Silicone Oil-Filled Transformers

Silicone oils became more common when polychlorinated biphenols were discontinued. They are mainly used in transformers inside buildings and are smaller than generator step-up transformers. Silicone oils have a higher fire point than mineral oils and, therefore, are used where fire concerns are more critical. As of this writing, there are no definitive published standards. IEEE has a guide, and Doble has some service limits, but there are no standards. Information below is taken from the IEEE publication, from Doble, from articles, from IEC 60599 concepts, and from Delta X Research’s/Transformer Oil Analyst rules. Silicone oil dissolved gas analysis is in the beginning stage, and the suggested methods and limits below are subject to change as we gain more experience. However, in the absence of any other methods and limits, use the ones below as a beginning.
Silicone oils used in transformers are polydimethylsiloxane fluids, which are different than mineral oils. Many of the gases generated by thermal and electrical faults are the same. The gases are generated in different proportions than with transformer mineral oils. Also, some fault gases have different solubilities in silicone oils than in mineral oils. Therefore, the same faults would produce different concentrations and different generation rates in silicone oils than mineral oils.
As with mineral oil-filled transformers, three principal causes of gas generation are aging, thermal faults, and/or electrical faults resulting in deterioration of solid insulation and deterioration of silicone fluid. Overheating of silicone oils causes degradation of fluid and generation of gases. Generated gases depend on the amount of dissolved oxygen in the fluid, temperature, and how close bare copper conductors are to the heating. When a transformer is new, silicone oil will typically contain a lot of oxygen. Silicone transformers are typically sealed and pressurized with nitrogen. New silicone oil is not de-gassed; and, as a rule, oxygen concentration will be equivalent to oxygen solubility (maximum) in silicone. The silicone has been exposed to atmosphere for some time during manufacture of the transformer and manufacturer and storage of silicone oil itself. Therefore, carbon monoxide and carbon dioxide are easily formed and dissolved in the silicone due to the abundance of oxygen in the oil, resulting from this atmospheric exposure. In normal new silicone transformers (no faults), both carbon monoxide and carbon dioxide will be generated in the initial years of operation. As the transformer ages and oxygen is depleted, generation of these gases slows, and concentrations level off. See Picture 1 for the relationship of decreasing oxygen and increasing carbon monoxide and carbon dioxide as a transformer ages. This curve is for general information only and should not be taken to represent any particular transformer. A real transformer with changes in loading, ambient temperatures, and various duty cycles would make these curves look totally different.

Picture 1: Relationship of Oxygen to Carbon Dioxide and Carbon Monoxide as Transformer Ages


After the transformer is older (assuming no faults have occurred), oxygen concentration will reach equilibrium (Picture 1). Reaching equilibrium may take a few years, depending on the size of the transformer, loading, ambient temperatures, etc. After this time, oxygen, carbon monoxide, and carbon dioxide level off, and the rate of production of these gases from normal aging should be relatively constant. If generation rates of these gases change greatly (seen from the DGA), a fault has occurred, either thermal or electrical. Rate of generation of these gases and amounts can be used to roughly determine what the fault is. Once you notice a significant increase in rate of generation of any gas, it is a good idea to subtract the amount of gas that was already in the transformer before this increase. This ensures that gases used in the diagnosis are only gases that were generated after the fault began.

Comparison of Silicone Oil and Mineral Oil Transformers

Some general conclusions can be drawn by comparing silicone oil and mineral oil transformers:

1. All silicone oil-filled transformers will have a great deal more CO than normal mineral oil-filled transformers. CO can come from two sources—the oil itself and from degradation of paper insulation. If the DGA shows little other fault in gas generation besides CO, the only way to tell for certain if CO is coming from paper degradation (a fault) is to run a furan analysis with the DGA. If other fault gases are also being generated in significant amounts, in addition to CO, there obviously is a fault, and CO is coming from paper degradation.
2. There will generally be more hydrogen present than in a mineral oil-filled transformer.
3. Due to “fault masking,” mentioned above, it is almost impossible to diagnose what is going on inside a siliconefilled transformer based solely on DGA. One exception is that if acetylene is being generated, there is an active arc. Look also at gas generation rates and operating history. Look at loading history, through faults, and other incidents. It is imperative that detailed records of silicone oil-filled transformers be carefully kept up to date. These are invaluable when a problem is encountered.
4. If acetylene is being generated, there is a definitely an active electrical arc. The transformer should be removed from service.
5. In general, oxygen in a silicone-filled transformer comes from atmospheric leaks or was present in the transformer oil when it was new. This oxygen is consumed as CO and CO2 are formed from the normal heating from operation of the transformer.
6. Once the transformer has matured and the oxygen has leveled off and remained relatively constant for two or more DGA samples, if you see a sudden increase in oxygen, and perhaps carbon dioxide and nitrogen, the transformer has developed a leak.