The Big Easy?

Editor's note: This month we take a left turn, so to speak, into the civil world, examining an application of automated switchgear, not in an industrial or commercial setting, but on a lake-spanning bridge.

Those who have visited New Orleans know that Crescent City citizens draw their drinking water from a vast body of water known as Lake Pontchartrain. And even though New Orleans is renown as "the Big Easy," those living outside the city have the onerous task of traversing this lake across a 24-mi.-long causeway.

In fact, the structure—the longest overwater bridge in the world—has some 30,000 vehicles cross over it every working day. To better accommodate commuters , numerous cell phone towers and variable message boards have been erected along the bridge's length. And the need for reliable electric service for these communication devices is key to the bridge's owner-operator, the Greater New Orleans Expressway Commission, as the bridge serves as a major hurricane evacuation route.

Recognizing these factors, the commission initiated a $79-million capital improvements master plan in late 1995. One of the largest projects in the rehabilitation  program was the complete replacement and upgrade of the causeway's high-voltage electrical supply, first installed in 1956 when the initial span opened. The original supply consisted of a 4-kV feeder from Tammany Parish on the north  shore, and a 4-kV feeder from Jefferson Parish on the south shore. Each power supply was generated by separate utilitie s.

The open point between the two feeders was the bascule drawbridge located at the 16-mi. point of the structure. To provide manual sectionalizing in the old scheme, switchgear was located at each of seven traffic crossovers that connect the 80-ft. gap between the spans. These connections also function as emergency pullover areas.

But with the addition of the message boards and particularly the cellular antennas, the power loading levels were far beyond the original capacity. In fact, the voltage drop increased to 40% at the end of the feeders—insufficient to operate the four 100-kW motors that powered the bascule. As a stop-gap measure, a generator was installed to power the motors.

Besides an undersized power supply, the bridge's high-voltage cable itself was nearing the end of its useful life. The aerial cable, strung to a messenger wire under the bridge, began to experience an increasing number of faults due to insulation degradation.

Furthermore, in the original electrical system, there was no mechanism, either automatic or manual, for tying the two feeders together. In the event of a fault along one of the feeders, maintenance personnel could isolate the faulted section using the switchgear located at each of the seven crossovers. Service could then be restored up to the faulted section, but not beyond. If an outage occurred on the utility feeder, the load supplied by that feeder would remain out of service until the utility's repairs were complete. 

In assessing the situation, four essential components were identified:

• Increase the voltage level of the supply feeders.
• Replace all high-voltage cable.
• Provide the capability to tie between the north and south feeders.
• Automate fault isolation and restoration.

For the first part of this undertaking, the voltage supply feeders were increased to 24.9-kV with wye configurations, coming from both the north and southshore stations. One problem in working with two different utilities was that they have different distribution voltage standards. The issue, however, was addressed by means of step-up transformers.

Additionally, at each shore a loadbreak switch with a resettable vacuum fault interrupter (RFI) has been installed to provide fault isolation and sectionalizing between the utilities and the bridge. Along the bridge itself, nine other switchgear units have been installed to provide fault isolation and restoration. These units are also configured with the loadbreak switches along the mains. Depending on the specific loading, one or three 3-phase taps are protected by the RFIs.

Controlling the system, of course, was another major factor. A supervisory control and data acquisition (SCADA) master station, installed at the Causeway Commission's south shore maintenance facility, communicates to each switchgear unit loc ation via a second fiber-optic loop using a Mirror Bits DNP 3.0 protocol. A full-graphic operator interface with event recording and alarming is provided. Maintenance personnel can put the automated restoration system into a manual mode to temporarily modify the configuration or to return the feeders to normal after some automatic operation (see "SCADA at a Glance," below).

While the solution certainly breezes along from a reader's perspective, the project was far fro m easy. In fact, having to work 24 miles directly over water made the job unique.

The most significant of these challenges involved the following:

• Voltage level. One of the first decisions involved a determination of the most efficient voltage level to supply the bridge. A number of engineering studies and cost-benefit analyses were conducted to decide between 14.8-kV and 24.9-kV supplies. At 14.8 kV, the maximum steady-state voltage drops were calculated to be 7.5%, while at 24.9 kV they were calculated to be 1.5%. Both compare favorably to the previous 40% voltage drop. However, the 7.5% voltage drop proved excessive for a new system that may be required to take on additional loading in the future. The cost increase of moving to 24.9 kV was determined to be reasonable.
• Lack of real estate. The question of installing anything on a 24-mi.-long bridge is "Where do you put it?" A number of options were evaluated before designing the final solution. As with any replacement project, the existing facilities must remain in service during the construction, so the location of the old switchgear and aerial cable was not a choic e. Besides, the existing 5-ft. x 7-ft. x 4-ft. enclosures located at the crossovers were too small. An expansion of the crossovers was considered, but ruled too costly. Another option was the installation of a new platform between the spans. Getting a crane to that location to drive the pilings, however, was not possible. Installation of the switchgear below the bridge was considered as well, but that option was also infeasible because of instances of high water.

The solution lay in installing structures off the side of the bridge itself to hold the new automated switchgear. Complicating the choice, however, was the requirement that these structures be located 30 ft. away from the bridge to allow for future expansion. As a result, four 90-ft. pilings were driven into the lakebed to support a 15-ft. x 20-ft. concrete pad, upon which sits a fiberglass electrical distribution vault.

• Installation of new cables. As in placing the new switchgear, the same issue existed with the new high-voltage cable and new fiber-optic control cable. Various options were explored, including installation of conduit or hanging the new cable from a messenger wire. Neither solution, however, facilitated future expansion. In the end, a cable tray was designed that resides under the overhang off the side of the bridge.
• Automated restoration. To maximize the availability of the bridge, the project included a performance specification requiring the new high-voltage electrical system to be fully automated, both for fault isolation and for restoration of service (see "Breaking Down the Automated Control System," p. 46).

Developing unusual solutions was just the first part of the job. Extensive testing was also required. Fortunately, this was accomplished within a short nine months leading to factory acceptance testing (FAT) in September of 2001. Gulf Engineers and Consultants witnessed the work at the Canada Power Factory—the project's electrical supplier and system integrator—where all switching and fault scenarios in normal conditions, as well as fault scenarios in abnormal conditions, were tested thoroughly, including controllers, software and master station functions. The system performed well in all aspects, providing a high degree of confidence in the long-term success of the project.

Now, whether it's the daily commute or folks evacuating from a tropical storm, causeway officials are confident traffic—and power—will keep rolling.