Effect of surface modification of nanomaterial on insulation performance for HVDC transformer

Nanomaterials have provided their superior properties on the insulation performance of HVDC transformer, which is helpful to ensure the safe and stable operation of the ultra-large-scale transmission system effectively. Especially, nanomaterials conduce to reduce the size and cost of the HVDC transformer. Their effectiveness is related to the microscopic properties of nanomaterials. However, the influences of nanomaterial microscopic properties on insulation performance are lacking. This paper focuses on the effect of nanomaterial surface modification on the insulation performance of transformer oil. Titanium dioxide nanomaterials with different surface modification (acetic acid, caproic acid and oleic acid) were prepared and added into the transformer oil evenly. The breakdown property and discharge streamer propagation of transformer oil and nanofluids under positive lightning impulse voltage were measured. The results show that the surface modification of nanomaterials has a significant impact on the lightning breakdown performance of transformer oil. Compared with transformer oil, the breakdown voltage and time to breakdown of nanofluid with acetic acid modifier are increased by 37.4% and 67.7%, respectively. The breakdown voltage of nanofluid increases with the decrease of the chain length of carboxylic acid molecule. Moreover, As the chain length of carboxylic acid molecule of the nanomaterial modifier decreases, the streamer branches are denser and the development speed is slower. Therefore, the nanomaterials with acetic acid modifier inhibit the development of the streamer propagation more obviously, thereby generating a better performance on the breakdown voltage of transformer.