EPR (Part 2 of 3)

In yesterday’s post, EPR (Part 1 of 3), I provided a response to the inquiry of Ethel P. Reliability on whether it made sense to rejuvenate aging EPR cables.  The short answer was yes and I dispelled some EPR myths along the way.  This post explores the chemistry of EPR and dispels another myth suggested by a physicist at the ICC meeting.  I won’t use his name to spare embarrassment, but he is often critical of others who speak without actual knowledge or evidence … a sin this frog does not commit.  The gentlemen suggested that the silanes in rejuvenation fluid might interact in some nefarious way with the silane surface treatments employed in the manufacture of EPR compounds. 

First some background

EPR or ethylene propylene rubber comes in several flavors – often designated with colors. All EPRs are roughly half rubber – the balance is filler.  Modern EPR materials use treated clays as the filler and their color varies from gray to brown to pink.  Some early EPR compounds were filled with carbon black and these are referred to as Black EPR.  Today we are going to limit our discussion to clay-filled EPR.  Those who manufacture EPR hold the details of their clay formulations close to their vests.  There are several clays including hectorite, beidellite, montmorillonite, and kaolinite.  All share several chemical properties.  All include silicon as their most prevalent low electronegative atomic constituent and all include hydroxyl groups (oxygen bonded to hydrogen represented as “-OH”).  The hydroxyl groups are connected most commonly to silicon, and less commonly to aluminum, magnesium, or lithium.  These hydroxyl groups, illustrated nearby are polar in nature and incompatible with the organic ethylene-propylene polymer. To improve the compatibility of the clay with the rubber, compounders treat the surface of the clay with silanes very similar to rejuvenation compounds. These silanes form oxane bonds, largely eliminating the hydroxyl groups. For all practical purposes these reaction are permanent – that is the silanes are permanently bonded to the clay surface. One purpose of this surface treatment is to ensure uniform dispersion of the clay in the polymer during processing. Another advantage of silane treatment of the clay filler is a reduction in the effective solubility of water in the EPR compound, because of the replacement of the hydrophilic hydroxyl groups with hydrophobic silicone. 

There is no evidence that anything in any treatment fluid interferes in any way with EPR. In fact, the opposite is true.

EPR (Part 1 of 3)

Dear Grandest of Frogs,

I was in the audience of an ICC (Insulated Conductor’s Committee) discussion group C30 (Extending the Life of Field Cables) and was confused by the discussion about treating EPR cables. There were some in the room who seemed to be of the opinion that rejuvenating EPR cables is not appropriate.  Does it ever make sense to rejuvenate EPR cables?

Regards,

Ethel P. Reliability

Dear E.P.R.-

I was not at the meeting in question, but I have several very good friends who were, so I got the straight scoop.  First there was a bizarre suggestion that those who originally invented rejuvenation technology never intended to treat EPR cable.  Two gentlemen who were actually there for the original inventive step of modern alkoxy-silane-based rejuvenation technology were founders of Novinium and are able to confirm that claim is without merit.  The target of the development effort was solid-dielectric cables – EPR cables fall within that genre.  There were several very vocal proponents of EPR cable at that meeting and they were united in a common narrative.  To wit, EPR cables do not fail, they do not suffer water treeing, and hence rejuvenation is counterproductive.

Poppycock!  Poppycock!  Poppycock! Let’s take those assertions one at a time.

EPR cables do not fail

EPR cables do fail.  One laboratory (Cable Technology Laboratory or CTL) that studies cable failures on a routine basis reports that over three decades they have received about 600 samples of failed XLPE cable and about 10 samples of failed EPR cables.  At first glance, one might say that this anecdote suggest a 60-fold reliability advantage for EPR cables over XLPE – that assumption would be exagerated, because about four-times more XLPE cable was deployed from 1964 to 1980 (See Forrest, “Predicting Medium Voltage Underground Power Cable Failures and Replacement Costs,” Western Electric Power Institute, Apr. 8, 1997).  What would be fair to say is that over the course of the last three decades, the reliability of 1960-1980 vintage EPR was about 10 to 15 times better than the same vintages of XLPE cables.  Historically more reliable, yes … invulnerable, no … in need of rehabilitation at some point, yes.

EPR cables do not suffer water treeing

It is true that imaging water trees in EPR cables is quite challenging. EPR cables are filled with clay and as a consequence even very thin samples are opaque.  Traditional microscopic examination with staining often fails to reveal water trees.  A colleague at CTL has figured out a way to image trees in EPR.  Bogdan Frysczyn’s annotated images nearby show bow-tie and vented water trees right at the failure breakdown channels in so-called “pink” EPR.  Similar images have been made for brown and black EPR too.  So much for that narrative.

Rejuvenation of EPR would be counterproductive

Now that we have established that EPR cables do fail and that they suffer the same water treeing phenomenon as XLPE cables, it would seem to be self-evident that rejuvenation would benefit aging EPR cables.  Over a  decade ago, my friends over at CTL together with EPRI (Electric Power Research Institute) and Reliant Energy (Houston) wrote a paper demonstrating just this entitled, “Extending the Service Life of Ethylene Propylene Rubber Insulated Cables.”  In this paper, the authors concluded:

• It is feasible to upgrade early vintage black EPR cable and achieve a significant increase in ac and impulse voltage breakdowns.
• It is feasible to upgrade current vintage pink EPR cable and achieve a significant increase in ac and impulse voltage breakdowns.

These results were based on treatment with phenylmethyldimethoxysilane (PMDMS), which is the primary ingredient in both CableCURE^®/XL fluid offered by UTILX^® Corporation and Perficio™ 011 fluid provided by the Masters of Reliability™ at Novinium. In two subsequent posts, this frog will explain another misconception perpetrated at the last ICC meeting (EPR 2 of 3) and how the technology has advanced over the last decade to improve the post-injection reliability of EPR cables specifically (EPR 3 of 3).

Eternally Proactively Reliable,

Thermonuclear B. Frog