User Manual Liebert Hipulse E Chapter 7 - General Description
Single or '1+N' UPS System Design Concept
(07/04) Page 7-3
7.2.3 System Control Philosophy
Normal operation
During normal operation, i.e. when the UPS input supply is present and within specification, both the rectifier and
inverter sections are active and the static switch is turned on to connect the inverter output to the critical load busbars.
The battery circuit breaker is also closed and the battery is therefore permanently float charged at the d.c. busbar
voltage level.
(1+N Parallel UPS System) Note: As the unit outputs are connected in parallel, the System checks that the inverter
control circuits are perfectly synchronised with one another and with the Bypass Mains in terms of both frequency and
phase and that they have the same output voltages. Current supplied to the load is automatically divided among UPSs.
A warning message appears while synchronisation is in progress.
A module's static switch cannot close until these conditions are satisfied
Mains Failure
If the power mains has a failure or is out of tolerance the rectifier will stop automatically, while the Inverter will
continue to operate on power from the battery for a period of time which depends on the load and the capacity of the
battery. If the mains supply has not returned within this time, the Inverter will stop automatically and an alarm message
will appear on the UPS operator control panel display.
Critical load will not be interrupted in the event of a drop or return of the AC power mains.
Return of power mains
When the mains returns within the required tolerance, the Rectifier will start up again automatically and gradually
(power walk-in), supplying power to the Inverter and recharging the battery at the same time. There will be no
interruption of the critical load.
Input Power Walk-in
The rectifier/charger provides a feature that limits the total initial power requirements to 20% of rated load and
gradually increases power up to 100% of full rating over an hardware selectable time (slow or fast).
Off-Battery
If the battery system only is taken out of service for maintenance, it is disconnected from the rectifier/charger and
inverters by means of (an) external disconnect breaker(s). The UPS shall continue to function and meet all of the
specified steady-state performance criteria, except for the power outage back-up time capability.
Power Walk-in delay timer
On 1+N parallel systems, it is possible to select the start of each UPS after a power failure, by setting a suitable time
from the operator's display (ranging between 0 and 120 seconds). This function is particularly useful when a generator
is present on the input side because it enables to acquire power gradually.
UPS Module fault
In the event of an Inverter fault, the Static Transfer Switch will automatically transfer the load onto the bypass mains
with no interruption. In such an event, request qualified technical assistance.
(1+N Parallel UPS System) In the event of a fault in a unit, the unit’s Static Transfer Switch will automatically exclude
the unit from the system. If the system is still capable of providing the required load, the remaining units will continue
to supply the load with no interruption. When the units still present in the system are no longer capable of fulfilling
power requirements, the load will automatically be transferred onto the bypass mains. The load will be transferred with
no interruption if the Inverters are synchronised with the network; if this is not the case, there will be an interruption
lasting about 20 msec.
Overload
In the event of an overload at the Inverter output which lasts longer than the typical time/current (refer to Installation
Manual – Specifications), the Inverter will shut down and the Static Transfer Switch will automatically transfer the
load onto the bypass mains with no interruption. If the overload falls within the typical time/current that has been
specified, the load will be returned to the inverters when the power drops to a level which can be supported by the
number of active units in the system (parallel 1+N).
In the event of a short circuit in the output, the load will normally be transferred onto the bypass mains, which will
cause the Inverter to shut down; this switch is determined above all by the features of the protective devices in use in
the system.
In either case, an alarm message will appear on the UPS operator control panel display.
(1+N Parallel UPS System) The control logic system constantly monitors load requirements and controls the power
supplied by the UPS modules. In the event that an overload condition is sustained for greater than a preset time, the
load will transfer to the mains bypass supply, when the number of active modules are unable to satisfy load
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