Electric field grading using insulators with microvaristor filled silicon rubber

The lifetime of insulators for Ultra High Voltage Overhead Transmission Lines (UHV OHTL) for voltages of 765 kV or 1100 kV is currently restricted to limited years due to unknown effects regarding the onset of streamer discharge. To overcome this limit special insulators are developed that incorporate a microvaristor layer thus restricting the peak field strength to 0.5 kV/mm (according to the switching point to be determined by the microvaristor microstructure). In this paper the current status including most recent FEM modelling and HV laboratory test results is presented. This resulted in the development of a prototypical design to be tested in a forthcoming field test.