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.