Undesired Locus of Operational Impedance and Consideration of DC Decay Testing Method for Large-Capacity Alternating Current Generator

Abstract

The DC Decay Testing Method is quite useful at testing large-capacity machines, because it is a simple standstill response testing method and needs only a small capacity of dc power supply equipments. When authors held a field test of the large-capacity alternating current generator (6000KVA-18P), we found that there was an unnecessary small curl in operational impedance locus. And it lead to the error at predicting the machine constants of its generator. So we consider that it is a problm in practice.In this paper, the investigation of measured data and approximate analytical calculation shows that the curl in operational impedance locus is caused by truncating the dc decay current data. So, authors propose a data processing method by data extrapolation which predicts the decay current after the measured data. And the result of applying the Expanded Dalton-Cameron Method to the total decay current is good agreement with the result of the other measuring methods. The proposed method is also applicable in testing the small and middle-capacity machines and expands the usefulness of the DC Decay Testing Method.