Occasionally, we get questions from customers regarding whether two particular insulating liquids are compatible. For example, they may be contemplating retrofilling one of their transformers with a different fluid. In other cases, they just need to add fluid to a unit (“top-off”), but the original fluid is no longer being made, so they need to identify a compatible substitute fluid. In still other cases, the fluid in their transformer may have become inadvertently contaminated with another fluid and they would like to know whether this is a potential problem.
Before addressing some common specific questions about fluid compatibility, let us briefly discuss the primary reason that two dielectric fluids would be considered incompatible. The most important and critical reason that two dielectric fluids would be considered incompatible is if the two fluids do not mix well. If two fluids that do not mix well are combined, it can result in a phase separation in the transformer – two layers and/or droplets of one fluid in the other. Any phase separation in an energized transformer creates a risk that a phase boundary may come in contact with the energized conductor. Even if the two phases are both dielectric liquids, contact with the phase boundary has the same general effect as if this were a globule of free water or a bubble of free gas coming in contact with the energized conductor. Therefore, a phase separation of two liquids due to incomplete mixing, whether in the form of layers or droplets, can result in the sudden and violent failure of the transformer. If two fluids in a transformer are not completely mixed, the transformer should be considered unreliable, and is thus a candidate for replacement.
In some cases, two fluids may be readily miscible (they will easily mix), but they still may not be considered good candidates to combine. For example, a particular fluid may be miscible with the fluid in your transformer, but it may have a significantly lower flash point, and thus may not be the best candidate for a retrofill or top-off fluid. In another example, a particular fluid may be miscible with the fluid in your transformer, but viscosity and/or heat transfer considerations would dictate better choices for a retrofill or top-off fluid.
Let’s now review some common questions regarding fluid compatibility:
Q. The liquid level in my silicone-filled transformer is low, so I need to add some fluid. Are there any other fluids that will work, or should I use the same silicone fluid?
A. You should use the same silicone fluid. Other insulating liquids do not mix easily with silicone fluid, so to avoid potential sudden failure of your transformer due to phase separation, only silicone fluid should be added to a silicone-filled transformer.
Q. My silicone-filled transformer has become contaminated with oil. Is this a problem?
A. If the oil is not completely mixed with the silicone fluid, a phase separation can result in the sudden and violent failure of the transformer. A representative sample of the fluid should be taken and carefully observed for any signs of phase separation – either in the form of layers of fluid or cloudiness. If any such evidence is found, the transformer should be considered unreliable, and a candidate for replacement. Further, if the two fluids appear to be fully mixed, then depending on the concentration of oil contaminating the silicone fluid, the flash and fire points of the silicone fluid may have been significantly lowered. A flash point test should be performed if this is a concern.
Q. My oil-filled transformer has become contaminated with silicone-fluid. Is this a problem?
A. If the silicone fluid is not completely mixed with the oil, a phase separation can result in the sudden and violent failure of the transformer. A representative sample of the fluid should be taken and carefully observed for any signs of phase separation – either in the form of layers of fluid or cloudiness. If any such evidence is found, the transformer should be considered unreliable, and a candidate for replacement. Further, if the two fluids appear to be fully mixed, then it should be noted for future reference that the mixture will most likely foam excessively if subjected to heat and vacuum processing, making processing very difficult.
Q. The liquid level in my Askarel-filled transformer is low. What fluid do you recommend I use to top it off?
A. Askarel fluid is PCB fluid, and as such, has not been manufactured as a dielectric fluid since 1979 when its production was banned. We recommend polyalphaolefin fluid to top-off an Askarel-filled transformer. Alpha-1 fluid, made by DSI, is one well-known brand of polyalphaolefin fluid.
Q. I’ve thought about retrofilling my oil-filled transformer with a natural ester fluid. Are these two fluids compatible?
A. Yes, mineral oil dielectric fluid is compatible with natural ester (vegetable oil) dielectric fluids such as Cooper’s Envirotemp FR3 and ABB’s Biotemp. Some manufacturers of natural ester fluids offer retrofilling guides covering this topic.
Q. I’ve thought about retrofilling my silicone-filled transformer with a natural ester fluid. Are these two fluids compatible?
A. No, silicone fluid is not compatible with natural ester (vegetable oil) dielectric fluids such as Cooper’s Envirotemp FR3 and ABB’s Biotemp. Natural ester fluids do not mix easily with silicone fluid, so to avoid potential sudden failure of your transformer due to phase separation, do not retrofill a silicone-filled transformer with a natural ester fluid. In general, if a silicone-filled transformer ever needed to be retrofilled, it should only be retrofilled with silicone fluid.
Q. The liquid level in my R-Temp-filled transformer is low. What fluid do you recommend I use to top it off?
A. R-Temp fluid is an example of a brand of high fire-point mineral electrical insulating oil. It was originally made by Cooper, but is no longer manufactured. Mepsol and Wemco FR are other brands of this type of fluid that are also no longer manufactured. Transformers filled with these discontinued fluids can be topped-off with Beta fluid, made by DSI. Beta fluid is a high fire-point mineral electrical insulating oil that is still being manufactured. In addition to Beta fluid, other fluids that would work in this application are natural ester (vegetable oil) dielectric fluids such as Cooper’s FR3 and ABB’s Biotemp; synthetic ester dielectric fluids such as Cooper’s Envirotemp 200 and M&I Materials’ Midel 7131; and polyalphaolefins such as DSI’s Alpha-1.
