Which tap changer gases are normal?

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.

1. 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.
2. 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.
3. Optionally add up to 12 earlier samples, your own concentration limits and your own ratio thresholds, and the gases of the main tank.
4. 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.

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.

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.

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