Comparison Among Ester Liquids for Streaming Electrification of Power Transformers

Ester liquids show better flame resistance and higher biodegradation than mineral oil and are now being considered as liquid insulating material in 500 kV power transformers in the Southern Power Grid of China. Streaming electrification characteristics among different types of alternative liquids must be investigated to eliminate insulation failure of large power transformers. In this paper, a mini-static tester acknowledged by CIGRE WG A2 and D1 was established to investigate the streaming electrification of three different ester liquids that are widely used around the world: synthetic ester (MIDEL 7131), natural ester (FR3), and modified ester (Palm Fatty Acid Ester, PFAE). Mineral oil was also tested to compare the streaming electrification properties with ester liquids. Liquids were tested under different liquid temperatures from 20°C to 90°C and a liquid flow velocity range of 0.4-2.0 mL/s. Results showed that synthetic ester presented the highest streaming current, which is then followed by natural ester. The modified ester showed the lowest streaming current under every condition. However, the streaming current of all ester liquids was a thousand times higher than that of mineral oil. All liquids presented the same property in that the liquid flow velocity had a positive correlation with streaming current, but modified ester differed from other liquids in thermal properties. The streaming current of modified ester only decreased when the liquid temperature increased, but the streaming current of other liquids increased first and then decreased. The electrostatic charging tendency (ECT) measurement also presents the same order of magnitude difference as the streaming current.