A redundant (N+1) system is sized with an additional Multiple Module Unit. In this configuration, the loss of one MMU will not cause the system to drop the critical load. The malfunction of one of the Multiple Module Units will cause the Multiple Module Unit with the fault to be disconnected from the critical load. The remaining Multiple Module Units will continue to carry the load. After the Multiple Module Unit has been repaired, the unit can be reconnected to the critical load. The System can then resume redundant operation.
If more than one Multiple Module Unit is removed from the system and if the load exceeds the capacity of the Multiple Module Units that are remaining on-line, there will be an automatic transfer of the load to the bypass line without an interruption of the power. Any redundant Multiple Module Units can be taken off the critical load manually for maintenance without disturbing the critical load bus. In this case, the isolation switches are used.
Systems without a Redundant MMU
In a system without a redundant MMU, all the Multiple Module Units will supply the full rated load. If the Multiple Module Units malfunction, there will be a transfer of the load to the bypass line without an interruption of power. Any Multiple Module Units can be taken off the critical load manually for maintenance without disturbing the critical load bus.
In parallel operation, all of the inverter units automatically share the load at all times. The output current of individual Multiple Module Units will be no more than 15% unbalanced. The parallel load sharing function is programmed within each MMU. In (N+1) systems, the redundant MMU only shares the reactive current.
Each MMU has a self-diagnostic capability. If a failure of a Multiple Module Unit occurs, the faulty Multiple Module Unit will be removed from the critical bus. Selective tripping does not rely on information that is shared among Multiple Module Units.
A manual load transfer between the output of the UPS and the alternate bypass AC source can be initiated from the control panel of the System Cabinet. Manually initiated transfers will make the connection to the alternate bypass AC source before breaking the connection. These transfers utilize the output of the UPS and the bypass circuit breakers (system).
An automatic load transfer between the outputs of the inverter and the alternate bypass AC source is initiated if an overload condition is sustained for a time period in excess of the capability of the system output. An automatic transfer of the load may also be initiated due to a malfunction that would affect the output voltage. Transfers that are caused by overloads initiate an automatic transfer of the load back to the system again only after the load has returned to a level within the rating of the UPS. The internal programming of the UPS allows a transfer of the load back to the UPS up to three times. The transfers of the UPS back to the system are adjustable within ten minute periods in order to prevent cyclical transfers which are caused by overloads. On the fourth transfer, the system will lock into the bypass mode.
The static bypass switch will connect the bypass AC source to the load and the static bypass switch will close the bypass circuit breaker (system) in one of the following cases:
- inrush of Load current
- The fault (branch load circuit) is in excess of the rating of the system. The transfer of the load to bypass mode will be uninterrupted.
Protection and Backfeed Prevention
The critical output bus is protected from the flow of excess current fuses. Each phase of the bypass circuit is protected by the bypass circuit breakers. Monitors for blown fuses indicate when a blown fuse will prevent the path for the static bypass switch from being available for automatic transfers.
The static switch (bypass) will not feed the power back from the UPS to the distribution system (bypass) while the UPS is operating on flywheel mode during a outage of the source voltage for bypass. The static switch (bypass) is provided with redundant circuits that sense outages in the bypass power. The system for backfeed prevention operates even if two component failures exist simultaneously. If a shorted SCR is detected, the static bypass switch is isolated. An alarm message will be annunciated at the control panel on the System Cabinet. The load shall remain on conditioned power and protected power after the detection of a shorted SCR and after the isolation of the static bypass switch.