Lessons learned from twenty-year operation of the Large Helical Device poloidal coils made from cable-in-conduit conductors

•The LHD poloidal coils have been operated stably for twenty years.•Long-term monitoring of AC loss and hydraulic characteristics has been performed.•The quench detection system generated false detection events ten times.•One of the 120 cryogenic insulation breaks suddenly leaked helium.•These lessons learned will help in the design and maintenance of future systems.