ABB HiPerGuard MV UPS User Manual page 25

The process is the same for a single or several generators connected to the MV UPS
input busbar.
The minimum walk-in time is defined by the capacity of the generator (or generators) to support the
load based on the frequency error. There will be a tradeoff between how fast the generator governor
responds to a frequency error and the maximum frequency error the load can tolerate. Typically, 2% of
the nominal frequency is chosen as a suitable tradeoff for fast generator response while having minimal
disturbance on the load.
Additionally, the maximum duration of the walk-in is limited by the capacity of the energy storage.
The MV UPS will share the load with the generator until the energy storage is depleted. If the generator
cannot successfully support the load during the walk-in procedure, and the energy storage is depleted,
the MV UPS will transfer the load to the auto-bypass breaker,
3.2.5.1. Single common generator bus
A common configuration is to have multiple parallel generators connected in parallel to create a single
common generator bus, which when the grid is lost, the entire site switches over to the generator bus.
The single bus may feed both numerous different loads and MV UPSs.
Figure 3.6: Single common generator bus
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