The plant was operating at 100% power when a small fire occurred on the exhaust manifold lagging of an emergency diesel generator that was started for testing. An operator quickly extinguished the fire using a hand held extinguisher. No other systems were affected by the fire and the operators continued to operate the emergency diesel generator until the test was complete. The diesel generator remained operable throughout the event. The source of the fire was minor oil leakage past a flange gasket between the exhaust manifold and the turbogenerator.