Winding short-circuit fault modelling and detection in doubly-fed induction generator based wind turbine systems

Student thesis: Doctoral Thesis

Abstract

This thesis deals with the operation of and winding short-circuit fault detection
in a Doubly-Fed Induction Generator (DFIG) based Wind Turbine Generator
System (WTGS). Both the faulted and faultless condition of operation has been
studied, where the focus is on the electrical part of the system. The modelled
electrical system is first simulated and the developed control system is then
validated on a test bench. The test-bench component dimensioning is also
discussed.
The faultless condition deals with the start-up and power production mode of
operation. Control design based on the Proportional Integral (PI) control
technique has been compared for power and torque control strategies against
the Linear Quadratic Gaussian (LQG) control technique, at different operating
points through the variable-speed region of WTGS operation following the
maximum power curve of the system. It was found that the torque control
strategy offered less degradation in performance for both the control techniques
at operating points different for the one for which the control system was tuned.
The start-up procedure of the DFIG based WTGS has been clarified and
simplified. The phase difference between the stator and the grid voltage, which
occurs due to the arbitrary rotor position when the rotor current control is
activated, is minimized by using a sample-and-hold technique which eliminates
the requirement of designing an additional controller. This method has been
validated both in simulation and experiments.
The faulted condition of operation deals with the turn-turn short-circuit fault in
the phase winding of the generator. The model of the generator, implemented
using the winding-function approach, allows the fault to be created online both
in a stator and a rotor phase. It has been demonstrated that the magnitude of the
current harmonics, used extensively in literature for the Machine Current
Signature Analysis (MCSA) technique for winding short-circuit fault detection, is
very different when the location of the fault is changed to another coil within the
phase winding. This makes the decision on the threshold selection for alarm
generation difficult. Furthermore, the control system attenuates the current
harmonics by an order of magnitude. This attenuation property is also
demonstrated through experiments. The attention is then shifted to the negativesequence
current component, resulting from the winding unbalance, as a
possible fault residual. Its suitability is tested in the presence of noise for
scenarios with different fault locations, fault severity in terms of the number of
shorted-turns and grid voltage unbalance. It is found that due to the presence of
a control system the magnitude of the negative-sequence current, resulting from
the fault, remains almost the same for all fault locations and fault severity. Thus,
it was deemed more suitable as a fault residual. In order to obtain a fast
detection method, the Cumulative Sum (CUSUM) algorithm was used. The test
function is compared against a threshold, determined on the basis of expected
residual magnitude and the time selected for detection, to generate an alarm. The
validation is carried out with noise characteristics different from the ones used
during the design and it is shown that the voltage unbalance alone is not able to
trigger a false alarm. In all the scenarios considered, the detection was achieved
within 40 ms despite the presence of measurement filters.
Date of Award13 Oct 2011
Original languageEnglish
SupervisorJohan Gyselinck (Promotor), Michel Kinnaert (Jury), Philippe Lataire (Jury), Jean-Claude Maun (Jury), Pierre Mathys (Jury), Raymond Hanus (Jury) & L. Van De Velde (Jury)

Keywords

  • wind turbines
  • fault detection
  • electric machines

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