The importance of lightning impulse polarity in transformer liquid insulation

Recent publications [1-5] have shed light on how streamer propagation in insulating liquids (such as those based on hydrocarbons or ester liquids) can vary substantially, and that the speed of propagation is significantly faster in positive polarity than negative polarity. In terms of the suitability of a transformer design to lightning impulse stresses, one must therefore consider the possibility of faster propagation speed in the alternative liquid under consideration compared to traditional mineral insulating oils. Whilst the phenomenon of propagation governed breakdown is more apparent in areas of the insulation system with more inhomogeneous field distribution [6] equipment manufacturers and utilities may still have to take this into account. Specifically, the IEC 60076 suite of standards for power transformers and related equipment only mandates negative polarity testing, but if positive polarity lightning impulse is applied, the risk of breakdown may be higher, especially if the insulating liquid tends to have higher propagation speeds than the reference design case [2]. The prevalence of positive lightning in nature is often stated of being around 10% [7], but the percentage of strikes that in turn cause impulse stresses on equipment in electricity networks may be higher [7]. It may be important for utilities to consider the connection between positive lightning in nature and positive impulse stresses on their actual equipment. Some results in point-plane gap studies with pure liquids are also presented and help illustrate how, in inhomogeneous fields, the propagation speed of the streamer in the insulating liquid is the key deciding factor whether breakdown will occur with the gap size and impulse wave shape as key parameters. Utilities and equipment manufacturers should investigate whether specifying the lightning impulse test at both polarities will improve their equipment reliability.