Alternating Voltage, Direct-Voltage Regulation and Power Factor of Convertor Stations operating on A.C. Systems of Finite Short-Circuit Capacity

The paper deals with the effect of the impedance of the a.c. system on the voltage and power factor at the a.c. terminals of a convertor such as a mercury-arc-rectifier group, and on the direct-voltage regulation of this group. For convenience, the results are given as charts of curves, which are also being incorporated in a forthcoming I.E.C. publication, and the paper gives the derivation of these. For the investigation an equivalent connection diagram is used, the impedance of the a.c. system, including that of the generators feeding the system, being shown on this. A formula is derived relating the r.m.s. value of the alternating voltage, UL, in the convertor station and that at the imaginary feeding point, where the voltage, even with convertor loading, remains sinusoidal. The results obtained are then used in the deduction of the additional direct-voltage regulation, due to the a.c. system, which must be introduced on the d.c. side if the conventional theory of convertors has been used to calculate the direct voltage, thereby the convertor-station voltage being assumed to be sinusoidal and of r.m.s. value equal to UL. Curves are given showing the voltage regulation over a range covering most practical cases. A formula for calculating the fundamental wave of the alternating voltage in the convertor station is also derived in the paper. By means of this formula, together with that for the r.m.s. value of the convertor-station alternating voltage, it is shown that the displacement factor and power factor in the convertor station are determined by factors which depend entirely on known plant and a.c.-system data, and consequently that acceptance-test measurements of power factor in convertor stations actually serve no useful purpose, since they merely confirm this dependency.