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System Description
5
1.4 Control System Features
A key control feature necessary for successful momentary or continuous tie operation of two UPSs is
the dynamic load current sharing function.
A key design requirement for maximizing multiple system reliability and inverter availability in tied
configurations is assuring maximum system independence under the normal, separated operating
conditions.
Liebert has incorporated a number of unique solutions in your Power-Tie System to accomplish these
high availability/high reliability performance goals:
Digital Controls
Digital Controls provide more precise control than analog techniques. They permit the direct use of
fiber-optics for the most critical inter-system control functions without the need for the additional
complexity of A/D and D/A converters. Digital Controls are very reliable—implemented using inter-
nally redundant ASICs (application specific integrated circuits) with millions of field-proven trouble-
free operating hours—and provide rock-solid UPS load sharing and synchronization control under
dynamic load conditions.
Digital Phase Lock of Uninterruptible Power Module (UPM) Inverters
Parallel module inverters are all tightly phase-locked together by redundant digital oscillators. No
master oscillator is required. This feature, coupled with closely matched module output impedances,
provides the primary module-to-module load sharing function to approximately ±10%. This degree of
load sharing is good enough, under emergency conditions, to provide acceptable operation for system
load sharing and transfer operations.
System Output Voltage Control
A separate, independent and isolated system control function for output voltage which also makes a
secondary contribution to the load share function of an additional ±3%.
Module Load Sharing
An additional, separate, independent secondary control function contributes another ±3% of inter-
module load sharing.
These three independent control functions are additive in contribution to the module load share
function, and provide an additional level of redundancy to this function without simply duplicating a
circuit.
In combination, these three control functions provide module load sharing to better than ±5%. The
system will continue to operate acceptably well with the loss of one or more of these control functions.
The loss of which will be annunciated through the diagnostic system.
In a tied system configuration these critical functions, coupled with the Tie Control Unit, provide the
degree of load share control required, while maintaining system-to-system independence and
isolation.
1.5 Operational Description
1.5.1 Operational Choices
See Figure 3.
Momentary Tie provides intersystem transfer capability without bypass required, permitting on-line
maintenance of the unloaded UPS and distribution system.
Continuous Tie permits tying the available modules in both systems together to act as a single multi-
module system, in the unlikely event that multiple module failures have reduced the capacity of the
UPS system(s).