Three dimensional FEM electrical field calculations for EHV composite insulator strings

Numerical simulations play a key role in various fields of engineering as approach to verify and optimize functional properties for complex configurations which cannot be studied analytically, starting from the early stages of design even before construction of the first prototype. Numerical methods can be adopted to calculate electric field magnitude on the components of high voltage composite insulating strings: resulting data can be subsequently used to verify and if necessary to optimize the design of corona rings and field grading hardware, which constitutes a key feature especially for insulator sets operating at the highest voltage levels. Three dimensional Finite Element Method (3D-FEM) is a particularly suitable tool for such purposes since both symmetrical (mirroror rotational-) as well as non-symmetrical geometries can be taken in account in the field calculation. Overhead Transmission Line (OHTL) composite insulator sets operating within and above EHV system voltages (>;345kV) are always equipped with field grading hardware in order to reduce peak field stresses along the insulator string. String configuration (V-string, single or double suspension and tension sets etc.) geometry of conductor bundle, presence of metal structures in the vicinity like i.e. the lattice tower and cross arms as well as distance of insulator sets from the ground may have remarkable influence on the actual field magnitudes encountered under operating conditions. When all these details of geometry are considered, resulting geometry is typically neither mirror- nor rotational symmetric, thus complete study can only be performed by 3D-analysis. Beside an accurate knowledge of electric field magnitude around the insulator string, accurate information about critical field level is necessary in order to perform design evaluation properly. Stabile partial discharge on polymeric insulating materials (Silicone, EPDM, etc.) may generate long term degradation of material properties and consequent reduction of reliability in service. The simulation procedure in its main steps including modeling, meshing, numerical solving and post-processing is illustrated in the paper and some of the critical features are reported and discussed. Since the number of Degrees of freedom (Dof) growths quickly with model complexity, advanced modeling and simulation techniques for electrostatic or electro-quasistatic field distributions are necessary to perform bulk calculations in a reasonable amount of time. In this regard, a full 420 kV case study is presented.