ABB M2004HW industrial control chassis control unit
xcitation systems are typicallyused in power-plants for generator
control. This is an application in
which reliability is the most important
requirement. As opposed to the previous example, these systems can be
very large and incorporate several
subsystems.
In this case, the introduction of the
PP D104 permitted the division of the
entire system into several independent
sub-systems, each dedicated to a particular subset of tasks, and with each
subsystem being controlled by a separate controller. The total system is
then controlled and coordinated by a
powerful main controller based on the
PP D103 processor module.
This modularization not only greatly
reduced the complexity of the overall
system, but presents two further
main advantages: scalability and reliability.
Traditionally, systems that are scalable
over a very broad range of sizes come
at the cost of a complex architecture
and hence impose difficulties for the
engineering staff. The modularization
made possible by the low-end extension of AC 800PEC allows the implementation of a very natural scalability
over a broad range. Each sub-system
can be instantiated several times without further complicating the software
in the main controller.
As already mentioned, reliability of
the system is a key issue in powergeneration, and often full redundancy
is required. Whereas in slower systems, the controllers themselves can
be realized as redundant devices, the
cycle-times found in and required by
power electronic systems make traditional redundancy concepts for controllers unworkable.
Here, the solution of choice to
achieve redundancy is no longer on a
device level, but on a system level.
For the redundancy concept implemented in ABB’s UNITROL® excitation
systems, each subsystem is available

n-times. In case of a problem in one
subsystem, the main controller switch es over to the remaining subsystems,
which are scaled in such a way that
the overall task can still be fulfilled 4 .
Should the main controller fail, there
is always a second controller available
in hot-standby