Methodology of modern E-field calculations — Case study for insulated cross-arm

In the past superior pollution performance was the main driving force for composite insulator application, and therefore their use in polluted areas was considered as typical. Based on service experience surveys it is however found at present that in many countries composite insulators are installed in light polluted areas. In this case they are typically used in new, compact designs (insulated cross-arms, etc.). Therefore, insulators should be well protected from possible long-term ageing. Corona rings are used to grade the E-field along and around the insulator and normally are defined as grading rings. Both dry and water-induced corona activity might deteriorate the insulator and should be avoided at the design stage. Because of this, an important part of the development of new overhead line, equipped with composite insulators, is to verify that no corona activity will appear. This is done easiest by doing 3-D calculations with a FEM/BEM program. This article presents the results of typical case study for the rather complicated dual three phase tower structure (insulated cross-arm) and rather high AC voltage level (420 kV). The configuration also includes both grading rings and arcing horns. A comparison between electrical field calculations is presented for two different FEM/BEM programs, i.e. Comsol Multiphysics and Coulomb 3D. This paper also presents a need for laboratory test for the final verification of the insulation system design. Ideas on how this laboratory test, intended to reproduce water-induced corona, can be performed are listed.