Shields Scotty

Inquiry

What impacts do rejuvenation fluids have on the conductor and insulation shields?

Response

With Star Trek Into Darkness in the theaters it was appropriate to revisit the question of shields. Not the shields that deflect Klingon disrupter blasts, but the conductor and insulation shields used to smooth electrical stress in underground power cable. When rejuvenation technology was first introduced about 25 years ago, that was one of the first questions that had to be resolved.

The table below provides some publically available data from a report prepared by Cable Technology Laboratories titled “Testing of VEPCO 35kV cable 5 Years After Upgrading,” dated May 28, 1999. The data examined the impact of a 70:30 mixture of phenylmethyldimethoxysilane and trimethylmethoxysilane (the white phase) and dimethyldimethoxysilane (the blue phase) in an application at Virginia Electric Power, compared to the untreated control (the green phase).

In short, both rejuvenation fluids show slight, but insignificant increases in volume resistivities of the conductor and insulation shields, four year after treatment at both 22°C and 90°C. In all cases, the increase is well below the limit set for cables of that vintage. The more modern limit is even more forgiving than the 1971 values. Dozens of such measurements were made in the last century, but they ceased to be interesting, because the impact was repeatedly insignificant.

Fabric Tape Conductor Shield

Dear Amiable Amphibian,

I am wondering if you can provide some thoughts or comments on a cloth fabric semi-con we have here on some older #2 Cu cables, 15kV. Does the Ultrinium fluid harm this fabric?  Does the fluid react with any semi-conducting materials like carbon? Does Novinium think the injection process disturbs or harms the fabric?

Conducting Query

Dear Conducting-

The fabric tapes used on pre-1980 vintage cables were carbon-black dispersed on cotton fibers. Neither the cotton nor the carbon black in these semiconducting tapes react with the silanes used with Ultrinium™ fluid or Perficio™ fluid. In fact, cables with fabric tapes have been treated with alkoxysilane rejuvenation fluid for over two decades. Novinium has not experienced a single failure of a cable with a taped conductor shield.

The use of taped conductor shields in medium voltage distribution applications all but halted by the mid-1970’s in North America. Bartnikas and Srivastava relate in Power and Communication Cables, page 83 …

“Semiconducting carbon black tapes were … used as shields in the early linear polyethylene (PE) insulated cables. Due to poor adhesion between the tapes and the PE as well as occasional breaks or gaps between butting edges of carbon black tapes themselves, partial discharges often occurred within the voids formed at these faults. Polyethylene cables using carbon black shielding tapes were also found to be highly susceptible to tree growth.”

The fact that you have some of these cables in service today is testament to the absence of partial discharge and hence these particular cables do not suffer from the poor adhesion, occasional breaks or gaps which Bartnikas and Srivastava warned about in their book. However, there are certainly water trees and I’ll bet they’re doozies.  Fortunately, ameliorating the pernicious effects of water trees is precisely what Ultrinium and Perficio fluids are designed to do.

There are two operational considerations when injecting taped conductor shield cables. First, there is much more room in the strand interstices, so the fluid will flow faster and the cable will hold more fluid – these are both good things.  Second, when fluid flows through the strands it will entrain some un-adhered carbon black, so the outlet fluid will be black. Not to worry, there is much more where that came from and there is no need to try to flush it all out either.

Yours truly,

Thermo B. Frog

Middle East Query – Rejuvenation Impact on Conductor Shield

Dweller of the Desert asked 22 questions in his post …

Middle East Query – 22 Questions.

In this installment I address question 9.

9.   Will the injection affect the semicon around the conductor, since the fluid will penetrate through it?

Semi-conductive shields are carbon-black-filled polymers. Typically the carbon-black loading is about 50%.  Electrons flow through the carbon black, because the conductive carbon black agglomerates are physically touching each other.  The polymer in between the agglomerates is a dielectric. All rejuvenation fluids are quite soluble in the strand-shield. Of course, part of that solubility is because the fluids diffuse into and through the amorphous portions of the polymer. To learn how that happens, check out my 15-March, 2011 post Chain Entanglement. Even more significant than diffusion through the polymer portion is transport though the carbon black agglomerates. In the diagram nearby nano-me (my nano-sized alter ego) demonstrates the morphology of the carbon black at greater and greater magnification. Carbon black has a great deal of volume in micro-sized and nano-sized holes and pores that provide flow paths for fluid transport. Novinium fluids are selected and tested to verify that they have no detrimental effect on cable materials. Upon exposure to Novinium dielectric fluids there is a slight increase in the resistivity of the conductor shield, but well within the IEC and ICEA conductivity specifications. The typical increase is on the order of 1% of the ICEA S-94-649 2000 specification.  The agglomerate-to-agglomerate electrical connections are not perturbed in a substantive way. Novinium^® Ultrinium™ fluids include high dielectric constant stress grading components. High dielectric constant conductor shields are used by at least one EPR cable manufacturer. Ultrinium fluids improve the dielectric performance of the cable system including the conductor shield.

T. B. Frog