EPR (Part 1 of 3)
Rejuvenating Cables Made with EPR Part 1 of 3
Q: I’ve heard some people say that rejuvenation isn’t appropriate for EPR cables. Does it ever make sense to rejuvenate EPR cables?
A: It’s true—some have suggested that those who invented rejuvenation technology never intended to use it treat Ethylene propylene rubber (EPR) cable. That claim is without merit. The target of the development effort was solid-dielectric cables, and EPR cables fall within that genre.
Similar assertions are that EPR cables do not fail, they do not suffer water treeing, and hence rejuvenation is counterproductive for EPR cables. Let’s consider those assertions one at a time.
EPR Cables Do Not Fail
EPR cables do fail. Cable Technology Laboratory (CTL), which routinely studies cable failures, reports that over three decades they have received about 600 samples of failed XLPE cable and about 10 samples of failed EPR cable.
At first glance, one might say that figure suggests a 60-fold reliability advantage for EPR cables over XLPE. But that assumption would be exaggerated 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, April 8, 1997). What would be fair to say is that the reliability of EPR of 1960–80 vintage was about 10 to 15 times better than the same vintages of XLPE cables. But although it was historically more reliable, it wasn’t invulnerable, and it was still in need of rehabilitation at some point.
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.
Nevertheless, Bogdan Fryszczyn at CTL has figured out how to image trees in EPR. The annotated images 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.
Rejuvenation of EPR Would Be Counterproductive
Because EPR cables do fail and suffer the same water-treeing phenomenon as XLPE cables, it seems self-evident that rejuvenation would benefit aging EPR cables. In fact, over a decade ago, some professionals at CTL, Electric Power Research Institute (EPRI), and Reliant Energy (Houston) wrote a paper demonstrating just this. In “Extending the Service Life of Ethylene Propylene Rubber Insulated Cables,” 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.