Investigations on material structural changes on electrical joints at high contact temperature

The maximum levels of operating temperature pertaining to electrical joints employed in power system network are provided in international standards. It could be noted from these standards (IEC 61439-1:2009) that the highest temperature pertaining to a bus-bar and plugin-contact is limited to 140°C. It is well known that the conditions/mechanisms occurring due to ageing of an electrical joint is enhanced depending upon the high operating temperature and maximum service time. The high temperature and service time at the joints causes an increase of joint resistance by reducing contact force, interdiffusion, fretting, chemical reactions and electro migration. The resulting higher losses might cause the temperatures to reach critical values inducing contact failures. The phenomenon behind such effects remains well established for copper and is currently adopted in electrical power apparatus during last decades. The current trend is to substitute copper with other cost effective alternatives like aluminum. In this context, there is an immediate need for fundamental investigations on other materials (aluminum) at higher or elevated temperature levels. In this paper, the physical background behind the changes occurring in the material structure of different contact materials is studied by employing modern methods of microscopy. Following this, the influencing parameters of contacts with various surface treatments is investigated and the different stages of microstructure changes (recrystallization, grain coarsening) are obtained. Based on these results, the method of modeling is discussed.