Operation
76
6.1.1
Split-Bypass Input
Figure 63
illustrates the Liebert NX UPS in what is known as the
split-bypass
configuration
wherein a separate power switch to a dedicated bypass power source that also feeds the maintenance
bypass line connects the static bypass line. Where a separate power source is not available, the bypass
and rectifier input supply connections are linked.
6.1.2
Static Transfer Switch
The circuit blocks labeled
Static Switch
in
Figure 63
contain electronically controlled switching cir-
cuits that enable the critical load to be connected to either the inverter output or to a bypass power
source via the static bypass line. During normal system operation the load is connected to the
inverter; but in the event of a UPS overload or inverter failure, the load is automatically transferred
to the static bypass line.
To provide a clean (no-break) load transfer between the inverter output and static bypass line, the
static switch activates, connecting the load to bypass. To achieve this, the inverter output and bypass
supply must be fully synchronized during normal operating conditions. This is achieved through the
inverter control electronics, which make the inverter frequency track that of the static bypass supply,
provided that the bypass remains within an acceptable frequency window.
A manually controlled, maintenance bypass supply is incorporated into the UPS design. It enables the
critical load to be powered from the utility (bypass) supply while the UPS is shut down for routine
maintenance.
6.1.3
Battery Circuit Breaker
Any external battery is connected to UPS through a circuit breaker fitted inside the battery cabinet –
or located adjacent to the batteries where a battery cabinet is not used. This circuit breaker is closed
manually, but it contains an undervoltage release coil, which enables it to be tripped from the UPS
control electronics following certain detected faults. It also has a magnetic trip facility for overload
protection. The undervoltage release coil control is replaced by a battery contactor located inside the
UPS fitted with either internal batteries or with battery start option or both
6.1.4
Battery Temperature Compensation
For 30-40kVA UPS with internal batteries, a standard temperature probe is installed to measure the
internal battery temperature to optimize battery management. The measured temperature can be
displayed from the UPS front panel.
For UPS with external batteries, an optional battery temperature interface equally optimises the
external battery management by connecting up to four external temperature sensors from the battery
cabinet(s) to a control unit inside the UPS.
For details, refer to
Figure 27
.)
6.1.5
Redundant Control Power Supply Board
The UPS is equipped with two identical and fully redundant control power supply boards. Each of
them takes inputs from the AC and DC sources. When one of the sources or even if one of the control
power boards fails, the UPS system can still operate normally. This feature further enhances the reli-
ability of the system.
6.1.6
Socket Outlet
One single-phase Shuko-type universal outlet of 3A current handling capability provides nominal
UPS output voltage of up to 3A current capacity for the ease of testing, commissioning & servicing of
the UPS.
NOTE
When the UPS is operating in bypass mode or on maintenance bypass, the connected
equipment is not protected from power failures or surges and sags.
