Research On The Nano-modified Transformer Oil's Preparation And Characterization

With the increase of the voltage level in power system, how to improve the dielectric strength is a hot research topic in the area. Transformer oil is one of the most important insulating dielectric in power transfonner because it plays a dual role in improving the heat transfer and insulating charaters of the system. Therefore how to improve the thermal and insulating performance of the transformer oil has also been paid more and more attention. Recent studies have shown that nanofluids may possess significantly greater thermal conductivity compared to their base fluids. Based on these observations, the transformer oil was modified by different nanoparticles, and a systematic study was performed on the preparation, stability, thermal conductivity, electrical conductivity and breakdown characterization of the nano-modified oil. This study aims at building the theoretical and practical basis of nano-modified oil in actual application.Firstly, starting from the preparation of nano-modified transformer oil, the methods of synthesizing different nanoparticles with uniform size and morphology were employed. X-ray diffraction (XRD) was used to determine the crystalline structure of the as-synthesized namoparticles. Detailed morphology and microstructure of the samples were characterized by using transmission electron microscopy (TEM). In the experiment, the as-synthesized and purchased nanoparticles were dispersed into the 25# transformer oil by milling, stirring, ultrasonic vibration and surface modification. The optical image of nano-modified transformer oil reveals that the shade of colors is related to the volume fraction of the nanoparticles. The stability of the fluid was studied by measuring the optical transmissivity of the nanofluids. The results show that the nanofluids prepared from as-synthesized namoparticles are more stable than those prepared from purchased namoparticles, which is due to the smaller size and more hydrophobic surface of the nanoparticles. Based on the uniform nanoparticles different stable nano-modified transformer oil were prepared.Secondly, the thermal conductivity of different nano-modified transformer oil were measured. The results show that the thermal conductivity of all modified oil is improved. Further, the experimental results were compared to the values predicted by the four typical thermal conductivity models. Analysis indicates that there has large disparity between the measured and calculated value. Then a universal thermal conductivity model for nano-modified transformer oil was proposed by considering the factors such as the shape of nanoparticle, adsprption nanolayer, Brownian motion, particle aggregation and so on. The thermal conductivity predicted by this new model accurately reflects the variation of the experimental process.The electrical conductivity of different nano-modified transformer oil were measured by YX1154B oil electrical conductivity meter. The experimental results show that the electrical conductivity of all modified oil increases significantly. A linear dependence of the effective electrical conductivity on volumetric fraction has been observed. The temperature dependence of the electrical conductivity reveals a nonlinear relationship. However, it's found that the amplitude and the variation are nothing to do with the electrical conductivity of the base nanoparticles. Further, the electrical conductivity predicted by the classical Maxwell model were compared to the measured results and the comparison shows that the Maxwell model underestimates the electrical conductivity of the nanofluid. We suggest that the electrical conductivity involves two separated transport process, one of which is the static electrical conductivity while the other is the dynamic electrical conductivity. The static conductivity is based on the Maxwell model which neglects the above dynamic factors. For the dynamic conductivity, the electrophoresis and Brownian motion of the particles are the key factors that play an important role in the electrical transport process. Based on this, a new electrical conductivity model is proposed, and the theoretical results calculated by new model are in good agreement with the experimental results.Finally, a test circuit was designed to determine the AC breakdown voltage of the nano-modified transformer oil. The results show that the breakdown characteristic of all nano-modified transformer oils is systematically improved. When the volume fraction of the nanoparticle is relatively small, the breakdown voltage increases with the volume fraction. The mechanism of breakdown characteristics for nano-modified insulating dielectric was investigated. It's shown the charge relaxation time for the nano-modified transformer oil is extremely longer than that of the streamer propagation. Accordingly, after neglecting the surface charge effect, the potential distortion as well as its distribution caused by the surface polarization charge of the nanoparticles under an external electric field was analysed. The results show the breakdown characteristics will be strengthened when the induced trap depth is much deeper than that of the chemical defect (0.45 eV). Finally, the dependence of the breakdown characteristics for nano-modified transformer oil on the material dielectric constant, nanoparticle size and nanoparticle shape was studied. The results indicate that a better breakdown characteristic needs greater dielectric constant, larger particle size and more spherical particle shape.