Naphthenic versus paraffinic oils – which is better? The answer is not so straightforward because the application will drive the choice of oil.
In the case of transformers there is a lot more historical data and knowledge on naphthenics than on paraffinics, and naphthenic oil has traditionally been considered the gold standard. However paraffinic oil is gaining traction on account of it being virtually sulfur-free. But while naphthenic base oils cannot be made totally devoid of sulphur, they do offer two important and sought after properties: high solvency and excellent low temperature characteristics.
In naphthenic oil’s corner
The aniline point of paraffinic oil is much higher than a naphthenic meaning the solvency is worse. This translates into paraffinic oil forming sludge and sediment in the transformer over time as the oil oxidises rather than keeping the oxidation products in solution. This can result in sludge on insulating paper and shorter life for the transformer.
Paraffinic oil cannot cool transformers as well because the viscosity does not drop as fast as naphthenic when heating up. Paraffinic oil has a high viscosity index which is good for engine oil but bad for transformers. The naphthenic oil will reach a lower viscosity meaning more oil circulation in the transformer and better cooling. Tests have shown several degrees difference, which is enough to significantly impact the life of the paper and eventually the transformer. There is a well-known equation showing that a rise of about 6 degrees in the operating temperature of a transformer halves the life of the paper. Additionally naphthenic oils have a better heat transfer coefficient which is another reason why they cool the transformer better.
Naphthenic oils have superior dielectric properties to paraffinic, i.e. offer better insulation, and of interest to electrical engineers this mean lower propagation of streamers during partial discharge and less electrostatic charging tendency.
Rubber seal compatibly can be an issue too for paraffinic oils due to chemical interaction with seals. Some reports of leakages due to incompatible pairings have emerged, whereas naphthenic oils are used themselves in many rubber composition such as Styrene Butadiene Rubber for instance and the compatibility is excellent.
The case for iso-paraffinic oil
The issue of corrosive sulphur laden transformer oil has for some years now been a been a contentious issue. Documented cases of catastrophic failures have been traced to the presence of corrosive sulfur species including mercaptans or thiols, cyclic and aliphatic sulphides, thiophens and benzothiophens, disulphides and polysulfides. But not all the sulfur species present in mineral oil are corrosive in the same way, and some are actually good.
Naphthenic base oils cannot be made totally free from sulfur (but maybe as low as 0.005% sulfur). While metal passivators may be included as additives, the risk is only marginally reduced. Iso-paraffinic base oils are made with a combined process of severe hydrocracking and hydroisomerisation. These techniques remove sulfur completely and other undesirable compounds resulting in a very stable and ultra-pure base oil that does not require metal passivators.