How often should DGA monitoring be performed on a transformer?

For critical transformers, continuous online DGA monitoring is recommended — this provides 24/7 surveillance and captures developing faults between laboratory samples. For the broader transformer fleet, annual or semi-annual laboratory DGA testing is standard practice. Sampling frequency should be increased for transformers that have shown elevated gas levels, have a history of faults, are operating beyond nameplate rating, or are approaching end of design life. IEC 60599 and IEEE C57.104 provide guidance on adjusting sampling intervals based on gas condition levels.