Investigation on charge-carrier transport characteristics of transformer oil-based nanofluids

Adding nanoparticles to traditional transformer oil can improve their heat exchange properties as well as enhance their dielectric withstand characteristics. Investigations into the charge-carrier transport characteristics of these transformer oil-based nanofluids (TNFs) can explain their improved insulating characteristics, and clarify the mechanisms dictating these modifications. In this study, we measured the conduction current and velocity field of TNFs under high electric field excitation for the first time, for analysis of charge-carrier transport. We describe different transport processes according to the magnitude of the applied electric field, characterized as ohmic, tunneling, and space charge limited current (SCLC) stages. In the ohmic stage, characterized by very low electric field strengths, the increase in conduction current is due to the increase in the number of carriers from the addition of nanoparticles. In the tunneling stage (medium-to-high electric field strengths), the predominant charge carriers in the TNF change from ions and colloidal particles, to electrons emitted from the electrode. The thickness of the Schottky barrier at the metal-liquid interface increases on the addition of nanoparticles, which reduces the number of electrons that pass through to the interface region. The field strength required for electron transmission is enhanced, and the insulation strength is improved. In the SCLC stage (very high electric fields), the carrier mobility is reduced because of the larger trap density of TNFs and the electrical discharge is suppressed.