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Series 7400 Single Phase 1+N UPS
As previously mentioned, each of the two modules forming the
one-plus-one system is basically a standard single module 7400
series UPS. This section describes an individual module’s
operating principles - the effects of the additional parallel control
facilities on the standard module are described later.
The UPS basically operates as an AC-DC-AC converter (see figure
1). The first conversion stage (from AC to DC) uses a 3 Phase,
fully-controlled SCR bridge rectifier to convert the incoming mains
supply into a regulated 432V DC busbar.
The DC busbar produced by the rectifier provides both battery
charging power and power to the inverter section -which is of a
transistorised / IGBT based pulse width modulation (PWM) design
and provides the second conversion phase; i.e. reconverting the
DC busbar voltage back into an AC voltage waveform.
During normal operation both the rectifier and inverter sections are
active and provides regulated load power whilst simultaneously
float charging the battery. In the event of a mains power failure,
the rectifier becomes inoperative and the inverter is powered solely
from the battery. Critical load power is maintained under these
conditions until the battery is fully discharged, whereupon the UPS
shuts down. The end of battery discharge is assumed when the
battery voltage falls to 320Vdc. In case of 80 to 125 kVA, 1 PH
models the end of discharge is taken as 330Vdc.
The period for which the load can be maintained following a mains
power failure is known as the system’s ‘Autonomy Time’ and is
dependent upon both the battery A/Hr capacity and the applied
percentage load. It is usual in larger installations to provide an
alternative UPS input power source from a stand-by generator
when the mains supply fails. Once such a generator has been
brought on-line, and the UPS input power has been re-established,
the batteries immediately begin to recharge. Modern generators
can be started and brought on-line very quickly and where such a
facility is incorporated into the UPS installation it results in short
battery discharge periods and correspondingly rapid recharge
times.
Figure 1: Series 7400 UPS Single Module Block Diagram
1.2.2 7400 Module
Design
RECTIFIER
STATIC
SWITCH
INVERTER
BATTERY
Bypass
Supply
Mains
Supply
UPS
Output
Supply
(a.c) (d.c) (a.c)
(d.c)