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System Description
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Synchronizing activity takes place automatically once the preferred source is chosen and a manually
initiated “SYNCHRONIZE SYSTEMS” signal is given. Visual indicators show that the systems are
synchronizing and when the systems are ready for the manually initiated, automatically controlled
paralleling sequence.
Continuous Tie/Transfer/Parallel Control
The transfer load and tie controls operate the tie breakers between systems in a sequential manner to
allow paralleling and make-before-break transfer of loads. Once the synchronizing sequence is com-
plete, visual indicators show parallel operation is permitted. The operator initiates the closing of the
tie breaker. Once initiated, the tie command is executed when conditions of sync and voltage match-
ing are verified prior to tie execution. In essence, the tie breaker closes and two UPS systems are par-
alleled.
With the two systems operating in parallel, only one bypass circuit is active. Should something occur
that requires the UPS to automatically transfer to bypass, then all the load will be supplied through
the static switch and bypass circuit breaker of the system that was selected as the preferred source.
When the two systems are paralleled, it is possible to isolate and then shut down one system entirely
for maintenance. The system to be shut down must be the one that is not the designated preferred
source. Shutdown is accomplished by first opening the System Isolation Breaker of the selected sys-
tem. Once this action is taken, all the load remains on the preferred system.
The three load sharing control circuits between systems are disconnected with the opening of the Sys-
tem Isolation Breaker. At this time, sync control of the off-line system will be returned to its own local
bypass source, and the system can be operated in its normal manner for testing and maintenance.
Restart of the systems under maintenance is accomplished in the normal manner. Once the system
has been brought on-line, the resync to the on-line system can be initiated. The sequence from this
point is similar to the sequence that paralleled the two systems initially. As soon as the OK TO TIE
SYSTEMS indication is given the System Isolation Breaker can be closed, once again paralleling the
two systems and enabling the load-sharing loops between the two.
Taking the systems out of parallel operation is manually initiated. The tie breaker automatically
opens separating the loads onto their respective UPS systems. The intersystem power sharing is dis-
abled and the sync circuits revert to independent operation.
1.5.4 Tie System Components
See Figure 4.
A Liebert Power-Tie Uninterruptible Power System consists of the following basic components:
1. Two complete, independent UPSs with individual load buses, each with the capability to source
the combined critical load of the two load buses. Each of the two UPSs may be a single-module
system (SMS) or a multi-module system (MMS).
2. Two discrete system input sources, each with the capacity to source the combined critical load.
The two input sources need only be acceptable with regard to frequency and voltage. The two
sources do not require an in-phase relationship, although under this circumstance, a “V Lockout”
alarm will be indicated until the “Sync Systems” button is pushed. Then the “V Lockout” should
clear.
3. Two discrete bypass sources, each with the capacity to source the combined critical load. The two
bypass sources need only be acceptable with regard to voltage and frequency, such as the utility or
an emergency generator, and are not required to be in-phase. However, if make-before-break
downstream load switching equipment, such as automatic transfer switches, static transfer
switches (STS) or dual input power distribution units, is intended to be used, having the two
bypass sources in sync is preferable.
4. One system isolation and tie switchboard containing system isolation breakers and tie breaker(s).
5. Tie Control Unit, wall or floor mounted or installed in the tie switchboard.
6. Optional maintenance bypass breakers and test load distribution.