Hydrolysis of Kraft paper is indicative of the breakdown of the kraft paper system. It was found that five different acids may be present in aging transformers. These include Formic acid, Acetic Acid, Levulinic Acid, Stearic Acid, and Naphthenic acid. Low molecular weight acids are absorbed by the kraft paper, the high molecular acids remain in the oil. The three low molecular weight acids accelerate the aging of the kraft paper, there is also a clear synergy between these acids and water content. This proves the fact that the low molecular acids act as catalysts in the breakdown of the kraft paper. 
Overview 
 
We want to analyze the cost-benefit of maintenance on transformers as well as the risks involved in the aging cellulose insulation system. 
 
The aging of cellulose is due to three different processes
 
1. Pyrolysis
 
2. Hydrolysis
 
3. Oxidation
 
The hydrolysis reaction is driven by the H+ ion that acts as a catalytic, the H+ ion comes from the broken down carboxylic acid. The aging rate for this process is directly proportional to the concentration of the H+ ions. 
 
Acid content in transformer oils are measured by IEC 62021 method, this does not reflect the H+ activity. It only reflects the total sum of organic acids, phenolic compounds, oxidation products, resins, organometalic salts and additives [1]. Neither does it distinguish between the different types of acids. 
 
We have differentiated between five acids already, relevant and typical for aged transformer insulation [2] Formic, Acetic and Levulinic Acids are formed by aging cellulose and then Naphtenic and Stearic acid will be formed alongside low molecular weight acids in the oxidation of insulating oils. 
 
It was found the Formic, Acetic and Levulinic Acids would be absorbed by paper. 

Acid Solubility 

Kraft paper will absorb – Formic, Acetic and Levulinic acid (hydrophilic acids) thus we won’t pick up the exact concentration from oil analysis for these types of acids as the analysis determine the total acid number. Heavy weight acids are mostly concentrated in the oil and not absorbed by the cellulose paper as these components  are hydrophobic and less polar. We see a quick degradation in the DP value, this is possibly because of the degradation of the the amorphous regions of the cellulose fibre and then afterwards a slower degradation because the crystalline regions does not readily allow the penetration of water and acids. 
 
It should be noted that water extracted from transformers using low-frequency heating contain a large portion of acetic and formic acid. Most of the low molecular weight acids found are paper degradation products. 
 

Conclusions

* Low molecular weight acids – that may be extracted from paper and oil by water – is proven to have a significant influence on Kraft paper aging. 
REFERENCES:
 
1. IEC Oil maintenance guide IEC 60422
 
2. S Ingebrigtsen, M Dahlund, W Hansen, D Linhjell, L E Lundgaard, ” Solubility of carboxylic acids in paper (Kraft) oil insulation systems” IEEE CEIDP 2004, Boulder, Colorado, USA pp253-257
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