# DFIG modelling and winding fault analysis

```DFIG modelling and winding fault
analysis
y
By
S. Djurovic
j
School of Electrical and Electronic Engineering
The University of Manchester
Research
• Doubly fed induction generator (DFIG) is the most
commonly used type of generator in contemporary large
variable speed wind turbines
• One of the most common induction machine faults are
winding faults due to short-circuit or open-circuit or
abnormal
b
l connection
ti off phase
h
windings
i di
• Preferred fault diagnostic method is the steady state
stator current spectral analysis (MCSA) - non invasive as
windings are used as search coils
• Investigate the possibility of DFIG winding fault detection
through monitoring stator line current frequency spectra
Work so far
• An analytical model for a DFIG has been developed to enable
the dynamic behaviour of a generator operating with various
healthy and faulty winding configurations to be simulated.
• A 30 kW DFIG test-rig has been built to explore DFIG steadystate operation under a range of winding and supply fault
scenarios.
• The analytical model has been verified by comparison of
model predictions with test-rig measurements in both time and
frequency domain for a series of typical DFIG steady state
operating points.
• DFIG operation
ti
with
ith a range off typical
t i l winding
i di
f lt and
faults
d
supply asymmetries has been explored.
• Changes in the stator current spectrum arising from each fault
condition, have been identified.
DFIG modelling
d lli for
f condition
diti monitoring
it i
purposes
• Based on coupled
coupled-circuit
circuit approach
• A circuit is defined as ‘any series connection of
coils’
• Coupling inductances are calculated between
circuits
• This approach makes it possible to analyze an
arbitrarily connected n-phase machine while
taking into account higher order field space
harmonics
M d l equations
Model
i
• Machine voltage and flux equations (matrix form)
V s   R s I s   d  s 
 s   L ss I s   L sr I r 
V r   R r I r   d  r 
 r   L rs I s   L rr I r 
dt
dt
• Torque and speed equations
Te
d
 J
dt
1
T d L 
I 
I 

2
d
d
 
dt
DFIG test rig diagram
DC SPEED DRIVE
(MENTOR)
STATOR
DC MACHINE
SYNCHRONIZING
GRID
CONTACTOR
SUPPLY
AC TEST MACHINE
(doubly fed IM)
ROTOR
BACK-TO-BACK
CONVERTER
Test Rig Description
Laboratory test bed
(viewed from above)
Test Rig Description
DFIG Terminal box
Rotor back-to-back converter
Healthy and faulty DFIG winding configurations
used in this presentation
US 1
US 2
VS 2
WS1
VS 1
a) Stator healthy
US 2
WR 1
W S2
c) Stator open-circuit fault
WR 2
b) Rotor healthy
US
WS
W S1
VS1
UR 2
VR 2
VR1
WS2
VS 2
UR 1
VS
d) Stator short-circuit fault
Predicted and measured DFIG current rms values
for a typical operating point and a healthy DFIG
measured
measured
predicted
15
32
14
30
13
28
12
26
24
Current [A
Arms]
Current [A
Arms]
11
10
9
8
7
6
5
4
22
20
18
16
14
12
10
8
3
6
2
4
1
2
0
predicted
sU
sV
Phase
a) Stator line currents
 R  1684rpm
sW
0
rU
rV
Phase
b) Rotor line currents
rW
Predicted
P
di t d and
d measured
d DFIG currentt rms
values for a stator open-circuit fault
Measured
Measured
Predicted
30
28
10
26
9
24
Current [A
Arms]
Current [A
