Breakdown and streamer behavior in homogeneous synthetic trimethylolpropane triesters insulation oil

Abstract

A new method, which involves the addition of alkaline Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (3 wt%) and activated clay (5 wt%) as adsorbents for homogeneous synthetic trimethylolpropane triesters (TME), is reported here to not only improve electrical properties but also to shed light on future volume production. The results show that the acid value and the dielectric loss of TME oil is successfully reduced and meets IEC 61099. The viscosity of TME approaches that of mineral oil (MO) for transformers. In addition, TME oil with antioxidants exhibits fairly good oxidation stability that is evidenced by a notably high oxidation onset temperature of 226°C, which is 39 K and 14 K higher than that of commercial natural ester oil (NEO) (187°C) and MO (212°C), respectively. Furthermore, compared with the AC breakdown voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> ) of NEO (65.3 kV) and MO (70.4 kV), the TME oil has a higher V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> =73.2 kV. Besides, the positive lightning impulse (LI) V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> and streamer behavior of TME at small gaps are comparable to those of MO and NEO. However, the negative LI V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> of TME and NEO are significantly lower than that of MO owing to the combined field variation towards the plane electrode which will facilitate the propagation of the streamer. Therefore, additives without oxygen, fluorine and chlorine should be given priority when they are used to improve the properties of insulation oil.