Repairing or replacing failed underground cable is a matter of routine business for most utility companies. However, during the late 1990s, increased underground cable failures caused NV Energy crews in southern Nevada to scramble to stay ahead of the curve and search for a solution to slow down the cable failure trend. The problem was with 12.47-kV (line-to-line) non-jacketed, non-strand-filled, direct-buried 1/0 XLP cables installed in the early 1970s to late 1980s. This cable was the standard at the time but is now rather notorious for having problems with both electrical and water treeing within the polyethylene insulator causing internal faults.
When customers experience outages in neighborhoods served by underground cables, the traditional utility response is to pinpoint the fault, dig up the area to reveal the damaged cable, install a new spliced section of cable and then return the street, sidewalk or landscaping back to an acceptable state. When a segment of cable experiences repeated faults, the utilities may consider abandoning the segment and installing a new cable segment. These methods require substantial amounts of manpower and resources, and often are a significant disruption to customers, streets and neighborhoods.
Technology to the Rescue
To take control of the situation, NV Energy partnered with UtilX Corp. for a cable injection pilot program in 1997, targeting an area known to have a high number of cable failures. Cables in this area were injected and monitored while the cable injection program was in its experimental stages during the initial years. However, even after the initial pilot program, it was apparent that the program was heading in the right direction with no faulted segments recorded in the treated areas. In fact, to date, NV Energy has recorded less than 1% of injected cables faulting after they've gone through the injection process. Total success is measured over many years, but initial reliability improvements and process confidence resulted in NV Energy deciding to gradually increase its investment in this program with a current program budget of $12.5 million annually.
Earlier this year, the company celebrated crossing a 5-million-ft threshold of injected cable, which is more than one-third of its 14 million ft of primary cable that used the direct-bury standard of the 1970s and 1980s. Currently, the company is injecting nearly 1 million ft of cable annually, and calculates a 15-year savings to date of about $300 million, compared to costs associated with using traditional methods to replace or repair underground cables.
The Rejuvenation Process
In general, the process entails injecting a silicone-based fluid into the voids between strands of the conductor. This fluid is designed to diffuse into the polyethylene insulation. Once there, the fluid chemically reacts with any moisture, converting it into a stress-grading dielectric, ultimately increasing the cable's insulation strength. In most cases, NV Energy's distribution system is looped; this process can be done without the customer experiencing a power outage or traffic disruptions caused by excavation activities of traditional cable replacement.
The first step is a diagnosis of the section of cable that needs to be refurbished. Crews use a time-domain reflectometer to detect any previously installed splices, cable length and the integrity level of the concentric neutral.
The next step is to install Elastimold injection elbows from Thomas & Betts at both ends of the line. These elbows have an injection port specifically designed for this process and replace the original elbows. Once the elbows are in place, the line is tested by pumping nitrogen through the injection port to ensure good flow. The flow test is followed by a pressure test in the range of 20 psi to ensure all existing cable components are leak-free at the low injection pressure employed. Then a strand desiccating fluid is sent through the strands of the conductor to prepare the cable for injection. Finally, UtilX's CableCURE fluid is injected into one end of the line at approximately 20 psi while a vacuum is drawn at the other end. The vacuum ensures a void-free fill.
To ensure the proper amount of fluid is supplied to the cable, a lightly pressurized reservoir tank with a predetermined amount of fluid — based on the specific cable parameters — is left in place at the other end of the cable so the insulation can receive additional fresh rejuvenation fluid for an additional period of time. This allows the cable access to the optimum fluid quantity regardless of the volume available in the conductor strands. After the cable has been filled end to end, the vacuum end of the cable is permanently capped closed.
Reaping the Benefits
For the past decade, underground cable failures have topped the outage list for NV Energy's Southern system. At the height of the underground cable fault problem, the utility had nearly 600 underground cable faults annually that were backlogged and listed to be fixed either by fault repair procedure of splicing the cable or total replacement. Once the company's engineers and managers fully embraced a cable-injection program and dedicated crews and materials to the effort, the cable failure trend began to reduce. This improved service reliability and reduced crew call outs for faulted cable repairs. Customers experienced outages less frequently and were happier, and fewer customers complained about NV Energy crews digging up neighborhood sidewalks, streets and landscaping.
Injected cables have a 20-year warranty from the date of their injection and 100% of injection costs are reimbursed in case of an internal failure.
The cable injection program is regarded as the most impactful maintenance program southern Nevada administrates. Its success is a significant contributing factor in NV Energy's favorable reliability performance comparisons as measured in the Edison Electric Institute's Reliability Reports. In fact, NV Energy's southern Nevada service territory placed within the top decile for the 2011 EEI Reliability Report for both SAIDI and SAIFI metrics.
Christina Clark is the manager of transmission and distribution asset management for NV Energy. She overseas regional maintenance support services, GIS and mapping, operational research and analysis, and transmission and distribution standards departments.
Robert DerAshodian is supervisor of regional maintenance support services for NV Energy. He manages various NV Energy maintenance programs including cable injection and replacement, overhead circuit rebuild, pole inspection and treatment, risk management and thermal vision, substation RCM database, transmission line patrols and vegetation management.
NV Energy | www.nvenergy.com
Thomas & Betts | www.tnb.com
UtilX Corp. | www.utilx.com