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Operation
75
6.0 OPERATION
6.1 Introduction
The Liebert NXa Uninterruptible Power System is connected between the Mains AC input source and
the critical load to provide uninterruptible power to the latter. The power from the UPS is free from
voltage and frequency variations and from disturbances experienced at the Mains AC input supply.
This is achieved through high frequency double conversion power pulse width modulation (PWM)
associated with full digital signal processing control (DSP).
As shown in Figure 63, the AC input mains source is supplied at Q1 and converted into a DC source.
This DC source feeds a DC/DC Bidirectional Battery Converter (that ensures the battery to remain
charged at all times) and at the same time feeds the Inverter that converts the DC source into a clean
and input independent AC source. The battery powers the load through the bidirectional battery con-
verter and through the inverter in case of an AC input mains power failure. In case of Inverter
unavailability or excessive overload, the load may also be powered from an external AC Bypass source
through input isolator Q2 and the static bypass. Besides this, if maintenance or repair of the UPS is
necessary, the UPS can support the load through the internal and manually controlled maintenance
bypass isolator Q3. With the exception of the maintenance bypass isolator, all the isolators shown are
closed during normal UPS operation.
Figure 63 Single unit block diagram with split-bypass input
!
WARNING
Hazardous mains and / or battery voltage present behind covers.
No user-serviceable parts are located behind covers that require a tool for their removal. Only
qualified service personnel are authorised to remove such covers.
1
Battery
Circuit
Breaker
2
Battery
UPS
Output
Battery
Converter
Input
Static
Switch
Input
Isolator
Q1
Rectifier
Inverter
Static Switch
Output
Isolator
Q5
DC Bus
Bypass Isolator Q2
Maintenance Bypass Isolator Q 3
UPS Module
Bypass
Mains
Supply
Input
Mains
Supply
N
Bypass Side
CB