IEEE Electrification Magazine - December 2019 - 57

N PHYSICS, INERTIA IS DEFINED AS THE
tendency of an object to resist changes in
its state of motion: in other words, the
tendency to resist changes in speed and
direction. It is a property of matter to
remain at rest or uniform motion unless an external
force is applied to it. In classical electric power systems,
the system frequency is directly coupled to the rotational speed of all rotating equipment directly connected to
the grid (as in the cases of synchronous and induction
generators, induction motors of big industrial drives,
pumps, fans, and so on). Therefore, we can define the
inertia of a power system as the resistance of the system
to variations in its frequency, which is directly related to
the resistance to variations in the angular rotating
speed of all of the rotating mass connected to the system. It is then evident that the inertia of the power system is determined by the total rotating inertia directly
connected to it.
In classical power systems, most of the rotating inertia
is given by synchronous generators and turbines. Besides,
the generation is centralized in big power-generation
plants, which are the most important elements for regulating the electric grid. However, in recent years, more
power is being generated from renewable energy sources
(RESs) distributed along electric grids. The characteristics
of an RES differ significantly from the typical systems
connected to a classic large power plant. The power generated by these power sources is inherently variable. In
addition, unlike synchronous machines, they inherently
lack an inertial response. The reason is most RESs are
connected to the grid using power electronic converters
that decouple them from the grid frequency. Advancements in power electronic, information, and communication technologies are making it possible to connect more
and larger RES-based generation systems and energy
storage systems (ESSs) to the grid. As this occurs, we are
seeing electronic devices that inherently do not provide
any support to the grid as they replace classical rotating
machines attached to large mechanical inertias. This is
posing certain challenges that the industry has not faced
since the first electric grids were established.
The first electric power systems at the end of the
19th and the beginning of the 20th centuries would be
considered microgrids today. Most of these systems
were point-to-point power lines connecting a single and
relatively small generation unit with certain loads. Even
though the so-called War of the Currents officially came
to an end in the late 1880s, many of those lines were still
being built in dc. The total inertia connected to those
power systems was quite low, so the connection and disconnection of loads generated severe frequency variations (voltage variations in dc systems) that jeopardized
the stability and reliability of those first grids. The engineers of the time facing these problems devised ingenious inventions to improve the behavior of the system.

I

The most successful solution was to use devices known
as load equalizers, which consisted of electric machines
with big flywheels attached at their shafts. These flywheels were connected using synchronous or induction
machines to ac systems and using dc machines to dc
systems. This means that for more than a century, engineers have understood the benefits of increasing the
inertia connected to power systems to decrease the rate
of change of frequency (RoCoF) in ac systems and the
rate of change of voltage (RoCoV) in dc systems under
big load variations.
As the number and power of loads increased, those
first electrical systems evolved toward interconnected systems using increasingly powerful generators. This gave
rise to the meshed electric power system concept, which
comprised large generation power plants and huge inertial values. This is now conventional. However, as mentioned previously, the development of power electronic
converters is enabling RES generation systems and ESSs to
take the place of big power plants. As a consequence, the
inertial response of the power system is being reduced.
These issues threaten the stability of the system. In fact, it
is the origin of some of the most significant problems suffered at modern power systems, such as the South Australian blackout of 2016.
Meanwhile, these new power electronic converters,
because of their controllability and flexibility features, are
enabling the expansion of microgrids or subgrids connected through interlinking converters (ICs) to the main grid.
These microgrids are lauded as among the best options for
grid integration and controlling and managing local energy
resources. However, these microgrids are decoupled from
the main grid with very low inertial values, making the
controllability and stability of these microgrids a hot research topic.
Progress in the development of electronic power converters is also expected to lead to the replacement of distribution transformers by solid-state transformers (SSTs),
which can also be considered ICs for interconnecting subgrids that have been decoupled.
One way to address the challenges presented by the
integration of power converters is to take advantage of
their controllability to help regulate the power system. The
control of power converters-especially as it relates to
their role in the primary regulation and in the inertial
behavior of power systems-has received much attention
from the research community. This article provides an
overview for the emulation of inertia via power converters,
focusing not only on converters interfacing RESs or ESSs,
but also on ICs, the converters that interconnect different
parts of the grid. We show that it is possible to employ a
unified framework for the emulation of inertia regardless
of the type of grid to which the converter is connected. We
propose a new generalized control strategy for ICs to contribute to the primary regulation and inertial behavior of
the power systems they connect.
IEEE Elec trific ation Magazine / D EC EM BE R 2 0 1 9

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IEEE Electrification Magazine - December 2019

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