Case study: Defining hospital loads
An example of developing the system architecture of the PSG system using a hospital campus project.
When developing the system architecture of the PSG system, it is important to understand the types of loads served and how they are segregated and configured. A hospital campus project will be used as an example of segregating and configuring loads.
The campus energy center and first four phases were developed by another engineering consultant. The project included a central standby generator system to serve all future phases. Another consulting firm was going through final testing of the system when JBA Consulting Engineers started the design for its phase of the project. There were challenges with the initial sequence of operations and closing the required number of generators onto the PSG generator bus to supply the life safety systems within the code-prescribed 10-sec timeframe.
The configuration of the downstream systems required all of the PSG feeder breakers to close within 10 sec because each radial feeder served several life safety ATSs via unit substations that also served critical, equipment, and optional ATSs. Each of the four radial feeders out of the PSG served, on average, four unit substations each rated at 1500 to 2500 kVA. This amounted to approximately 8 MW of total transformer inrush on the generators when each feeder breaker closed. If all of the generators were synchronized and closed into the bus, this was achievable, but not within the 10-sec requirement. Ten sec was not enough time to include the delays required to keep the generators that were currently synchronized from being subjected to excessive inrush current resulting in generator shutdown to protect them from damage.
Options for modifications to the PSG system were limited. Fortunately, the campus energy center had been provided with unit-mounted battery packs in all egress lighting and all life safety systems included integral battery backup. All of the life safety systems in the energy center were reclassified to the equipment system. The energy center was served entirely by one of the four radial feeders out of the PSG, and reprioritizing the energy center feeder to a lower priority enabled the balance of the system to meet the 10-sec timeframe.
A more prudent approach for this system would have been an aggregate generator bus and separate load buses. This would have avoided the challenges experienced while also improving system reliability by reducing (and in the best case, eliminating) single points of failure. See Figure 5 for an example configuration. Initially, the system could operate as two separate systems to bring the life safety systems online within 10 sec. As more generators are synchronized and the two separate generator buses close together, the remaining ATSs can transfer load onto the system. With separation for the feeders between separate load buses, the generators are subjected to lower levels of transformer inrush current.
Robert R. Jones Jr. is an electrical project engineer at JBA Consulting Engineers with more than 10 years of design experience. Jones has experience in market sectors including hospitality, commercial, medical, and government projects. He specializes in medium- and low-voltage distribution systems, emergency/standby power systems, renewable energy design and implementation, circuit analysis calculations, and equipment space planning.
See how generators and transfer switches are used for mission critical facilities below.