Introduction
The stator winding insulation system in generators rated 6 kV and above is usually very reliable, with an expected life of 30 years or more. However, there are a large number of failure processes that gradually reduce the electrical and mechani-cal strength of the insulation to the point that insulation failure will eventually occur. Some of the failure processes are the result of various design or manufacturing issues. Most other failures that gradually age the insulation are the result of the thermal, mechanical, electrical, and environmental stresses that occur in operation [1], [2].