Stress Control for Polymeric Outdoor Insulators Using Nonlinear Resistive Field Grading Materials Operating Under Different Conditions

Strong electric field enhancements near both end fittings of the high-voltage polymeric outdoor insulator contribute to premature degradation and aging. Nonlinear resistive field grading composites offer a promising possibility to stress control and more competitive products, especially with the increasing power density and voltage levels. Recently, new nonlinear resistive field grading materials (FGMs) based on zinc oxide (ZnO) microvaristors offer superior characteristics that enable using them in high-voltage systems. Moreover, their switching parameters can be tailored across an extensive range for a given application. This article discusses the effectiveness of nonlinear FGMs based on ZnO microvaristors for stress control along a 230-kV polymeric outdoor insulator using the finite-element method and COMSOL Multiphysics. Investigations are conducted to appropriately find out the electrical properties of the nonlinear FGMs to get an effective stress control along the insulator surface. The different areas in the
${E}$
–
$\sigma $
plane where the conductivity of FGMs should lie are determined based on the criteria relevant to the polymeric outdoor insulator under normal and abnormal operating conditions. A new mathematical formula to express the field-dependent conductivity of FGMs is proposed to consider the saturation effect in high-field regions. Two different insulator models equipped with nonlinear resistive FGMs are presented and evaluated under different operating conditions, ac and overvoltages. In addition, the generated resistive loss within the FGMs and the leakage current under power frequency operation are evaluated. Finally, we studied the possibility of using nonlinear FGMs for polymeric outdoor insulators in HVDC systems.