Alternate Utility Facility – Port of Seattle, SeaTac International Airport – SeaTac, WA


Port of Seattle


SeaTac, WA

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Design Build Implementation of a Fully Automated 30MW Standby Power Generation Facility

Alternate Utility Facility – Port of Seattle, SeaTac International Airport – SeaTac, WA

The Alternate Utility Facility (AUF) primary focus was to provide a backup power source for the entire SeaTac Airport in the event the 115KV utility feed is disrupted. Special consideration was given to ensure the facility could operate if any single device was to fail. The facility was also built to withstand major earthquakes and lightning events. The AUF project was a Design Build project with Mortenson being the primary contractor on the project.

The Port of Seattle identified a 1.2 Acre site that the AUF was to be built. This site is situated approximately 1,200’ from the SeaTac 12.47KV substation that is the connection point to the SeaTac 12.47KV system. Casne designed and worked with the civil and structural engineers to layout ten (10) 3MW Cummins Generators on the 1.2 Acre site. The site also houses a 16 bay 12.47KV Switchgear building, 3MW load bank, and a control building that has quarters to house Port of Seattle employees for extended period of times. Casne designed a parallel 1,800A duct bank from the AUF site to the SeaTac Substation. The design was to allow for the AUF to feed either the A or B bus in the SeaTac Substation. To provide a soft transition (make before break) connection to the local service provide Puget Sound Energy (PSE), Casne designed an 8 bay 12.47KV ATS Switchgear Building that had the capability of transitioning the SeaTac load onto the AUF within 100msec. The ATS Switchgear Building is made up of 2 individual automatic transfer switches, one for each bus in the SeaTac Substation.

Casne implemented a centralized control system for the AUF site that includes redundant PLCs, a redundant ring ethernet network with hardwired IO for critical functions, and Modbus monitoring of non-critical systems. The architecture allows the central PLC to control and monitor the 3MW generators, switchgear breakers, power synchronization modules, battery monitors, the load bank, and ancillary building systems.

Casne wrote, tested, and commissioned a master PLC program that controls the AUF site and the ATS Switchgear Building. The program is responsible for monitoring the PSE feeds in the SeaTac Substation, starting up the generators if both PSE feeds are de-energized, switching breakers to connect the generator system to the SeaTac Substation, and shedding generators to match load. The PLC program ensures the 100 msec transfer time between AUF generators and the PSE feeds, and reacts to fault conditions by utilizing the inherent system redundancy. Casne also programmed HMI’s to provide the operator with manual system override capabilities, diagnostic trending, and to report on system events and alarms. This includes alarming if the power quality from PSE is outside of acceptable measurements. The Port of Seattle also requested that the PLC be capable of automatically running each generator through a monthly load test; Casne programmed the PLC to allow for automatic and manual load testing. The program will discontinue a load test if the SeaTac Substation loses the PSE feeds.

Casne performed the Arc Flash, protective coordination and short circuit study for the AUF site using the Port of Seattle’s modeling software, EasyPower. Casne coordinated with PSE to determine the appropriate settings of the main breakers to coordinate with the upstream breakers. From the main breakers, Casne identified the individual breaker settings for the 12.47KV, 480V, and 120V breakers in the AUF Site. Casne then programmed the SEL medium voltage breakers in the AUF Switchgear and the ATS Switchgear with the appropriate settings and parameters.







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We strive to provide engineering at the highest level of quality. We recognize that project success is the result of team members working together toward a common goal. Our team is comprised of individuals with the highest integrity. They are joined as a team with each other, the client and other engineering disciplines by commitment and a relationship based on trust. We believe that this attitude is necessary for successful outcomes.

Curtis Clute

Curtis Clute, President


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