Nonlinear Characteristics and Effecting Factors of Low-Frequency Dielectric Response Currents of Damped Oil-Paper Insulation

Frequency-domain dielectric spectroscopy (FDS) is a nondestructive testing method that is effective in assessing the moisture state of transformer oil-paper insulation. FDS generally treats the sample as a linear system and uses the dielectric loss curve (tan<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\delta-f$ </tex-math></inline-formula>) as the basis for evaluation. However, this method is substantially affected by the insulation structure, and the oil-paper insulation will show nonlinear characteristics under certain conditions, affecting the accuracy of testing and evaluation. This article explores the effects of excitation voltage frequency, field strength, sample moisture content, and insulation structure differences on dielectric response current waveforms. Results indicate that the oil-paper insulation will demonstrate nonlinear characteristics after damping, and the response current waveform is gradually distorted. The distortion degree is increasingly observed with the rise in water content, the improvement in electric field strength, and the reduction in frequency. The response current contains a large number of odd harmonic components, with the third harmonic accounting for <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta_{3}$ </tex-math></inline-formula> being the largest, and the Poole–Frenkel effect is used to explain the mechanism of this phenomenon. Furthermore, a new characteristic parameter, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta^{\ast }_{3}$ </tex-math></inline-formula>, is proposed to characterize the moisture content with the per unit value of the third harmonic ratio. A power function relationship also exists between <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta ^{\ast }_{3}$ </tex-math></inline-formula> and the moisture content. Compared with the existing dielectric characteristic parameters, the quantitative relationship between <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta ^{\ast }_{3}$ </tex-math></inline-formula> and moisture content is minimally affected by the difference in insulation structure and has better applicability in the case of missing insulation structure information for onsite equipment.