IEEE Power & Energy Magazine - March/April 2021 - 81

System Fragility and the Risk of Cascading
Also linked to decreasing system strength and inertia, many
renewables, DERs, and their protection systems may generally be more sensitive and vulnerable to both frequency
and voltage excursions and, therefore, be more prone to cascading (what we can generally indicate as " fragility " ). This
includes inadequate ride-through capability, insufficient
local reactive support in response to voltage transients, or
unsuitable or not fully coordinated protection settings.
Additionally, there are risks related to interlinkages
between system parameters and security conditions, such as
voltage-induced frequency dips and uncoordinated or poorly
understood dynamic response from new technologies (see
the case study next). All of these situations highlight the
greater fragility of an increasingly complex grid with large
shares of IBRs and multiple interlinked dynamics, which
calls for new operational tools and technologies to manage
system security.

The Role of New Operational Security Tools
In the face of all of the emerging technical challenges illustrated
previously, several advances have been proposed to deal with
fast changing system conditions, including new optimal power
flow and scheduling tools to securely commit and dispatch
resources in low-inertia systems as well as machine-learningbased tools to deal with the variability and uncertainty of differmarch/april 2021

ent natures. For example, the frequency-response security maps
developed during the Finkel Review (Figure 2), which aimed to
review the status of the NEM following the South Australian blackout of September 2016 and move forward toward a
renewables-rich future, can be used to represent the effects of
different parameters and the utilization of resources on security requirements. Such maps depict the secure area (shaded)
in which the relevant three-frequency-response parameters
would be constrained to meet suitable security criteria (e.g.,
maximum acceptable ROCOF, minimum acceptable nadir,
and the minimum level of quasi-steady-state frequency)
for different levels of PFR and system inertia. The vertical line depicts the level of inertia that corresponds to the
desired ROCOF limit while the horizontal line corresponds
to the static PFR requirement (currently implemented in the
NEM and in most jurisdictions worldwide) for the desired
quasi-steady-state frequency limit. The hyperbolic-like
curve depicts the desired limit for the frequency nadir for
under-frequency events (similar maps can be drawn for overfrequency events and even for multiple areas in the case of
system separation).
For typical parameters that might be used to constrain the
system (e.g., a postcontingency frequency nadir not lower than
49 Hz) and typical load-damping factors (e.g., 3%/Hz) highinertia systems with plentiful synchronous generation are
typically constrained by static requirements that are independent of the system inertia, i.e., the secure operating envelope
is bounded by the horizontal line. However, when transitioning into low-inertia systems, both the frequency nadir limit
(the hyperbolic curve) and the ROCOF limit (the vertical line)
may become binding to meet the operational security requirements. Therefore, the scheduled PFR should be higher than
the static requirement typically adopted in traditional systems with plentiful inertia and should also be sized (or cooptimized), taking into account the incumbent system inertia.
This approach also provides an intuitive way to visualize (and
even formally quantify) the transition into low-inertia conditions from the perspective of frequency management.

Nadir Requirement
Static Requirement
ROCOF Requirement
Secure Region

PFR

operating conditions, including following an outage. System
strength has typically been associated with the fault current
level available at a given location and measured via metrics
such as the short circuit ratio.
However, several emerging instability issues have also been
recently witnessed in the presence of IBRs and fast control
loops, calling for new considerations and metrics besides fault
current level. For example, in strong grids, IBRs normally use
a phase-locked loop control as the voltage synchronization
mechanism that makes up for the lack of inherent synchronism. However, as very weak grids with large shares of IBRs
may experience significant voltage oscillations and distortions
during and after a disturbance, existing controls may be inadequate to track the voltage angle and maintain synchronization.
This has led the Australian Energy Market Operator (AEMO)
to make system-strength-related security interventions, such as
the enforcement of a minimum number of synchronous generators that stay online, the curtailment of renewable output,
and the procurement of synchronous condensers.
Other solutions to reduce the negative impacts of low
system-strength conditions include the retuning of IBR controllers and fast reactive power support. Several ongoing,
large-scale battery-storage projects are also being trialed
throughout Australia. Some of these include newer grid-forming converters, which do not use a phase-locked-loop-based
architecture but generate the reference voltage themselves,
and virtual synchronous machines that emulate the dynamic
characteristics of conventional synchronous generators.

High Inertia
Low Inertia
Aggregated Inertia After Contingency

figure 2. A frequency-response security map for lowfrequency events with an illustration of the conventional
transition from " high-inertia " to " low-inertia " conditions
(adapted from Mancarella et al., 2017).
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IEEE Power & Energy Magazine - March/April 2021

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