Each independent power producer connecting to the grid seems to have a unique story to tell. In this case, two stories emerge. Entergy Wholesale Operations (New Orleans, Louisiana, U.S.) and PPG Industries (Pittsburgh, Pennsylvania, U.S.) each own a 50% share of RS Cogen, which owns a 425-MW cogeneration facility being constructed in Lake Charles, Louisiana.

The natural gas-fired, combined-cycle power plant will be in operation in late 2002. Half of the facility's electrical output will supply the PPG plant, and Entergy's power marketing group will sell the other half through the wholesale power market. Earlier, the Louisiana Public Service Commission (PSC) decided that power from cogeneration facilities that is consumed by wholesale owner-manufacturers is not subject PSC regulation. The facility will provide process steam to PPG's chemical plant as well as other industrial customers.

Within this bigger story exists an underground engineering challenge. Engineering constructor Black & Veatch, (B&V, Overland Park, Kansas, U.S.) was asked to develop a design to bring power from this new generating station to a substation undergoing refurbishment. Space was limited, making it difficult if not impossible to run the needed 69-kV line overhead using traditional construction practices. Instead, the company installed a 1000-ft (305-m) run of three-phase, 69-kV, 500 kcmil extruded-dielectric cable from the generating facility to a new transformer being added to the existing substation with very tight physical constraints.

The Cable Problem

Multiple existing overhead line crossings made underground cable the only viable solution. Still, existing underground facilities negated the opportunity for a continuous traditional trench. Instead the underground cable traversed a circuitous path above and below ground within existing facilities. In some locations the cable was placed 6 ft (2 m) below grade and in others the cable was placed 40 ft (12 m) above grade. Along the 1000-ft route, only 100 ft (30 m) of cable was encased in duct bank, with the remaining 900 ft (274 m) located in overhead cable tray.

In an area where the route crossed a major access road, B&V designed a shallow duct bank with a cable burial depth of 15 inches (38 cm) as opposed to a more traditional depth of 30 to 36 inches (76 to 91 cm). Heavy construction vehicles crossed this access road, so B&V needed a duct bank that could accommodate additional load and, therefore, increased the amount of rebar as well as the rebar layout. It also added a red dye to the concrete to reduce the potential for damage in future excavations.

On the substation grounds, many existing facilities were already placed below grade level, so constructing an additional duct bank was not feasible. B&V engineers decided to place the cable on an above-grade outdoor cable tray located on tall trestles.

The cable tray extended from the trestle, running adjacent with the terminator riser structure. Concrete sleepers (shallow foundation with wall to support tray) supported the tray the entire length. This solution greatly reduced excavation requirements. The cable tray was placed at a height such that workers could not easily walk on the cable or straddle the cable tray, which reduced the risk to workers, the cable tray and the cable.

With steam lines, electrical distribution lines and many other utilities paralleling the 69-kV cable, it became quite a challenge to maintain required electrical and physical clearances. Steam and electric utilities, in particular, can have negative effects on underground cable systems as they also generate heat. They were continuously making revisions to accommodate ever-changing trestle design and utility conflicts.

The route of the cable tray had become increasingly difficult because the utilities had to account for the 52-inch (132 cm) minimum-bending radius of the cable, resulting in the introduction of many horizontal and vertical bends into the design.

As with many projects related to independent generators, this project also has an aggressive construction schedule but the project is well under way and on schedule to meet the delivery date.

John Rector is a project manager in the Power Delivery Division of Black & Veatch. He has been with the company for 25 years. He is a senior member of IEEE and a voting member of the Insulated Conductors Committee (ICC). Rector received the BSEE degree from Kansas State University and is a registered professional engineer.

Shane Miller is an engineering technician in the Power Delivery Division of Black & Veatch. He has been in the underground transmission line group designing high-voltage insulated cable systems for six years.

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