A redundant (N+1) system is sized with an additional MMU. In the MMU configuration, the loss of one MMU will not cause the system to drop the critical load. The malfunction of one MMU will cause the MMU with the fault to be disconnected from the critical load. The remaining units will continue to carry the load. After the MMU has been repaired, the unit can be reconnected to the critical load. The system can then resume redundant operation.
There will be an automatic transfer of the load to bypass without an interruption of the power if both of the following conditions exist:
- If more than one MMU is removed from the system
- If the load exceeds the capacity of the units that are remaining on-line
Any redundant 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 units will supply the full rated load. If the units malfunction, there will be a transfer of the load to the bypass line without an interruption of power. Any MMU 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 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 an MMU occurs, the faulty MMU will be removed from the critical bus. Selective tripping does not rely on information that is shared among units.
A manual load transfer can be initiated from the control panel of the system cabinet. The manual load transfer can be made between the output of the UPS and the alternate bypass AC source.
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. The transfers utilize the output of the UPS and the bypass circuit breaker (system).
An automatic load transfer between the outputs of the inverter and the alternate bypass AC source is initiated for the following reasons:
- If an overload condition is sustained for a time period in excess of the capability of the system output
- A malfunction that would affect the output voltage
The following rule applies to transfers that are caused by overloads.
- Automatic transfer of the load back to the system will occur 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. Transfers of the UPS back to the system are adjustable within 10 minute periods. The adjustable time is use 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 will close the bypass circuit breaker for:
- Inrush of load current
- Fault (branch load circuit) that is in excess of the rating of the system. The transfer of the load to bypass mode will be uninterrupted.
Protection and Back Feed 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 provide power from the UPS to the distribution system (bypass) while the condition exists:
- The UPS is operating on flywheel mode during an 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 back feed 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 for the following reasons:
- After the detection of a shorted SCR
- After the isolation of the static bypass switch