Harmonic domain modelling of direct connected generator and HVDC convertor units.

This thesis describes a new method of analysing the harmonic content of direct connected generator-HVdc convertor units. Existing analysis techniques are reviewed, and their strengths and weaknesses are discussed. A set of simultaneous equations that fully describes the harmonic interaction among the generators, convertors and the dc system is derived. The simultaneous equations are solved using Newton's method. By exploiting the sparse nature of the Jacobian matrix of partial derivatives, the solution is obtained quickly. The solution of a test system is verified by means of dynamic simulations. The developed unified solution is a significant improvement on existing harmonic analysis. A harmonic model of the salient-pole generator is developed and validated against time domain simulations and harmonic measurement results obtained from the Benmore convertor station when being operated as a group connected unit. The relationship between the generator terminal voltages and currents in the presence of saliency can be represented by analytical expressions in sequence components, derived from normal machine parameters. The effect of generator saturation on the harmonic content is not significant. The effect of rotor angle on the commutation process in the presence of saliency is significant in direct connected units. By choosing the generator terminal voltage as the firing angle reference, existing firing control for conventional HV dc schemes can be used for direct connected units with practically no modification. Provided that the nominal firing angles are kept within the restriction limits, imposed by the overlap angle, generator terminal power factor, equivalent continuous negative-sequence harmonic loading and the number of generators, practically no extra capacity for harmonic current loading is required in direct connected units. The generator harmonic ratings can be taken at the nominal speed and rated power output. Conventional tuned dc filters are ineffective in adjustable speed operation. Finally, detailed three-phase harmonic information of the Benmore convertor station when being operated in the group connected mode is collected and discussed. The test results are used for validating the developed models, and they also provide valuable evidence of the feasibility of direct connected units.