A study of oil-paper insulation voltage dependency during frequency response analysis

Abstract

People often believe that power factor/dissipation factor testing at power frequency (50/60 Hz) usually exhibits a flat response as a function of test voltage if the insulation is in good condition. Dielectric Frequency Response, DFR is the extension of power factor testing except that the measurement is performed from 1 kHz down to typically 1 mHz. It is a very useful tool for evaluating the moisture content in solid insulation of HV and EHV components such as power transformers, bushings, instrument transformers and PILC cables. The voltage dependent phenomenon also called “the Garton effect”, caused by paper absorbing electric charges in oil is investigated. The application of DFR in HV and EHV substations required a conceptual analysis of the phenomenon to better interpret the condition of the insulation system while increasing the signal to noise ratio to minimize the effect of surrounding interference. As a result of this work, authors provide practical recommendations regarding test voltages and frequency ranges to be used under high interference environments. The wide application of the method is supported with experimental field data.