Hybrid and Thin Power Electronics for Electrical Power Networks

Abstract

A new hybrid diverter design for an On-Load Tap Changer (OLTC) is presented and experimentally validated. The design differs from existing semiconductor-assisted OLTC systems in that the part of the system containing semiconductor devices is connected in a purely shunt con guration to the main current path, resulting in a system that is electrically robust and very low loss. The new design provides zero-current, zero-voltage operation of both diverter switches at all times, eff ectively eliminating arc-induced contact wear. Contact lifetime of over twenty-five million operations is demonstrated. Contact wear rates under the new design are compared experimentally with those under alternative contact protection schemes and are shown to be dramatically reduced. A fast electromechanical switch intended for use under the new hybrid diverter is presented. The low-wear conditions created by the new diverter allows a dramatic reduction in the switch moving mass when compared to that of the standard OLTC, allowing sub-half-cycle actuation times to be achieved. A study of switch topology is made in order to guide the design process. An analysis of a magnetic actuator providing both high actuation and static contact forces is also presented. In a second strand of this thesis, a general method of formulating optimal modulation problems for thin power electronic systems incorporating a buck converter is presented. The method is employs a frequency domain representation of the buck converter where the describing equations are formed into a square matrix relating a set of input harmonics to sets of output harmonics. This allows the interaction between the buck converter and a set of linear filters to be modelled in a systematic way. Two example circuits, the Inverter-Less Active Filter and the Controllable Network Transformer, are used as example problems. The use of general-purpose optimisation software for finding optimal modulation waveforms for these circuits is demonstrated.