How did PCBs improve transformer oil performance and why were they phased out?

PCBs improved performance by offering excellent dielectric strength, high thermal stability, and superior fire resistance compared to standard mineral oils. They reduced flammability risk and tolerated high stress conditions. They were phased out because they are toxic, persistent, and bioaccumulative, posing long term health and environmental hazards. Leaks and improper disposal caused widespread contamination. Regulations banned most PCB uses and required strict management of legacy equipment, leading to replacement with mineral and ester fluids that are safer.