Measurement of ion mobility in dielectric liquids
Examensarbete för masterexamen
Zadeh, Mohsen Shafiee
Rising energy demand and at the same time global effort to reduce greenhouse gases are strong pushes for harvesting renewable energy sources in remote areas far away from consumption centers. Ultra High Voltage Direct Current is a highly efficient way to transfer bulk power over long distances. One key component for the reliable energy transfer in this technology is HVDC converter transformer. The insulation system of HVDC converter transformers, which basically consists of oil and paper/pressboard with different dielectric properties, needs to withstand AC and DC stresses. In particular for the UHVDC application the transient phenomenon of electric stress due to space charges needs to be further understood in order to design an insulation system for safe operation during long service life time. A physical model commonly called the ion drift model describes space charge accumulation on solid-liquid interface by ion transport in liquid. One important parameter in this model is the ion mobility. An investigation was made to evaluate the experimental set up for ion mobility measurement in COMSOL Multiphysics®. Charge transport, electric field distributions and current curves were studied in a bulk of oil under dc voltage application. After analyzing the simulation result decision concerning the set-up type was made by having two factors in mind: accuracy and large enough signal to determine the mobility. Based on the physical picture of the ion movement in the transformer oil obtained by the numerical calculation, the plane parallel electrode geometry had been chosen for experiment. The evolution of instrument and software for data acquisition during the experiment enabled us to get the desired data. The measurements were carried out for two types of dielectric liquid: mineral oil and natural ester oil. By obtaining information on the time of flight from the current curves, the mobility of ions in positive and negative voltage application for the respective liquids were determined. For mineral oil the measured mobility was around 2.61±0.175×〖10〗^(-9) m^2/Vs and for natural ester oil was 1.67±0.18×〖10〗^(-10) m^2/Vs.
Energi , Hållbar utveckling , Elkraftteknik , Energy , Sustainable Development , Electric power engineering