Protective Relays Keep the Water Clean

Few services are as vital to a community as water treatment, and few treatment plants see the workloads that Denver's Metro Wastewater Reclamation District faces. As the largest water treatment plant between the Mississippi River and the West Coast, the facility serves a 380-sq.-mile area and handles some 150 million gallons of water per day.

09/01/2003


Few services are as vital to a community as water treatment, and few treatment plants see the workloads that Denver's Metro Wastewater Reclamation District faces. As the largest water treatment plant between the Mississippi River and the West Coast, the facility serves a 380-sq.-mile area and handles some 150 million gallons of water per day. And its purified outflow provides the South Platte River 90% of its annual flow.

In order to continue guaranteeing this service, the plant undertook a study and redesign of its power system. Wastewater plants are harsh environments for electrical equipment, because the high quantities of hydrogen sulfide present can corrode copper and silver-plated copper wiring.

Of particular concern were protection relays connected to several motors powering a number of 15-kV and 5-kV aeration blowers. If just one of these motors goes down, it can disable a part of the plant's treatment capacity. If undetected, it can result in a U.S. Environmental Protection Agency violation. The in-place industrial-grade relays were proving unreliable, with occasional relay and control logic failures.

Engineers from NEI Electric Power Engineering suggested a utility-grade replacement for medium-voltage applications such as the blower motors. In addition to added reliability, the new models incorporated the communications, monitoring and metering capabilities plant managers required.

The first relay retrofit was for an emergency that occurred during the redesign process when an existing relay failed. High cost estimates from the original relay's manufacturer, along with lengthy delivery lead-times, forced plant personnel to consider an alternative that NEI engineers had suggested. The replacement proved more robust and less expensive than the original. Features included a sequential events recorder for fault recording and analysis, along with integral metering and communications functions.

As planned-for improvements progressed, designers incorporated the new units into the plant's cogeneration facility. Methane produced during wastewater processing powers two gas-turbine units, producing about $1.6 million worth of electricity that the plant sells back to Public Service Colorado. The new relays and communications processors met all utility-interconnect requirements. Further, a new fiber-optic network will be installed soon to connect all the new equipment to a distributed control system.





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