The modelling of a squirrel-cage induction generator in an oscillating-water-column wave-energy converter
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Date
2015-04-28
Authors
Pestana, Ronaldo Jorge
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Abstract
The research is focused on the modelling of a squirrel-cage induction generator in dynamic
generation involving ocean-wave energy. The chosen application includes an oscillating
water column fitted with a Wells turbine.
The modelling approach is based on the evaluation of existing generator models. These
include the equivalent steady-state and dynamic models which are considered from a timedomain
(differential equation) perspective. Since generation is dynamic in nature, model
stability is an important component of model evaluation.
The evaluated models provide information regarding the electrical and mechanical
operational variables of the generator. Power flow and energy loss between the mechanical
and electrical subsystems are easily calculated from these variables.
The wave-energy converter excluding the induction generator is not explicitly considered.
The generator models are evaluated by considering typical generator inputs which are
representative of the given application. These dynamics are reproduced experimentally and in
simulations with a comparison of generator response allowing for a conclusion on model
performance. Generator inputs include the stator voltage excitation and turbine torque with
the generator response given by the stator currents and rotor velocity. Electrical and
mechanical power are also considered.
Dynamic generation is broken down into two modes of operation: the first mode involves
generation for a constant sea state and the second mode involves generator operation for a
change in sea state. The dynamics for the first mode involve a set generator speed (set voltage
supply) and a sinusoidal prime-mover torque. Dynamics for the second operating mode are
not well-defined owing to system variations. Since only the generator model is considered, an
informative dynamic is tested providing an indication of possible model performance. The
tested dynamic involves a sinusoidally-varying stator frequency and prime-mover torque.
The steady-state model considered from a time-domain perspective is found to be unstable
for all generating slip values and is, therefore, unsuitable for the given generation application.
The dynamic model shows good agreement between experimental and simulated generator
response for the two operating modes identified. In conclusion, the model is applicable for a
constant sea state with a wave period of up to 0.2 s. Furthermore, it is suspected that the
dynamic model is applicable in the case of a change in sea state. Cases involving magnetic
saturation and parameter variation are left for future development.
The dynamic-model evaluation assumes a balanced stator-voltage excitation – strange
electrical transients including electrical faults are not considered. An important simulation consideration involves the quantification of state-variable initial
conditions. Initial rotor currents are problematic as these are not easily measured or defined
in a practical squirrel-cage rotor construction. The initial rotor currents are approximated by a
phasor analysis of the steady-state circuit model at zero time.
The use of an inverter-based generator excitation for the experimental work poses an analysis
problem owing to the pulse-width-modulation-based voltage supply (not truly sinusoidal).
This is solved by considering only the fundamental component of the stator voltage and
current. Second-order low-pass filters are used to facilitate such measurements.