NPP: Power Grid Mutual Safety Assessment - Nuclear Power Plant Instrumentation and Control Systems for Safety and Security - page 413

Figure 10. The stages of power grid system' dy-
namical criticality CWW- based analysis

and adjustment of criticality matrices (Kharchenko
et al., 2009), taking into consideration probable
changes, are carried out.
The principle of hierarchy assumes represen-
tation of grid structure as a hierarchy. The set of
criticality matrices of subsystem failures groups
in clusters.
The principle of influence of subsystem fail-
ures of i-level (on subsystem failure criticality of
the same level) and influence on subsystems of
(i-1)-level (higher) is important.
The safety of all influenced subsystems must
be reconsidered.
The principle of uncertainty takes into
consideration information incompleteness and
uncertainty related to the conditions that cause
PG accidents.
The stages of PG dynamical criticality CWW-
based analysis are shown in Figure 10.
The power grid is a very complex system. It
is characterized by huge number of nodes and
links between nodes with increasing structural
complexity; links between nodes could change
over time, have different weights, directions, etc.
The safety is one of important attributes of
PG. The PG safety is an integral value composed
of grid systems safety values. The grid safety is
determined by the uncontrolled mutual influ-
ence among grid systems. It is worth to note that
influence exists on all grid levels and have to be
taken into consideration when providing grid
systems safety.

changes in NPP state and characteristics stipulate
the changes in the influence value.
The formalization of influences between PG
systems is very helpful for its safety assessment
and might be based on criticality matrices. For
example, if PG system S 1 consists of three sub-
systems S 11 , S 12 , S 13 , then criticality matrix, which
represents the system S 1 , might be presented as
shown in Table 5.
According to the principle of hierarchy, the
grid structure might be represented as a hierarchy.
In this case the safety of PG systems of higher
level hierarchy might be evaluated as a sum of
criticalities of power grid systems of a lower

Types of Influences Between
Power Grid Systems

According to the principle of influence, all influ-
ences (or relationships) existing in PG can be di-
vided into several hierarchy levels. The influence is
an ability of one PG system to determine the state,
characteristics or processes in other systems. Any
type of influence is a time dependent value. The