Alternate Utility Facility

Location: Seattle, Washington

Project Description 

The primary focus of the Alternate Utility Facility (AUF) was to provide a backup power source for the entire SeaTac Airport in case the 115 kV utility feed was disrupted. Special consideration was given to ensure the facility could operate if any single device failed. Additionally, the facility was built to withstand major earthquakes and lightning events. The AUF project was a Design-Build project, with Mortenson as the primary contractor.

The Port of Seattle identified a 1.2-acre site where the AUF was to be built. This site is approximately 1,200 feet from the SeaTac 12.47 kV substation, the connection point to the SeaTac 12.47 kV system. Casne designed and worked with the civil and structural engineers to lay out ten (10) 3 MW Cummins generators on the 1.2-acre site. The site also houses a 16-bay 12.47 kV switchgear building, a 3 MW load bank, and a control building with quarters to accommodate Port of Seattle employees for an extended period. In addition, Casne designed a parallel 1,800 A duct bank from the AUF site to the SeaTac Substation. The design allowed the AUF to feed the A or B bus in the SeaTac Substation. Finally, to provide a soft transition (make before break) connection to the local service provider Puget Sound Energy (PSE), Casne designed an eight-bay 12.47 kV ATS switchgear building with the capability of transitioning the SeaTac load onto the AUF within 100 msec. The ATS switchgear building is made up of two 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 3 MW 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 the 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 HMIs to provide the operator with manual system override capabilities, diagnostic trending, and reports 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 also performed an arc flash, protective coordination, and short circuit study for the AUF site using the Port of Seattle's modeling software, EasyPower. They 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 12.47 kV, 480 V, and 120 V 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.

The Alternate Utility Facility project was completed in 2018 and was a successful example of a Design-Build project. The facility provides SeaTac Airport with a reliable backup power source, ensuring that the airport can continue to operate in the event of an unexpected power outage. The AUF's design allows it to operate even if a single device fails, and it was built to withstand significant earthquakes and lightning events.

The Port of Seattle and Casne Engineering collaborated to create a highly efficient and dependable facility that will serve the airport for years to come. The centralized control system implemented by Casne provides the operator with the capability of overriding and monitoring the AUF's systems, ensuring that it continues to operate at peak performance. The AUF project is an excellent example of how collaboration between a contractor and a client can result in a successful outcome.