Effect of Dielectric and Magnetic Material Characteristics on Frequency Response of Power Transformers

Doctoral thesis, 2007

Frequency response analysis (FRA) technique has become increasingly popular for the assessment of mechanical integrity in power transformers. This method is based on the fact that the shape of the frequency response at higher frequencies is uniquely determined by the geometrical construction of the transformer. However, characteristics of the insulation system (pressboard, paper and oil) and the laminated ferromagnetic core may influence the distributed inductances and capacitances, resulting in deviations of the frequency response. The main goal of this thesis is to investigate the influence of properties of the core and insulation materials on the frequency response of power transformers. To be able to realize this goal, a small-signal high frequency model based on lumped circuit approach, of which all the circuit parameters are calculated by considering the frequency dependent phenomena related to transformer insulation and core materials, is developed. Capacitances and conductances are calculated based on analytical expressions derived from approximated geometrical parameters and measured frequency dependent permittivities of paper and pressboard. Hierarchical multi-scale modelling approach is adapted for the purpose of calculating inductances and resistances, including anisotropic properties of the core and diamagnetic properties of the windings. Complex permeability of silicon steel laminations, which is a vital input parameter in inductance calculations, is estimated using an optimization algorithm as well as by means of a single sheet tester. The developed circuit model is implemented in MATLAB as a state space model, which is used to quantify the influence of insulation condition on the frequency response and to explore a possibility of using FRA technique for characterization of the transformer insulation. According to the model prediction, the moisture content in transformer insulation can influence the response most, since it modifies both the real part of dielectric permittivity and the losses. Measurements were performed on two transformers at different magnetization levels to show effects of remanent magnetization in the core due to removal of 3-phase supply, relaxation demagnetization and DC flux on the frequency response of winding impedance. It was found that the low frequency response (<10 kHz) is affected by magnetic viscosity yielding relaxation of permeability over a long period of time. Therefore, to avoid the effects of core magnetization, diagnostic FRA measurements on power transformers should not be performed directly after disconnection from the network.