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.
