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Supersaturation

September 27, 2013
Inquiry

Under what circumstances does supersaturation or over-saturation occur in rejuvenated power cables? Do supersaturation or over-saturation cause cables to fail?

Response

I think we all have experience with supersaturation. What kid didn’t make rock candy? Warm water can dissolve more sugar than cold water. As the temperature of sugar-saturated hot water falls, the sugar becomes supersaturated and it will crystallize out of solution onto a seed crystal on a stick. The result is a pure-sugar treat in an attractive crystalline form.

Cable engineers need not look back to their childhood for examples of supersaturation, because water halos are a phenomenon that occurs in all polyethylene power cables subjected to thermal cycling. In the photograph nearby the circular halo is visible along with the vented water tree. The formation of the halo is explained in “Molecular Thermodynamics of Water in Direct-Buried Power Cables” (IEEE Electrical Insulation Magazine, Nov/Dec 2006, p. 20), Bertini explains how halos form:

“When the temperature decreases [in a cable], the thermodynamic driving force reverses, and the water is driven out of the PE and out of the vapor phase within the voids to the liquid phase. If the water molecule is close to the outer cable surface, it can exit the cable into the soil. If the water molecule is close to a stranded and unblocked conductor, it can exit the polymeric layers and enter the strand interstices. However, if the water is near the center of the insulation, the water cannot exit the insulation fast enough and the insulation becomes supersaturated with water. If a solid-core or effectively strand-blocked conductor is used the region of supersaturation is likely expanded radially inward. The water phase creates a large number of voids near the center of the insulation that are collectively referred to as a halo …”  Click here to see the entirety of the paper.

Supersaturation of water occurs in nearly all power cables and does not cause a cable to fail. The reason that water supersaturation does not create a serious reliability issue is that the solubility of water in the polyethylene is very low. In order for there to be a reliability issue associated with supersaturation, three elements are required.

  1. The cable must undergo substantial thermal cycles. Thermal cycles are greatest when the amplitude of the change is large and the frequency of the change is short.
  2. There has to be an excess supply of the soluble fluid.
  3. The solubility of the fluid must be sufficiently high at higher temperatures.
Thermal Cycles

If cable load stays below 50% of the cable ampacity the risk of over-saturation is close to zero. In order to drive over-saturation large temperature swings are required to occur within a 24-hour period. Such swings are rare in typical underground residential distribution environments, but are much more common in large conductor feeder cables.

Excess Supply

U.S. Patent 6,162,491 conceived and reduced to practice by a Novinium founder, defined the relationship between the amount of fluid required to properly treat a cable and the amount of fluid reservoir that is available within the cable strands. The graph nearby summarizes the relationship for all conductor sizes and for the three most common strand compressions. For any given conductor, where the interstitial fluid quantity is less than or near the fluid requirement the chance of over-saturation is very close to zero. At Novinium it is zero, because Novinium technology does not require a soak period. Other suppliers leave soak bottles attached to the cable for months that run the risk of over-supply if the field technicians do not properly limit the size of the reservoir.

For conductors larger than 500 MCM (about 250 mm2) that are not compact conductors, the volume in the strands is greater than the amount of fluid required to treat the cable. In this case over-saturation is possible if the other two elements are present, namely Thermal cycles discussed above and Solubility discussed below.

Solubility

There are three solubility classes of alkoxysilanes used to rejuvenate power cables.

Alkoxysilane Class Commercially Significant Examples

Solubility

g/cm3 @ 80°C

Alkane dimethyldibutoxysilane predominant component of Cablecure® DMDB fluid

0.41

Cyclic-aromatic phenylmethyldimethoxysilane predominant component of Cablecure XL fluid and Cablecure iXL fluid

0.15

tolylethylmethyldimethoxysilane significant component of Cablecure 732 fluid

0.15

Cyano (pronounced sigh-an-oh) cyanobutylmethyldimethoxysilane significant component of Cablecure 732 fluid

0.014

The greater the solubility, the greater the risk of over-saturation. Consider the case of the Alkane class of materials. At 80°C polyethylene swells over 40% in the presence of excess Alkane fluid. A 40% swell places substantial mechanical stress on the cable and the risk of catastrophic failure is relatively high. Novinium utilizes only low concentrations of these materials to avoid the risk of over-saturation.

The Cyclic-aromatic class of materials enjoy almost 3-times less solubility, and hence the conditions required to create a problem are about 3-times more severe. Even though supersaturation is a rare event with Cyclic-aromatic materials, Novinium does not recommend their use where the temperature of the cable might be above 40°C unless they are utilized with a lower solubility buffer such as a Cyano class material.

The Cyano class of materials is an order of magnitude less soluble than the Cyclic-aromatic materials and the chance of over-saturation is nil. Novinium is the only firm in the world that can practice the technology of U.S. Patent 7,611,748 to tailor a rejuvenation formulation to the cable. One of the key parameters that we tailor is the ratio of Cyclic-aromatic and Cyano class materials to the thermal condition expected of the cable to be treated. The higher the temperature and the greater the temperature cycling, the greater the ratio of Cyano class materials to Cyclic-aromatic class materials.

Summary

Failure due to over-saturation is a pretty rare event, even with legacy approaches. Novinium avoids Alkane class materials and utilizes formulation tailoring to stay well away from the conditions conducive to over-saturation. At Novinium our failure rate due to over-saturation is zero. Our overall failure rate from all causes is less than half that of legacy technologies.