EN
Dissolved Water Analysis
Dissolved gas analysis (DGA) in transformer oil: chromatographic determination of gases; fault classification and maintenance priority, with accredited reports.
Dissolved gas analysis (DGA) determines and interprets the gases that have entered the insulation oil (hydrogen, methane, ethane, ethylene, acetylene, carbon monoxide and carbon dioxide) through gas chromatography. The gases in the oil are the fingerprint of the transformer's internal state: normal ageing, overheating, discharge or arcing faults each form distinct ratios in the gas set. DGA counts as the single most valuable tool of planned maintenance; the early warning window can extend weeks to months ahead.
Sampling Discipline of Its Own
- Syringe-type or special metal-capped bottles prevent gas loss; oil-air contact is blocked during drawing.
- The draw point is the lower valve; during transport, temperature-vibration control and delivery time are critical — delay causes gas loss and falsely low results.
- Drawing conditions (load, temperature) enter the record; trend comparison must run from the same point-condition chain.
Interpretation Methods
Beyond raw gas ppm values, gas ratios classify the fault type. Typical signs: hydrogen rise marking breaker duty or partial discharge, acetylene marking high temperature arc (winding contact fault), ethylene-ethane marking thermal degradation trend, CO-CO₂ marking cellulosic paper breakdown. Interpretation uses the Rogers ratio method and the Duval triangle; the sum and rate of change of gases determine fault severity (energetic/incipient) and threshold exceedance. Exceedance or a rising trend calls for a de-energisation decision or in-test at a planned shutdown. On aged units DGA is read together with winding insulation tests (Insulation and Isolation Test).
Legislation and Standards
DGA is an item of the oil analysis panel under the maintenance duty of the Regulation on High Current Electrical Installations. Technical reference: interpretation guide IEC 60599 (gas trend evaluation limits) and sampling by IEC 60567 / IEC 60475. Laboratory instruments are calibrated gas chromatographs; on critical equipment the result is signed with accredited analysis and engineering assessment. Recommendation: 6-12 month interval; on a trending unit the interval falls to 1-3 months.
Outcome
The report carries gas concentrations, ratio classification, the triangle diagram and a maintenance action proposal, adding a trend table against previous results. The greatest value of DGA is giving a condition picture without de-energising the equipment: when a fault signal arrives, the urgency of intervention (immediate de-energisation, load restriction, increased monitoring frequency) is read from the gas species and their rate of change. Unexpected explosion-rupture failures are thereby replaced by planned, controlled shutdowns, converting directly into insurance and continuity gains. This analysis sits under Transformer Tests and Analyses and the Insulation Oil Test panel; for other parameters see Dielectric Strength and Oil Leak Inspection, or contact us for an inventory-specific programme.
FAQ on Dissolved Water Analysis
Beyond lowering the dielectric strength of the oil, moisture accelerates the polymerisation of the insulation paper and permanently weakens cellulose strength. Water in the paper is far more harmful than in the oil and only recoverable by drying. Dissolved water rises proportionally with temperature: in a hot-running transformer the oil moisture is read as a messenger of paper moisture, so oil and computed paper water are evaluated together.
With Karl Fischer titration, dissolved water in the oil is measured in ppm (mg/kg); the result is given with the sample temperature and oil type reference. Complementarily, furan analysis is taken for paper polymerisation-state and tan delta for the moisture-insulation relation. Sampling must be air-free and representative; otherwise external moisture raises the result.
Acceptance thresholds vary by voltage class-age and working temperature; in operating oil, rising ppm moisture with falling breakdown strength is a drying signal. In furan determination, rising values show cellulose degradation, and tan delta shows moisture-contamination with ageing. The aim is to remove the moisture with planned vacuum drying before the paper water reaches the strength threshold, thus protecting the transformer life.
Vacuum hot drying (pulling moisture from oil and paper), oil change-filtration where needed, and sealing repair (conservator-gasket-breather-dehumidiser) are planned. A verification sample is taken after drying; the moisture-temperature-furan thresholds are rechecked. If the moisture source is external (leak-breathing) the treatment is temporary until the root cause is removed. The report carries the moisture-value-drying decision-verification chain as a trend.