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.

Nonconventional measurements on insulation materials high voltage bushings and power cables using frequency response analysis method

“…In this context, theoretical and experimental investigations that minimizes the intricacies on transformers has been already investigated. 5,6,9 The former theoretical part provides scientific evidence 5 while the later experimental part 6,9 validates the same on actual onsite transformers. Nevertheless, the material-based investigations and further extension of the same to other power apparatus such as bushings, cables, etc., are yet to receive attention.…”

“…In this context, there is an increasing interest in extending frequency response analysis (FRA) as an alternative (nonconventional) method for determining the insulation status of power and distribution transformers. [1][2][3][4][5][6][7][8][9][10] In addition, there are few more attempts made to expand the potential of FRA method to assess the condition of bushings and power cables. [10][11][12][13][14][15][16][17][18] This approach is reasonable as the presence of moisture and material degradation in the insulation geometry of transformers and bushings can be identified at very low frequencies (μHz till Hz) and at very high frequencies (few kHz till MHz, in some cases GHz).…”

“…3,4,[19][20][21][22][23][24] So, a simple comparison of the measured spectra (magnitude/phase response) with the respective "signature" data would reveal the insulation status of high voltage apparatus appropriately. 5 Nevertheless, the current attempts to correlate the insulation defect to the changes in the respective magnitude and phase frequency response function is still at its infancy. 5,9 In addition, extending the same to power apparatus other than transformers are not yet deeply investigated.…”

“…5 Nevertheless, the current attempts to correlate the insulation defect to the changes in the respective magnitude and phase frequency response function is still at its infancy. 5,9 In addition, extending the same to power apparatus other than transformers are not yet deeply investigated. The FRA is a universal test method widely applied as a diagnostic tool.…”

“…The FRA is a universal test method widely applied as a diagnostic tool. 1,2,5 This method is used not only for testing insulation, windings, etc., but also used for testing amplifiers, filters, electronic devices, printed circuit boards so on and the underlying principle is broadly grouped under frequency domain spectroscopy or simply the FDS method. 1,2 The FDS method employing frequencies ranging micro-Hz/milli-Hz to few kHz are used for dielectric response analysis which measures the slower polarization process and uses the same as an index to assess the insulation condition.…”