Rising Acetylene in a 20 MVA Substation Transformer
A 20/26.7 MVA, 138/13.8 kV conservator transformer with a nitrogen bladder supplies the only feed to a refinery process line. It has a separate-compartment OLTC, and the main-tank sample is taken from the bottom valve. Six months ago the main-tank DGA showed H2 90, CH4 120, C2H2 0, C2H4 70, C2H6 30, CO 500, and CO2 4200 ppm. The current sample shows H2 610, CH4 290, C2H2 14, C2H4 240, C2H6 55, CO 620, and CO2 4600 ppm. Moisture is 18 ppm, oil temperature is 62°C at 85% load, and the Buchholz relay has not operated. The sudden-pressure relay and differential relay are in service. No abnormal noise, vibration, or oil leak is reported. The next planned outage is in 10 weeks, and no spare transformer is available. Operations wants to keep the unit in service.
1. Interpret the DGA results, including the significance of the rate of change and the acetylene.
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The key change is acetylene: 0 to 14 ppm in six months, with H2 rising from 90 to 610 ppm, CH4 from 120 to 290, and C2H4 from 70 to 240. Acetylene is not a normal aging gas; it indicates arcing or pyrolysis above roughly 1000°C. The absolute values are in IEEE C57.104 Condition 2 territory, but the rate of change is more important than the condition number: gas generation is active. CO rose only 500 to 620 ppm and CO2 4200 to 4600 ppm, so there is no strong evidence yet of paper involvement. The separate OLTC compartment rules out the tap changer as the source for this sample.
No Buchholz operation means there is no large active gas pocket, but that does not rule out low-energy sparking or a hot spot. Possible causes include a poor bolted connection, core ground fault, static discharge, or an incipient winding fault. This is not a continue-to-normal-outage pattern; it needs investigation now.
2. Recommend actions and decision criteria for the next 24–72 hours.
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Within 24 hours: verify sample chain of custody and resample with a clean syringe to rule out contamination; run DGA, moisture, and furans; check core ground current and inspect the core ground bushings; perform infrared thermography of the tank, bushings, and connections; check oil and winding temperatures against load; confirm the Buchholz and sudden-pressure relays are functional and that alarms are announced. Increase DGA frequency to every 2–3 days.
Decision criteria: if C2H2 continues to rise, or H2 rises sharply, or a Buchholz gas accumulation appears, de-energize. If C2H2 is stable at 14 ppm on two or three consecutive samples, continued operation at reduced load with a documented contingency may be acceptable. The technician should state the risk and let the asset owner decide, but should not recommend waiting 10 weeks without enhanced monitoring.
3. If operations insists on running the transformer for the full 10 weeks, what conditions and monitoring should be required?
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Require weekly (preferably twice-weekly) DGA and a continuous DGA monitor if available; load limited to 80% or lower with cooling banks verified; all protective relays in service, including Buchholz alarm/trip, sudden pressure, differential, and overcurrent; no bypassing; fire protection and oil containment checked; a mobile transformer or spare capacity identified; operating procedures for a sudden gas alarm; and written risk acceptance.
Recognize that electrical protection may not detect a slowly developing turn-to-turn fault before it becomes a violent failure, so monitoring is the primary defense. Do not disable the sudden-pressure relay to avoid nuisance trips. If any new acetylene or a step change appears, trip the unit.