Under the context of the green transformation of electrical equipment

promoting environmentally friendly transformer insulating oils still faces bottlenecks in optimizing insulation performance. The existing understanding of liquid dielectric streamers and their discharge phenomena has difficulty guiding the improvement of transformer oil insulation properties. This paper investigates the development mechanism of the streamer formation in mineral oil

natural gas-based oils

and ester-based oils under pulsed voltages. First

this paper develops a shadow imaging system to measure the fast/slow streamer velocities

breakdown voltages

and development morphology of three types of oil samples in a 50mm needle-plate gap under lightning surge conditions. The streamer development process is divided into four stages: Ball I

Ball II

Cylinder I

and Cylinder II. Then

the branching characteristics of streamers are analyzed based on a multifractal parameter analysis method. The analysis of different oils’ streamer phenomena is conducted considering the differences in ionization energy

the competing mechanism between the Laplace electric field and space charge. Finally

numerical simulations of the streamer model

which includes carrier density fluctuation terms

are performed for quantifying the electric field component relationships at the streamer head. The results indicate that mineral oil and gas-based oils exhibit higher breakdown voltages under lightning surge conditions (approximately 130kV and 120 kV

respectively)

while ester-based oil has the lowest breakdown voltage (around 90 kV). Ester-based oil enters the fast mode at lower voltages

exhibiting the worst electrical endurance. The streamer morphology can be categorized into four types: “Ball I”

“Ball II”

“Cylinder I”

and “Cylinder II”. Mineral oil

due to its high ionization energy

exhibits a unique "Ball/Column coexistence" type

while oils with lower ionization energy are more likely to be dominated by space charge and exhibit branching phenomena. Overall

the dynamic interplay between the Laplace electric field and space charge determines the propagation speed and branching characteristics of streamers under different modes.