Case study: Moving to centralized power generators with utility backup
A centralized standby generator arrangement will be explored further in the following case study
An existing hospital campus is undergoing a master planning effort to determine strategies for growth and infrastructure improvements. The campus is currently fed from two utility sources, Circuit A and Circuit B at 13.8 kilovolts (kV). The two sources serve 15-kV switchgear that distributes power throughout the campus through underground duct banks with taps made in maintenance holes.
The health care buildings have double-ended unit substations that include 15-kV switchgear to allow for selection of either Circuit A or Circuit B. Each building has dedicated standby diesel generator sets serving transfer switches in each building. The generator sets and associated distribution is at 480/277 V, which matches the secondary side of the unit substation transformers in each building.
The master planning effort identified campus growth that would necessitate relocation of the existing utility service and the future building layout will require replacement of the existing 13.8 kV underground distribution. A new electrical service yard is planned for the center of the campus and new 13.8 kV underground distribution will serve both existing and future buildings. The route for the new duct banks is coordinated with the 20-year campus plan to avoid future disruptions and conflicts. Once the team solidified the utility plans, it moved the focus to campus backup and the existing standby generator sets.
The campus has a newer, 3-megawatt (MW) standby diesel generator set that is connected to utility Circuit A to provide campus backup of that circuit in the event of an outage. Circuit B is not connected to the campus backup generator. As a result, most of the buildings on campus are connected to Circuit A which has an average load of 1.9 MW while Circuit B has an average load of 0.5 MW.
The campus has a central utility plant (CUP) for heating systems, but the CUP is only fed from Circuit B. The master plan effort included a study to provide campus backup of Circuit B. A second 3-MW standby diesel generator set is planned, to be connected to Circuit B in new switchgear.
An understanding of phasing is required in order to mitigate long outages of campus power. The proposed project phasing is as follows:
Route new utility feeds from Circuit A and Circuit B to the new service location. Provide switchgear suitable for the full camps buildout.
Provide a new 13,8-kV, 3-MW standby diesel generator for backup of Circuit B. Provide necessary controls and switchgear to allow for campus backup.
Provide new 13.8-kV underground distribution across the campus, with new maintenance holes and duct banks. Provide new maintenance holes near each building feed.
Starting with buildings that already have two campus feeds, refeed the Circuit B side, then refeed the Circuit A side. These buildings will not experience an outage.
Refeed the remaining buildings, with coordinated outage plans as utility cables are switched over.
Once the campus is operating on the new 13.8 kV distribution, relocate the existing 13.8-kV, 3-MW generator to the new service yard and connect to Circuit A.
The existing distribution can now be demolished to make room for campus expansion.
The master plan effort includes two options for essential branch power for each building: centralized or decentralized. The first option is to maintain the current decentralized generator sets across campus and upgrade each as needed. New buildings would get local standby generator sets. For additional redundancy, tie breakers and tie feeders can connect essential branch distribution in adjacent/connected buildings. However, this method of providing redundancy is costly given the distances involved being between 1,000-1,500 feet and the necessary size of the tie feeders being 2,000-3,000 A.
The centralized option can be broken down into two sub-options, one of which uses the pair of 3 MW generators as both the essential branch power and utility backup power source. Providing separate standby diesel generators for centralized essential branch power is the typical approach. The operating voltage of the centralized generators can be 4,160 V or 13.8 kV, with associated distribution across campus in dedicated duct banks and maintenance holes at each building.
As is typical with centralized medium-voltage systems, new unit substations will be needed at each building to replace the existing decentralized generators. Because the existing generators are nearing end of useful life, this option is not cost-prohibitive because replacing the existing generators is necessary anyway.
The other centralized option, which uses the pair of 13.8-kV, 3-MW campus utility backup generators, saves both cost and space but has its own challenges. In a utility outage, the centralized generators need to prioritize the essential electrical branch first.
For example, when a Type 10, Level X is required by NFPA 110: Standard for Emergency and Standby Power Systems for a hospital, the transfer switches in each building need to transfer within 10 seconds for the life safety and critical branch transfer switches. After a predetermined amount of time has passed (usually 60 seconds) the various equipment branch transfer switches can transfer.
At this time, the centralized generator paralleling gear will evaluate the current load on the generators and, if the load is within predetermined values, the Circuit A transfer sequence will start. This will bring Circuit A online, powered from the centralized standby generators and will return “normal” power to the transfer switches fed by that circuit on the normal side.
Transfer inhibit signals are required in these transfer switches to prevent them from returning to “normal.” This inhibit signal is important because the Circuit A transfer sequence includes a “load shed” command which will remove the Circuit A load from the generators if they become overloaded (usually an under-frequency alarm). The same sequence and controls will follow for Circuit B, with the same load shed commands.
At this point, the entire campus will have power restored to the normal Circuits A and B, with all campus transfer switches in the “emergency” position. When utility power is restored, the first loads to transfer will be Circuits A and B. Then the typical retransfer to normal sequence will occur within the various transfer switches within each of the buildings.
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