In-situ high temperature study of the long-term stability and dielectric properties of nanofluids based on TiO2 and SiC dispersions in natural ester oil at various concentrations

Diverse types of nanoparticles dispersed in various mineral and natural oils are under systematic investigation over the last years, in an effort to develop insulating nanofluids towards enhancing the lifetime and improvement of the operation conditions of power transformers. The moderate stability of the nanofluids (NFs) is a major obstacle in advancing reliable solutions with strong application potential. Although typical stability studies, i.e. ageing against time at ambient conditions, have been undertaken, thermally-induced degradation of NFs are very scarce; whereas in-situ assessed stability and dielectric properties at temperatures similar to that of the operation conditions of a power transformer, have not yet been conducted. Here, we present a systematic study to evaluate in-situ the influence of high temperature on the stability loss of nanofluids and their dielectric properties. Two types of semi-conducting nanoparticles, an oxide (TiO2) and a carbide (SiC) have been employed in conjunction to a natural ester oil, to prepare NFs at various concentrations. Two sets of experiments were run in parallel, where dynamic light scattering and electrical measurements (AC Breakdown Voltage of the nanofluids and Partial Discharge of insulating paper impregnated with them) were conducted at ambient temperature and at 90 °C. It has been found that all NFs showed increased dielectric properties under thermal ageing with respect to the NFs aged at room temperature. In addition, TiO2-based NFs experience accelerated ageing at high temperature, while the SiC-based NFs exhibit better stability in comparison to their TiO2-based counterparts, when compared at similar conditions of time and thermal ageing. The NF with 0.004% w/w SiC nanoparticles exhibits the best dielectric response and the longer time stability for both sets of experiments i.e., ageing under both ambient temperature and 90 °C. These results demonstrate that results obtained from ambient temperature stability and dielectric properties studies, cannot be generalized or extrapolated at higher temperatures, where power transformers typically operate.