Development of Mixed Insulation Oil as Alternative Liquid Dielectric:A Review

Use of traditional mineral oil(MO)as a liquid insulation in transformers has spanned more than 130 years.However,MO has poor heat resistance,a low ignition point,and is a non-renewable resource,which does not meet development requirements for high-performance and environmentally friendly insulation oil.Consequently,researchers have explored alternatives such as natural ester(NE)and synthetic ester(SE)oils,as well as mixed insulation oils.Mixed insulating oil is a blend of diverse insulating oil types,with optimal performance achieved by adjusting proportions of base oils.This article summarizes the innovative achievements and development of mixed insulation oil in terms of development of mixed ratio,basic physical chemical properties,electrical properties,thermal stability,and application including operation and maintenance technology.Through these efforts,this article aims to provide recommendations for future development of mixed insulating oils to advance liquid dielectric research based on enhancement mechanisms.

Microstructure-ionisation potential relationship analysis for understanding the molecular ionisation difference of natural esters during the impulse discharge via DFT calculation

Understanding the impulse discharge behaviour of natural ester(NE)is crucial for its safe application in high-voltage transformers.The ionisation potential(IP)of triglycerides plays a significant role in the process of molecular ionisation during impulse discharge for NE.The correlation coefficients between molecular size,frontier orbital,electrostatic potential,polarity,and IP were analysed.And the quantitative microstructure-IP model for triglycerides were built,which revealed that IP is strongly related to the highest occupied molecular orbital(HOMO)energy EHM,as well as the average value of positive electrostatic potential V_(p)(t),the average value of negative electrostatic potential V_(p)(-),and the surface area of negative electrostatic potential S(-).The HOMO and negative electrostatic potentials of saturated triglycerides are predominantly linked to the carbonyl O atoms.Conversely,in unsaturated triglycerides,the influence of C=C double bonds on HOMO and negative electrostatic potentials is more pronounced.Therefore,the IP of unsaturated NE with C=C bonds is lower than that of saturated NE,rendering it more susceptible to ionisation under a strong electric field which is unfavourable for impulse breakdown voltage.The structure-property correlation analysis is valuable for understanding ionisation mechanism during the impulse discharge breakdown of NE at the microscopic level.

Experimental and molecular level analysis of natural ester delaying degradation of cellulose insulation polymer

Developing green insulation materials has become an important direction for future research.As a new environmental insulation liquid,natural ester can effectively delay cellulose insulation polymer ageing.Mastering the mechanism of natural ester delaying the cellulose ageing could promote its application in power transformers.In this study,based on the accelerated thermal ageing of pressboard immersed in natural ester and 25#mineral oil at 120°C for 221 days,the thermal ageing rate and the levelling off degree of polymerisation(LODP)for two kinds of oil-impregnated pressboards were compared.The molecular level mechanism of natural ester delaying cellulose degradation and keeping higher LODP value was proposed using molecular dynamics simulation.At the early stage of thermal ageing,cellulose with a higher degree of polymerisation(DP)can form many hydrogen bonds with hydronium ions,and their binding energy is stronger,which can accelerate the acid hydrolysis of cellulose,causing the DP of pressboard immersed in natural ester and mineral oil to decrease rapidly.With the breaking of the cellulose molecular chain,the number of hydrogen bonds and binding energy between cellulose and hydronium ions becomes smaller and the thermal ageing rate decreases gradually.At the end stage of ageing,it is difficult for the hydronium ions to enter the crystalline cellulose,acid hydrolysis of crystalline cellulose is hard to conduct,leading to the occurrence of the LODP phenomenon.The degradation rate of cellulose in natural ester is only 50%of that in mineral oil.The cellulose LODP value in natural ester is over 200 larger than that in mineral oil.Compared with the mineral oil-cellulose system,natural ester can form hydrogen bonds with hydronium ions and lead to fewer hydronium ions in cellulose,fewer hydrogen bonds and smaller binding energy between cellulose and hydronium ions,which can reduce the acid hydrolysis reaction of cellulose.This study provides a theoretical explanation for natural esters to delay the ageing of insulation paper.