Influence of Insulating Oil on Lightning Impulse Withstand Voltage Level for Instrument Transformers

Instrument transformers are a part of every standard high-voltage substation, used for accurate measurements of voltages and currents within the network. As such, they need to be designed robustly, having the capability of withstanding all the network situations, be they stationary or transient, continuously throughout decades. Nowadays, a lot of emphasis is put on designs that have a reduced environmental impact. With an array of different insulating oils present on the market, it is of great importance to adequately investigate the behavior of such liquids, in order to confirm their applicability in instrument transformers and similar products. This paper presents the direct comparison of three geometrically identical insulation systems impregnated with different insulating oils, tested under a standardized negative polarity lightning impulse test of 1.2/50 $\mu$s. The oils considered are NYTRO Lyra X, NYTRO BIO 300X, and MIDEL 7131. The main purpose of the paper is to investigate the behavior of different oils in homogenous paper-oil insulation systems. Furthermore, the purpose is to evaluate the effect of using the considered oils on design rules conventionally used for such insulation systems. As such, this work can aid manufacturers of such insulation systems in designing safer, more robust and more ecologically friendly devices.