How does furan analysis indicate paper insulation aging in transformers?

Furan analysis measures furanic compounds in oil that originate from thermal degradation of cellulose (paper) insulation. Rising furan levels typically indicate ongoing paper aging, localized overheating, or accelerated insulation breakdown, even when oil looks acceptable. Because paper condition is a primary life limiter, furan trends help estimate insulation health and remaining life, especially when combined with load history, temperature data, and dissolved gas analysis. The key is trending, not a single sample.