Filter
Top Products
System Description
7
The Tie Control Unit also contains interlock and monitoring circuitry necessary to assure successful
transfers of load between UPS systems. These consist of the following:
1. Sync Verification circuitry, similar to the circuitry employed in each UPS, provides the sync check
function in two independent modes:
a. Phase Difference detection for ±3 degrees.
Phase Difference inhibits a transfer if limits are exceeded.
b. Voltage Difference detection for ±3%.
Voltage Differential inhibits transfers if limits are exceeded (if phase error is greater than ±3
degrees, the Voltage Difference will also exceed ±5%), making the differential voltage detec-
tion a redundant phase detector.
2. Load Verification circuitry prohibits transfer if the resulting total system load will exceed the
capacity of one UPS System (the system which will assume all the critical load once the transfer is
complete).
1.5.3 Continuous Tie
See Figure 3.
The Liebert Continuous Tie system is a topology that extends the concept of a manually initiated
momentary tie of UPS critical load buses by providing the capability to continuously parallel the out-
put buses of two otherwise independent UPS Systems.
All the performance features of a momentary tie system are available in the continuous tie system. In
the unlikely event that one UPS system or both UPS systems have only partial available capacity
(e.g., loss of one or more modules in an otherwise functional system), this partial capacity system can
be continuously tied to a second full (or partial) capacity system, powering simultaneously the now-
tied two critical buses.
This results in a single, fully redundant UPS system powering two different distribution strings
through a single UPS critical output bus. When the off-line (whether for maintenance or repair) mod-
ules are brought on-line again, the two UPS critical buses are again separated, all in a make-before-
break manner, and without the need to transfer to bypass sources. For continuous-tied systems, mod-
ule kVA/kW ratings must be identical.
There are two main control functions associated with the Continuous Tie:
• Intersystem synchronizing and load sharing;
• Intersystem transfer/parallel control and switch gear.
Intersystem Sync and Power Sharing
Synchronization between the UPS systems is accomplished in a two step process:
First, a line sync source is chosen. This will be the bypass power source of the UPS system that is the
manually selected preferred bypass source. Generally, if one system is to be shut down for mainte-
nance after the two systems and loads are paralleled, the preferred bypass source is the system
remaining on line during the maintenance activities. Both systems are synchronized to the preferred
bypass.
Second, once both UPSs are in sync with one bypass, inter-module sync is enabled. This is a digital
sync bus that holds all modules of both systems in phase-lock operation, with or without a bypass ref-
erence. This ensures that the individual modules of both systems are tightly controlled to enable par-
alleling.
Load sharing between UPS systems is accomplished by interconnecting the load sharing circuitry—
the combination of the System Output Voltage Adjustment Control plus the Module Load Sharing
Control—of the two systems when the tie breaker between the systems is closed. This operation is
analogous to bringing a module on-line in a multi-module system
The voltage synchronizing and load sharing loops also back up each other. Together, they hold module
load sharing to within ±5% of load current. Should one or the other loop be inoperative, load sharing
can still be maintained within acceptable limits.