Underground transmission installations recently have become more commonly used for solving difficult river crossings in the utility industry. Duke Energy Progress’ installation of a 230-kV underground transmission cable crossing was not an industry first nor an evolution of the design process. What made this project special was the utility had no history with underground transmission in this service territory.

The project team rightly anticipated pumping plant maintenance would be both a new and ongoing task for the local electricians and relay technicians, so the employees accountable for maintenance were included in the engineering and design process to specifically address the project from an after-construction perspective. A conscious effort was made to consider and evaluate options that directly impacted the field personnel and maintenance of the line after construction was complete.

Justifying the Need

Potheads at Town Creek

In early 2009, Duke Energy Progress identified the need to mitigate the potential impact of a common tower failure on two 230-kV lines crossing the Cape Fear River in Wilmington, North Carolina, U.S. The two lines originate from the Brunswick Nuclear Plant and remain on separate structures except for the mile-wide river crossing on two 330-ft (101-m)-tall double-circuit towers. Failure of either of these structures would result in a significant output reduction to each 940-MW unit at the plant that could take several months to repair.

The project objective was to remove one circuit from the towers, cross the river at a new location and tie back into the original lines on each side of the river. Multiple overhead routes were considered, but, in the end, the utility decided an underground line would have less overall impact from easement acquisition, which would involve a lengthy environmental permitting process and acceptance from the U.S. Coast Guard, the U.S. Army Corps of Engineers and the Port of Wilmington. Although the cost estimates for the underground crossing were marginally higher, overhead concerns justified choosing the underground option.

The design decision was made to install a high-pressure fluid-filled (HPFF) pipe circuit, consisting of two pipes for rerouting the line. Two pipes were necessary because of the high line ampacity. The cable used for the crossing was 2500 MCM copper.

One interesting aspect to the project, the original overhead circuit crossing the river was to be left in place and act as a backup in the event the underground circuit needed to be de-energized. To facilitate this, two line switches were installed on each side of the river to allow transferring the load between the overhead and underground circuits.

Designing Through Collaboration

Inspecting cable pipe welding and weld coating

Duke Energy Progress used outside engineering expertise to complete the underground pipe, cable and pumping plant designs, while using internal transmission line, substation and relay engineering resources to integrate those designs. An independent consulting engineering firm provided technical oversight, and Duke Energy Progress transmission engineering personnel performed project management, environmental permitting, transition station/substation site design and overhead transmission design functions in house.

What occurs so often when utilities install new products, pieces of equipment or, in this case, an underground transmission line is this: engineering work is completed, projects are bid and awarded, contractors complete the installation and demobilize, the invoices are paid, the line is energized and, with a resounding yes, the project is considered complete. Often, the personnel charged with maintaining the system are left with the task of learning how to maintain the system on their own. This project proved to be an exception.

Duke Energy Progress’ transmission department includes an asset management group responsible for providing direct engineering field support to crews, creating preventive maintenance programs with justified inspection frequencies, writing specific maintenance tasks and related instructions, assisting with the development of training programs and providing guidance in specifying critical spare parts needed for field maintenance work groups in the department. Personnel from this group interfaced with the hands-on maintenance crew to identify areas that most impacted future periodic and preventive maintenance activities.

After making the decision to go with underground transmission on this project, and having no experience with that technology in North Carolina, Duke Energy Progress analyzed its Florida underground system and used best practices to integrate the two system philosophies.

Underground cable

At the start of the project, the utility had no previous experience with underground transmission technologies. Without legacy standards, practices and procedures in place, this allowed for the integration of the maintenance organization into the design process. After the merger with Florida Power Corp. in 2001, Duke’s technical field support personnel, who supported Florida and the Carolinas, assessed the underground transmission system located in St. Petersburg, Florida. This exposure made all the difference and prompted many discussions concerning options and preferences during the Cape Fear crossing design.

The Florida system was constructed primarily in the 1960s and includes three vintage Jerome pumping plants. Although the system is aged, it operates remarkably well and continues to be quite reliable. This assessment ensured the support personnel were prepared to contribute to the design discussion. A conscious decision was made to incorporate the best aspects of the Florida system into the Cape Fear crossing and improve on shortcomings.