Title:
|
Analysis of doubly-fed induction generator under electrical fault conditions
|
This thesis investigates the effects of winding faults on the doubly fed induction
generator (DFIG) operation. A DFIG is a wound rotor induction machine that is most
commonly used in present day wind power applications. In combination with a back-toback
converter in the rotor circuit the DFIG is capable of generating power at constant
frequency while operating at variable speed. The aim of the research in this thesis is to
develop analytical and experimental tools that would enable the work on establishing of
DFIG condition monitoring techniques.
DFIG winding faults are examined using the machine current signature analysis
(MCSA) method. This method is based on exploring the machine current frequency
spectra, and defining and monitoring the fault specific changes in the spectra harmonic
content. For this purpose a detailed analytical machine model is developed in this work.
The model is capable of representing the effects that various winding faults have on the
current spectra and is based on the generalized harmonic analysis. Higher order m.m.f.
~ ·harmonics are taken into account in the model calculations. Combining the model with a
connection matrix enables the machine current frequency analysis for various healthy and
faulty DFIG winding configurations. The model equations are solved in a time stepping
algorithm based on Euler's method.
For the purpose of experimental work a DFIG laboratory test rig was built. The
test rig is designed to enable the simulation of various DFIG operating regimes and
winding faults. The test machine stator winding terminals are taken out to a specially
constructed terminal box, where by simple reconnecting of the terminals different
winding configurations are achieved. The effect that open-circuit and short-circuit faults
have on the DFIG current spectra harmonic content, when compared to healthy operating
conditions, was analyzed in both the analytical model and on the test rig. The comparison
of the model results against the experimental results shows that they are in very good
agreement, thus verifying that the developed model is capable of predicting the real life
operating conditions that are present in the simulated system. Some fault specific
harmonic components are defined from model predictions and corresponding test rig
experiments for each of the winding fault scenarios and DFIG operating conditions
analyzed. !tis shown that the frequency of the fault specific harmonic components is
dependant on the DFIG operating speed.
|