IEEE Power & Energy Magazine - November/December 2021 - 21

engineered to provide various behaviors, such as instantaneous
active power injection in response to phase jumps,
an ability to operate without any other resources in the
network, and black-start capability. These functionalities
are subject to the available energy buffer and the inverter
overcurrent capability.
Impact of IBRs on
Power System Stability
Maintaining power system stability is not a new challenge. However,
novel and existing stability phenomena are manifesting
themselves during high-IBR conditions. Challenges related to
frequency, voltage, and angular stability as well as control stability
are discussed in the following, along with possible solutions.
Frequency Stability
In a bulk power system (BPS) with synchronous generators,
the inertial response determines the initial rate of change of
frequency (RoCoF) after a contingency. The generator governor
response assists in arresting the system frequency before
protective schemes, such as underfrequency load shedding
and overfrequency generation reduction, take effect. The frequency
is then stabilized and restored to nominal by using
additional reserves. Because GFL IBRs are displacing synchronous
machines, their inertial response may also be displaced,
resulting in a higher RoCoF after a contingency.
A high RoCoF may lead to the exceedance of frequency
limits before any countermeasures have time to respond, and
protective devices may trip the generation or load. Furthermore,
protective devices triggered by a high RoCoF may
aggravate these effects. State-of-the-art IBRs can provide
frequency control with performance superior to the governor
response of synchronous generators due to a faster response
speed. Too-aggressive controls may, however, introduce frequency
oscillations in low-inertia systems as well as other types
of instability. Since BPS characteristics are changing, system
operators should continuously review and revise frequency
response needs in terms of performance and reserve amounts.
The size of the largest generation or load contingency is
another important factor for frequency stability. While the size
of IBRs is generally smaller than that of synchronous generators,
with high shares of these resources, new common modes
of failure may be introduced, affecting several IBRs simultaneously.
For example, in geographically compact systems,
such as Ireland, voltage-induced frequency dips may occur
due to wider-area low-voltage propagation during faults. Such
events need to be understood and studied to determine appropriate
mitigation measures. In large interconnected systems, a
synchronous area split is a concern since it may be challenging
to maintain frequency stability in separated subsystems.
A high RoCoF after a contingency or system split could
pose a limit to the maximum instantaneous penetration of IBRs
unless a sufficient level of inertia is sustained. One option is to
maintain several must-run synchronous generators. However,
synchronous generators, particularly gas turbines, can be sensitive
to a high RoCoF and may not mitigate the concern. The
necessary inertia can also be provided by synchronous condensers
(SynCons) and GFM IBRs. While the inertia of SynCons is
relatively small, flywheels can be added to increase their contribution.
The inertial response of a GFM IBR is subject to the
available energy buffer and inverter overcurrent capability.
Voltage and Angular Stability
Long-distance power transfer from remote generation pockets
has been one of the major challenges in system planning
and operations. The maximum transfer from regions with
GFL
* GFM Versus GFL
Controlled Current
Injection
* Basic Trait
Versus
Grid Service
Requires Minimum
Grid Strength to
Establish Grid
Voltage Waveform
Closed-Loop Voltage
Control
Fast Frequency
Response
Primary Frequency
Response
Automatic Generation
Control and
Dispatchability
Stability Services
Unique to GFL
Inverters
Controlled Internal
Voltage Phasor
Voltage Persistence and Inertia:
- Intrinsic for Synchronous Machines
- Designed Service for GFM
Inverters
Black-Start and Island Operation
GFM and
Synchronous
Machines
Functionality Possible for All IBRs,
Independent of Inverter Type
Unique to GFM
Inverters
figure 1. The basic traits and grid services of GFL IBRs, GFM IBRs, and synchronous machines.
november/december 2021
ieee power & energy magazine
21

IEEE Power & Energy Magazine - November/December 2021

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - November/December 2021