Theoretical considerations while applying sweep frequency response analysis method in determining the interwinding capacitance of power transformers at higher frequencies

Abstract

This paper systematically investigates on theoretical and practical aspects of frequency response analysis method in evaluating the dielectric integrity of transformers. Current practices focus on interwinding capacitances as they contain sensitive diagnostic information of transformer windings, magnetic core, and insulation. However, these procedures are inadequate as they are experience based, frequency limited, and overlook the electromagnetic influence of winding core during interwinding capacitance measurements. Corresponding interpretations are subjective and require an expert's decision. Clearly, a method based on trials and involving subjective analysis procedures may render inaccurate and unreliable results. On contrary, if the influence of winding core is minimized, then the high-frequency cutoff limit can be widened, and the interwinding capacitances can be accurately determined. Presumably, the influence of winding core can be minimized via careful selection of terminal connection, system function pair. For this purpose, resistance-capacitance and ladder network models describing the impulse behavior of winding-insulation-core of transformers are simulated. A suitable terminal connection, system function pair that minimizes the influence of winding-core is identified. Subsequently, the sensitivity of the proposed method and the accuracy of interwinding capacitance measured are ascertained through comparative analysis and experimentally verified on model coils and transformers. Pertinent interpretations are objective and reveal the sensitive diagnostic information of transformers.