Equivalent Nonlinear Circuit Model with Interface Charge and Polarity Effect for Oil-Paper Composite Insulation

In the operation of a converter transformer, the interface of its oil-paper composite insulation is prone to charge accumulation under DC voltage. A simple non-linear equivalent circuit model is proposed based on the extended Debye model in this paper, which can calculate the electric field distribution and interface charge amount of oil-paper insulation more easily and accurately. Firstly, the capacitance and resistance parameters of the transformer oil and the oil-immersed paperboard are separately measured, and the electric field strength in the oil is measured based on the Kerr effect method at applied voltages from 5 to 25 kV respectively. The electric field distribution in the oil-paper composite insulation is calculated for different applied voltages as well as different voltage polarities, and the measured results are compared with the calculated results from the RC model. Secondly, the conductivity of the transformer oil and oil-immersed paperboard are measured under different electric field strength to establish a non-linear conductivity model for the oil-paper composite insulation. Then the polarisation currents of the transformer oil and oil-immersed paperboard are separately measured and fitted to extract the branch parameters of extended Debye model. Finally, the interface potential barriers are derived from the measured polarisation currents, and the polarity determined interface potential barrier is then added to the model to limit the current through the interface, thus establishing a non-linear equivalent circuit model for the transformer oil and oil-impregnated paperboard where the resistances are the electric field strength dependent. The electric field distribution in the oil-paper composite insulation calculated by the established non-linear equivalent model is compared with those by the RC model, extended Debye model and actual measured results. The time to steady state calculated by the equivalent model is reduced to about 500 s, which is shorter than that calculated by the RC and Debye models, and is closer to the measured results. As regards the steady state electric field in oil at different applied voltages, it can be seen that the error between those by the equivalent model and measurment is less than 5% for both positive and negative polarities. The error between the electric field in oil calculated by the equivalent model and the measured results is greatest at DC voltage of 25 kV, but the maximum error is 4.8%. These indicate that the accuracy of this model is greatly improved compared to the RC and series extended Debye model. Comparing the results of the three models with the measured values for positive and negative polarities, the equivalent model is much closer to the measured values than the RC and extended Debye model, and just the equivalent model reflects a polarity effect. The absolute value of the measured negative interface charge amount at 25 kV is merely 1.07% higher than the proposed equivalent model calculation, and the measured positive interface charge amount is 0.95% higher than the proposed equivalent model calculation. The following conclusions can be drawn from the above: (1) Compared with the RC and extended Debye model, the dynamics of electric field distribution of the oil-paper insulation calculated by the proposed model reach the steady state faster and is closer to the measured situation. (2) The proposed model takes into account the polarity determined interface barrier to limit the current through the interface, so its calculation can reflect the polarity effect and the error between the calculated and measured electric field is less than 5%. (3) The model has incorporated the influence of nonlinear conductivity, relaxation polarisation and interfacial potential barrier limiting current, so it has a good applicability to the calculation of the interfacial charge of oil-paper insulation under different conditions more easily and accurately. © 2024 China Machine Press. All rights reserved.