Continuous monitoring of power transformer solid insulation dry-out process — Application of dielectric frequency response

Power and distribution transformers are some of the most important components of the energy system, and the aging of these devices during normal operation is inevitable. From the beginning of the manufacturing process, a strict control over the quality of materials used for the construction and assembly of transformers is implemented. Special attention is given to the solid insulation components in order to minimize exposure to the environment where they might be contaminated. After completion of assembly of the active part of the transformer, the windings and core are subjected to different dry-out processes to dry the solid insulation without affecting the life expectancy parameters defined by the degree of polymerization of the cellulose. Throughout the life of the transformer and due to the normal aging effect, several byproducts will evolve within the insulation system, and field operators will encounter the challenge of re-conditioning the insulation system of the transformer removing as much as possible these aging byproducts on-line or off-line. A variety of methods are available; selecting the right method should be the first decision. The dryout process must later be evaluated, and remaining moisture in the insulating system requires accurate estimation before the transformer is put back on service. In this work, factory and field dry-out processes are continuously monitored using Dielectric Frequency Response (DFR), analysis and validation is carried out comparing results against other available methods. On-line DFR application is demonstrated to optimize the dry-out process, providing valuable information regarding process efficiency.