Transformer Dissolved Gas Analysis

[KJ Cheng]

I am working with a hydroelectric power plant with three generating units. The Generator Step-up Transformers for each unit are arranged in three single-phase with nine (9) phases in total.

Dissolved Gas Analysis was performed on each phase of the transformers on an annual basis (ASTM D3612). I noticed that the levels of Ethane (C2H6) is in an increasing trend as indicated below:

Gas 2013 2014 2015 2016 2017 2018 2019
Ethane (ppm) 69 127 151 162 133 177 235
Ethane (ppm) 53 111 168 175 181 191 234
Ethane (ppm) 61 61 143 155 157 170 225
Ethane (ppm) 65 85 134 146 149 159 206
Ethane (ppm) 57 98 114 119 129 139 179
Ethane (ppm) 51 67 113 121 125 141 192
Ethane (ppm) 60 58 147 68 167 187 237
Ethane (ppm) 76 99 154 165 144 180 227
Ethane (ppm) 82 120 152 166 172 191 211

Can anyone comments and share your experiences on what could be the problem associated with this increasing trend of Ethane?

On another note, is the analysis for the presence of corrosive sulfur necessary to check the health of the transformers?

Thank you very much.

KJ Cheng

 

[Palletjack]

Are these transformers overloaded and hot? What’s your CO to look like

 

[ScottyUK]

Ethane is usually a sign of a fairly high temperature fault within the transformer. A competent oil lab should also be providing advice on the transformer's condition based on the gas signature - speak to your lab and ask for some help interpreting the data.

Of far greater interest than individual ppm values is the ratio between several key gases, the 'Rogers Ratios' developed by a Mr. Rogers who worked for the CEGB many years ago. A Canadian gentleman named Duval also did some substantial work on interpreting DGA results, although it's not something we in the UK have widely adopted. Have a read about these.

 

[argotier]

For a proper analysis you will need to consider all relevant gases and not only Ethane.

I recommend you to check the following standards that will help you in interpreting the DGA results and identifying the problem (if any) or ruled it out:

- ANSI C57.104-2019: IEEE Guide for the Interpretation of Gases Generated in Mineral Oil-Immersed Transformers.

- IEC 60599-2015: Mineral oil-filled electrical equipment in service - Guidance on the interpretation of dissolved and free gases analysis.

But be aware that even in the standards there is no 100% mandatory specific values or trends to comply with, all the DGA analysis it's done based on statistical data and acceptable value ranges for most types of transformers are likely given.

Hope it helps.

 

[prc]

If only Ethane is increasing, but all other gases are steady, it can be a case of stray gassing ( see clause 4.3 of above IEC) due to type of oil used. It is not then a case to worry.

 

[KJ Cheng]

Thank you ALL for the responses and comments.

I have referred to the latest test reports of all transformers after the one mentioned above and the results showed that level of Ethane in each oil sample has reduced.

I have also referred to the publication: "Transformer Diagnostics by the United States Department of the Interior Bureau of Reclamation" and two tables namely Table 1: Dissolved Key Gas Concentration Limits in PPM and Table 2: Actions Based on Dissolved Combustible Gas. Each table gives 4 conditions based on Individual Combustible Gas Level (Table 1) and Total Dissolved Combustible Gas (Table 2). There was a CAUTION NOTE in the publication, which stated that:

Transformers generate some combustible gases from normal operation, and condition numbers for dissolved gases given in IEEE C-57-104-1991™ [4] (table 1 above) are extremely conservative. Transformers can operate safely with individual gases in Condition 4 with no problems, provided they are stable and gases are not increasing or are increasing very slowly. If TDCG and individual gases are increasing significantly (more than 30 ppm per day [ppm/day]), an active fault is in progress. The transformer should be de-energized when Condition 4 levels are reached.

In these cases above, the Total Dissolved Combustible Gas (TDCG) Levels of all samples fall under Condition 1 of Table 2 but the individual gas level of Ethane of 200 ppm falls under Condition 4 of Table 1.

Considering that the TDCG level is low coupled with the condition that the rate of increase of Ethane is very slow which is less than 10 ppm per day, the transformers can operate safely referring to the publication. Close monitoring will continue.

Once again, thanks everyone for the comments.

 

[prc]

The documents that you are referring are quite old and bit outdated. Latest standards are C57.104-2019 & IEC 60599-2015 on DGA interpretation.

 

[dik]

'CO' ???

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Palletjack]

CO2/CO ratio.
With those numbers I was curious if the transformers were ran hot and the cellulose insulation may be breaking down.

The numbers the OP gave, when taken alone may not mean much until some other numbers are taken into consideration.