IEEE Power & Energy Magazine - March/April 2021 - 80

renewables and inverter-based resources (IBRs). In particular, frequency control and balancing challenges are prominent in systems that are weakly interconnected, connected
to other grids via high-voltage direct current links, or even
completely isolated, such as Australia's system. Other challenges, generally associated with voltage sensitivity and stability, are more typical of systems where resources and load
centers are linked by long, relatively low-density networks,
which, again, is a widespread situation in Australia. Next
we discuss some of the major technical challenges of lowcarbon operation with real-case studies from Australia.

Frequency Control and Inertia
Frequency control is inherently more challenging in an environment with more variable and asynchronous resources. First
of all, lower system inertia means that the system frequency
becomes much more sensitive to active power disturbances.
In particular, large disturbances associated with contingencies may lead to very high initial rates of change of frequency
(ROCOF) and to undesired minimum (nadir) or maximum
(zenith) frequency values at time scales too fast for conventional primary or even emergency responses to alleviate.
For example, Figure 1 illustrates how the system response's
dynamic characteristics change with inertia levels while the
quasi-steady-state frequency level does not. The latter only
depends on the available primary frequency response (PFR),
which was assumed to be the same in the two cases. In the
case of the South Australian blackout in 2016, when South
Australia islanded from the rest of the grid and less than
40% of available generation was synchronous, the estimated
ROCOF was about -6 Hz/s. With that rate of frequency drop,
" under-frequency load-shedding " schemes were not able
to intervene. Also, with increasingly larger shares of loads
interfaced via power electronics, the frequency damping by
load will inherently reduce dramatically, again, calling for
both faster and larger amounts of frequency response. Such

faster and/or larger reserves might not be readily available, or
the market or regulatory setup may be unsuitable to provide
the right incentives in the short term.

Variability, Uncertainty, Visibility, and Flexibility
Systems dominated by renewable energy sources, particularly wind and solar, feature a much higher degree of operational variability and uncertainty (due to forecast errors) over
various time scales, from seconds and minutes (impacting
regulation and load-following reserves) to hours (potentially
requiring new types of flexibility and ramping reserves).
This situation is further compounded by the fact that many
distributed energy resources (DERs) embedded in distribution networks are not visible at the system level.
This means that the demand side, whose forecasting
has historically been quite accurate, becomes more difficult. Moreover, new forms of uncertainty are being witnessed, including " commercial contingencies " through selfcurtailment by generators within dispatch periods in response
to low or negative prices and lack of visibility on the curtailment of DERs.
All of these issues call for more active power flexibility and new flexibility service providers. Conventional
resources are also starting to respond to new requirements.
For example, in Australia, coal power plants are investing in
making themselves more flexible so that they can cycle on
and off during the middle of the day (due to, for example,
low prices resulting from large amounts of solar generation).
However, there is a system risk to this as well, once again
illustrating the complexity of the energy trilemma. This is
the time when such generators might be most needed to provide inertia as well as system strength.

System Strength and Local Stability Challenges

Frequency (Hz)

In addition to the issue of decreasing inertia, grids with
fewer synchronous generators and more IBRs are also
weaker and more susceptible to
voltage disturbances. In particular, the increased voltage sensitiv50
ity to reactive (as well as active)
Same Quasi-Steady-State
Lower
power d ist u rba nces a nd t he
49.9
Frequency Value
Inertia
decreasing availability of short
49.8
circuit current means that voltHigher
Higher
age management becomes much
49.7
Inertia
ROCOF
more difficult, especially in long,
49.6
high-impedance networks in rural
49.5
areas where many large-scale
Lower
Frequency
renewables are connected.
49.4
Nadir and
The term system strength has
Reduced
49.3
been
introduced as an umbrella
Nadir Time
concept
to indicate a general mea49.2
0
5
10
15
20
25
sure
of
power system stability,
Time (s)
particularly in terms of a system's
ability to maintain voltage wavefigure 1. The effects of lower inertia on system frequency response after a given
contingency event (adapted from Mancarella et al., 2017).
form and phase under generic
80

ieee power & energy magazine

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