Liquid Compatibility

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 liquid.  In other cases, they just need to add liquid to a unit (“top-off”), but the original liquid is no longer being made, so they need to identify a compatible substitute liquid.  In still other cases, the liquid in their transformer may have become inadvertently contaminated with another liquid and they would like to know whether this is a potential problem.

Before addressing some common specific questions about liquid compatibility, let us briefly discuss the primary reason that two dielectric liquids would be considered incompatible.  The most important and critical reason that two dielectric liquids would be considered incompatible is if the two liquids do not mix well.  If two liquids that do not mix well are combined, it can result in a phase separation in the transformer – two layers and/or droplets of one liquid 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 liquids in a transformer are not completely mixed, the transformer should be considered unreliable, and is thus a candidate for replacement.

In some cases, two liquids may be readily miscible (they will easily mix), but they still may not be considered good candidates to combine.  For example, a particular liquid may be miscible with the liquid 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 liquid.  In another example, a particular liquid may be miscible with the liquid in your transformer, but viscosity and/or heat transfer considerations would dictate better choices for a retrofill or top-off liquid.

Let’s now review some common questions regarding liquid compatibility:

Q.  The liquid level in my silicone-filled transformer is low, so I need to add some liquid.  Are there any other liquids that will work, or should I stick with silicone fluid?

A.  You should stick with silicone fluid (same type, not necessarily the same brand). Other insulating liquids do not mix easily with silicone fluid, so to avoid the 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 thoroughly mixed with the silicone fluid, phase separation can result in the sudden and violent failure of the transformer. A representative sample of the liquid should be taken and carefully observed for any signs of phase separation—either in the form of layers of liquid or cloudiness. If any such evidence is found, the transformer should be considered unreliable and a candidate for replacement. Further, if the two liquids appear to be thoroughly 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 and fire 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, phase separation can result in the sudden and violent failure of the transformer. A representative sample of the liquid should be taken and carefully observed for any signs of phase separation – either in the form of layers of liquid or cloudiness. If any such evidence is found, the transformer should be considered unreliable and a candidate for replacement. Further, if the two liquids 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 liquid do you recommend I use to top it off?

A. Askarel fluid is PCB fluid and has not been manufactured as an insulating liquid 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 liquid.  Are these two liquids compatible?

A.  Yes, mineral oil insulating liquid is compatible with natural ester (vegetable oil) insulating liquids such as Cargill's FR3, Prolec's VG-100, and M&I Materials’ Midel eN 1204 and eN 1215. Some manufacturers of natural ester liquids offer retrofilling guides covering this topic.

Q.  I’ve thought about retrofilling my silicone-filled transformer with a natural ester liquid.  Are these two liquids compatible?

A. No, silicone fluid is incompatible with natural ester (vegetable oil) insulating liquids such as Cargill's FR3, Prolec's VG-100, and M&I Materials’ Midel eN 1204 and eN 1215. Natural ester liquids do not mix easily with silicone fluid, so to avoid the potential sudden failure of your transformer due to phase separation, do not retrofill a silicone-filled transformer with a natural ester liquid. If a silicone-filled transformer ever needs to be retrofilled, it should only be retrofilled with silicone fluid.

Q.  The liquid level in my R-Temp-filled transformer is low. What liquid do you recommend I use to top it off?

A.  R-Temp fluid is an example of a brand of less-flammable mineral insulating oil. It was originally made by Cooper but is no longer produced. Mepsol and Wemco FR are other brands of this type of liquid that are also no longer produced. Transformers filled with these discontinued liquids can be topped off with Beta fluid, made by DSI. Beta fluid is a less-flammable mineral insulating oil that is still produced. In addition to Beta fluid, other liquids that would work in this application are natural ester (vegetable oil) insulating liquids such as Cargill's FR3, Prolec's VG-100, and M&I Materials’ Midel eN 1204 and eN 1215; synthetic ester insulating liquids such as Cargill's Envirotemp 360 and M&I Materials’ Midel 7131; and polyalphaolefins such as DSI’s Alpha-1.