One Plus One System; Redundant Vs Non-Redundant Configuration; One-Plus-One Parallel Control - Liebert 7400 Series User Manual

Chapter 1- General Description

One Plus One System

One Plus One System
1.3
1.3.1 Redundant vs
Non-Redundant
configuration
1.3.2 One-Plus-One Parallel
Control
1-8
Issue 2
(02/98)
6310018a.02.doc
The one-plus-one system comprises two standard 7400 series UPS
modules which are modified to allow their outputs to be connected in
parallel. These can then be used in a "redundant" or "non-redundant"
configuration as explained below.
In a non-redundant module configuration, the system is sized such that
both UPS modules are required to feed the potential load, and if one of
the two modules develops a fault, or is for some reason shut down, the
other module automatically shuts down also.
Note: In such an event the load is transferred to an unprocessed bypass
supply -as described later.
In a redundant module configuration the system is sized such that the
potential load can be provided by just one of the two modules. Under
normal circumstances both modules are operational and share the load
current equally; but if one module develops a fault, or is shut down, the
second module is able to take over the full load demand and continue to
provide it with processed, backed-up power. The advantages of a
redundant system over a non-redundant system in terms of overall system
reliability are self evident.
Changing a one-plus-one system's configuration between redundant and
non-redundant is quite straightforward, being carried out by
configuration links on the circuit board which governs the modules'
parallel control operation.
When two of the standard 7400 modules just described are connected
together to form a one-plus-one system, each module is fitted with an
additional circuit board which allows the two modules to communicate
with each other. Communication takes place via a single ribbon cable
connected between the modules as illustrated in figure 1-5.
The inter-module parallel control responsibilities are complex but can be
summarised as follows:
Synchronisation:
As the outputs from both UPS modules are connected together to provide
a single load supply, it is imperative that the inverters are fully
synchronised both in frequency and phase. This is achieved by digitally
locking the two inverter control oscillators. Similarly, as has already
been mentioned, it is necessary for the inverters to be synchronised to the
bypass supply to enable a "no-break" transfer to be achieved when the
static switch transfers the load to the bypass supply. The inverter control
oscillators are therefore not only locked together but are also made to
track the bypass frequency.
Current sharing:
The parallel control circuit compares the module's output current with
that of its partner and is thereby able to effect current sharing between
the modules by making fine adjustments of an individual module's
output voltage.
7400 Series UPS User Manual
Single Module and One plus One Systems