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EPR (Part 3 of 3)

December 14, 2011

EPR (Part 3 of 3)

Over the course of the last two days I delivered two posts. In the first, titled EPR (Part 1 of 3),” I provided a response to the inquiry on whether it made sense to rejuvenate aging EPR cables. The short answer was yes and I dispelled some EPR myths along the way. In the second, titled EPR (Part 2 of 3)we explored the chemistry of EPR and the clay fillers utilized in EPR formulations and dispelled a myth that silanes in rejuvenation fluid might interact in some unhelpful way with the silane surface treatments employed in the manufacture of EPR compounds. In this third installment, I explain how treating EPR cables is different from treating XLPE-insulated cable. I provide guidance on how one should choose the right rejuvenation fluid for the unique requirements of EPR cables.

As was demonstrated in the first post, an experiment at CTL (Cable Technology Laboratories) sponsored by Reliant Energy and EPRI demonstrated that even earlier generations of technology do a decent job of extending the life of EPR-insulated cables. If decent is good enough you can stop reading here. Any of the commercially available fluids will do a decent job. If you desire to learn how to fashion the best solution, read on …

Compound

EPR:PE

D

S

P

TEMDMS

1.3

3.4

4.4

CBMDMS
Only CBMDMS permeates at about the same rate in EPR as in PE

0.5

1.8

0.9

DMDBS

1.1

5.1

5.5

acetophenone

0.9

1.6

1.5

Ratio of Diffusivity (D), Solubility (S), and Permeability (P) for four compounds in EPR & PE (polyethylene) at 55°C.

EPR and polyethylene are not the same. At the micro-scale the biggest difference is the clay and/or carbon black filler in EPR that is absent from XLPE. The filler has a profound impact on the permeation properties of treatment fluids in the insulation. In the table nearby I illustrate how the permeation properties of several exemplary fluid components behave in EPR relative to their behavior in PE. TEMDMS is tolylethylmethyldimethoxysilane; CBMDMS is cyanobutylmethyldimethoxysilane. TEMDMS and CBMDMS are components of Cablecure® 732 fluid. DMDBS is dimethyldibutoxysilane and a component in Cablecure DMDB. Acetophenone is a common by-product of cross linking and is provided as a reference. While the ratios for the legacy fluid phenylmethyldimethoxysilane (PMDMS) were not measured, its performance will undoubtedly be very similar to that of the structurally similar, TEMDMS. PMDMS is the primary ingredient (>90%) in Novinium’s Cablecure iXL fluid and Cablecure XL fluid.

The diffusion coefficient (D) is a measure of how quickly molecules can move through the insulation matrix. TEMDMS (and by analogy PMDMS) and DMDBS experience slight increases in D. CBMDMS and acetophenone have lower diffusion coefficients in EPR than they do in PE. For solubility (S) the story is different. All materials are more soluble in EPR than they are in PE. This is almost certainly due to surface interactions on the filler particles and low to no crystallinity of the polymer phase. In the case of TEMDMS, PMDMS, and DMDBS the solubility increase is between 3X and 5X. Finally, permeation (P) is the product of D and S and provides an indication of how fast a material will exude through a membrane—the cable insulation is a thick cylindrical membrane. Thus with identical temperature and identical cable geometry, TEMDMS, PMDMS, and DMDBS will exude 4- to 6-times faster from an EPR cable than a PE cable. Put another way, an EPR cable treated with Cablecure iXL or Cablecure XL fluids would not enjoy the same life extension as a similarly treated XLPE cable—the treatment would be expected to last about one-quarter as long.

CBMDMS is the only material that actually permeates more slowly in EPR than it does in PE. The incredible CBMDMS is protected by U.S. Patents 7,658,808 and 8,101,034, and their foreign equivalents. CBMDMS is available only in Novinium® brand Cablecure 732/733 products. Novinium exercises the claims on another U.S. Patent, 7,611,748 to tailor the formulation of its Cablecure 732/733 product to specifically address EPR cables. There is not a single, one-size-fits-all, formulation that is optimum for all cable sizes and insulation polymers. Novinium’s patented process tailors the formulation to the unique circumstances of each cable including the substantial difference between EPR and PE cables. Specifically, the amount of CBMDMS is increased along with the amount of antioxidants at the expense of the TEMDMS. Tailored formulation™ is available only with Novinium’s Cablecure 732/733 technology.

Novinium’s technology is entirely transparent—no secret handshakes. The formulation adjustment described above is documented in Novinium Rejuvenation Instruction 20, step 9d. Click NRI20 to review “Power Cables Tailored Formulation™ & Tailored Pressure™.”

There is another important factor that impacts the post-injection life extension of EPR cables. As described in my December 29, 2010 post, “Catalytic Considerations—Component I,” Novinium’s patented catalyst technology (U.S. Patent 7,700,871, pending applications, and their foreign equivalents) keeps more of the supplied fluid in the cable longer. Click here to check out that technology. Thus while I could not recommend Cablecure iXL fluid for EPR life extension because of the permeation multiplier described above, at least it would last longer than other treatments that do not benefit from the improved catalyst technology both Cablecure iXL and Cablecure 732/733 fluids employ.

EPR cables age and fail with mechanisms similar to those that affect their PE cousins. Historically EPR cables have enjoyed a longer reliable life, but they do have a finite life. It has been shown unambiguously in both the laboratory and in field applications that rejuvenation improves the dielectric performance and extends the life of EPR insulated cables. Even less advanced formulations provide benefit when properly applied, but because these earlier generation fluids exude so quickly from EPR cable, those benefits are short-lived. Life extension of 40 years is only possible with Cablecure 732/733 fluids that are tailored to the individual cable and incorporate chemistry specifically optimized for EPR.