Nonlinear Polymer Nanocomposite for Field Grading in Medium-Voltage Power Converters under High-Altitude and Humid Environments

Abstract

This thesis presents the development and characterization of a nonlinear resistive polymer nanocomposite (PNC) coating designed to enhance insulation within medium-voltage (MV) power modules and suppress flashover on printed-circuit boards (PCBs) at high altitudes. Electric field simulations of the triple-point (TP) region revealed strong E-field intensification at conductor-ceramic-silicone and conductor-FR4-air interfaces, leading to premature partial discharges and breakdown. To mitigate these effects, a PNC coating composed of a polymer matrix with dispersed conductive nanoparticles was applied as a conformal field-grading layer. Electrostatic force microscopy (EFM) measurements exhibit an average distance of 135 nm between nanoparticles within the polymer matrix. Finite element simulations conducted in COMSOL demonstrated that the nonlinear conductivity of the PNC effectively redistributed the local electric field, reducing the peak intensity at the TP by approximately 50% compared to an uncoated interface. Experimental validation through partial discharge inception voltage (PDIV) and breakdown voltage (BV) tests confirmed that the PNC coating increased surface flashover voltage by approximately 30% under both ambient and low-pressure conditions when exposed to air. Humidity aging and condensation tests were performed to assess the long-term reliability of the coating within power modules. The PNC maintained its insulation improvement ability under prolonged high-humidity exposure, showing no measurable degradation in insulation strength. Overall, this work demonstrates a robust and environmentally stable nonlinear coating for surface field grading in MV power modules and converters. The PNC provides a promising pathway toward improving partial discharge immunity and insulation reliability in high-power, high-voltage electronic packaging applications.