DGA is a powerful diagnostic technique that detects transformer faults at the INCIPIENT stage, before they grow into major failures — a genuine early-warning capability the conventional Buchholz relay simply doesn't have.
Exam Focus — Why Buchholz Relay Isn't a Diagnostic Tool
The Buchholz relay's real limitation: enough gas must FIRST saturate the oil, and then either come out of solution or produce a genuine gas surge, before the relay can operate. It was never meant to be a preventive-maintenance diagnostic device — only a protective one that reacts after the fact. This is exactly why DGA exists as a separate, proactive technique.
5 Real Gas-Formation Mechanisms
- Oxidation — carbon dioxide is predominantly liberated when small quantities of oil chemically combine with dissolved oxygen, forming organic acids that react with transformer metal to form metal-based soaps, which act as a CATALYST accelerating further oxidation.
- Vapourisation — oil vapourises at about 280°C, water at about 100°C. A Buchholz relay FALSE ALARM can genuinely occur when excess moisture in the tank vapourises from a heat source, or when hydrocarbon constituents of the oil itself vapourise.
- Insulation decomposition — solid insulants (cellulose/resinous types: paper, pressboard, resins, varnishes) contain oxygen, carbon, hydrogen. Between 150°C and 400°C, breakdown liberates hydrogen, CO2, and CO; above 400°C, relatively FEWER gases form.
- Oil breakdown — direct breakdown by arcing cracks the oil; aromatic contents break down into hydrogen, with acetylene and methane as major constituents.
- Electrolytic action — hydrogen and oxygen liberated when minute fibre particles in the oil cause electrolysis; light hydrocarbon gases may also appear if solid insulation is involved.
3 Real Fault Types & Their Gas Signatures
Exam Focus — The CO2/CO Ratio
For OVERHEATING faults: thermal degradation of paper produces CO2 and CO in a ratio TYPICALLY AROUND 5. But if that ratio falls BELOW 3, it indicates SEVERE overheating of the paper — a real, specific diagnostic threshold, not just 'more CO2 is bad.' Separately, methane/ethane appear above 120°C, ethylene above 150°C, and acetylene only at several hundred degrees — meaning the specific gas mix genuinely indicates how hot the fault is running.
- Partial Discharge — occurs from ionisation of oil in highly-stressed areas with gas/vapour-filled voids, or moisture-containing insulation. Main product is HYDROGEN, with small amounts of methane and other gases depending on thermal degradation.
- Arcing — occurs between leads, between lead and coil, and other highly-stressed regions weakened by fault conditions. High temperatures from arcing produce ACETYLENE and HYDROGEN.
The Real Key Gas Table
| Fault/Pattern | Key Gas |
|---|---|
| Conductor Overheating | CO / CO₂ (Carbon Oxides) |
| Oil Overheating | C₂H₄ (Ethylene) |
| Partial Discharge | H₂ (Hydrogen) |
| Arcing | C₂H₂ (Acetylene) |
Memory Aid — Conductor vs Oil Overheating Are Different Gases
A genuinely easy mix-up: 'overheating' isn't one single key gas — CONDUCTOR overheating shows CO/CO2 (from paper/cellulose breakdown), while OIL overheating shows ethylene (C2H4) instead. Knowing which part of the transformer is overheating changes which gas you'd expect to see dominate.
Solubility & Rogers' Method
Gas solubility varies with temperature and pressure: H2, N2, and CO solubility INCREASES with temperature, while CO2, C2H2, C2H4, and C2H6 solubility DECREASES with temperature — methane (CH4) solubility stays essentially constant. All gases become MORE soluble with increased pressure. This solubility knowledge is essential for correctly interpreting gas-analysis results.
Exam Focus — Absolute vs Relative Gas Concentration
A genuinely important distinction: ABSOLUTE concentration of fault gases indicates the general STATUS of transformer insulation (how degraded overall) — but RELATIVE concentration between different gases is what actually gives a clue to the TYPE of fault. For fault-type diagnosis specifically, Rogers' Analysis is the adopted method, based on evaluating 4 real gas ratios: Methane/Hydrogen, Ethane/Methane, Ethylene/Ethane, and Acetylene/Ethylene.