Vertiv Liebert APM Plus - Bypass; System Control Principle

2.2.2 Bypass
Through the intelligent control of the ‘Static Switch’ module (as shown in Figure 2.1 on the previous page) containing the
controllable electronic switch, the loads can be supplied by the inverter or the bypass. In normal situation, the loads are
supplied by the inverter, in that case the automatic inverter switch at inverter side is closed. In the case of overload (the
overload delay time expires) or inverter failure, the automatic inverter switch is opened, and the Static Switch module will
automatically transfer the loads to the bypass.
In normal operating state, to realize the uninterrupted transfer between inverter and bypass, the inverter output must be
synchronized with the bypass.
Therefore, when the bypass frequency is within the synchronization range, the inverter control circuit will synchronize the
inverter output frequency with the bypass frequency and phase.
Besides, the UPS has a manual maintenance bypass switch (without switch excluded) for shutdown of the UPS upon
maintenance. In this situation, the bypass will directly supply the critical loads through the maintenance bypass.
N O T E: W hen the load is supplied by the bypass or m aintenance bypass, the quality of the pow er
supp ly will be uncertain.
2.2.3 System Control Principle
Normal operation
Normal mode: It means that the UPS has normal input mains, the rectifier and inverter operate normally, the load is supplied
by the inverter, the battery circuit breaker is closed, and the battery is in stable floating charge state.
(Parallel System) As the UPS single module outputs are connected in parallel, the system checks that the inverter control
circuits are perfectly synchronized with one another and with the bypass 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 synchronization is in progress.
Mains abnormal
When the mains fails or is abnormal, the rectifier will stop working automatically, and the system will transfer to battery output
(through inverter). The length of the operation time in battery mode depends on the load and the battery capacity. During this
period, if the battery voltage falls to the EOD voltage and the mains still has not been recovered, the inverter will stop working
automatically, and the UPS touch screen will display corresponding alarm messages. If the system uses split bypass
configuration and the bypass is normal, the system will transfer to bypass.
Mains recovery
When the mains resumes normal within allowable time, the rectifier will start automatically and supply the load and charge the
battery again. Therefore, the power supply to the load will not be interrupted.
Battery disconnection
To disconnect the external battery from the UPS system for maintenance, use the external isolating switch. At this time,
except for the battery backup function upon mains failure, other functions and all the steady state performance of the UPS
will not be affected.
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Vertiv™ Liebert® APM Plus User Manual