Comparison of Generator Same Insulation Class F and Different Temp Rise

Class of insulation has to do some thing with thermal aging and therefore its life. Thermal ageing of insulation depends on the operating temperature of the insulation. Hot spot temperatures causes faster reduction of the insulation life. The temperature rise depends on amount of losses (cooling conditions remaining same which would be very much applicable for a 25MW set). Therefore machine with Class B temperature life will have less losses for the same output. Lesser losses can result from lower resistance and lower flux density in core. In turn, lower resistance and lower flux density would require higher cross section of copper and core iron. This would means more material in the generator.

As regards margin, if machine with class B rise, its margin can only be utilized if class limit is exceeded but in that case higher reliability and higher life of insulation will not be available.

No manufacturer would design a new machines just because of demand that temperature rise is to be limited to that under class B. Instead next higher rating machine , may be 27-28MW will be offered. If you are asking as a customer, let me tell that machine with class F insulation and class B rise will be definitely costlier. Reliability, margin etc., every thing costs. Nothing comes free.