Low-cost power generating technology for small-scale stand-alone applications.

Abstract

This thesis presents a theoretical basis and the practical implications for using small-scale (up to 50 kW) low-cost power generating systems comprising an ungoverned turbine, a three phase self-excited induction generator, a variable VAr source for voltage regulation, and an electronic load governor. The thrust of this work is towards the reduction of cost by replacing the more conventional synchronous generator by an induction machine and, by eliminating the site-specific fully-governed turbine. An electronic load governor, utilising zero-voltage switching to minimise waveform distortion, is used to regulate the system frequency by controlling the generator loading. The performance of the load governor is analysed in conjunction with both synchronous and induction generators, and the conditions under which the governed system is stable are determined. The induction machine is shown to have better operating characteristics than the synchronous machine and has the further advantages of low maintenance and ready availability making it particularly suitable for isolated applications. A theoretical explanation of the process by which an induction machine self-excites when capacitance is connected across its terminals is developed, and methods of guaranteeing excitation are proposed. It is shown that the remanent magnetism, present in the rotor, plays a significant role in the machine excitation and the conditions leading to loss of remanence are investigated. Once excited, the operation of the induction generator is analysed for both balanced and unbalanced loadings. The practical implications of these results are taken into account in proposals for single phase supplies, these being more desirable in smaller systems. A range of static VAr sources, including a switched capacitor VAr source and an inductively loaded ac-to-dc converter, are considered for controlling the machine’s excitation. The VAr sources are compared on the basis of harmonic distortion and system stability. The thesis considers the application of these devices to micro-hydro and wind generating systems. Preliminary investigations have been carried out into the use of two-speed induction machines to improve the efficiency of CSCF Constant Speed Constant Frequency wind generating schemes and into the use of centrifugal pumps as ungoverned turbines. Practical results obtained from laboratory experiments and tests on existing micro-hydro installations are included. It is shown that significant reductions in cost are obtainable by employing the methods discussed in this thesis. The resultant systems may incur a reduction in efficiency but there is little reduction in performance and in many respects the quality of supply is improved.