Case study: University of Wyoming energy plant upgrade
The University of Wyoming decided to upgrade its aging normal and standby power system at its central energy plant. This plant produces chilled water with redundant chillers and steam with three coal-fired boilers, which support the Laramie, Wyo., campus’ energy needs. As suggested by the name, the central energy plant is the dominant energy source for the campus, so a robust standby electrical power system is crucial.
A study was performed to determine the best approach for the upgrade. Several options were evaluated to determine the project’s characteristics including generation capacity required, fuel for standby generation, distribution voltages, and the location of the generators. A load study was performed using historical data from existing metering and the university’s anticipated load growth.
The new standby system has to carry the entire plant load, in both the summer and the winter, so around 2500 kW of capacity is required. Diesel generators were chosen over natural gas because they are much more cost- and space-efficient at the capacity required. Plus, the facility has an existing bulk-diesel storage tank that could be leveraged for the new system.
A 480-V paralleling switchgear was chosen because space is constrained, making medium-voltage systems more difficult to implement. Additionally, the plant loads are primarily 480 V, so 480-V generation minimizes the number of transformers and breakers between the generators and the loads, increasing reliability.
The paralleling switchgear was arranged in a main-tie-main configuration to accommodate redundant utility sources (Figure 3). Two new 13.8-kV/480-V utility transformers comprise the normal supply for the system (Figure 4).
Next, it was determined to locate the generators inside the existing building. Space in the existing building is limited and could not accommodate the length required for a single, larger generator, so two paralleled 1,250-kW units were used to meet the capacity requirements (Figure 5). This also has an added reliability benefit because, depending on the plant load at a given time, one generator can supply all the power required. The study considered environmental requirements that depend on the anticipated hours of operation and the application type (for example, this application was standby as opposed to continuous).
After the study solidified major characteristics, detailed design could begin. Large variable frequency drives exist at the plant, requiring an assessment of potential harmonic impacts. Additionally, Laramie is 7,200 ft above sea level, requiring machine-power derating. A 4-wire system was selected with paralleled neutrals and a single-point ground. This single-point ground was relayed for ground-fault protection.
A master control cabinet was required for the application. Automatic load-demand functionality was implemented, which keeps only the generation required to carry the load online. This helps to optimize operation and prevent problems associated with running generators at low loads, such as wet stacking.
The master control cabinet also facilitates load-shedding and load-adding automation, which ensures that critical loads are prioritized in the event of diminished generator capacity if, for example, one of the generators fail to start. Smart devices in the generator control cabinets, master control cabinets, and switchgear are all networked together for connection to the plant’s distributed control system (DCS).
The plant had to remain running during the construction process, so careful planning was required in addition to close coordination between the contractor and the plant operators. To maintain power to the plant load, the existing utility transformers had to be replaced one at a time. First, the secondary utility transformer was removed. Then one of the new transformers was installed in its place, at which time the load could be switched over to the new transformer. Finally, the other existing transformer could be replaced, establishing the redundant utility feeds.
The owner, contractor, engineer, and equipment manufacturers collaborated to commission the system. First, onsite portable load-bank testing was performed on each individual unit. Next, the whole system was tested with the plant load. This process had to be closely coordinated with plant operations as to not impact steam production for the facility. When a power loss at the plant was simulated to test the standby system, steam-header pressure would decrease. This was tolerable for only a short time, so the system had to remain in steady state for a period of time to re-establish pressure before testing could resume.
Open communication and cooperation among the owner, engineer, contractor, and equipment vendors were critical for this project. All stakeholders played their part to provide the facility with a robust paralleled generation system.
Joseph Thornam is a principal electrical engineer at Stanley Consultants.
Stanley Worcester is the chief electrical engineer at Stanley Consultants.