New technique for improving the electric field distribution at HV winding ends of power transformers

In this paper, the electric field at HV winding ends of UHV power transformers was analyzed by using the isoparametric finite element method. The analysis indicates that the maximum electric field (E_max) in a wedge-shaped oil gap close to the surface of an electrostatic ring may be expressed as a function of the main insulation distance (m) between the HV and LV windings, the end-insulation distance (h), the radius of curvature (r) and the solid insulation thickness (d) of the electrostatic ring. A new type of winding end insulation system without a wedge-shaped oil gap is proposed, for which the partial discharge inception voltage is 94% higher than that of a conventional system. Another technique for improving the electric field distribution at HV winding ends is the application of a SiC semiconducting coating, with which the partial discharge inception voltage of a HV winding end system increases by 35%. Experimental results show that the SiC coating is compatible with transformer oil at 105°C, and may be used in UHV power transformers