Generation for a Constant Sea State

The first dynamic generation test presented in chapter 5 involves a dynamic mechanical input to a SCIG. This test is used to evaluate the dynamic model for generation in the case of a constant sea state. The stator voltage excitation is constant in amplitude and frequency (near rated in this case) corresponding to some system operating speed. A sinusoidal prime-mover (turbine) torque drives the induction machine between light motoring and deep generation. This is the worst-case generation dynamic for a constant sea state as described in chapter 1. The tested dynamic involves a prime-mover torque oscillation with a period of 0.1 s (10 Hz) which is faster than the typical dynamic of 4 s − 5 s. Furthermore, the tested torque dynamic is rather fast considering that it is a mechanical dynamic. It is unlikely that a faster realistic dynamic should occur in practice.

Based on the agreement between simulated and measured generator response in terms of stator current, the model is found to be applicable for the tested dynamic generation. The agreement in corresponding electrical and mechanical power further supports this conclusion. Therefore, the model is applicable for typical SCIG operation as driven by a Wells turbine in an OWC WEC for a constant sea state with a dynamic torque oscillation up to a frequency of 10 Hz. This is provided magnetic saturation and parameter variability (as discussed in chapter 2) are not present. Otherwise, model parameters become a function of the generator operating point.

101 6.3.2 Generation for a Change in Sea State

As previously mentioned, quantification of the generator input dynamics in the case of the system response to a change in sea state is difficult to achieve. This is especially true for the simplistic approach adopted where the rest of the WEC system is not explicitly modelled. What is known is that some change in generator electrical excitation is used to move the generator to a new operating speed while experiencing some turbine torque response. In light of this, an informative dynamic test is carried out in chapter 5 ("Dynamic Generation – Electrical Input Dynamics") which provides an indication of the model performance.

The tested dynamic involves a sinusoidal variation in stator excitation frequency. This mimics the case of an oscillating desired system speed. The prime-mover torque is also sinusoidal in nature and is simply the response of the experimentally coupled induction motor excited with a steady-state voltage. For experimental purposes the induction generator oscillates between light generation and deep generation. Obviously the exact WEC system response has not been considered. However, the objective is to observe model performance for a dynamic electrical excitation with some dynamic prime-mover torque.

The electrical-frequency and torque oscillations have a sinusoidal period of 0.25 s (4 Hz). Such a dynamic is expected to be much faster than any typical dynamic for the given application. For perspective, consider the typical wave period of 8 s − 10 s. This is much slower than the tested dynamic. Furthermore, changes between sea states are not expected to occur at anywhere close to the typical wave period.

Since there is good agreement between the simulated and experimental generator response (given by the stator current and electrical and mechanical power), the model is applicable for the tested dynamic. Owing to the nature of the tested dynamic (sinusoidal electrical frequency and prime-mover torque with a relatively fast oscillation), it is suspected that the dynamic model would also be applicable for actual generator operation as a result of a change in sea state. Again, cases involving magnetic saturation and parameter variation still require further consideration.

6.4

Research Context

As discussed in chapter 1, the work in [1] is focused on reducing the fluctuation in generated power in the case of an OWC WEC fitted with an induction generator. The work is based on a phasor analysis (frequency domain) of the per-phase steady-state circuit model. The research question answered by the work presented in this dissertation is whether the same model may be considered from a differential-equation (time-domain) perspective.

The work presented in [2] is concerned with improving the Wells turbine efficiency in an OWC WEC by avoiding aerodynamic stalling. The work assumes a DFIG and makes use of the equivalent dynamic model. The research presented in this dissertation is based on proving/supporting the use of the same model in the case of a SCIG.

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6.5

Final Remarks and Future Work

In modelling the SCIG for the given test cases, magnetic core loss is neglected and magnetic saturation and parameter variability are not considered/included. Based on the good agreement between simulated and experimental results, these assumptions are justified. As previously mentioned, cases involving magnetic saturation and parameter variability are left for future development.

System starting and shutdown are not considered. Such operation would require the incorporation of the entire WEC model. This is left for future development. Based on the model performance for the given dynamic test cases, one might expect the generator model to perform suitably for starting and shutdown conditions. However, it is important to note that the tested dynamics have relied on a balanced, albeit dynamic, stator voltage supply. Cases involving some strange electrical-supply transient such as an electrical fault or phase- switching have not been considered. Again, model evaluation of such cases requires further research.

As discussed in chapter 1, experimental scaling has no bearing on the research results/conclusions. In evaluating the SCIG dynamic model, the experimental and simulated results show good agreement – especially considering possible system non-idealities:

• Stator voltage supply which is not perfectly balanced with some source impedance • Generator which is not perfectly symmetrical across all three phases and where windings

are not continuously distributed

• Non-ideal generator/prime-mover mechanical coupling

The inclusion of system non-idealities in the generator model is not considered part of the research scope and is left for future development.

The SCIG models considered are focussed on providing information regarding the electrical and mechanical variables of the generator including terminal voltages and currents, mechanical torque and rotor velocity. Important power flow and energy loss between the mechanical and electrical systems can also be obtained from the model simulation while giving an indication of the machine mode of operation. Thermal considerations including heat dissipation and ventilation as well as mechanical wearing and vibration are not solved in the given models.

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6.6

References

[1] Muthukumar S. et al., "On Minimizing the Fluctuations in the Power Generated from a Wave Energy Plant," in 2005 IEEE International Conference on Electric Machines and

Drives, San Antonio, 2005, pp. 178-185.

[2] Amundarain M., Alberdi M., Garrido A. J., and Garrido I., "Modeling and Simulation of Wave Energy Generation Plants: Output Power Control," IEEE Transactions on