NonStop NS2200 Series Planning Guide

NonStop System with Cascading Rack-Mounted UPS and Data Center UPS
Figure 66 shows an example of a non-supported configuration in a NonStop system with the left
PDUs connected to the rack-mounted UPS, and the right PDUs connected to a data center UPS. To
create a cascading UPS configuration, the rack-mounted UPS is connected to the data center UPS.
Figure 66 NonStop System With Cascading UPS
A cascading UPS configuration presents potential problems. Problems attaining stability between
the pair of cascaded UPSs can cause unexpected and undesirable behavior. The control loops of
each UPS can interfere with the other.
A typical scenario where this behavior occurs is the failure of the smaller downstream UPS to
recognize a stable input from its upstream source. In the event of an upstream UPS failure or output
disturbance, the downstream UPS switches the load to battery. Once the upstream UPS regains
full function, the downstream UPS should recognize a stable input and switch to pass-through mode;
but this does not happen in all cases and some cases fail.
In a failing case, the downstream UPS fails to switch back to pass-through mode, instead running
from the battery until the battery set is drained. Once the battery is drained, the downstream UPS
must attempt to switch back up to pass-through mode. At the minimum, this leaves the downstream
UPS with depleted batteries.
208 UPS and Data Center Power Configurations