition
is the process of sampling signal that measure real world
physical conditions and converting samples into digital
numeric values that can be manipulated by a computer. It
converts analog waveforms into digital values for
processing. It includes sensors to convert physical parameter
to electrical signal and signal conditioning circuitry to
convert sensor signals into a form that can be converted to a
digital form.
5. SIMULATION RESULTS
For simulation results, inverter should be simulated in
MATLAB for every fault mode. In this paper three distinct
faulty operations are considered, a single IGBT open-circuit
fault, a single-phase open-circuit fault (double fault in the
same inverter leg), and a double switch open-circuit fault.
Open switch fault can be introduced by removing respective
gate signal of switches.
a) Single IGBT open switch fault
Fig. 4 shows the time-domain waveforms of the three phase
inverter currents, the diagnostic variables and the
normalized current’s average values. An IGBT open-circuit
fault in switch S1 is introduced by removing its gate signal.
When the fault in IGBT S1 occurs, the diagnostic variable of
the corresponding phase 𝑒 𝑎 immediately increases,
converging to a value of 0.23. The other two remaining
errors will decrease until they reach a value of
approximately −0.08. S1 is upper switch of phase a,
therefore the current flows through bottom IGBT switch
which is large negative average value of current.
b) Single phase open circuit fault
Fig. 5 shows simulation results for single phase open circuit
fault in IGBT S1 and S2. In this case, the fault is introduced
in S1 and S2 by removing corresponding gate signals. As a
result, the diagnostic variable 𝑒 𝑎 will immediately increase
to a final value of 0.51. Remaining two diagnostic variables
will decreases, converging both to a value of about −0.18.
c) Double power switches open circuit fault
When the faults in transistors S1 and S3 are introduced, both
the diagnostic variables ea and eb will increase and reach
values higher than kf. The diagnostic variable of the normal
phase will decreases to negative value.
(a)
(b)
(c)
Fig -4: Simulation results of (a) inverter three phase
current,(b) diagnostic variables and (c) normalized currents
average values when open switch fault in switch S1.
(a)
(b)
(c)
Fig -5: Simulation results of (a) inverter three phase
current,(b) diagnostic variables and (c) normalized currents
average values when open switch fault introduced in switch
S1 and S2.
CONCLUSION
In this paper, a new diagnosis scheme for three phase
inverter IGBT open switch faults is presented. In proposed
method, three phase current of inverter is used as input to
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
-5
-4
-3
-2
-1
0
1
2
3
4
Three phase current of inverter
Ia
Ib
Ic
0 1000 2000 3000 4000 5000 6000 7000 8000
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
Diagnostic Variables
ea
eb
ec
0 1000 2000 3000 4000 5000 6000 7000 8000
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Normalized current average values
<IaN>
<IbN>
<IcN>
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
-4
-3
-2
-1
0
1
2
3
4
Three phase current of inverter
Ia
Ib
Ic
0 2000 4000 6000 8000
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
Diagnostic Variables
ea
eb
ec
0 1000 2000 3000 4000 5000 6000 7000 8000
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
Normalized current average values
<IaN>
<IbN>
<IcN>

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diagnostic method. This method avoids extra sensors and
therefore reduces cost of manufacturing. The diagnostic
variables provide the information about faulty phase. Along
with this, average normalized three phase current, faulty
switches can be detected. This method gives detection of
single as well as multiple faulty switches. Because of
normalization method, this method becomes independent on
load level or motor speed.
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[9] Jorge O. Estima and Antonio J. Marques Cardoso, ―A
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IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 |
pISSN: 2321-7308
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Open switch fault diagnosis in three phase inverter using diagnostic variable method

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