Single conductor insulation of high voltage machines using mica tapes

In the last few years considerable improvments of insulation techniques in high voltage machines have been achieved by the introduction of mica tape enameled copper bars. With respect to corona and thermal resistance requirements mica has shown to be the most suitable insulating material. Various mica tapes * are made of polyester film as a substrate, mica paper and a modified epoxy resin as a binder, which keeps the tapes flexible even after a longer treatment with high temperatures. In a typical case the breakdown voltage between turns of wires after 1000 hours thermal ageing at 180?C has dropped from 8 kV to only 6 kV at minimum and after 1 million hours thermal ageing at this temperature a MICALASTIC? insulated coil** shows still a breakdown voltage of 4,2 kV.