Effect of immersion time on the combustion characteristics of oil‐impregnated transformer insulating paperboard

Abstract

Transformer insulating paperboard is an insulating material commonly used in the oil-filled equipment in substations, which is usually immersed in the transformer oil.In this study, the effects of immersion time on the combustion characteristics of oil-impregnated transformer insulating paperboard are studied using the thermogravimetric, UL94 vertical flammability, and cone calorimeter experiments. The experimental results show that the total oil content increases with increasing immersion time and reaches the saturation state in 240 h, with a saturation content of 14.7% ± 0.5%. However, the distribution of oil content inside the paperboard is uneven. At the saturation state, the surface oil content is 35.5%, being about 3 times of the core oil content. The results of UL94 vertical flammability experiments show that the flame spread rate increases with increasing immersion time and is controlled by the total oil content. In the cone calorimeter experiments, the time to ignition is significantly reduced compared to the clean paperboard. Unlike the flame spread rate, the ignition time is controlled by the surface oil content and barely varies with immersion time. The critical heat flux of oil-impregnated transformer insulating paperboard is found as 3.9 kW/m 2 , being only 1/3 of clean paperboard. With increasing immersion time, the peak heat release rate increases, while the total heat release, fire growth index, and effective heat of combustion present linear correlations with the total oil content. The CO yield increases with increasing immersion time, and the released CO mainly comes from the combustion of transformer oil.

Scaled experimental study on the initial oil thickness effect on the combustion characteristic of the typical transformer insulating oil

“…In addition, mineral insulating oil is difficult to degrade and will cause ecological harm when it flows into the natural environment [3] . In contrast, the burning point of natural ester plant insulating oil is mostly above 360℃, which is a K-class difficult fuel, and it has a higher burning point than traditional mineral insulating oil [4] . Moreover, plant insulating oil has the advantages of being widely available, renewable, degradable, pollution-free, etc., which is expected to replace traditional mineral insulating oil [5] .…”

“…In 2021, Huang Qingdan used a cone calorimeter to carry out experimental research on the combustion characteristics parameters of plant insulating oil for transformers under different external radiant heat currents and concluded that the fire risk of plant insulating oil increased with the increase of external radiant heat flow. The order of fire risk from high to low was concluded: synthetic ester vegetable oil >> soybean oil > rapeseed oil > camellia seed oil [4] . However, the comparative analysis of mineral oil and natural ester insulating oil is still lacking, and so is the effect of initial fuel thickness on combustion characteristics.…”

“…In 2020, Wang Yalong studied the effects of initial oil temperature, fuel thickness, and oil pool diameter of KI50X UHV transformer on combustion rate, flame height, and plume axial temperature, and found that fuel thickness had a great influence on mass loss rate. When the fuel thickness was 20 mm, three typical combustion stages can be observed that obtained initial growth, quasi-steady combustion, and decay to extinction [8][9] . In 2021, Huang Qingdan used a cone calorimeter to carry out experimental research on the combustion characteristics parameters of plant insulating oil for transformers under different external radiant heat currents and concluded that the fire risk of plant insulating oil increased with the increase of external radiant heat flow.…”

Experimental Study on Fire Suppression of the Outdoor Oil-Immersed Transformer by High-Pressure Water Mist System

“…Chen [11] proposed that there were five main causes leading to transformer fire accidents, i.e., bushing malfunction, winding short circuit, iron core spoilage, deterioration of insulation oil quality and natural disasters. Additionally, the study of small oil pool fires was the most popular method to analyze the combustion characteristics of the transformer oil in terms of the initial oil temperature [6,12,13], the fire source power [14], the size of the oil pool [15][16][17][18][19], the level of oil pool [20], the burning rate [21], the ignition mode [22], the pulsating characteristics of the flame [23,24], etc. However, the transformer fire is a collection of multiple fire scenarios, and the burning rule obviously changes with its geometric scale.…”

Numerical Simulation of 35kV Oil-Immersed Transformer Fire and Extinguishing Effects of High-Pressure Fine Water Mist

“…Insulation oil is the main insulating and cooling medium for oil-immersed transformers, which are filled with large quantities of combustible transformer oil inside and on top of the oil pillow. Oil-immersed power transformers in the long-term operation process, its internal insulating oil paper will be affected by electrical, thermal, mechanical stress and other factors and gradually deteriorate, resulting in its insulation performance decline, in the fault, high temperature conditions fire, often leading to oil-immersed transformers themselves and other adjacent electrical equipment damage, serious damage to substations, once a fire will not only affect the safe and stable operation of the power grid, and even to the safety of society has a great impact [6][7][8][9][10][11][12]. In a 220kV transformer fire true type experiment, the temperature around the fire was up to 800℃, causing serious damage to the facilities around the substation [13].…”