Filter
Top Products
Theory of Operation
10
2.0 THEORY OF OPERATION
2.1 General Component Descriptions
The UPS system includes all of the equipment necessary to continuously provide computer-grade AC
power to a critical load, even when there is an interruption of the utility power. It consists of a System
Control Cabinet (SCC), UPS modules and a back-up battery plant. Refer to Figure 6 and Figure 7.
Except where otherwise noted, the configuration is presumed to be parallel-redundant (with at least
one more UPS module than required to support the rated load).
2.1.1 System Control Cabinet
The System Control Cabinet (SCC) includes system controls, static bypass switch, load transfer con-
trol, protective devices and other accessories. The system controls simultaneously monitor two or
more parallel UPS modules sharing a critical load for increased capacity and/or redundancy. The two
(2) motor-operated system circuit breakers (system bypass breaker—SBB and UPS output breaker)
may be contained within the SCC, depending on the configuration designed for your installation.
System Controls: The system control logic automatically manages critical bus operation and moni-
tors performance of the UPS modules. Microprocessor technology and dedicated firmware provide
advanced logic control and a comprehensive display of information. The system control logic synchro-
nizes the output of the UPS system to the bypass source. The SCC includes ports for communicating
with external devices. Liebert Multi-Module Units do not require an SCC for load sharing. Automatic,
parallel module load sharing is a function built into the modules themselves; however interconnection
through the SCC enables the modules to share data for more precise control. Modules do not require
master clocks or controls in order to load share or free-run at 60.0 Hz.
Static Bypass Switch: The static (solid-state) bypass switch immediately transfers the load from
the inverter to the bypass AC power source in the event of a severe overload on the system or a failure
within the UPS. This transfer takes place without any interruption of the power supplied to the load.
In all SCCT control cabinets 1600A or larger and in all SCCI and SCCC control cabinets, the system
includes redundant circuits to detect and isolate shorted SCRs in the static switch.
Fuses are installed in series with the static bypass circuit (with the exception of the continuous duty
static switch installed in the SCCC control cabinet) to ensure reliable overload protection in the
unlikely event of a catastrophic output condition (e.g., a dropped wrench) electrically close to the out-
put of your UPS system. The static switch SCRs themselves are rated to easily handle the fuse-blow-
ing current.
Bypass Circuit: The bypass circuit consists of electrically operated circuit breakers and associated
synchronizing and control circuitry to transfer the load to/from the bypass source.