Harmonic Analysis of
a DFIG for a Wind
Energy Conversion
System
Lingling Fan, Ph.D., P.E.
Assistant Professor
Dept. Electrical Engineering
University of South Florida
Tampa, FL 33620
Linglingfan@usf.edu
April 20, 2010
2010 IEEE Transmission & Distribution
Conference and Exhibition
New Orleans, LA
Outline
 Objective
 Principle
 Case studies
– Rotor injection
– Unbalanced stator conditions
 Conclusion
Objective
 Develop a steady-state circuit to give
quantitative analysis for harmonics in DFIG
 The work is useful for understanding of
– DFIG behavior during non-sinusoidal rotor
injection
– DFIG under unbalanced grid conditions (fault
ride through)
Doubly Fed Induction Generator
Ps
fm
RSC
AC/DC
GSC
DC/AC
Pr = sPs
5
A generalized circuit for harmonic
analysis in DFIG
Case study 1 – non-sinusoidal rotor
injection
Frequency components in rotor voltage
and stator current
 Stator currents
– (6n+1)fr+fm, -(6n-1)fr+fm,
Electromagnetic torque
 Interaction of stator and rotor currents
Stator
Is1 (60 Hz)
Is2(-24*5 +36 Hz)
Is3 (24*7
+36 Hz)
Rotor
Ir1(24 Hz)
Ir2 (-24*5 Hz)
Ir3 (24*7 Hz)
Ex: Is1, Ir2  torque (60- (-120 +36)) = 144 Hz = 6*24 Hz  6fr
Is1, Ir3  torque (60- (168 +36)) = -144 Hz = -6*24 Hz  6fr
Torque
+
Case study 2 – unbalanced stator
conditions
 Rotor injection – programmable power
source –sinusoidal 3-phase
 Stator phase a resistance is reduced
Analysis
Unbalanced stator currents
Positive sequence
(fe)
Negative sequence
(-fe)
Zero sequence
Rotor currents
Ir: fe-fm=sfe
Ir: -fe-fm=-(2-s)fe
s: slip = 1-fm/fe
60 Hz (stator) –
-60 Hz (stator) –
fm: electric frequency
corresponding to 50 Hz (rotating
50 Hz (rotating
rotating speed.
speed) = 10 Hz
speed) = -110 Hz
fe: nominal frequency
13
Torque
Positive
Negative
Stator
Is1 (+60 Hz)
Is2 (-60 Hz)
Rotor
Ir1 (10 Hz)
Ir2 (-110 Hz)
1. Te1 Te2 – dc components
2. Te3, Te4 – pulsating components 120 Hz
Reference frames
Pos. Neg.
Sequences can
be separated
by a dc filter!
15
Extraction scheme
16
Lab setup
Case 3 – Grid interconnected DFIG
Simulation results
Confirm simulation results with the
analysis
Conclusion
 This paper develops
– A generalized steady-state DFIG circuit for
harmonic analysis
– A systematic method to compute torque by
computing the interactions of stator and rotor
currents
– The sequence network based on DFIG pos,
neg circuits which facilitates the analysis under
unbalanced stator conditions.