Arms]
11
8
7
6
5
4
22
20
18
16
14
12
10
8
3
6
2
4
1
0
Predicted
32
12
2
0
sU2
sV1
sV2
sW1
sW2
Phase
a) Stator winding group currents
 R  1556rpm
rU
rV
Phase
b) Rotor line currents
rW
Frequency content of predicted and measured
DFIG stator line current for a DFIG operating with
balanced windings and unbalanced supply
0
286 Hz
-10
-5
386 Hz
150 Hz
622 Hz
250 Hz
Normalized sp
N
pectrum [dB]
Normalized spectrum [dB]
N
0
722 Hz
-20
-30
-40
286 Hz
386 Hz
-10
-15
-20
622 Hz
722 Hz
-25
-30
-35
-40
-50
-45
-60
-50
0
100
200
300
400
500
600
700
800
0
100
200
300
400
500
600
700
Frequency [Hz]
Frequency [Hz]
a) Experimental current spectrum
b) Predicted current spectrum
800
Frequency content of predicted and measured DFIG
stator line current for a stator short-circuit fault
0
0
118 Hz 218 Hz
454 Hz
-10
10
454 Hz
-10
Normalized spectrum [dB]
N
Normalized sp
N
pectrum [dB]
386 Hz
118 Hz 218 Hz
286 Hz
554 Hz
722 Hz
622 Hz
-20
-30
-40
-50
554 Hz
622 Hz
-20
722 Hz
-30
-40
-50
386 Hz
-60
250Hz
286 Hz
150Hz
-60
0
100
200
300
400
500
600
700
800
-70
0
100
200
300
400
500
600
700
Frequency [Hz]
Frequency [Hz]
a) Experimental current spectrum
b) Predicted current spectrum
800
Frequency content of predicted and measured DFIG
stator line current for a stator open-circuit fault
0
0
264 Hz 364 Hz
-10
364 Hz
521 Hz
107 Hz
578 Hz
421 Hz
678 Hz
-20
-30
-40
150 Hz
-50
-60
264 Hz
-10
10
207 Hz
Normalized spectrum [dB]
N
Normalized spectrum [dB]
N
107 Hz
521 Hz
421 Hz
207 Hz
578 Hz
-20
678 Hz
-30
-40
-50
150 Hz
-60
0
100
200
300
400
500
600
700
800
-70
0
100
200
300
400
500
600
700
Frequency [Hz]
Frequency [Hz]
a) Experimental current spectrum
b) Predicted current spectrum
800
Frequency content of predicted and measured
DFIG stator line current for a healthy DFIG
0
0
Model 1
-5
150 Hz
250 Hz
-10
622 Hz
956 Hz
722 Hz
-20
-30
-40
Normalized sp
N
pectrum [dB]
Normalized spe
N
ectrum [dB]
286 Hz
386 Hz
286 Hz
386 Hz
-10
-15
-20
622 Hz
722 Hz
-25
956 Hz
-30
-35
-40
40
-50
-45
-60
-50
0
100
200
300
400
500
600
700
800
900
1000
Frequency
q
y [[Hz]]
a) Experimental current spectrum
0
100
200
300
400
500
600
700
800
900
1000
Frequency [Hz]
b) Predicted current spectrum,
Includes high order field harmonics
Frequency content of predicted DFIG stator line
current for a healthy DFIG
0
0
Model 1
Model 3
-5
Normalized sp
N
pectrum [dB]
Normalized spe
N
ectrum [dB]
-5
-10
-15
-20
-25
-30
-35
-40
40
-15
-20
622 Hz
722 Hz
-25
956 Hz
-30
-35
-40
40
-45
-45
-50
286 Hz
386 Hz
-10
-50
0
100
200
300
400
500
600
700
800
900
Frequency
q
y [[Hz]]
a) Predicted current spectrum,
includes fundamental only
1000
0
100
200
300
400
500
600
700
800
900
1000
Frequency [Hz]
b) Predicted current spectrum,
Includes high order field harmonics
Conclusions
• A detailed DFIG analytical model developed
• DFIG Test Rig built
• The developed model's validity is verified through
comparison
i
off model
d l predictions
di ti
against
i t th
the
experimental results obtained from the test rig
• Advantages of modelling the effects of higher order field
harmonics demonstrated
• Some stator current fault-specific harmonic components
are identified. These are slip dependant.
Th k You
Thank
Y
```