Comparing mineral and ester oil aging behavior

Electrical, thermal, mechanical, and chemical stress leads to the aging of insulating liquids. 
 
Breakdown of the hydrocarbon chain of transformer liquids generates soluble gases and a colloidal suspension. This suspension aids the further oxidation of the oil. Oxygen contributes to the formation of soluble oxidation products as well as insoluble sludge which are detrimental to the solid insulation. Both of these are unnoticed, retained by cellulose insulation stopping heat dissipation. This will lead to overheating and a bunch of problems comes from overheating. 
 
The sludge forms a physical barrier restricting the flow of heat from the fluid to the cooling fins or cooling unit, as well as from the coils to the cooler part of the transformed, thus dissipating the heat and decreasing the overall operating temperature of the unit. We will have a closer look at the decay products in mineral oil, natural and synthetic esters.
 

Overview

There are three sources of energy that are capable of splitting the covalent bond in oil molecule chains.
 
* Electromagnetic field in the transformer
*Thermal energy generated by active parts
* Dissolved oxygen
 

Electrical stress

Short voltage surges facilitate the formation of free electrons on the conduction band of metal conductors in the transformer. These free electrons are accelerated by the electrical charge in the unit, the collide with molecules in their path, a pair of free radicals can be formed from this reaction. 
 

Thermal stress

The magnetic core and windings produce heat and this contributes to the decay of the insulating liquid. High operating temperatures accelerates oxidation processes, increasing the movement of free radicals formed by the gassing of oil, this promotes random secondary chemical reactions that precede the formation of decay products. 
 


Chemical stress

 
When the concentration of dissolved oxygen is high, like in free-breathing transformers, the energy of the oxygen molecule can be transferred to other molecules. Oxidation accelerated by temperature, moisture, or chemicals, resulting in the formation of peroxides which will break down and form free radicals as part of the breakdown products.
 
 

Decay products

All “broken” molecules and Hydrogen atoms are labeled as free radicals. As the population of free radicals increases, some gaseous and liquid fractions may capture a free electron and form an ion
 
An accumulation of ionized molecules increases the dissipation factor of oil-paper insulation. Large free radicals may combine to form large colloidal compounds that form sludge. Also called X-Wax – this is an insoluble compound. With an increased free radical concentration the formation of soluble and insoluble decay products are are the outcome of random chemical reactions. 
 
The final stages of deterioration is marked by sludge and acid in sufficient quantities to impair the heat transfer as well as poor dielectric properties. The oil has changed color at this stage, from a bright yellow for new oil to an amber color that marks the endpoint for the oil. The colloidal suspension and acids are absorbed by the large paper surface, the cellulose fibres and metal surfaces are attacked, the leads to the formation of metallic soaps, lacquers, aldehydes, alcohols and ketones. Heavily loaded units are more prone to sludge formation, this cause shrinkage of the insulation through the leaching out of varnishes and cellulose materials. It should be noted that the decay process start as soon as the unit is switched on for the first time, at first there is little degradation. It is only after a considerable period of time that we notice a real change in the system and then it is too late, acid and sludge, and the impact on the transformer cellulose paper system is irreversible. 
 

Conclusion

The decay products are the result of progressive damage to the insulation system, they are formed from secondary chemical reactions between decomposed molecules under the impact of electrical, chemical, and thermal stresses. 
 
Synthetic esters were found to age slower than natural ester or mineral oil. 
 
It is important to use different testing methods to determine the concentration of decay products in the oil as Total Acid Number and Interfacial tension does not give an accurate indication of the state of the oil. ASTM D6802 and D6181 methods were found to be much more effective for monitoring of decay products, even from an early stage. 
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