IEEE Power & Energy Magazine - March/April 2021 - 38

A large interconnected power transmission system
is probably the single most complex engineering device
to be created by mankind.
and evolution of electricity markets. These factors contribute to fundamental changes in generation patterns and power
transfers in ways that were not anticipated.
This article focuses on the impacts of renewables on
operational security. Since renewable generation is inherently intermittent due to the countless combinations of
loads, weather, and electricity market conditions, the variety
of generation patterns and transfers is virtually unlimited.
Therefore, base case and contingency case scenarios may
significantly vary on a minute-to-minute basis in terms of
the following:
✔ online generation portfolios
✔ power flows
✔ voltage profiles
✔ parameters of power system dynamics.
Thus, to adequately support the operational security of a
power system, offline analyses and studies should be complemented by online, close-to-real-time evaluations. While
attempting to cover all possible scenarios, offline studies
produce estimated security limits that can be extremely conservative for one setting and insufficient for others. Transmission system operators (TSOs) whose networks have a
high penetration of renewable generation face several technical challenges, including the following:
✔ Variability: The output of most types of renewable
generation changes in time frames measured from
seconds to hours.
✔ Uncertainty: Variable generation is less predictable
than conventional generation.
✔ Location: Renewable sources of energy are often located in remote areas that have small or no local loads,
and they can even be situated offshore (e.g., wind and
wave generation). This requires long transmission
paths to deliver power to load centers.
✔ Nonsynchronicity: Renewable generation normally
uses convertor-based technologies to couple with the
grid. As a consequence, it does not inherently provide
an inertial contribution, which is crucial for synchronous power system stability.
✔ Service capabilities: There is a lack of the traditional
system service capabilities that conventional generators normally have, such as
* sufficient voltage and frequency regulation
* an operational (rotating) reserve
* adequate fault ride-through (e.g., the slow active
power recovery of a wind generator following a
transmission fault clearance).
38

ieee power & energy magazine

✔ Complexity: Additional complexity is introduced into

the system configuration and operation through the
necessity of solving special problems, such as harmonics, subsynchronous resonances, interactions with
other control systems, and the performance of special
protection systems.
Therefore, it is critical to implement additional measures
to ensure the security of a power system under all conditions, especially when a high level of renewable and other
unconventional generation is integrated. In recognition of
that fact, the European Network of Transmission System
Operators for Electricity (ENTSO-e) included a requirement in " System Operation Guideline on Dynamic Stability
Monitoring and Assessment " (European Commission regulation 2017/1485). According to Article 38, " each TSO shall
perform a dynamic stability assessment at least once a year
to identify stability limits and possible stability problems in
its transmission system. All TSOs of each synchronous area
shall coordinate the dynamic stability assessments, which
shall cover all or parts of the synchronous area. " However,
the pace of penetration of renewables and other new technologies into power systems is such that the minimum requirement set by the regulation is insufficient, especially when
saturation levels reach 70-80% (i.e., 70-80% of the total
generation fleet in a synchronous zone is nonconventional).
One response to these challenges can be to use online, " realtime " dynamic security assessment technology.
The technology includes three main components. The
first is a digital model of a power system that represents the
real network at selected points in time; this includes the network topology, operating conditions, and all other relevant
factors. The second is a system that, utilizing the real-time
digital model, runs millions of simulations on a digital
model of a power system (contingency analyses). The third
is an analytical tool that produces system security and stability assessments based on the simulations. These assessments
can be made for any selected point in time-the present and
the future-and they will be used by system operators to
maintain operational security.
In the following, we will answer these questions:
✔ What is the operational security of a power transmission system?
✔ How is it affected by significant penetrations of renewable and nonconventional generation?
✔ How does online dynamic security assessment help
manage ever-increasing levels of renewable and nonconventional generation in transmission systems?
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

Contents
IEEE Power & Energy Magazine - March/April 2021 - Cover1
IEEE Power & Energy Magazine - March/April 2021 - Cover2
IEEE Power & Energy Magazine - March/April 2021 - Contents
IEEE Power & Energy Magazine - March/April 2021 - 2
IEEE Power & Energy Magazine - March/April 2021 - 3
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IEEE Power & Energy Magazine - March/April 2021 - Cover3
IEEE Power & Energy Magazine - March/April 2021 - Cover4
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