Why Is Hydrogen High in Transformer Oil DGA?

Published 27 Aug 2026Updated 27 Aug 20268 min readIntermediateDiagnostics
Quick answer — Hydrogen (H2) is produced whenever the oil or solid insulation is stressed thermally or electrically. A rise confined mostly to hydrogen — sometimes with methane and ethane — is the classic signature of partial discharge (corona) or low-energy sparking. Hydrogen plus acetylene points to higher-energy arcing, while hydrogen plus carbon oxides or methane points to cellulose overheating.
Contents
What causes high hydrogen in transformer oil dissolved-gas analysis?

What DGA measures

Dissolved-gas analysis samples the oil and measures the gases dissolved in it: hydrogen (H2), methane (CH4), ethane (C2H6), ethylene (C2H4), acetylene (C2H2), carbon monoxide (CO), carbon dioxide (CO2), oxygen (O2) and nitrogen (N2). The pattern and the rate of change diagnose the fault, not a single gas in isolation.

Why hydrogen appears

Hydrogen is released by low-energy discharges: partial discharge in voids, bubbles or on sharp edges, and by cracking of oil molecules under modest thermal stress. It is the gas that rises earliest and fastest in PD situations, which is why a "hydrogen-dominant" trend is a red flag.

Interpreting the ratios

Use ratios (e.g. Duval or the classic Rogers/Doernenburg ratios) rather than absolute values: acetylene/hydrogen and methane/hydrogen distinguish thermal from electrical faults; ethylene/ethane separates low- and high-temperature thermal faults; carbon monoxide and carbon dioxide indicate paper involvement.

What to do

Repeat the sample to confirm the trend (gases get worse, not better), consider online monitoring for rate tracking, inspect the tap changer if acetylene is present, and look for the PD source (loose connection, partial-discharge site, moisture or contamination). Do not open the sealed unit without de-energising and a plan.

Worked example

A sample reads H2 600 ppm, CH4 120 ppm, C2H2 < 1 ppm. Hydrogen and methane dominate with no acetylene: consistent with partial discharge rather than arcing. The gas ratio methane/hydrogen < 0.1 confirms a PD/discharge signature.

References & standards

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Technical content is prepared from engineering references, standards frameworks, manufacturer documentation and industry sources. Contractual or compliance decisions should refer to current official standards. TransformerPath Technical Editorial.
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