Emerson 480V user manual Bypass Supplies, Operating Modes, Normal Mode, Battery Mode, Bypass Mode

Operation

7.1.1Bypass Supplies

The Liebert NX contains an electronically controlled switching circuit that enables the critical load to be connected to either the inverter output or to a bypass power source via the static bypass line. Dur- ing normal system operation the load is connected to the inverter and the inverter contactor is closed; but in the event of a UPS overload or an 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 bypass 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.

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.

In dual input configuration, the neutrals of each source must be connected to each other, to guarantee that the dual source has the same voltage potential to earth.

7.1.2Operating Modes

The UPS is designed to operate as an on-line, double-conversion, reverse-transfer system in the fol- lowing modes:

Normal Mode

Operating in normal mode, the Liebert NX’s rectifier derives power from a utility AC source and sup- plies regulated DC power to the inverter, which regenerates precise AC power to supply the connected equipment. The rectifier also uses the utility source power to charge the batteries.

Battery Mode

When utility AC power fails, the Liebert NX protects the critical load by instantaneously channeling battery power to the inverter, which continues supporting the critical load without interruption. When utility power returns and is within acceptable limits, the Liebert NX automatically shifts back to Normal mode, with the rectifier powering the critical load.

Bypass Mode

When the Liebert NX is in bypass mode, the load is directly supported by utility power and is without battery backup protection.

The Liebert NX’s inverter static switch and bypass static switch will shift the load from the inverter to bypass mode without an interruption in AC power if the inverter is synchronous with the bypass and any of the following occurs:

•inverter fails

•inverter overload capacity is exceeded

•inverter is manually turned off by the user

NOTE

If the inverter is asynchronous with the bypass, the static switch will transfer the load from the inverter to the bypass WITH interruption in AC power to the critical load. This interruption will be less than 10ms. This interruption time may be altered by modifying the Output transfer interrupt time setting.

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