What do furans in transformer oil indicate about insulation condition?

Furans in transformer oil generally indicate the condition of the cellulose based paper insulation. Higher furan levels usually mean more paper depolymerization, which reduces mechanical strength and increases risk during short circuit events. Trending is critical, because a stable baseline can be acceptable while a rising trend suggests accelerated aging due to overheating, moisture, or oxygen exposure. Furan results are best used together with moisture in paper estimates, DGA, acidity, and operating temperature history to decide maintenance or refurbishment plans.