Electric field computation

Finite Element Method (FEM) is a common method that is used for modeling of insulation systems for determination of the electric field distribution. The method can be used in either the time or frequency domains. The transient finite element method is the best option for fast rise time pulses but efficient algorithms need to be used in order to solve the very non linear problem associated with semiconductive materials that have properties that are both dependent on electric field and temperature. Considerable experience and expertise has been developed in using these methods through various projects that are briefly described below.

Corona rings

corona rings
The field distribution on non-ceramic insulators affects both the short and long time performance and corona rings are applied to prevent radio and audible noise and possible degradation of the polymeric housing. Corona rings are generally designed and tested for standard line applications but these generic recommendations may not apply to non-standardized configurations and modeling and testing may be required. An example of this is inter-phase spacers where both ends of an insulator are energized and the physical relationship to the conductors must be taken into account.

Corona from water droplets

corona from water droplets
Under wetting conditions on outdoor insulation the high relative permittivity of water gives rise to localized distortions in the electric field above the corona onset level. The electric field enhancement factor varies with the size and number of droplets but for single droplets the threshold field for corona is between 3 and 3.5 kV/cm. Water droplet corona has been shown to age polymeric insulation and the reduction of the surface electric field in the design of outdoor insulation is imperative for long life.

Cable terminations

cable terminations
The insulation shield on power cables is at ground potential at power frequency and voltage gradually rises in the stress control layer. At high frequency or during the rise time of fast pulses the voltage does not penetrate the stress control layer and the high field at the junction between the insulation shield and the stress control layer results in significant ohmic heating. In addition, pollution in the form of conductive particles or water droplets give rise to enhanced electric fields resulting in surface partial discharges or corona that eventually will destroy the stress control layer.