Alternative exciter supply for use on synchronous alternators in micro-hydroelectric applications

Publication Type:
Thesis
Issue Date:
2006
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This thesis develops an understanding of the context, and operating nexus linking synchronous machine excitation requirements and ballast load frequency controllers in remote area power supplies in developing communities in the Pacific. A framework has been developed to serve as a guide for this study as well as future technology transfer projects. Remaining chapters in the thesis coalesce to offer confidence in the hardware solutions presented: an alternative, simpler exciter supply circuit; and a novel method for drying alternator windings. The thesis stems from an ongoing need for the APACE-VFEG technical team to develop dedicated systems and hardware solutions for micro-hydroelectric systems. The close relationship between system designer and rural community users has afforded a wealth of valuable ‘technical’ experience and knowledge. Much of this ‘intellectual property’ remains in-house. We have evidence to show there is merit in the models and community partnership approach adopted; this study affords an appropriate vehicle to research this ‘contextual’ material with more rigour. A void between two standpoints is apparent: one where researchers identify barriers to technology transfer at a systems or sociology level - and lack depth in technical design aspects; the other standpoint where electronic systems designers typically remain focussed on component level analysis of their equipment - and hence fail to consider the broader contextual issues. The study promotes the thesis that, beyond the barriers to technological diffusion at institutional, social, and political level (which appear to dominate the issues normally considered), problems remain in providing sustainable technical solutions for this small Pacific island context. The framework is developed to provide guidance for this study and for future technology transfer projects. Soft Systems Methodology, STEEP analysis, the concepts of Actor Network Theory, and technology compatibility, readiness, and maturity afford pertinent insights to the framework at the institutional, actor, and technical levels. A review of literature pertinent to this study has been completed and gaps identified in the general body of knowledge associated with micro-hydroelectric systems for consideration in this study. Focus areas include the assumption that an Automatic Voltage Regulator (AVR) is the only solution, the requirements for flywheels, and the various explanations for what causes the unsatisfactory operation of (AVR excited) ballast load controlled micro-hydroelectric systems. This thesis shows that despite the complexity of interconnections and time constants involved, a complete and verified control systems model developed from first principles is achievable. Relationships between the system steady state operating points and the equations describing the system parameters (accelerating torque, moment of inertia, armature current, alternator rotor speed) are modelled, simulated, and verified with measured responses on a full scale micro-hydroelectric test rig. Further system testing shows an alternative solution to the AVR exists, flywheels may not be a necessity, and the cause of the unsatisfactory operation (with AVR excitation) stems from the similar time constants of the two feedback control systems (the frequency controller and voltage regulator) as well as the Under Frequency Roll-Off characteristic of the AVR. Two hardware solutions are specified, designed, simulated, implemented, and verified: the alternative exciter field supply; and a novel method for drying alternator windings. The specifications prepared have sufficient detail to afford confidence in the hardware solutions developed - they address the technical requirements and broader contextual requirements of a technology to be transferred into a developing community. Two feedback control circuits have been completed: one to replicate the constant field current design of the existing APACE constant current exciter supply; the other to incorporate voltage regulation attributes (which address the constraints of the existing APACE solution). Experiments have been conducted to verify the predicted (simulated) operating characteristics. Although designed for the Renewable Energy Laboratory alternator, the alternative exciter supply is based on the specifications for a range of AVRs and hence the solution is expected to be transferable to a broad range of machines, as well as applications beyond those described in this study. A review of the new technology developed has been completed, with a view to verifying the designs against the specifications as well as the framework guidelines. Moisture is the prime cause of machine winding insulation deterioration and corrosion of metallic parts, and this is certainly the case in humid or rainy conditions, such as those prevalent in the wet tropics. Moreover, moisture can demonstrably have catastrophic impacts if it is allowed to expand in the confined space of the windings during start-up. Given that the mechanism for moisture ingress (and egress) is not perceptible to users in this context, the existing remedy has been counter-intuitive and difficult to transfer. A novel method for drying machine windings has been developed and implemented to address this essential requirement. The method is safer and more intuitive for the system operators and reduces risks of damage to equipment. Finally, this study is considered an incremental step towards improved system sustainability. That is, rather than being seen as an exit point, the outcomes from this study have proven the concept at component level. The recommendation is therefore to commence planning on the next phase, which may be to consider combining all the various control system components together into a single package. Consideration of such a design integration will benefit from the contextual framework suggested here.
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