OLTC DGA Check: interpret tap changer gases with IEEE C57.139-2015 and Duval Triangle 2
The Seetalabs® OLTC DGA Check is a free tool for engineers who need to read the dissolved gas analysis of an on-load tap changer compartment in mineral oil. It returns Normal, Caution or Warning following IEEE C57.139-2015, with the Duval Triangle 2 zone, the gas ratios, the trend over earlier samples and the clause behind each result.
Why tap changer gases need their own rules
In an arcing tap changer, the arc in oil produces combustible gas at every operation, above all C2H2 and H2. Applied to that oil, the main-tank limits of IEEE C57.104-2019 or the typical values of IEC 60599:2022 would flag a fault in almost every sample. IEEE C57.139-2015 gives a separate method: gas ratios, Duval Triangle 2 and limits derived from the population of similar tap changers.
How it works
- Describe the tap changer: arcing in oil or vacuum; resistor or reactor type if known; for vacuum types, whether the selector arcs; free-breathing or sealed.
- Enter the latest sample in ppm: CH4, C2H4 and C2H2 are required, the other gases are optional. Add the number of operations and whether the oil was changed.
- Optionally add up to 12 earlier samples, your own concentration limits and your own ratio thresholds, and the gases of the main tank.
- Read the result: Normal, Caution or Warning with the reasons and clauses, the recommended action, Duval Triangle 2 with the path of earlier samples, the ratio table, data checks, the trend and the main-tank check.
A value such as "<5" is not accepted: enter a number, or leave the field empty if the gas was not measured. An empty field is never read as zero.
Method and standards
The tool follows IEEE C57.139-2015, with CIGRE TB 443 (2010), CIGRE TB 771 (2019) and IEC 60599:2022 where C57.139 is silent.
- TDHG. Always recalculated as CH4 + C2H6 + C2H4 (C57.139 clause 3). Without C2H6 the total is partial, and the page says so.
- Reliable minimum. A gas below 10 ppm, or a TDHG below 15 ppm, does not enter ratios or concentration checks (C57.139 §5.2.1). If all combustible gases are that low, the result is Normal with "gases too low for a diagnosis".
- Ratios. C57.139 §4.3 ratios are all shown. Only C2H4/C2H2 is scored by default, with the Annex B.3 thresholds of 0.5 for Caution and 1.0 for Warning, and only for arcing-in-oil types. TDHG/C2H2 and concentrations are scored only against limits you enter, since C57.139 derives them from the population of similar tap changers.
- Duval Triangle 2 (C57.139 Annex D). Shown when C2H2 or C2H4 reaches 10 ppm; it counts in the result only when CH4, C2H4 and C2H2 are all at 10 ppm or more.
- Result. The highest level among the triangle zone, the ratios and the concentrations, following C57.139 §6.3.1.
- Borderline. If the zone would change within the measurement uncertainty of C57.139 §5.2.1 (the larger of 1 ppm and 10 %), the recommended action is to resample within 90 days.
- Trend. Gas increase factors, gas per 1000 operations between samples of the same oil, and confirmation when gas per operation rises more than 50 % over a previous Normal interval (CIGRE TB 771 §4.1). After an oil change, a new baseline with a residual-gas threshold (CIGRE TB 443 §4.5).
- Main tank. C2H2/H2 above 3 as a likely sign of contamination from the tap changer (IEC 60599:2022 §5.7), and the OC zone of Duval Triangle 1.
Where Seetalabs® interprets. Neither IEEE C57.139-2015 nor IEC 60599:2022 assigns a Normal, Caution or Warning code to a Triangle 2 zone. The tool treats a zone outside N as Caution and lets it enter the highest-level rule of §6.3.1; Warning comes only from the ratios. This zone-to-code mapping is our reading of CIGRE TB 771 §4.1, not a normative table. Three other rules are also our reading: the strict 10 ppm filter on all three triangle gases (the sources speak of about 10 ppm, or about ten times the detection limit), a "significant" gas in a reactor-type vacuum tap changer, and the split at 3 of the C2H2/H2 ratio, where IEC 60599:2022 says "higher than 2 to 3". Each of these carries the label "Seetalabs® interpretation of" and the clause on the result.
