IEC 60391: Insulated Bushings for Power Transformers — Capacitive Grading Design of Condenser Bushings

Condenser Bushings — The Elegant Design Using Multi-Layer Metal Foils to Force Uniform Voltage Distribution

IEC 60391:1972 specifies insulated bushings for power transformers. The condenser bushing core design embeds multiple concentric metal foils within the insulation body — each foil layer forms a series capacitor. By precisely controlling radial widths, the voltage is forced to distribute uniformly along the bushing length. Without capacitive grading, the voltage concentrates near the flange (highest field stress), causing partial discharge and flashover.

TNLab — The condenser bushing is a classic HV engineering case study of “using conductors to actively control electric fields.”