Salt River Project treats underground residential distribution cable by injecting silicone fluid to extend life and enhance reliability.
CABLE INJECTION TECHNOLOGY HAS BEEN USED TO TREAT CROSS-LINKED POLYETHYLENE CABLE since 1998 at Salt River Project (SRP; Phoenix, Arizona, U.S.). To date, the utility has successfully treated approximately 4.6 million ft (1400 km) of underground residential distribution (URD) cable. Approximately 96% of the successfully treated cable remains in service and failure free.
More than 35 years ago, SRP began direct-buried installation of URD cables. Eventually, the utility determined that the increasing level of fault activity on the system would require some form of remedial action to ensure continuity of reliable service as the cable reached the end of its effective life. In 1992, SRP made an attempt to remedy the situation by installing URD cable in a rigid conduit system. However, faults on the direct-buried cable system continued to increase until it was no longer a manageable occurrence.
By 1996, the total number of 12-kV failures on direct-buried cable had increased four-fold to nearly 800 per year. Engineers predicted the faults on this aging cable would grow to more than 1500 faults per year in the near future. At that time, the standard practice was to replace each direct-buried cable segment with cable installed in conduit after three faults. However, with limited resources and the high cost associated with the replacements, an alternative was necessary to prevent the problem from turning into a customer satisfaction, maintenance and financial nightmare.
Cable-injection technology appeared to be one alternative to cable replacement. To study this option, maintenance personnel reviewed hundreds of samples of failed cable. While water-treeing was not a primary reason, the personnel found that numerous voids and contaminants in the insulation, as well as protrusions of the semiconducting strand shield, produced a significant number of failures. The utility concluded that if the imperfections in the insulation could be remedied by injection, costly replacement could be deferred.
In late 1997, SRP organized a pilot project to determine whether this injection technology could rejuvenate poorly performing cable, while also proving to be a practical and cost-effective method in the field. Of the 31 segments of failure-prone cable selected for this test, 70% were successfully injected. While the fault history indicated that at least four faults should occur on the injected segments during the following summer, no new faults occurred. At that time, the cost of cable replacement was about three times more expensive than the cable-injection trial.
As a result, SRP began a systemwide program to inject direct-buried, looped single-phase cables with the silicone fluid, which was enhanced by the guarantee from the manufacturer to reimburse SRP the full cost of the injection fluid if any successfully treated cable failed within 10 years. Today, UtilX Corp. (Kent, Washington, U.S.) guarantees injected cables for 20 years, and SRP's cost benefit for injection over replacement has improved to almost four times.
In selecting areas for cable injection, engineers first consider the fault history of the area through a weighting program that gives greater value to more-recent failures. The areas with the highest weighted value are then plotted in map form, with an individual segment's fault history added to the plot for review. If the fault history appears to be widespread across the area, the individual segments have less than three faults and most of the segments are less than 1320 ft (402 m), single phase and of looped construction, the area is considered a prime candidate for injection.
The flowchart in Fig. 1 shows the process when an individual cable segment is being evaluated in the field for injection. After a series of tests have been completed, a decision is made to inject, abandon or replace a blocked splice on the segment under consideration. If this evaluation confirms the cable is suitable for injection, the injection process is initiated.
In the testing phase, the cable segment is de-energized, tested, tagged and grounded, and clearance is taken from the distribution operations center. The electrical equipment at both termination points of the cable segment, and the cable itself near these termination ends, is then visually inspected, with the inspector looking for issues like a loose semiconducting shield, missing concentric neutral, connector damage, discolored insulation or broken cable strands that may prove the cable to be unworthy of injection. A time-domain reflectometer (TDR) test is performed to determine cable length and neutral condition, and to identify the number and location of splices in the segment (Fig. 2). Next, either injection elbows (dead-front application) or cable-injection adapters (live-front application) are installed.
The UtilX technicians perform an airflow and pressure test through special injection ports on the elbow, using nitrogen to determine if splices are blocked (Fig. 3). The CableCURE fluid will flow if splices are not blocked. There has been good success with injection through SRP's 200-A pre-molded splices from the direct-buried construction era (1968 to 1993). In a relatively small number of cases, SRP has decided to dig up and replace a blocked splice. Each situation is evaluated for cost and customer impact. If there is more than one blocked splice in a cable segment, or the location is not easily excavated, no attempt is made to inject the cable and the segment is abandoned.
