Comprehensive calculation of convertor harmonics with system impedances and control representation

The paper describes a computational method for finding the harmonics in large convertor systems, with particular reference to the interdependent effects of convertors, transformers, AC and DC system representation and convertor controls. This work is of particular concern as the number of HVDC schemes and large drive systems increases within AC power systems. The method described is comprehensive in that it can model, for example, unbalanced AC system voltages and impedances; any type of convertor transformer arrangement; virtually any pulse number; harmonic filters, a non-infinite AC system representation; it does not assume a smooth DC side current and can include the effect of system resonances in both the AC and DC system. Within the bridge convertors, such features as firing unbalance, convertor transformer saturation, unequal commutation overlaps and the effect of the two principal control methods are all easily modelled. If a comprehensive method is not used, the results of an harmonic study could give unrealistic results. This is shown by example.