Old meets new - Nanotechnology enables new record for transformer insulation

“By turning wood’s natural structure into a precision-engineered insulation architecture, this research can help the aging grid survive and thrive in the 21st century,” said Yale’s Liangbing Hu, who led the study, and is the Carol and Douglas Melamed Professor of Electrical & Computer Engineering & Materials Science. 

Most large power transformers in the U.S. are over 25 years old, with a typical life expectancy of just 30 years. Insulation breakdown costs consumers tens of billions of dollars annually, as well as causes lengthy power outages and safety hazards. Large power transformers today feature oil-impregnated Kraft insulation paper (OIP), a technology dating back to the 1890s that consists of cellulose fibers soaked in insulation oil. Its internal structure forms a three- dimensional, micron-sized network of oil pockets. Because oil has much lower dielectric strength than cellulose, electrical breakdown paths easily travel through these interconnected oil regions, limiting the paper’s overall performance as an insulator. Moreover, OIP doesn’t withstand physical stress well and is a poor conductor of heat, which increases the temperature of the equipment and speeds up the material’s degradation.

“While the multidisciplinary research was motivated by the global need to improve transformers, the thermal, mechanical, and electrical properties of this unique material promise to be a difference-maker over a wider range of applications,” said Robert Hebner, the Research Professor Emeritus at the University of Texas at Austin, who coordinated the testing, observed. 

The research team, featuring researchers from Yale, the University of Maryland, the University of Texas at Austin, Brookhaven National Laboratory, the USDA Forest Products Laboratory, and Rensselaer Polytechnic Institute, have taken a completely different approach. Instead of conventional pulp paper, they started with natural wood veneer, which possesses a naturally anisotropic, aligned pore structure. They applied mild alkaline treatment to remove some of its natural compounds—lignin and hemicellulose—and then soaked the wood in insulating oil. Finally, they put the wood under pressure to make it denser. The result is a new material they call oil-impregnated densified wood (ODW).

During densification, the originally micron-sized oil-filled channels shrink into isolated, one-dimensional nanosized channels. These nanometer-scale oil channels are separated by dense, highly aligned cellulose walls. In contrast, conventional OIP contains a continuous 3D oil network with no such isolation. As a result, ODW is a very robust electrical insulator. Because the oil exists in isolated nanochannels, electrical breakdown cannot propagate continuously through the oil and the surrounding high-strength cellulose.

The material also excels in mechanical and thermal properties. ODW has a tensile strength of 3.5 times that of high-density OIP and a through-plane thermal conductivity 1.6 times higher. These properties are critical for withstanding the winding stress and heat generated inside transformers. Accelerated thermal aging at 150°C for six weeks showed that ODW retains more than 70% of its tensile strength, still far above the initial strength of conventional OIP. The improved thermal conductivity directly lowers operating temperatures, which slows the chemical degradation of cellulose.