Harmonics in multiple convertor systems: a generalised approach

This paper describes a method which has been developed (and used) to calculate the harmonics associated with systems containing a large number of convertors and other sources of harmonics, e.g. steel mills or large h.v.d.c. convertor stations. The method used is a steady-state (frequency-domain) analysis in which each convertor can be individually described in detail. The effect of the a.c. system impedance and harmonic filters is catered for by using an iterative procedure. By this method, voltage harmonics and unbalances caused by, for example, arc furnaces, system impedance, system unbalances, transformer saturation, firing unbalances and the effect of firing angle and load, can all be fully represented to give a final level of harmonic current injection into the a.c. system, or the voltage distortion of the a.c. busbars. The multiconvertor aspect of the problem, involving different convertor transformer phase shifts, is handled using a transformer/convertor matrix method. This method is therefore useful in examining new and planned extensions to existing systems to check whether they meet the requirements of standards, (such as Electricity Council recommendation G5/3).