IEEE Power & Energy Magazine - March/April 2021 - 98

and challenges with relevance for system security in different systems have
been presented together with solutions
and remedial actions. Many innovative designs have been suggested and
implemented as a response to new difficulties, and they are good examples
of how power engineers perpetually
work to guarantee the security of supply. In my view, there are a couple of
main causes behind the development
described here, as I will explain.
First, with the large-scale introduction of new weather-dependent
renewables [mainly wind power and
photovoltaics (PVs)], power system
planning, operation, and control face
new problems, and the ongoing studies on the topic are plentiful. With the
ever-increasing penetration of renewables, new solutions are proposed, and
I do not think we have seen the complete answer yet.
Considering the total electric energy production by wind power and PVs
from a larger geographical area and
over a longer period, e.g., a year, this
production will not show any larger
variations from year to year. The yearly
energy production of the large area can
be assumed to be fairly constant (although a changing climate might imply
that this will not be true in the future).
However, the local variations from year
to year can be substantial, and so can
the temporal variations over the year.
There are, in principle, three ways to
cope with these problems: transmission lines, storage, and demand-side response. Depending on the nature of the
variations (geographical or temporal)
and other system characteristics, one
or the other (and in many cases a combination of these means) must be employed to guarantee the desired system
operation requirements and security.
Several articles in this issue describe new ways to master this problem
in different systems. Note that in some
systems, e.g., the continental European
system, interconnections were originally built and designed primarily to
enable the sharing of reserves and providing redundancy. Therefore, even if
interconnecting transmission lines ex98

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ist, these might not always be able to
fulfill the tasks required to integrate
renewables securely. A fourth possibility would be to utilize the flexibility of other power plants, e.g., hydropower, if available, or flexible thermal
power plants.
A new security threat that has attracted a lot of attention lately is related to cybersecurity. We have already
seen cyberattacks in power systems
and other technical systems. With the
increasing penetration of control and
monitoring systems, e.g., based on phasor measurement units using Internetbased communication channels, the exposure to such attacks will grow. I am
convinced that cybersecurity will be
an integrated part of the overall system
security assessment in the future, and
this must be done in cooperation with
other engineering disciplines.

How to Achieve Security
To achieve the desired security level,
one can, in principle, use two design
approaches: make the system either
robust or resilient. The difference can
be captured in the following somewhat simplified descriptions: a robust
system is designed so that its users
should not be surprised, whereas a
resilient system is designed so that
its users are prepared to be surprised.
The article " Toward a Consensus on
the Definition and Taxonomy of Power System Resilience " offers the following definition:
The ability to withstand and reduce the magnitude and/or duration of disruptive events, which
includes the capability to anticipate, absorb, adapt to, and/
or rapidly recover from such
an event.
A robust system is designed to cope
with several predetermined events
and disturbances within a specified
space. These events are presumed to be
known a priori, and their magnitudes,
in some metric, are assumed to be
bounded. The events to be considered
in the design and their possible maximum magnitudes are based on experience and system knowledge. In a robust

system, we try to contain the impacts
of the " known " unknowns in such a
way that the risk level of the system is
within desired limits, and this can be
said to be the traditional way of designing power systems.
However, due to the increased system complexity because of new technologies, new players, new types of
power plants, and so on, the ability to
foresee all possible threats is increasingly difficult. This is corroborated
by the analyses of many of the major
power system blackouts during the last
decades. A system can be designed
to withstand many individual disturbances, but a specific combination and
temporal occurrence of these events
might bring the system to a complete
or a partial blackout. One could say
that an " unknown " unknown, i.e., the
combination of events, was the cause
of the power outage. Furthermore, designing a system to withstand all lowprobability events, even if they can
have a very severe impact on the system, is not economical. Examples include hurricanes or ice storms hitting
a power system with disastrous consequences. History tells us that such
extreme weather events can be very
different, so it is not realistic to aim at
designing and building a system that is
robust against all these events. Thus,
other means to cope with high-impact,
low-probability events are needed,
and here, a resilience-based approach
would be more promising.
I believe that the future solution
would be to combine robustness and
resilience in system design and operation. Actively introducing resiliency is
fairly new in the design of power systems, at least when compared with robustness, and a lot of research is going
on in this area. I am sure that, in the
future, many new and innovative ideas
inspired by resiliency will be implemented in the power system to increase
system security. As indicated in the
previous definition, an effective restoration process will add to the resiliency
of the system. Restoration plans have
always been important to ensure system security, and I am convinced that
march/april 2021

IEEE Power & Energy Magazine - March/April 2021

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