DGA Glossary | 60+ Dissolved Gas Analysis Terms

A–Z Glossary of DGA Terms

60+ essential terms organized alphabetically with cross-references to detailed guides.

Acetylene (C₂H₂): The most diagnostic fault gas. Formed above 700°C, definitive marker of arcing faults (D1/D2). Even 2 ppm is significant. See: Fault Types
Arcing Fault: High-energy discharge (>1,000°C at arc channel). Generates C₂H₂ as primary diagnostic gas. D1 (low-energy) or D2 (high-energy).
ASTM D3612: Standard test method for DGA by gas chromatography. Vacuum extraction (Method A) and headspace (Method C).
Buchholz Relay: Mechanical fault detector in the conservator pipe. Detects gas accumulation and oil surge — complement to, not replacement for, DGA.

Carbon Monoxide (CO): Thermal decomposition of cellulose (paper) insulation. CO₂/CO ratio <3 suggests significant paper pyrolysis. See: DGA Fundamentals
CIGRE: International Council on Large Electric Systems. Publishes TB 771 (DGA Interpretation) and TB 783 (DGA Monitoring Systems).
Condition-Based Maintenance: Maintenance triggered by measured condition rather than fixed intervals. Online DGA is the foundational CBM technology.
Dissolved Gas: Gas molecules physically dissolved in transformer oil per Henry’s Law. DGA measures dissolved gases extracted from oil sample.
Doernenburg Ratio: Early DGA method using 4 gas ratios. Requires at least one gas to exceed 2× LOD. Largely superseded by Duval/IEC.
Duval Triangle: Graphical method plotting relative % of CH₄, C₂H₄, C₂H₂. Seven fault zones: PD, T1–T3, D1, D2, DT. Always produces a classification. See: Duval Guide

Ethane (C₂H₆): Low-temperature thermal decomposition (150–300°C). Used in C₂H₄/C₂H₆ ratio for IEC/Rogers diagnostics.
Ethylene (C₂H₄): Moderate-temperature thermal decomposition (300–500°C). Dominant hydrocarbon in T2/T3 faults.
FTIR: Fourier Transform Infrared — optical DGA using broadband IR. 3–5 gases. Lower sensitivity than PAS/GC for acetylene.
Gas Chromatography (GC): Reference laboratory method. Gold standard recognized in all major standards. Requires skilled operator, consumables. See: PAS vs GC
Headspace: Gas volume above oil sample in sealed container. Headspace gas concentration relates to dissolved gas via Ostwald coefficient.
Henry’s Law: C = k × P — dissolved gas concentration proportional to partial pressure. Ostwald coefficient is the equivalent solubility coefficient for transformer oil.
Hydrogen (H₂): Universal early-warning gas — produced by all fault types. IEEE C57.104 L1 limit: 100 ppm. See: H₂ Monitoring

IEC 60567: Sampling and analysis of gases in oil. 2024 edition. Specifies extraction methods and quality control.
IEC 60599: DGA interpretation standard. 2022 edition: Basic Gas Ratios, concentration norms, Duval Triangle (Annex B). See: Standards Reference
IEEE C57.104: IEEE DGA interpretation guide. 2019 edition: percentage-of-L1-limit assessment using 1.1M-record database. See: IEC vs IEEE
Incipient Fault: Early-stage fault generating detectable gases but not yet causing dielectric failure. The primary value of online DGA.
Key Gas Method: Simplest DGA interpretation: dominant gas → fault type. Quick but less accurate than ratios/triangles.
Laser Photoacoustic Spectroscopy (LPAS): 4th-generation DGA: tunable laser → gas absorption → acoustic wave detection. Sub-ppm, zero consumables, solid-state. See: LPAS Guide
L1/L2 Limits: IEEE C57.104-2019: L1 = 90th-percentile healthy population. Levels 1–4 assessed as % of L1.

Methane (CH₄): Low-temperature thermal decomposition (150–300°C). Often first hydrocarbon to appear.
MODBUS RTU/TCP: Universal industrial protocol. All PAS monitors support MODBUS RTU (RS-485) and TCP (Ethernet).
NDIR: Non-Dispersive Infrared — optical DGA, 3–5 gases. Lower cost than GC/PAS, higher detection limits.
Offline DGA: Laboratory DGA on manually collected oil sample. 1 data point/year. Highest accuracy (GC reference). See: Online vs Offline
Online DGA Monitor: Permanently installed instrument providing 365–8,760 data points/year. Technologies: PAS, NDIR, FTIR, GC.
Ostwald Coefficient: Dimensionless solubility coefficient relating oil-phase to gas-phase concentration. Temperature and gas-type dependent.
Palladium Thin-Film Sensor: Solid-state H₂ sensor: Pd alloy adsorbs H₂ → resistance change. Sub-ppm, 10+ yr life, zero consumables. See: Pd Thin-Film
Partial Discharge (PD): Localized dielectric breakdown in gas-filled void. Generates primarily H₂. Progressive — each event erodes insulation.
ppm: Parts per million. 1 ppm = 1 µL gas per liter of oil. Standard DGA unit.

Rogers Ratio: DGA method using same 3 ratios as IEC but different coding boundaries. Developed by R.R. Rogers (1978). See: Diagnostic Methods
Stray Gassing: Fault gas generation at normal temperatures without actual fault. Oil oxidation phenomenon. Known false-positive source. IEC 60599:2022 addresses.
T1/T2/T3 Thermal Faults: T1: <300°C, T2: 300–700°C, T3: >700°C. Higher T → CH₄ shifts to C₂H₄ → traces of C₂H₂ at T3.
TDCG: Total Dissolved Combustible Gas = H₂+CO+CH₄+C₂H₂+C₂H₄+C₂H₆. Key severity indicator. Rate >30 ppm/day = Level 4 (urgent).
TDLAS: Tunable Diode Laser Absorption Spectroscopy. Single-gas, extremely selective. Niche: high-precision C₂H₂ monitoring.
Transformer Oil: Mineral insulating oil (IEC 60296). Types: uninhibited (I), trace-inhibited (II), inhibited. Alternatives: natural/synthetic ester (IEC 62770). Different DGA interpretation for esters.