Investigation into High Efficiency Dc-Dc Converter Topologies for a Dc Microgrid System
thesisposted on 09.03.2012, 12:20 by Oday Ali Ahmed
Distributed generation in the form of DC microgrids has recently attracted increasing research interest. For integrating primary sources and energy storage devices to the DC bus of a DC microgrid power electronic converters are necessary, but the associated losses may degrade the microgrid efficiency. Therefore, the aim of this work is to develop high-efficiency converters, particularly for fuel cell generators and ultracapacitors energy buffers suitable for use in a stationary distribution system. Based on the evaluation of the fuel cell dynamic performance, a current–fed DC–DC converter design with a lower voltage rating of the switching devices and a higher DC voltage conversion ratio is proposed. A number of optimisation approaches have been applied to further improve the converter efficiency over its full power range. The periodic steady state operation of the converter is analysed in detail; state-space averaging is then used to determine the small signal equations and derive transfer functions. A closed loop controller has been designed and verified by a novel PSpice/Simulink/actual processor co–simulation approach, where the modelling results are validated by experimental results using a model–based design method. To sustain the charging and discharging states of the ultracapacitor, a bidirectional DC–DC converter is required. Based on a comprehensive overview on different DC–DC converter topologies, the research presented here has shown that, bidirectional voltage–fed topology is better suited for dealing with the fast dynamic response of the ultracapacitor. But for a wide input voltage variation, this topology exhibits a higher circulating power flow and higher conduction losses as a consequence. Therefore, a detailed analysis of the bidirectional converter exploring the impact of the circulating power flow interval is developed in this study. Analytic methods have been applied to establish the optimal operation of the bidirectional voltage–fed converter for an ultracapacitor to improve its performance and efficiency. Based on these methods, a novel modulation scheme is proposed that minimises the circulating power flow in the converter, that has been verified by detailed simulation.