Naphthenic versus Paraffinic

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Naphthenic versus Paraffinic
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• 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.

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In naphthenic oils 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.

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• 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.

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• 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.

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• 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.

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• 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.

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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.

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• 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.