Novel systems for the prolongation of the lifetime of power transformer insulations

Although solid and gaseous insulations have become increasingly popular during the last decades the application of insulating liquids together with a solid insulant immersed therein is still essential for some kinds of applications like power transformers. A significant risk, reducing the strength of such insulations, is water, thus drying procedures are required to extend lifetime and operation reliability. As actually used systems for this purpose comprise several imponderabilities new systems are required. 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 thus is time consuming and expensive. In this contribution an innovative procedure for the dehydration of the transformer insulating system is presented, working online during transformer operation. Thus it is well-priced and ideally suited for transformers with a moderate water strain, where an immediate drying is not yet indispensable. The procedure not only performs a complete desiccation, it also purifies the liquid without influencing the dissolved gas analysis (DGA) or any other physical property. The application of the system on power transformers has shown, that the procedure is beneficial for insulating liquids as well as for solid insulation immersed therein. As the insulating liquid operates as a “water transport medium”, disposing the water out of the transformer insulating system, the efficiency of this system can be increased by a hygroscopic insulating liquid like ester liquid, that is capable to comprise more than 30 times the rate of water compared to mineral oils. Beside the acceleration of the drying process together with the presented system they also have a beneficial effect on the aging of cellulose impregnated therewith. As ester liquids and mineral insulating oils are completely miscible also blends of esters and insulating oils are advantageous. The contribution presents these beneficial effects by means of some examples gained during aging tests in laboratory and also discusses the applicability of the test results for transformers in operation. INTRODUCTION For power transformers the combination of an insulating liquid and a solid insulation impregnated therewith is almost the only kind of utilized insulation. Although long term experience has shown, that this kind of insulation is very reliable, the market demands for an extended efficiency and reliability of the equipment forces the utilities to increase the transformer load while avoiding malfunctions. One of the most significant influences that decreases the insulation reliability and lifetime is water in the insulating system which reduces the breakdown strength of the insulating liquid and increases the aging of the solid insulation [1]. Water is not only absorbed from the environment as it happens especially for breathing transformers, it also is generated inside the device due to the depolymerisation of cellulose as it is schematically illustrated by Figure 1. The solid insulation materials are commonly used as wrapping around the conductors and spacers, thus a refurbishment of these structures during a maintenance action would require an almost complete dismantling of the device. Due to the costs of such a treatment a destroyed paper insulation therefore represents in most cases an unrecoverable failure which leads to the disposal of the transformer. Especially for aged transformers, that have already aggregated high amounts of water, a drying of the insulation therefore seems to be an obligation to increase operation reliability and residual lifetime. With most of the actually available systems for drying transformers a desiccation is performed as a C O