This thesis describes the development of a general method for the analysis of integrated a.c. and d.c. systems under normal, but not necessarily balanced, steady state operation. Phase component three phase system modelling is reviewed and the relationship of the well known symmetrical components to the three phase modelling is discussed. Using as a reference the single phase fast decoupled algorithm the modifications required to produce an efficient three phase fast decoupled load flow are described. It is demonstrated that the three phase fast decoupled load flow displays all the characteristics of the original single phase version. Single phase balanced convertor modelling is reviewed and several techniques for the integration of such models with the single phase fast decoupled load flow are developed and their performance is compared. The methods for single phase convertor modelling are extended to allow unbalanced convertor operation to be analysed. The integration of the unbalanced convertor model into the three phase fast decoupled load flow is described. Convergence properties are examined and detailed results given. The extension of steady state analysis techniques to the consideration of harmonic frequencies is discussed. The unbalanced convertor model is used as a basis to enable the harmonic interaction of d.c. convertors and the a.c. system to be studied.