Dielectric Evaluation of Tap-changer

Abstract

Abstract Tap-changers are electro-mechanical devises typically inbuilt into power transformers used for voltage regulations in power systems. They operate under extreme electrical stresses and must be reliable to ensure un-interrupted power flow and voltage stability in the power system. Thus, according to the recent statistics, around 30% of failures of transformers in Europe are caused by failures in tap-changers. The functionality and lifetime of a tap changer is directly affected by the ability of its insulating materials to withstand high voltages between the elements providing necessary clearances. The latter are defined by the geometry of the internal structures and the intensity of the electric field related directly to the magnitudes of voltage differentials. The traditional methods of tap-changer dielectric analysis have used a combination of rules of thumb that usually leads to oversizing of the insulating system. A more accurate approach to the high-voltage design is to use simulation tools to compute the electric field stresses on and in the components of tap-changers. The aim of the present work is to perform the dielectric analysis of parts of tapchangers designed at the Tap-changer department of Hitachi Energy in Ludvika. Two types of tap-changers have been considered for the study. The mechanical models of the tap-changers have been provided by the department and used for setting up dielectric simulations in COMSOL Multiphysics software. Since a tap-changer has a very complex geometry consisting of hundreds of parts in different sizes and shapes, which make the simulations challenging and time consuming, possibilities for simplifications of geometrical shapes of certain elements (different types of screws, various kinds of contacts and jumpers, etc.) have been evaluated. The results of electric field imulations of most critical parts are presented in this report. Furthermore, the calculated results have been evaluated against internal dielectric design rules of Hitachi Energy and necessary modifications have been introduced to fulfill the requirements. In additions, the sources of errors in the simulations have been considered focusing on possible errors in the original mechanical models, errors introduced while importing of the models into COMSOL Multiphysics as well as errors due to unproper meshing of elements’ geometries. It has been found that among the mentioned errors, meshing errors and the errors in the mechanical models have had a significant impact on the simulation results. The conducted simulations have resulted in an improved designs of the tap-changer that fulfills the dielectric requirements. A prototype of the devise has been manufactured and sent to a high-voltage lab for dielectric testing.