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System Description
9
1.5.5 Modes of Operation
See Figure 4.
The Power-Tie Uninterruptible Power System is designed to operate in the following modes:
1. Normal - Each load bus is fed from its respective UPS and the tie breaker is open. Each inverter
is synchronized to its respective bypass source. Each load is supplied by its inverter(s), with its
bypass available as an alternate source.
2. Momentary Tie - The load bus and the two UPSs are momentarily paralleled through the tie
breaker. Both UPSs are on-line, and the bypass source of one UPS will be selected as the primary
sync source. When one UPS drops off-line, then operation reverts to the Combined Loads Mode
below.
3. Combined Loads - Both loads are running on one UPS through the tie breaker. The combined
loads are supplied by the selected UPS’s inverter(s), with that UPS’s bypass available as the
alternate and sync source. This mode is typically used for servicing a UPS system, including its
bypass and bypass transfer controls, without placing that UPS’s load on bypass.
4. Continuous Tie - The load bus and the available modules from the two UPS are continuously
paralleled through the tie breaker. Both UPSs are on-line, and one UPS’s bypass source will be
selected as the alternate bypass and sync source. Although not normally done from this
operational mode, one of the two UPSs could be shut down and isolated at this point as well.
5. System Isolation - A selected load bus which has had all load removed (via shutdown, external
maintenance bypass or downstream critical load switchover devices—dual input PDU, STS or
ATS) may be isolated from one or both UPSs for maintenance.