An innovative online drying procedure for liquid insulated high voltage apparatus

Although solid and gaseous insulations have become increasingly important during the last decades, the application of insulating liquids together with cellulosic paper is still essential for some kinds of applications like power transformers. A dominant risk, reducing the strength of such insulations, is water; thus drying procedures are required to extend lifetime and operation reliability. Actually transformer drying is in most cases performed as a maintenance action where it is tried to remove water by vacuum and heat. Such a procedure has to be performed offline and also comprises the risk of insulation endangerment. Such disadvantages can be avoided by an online drying procedure which is not only valuable for aged insulations where it increases the residual lifetime but also for new devices where it reduces the risk of early outages. In this contribution an innovative procedure for the dehydration of mineral insulating oils is presented, working online during transformer operation. This procedure not only performs a complete desiccation, it also purifies the liquid without influencing the dissolved gas analysis (DGA) or any other physical property. This paper presents investigations into operational parameters like temperature and flow rate which have shown that the procedure is beneficial for insulating liquids as well as for solid insulations immersed therein.