Acetylene in Your DGA Results: Understanding Arcing Faults and How to Respond

Few DGA findings demand more immediate attention than acetylene in a transformer oil sample. Acetylene (C₂H₂) is generated by the thermal cracking of hydrocarbon molecules at temperatures above approximately 700°C in mineral oil, temperatures that are not reached under normal transformer operation and that are reliably associated with high-energy electrical discharge, specifically arcing [1]. A transformer that has never shown acetylene and then appears with even a few ppm represents a qualitative change in condition that requires structured investigation.

However, "immediate attention" and "emergency shutdown" are not synonymous. The appropriate response to acetylene in a DGA result depends on understanding what generated it, what the trend is, and whether the current condition is stable or actively deteriorating. This article provides the framework for making that determination.

The Physical Basis for Acetylene's Diagnostic Specificity

Acetylene's diagnostic importance derives from the thermodynamics of its formation. In mineral oil, acetylene is generated when C–C and C–H bonds are dissociated at temperatures above approximately 500–700°C, with significant generation rates above 700°C [1]. This temperature range is far above that of normal transformer operation (oil temperature rarely exceeds 100°C under normal load; winding hotspot temperature may reach 110–130°C at high load).

The processes that can produce temperatures above 700°C inside a transformer are limited:

• Sustained electrical arcing between winding conductors, between winding and core, or between any conducting surfaces with a sufficient voltage difference to sustain an arc through the oil
• Very high energy transient discharge associated with a developing insulation failure in the winding
• Tap changer arcing in load tap changers that share oil with the main tank or whose compartment seal has been compromised

The first two are the fault modes of concern. The third is a maintenance concern but does not indicate a winding fault.

IEEE C57.104-2019 [2] and IEC 60599:2022 [3] both identify acetylene as the gas most strongly associated with electrical arcing, with even trace amounts (above 1–2 ppm where the previous sample showed zero) warranting immediate follow-up. Both standards recommend a confirmatory sample within days to weeks, not the next scheduled interval.

Interpreting Acetylene in Context

The appropriate response to acetylene depends on several contextual factors that must be assessed before action is decided.

Concentration and Appearance History

New appearance from zero. A transformer with no previous acetylene history that shows 5 ppm represents a qualitatively different and more concerning situation than one that has been at 5 ppm stably for two years. New acetylene indicates a new high-energy discharge event. A follow-up sample within one to two weeks is warranted regardless of absolute concentration.

Existing trace level, stable. Some transformers carry trace acetylene levels (1–5 ppm) that are stable over years. This pattern is typically attributable to a historical event, such as a through-fault that has been absorbed without further consequence, or residual OLTC contamination. Stability, not the absolute level, is the key indicator. Stable trace acetylene warrants monitoring but not necessarily urgent intervention.

Rising trend. Acetylene that is increasing on consecutive samples, particularly combined with rising hydrogen and ethylene, indicates active, ongoing arcing activity. This pattern requires urgent investigation. The combination of C₂H₂ + H₂ + C₂H₄ all rising in the same direction places the Duval Triangle interpretation firmly in the D2 (high-energy discharge) zone and indicates sustained arcing, not a past transient event [1].

The Accompanying Gas Profile

Acetylene does not occur in isolation in a transformer with active arcing; it appears alongside other gases generated by the same high-energy process.

Active arcing profile. High-energy arcing generates acetylene alongside substantial hydrogen (H₂) and ethylene (C₂H₄), with the ethylene/methane ratio indicating the sustained thermal component of the arc. This combination plots firmly in the D2 zone of the Duval Triangle [1]. In the Duval Pentagon, the high H₂ contribution further confirms electrical discharge.

D1/D2 boundary. Lower acetylene concentrations with primarily hydrogen and methane, and modest ethylene, may plot on the D1/D2 boundary, indicating lower-energy electrical discharge activity that may or may not have progressed to sustained arcing. This pattern warrants investigation but may allow a slightly longer follow-up window than confirmed D2.

Mixed thermal/electrical (DT zone). The Duval Triangle includes a DT zone at the boundary between thermal fault classifications and the discharge zones. This pattern indicates faults that involve both thermal and electrical components, potentially a hot connection that has progressed to initiating discharge activity. The maintenance response depends on which component is dominant and which is increasing faster.

OLTC Contamination

On-load tap changers (OLTCs) operate by arcing: each tap change involves a brief arc in the tap changer compartment, and this arc deposits acetylene into the OLTC oil. In transformers where the OLTC oil is in circuit with the main tank oil (as in some designs) or where the OLTC compartment seal has deteriorated, acetylene from normal tap changer arcing can appear in main tank DGA results.

OLTC-sourced acetylene typically appears alongside main tank DGA results that are otherwise clean, with no concurrent rise in ethylene or hydrogen from winding fault activity. The distinction matters: a clean winding DGA profile with trace acetylene that correlates with operational tap change frequency is a different situation from winding DGA gases rising alongside acetylene.

Separate DGA sampling of the OLTC compartment (where the design permits it) should be part of any investigation of main tank acetylene.

Post-Through-Fault Events

A through-fault, defined as a fault on the external network that drives high short-circuit current through the transformer without the transformer itself failing, can produce transient arcing in the winding during the high-current event. A DGA sample taken shortly after a confirmed through-fault event may show elevated acetylene that is a record of that event rather than of ongoing internal fault activity. If subsequent samples show declining or stable acetylene and no rising companion gases, the event may be historical.

IEEE C57.152-2013 [4] recommends post-fault electrical testing (frequency response analysis, winding resistance, power factor) to assess whether the transformer sustained mechanical or dielectric damage from the through-fault event. DGA and electrical testing together provide a more complete picture than either alone.

The Systematic Response Protocol

A structured response to acetylene in a DGA result:

Step 1: Verify the result. Before any action, confirm that the sample was collected and handled correctly, with no air contamination, correct sampling point, and correct laboratory. An anomalous result from a sampling or laboratory error should not drive a major maintenance response. Collect a follow-up sample immediately, from a correct procedure, to verify.

Step 2: Review the historical DGA record. Was acetylene present in previous samples? What is the trend over the past three to five samples? Is it new, stable, or rising? Are companion gases (H₂, C₂H₄) also rising? The trend context is more important than any single result.

Step 3: Check for event history. Has the transformer experienced a through-fault event since the last sample? Has the OLTC been serviced or experienced a malfunction? Are there any operational events that might explain a one-time acetylene appearance?

Step 4: Apply Duval Triangle and R-DGA analysis. Plot the result and the historical sequence on the Duval Triangle. Does it plot in the D1, D2, or DT zone? Is the Hazard Factor (HF) in TOA elevated and rising? [5] Both the fault type classification and the severity metric are needed for a complete assessment.

Step 5: Calibrate the response to the findings. Stable trace acetylene with no rising companion gases and a consistent OLTC explanation: monitor at accelerated intervals. New acetylene with no event explanation or rising companion gases: immediate follow-up sample within days, consider operational review. D2 pattern with rising HF: treat as urgent fault event; prepare for possible derating, targeted inspection, or outage.

Step 6: Consider online monitoring for continued visibility. For high-consequence transformers with confirmed or suspected active acetylene generation, continuous online monitoring through Monitor Watch provides the real-time gas trend tracking between sampling intervals. An arcing fault that is accelerating can progress to failure in days; quarterly sampling cannot provide adequate warning. Contact us to discuss Monitor Watch deployment for critical assets.

For technical background on DGA fault classification and R-DGA severity assessment, visit the Science page. For product information, visit the TOA page or Monitor Watch page.