Abstract:
Small scale single-phase renewable distributed generation system is expected to play an important role in meeting electricity demand. Because, such systems can provide low-cost electricity for homes, businesses and farms in remote areas without burning fossil fuels. The standard 3-phase squirrel cage induction machine (SCIM) is widely used in these systems due to its low unit cost, ruggedness and virtually free maintenance. However, in order to generate single-phase electricity, the standard 3-phase SCIM needs to operate in the single-phase mode. Thus, various techniques of using 3-phase SCIMs for single-phase electricity generation have been widely explored. Among these techniques, SCIM operated in the two series-connected and one-isolated (TSCAOI) winding arrangement, which provides well-regulated output voltage and frequency with a low cost, has drawn attention. There are, however, three major unsolved difficulties associated with the TSCAOI configured generator. This thesis therefore proposes the following solutions to these three difficulties, contributing to the ongoing research on the TSCAOI configured generator. The first difficulty is that the TSCAOI configured generator lacks a unified equivalent circuit model, through which the steady-state characteristics of this particular generator can be comprehensively investigated. Therefore, this thesis proposes the steady-state equivalent circuit models of the standalone TSCAOI configured generator, using the method of symmetrical components. Through the investigation of the proposed models, this thesis identifies the impacts of system parameters on its load and excitation characteristics, as well as on its level of unbalanced operation. Simulated and experimental results for a prototype generator are presented to demonstrate both the effectiveness and usefulness of the equivalent circuit models. The second difficulty is that the integration of the TSCAOI configured generator into a single-phase grid is yet to be proposed. Therefore, after investigating the behaviour of the grid-connected TSCAOI generator, this thesis proposes a control scheme to integrate the TSCAOI configured generators into the single-phase grid. The proposed control scheme can regulate the generator to provide electricity to the grid at near-unity power factor, while, minimising the unbalanced operation of the generator. The feasibility of the concept is demonstrated using both simulations and experiments of a prototype generator. The last difficulty is that the balanced operation condition and fast dynamic response are difficult to achieve for the TSCAOI configured generator, because of its inherent limited control degrees of freedom. In order to realize the balanced operation and fast dynamic response, this thesis proposes a topological modification to the TSCAOI winding configuration. The modified winding configuration, named NPC-TSCAOI (Neutral-Point-Connected TSCAOI) configuration, connects the neutral-point of the two series-connected winding in the TSCAOI winding configuration to an additional converter leg, thus adding an additional control degree of freedom. This additional control degree of freedom enables the stator flux control, which can realize both the output voltage and frequency regulation under balanced operating condition. The feasibility of this concept is also verified using both simulations and experiments of a prototype generator.