Ionization Field Dependence on Streamer Development Properties for Mineral Oil and Different Types of Natural Esters

The breakdown of insulating oil under the lightning impulse is crucial for oil-immersed transformers. Lightning breakdown in insulating oil is typically explained by the streamer theory which is related to the ionization properties of molecules. Therefore, Density Functional Theory (DFT) was used to simulate the ionization potential (IP) of oil molecules, and the finite element method (FEM) and experimental were used to study the impact of IP differences on streamer development and breakdown velocity under lightning impulse. It can be seen that in mineral oil (MO), the molecules are less likely to ionize under electric fields. While in natural ester (NE), IP significantly decreases under electric fields, making it easier for streamer branching to occur and the breakdown velocity increases under lower voltage conditions than MO. The IP of PFAE changes with the electric field in a manner similar to MO. The development of streamer is not as significant as NE.

I. Introduction

As core components of the electrical grid, oil-immersed transformers play a vital role in ensuring the insulation stability for the reliable operation of the grid. Traditional oil-immersed transformers have a history of over 100 years of using mineral oil as the liquid insulation dielectric. However, mineral oil (MO) is a non-renewable resource and has poor biodegradability. In the event of a leak, it can cause environmental damage to soil and other ecosystems. In recent years, naturally derived esters, such as FR3 and BIOTEMP, obtained through the refining of vegetable oils, have been extensively researched and successfully applied in 420 kV transformers [1, 2]. These natural esters (NE) have higher flash points, higher breakdown voltages, and when used in combination with solid insulation, they can delay the aging of oil-paper insulation [3]. However, they have relatively high viscosity due to their larger molecular weight. In addition to natural esters, synthetic esters also offer biodegradability benefits. For instance, PFAE-modified esters reduce viscosity by shortening the carbon chains. As a result, environmentally friendly alternatives for insulation fluids have become a prominent area of research.