Modeling and optimization of leakage current in polypropylene dc-link capacitors

Thin polypropylene film is having a strategic role at present because it is the dielectric of choice for high-energy density and high-power density dc-link capacitors. The fast evolution of green energy applications is demanding for higher energy and power density, with expected operating temperature and electric field up to 115°C and above 250V/µm respectively, at which the insulation resistance becomes a key performance factor. Studies on thin film polypropylene are rare in literature and the characterization of ultra-thin films poses several technical challenges. In this work, the use of relatively large capacitance wound film capacitors allowed to improve the sensitivity of conductivity measurements, and to perform multiple breakdown analyses. Two different commercial polypropylene films were tested. Films and capacitors were submitted to processes aimed at reducing the leakage current at high temperatures and high electric fields. The selected treatments made possible to improve the high temperature performance, with significant reduction of leakage current. One relevant observation highlighted in this work regards the constant decay of current during dc polarization experiments, combined with the absence of a steady state condition, as reported by few other authors before. The explanation of such observation required the formulation of a novel mechanism, deviating from a pure conductivity model. A theoretical model based on charge injection and electric field distortion was developed to explain the experimental leakage current results, with focus on the decay of leakage current with time. Numerical simulation was implemented and used to validate the model and qualitatively simulate experimental results. According to the proposed model, leakage current includes a dissipative contribute from conduction through the material and a reversible contribute due to injection of charge with formation of space charge and distortion of the internal electric field.