Relays Bridge the Gap in Causeway's Power-Protection Plan

The 24-mi. Lake Ponchartrain Causeway provides a vital link between New Orleans and communities to its north. In addition to serving some 30,000 vehicles on any typical workday, it also is a primary evacuation route for coastal areas in the event of hurricanes and other severe storms. When modernizing the twin-span's infrastructure, the Greater New Orleans Expressway Commission recognized the need for a redundant power design incorporating the latest power-control capabilities.

The causeway's high-voltage system serves drawbridge operations, cell-phone towers, toll facilities and a series of variable-message warning signs that alert drivers during frequent fog conditions and other emergency situations. Engineers at Baton Rouge, La.-based Gulf Engineers and Consultants designed an automated distribution system to meet causeway needs.

The system is capable of automatic fault detection, isolation and restoration, with communications over fiber-optic cable. It includes 11 resettable fault-interrupting switches rated 275-kV, 600-amp continuous, installed every two miles in a loop plan. A three-way switch is located mid-span as a tie point, which is normally open. Power sufficient to support the entire causeway is available from either of the utility's serving the span's endpoints in the case that either utility experiences an outage. In the event of a fault between a sectionalizing switch and a load, the plan uses a resettable fault interrupter (RFI) to isolate the faulted cable section. If a fault occurs between two sectionalizing switches on the main cable, one of the shore-based switches isolates the bridge from the main power source, and then reconfigures the system and restores power to the isolated segment by feeding from the alternate source.

When selecting relays for the project, engineers were looking for flexible, programmable units, because designers knew they would need to create a customized database that could be spread across the entire bridge locally, rather than being centralized in a single location. The devices chosen incorporate patented technology that allows high-speed, point-to-point communication of relay contact status, and high-speed bus protection, sectionalizing, restoration and interlock schemes.

The relays were programmed using the manufacturer's proprietary software. Design specifications require power restoration within 30 seconds. If one of the two endpoint utilities goes down, the shore switch at that service end has to open and the midpoint tie switch has to close to allow the other utility to supply the bridge's entire needs. If the system isolates a fault on the bridge—whether in a current transformer bus, cable or switch—it is re-energized from the tie point back to where the fault has been isolated.