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Operation Description
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2.4 Combined Loads on One System from Continuous Tie Operation
To combine two loads onto one system, perform a LOAD TRANSFER BETWEEN UPS SYSTEMS
(2.3 - Load Transfers Between UPS Systems). The operator selects the system he wants to main-
tain both loads using the TRANSFER LOAD TO buttons. The operator can then proceed with the rest
of the procedures in 2.3 - Load Transfers Between UPS Systems.
2.5 Combined Loads on One System to Continuous Tie Operation
To create a continuous tie from combined loads on one UPS system, perform a CONTINUOUS TIE
operation (see 2.1 - Continuous Power-Tie).
The Power-Tie logic selects the proper circuit breaker to operate. Once the tie operation begins, the
two UPS systems synchronize to the selected primary bypass source.
The Power-Tie logic:
• Checks the system configuration
• Verifies synchronization
• Verifies system voltages are within limit
• Verifies both the on-line UPS systems have sufficient capacity to maintain the combined loads
before allowing the systems to tie
2.6 Design Criteria
The ultimate power goal in the design of the Liebert Continuous Tie System is to provide maximum
UPS availability and maintainability without compromising the inherent reliability of the UPS sys-
tems. We have designed the Continuous Power-Tie system to minimize or eliminate possible failure
modes and their effect on the system.
Isolation and Fault Tolerance
See Figures 6 and 7.
Signal isolation prevents an internal failure from propagating between systems. By using fiber-optics,
optical isolators, and isolation amplifiers, we isolate all control signals between each UPS system and
the Power-Tie Controls.
To parallel systems requires some amount of intersystem information. The controls have been
designed to be as fault tolerant as possible.
We use a parallel module sync bus means because the system will still function if a 60Hz clock in an
individual module fails. The parallel module sync bus eliminates the need for a system master oscilla-
tor function.
In tied systems we use a parallel module sync and fiber-optics. Fiber optics transport the digital mod-
ule sync signals between systems to maintain isolation. We use fiber optics for these signals since
they are the most critical intersystem signals in parallel systems.
Load Share Signal
The Load Share signal is sent between UPS systems when they are in parallel. Isolation amplifiers
isolate the Load Share signal levels between the systems. The load sharing loop and the voltage con-
trol improves the module load sharing capability of the modules to within ±5% of module load.
Because a Continuous Tie system uses modules of the same kVA/kW rating, the modules have some
inherent capability to share loads due to their similar output impedance. Load Share signals are not
as critical as module sync signals.
System Voltage Control Signal
The System Voltage Control signal is sent between UPS systems when they are in parallel. Isolation
amplifiers isolate the System Voltage Control signal levels between the two systems. The System
Voltage Control signal helps to maintain load sharing by giving all the modules a common reference
signal for output voltage. The System Voltage Control signal is not a critical signal. Loss of the Sys-
tem Voltage Control signal should only cause a minor degrade in module sharing capability, if at all.