IEEE Electrification Magazine - December 2019 - 66

The replacement
of classical
interconnection
methods by ICs is
gaining a lot of
interest to improve
the properties of
power systems.

3) Dual IE (DIE): This is IE by considering both sides of the IC [Figure 10(b)]. In this case, the IC
observes the deviations at both
sides of the converter and provides inertial behavior based on
the differences between these
sudden deviations. This means
that the IC reacts to power variations on both sides of the converter. The IE loop is then based
on the derivative of the variable
of interest similar to what is done
with IE and CE techniques. This
concept of control has not been
proposed in the literature, but it
might be interesting for scenarios in which both grids
are similar in terms of inertial response [Figure 9(a)
and (b)]. One of the advantages of this approach is
that the controller can be generalized by emulating a
different inertia at the deviations of the primary and
the secondary sides (H V and H V , respectively). When
the two grids are similar in terms of inertial response,
we can define H V = H V = H V to contribute equally
under variations at any side of the IC. However, if one
of the grids is much stronger than the other [as in Figure 9(c)], we can modify the inertia emulated on each
side to prioritize support for the weakest grid. For
instance, if we only provided inertial response under
perturbations on the secondary side (by making
H V = 0), this controller would become an SIE technique similar to the one shown in the previous case.
By following the unified framework presented in this article, we can easily employ SIE and DIE controllers regardless of the type of grids the IC is connecting.
In Figure 11, we illustrate the behavior of the grids
interconnected by ICs for the three cases described (no IE,
SIE, and DIE) in different load-perturbation cases. The aim
is to understand the concept of operation of ICs depending on the location of the perturbation and the type of IE.
The drawings on the left and right correspond to load perturbations occurring at the primary and secondary side of
the converter, respectively. In the third case [Figure 11(c)],
we also show a particular case in which the perturbations
occur on both sides of the converter in opposite directions.
The curves of the first case [Figure 11(a)] show that no
matter what side of the converter is disturbed by the load
power increment, only that side of the grid will be transiently
affected. The RoC of the bus variable of this grid (frequency
for ac grids; voltage for dc grids) is directly proportional to the
equivalent inertia on that grid. The grid on the other side of
the converter is not dynamically affected by the perturbation
because the IC does not react to that transient deviation.
In the second case, the situation is different because
the IC is controlled with an SIE technique and therefore
can react under transient deviations on its secondary
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I E E E E l e c t r i f i cati o n M agaz ine / DECEMBER 2019

side. In that case, if the perturbation
is on the primary side, the behavior of
both grids is the same as in the first
scenario. However, when the load
perturbation occurs on the secondary
side, the IC reacts to reduce the RoC
of that grid by providing additional
inertial behavior. As this energy is
extracted from the primary side of
the IC, the grid connected at this side
will also encounter a dynamic perturbation at the bus. The SIE control for
ICs is interesting in case, for instance,
one of the grids is much stronger
than the other [Figure 9(c)]. In that
event, the strong grid supports the
weak one in terms of inertia and is not significantly
affected by the dynamics introduced by the IC.
In the third case, the IC is controlled with a DIE technique, meaning that it is prepared to react under sudden
deviations on either side. This type of control might be
interesting for cases in which the two interconnected grids
are similar in terms of inertial response [e.g., two strong or
two weak grids as shown in Figure 9(a) and (b)], because the
IC will tend to balance the transient deviations on both
grids dynamically. To illustrate the concept of operation of
this case, we consider that H V = H V = H V as explained in
the DIE technique. When one of the grids has a steeper
deviation than the other, the IC will transmit power to the
former to improve its dynamic response at the cost of slowing the response of the latter. The behavior is the same no
matter the location of the perturbation. However, when the
two grids have deviations in opposite directions, as shown
in Figure 11(c), the DIE-controlled IC will benefit both grids
simultaneously by dynamically transferring power from the
grid with an excess of power to its counterpart.
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Comparison and Research Trends
The key role of power electronic converters in the primary
regulation and inertial response of modern power systems
is clear. To ensure adequate operation of these power systems, the adoption of advanced control strategies is fundamental. This article has surveyed some of the most relevant
techniques for the emulation of inertia in ac and dc systems, highlighting the role of ICs. The key aspects of the
discussed techniques are presented in Table 2.
Several problems must be tackled to take full advantage of the control capabilities of power electronic converters connected to the power system. Some of the most
relevant challenges include the following:
xx
Consideration of voltage drops for the emulation of inertia in dc
systems: The literature includes several approaches to
account for voltage drops. Among these are measures for
carrying out accurate reactive power regulation in ac systems or carrying out primary regulation in dc systems.
These strategies need to be transferred to the IE level.

IEEE Electrification Magazine - December 2019

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