Degradation of Mica in Stator Insulation in Service 23 Years for A Large Generator

Degradation behavior of mica in stator insulation under operating stresses was invested through chemical formula analysis of crystal based on X-ray diffraction method. The insulation specimens were cut from a standby and an aged stator bar in operation for 23 years in a 300MW/18kV generator. Complicate changes were found on all mica crystal in aged insulation. Firstly, atom count of potassium in mica crystal decreases to 0.75 from 1. Secondly, the aluminum ion in mica crystal of aged insulation is replaced with ferrum and lithium ions, and the hydroxyl (OH) is with fluorine. Lastly, there is a trend of crystal lattice parameters increasing. Degradation mechanism corresponding to the degradation behavior of mica was discussed.