In order to place this research into context with respect to existing work in the field OWC WECs, the use of existing induction generator models is now considered. The focus here is on electrical-component modelling.
1.3.4.1 Per-Phase Steady-State Equivalent Circuit Model
The work presented in [4] is concerned with reducing the fluctuation in generated power which is typical of the wave-energy problem as illustrated in figure 1.1. The work assumes an OWC WEC fitted with an impulse turbine which drives an induction generator. The effect of increasing system inertia together with different generator parameters is observed.
In this analysis, the per-phase steady-state equivalent circuit model is assumed for the induction generator. This is because the wave period and the period at which the sea state varies are both large compared with the generator electrical response [4]. As presented in [2], the model is used from a phasor (frequency-domain) perspective. At any given point in time, voltage and current signals are represented by their corresponding RMS equivalents which allows for the calculation of a "varying" average power. The chosen mechanical model is the typical model used for a rotational system as previously described.
16 Based on this approach, the only system dynamics that are considered are the mechanical dynamics as given by the mechanical system differential equation. The electrical system is evaluated by a simple phasor calculation (no differential equations) based on the variables for the given time-step.
The approach of the proposed research involves implementing the given steady-state circuit model from a time-based/differential-equation perspective so as to consider the electrical dynamics of the corresponding circuit model. The simulation of instantaneous-time voltage and current waveforms is desirable for dynamic generation conditions so that generator transients may be observed – this is not possible with a phasor approach. The time-based approach would also allow for the calculation of instantaneous per-phase power as opposed to just an average power which is the case for the phasor approach. Knowledge of instantaneous per-phase powers allows for the calculation of instantaneous total power and instantaneous electromagnetic torque as opposed to average quantities using the phasor approach.
1.3.4.2 Equivalent Dynamic Circuit Model
The purpose of the research in [8] is to establish the improvement in WEC output power as a result of avoiding Wells turbine stalling in the case of an OWC WEC. The research considers the effect of turbine speed control as well as airflow control achieved through valve action. The work assumes a DFIG.
In generating the various simulation results, the equivalent dynamic model expressed in terms of "qd0" components is assumed. This is the same electrical model to be evaluated in the proposed research. In [8], the electrical model is used in simulating the electrical power output of the WEC. In the proposed research, the model suitability is not assumed and is evaluated by considering generator response as previously described.
The analysis in [8] also assumes a typical mechanical model for rotating systems.
1.3.4.3 Final Remarks
An important difference between the proposed research problem and the work presented in [4,8] is that the research problem does not assume the use of the generator models adopted in [4,8]. Instead, experimental and simulated generator responses are compared for model evaluation so as to allow for a conclusion regarding model applicability considering the given generation application. Furthermore, a comparison of model performance is desirable if possible.
It is desired for time-instantaneous generator variables (such as stator voltage and current) to be modelled. An instantaneous-time approach allows for a direct comparison of model/simulation results with experimental results.
17 The steady-state circuit model is tested for a case of dynamic generation. In order to model dynamic operation, the model is considered from a time-based/differential-equation perspective. As presented in [4], the model may be implemented from a phasor perspective and evaluated at each point in time; however, the model dynamics are not considered in such an analysis. Therefore, the model is not evaluated with this approach together with the reasons given in section 1.3.4.1 – the phasor approach of [4] is not a true instantaneous-time approach.
1.4
Dissertation Breakdown
The remainder of the dissertation is broken down as follows:
• Chapter 2 presents the SCIG electrical and mechanical models to be evaluated together with the means of approximating the various initial conditions for simulation purposes. • Chapter 3 reveals the experimental and measurement setup used in establishing the
experimental generator response – the use of inverter-based generator excitation as well as the effect of experimental scaling is discussed.
• Chapter 4 documents the parameters of the experimental SCIG to be used in the model simulations while highlighting the parameterisation process.
• Chapter 5 presents an evaluation of the given SCIG models for dynamic generation. This evaluation is based on model stability and a comparison of experimental and simulation test results. The effect of an inverter-based generator supply as opposed to a truly sinusoidal supply is also observed.
• Chapter 6 concludes on the work presented in this dissertation with a focus on the modelling of a SCIG given the dynamics of an OWC WEC.
• Appendix A highlights basic induction-machine construction and operation.
• Appendix B is a summary of the development of the equivalent dynamic circuit model. • Appendix C details the measurement system design and implementation including the
measurement circuit layout and equipment list.
18
1.5
References
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[10] Cashman D. P., O'Sullivan D. L., Egan M. G., and Hayes J. G., "Modelling and Analysis of an Offshore Oscillating Water Column Wave Energy Converter," in 8th European
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[11] Suzuki M., "Design Method of Guide Vane for Wells Turbine," Journal of Thermal
19 [12] Thakker A., Frawley P., Khaleeq H. B., and Bajeet E. S., "Comparison of 0.6 m Impulse and Wells Turbines for Wave Energy Conversion Under Similar Conditions," in
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2.1 Introduction _____________________________________________________________________ 20