Emerson 7400 - Manual (Page 16)

7
Series 7400 Single Phase 1+N UPS
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 4.
Note: - Modifying a standard module for use with a one-plus-one system also
involves replacing certain other circuit boards with modified versions, and
relocating certain minor assemblies. This means that although it is not
impossible to modify an existing 7400 module to form part of a one-plus-one
system it is not a straightforward proposition.
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.
Redundancy configuration:
A link in the parallel control logic determines whether the one-plus-
one system operates in a “redundant” or “non-redundant”
configuration. If a non-redundant mode is selected the two static
switch sections are effectively locked together in that both the
static switches are turned off or on by a single control signal. Thus
if one module develops a fault, when running, its static switch
control logic will transfer its output from the inverter to the static
bypass line and simultaneously send a signal to the static switch
control logic in the second module to do likewise. This does not
happen if the system is configured as a redundant system, in
which case the second module is allowed to continue supplying the
load from its inverter when the first module trips its inverter off line.
Reverse current:
A reverse current monitor circuit detects current flowing into, rather
than out of, the module’s output terminals. Such a condition can
arise if a module develops an internal power fault or if for some
reason the two modules become unbalanced, and is liable to
further damage the module and also degrade the load supply. If a
reverse current is detected the inverter on the affected module is
immediately shut down and load transferred to the bypass supply
depending on the system redundancy configuration.
1.2.6 One plus One
Parallel
Control