2 Chapter 1 Overview
NX UPS Single Module And Parallel System 250kVA ~ 800kVA User Manual
1.2.2 Bypass
The circuit block labeled static switch in Figure 1-1 contains an electronically controlled switching circuit that enables
the load to be connected to either the inverter output or to the bypass power source through 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
fault, the load is automatically transferred to the static bypass line.
During normal operating conditions, the inverter output and bypass supply must be fully synchronized so as to
achieve a clean (no-break) load transfer between the inverter output and static bypass line. The synchronization
between the inverter output and static bypass 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 maintenance bypass supply while the UPS is shut down for routine maintenance and repair.
Note
When the UPS is operating in bypass mode or on maintenance bypass, the load equipment is not protected from power failures,
surges, or sags.
1.2.3 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 BCB is closed, and the battery is in stable floating charge state.
(Parallel system)
Note: As the unit outputs are connected in parallel, the system checks that the inverter control
circuits are perfectly synchronized 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 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 operation control panel will display corresponding
alarm message.
Mains recovery
When the mains resumes normal within allowable time, the rectifier will start automatically (at this time its output
power will increase gradually) 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 battery from the UPS system for maintenance, the external isolating switch can be used. At this
time, except for the mains failure battery backup function, other functions and all the steady state performance of the
UPS will not be affected.
UPS single module failure
In case of an inverter fault, output fuse blowout and bypass SCR fault, the load will be automatically transferred to the
bypass, and the output power supply will not be interrupted. In this case, please contact the local customer service
center of Emerson for technical support.
(Parallel system)
In the event of a fault in a unit, the unit will be automatically excluded 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 the power requirements, the
load will automatically be transferred to the bypass mains.