Study on Estimation Method of Oil-Immersed Transformer Insulation

Abstract

To investigate the aging effects of transformer oil and insulation paper on the extended Debye model parameters and determine oil change time, accelerated aging experiment at 130 °C and PDC test on oil-paper insulation samples were carried out in laboratory. Then corresponding extended Debye models corresponding to every sample were established, and the changing rules of those models’ branch parameters with aging of transformer oil and insulation paper were analyzed respectively. It is suggested that the capacitance variation rate of intermediate time constant branch be taken as a threshold for changing transformer oil. It is demonstrated that: (1) transformer oil mainly influences the branch parameters with time constant less than 500 s, oil aging products will increase steering polarization of dipoles in oil and oil-paper interface polarization, the initial part amplitude of depolarizing current rises, the resistance of smaller time constant branch decreases while the capacitance increases; (2) insulation paper mainly influences branch parameters with time constant larger than 500 s, paper aging products only enhance the overall polarization in paper with aging time increasing, the end of depolarizing current curve rises, the branch resistance with larger time constant decreases while the capacitance increases; and (3) the capacitance variation rate of intermediate time constant branch of extended Debye model can be taken as a threshold for changing oil. When capacitance change rate rises over 0.36% and dielectric loss of transformer oil reaches 4.0%, oil should be changed.