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Better with Pressure

August 3, 2010

Q:

How can we be sure that the higher pressure does not damage the polymer insulation or impact the contact between the semiconducting conductor shield and the conductor? Also, does the expansion affect the interface adhesion between the insulation and the insulation shield, especially for older cables with thermoplastic EIS?

A:

First of all, Novinium is the only company in the world that can use all of the available injection paradigms. The state-of-the-art in rejuvenation technology is Sustained Pressure Rejuvenation (SPR). The folks at Novinium invented that technology. The old method is called Unsustained pressure Rejuvenation or UPR and Novinium founders invented that approach too. To learn more about the differences between SPR and UPR check out my blog entry, “How to inject cables”.

To get a direct and comprehensive answer to your question read the paper, “Silicone Injection: Better with Pressure”.

I have reproduced the summary of that paper below …“Rejuvenation injection pressures up to 1000 psig have been in use for over two decades.

Thousands of cables have been treated with moderate pressures in the 100 psig to 400 psig range. Every time moderate pressure injection has been examined against a lower pressure control, the higher pressure injection has outperformed the lower pressure control.

Cables are designed to accommodate the radial stresses that occur throughout their service. A cable warms with increasing load and cools when the load decreases. The 5.8-times and 6.8-times differences between the linear expansion with temperature of conductors and their insulating polymers create transient gaps between the conductor and the conductor shield. These transient gaps are a normal part of daily operation and deflections of 1% of the cable radius are common. The very similar deflection experienced when a cable is injected a single time creates no forces that are materially different and induces no geometrical deflections that are greater than a single temperature escalation from ambient to a cable’s maximum operating temperature. In contrast to the daily temperature cycles endured by a cable, sustained pressure rejuvenation involves a single cycle. Because sustained pressure rejuvenation utilizes an injection adaptor with shrinkback restraint, the only possible manifestation of the single pressure cycle or future daily temperature cycles is entirely eliminated. Experiments confirm that there are no significant changes in the geometry of a cable treated with the SPR process. Field observations such as that memorialized in Figure 8 (of the paper) confirm the laboratory measurements.

Not only does moderate pressure injection provide higher performance (even with older technology fluids), but it also lowers the cost of injection and makes possible the use of advanced rejuvenation materials. Few circuit owners would consider buying a cable today, which did not have anti-oxidant or tree-retardant properties. The newest generation of rejuvenation fluids has about two to three times the viscosity of the first generation fluid, because these fluids include advanced functionalities that provide 2-to-4 times more post-injection life than the lower viscosity 22-year-old technology.

Sustained Pressure Rejuvenation (SPR), Thermally Enhanced Rejuvenation (TER), and Unsustained Pressure Rejuvenation (UPR) are tools that circuit owners should have available for their rehabilitation programs. No one tool works best in every circumstance.”