VSC HVDC power transmission control

The problem of designing linear controllers for nonlinear plants is investigated in this thesis through a practical example from electrical transmission engineering, the control of a terminal of a VSC HVDC (voltage source converter high voltage direct current) transmission system. VSC HVDC is a technology for transmitting electrical energy between two AC systems in which high power transistors are switched at high frequency to control the voltages and power flows across the link. An analytical nonlinear model of the plant is developed and validated against a rigorous time domain model which explicitly includes the plant's switching behaviour. From an assessment of the nonlinearity of this model it is concluded that linear controller design and analysis is a pragmatic approach to take for the plant. The nonlinear model is used to generate a linear model representing a nominal operating point of the plant, a discrete set of linear models representing the range of operation of the plant, and uncertain linear models. Issues relating to the selection and construction of each of these representations and the conservatism of the uncertain linear model are explored. A new result that makes explicit the connection between the degrees of freedom in an SVD based approach to uncertain model construction and the multipliers of robust controller synthesis is introduced.