The effect of ageing on the thermal conductivity of stator coil overhangs in medium generators

The authors evaluate coil overhang conductivity and see how it varied with cyclic operation at up to 155°C under accelerated conditions in a laboratory environment. The coils on a typical Class F 155°C generator of about 25 MW rating are cooled by air at a speed of 10-20 m/s and at a maximum temperature of 40-45°C. Heating in the overhang region derives from I²R losses in the copper. Typically the heat dissipation from the coil will be in the region of 2000 W/m² of copper surface area. These conditions were reproduced as closely as possible in the ageing of two samples of overhang from a Class F machine. They were cycled on a two-hourly programme between 40°C and 150°C and for 2500 hours at this simulated the most severe feature of their operating regime over the period of their service lives. The coils were suspended at the end of a short wind tunnel. The air speed over the samples was approximately 10 m/s. Thermal cycling appears to have consolidated the insulation and increased the high temperature thermal conductivity