Design and optimization of inter-coil insulation system of a Cast resin transformer using FEM

In this paper, a 400kVA, 11/0.433kV Cast resin transformer (CRT) insulation system is considered for optimization. Due to its numerous advantages, the design and manufacturing technology for CRT is demanding, which can only be fulfilled if the design and manufacturing is free from any defects. The reliability and life of the CRT is defined by the life of its insulation system which is again a function of thermal and electrical stresses. To meet the allowable criteria for temperature rise is relatively an easy task for design engineers. Thus, within a window of 30 years, the reliability and life of CRT remains the function of electrical stresses only. Any activity of partial discharge (PD) in solid insulation leads to degradation of the solid insulation and premature failure. In the event of PD, time to failure depends on PD energy, PD resistivity of the materials and dielectric gap. Optimization of electric field and ensuring a void-free insulation system is must to avoid the premature failure of CRT. The electric field in CRT; more specifically, in high voltage winding depends on the electrical stresses in inter-layer insulation and end insulation. Insulation between two layers of high voltage winding is termed as layer insulation, whereas insulation between coils of high voltage wingdings and insulation to earth is termed as end insulation. At microscopic level, turns per layer (differential turns), insulation thickness between layers, and conductor radius decides the electrical stresses. Whereas for end insulation, the spacing between coils and coil-to-earth potential is deciding factor for electrical stresses. This paper addresses the design and analysis of inter-coil insulation system with finite element method (FEM) using 2D-ElecNet software by Infolytica Corporation. This software can be used for all types of electrostatic and electromagnetic analysis.