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Availability concerns are very
familiar to data center managers. It
comes down to simple economics:
given the high cost of downtime,
availability naturally became the
most important metric on which
data centers were evaluated in
recent years. It wasn’t uncommon
for data center managers and
corporations to operate under an
“availability at any cost” approach.
This mindset also dominated the
selection criteria for uninterruptible
power supply (UPS) systems.
New technologies and increasing
demands are driving changes in the
power systems that support data
centers. While the importance of
availability has in no way
diminished, it has been joined by
two new metrics: efficiency and
flexibility (or scalability), as data
center managers increasingly
struggle to keep pace with growing
capacity needs while working under
the constraints of tightened budgets
and energy efficiency initiatives.
This article is designed to provide
data center managers with a clearer
understanding of key factors and
considerations involved in selecting
the right UPS to meet current and
future needs, and optimize the
performance of their facilities.
UPS ToPoloGy
UPS topology, along with power
system architecture, impacts data
center availability and efficiency.
Because the power system uses a
relatively small amount of total data
center power and is so critical to
ensuring uptime, more robust
topologies are generally preferred,
especially because continued
improvements in power technology
allow robust power systems to
deliver high efficiency.
UPS internal topology
(configuration) determines the
relationship between the UPS
output power quality and incoming
utility power and, ultimately, the
effectiveness of the UPS at
protecting against certain types of
power disturbances. Topology is a
primary determinant of system
reliability and functionality, two key
elements of availability. Historically,
there have been three basic types of
UPS topology used in the data
center: passive standby, line
interactive and on-line double-
conversion.
PASSIve STAndBy
The simplest type of UPS is the
offline or passive standby topology
(Figure 1). Under normal operating
conditions, AC power from the
utility passes straight through the
UPS to the critical load. A charger
converts AC power to DC to
charge the battery. The inverter is
7X24 MAGAZINE FALL 2014
By Peter Panfil
Key FACToRS To exAMIne when SeleCTInG A UPS To MeeT PReSenT And FUTURe ReqUIReMenTS
seleCting the Right
uPs
fOR yOuR data CenteR
