Dielectric Frequency Response of Transformers - Field Experience

Dielectric Frequency Response (DFR), well-known as Frequency Domain Spectroscopy (FDS), is now a days becoming more popular as effective tool for diagnosis of power transformer insulation and estimation of moisture content. Dielectric Frequency Response is a non-destructive test. Dielectric Frequency Response of the composite insulation system of power transformers depends on different parameters i.e. properties of insulating oil, paper, hardboard and Transformer design. Power Transformer is a key element of power system network. The power transformers are installed outdoors and exposed to varying atmospheric conditions. Probability of moisture ingress in power transformers is high in the events of breathing defect, oil leakages, exposure of transformer active parts during internal inspection or repair/replacement work at site. The presence of moisture in insulation system accelerates the ageing effect and deteriorate it thereby reducing service life of power transformer significantly and may prove quite detrimental. If timely care is not taken, then failure of power transformers may also take place. The various tests, e.g. Insulation Resistance test, Polarization Index test, Tan Delta test, Oil Breakdown Voltage, Water content in oil, Relative saturation measurement test etc. can reflect the moisture ingress in insulation system of power transformer but none among these tests provides adequate information about total moisture content in power transformer insulation. Dielectric Frequency Response diagnostic test gives effective estimation of moisture content in liquid and solid insulation - oil, press board, paper etc. It also provides information about oil conductivity. In Dielectric Frequency Response test frequency range is generally 0.002Hz – 1000Hz, specifically the lower frequency and higher frequency range results reveal more information about moisture content. This paper presents various field experiences of DFR testing showing how DFR measurement results/outcomes of field testing found encouraging means for effective estimation of moisture content in transformer insulation.