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Dielectric III

June 21, 2012

Dielectric III

In Dielectric I, I provided the first part of a four-part answer to eight questions. The eight questions can be summarized: To what extent does the introduction of injection equipment into energized devices impact the safe operation of medium voltage circuits? We learned in the first installment that there are great differences in the extent of this risk depending upon the injection paradigm employed. There are three injection paradigms and at Novinium we use only the safest processes. With Novinium’s patented Sustained Pressure Rejuvenation (SPR) there is zero risk associated with the interaction of energized circuits and injection equipment. With Novinium’s Improved Unsustained Pressure rejuvenation (iUPR) process, the risk is many times less than the legacy approach employed by others. The legacy paradigm is called UPR (unsustained pressure rejuvenation). In my second post, Dielectric II, I provided data that demonstrated why the feed end of an iUPR injection is not a safety issue—unfortunately the same conclusion is not true for the legacy UPR process. The purveyors of UPR acknowledge safety shortcomings of UPR in their U.S. Patent 7,704,087. I provide a relevant excerpt in my April 15, 2011 post, “Soaking II: Safety First.”

In the illustration nearby I am propped up against iUPR vacuum equipment. When the iUPR injection method is utilized, fluid flows unidirectionally from the iUPR feed tank through the cable and into the iUPR vacuum tank. While the dielectric properties of the fluid flowing into the cable are well known and stable, there is no way to be certain what will come out the other end. From a design perspective the Novinium engineering masters must assume that conductive water will flow from the outlet termination and into the iUPR vacuum tank. The same 80 kV rated tubing is used on the vacuum side as was described in the previous post. The rating of the vacuum tank itself is higher still.

Of course, 80 kV is more than enough over-design for medium voltage applications, but there are other factors which provide the iUPR process a “belt and suspenders” robustness. Consider these three …

(1) It is very unusual for water to be in the strands of URD cables with 19 or fewer strands. I know that many believe the opposite is true, but the masters at Novinium have been injecting cables for over 25 years. If liquid water in URD cable strands were common we would see it—we don’t! The occurrence is less than 1 percent. Where water is found in the strands is on pole terminated cables where the terminator design has a leak so that every time there is precipitation, water finds its way into the strand interstices. Don’t purchase pressed lugs—buy only solid lugs. If you want to know why there generally is not water in cable conductors, read “Molecular Thermodynamics of Water in Direct-buried Power Cables” from the Nov/Dec issue of IEEE Electrical Insulation Magazine.

(2) In the rare case where there is water, the time in which water will be in the tube is very limited. While cables are designed for continuous operation at operating voltage, the iUPR vacuum equipment is exposed for zero to perhaps 60 minutes. Any short duration exposure is eliminated when dielectric fluid flushes the last of any water from the strands and tubing.

(3) Effervescence limits the conductivity of fluid effluent along the vacuum tube interior. Carbon dioxide (CO2) is liberated when you open a soda or beer bottle, because the pressure on the fluid is released. CO2 is used in iUPR to provide the driving force to the rejuvenation fluid. CO2 is even more soluble in Cablecure 732/733 [Ultrinium™] and Cablecure iXL [Perficio™] rejuvenation fluids than it is in beer and soda. As CO2-saturated rejuvenation fluid flows through the strand interstices and down the length of a cable the absolute pressure decreases almost linearly along the length. As the pressure decreases CO2 is liberated. The viscosity of CO2 is orders of magnitude lower than the liquid phase from which it effervesces, so it bubbles ahead of the fluid and rushes to the vacuum tank. Any fluid exiting the cable is interspersed between much more voluminous CO2 bubbles. Of course, gaseous CO2 is a great dielectric and its presence disconnects adjacent droplets of fluid and prevents there being a contiguous path for current to flow. Water, if present, does not wet the surface of the polyethylene tube, but instead stays as discrete droplets. The conductor voltage is not efficiently conveyed along the tubing length.

iUPR has simply never had issues arise in thousands and thousands of field applications. iUPR is not as safe as SPR, but it comes in second place.

In my final post, Dielectric IV, I will address the equipment separation issues raised in questions 6 through 8.