# Quantifying the Effect of Catalysts on the Lifetime of Transformer Oil
## 5 Discussion  
The causes of changes in the monitored parameters described in the previous chapter are various chemical reactions accompanying the process of thermal-oxidative aging of mineral oils in power transformers. This process is primarily accelerated by the presence of an oxidant (dissolved oxygen) and is highly heterogeneous. In particular, mineral oils are mixtures of linear saturated hydrocarbons (paraffins), cyclic saturated hydrocarbons (naphthenes), aromatic hydrocarbons, and small amounts of nonhydrocarbons (sulfur, nitrogen, and oxygen combined with carbon and hydrogen) [39; 40], whereas the selected oil may contain thousands of different types of paraffinic, naphthenic, and aromatic hydrocarbon molecules [40]. According to the amount ratios of basic types of hydrocarbons, and subsequently, according to the presence of specific chemical substances, the reactions that occur during thermo-oxidation of oil differ [39]. Chemical reactions may also vary significantly depending on the type of mineral oil (e.g., based on the production process and intended for the defined application) and on the environment of actual usage. Many factors influence the continuity and interlacing of ongoing chemical reactions. Elevated temperature, high voltage [39], and the presence of impurities, including dissolved gases (not only oxygen), water, polar and ionic contaminants (traces of sulphate, sulphonates, furfural, etc. ), and particulate solid contaminants (metals, metal oxides, fibers of cellulose, etc.) [40] contribute to thermo-oxidation in oil-filled transformers. The basic description of the thermo-oxidation mechanism as a standard radical mechanism generally applied to hydrocarbon compounds (other types of oils, polymers, etc.) that is described in [8; 41; 42; 43] is sufficient. Radical reactions occur gradually in the phases of initiation, propagation, chain branching, and termination of the standard thermo-oxidation mechanism. Several different types of radicals and other intermediate products are formed during this process. Alkyl and peroxy radicals are formed in the first stages. After that, hydroperoxides are formed and decomposed into alkoxy and hydroxy radicals during consequential radical reactions. The thermo-oxidative aging products of mineral oils are aldehydes, ketones, alcohols, water, carboxylic acids, peroxides, etc. [39; 40]. The increasing concentrations primarily of aldehydes and ketones cause condensation reactions, which lead to the formation of low-molecular-weight oligomers and polymers. The result of the presence of these compounds is an increase in the oil viscosity [43], as well as a gradual change in the oil color from bright yellow to amber [10]. Describing the real thermo-oxidation of oil in a power transformer also requires consideration of the presence of Kraft paper (winding insulation), whose main component is cellulose. The main products of cellulose degradation in oil are furfural formed from formaldehyde, carbon oxides, and acetone; these degradation products may significantly influence the thermo-oxidation of the oil [44]. As mentioned, phenol-based antioxidants, often referred to as radical scavengers, are used to attenuate the thermo-oxidation of mineral oils. Phenols directly enter the radical reactions of the thermo-oxidation mechanism of the oil, and the reaction of free radicals with radical scavengers leads to the formation of stable chemical compounds up to a certain temperature [42]. The result of radical scavenger activity is a reduction in thermo-oxidation speed until the added amount of the radical scavenger in the oil is consumed. As is evident from the achieved results, all the chemical processes discussed above significantly influenced the observable parameters measured by UV-Vis spectroscopy and Fourier transform infrared spectroscopy within a long-term experiment. Despite their different principles, both applied methods were able to detect and quantify the effect of catalysts on the lifetime of the tested mineral oil.