Investigation On Charge Carrier Mobility In Transformer Oil-Based Nanofluids

Transfomer oil-based nanofluids were widely concerned and researched, as it exhibited excellent thermal and insulationg properties. In order to explain the extraordinary insulation properties of nanofluids, numerous experimental studies on breakdown phenomenon and several mechanism analyses have been put forward at home and abroad. The mechanisms widely accepted were on the base of how nanoparticles influence the charge carriers in transformer oil. Neverthless, the influence methioned above have only been reflected by simulation or some other concept indirectly. Hence, it’s necessary to investigate how the nanoparticles affect the transportaion of charge carriers in transformer oil in a direct way, which may offer some help for explaining how nanoparticles influence the insulation properties of transformer oil.Charge carrier mobility is an parameter that can show the transporting characteristics of charge carriers, which includes ion mobility and electron mobility. Polarity reversal method was used to measure the ion mobility, that is, applying a DC voltage to the sample and after reversing polarity for two times, the ion mobility value could be deduced from the time-varying current figure. Barrier potential of ion transportation could be calculated from ion mobility values under different temperature. The electron mobility can be calculated from breakdown time lags, which can be obtained while imposing a step voltage to a small oil sample gap under quasi-uniform electric field.During the measurement of ion mobility, the ion mobility values of transformer oil were field independent to some degree, equaled about 1×10-9m2 V-1s-1. However, the ion mobility of nanofluids decreased with the increasing of electric field strength, whether the transformer oil were modified by titanium dioxide nanoparticels with different concentration or ferromagnetic nanoparticles. The ion mobility value of nanofluids was confirmed to be 1.2-2.5×10-10m2 V-1s-1, through the analysis of barrier potential values, diameter of nanoparticles and simulation results. During the electron mobility measurement, it was found that the electron mobility of pure transformer oil was about1.02×10-7 m2 V-1s-1, and the high humidity will cause a lower mobility value. However, the titanium dioxide nanoparticles could improve the electron mobility of transformer oil up to 1.54×10-7 m2 V-1Vs-1. Draw a conclusion, the titanium dioxide nanoparticles can reduce the ion mobility but increase the electron mobility of transformer oil, and this feature may be caused by double-layer on the surface of nanoparticles. Nanoparticles as a kind of charge carriers may reduce the ion mobility, but the double-layer structer formed in transformer oil. The charge carriers’ transfer characteristic in nanofluids may be an immediate explanation for the breakdown strength improvement of nanofluids.