Electric Field Calculations on Dry-Type Medium Voltage Current Transformers

This research presents potential and electric field calculations on medium voltage (MV) epoxy insulated outdoor current transformers (CTs) using a numerical calculation approach. Two designs of MV dry-type epoxy insulated CTs were modeled using 3D field simulation software COULOMBRTM 9.0. Potential and electric fields were calculated based on boundary element method. Different conditions such as dry exterior surface, wet exterior surface and internal voids were considered. The research demonstrates that the presence of internal conductors in CTs results in a less severe surface electric field distribution when compared to outdoor insulators of the same voltage range and type. The high electric field near the exited end triple-point of the CT reduces. This remained true even under wet conditions establishing better outdoor performance of CTs than outdoor insulators which have no internal conductors. The effect of internal conductors on voids within the insulation structure was also established. As a down side, internal voids in CTs experience higher electric field stress than in conductor-less insulators. The work recognizes that internal conducting parts in dry type CTs improves their outdoor performance when compared to electrical equipment without internal conductors.