The nonlinear behavior of the transformer insulation system

Dielectric Response Analysis of power transformer is widely used in a number of field tests and research projects. Both Dielectric Frequency Response (DFR) and Polarization and Depolarization Current (PDC) combined with proper paper-oil modeling are able to give more reliable condition assessment of power transformers. Power factor/dissipation factor test at power frequency (50/60 Hz) usually exhibits a flat response as a function of test voltage if the insulation is in good condition. However, the voltage dependent behavior starts to appear if the existence of the polarization voltage is prolonged. The mineral oil and cellulose of the transformer/bushing insulation system form the interface where electric charges accumulate and thus the nonlinear property shows. The nonlinear property depends on the electric field strength as well as the geometry of the insulation system. Several transformers with different power ratings and a condenser bushing are tested by the Dielectric Response method from 20 V to 1.4 kV_rms. The measurements show that the nonlinear behavior may exist at low frequencies while the high frequency parameters are voltage independent. Furthermore, lager power transformers are less sensitive to the test voltages. The high voltage distortion of the PDC method could be more than the DFR method because of its longer polarization period. The test voltage and frequency need to be carefully chosen before carrying out any insulation test.