Electrical machine characterisation and analysis for renewable energy applications

There has been an increased use of the doubly-fed induction machine (DFIM) in ac drive applications in recent times, particularly in the field of renewable energy systems and other high power variable-speed drives. The DFIM is widely regarded as the optimal generation system for both onshore and offshore wind turbines and has also been considered in wave power applications. Wind power generation is the most mature renewable technology. However, wave energy has attracted a large interest recently as the potential for power extraction is very significant. Various wave energy converter (WEC) technologies currently exist with the oscillating water column (OWC) type converter being one of the most advanced. There are fundemental differences in the power profile of the pneumatic power supplied by the OWC WEC and that of a wind turbine and this causes significant challenges in the selection and rating of electrical generators for the OWC devises. The thesis initially aims to provide an accurate per-phase equivalent circuit model of the DFIM by investigating various characterisation testing procedures. A novel testing methodology based on the series-coupling tests are employed and are found to provide a more accurate representation of the DFIM than standard IEEE testing methods. The series-coupling tests provide a direct method of determining the equivalent-circuit resistances and inductances. The tests allow for these parameters to be determined independently in contrast to the standard IEEE tests that require assumptions to be made in order to determine the parameters. A third approach known as the extended short-circuit test is also presented as an alternative characterisation method. I Experimental a 1.1 kW DFIM and a 30 kW DFIM utilising the various characterisation procedures is presented in the thesis. The various test methods are analysed and validated through comparison of model predictions and experimental results for torque-versus-speed curve for each induction machine. Sensitivity analysis is also used as a means of quantifying the effect of experimental error on the results from each of the testing procedures. This analysis is used to determine the suitability of the test procedures for characterising each of the devices. The research then focuses on the OWC WEC and the modelling of this device. A software model is implemented based on data obtained from a scaled prototype device situated at the Irish test site. Test data from the electrical system of the device is analysed and this data is used to develop a performance curve for the air turbine utilised in the WEC. This performance curve was applied in a software model to represent the turbine in the electro-mechanical system and the software results are validated by the measured electrical output data from the actual scaled device. Finally, once both the DFIM and OWC WEC power take-off system have been modelled succesfully, an investigation of the application of the DFIM to the OWC WEC model is carried out to determine the electrical machine rating required for the pulsating power derived from OWC WEC device. Thermal analysis of a 30 kW induction machine is carried out using a first-order thermal model. The simulations aim to quantify the limits of operation for the machine and give an insight into the rating requirements of an electrical generation system for the OWC WEC.