New method for digital-computer evaluation of convertor harmonics in power systems using state-variable analysis

A detailed assessment of the frequency-domain approach normally used for evaluation of convertor harmonics is presented, limitations and inaccuracies are identified, and the need for an improved method is established. The theoretical basis of a new method, which simulates the individual switching processes within each convertor, is described. The method uses a modified state-variable approach to automatically formulate the 3-phase network equations, which are then linked with the convertor equations to give a time-domain solution. The computer program developed is capable of analysing power systems with interconnected components of any desired configuration, and the simultaneous operation of several convertor loads may be studied. When the time-domain solution reaches a periodic condition, the harmonic components are evaluated using a special-purpose Fourier-analysis program. The computed results of selected cases are presented to demonstrate the inherent accuracy of the method and to show its potential usefulness for other applications