IEEE Power & Energy Magazine - March/April 2021 - 100

in my view

Göran Andersson

power system security
why & how

A SECURE, RELIABLE, EFFICIENT,
and sustainable energy system is a prerequisite for a well-functioning society
that can satisfy the needs of its citizens
affordably, and this applies to the electric power system in particular. This is
obvious to most of us, and I will not
discuss and motivate the correctness of
this statement further in this column
since numerous papers and articles discuss it in detail. Rather, I will take this
premise as a starting point for a discourse on some aspects of the security
of energy systems and, more specifically, the electric power system.
During recent years, many studies
and other initiatives have been directed
to multienergy systems, also referred
to as energy system integration or sector coupling, aiming at optimizing the
overall energy system. By considering
not only one energy-delivery system,
such as electricity, in the system analysis and optimization but also including other interrelated energy-delivery
systems, such as gas and heat-delivery
systems, the features of the different
energy-delivery systems can complement each other, resulting in a more efficient and reliable overall system. I am
convinced that future system designers will increasingly consider this option. However, due to the limited space
available, I will focus on the electric
power system. Another motivation is
that in most studies of future multienergy systems, the scenarios where
Digital Object Identifier 10.1109/MPE.2020.3043679
Date of current version: 19 February 2021

100

ieee power & energy magazine

electric power systems constitute the
" backbone " usually come out as the
most advantageous ones, due mainly
to the versatility and attractiveness of
electricity for the end consumer.

What Is System Security?
Formal definitions of system security
and reliability are worthwhile to establish as different engineering disciplines
use somewhat different definitions. A
joint CIGRE and IEEE task force published a paper in 2004 that formulates
a number of definitions used in power
systems dynamics, but I think that they
are also useful in the more general context we are dealing with here. The paper states:
Reliability of a power system
refers to the probability of its
satisfactory operation over the
long run. It denotes the ability to
supply adequate electric service
on a nearly continuous basis,
with few interruptions over an
extended time period.
Security of a power system
refers to the degree of risk in its
ability to survive imminent disturbances (contingencies) without interruption of customer service. It relates to robustness of
the system to imminent disturbances and, hence, depends on
the system operating condition
as well as the contingent probability of disturbances.
Stability of a power system
(...) refers to the continuance of
intact operation following a dis-

turbance. It depends on the operating condition and the nature of
the physical disturbance.
As a comment on the definition of
security, it is further postulated that
System security may be further
distinguished from stability in
terms of the resulting consequences. For example, two systems may both be stable with
equal stability margins, but one
may be relatively more secure
because the consequences of instability are less severe.
Other terms that one sees are " security of supply " and " system security, "
where the latter applies specifically to
power system dynamics, as was the focus of the aforementioned paper. However, I think that we can use the previous
definition with some minor modifications. Thus, I propose the following
definition of power system security:
✔ Security of a power system refers to the degree of risk in its
ability to withstand disturbances, on all connected equipment
and subsystems, without interruption of customer service. It
relates to the design of the system and connected subsystems
as well as the contingent probability of disturbances.
A few words and phrases have been
modified. These changes will be elaborated in this article. I want to emphasize that the risk concept is kept, which
means that when assessing security, we
(continued on p. 97)
march/april 2021

IEEE Power & Energy Magazine - March/April 2021

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - March/April 2021