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C57.161 - IEEE Draft Guide for Dielectric Frequency Response Test

This guide is relevant to the Dielectric Frequency Response (DFR) test methods of liquid immersed transformers. The guide includes recommendations for instrumentation, procedures for performing the tests and techniques for analyzing the data. DFR measurement is a non-intrusive, non-destructive off-line testing technique developed as an advanced diagnostic tool. It can be performed in the frequency domain to determine the moisture content of the transformer’s solid insulation and conductivity of the liquid insulation. For a DFR measurement, the capacitance and power factor/dissipation factor is measured over a wide frequency range, typically from an upper limit of 1 kHz down to a lower limit which ranges between 10 mHz and 0.1 mHz. The frequency range may be subject to change based on the physical and thermal conditions of the insulation under test. The measured data collected in a DFR test include: capacitance, percentage power factor and /or percentage dissipation factor at a constantly applied test-voltage over a wide range of frequencies. DFR displays the fundamental polarization and conduction phenomena in transformer winding insulation, which is a combination of oil gaps and solid insulation dielectric responses. As a result, it is influenced strongly by both solid insulation moisture content and oil condition. Mathematical modeling was seen as the key to determining how measured responses are affected by oil conductivity and moisture content. Once accurate and reliable test data has been acquired during the test, mathematical algorithms and a reference database are used to process the data. This guide describes factors affecting the DFR test moisture estimate including winding configuration and measurement set-up, cellulose material type, low molecular weight acids, and background AC/DC noise interference. This guide is applicable to both field and factory applications. The purpose of this guide is to provide the user with information that will assist in performing Dielectric Frequency Response 68 measurements and interpreting the results from these measurements.
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