INJECTING THE FLUID
During the injection phase, vacuum and feed tanks are attached to the same injection ports used for testing (Fig. 4). Desiccant and CableCURE dielectric fluid comes into the cable through the ports. The vacuum being applied to the cable helps accelerate the progress of the fluid through the cable and ensures a thorough fill. Fluid is injected at 10 psi to 20 psi (70 kPa to 140 kPa) and monitored for flow (Fig. 5). The injection time for the average 400-ft (122-m) #2 AWG conductor cable segment can take up to a day to complete. However, using low pressure allows the crew to re-energize cable segments being injected and continue the process on other segments. Once the fluid travels from the feed tank and reaches the other end of the cable segment, the vacuum tank is removed and replaced with a permanent cap.
The feed tank remains connected to the cable for a 60-day soak period, allowing the fluid to completely diffuse from the strands into the insulation, where it polymerizes with water and fills the voids in the insulation with dielectric fluid. After the soak period, the feed tank is also removed and replaced with a permanent cap.
Monthly reporting from UtilX's CTS system is downloaded into SRP's geographic information system (GIS) mapping product. Segments are marked as successfully cured or abandoned. Engineers refer to the GIS maps for planning and replacement purposes. This information also becomes valuable during the warranty collection process.
THE BOTTOM LINE
At SRP, the cable-injection program is headed by the cable asset manager and supported by a scheduler. A field inspector performs quality-assurance and quality-control activities after the work is completed by the UtilX crew, which consists of two injection technicians and two journeyman linemen provided by a local subcontractor. Almost all of the cable SRP has injected to date is #2 AWG URD, treated for about a quarter of the cost of replacement.
Since the start of the program, SRP and UtilX have worked accident- and injury-free while attempting to inject 6.8 million ft (2073 km) of cable. UtilX has been able to successfully cure 4.6 million ft of this cable, giving the program an approximate 68% (1402 km) success rate on attempted injection. Today, the remaining direct-buried primary distribution system consists of approximately 25 million cable ft (7620 km) of #2 AWG URD, 3 million cable ft (914 km) of 4/0 AWG and 8 million cable ft (2438 km) of 500-kcmil feeder cable. All of these cable types are stranded cross-linked polyethylene (XLPE) without strand block. All URD cable installed after 1984 is jacketed, but a majority of the direct-buried #2 AWG cable is unjacketed and predates this change in standard.
SRP continues to monitor the performance of successfully injected cable. While fault rates for successfully cured cable segments are approaching 4.5%, the program continues to meet expectations for the mitigation of faults in problem areas. A more-recent assessment of the program concluded that cable injection pays for itself in just five years. As a result, nearly half of SRP's successfully injected cable has already paid for itself and continues to provide value to the utility's bottom line. The deferral of the replacement cost for this cable would exceed US$180 million and has allowed SRP to focus its attention and money on other critical projects that would not have had available funding. In addition, it has deferred probable faults in these areas, and the resultant customer dissatisfaction, extending the life of an aging asset until such time that all direct-buried cable in SRP's system can be replaced.
Jacki Kenney Feldhahn (Jacki.Kenney@srpnet.com) manages the Cable Asset Program at Salt River Project. She has 30-plus years of utility management experience in the areas of distribution design, construction, streetlight management, and now primary and secondary cable replacement and primary cable injection. A graduate of Augustana College (Rock Island, Illinois), she was first employed by Iowa-Illinois Gas and Electric Co. (now MidAmerican Energy) for 10 years and has spent the last 20 years at SRP.
Rick Hudson (Rick.Hudson@srpnet.com) is a principal engineer in Line Maintenance Engineering at Salt River Project. Most of his 21-year career at SRP has been spent in the engineering, design and maintenance of the electrical distribution system. He received a BSEE degree from Arizona State University and is a registered professional engineer in the state of Arizona.
Editor's Note: In 2006, SRP first reported in T&D World about using cable-injection technology. Since then, the utility has almost doubled the amount of cable it has treated.