IEEE Power & Energy Magazine - March/April 2021 - 45

becomes increasingly difficult in a system with a high penetration of renewables. In areas with significant renewable generation, voltage collapses become more probable.
Consider the ideal power voltage characteristics of a transmission corridor between two parts of a system, with the
receiving part represented by the load in the form of impedance. This characteristic is described in Figure 6(a). In a
properly designed ac power system, where all generation is
being conducted by synchronous machines, the distance from
the area of normal operation (point A) to the site of instability (point B), the " nose " of the power voltage characteristic is
large enough to accommodate possible contingencies, such as
outages in parallel circuits. Historically, the thermal conventional generation portfolio was planned to be placed as close
as possible to load centers; thus, large amounts of electricity
were not designed to travel long distances. At high levels of
renewable penetration, when a significant part of the thermal
portfolio is displaced, power needs to travel farther because
renewables are normally placed away from load centers.
Consider the same transmission corridor for a system
where the conventional generation near the load center is
scheduled to be offline so that it can be replaced by distant
renewable generation. The power transfer would change
directions, and its absolute value would increase. Thus, the
power-voltage characteristic would change due to the additional impedance associated with importing electricity from
remote areas, as in Figure 6(b). Operating point A now will
be closer to instability point B because of the greater impedance and the increased transfer.

Effect of Renewables on Frequency Stability
The main effect of renewable generation on frequency stability is related to the fact that renewable generation provides
little inertia to a system because it is partially or completely
50.1
49.9

Frequency (Hz)

49.7
49.5
49.3

49.1
48.9

48.7
48.5

48.3
48.1

20
30
Time (s)

figure 8. Frequency deviations due to the sudden trip of
a large generator for (a) a traditional generation portfolio,
(b) a portfolio with a large nonsynchronous generation
penetration, and (c) a portfolio with a significant nonsynchronous generation penetration.
march/april 2021

mechanically decoupled. With the progressive replacement
of conventional generation with nonsynchronous renewable
generation (Figure 7), the available system inertia decreases,
meaning that if a power unbalance occurs, the frequency
deviation for the same unbalance will be bigger in a system
that has less available inertia (Figure 8). To an extent, the
issue can be alleviated by the introduction of fast frequency
response mechanisms (some of which are called synthetic
inertia), but the genuine inertia provided by a synchronous
plant can be fully replicated only by a converter-based plant
if the power imbalance on the system is directly assessed
and compensated in real time. Curve C in Figure 8 corresponds to the activation of an underfrequency load shedding
scheme. This is the system's integrity defense mechanism
designed to automatically disconnect a percentage of the
load. This is a last defense, and it is not accepted for routine
operation in modern society.

Real-Time Assessment of
Operational Security
Assessing the operational system security of a power system
becomes compulsory for TSOs that integrate large amounts
of renewables. Such assessments can be performed by using
online dynamic security assessment technology: software that
takes snapshots of power system conditions, performs security evaluations (including the determination of stability limits) in near real time and for future time frames (from hours
to days), and provides warnings about, and remedial measures
for, abnormal situations. An example of an online dynamic
security assessment system can be found in the article " Safety
in Numbers " in the " For Further Reading " section.

For Further Reading
Eirgrid. All Island TSO Facilitation of Renewables Studies.
Dublin: Eirgrid, 2010.
I. M. Dudurych, " On-line Assessment of Secure Level of
Wind on the Irish Power System, " in Proc. 2010 IEEE PES
General Meeting, Providence, RI, July 25-29, 2010, pp. 1-7.
doi: 10.1109/PES.2010.5588059.
I. M. Dudurych, A. Rogers, R. Aherne, L. Wang, F.
Howell, and X. Lin, " Safety in numbers, " IEEE Power
& Energy Mag., pp. 62-70, Mar.-Apr. 2012. doi: 10.1109/
MPE.2011.2178283.
ENTSO-e. in Proc. 1st DSA Stakeholder Workshop, Brussels, May 23, 2018. [Online]. Available: https://docstore.entsoe
.eu/Documents/Events/2018/180523_DSA_SH_Workshop
-Joint_presentation_with_conclusions.pdf
Eirgrid, " Operational constraints update version
1.80, " July 31, 2020 [Online]. Available: https://www
.eirgridgroup.com/site-files/library/EirGrid/Operational
ConstraintsUpdateVersion1_96_July_2020.pdf

Biography
Ivan M. Dudurych is with the Military Technological Colp&e
lege, Muscat, 111, Oman.
ieee power & energy magazine

https://www.docstore.entsoe.eu/Documents/Events/2018/180523_DSA_SH_Workshop-Joint_presentation_with_conclusions.pdf https://www.docstore.entsoe.eu/Documents/Events/2018/180523_DSA_SH_Workshop-Joint_presentation_with_conclusions.pdf https://www.docstore.entsoe.eu/Documents/Events/2018/180523_DSA_SH_Workshop-Joint_presentation_with_conclusions.pdf http://www.eirgridgroup.com/site-files/library/EirGrid/OperationalConstraintsUpdateVersion1_96_July_2020.pdf http://www.eirgridgroup.com/site-files/library/EirGrid/OperationalConstraintsUpdateVersion1_96_July_2020.pdf http://www.eirgridgroup.com/site-files/library/EirGrid/OperationalConstraintsUpdateVersion1_96_July_2020.pdf

IEEE Power & Energy Magazine - March/April 2021

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