Frequency depends on criticality, loading, and condition history. Critical transmission transformers may be sampled quarterly or semiannually, while less critical distribution units may be annual or event based. After abnormal DGA, overheating, relay operations, or oil leaks, sampling is performed sooner and more frequently to trend changes. Online monitoring can reduce manual sampling frequency but still needs periodic validation. The main goal is trend quality, because consistent intervals and methods produce reliable condition based maintenance decisions.
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