Anderson Island is a Small Island in South Puget Sound about 2 Miles from the Coast near the town of Lacey, Washington, U.S. The island is served by 2.3 miles (3.7 km) of splice-free 12.47-kV, 3-phase copper 4/0 ethylene propylene rubber (EPR) cable. In 2006, this cable had been in service for 30 years. At that time, Tanner Electric Cooperative (North Bend, Washington) realized the potential risk of lost service if the cable failed and began to look into what it could do to minimize this risk.

THE RISK OF FAILURE

In the late 1950s, the then 44 residents on Anderson Island, who obtained their electricity from diesel generators, approached the Rural Electrification Administration (REA) to see if it could develop a way to eliminate the generators. The REA contacted Tanner Electric to see if it could provide service. In 1961, the utility placed the first cable to serve the island, followed in 1975 by the present submarine cable, which has performed flawlessly.

Tanner Electric serves 4350 customers in North Bend, Ames Lake and Anderson Island. Although the cable has worked reliably all these years, failure at any time would be a significant problem because it serves one-third of the territory.

Reliability discussions about this cable, whether being discussed locally or at Tanner board meetings, have been ongoing for years. It is not merely a question of load, as this cable is operated at 20% of rated capacity in the summer. If the cable failed in the summer, the utility could back feed the island from rental generators as a temporary fix. However, if the cable failed in the winter, when the load is three times higher, it would not be possible to serve the total island load with available backup generators.

If Tanner had to repair this cable, it would likely take a minimum of 60 days to get a barge on location and perform the splicing operation. Such a deep-water splicing operation is also quite expensive.

POTENTIAL ALTERNATIVES

Tanner Electric investigated potential solutions to mitigate this risk, including the cost of installing a second feed. Initial estimates indicated it would take at least a year to perform the environmental studies at a cost of US$2 to $3 million. Because of the substantial time and cost constraints, the utility considered other options as well.

The utility investigated traditional cable-rejuvenation techniques that included a 20-year warranty. Upon further investigation, it determined the cost to be about 25% of the cost of replacement.

Novinium (Kent, Washington) presented Tanner with its second-generation cable-rejuvenation product. Tanner found the Novinium staff to be quite technical, with an in-depth understanding of the issues that must be addressed when using silicone injection for long cable lengths. Novinium put forth a proposal to inject the utility's 2.3-mile 4/0 EPR cable with a formulation tailored for the cable length in question. Historically, most cable that has been injected has been insulated with cross-linked polyethylene, but Tanner learned that EPR-insulated cables also could be successfully rejuvenated. Novinium tailors its formulation and adapts the injection process to optimize the treatment for EPR cable.

Before proceeding, Tanner contacted both Tacoma Power and Alameda Power & Telecom, utilities who had performed submarine cable-crossing treatments similar to what Tanner was considering. These utilities shared their experiences, which proved valuable in justifying this project.

TREATMENT DETAILS

Earlier injections on long cable crossings using traditional techniques have shown that as excess water in the conductor shield mixes with the silicone fluid during the process of injection, the viscosity of the fluid mixture increases. After several thousand feet, depending on the amount of water in the strand shield, the increased viscosity encountered increases pumping pressures until a point is reached where no more fluid can be injected and the rejuvenation process fails.

This time-dependent thickening or dynamic viscosity makes traditional treatment unsuitable for the longer-length underwater crossings. For the Tanner 4/0 EPR cable, traditional injection methods would not have been feasible. Novinium developed the N-Rex process for long submarine crossings.

The N-Rex process addresses this problem taking a two-step approach. The water is first removed from the interior portion of the cable by injecting a mixture of propylene carbonate and acetophenone. The propylene carbonate acts as a carrier and does not increase in viscosity as it encounters water. As the acetophenone soaks into the strand shield and the insulation from the inside out, it pushes water radially out of the cable. This water-exclusion process occurs because the polymeric layers of the cable are preferentially wetted by the acetophenone at the expense of the less-soluble water. The exclusion process continues until virtually all the water is removed from the inner half of the cable insulation. At this point the cable has dielectric strength close to that of new cable.

Once the water is removed, the inner half of the cable is dry. Next, the Ultrinium 732 fluid is injected to complete the cable rejuvenation process, enabling Novinium to guarantee an additional 40 years of cable life. The Ultrinium 732 fluid is a mix of chemicals that provides extended cable life, stress grading, hot electron absorption, ultraviolet protection and partial-discharge suppression.

The decision to rejuvenate the existing cable was made easier knowing that the treatment compounds are not flammable and do not pose an environmental hazard. When the injection is complete, the fluid is sealed inside the cable, so that the silicone fluid cannot degrade the terminations after injection.

Novinium presented a project plan providing a timetable to perform each step in the process. Leaks were located in the plumbing, requiring the utility to take an additional outage. But otherwise the project went smoothly. Novinium managed the project and worked with Tanner's local foreman on Anderson Island to complete the project in 121 days.

Tanner Electric is quite satisfied with the outcome of the rejuvenated cable project that carries a 40-year warranty. The project provided an attractive cost benefit when considering the loss of revenue, the cost to operate the diesel generators, the cost to repair the cables and the disruption to customers. The utility decided “buying an insurance policy” by rejuvenating the Anderson Island cable was a very good investment.

With this project completed, Tanner is planning an additional feed to Anderson Island and will implement this plan when funding is available.

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Steven W. Walter has been the Tanner Electric Cooperative general manager for two years. He has been in the electrical industry for 20 years having worked his way up from maintenance man, meter reader and then through the operations side of the business. steve@tannerelectric.coop