Abstract :
The technique of existence functions is applied to the analysis of effects of de terminal ripple on the ac line current of a 2 pulse converter/inverter, which effects have been neglected in previous analyses. Assuming that ripple in the de terminal current is the result of the action of de terminal voltage ripple on a finite inductance, the analysis proceeds by taking the products of the Fourier expansion of the thyristors´ existence functions and the ripple current. The resultant matrix of ac line current component terms is manipulated into solution vectors, each representing an individual component of the ac line current attributable to the dc terminal ripple current. It is clearly demonstrated that only harmonic and quadrature fundamental components are generated. These quantitative expressions are further manipulated to yield the magnitudes of the ac line current components in terms of the per-unit peak-topeak value of de terminal ripple. Since the expressions are not readily interpretable except in certain special instances, a computer program was created for their numerical evaluation. Selected results of these evaluations are presented. They show that, except at delay angles close to the theoretical rectification and inversion end-stops, the ac line harmonics generated by de terminal ripple tend to be antiphase to, and hence suppress, those produced by the de current. In contrast, the quadrature fundamental current produced by de terminal ripple always adds to that produced by the de current. These results indicate that the displacement and power factors of a converter operating with appreciable de terminal ripple are always less (worse) than those calculated assuming a perfectly smooth de current. However, the harmonic currents in the ac lines are reduced when de terminal ripple is allowed, both in absolute and relative magnitude. The greater the peak-to-peak de terminal ripple current, the greater these effects. It is concluded that there may be operation- l benefits, to wit harmonic reduction in the ac lines, when relatively large amounts of de terminal ripple current are permitted. However, the harmonic problem is transferred to the de side by so doing, and the quadrature fundamental current demand of the converter is significantly increased. Dependent on the specifications obtaining at the ac and dc interfaces and the relative economics of power factor correction and de side filtering versus ac side filtering, operation with high peak-to-peak de terminal ripple current can be preferable.
