Auto-expansion circuit breakers (also known as the self-blast circuit breakers) are an advanced generation of high voltage switchgear. This type of circuit breaker uses the arc's energy to generate a high pressure SOurce in the expansion volume (also known as the heating chamber) to create the required thermal and aerodynamic conditions for interrupting the circuit at a current zero. Ablation of the arc confining nozzle at high current is the main mechanism for pressurisation of the expansion volume. The operation of such a breaker is extremely complex and its performance depends on the whole arcing history as well as a number of geometrical factors characterising the geometry of the interrupter. It is a complex system with inter-related mechanical parts (moving pistons, valves, ablating nozzles, and moving contact) and physical processes (radiation, thermodynamics, aerodynamics, turbulence and ablation of solid materials). The relationship between all the constituent parts and physical processes gives rise to the collective behaviour of the whole system. Computer simulation of the arcing process in such a breaker has been carried out in the present work. Because of the very high power level encountered in such a breaker the arc interacts intensely with its surroundings. The objective of the work is to establish a computer model to simulate the whole arcing process, validate the model and then perform an extensive analysis of system behaviour to extract useful information for the optimisation design of such devices. The history of circuit breaker development, fundamental aspects related to the operation of high voltage circuit breakers, and history of computer modelling of switching arcs are first reviewed in chapter I, which provides an overall background picture for the present work. The mathematical description of the important physical processes is then given in chapter 2 which includes the governing equations for arc flow, the modelling of radiation and turbulence, the calculation of nozzle ablation, and the computation of electrical and magnetic fields for Lorentz force and Ohmic heating. The temperature and pressure encountered in computer simulation of the autoexpansion circuit breaker arc covers a wide range, from 300K to possibly 40, OOOK and from atmospheric pressure to 100 bar. The material and transport properties of the mixture of the working gas and ablated nozzle vapour are highly non-linear functions of plasma parameters. Thus a robust computational fluid dynamics (CFO) solver is essential. In the present work, a commercial CFO package, PHOENICS, is used for the simulation. The practically important issues, such as the implementation of the arc model with input of material properties into the solver, the specification of initial and boundary conditions, the approximate of the circuit breaker geometry, and choice of time step and control of convergence, are discussed in chapter 3. In the operation of an ABB auto-expansion circuit breaker, there are a number of mechanical parts that move with time during an operation process. The operation of over pressure valves, with one of them attached to the moving piston, has to be correctly modelled. This is detailed in chapter 4 where validation of the numerical meth~ ' ods is provided by comparing the prediction with analytical results from isentropic compression and also with measurement from ABB. Results show that the proposed numerical scheme can satisfactorily model the valve operation and the piston movement. Typical results of the gas flow in such a circuit breaker without the presence of an arc (no-load operation) are presented and discussed. In chapter 5 the operation of the ABB breaker under specified arcing current is then simulated for almost a whole arcing period. Results indicate that Lorentz force has a profound effect on the flow field as well as the arc shape. Detailed energy and mass balance calculation is performed for the arcing space and also for the expansion volume, which clearly shows the importance of radiation transfer, convection at different nozzle exits and the change of energy and mass storage at different instants in the arcing process. It is also shown that the pressurisation of the expansion volume is due to the influx of thermal energy, not the mass influx. The predicted arc voltage overally agrees with the test results within 15% for all three cases simulated with different breaker geometry. The predicted pressure at current zero is within 10% of the test results. On the whole the prediction is considered satisfactory in consideration of the approximations that have been introduced in the geometry and radiation model. It has been found that for the auto-expansion circuit breaker the pressure in the arcing space can fluctuate rapidly in the period shortly before the thermal recovery period. Pressure fluctuation with several bars around the current zero period results in a scatter of thermal interruption and dielectric recovery performances. Large pressure variation is therefore not desirable. Optimisation of design parameters is necessary in order to avoid pressure variation and to ensure maximum pressure and lowest temperature possible in the arcing voll!-me. A systematic study of the mechanisms responsible for the pressure fluctuation is therefore carried out in chapter 6. It has been found that the evolution of pressure and temperature fields in the arcing space around current zero depend on the supply rate of gas from the expansion volume and the exhaustion rate at the nozzle exits. Thus, an optimum design is directly linked with the design of the expansion volume and the link channel between the arcing space and the expansion volume. A systematic study of the influence of various design parameters is also carried out to identify the most influencing parameter, which is the dimension of the channel link. Based on the knowledge and understanding derived from this study a new design has been simulated which produces very promising result in smoothing the pressure fluctuation in the arcing space. Pressure and temperature fields at current zero depend on the whole arcing history as well as the contact movement which determines the gas exhaust passage. Arcing processes with different arcing time (altogether three cases with different arcing times) are finally performed to assess the efficiency of the new design. In all cases it has been shown that with the addition of a buffer volume the pressure smoothly changes in the period approaching the final current zero. In summary, the three objectives stated in chapter 1 have all been achieved by the work presented in chapters 2 to 6. Nevertheless, there are still several aspects of the model that need to be improved. This is discussed in the final chapter.

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This thesis examines various HVDC converter technologies that could be used in offshore Multi-Terminal DC (MTDC) grids. MTDC grids rely on AC/DC converters to interface with AC systems and also for control services. Two AC/DC topologies were compared, the half-bridge Modular Multi-level Converter (MMC) and the Alternate Arm Converter (AAC). As new DC system voltages emerge the DC/DC converter could be an enabling technology for interconnection and future MTDC networks. As yet there is no consensus on DC/DC converter topology and a critical comparison of several potential designs was conducted. An MMC based DC/DC converter had distinct advantages compared with other designs. Several average value converter models of the converters were developed to allow efficient simulation of MTDC networks, while maintaining a high level of accuracy of the converter characteristics. These models were verified with full switching models for steady state and fault conditions. Two o shore MTDC networks were studied; a four-terminal network, and a MTDC network. The four-terminal network used a normally open point to connect two existing point-to-point links, allowing reconfiguration in the event of a DC fault. The MTDC network uses a DC/DC converter to interconnect a bipole HVDC link with the previously studied four-terminal network. Several simulation studies show how new converters can improve the operation of a MTDC and provide additional capabilities such as DC fault blocking.

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