Transient, finite element-boundary element methods for modeling high field effects in nonhomogeneous solid dielectrics

A novel transient Finite Element-Boundary Element (FE-BE) numerical code is presented. This FE-BE method code calculates electric field distributions in high voltage insulation systems with complex boundaries and with nonhomogeneous characteristics. The very accurate representation of the 3-D space distribution of electric fields is achieved by using, concomitantly, the finite and the boundary element methods-in time-thus eliminating the drawbacks characteristic to each of the methods when applied separately. The boundary element method (BEM) is applied to the lossless regions and the finite element method (FEM) is applied to the lossy and nonhomogeneous domains of the field. Undesirable phenomena, due to high voltage gradient, are usually localized at the onset. To accurately model them, separate temporal and spatial scales, different from the scale for regular discharges, are adopted. On the temporal side, a particularly small time step to accommodate the frequency is chosen. For the space, a rezoning capability is adopted to gain fine grid solution. The coarse grid solution is used to interpolate the boundary conditions for the newly created points in a recursive refinement process based on an adaptive procedure. Several practical examples of the transient modeling of high field effects are presented.