What does hydrogen in transformer oil indicate during fault analysis?

Hydrogen is a key dissolved gas that often indicates partial discharge, low energy electrical discharges, or early stage overheating. Hydrogen alone is not enough to identify fault type, it must be interpreted with methane, ethane, ethylene, acetylene, and carbon oxides. Rising hydrogen trend can signal increasing discharge activity, insulation voids, or localized overheating. Rapid increase warrants investigation, correlation with load and temperature, and possible complementary tests such as moisture, furan, and electrical diagnostics.