Fault Tolerant Power Systems

This Ph.D. thesis documents the research work performed in partial fulfillment of the requirements for obtaining the Ph.D. degree from the Technical University of Denmark. Among the additional requirements for obtaining the Ph.D. degree is teaching activities and course participation both of which have been fulfilled. The topic of this project is fault tolerant power systems although a more universal classification would be reliability enhancement techniques for power electronic systems. The research originated with a state of the art examination in order to establish a foundation on which the techniques could be based. To assist in this examination a database was created. In addition to the database a solid theoretical foundation for reliability assessment is established. Both of these topics are covered in chapter ‘3 Concept clarification and point of origin’. From the theoretical examinations a more practical approach in system design was taken, which lead to the array-based redundancy concept presented in chapter ‘4 Array-based redundancy’. Although this work is mainly theoretical, suggestions for real-world power system implementations are presented and discussed. Based on the promising results from the array-based redundancy, the concept of digital control in power electronic systems are further examined in chapter ‘5 Digital control of DC-DC converters’, where the major elements are timing issues, software execution speed and analytical redundancy. The results in this chapter show that the use of low-cost microcontrollers allows for easy, cheap and relatively high performing converter implementations. Having explored the capabilities of digital converter control, the focus of the research work is turned towards analog system implementations utilizing parallel-connection as a means to achieve high reliability. This work is presented in chapter 6 and chapter 7. Chapter ‘6 Load sharing’ focuses on the use of dedicated load share controllers and the information needed for optimum system reliability. Chapter ‘7 Thermal droop load sharing’ concerns the implementation of power systems for high current/low voltage applications. The techniques presented in this chapter provide a simple means for parallel-connecting multiple power converters to form a single high-power system. A consequence of the proposed techniques is an equalization of the individual converter temperatures, which in turn results in improved overall reliability. Finally, the research work performed at University of California, Berkeley is described in chapter 8. This work included participation in a major project managed by Partners for Advanced Transit and Highways. Among other things this work resulted in examination of real-world problems associated with operating heavy machinery in urban areas. The overall conclusion of the the work presented in this thesis is that several reliability techniques apply to modern power system. Among the techniques considered during the Ph.D. project, several result in significant improvements in system reliability. A technique that unfortunately did not provide the anticipated reliability increase is the digital control of DC-DC converters. The overall failure rate of the digital controller is simply too high to be compensated by the additional features provided by increase in ‘intelligence’. Due to the rather high volume of data related to this project it has been decided to include all information on a CD, including a pdf-version of this thesis. A detailed table of contents for the CD can be found in the appendix.