Worked example
Example 1 of IEEE C57.139-2015 Annex C: an arcing tap changer reaching Warning
An arcing tap changer in oil, no concentration limits entered. From 1999 to 2005 every sample is Normal. The 2006 sample reads H2 0, CH4 7, C2H6 10, C2H4 42, C2H2 10, CO 121 and CO2 903 ppm.
- The point falls in zone X3 of Duval Triangle 2, but the zone does not count: CH4 is 7 ppm, below the 10 ppm minimum.
- C2H4/C2H2 = 42 / 10 = 4.2, above the Warning threshold of 1.0 in C57.139 Annex B.3.
- TDHG/C2H2 is shown without a verdict, because no limit was entered.
- Result: Warning (3), from the ratio. Recommended action: take a check sample now.
Two more cases from Annex C
With the gases of Example 2 and no limits, every sample from 1998 to 2006 is Normal. Entering the population limits of Annex C Table C.1 (C2H4 653, C2H2 1773 and TDHG 1213 ppm) turns the 2005 and 2006 samples into Caution: the result depends on limits that fit your tap changer population. In Example 3, the 2004 sample sits in zone X3 close to a boundary, so the tool recommends resampling within 90 days rather than a verdict on the zone.
Limits
- Mineral oil only: IEEE C57.139-2015 does not cover esters or silicone in tap changers.
- One compartment at a time. The tool does not derive population limits for you, and has no generic population limits.
- In a vacuum tap changer, very high concentrations remain Caution, as in the formal method.
- It is a screening result, not a diagnosis, and it does not assess the oil quality of the compartment (breakdown voltage, water).
- For the main tank of the same transformer, use the DGA Calculator and choose "main tank with communicating OLTC", which adds the OC zone of Duval Triangle 1. For a list of transformers from a laboratory CSV, use Fleet DGA Triage.
Your data stays in your browser
The tool runs entirely in your browser. Its Content Security Policy sets connect-src 'none', so the values cannot be sent to any server. It uses no cookies and no browser storage. You can save your inputs to a file on your own computer, load them again later, or print a dossier of the result.
The tool runs in your browser. Seetalabs® does not store or see the values you enter.
Frequently asked questions
Is acetylene in the tap changer compartment always a problem?
No. In an arcing tap changer, C2H2 comes from normal switching. What matters is how it compares with the other gases: with the default thresholds of IEEE C57.139-2015 Annex B.3, C2H4/C2H2 above 0.5 is Caution and above 1.0 is Warning.
Why did the Duval Triangle 2 zone not change my result?
Because at least one of CH4, C2H4 and C2H2 is below 10 ppm. The tool still plots the point, but a zone built on gases that low is not reliable enough to score.
Is "zone outside N = Caution" part of the standard?
No. IEEE C57.139-2015 and IEC 60599:2022 do not assign codes to Triangle 2 zones. The tool applies this rule as a Seetalabs® interpretation of CIGRE TB 771 §4.1 and labels it on the result.
Can I use it for a vacuum tap changer?
Yes. For reactor-type vacuum tap changers without limits, any reliable C2H2, C2H4 or TDHG gives Caution (CIGRE TB 443 §4.3.3). Resistor-type vacuum tap changers are scored only against limits you enter (§4.3.4).
When you follow many transformers
This check reads one tap changer compartment at a time. In the main tank, Ronin AI reads acetylene from a communicating tap changer as contamination, not as arcing (IEC 60599:2022 §5.7), on every transformer you follow. Tell us how many units have a communicating tap changer.
Tool version 1.0.2. Seetalabs®.
