Computer software to automate the graphical analysis of sudden-short-circuit oscillograms of large synchronous machines

Computer programs based on the IEEE Std-115 graphical procedure have been developed to automate the analysis of digitized sudden-short-circuit oscillograms. Using a number of basic building blocks widely available in commercial spreadsheet and matrix analysis software, it has proved possible to develop a new approach combining graphic displays and interactive processing. This allows a relatively inexperienced user to execute a full analysis and calculate reactances and time constants in less than three hours, starting from the raw data. Upper and lower peak time-constant envelopes are fitted using high-order polynomials or spline interpolation procedures. The latter are then used in compliance with IEEE and IEC recommendations to determine numerical values of T"_d, T"_d, x\´_d, x"_d and T_α by graphical and heuristic reasoning. Initially, simulated data was used to validate the new software and assess the effect of different time-scale decoupling windows on the parameter estimates. Despite its simplifying hypotheses and other inherent pitfalls discussed in the article, the present graphical method, we believe, should continue to be used, particularly as it is simple and fast enough to be applied directly in the field for test validation, or quick initialization of more advanced estimation procedures