A UPS with the (N+1) Option is sized with an additional Multiple Module Unit. In this configuration, the loss of one Multiple Module Unit will not cause the system to drop the critical load. The Multiple Module Unit will be disconnected from the critical load when a malfunction occurs in the Multiple Module Unit. 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 and the Multiple Module Unit can resume redundant operation.
The load will be transfered to the bypass line without an interruption in the power 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.
Any redundant Multiple Module Units can be disconnected from 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 Unit malfunctions, the load will be transfered to the bypass line without an interruption.
In parallel operation, all inverter units automatically share load at all times. The output current of individual Multiple Module Units will be no more than ±5% unbalanced. Parallel load sharing is programmed within each Multiple Module Unit. In systems with the (N+1) option, the redundant Multiple Module Unit only shares the reactive current.
A manual transfer of the load between the output of the UPS and the alternate source of power can be initiated from the control panel on the System Cabinet. Manually initiated transfers are "make-before-break". A manual transfer utilizes the following devices: the output of the UPS and the circuit breakers for the bypass circuit (system).
Automatic Transfers of the Load
An automatic transfer of the load between the inverter outputs and the alternate source of power 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 are transfered back to the system again, only after the load has returned to a level within the rating of the UPS. The logic of the UPS will allow the UPS to transfer back to the system up to 3 times. The time period for the UPS to transfer back is adjustable within ten minute periods in order to prevent cyclical transfers which are caused by overloads. When the UPS attempts to transfer the load on the fourth time, the system will be locked out in bypass mode.
The static switch will connect the bypass AC source to the load. Additionally, the static switch will close the circuit breaker for the system bypass in one of the following cases:
- inrush of load current
- excessive load
The transfer of the load to bypass will be uninterrupted.
Each Multiple Module Unit has a self-diagnostic capability. If a failure occurs in the Multiple Module Units, the Multiple Module Unit that is faulty will identify the internal failure. This Multiple Module Unit will subsequently be removed from the critical bus. Selective tripping does not rely on information that is shared among the Multiple Module Units.
Communicating Between Multiple Module Units
Communicating between the Multiple Module Units is provided by the system I/O board. The Multiple Module Units use Category 5, 4 pair UTP-24 AWG shielded wire.
One Multiple Module Unit is the processor for the serial communications to the outside world. The system cabinet has a parallel cabinet interface board with a rotary switch that allows you to select the Multiple Module Unit that will be used for external communications. External communications can be done via one of the following methods: RS-232/RS-485 port, ethernet and modem.
Protection and Backfeed Prevention
The critical output bus is protected from the flow of excess current by the 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 switch from being available for automatic transfers.
The static switch will not feed the power from the UPS to the bypass distribution system while the UPS is operating in the flywheel mode during an outage of source power. The static switch is provided with circuits that sense a loss of redundant bypass power. Additionally, the static switch is provided with disconnect switches that become active when there is a loss of redundant bypass power. The backfeed prevention system 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 for the System Cabinet. The load will remain on UPS power after the detection of a shorted SCR and after isolation of the static bypass switch.