An analysis of non-ideal HVDC convertor behaviour in the frequency domain, and a new control proposal.

This thesis describes a linear and direct method of analysing the interaction of waveform distortion around an HVdc convertor. Existing analysis techniques are reviewed, and their strengths and weaknesses outlined. Both time domain simulation and iterative harmonic domain solutions are potentially very accurate. However, the lack of insight they provide into the important mechanisms of waveform distortion interaction, coupled with the computational expense (time domain), or limitation to steady state integer harmonics (harmonic domain), are considerable shortcomings. The frequency domain transfer function approach is chosen, and extended to cover the most important mechanisms of frequency transfer. The developed approach is used first to consider the convertor in isolation, secondly to consider the convertor coupled with either the ac system or the dc system impedance, and finally the convertor embedded within both an ac and dc system. In this way the importance of the different mechanisms of distortion transfer through a convertor are established, the frequency dependent impedance of a convertor is developed, and a description of waveform distortion dynamics around a convertor with realistic ac and dc conditions is achieved. A new indicator, the Saturation Stability Factor, is used to describe the dynamics of transformer core saturation instability. The linearised frequency domain convertor model is demonstrated to have useful accuracy, and with its ease and speed of use should prove a useful complement to existing analysis techniques. Finally, a new control system structure for back -to-back HVdc links is proposed, fully utilising the proximity of the rectifier and inverter. The proposed control system lends itself to a number of strategies, and two new criteria are developed for their evaluation