IEEE Power & Energy Magazine - May/June 2021 - 31

since it facilitates direct information exchange between
protective devices. The protocol accepts any type of data
(binary or analog signals) and guarantees that they are
transmitted at a speed on the scale of a few milliseconds.
High-bandwidth communication networks are required
to facilitate such fast interaction using the appropriate type of
medium, such as optical fibers. Thus, one major disadvantage
of differential protection schemes is the high expected costs
associated with such a communication infrastructure. Especially in a microgrid with a considerable number of nodes
and lines, the required communication infrastructure can be
quite complex.
Last but not least, the protection scheme cannot be based
entirely on differential protection since the communication
networks can fail or suffer from cyberattacks. Therefore,
even when differential protection is used, redundant protection schemes based on typical relay functionalities that
respond to local measurements need to be installed.

Adaptive Microgrid Protection
One of the most promising approaches for microgrid protection is adaptive protection. It requires numerical relays,
mainly directional overcurrent relays with several setting
groups that can be parameterized locally or remotely by
control signals according to the current microgrid topology.
The communication infrastructure must also implement an
adaptive protection system; however, communication interaction is not needed during faults. Protection is therefore
based on the local functions of the relays. Communication
is necessary only to update the relays' setting group when
the configuration of the microgrid changes. Since low-bandwidth communication technologies can be used, the cost and
complexity of the communication network are reduced compared to differential protection schemes.
Adaptive protection schemes can be implemented with
centralized and distributed architectures. In the centralized
architecture, the setting groups can be updated based on
calculations performed online or offline. For online calculations, a central controller receives the required signals, e.g.,
circuit breaker statuses, to identify the current microgrid
topology. Then, a local computer executes fault simulations
at different microgrid locations, and, according to the calculated fault currents, the proper setting group is activated, e.g.,
by solving an optimization problem that aims to minimize
the operation times of the protective devices. Finally, digital
signals are sent to the relays via a low-bandwidth communication system. The main drawback of this approach is the
requirement for a simulation engine in the central controller
to perform the short circuit calculations in real time, which
might increase the total cost and adversely affect the reliability of the protection system. Also, it requires new calculations when a change in the microgrid occurs, thus delaying
the setting group update in the relays.
For the offline approach of a centralized adaptive protection scheme, short circuit calculations for all possible
may/june 2021

operating conditions of the microgrid are performed before
the adaptive protection system deployment. Based on the
solution of an optimization problem, an adequate set of setting groups for the protective devices for each operating
state is determined offline. During the online operation, the
central controller receives the required signals, e.g., circuit
breaker statuses, to identify the current microgrid configuration and, if necessary, change the active setting group of the
relays via a low-bandwidth communication channel as presented in Figure 8(a).
The main advantage of centralized adaptive protection is that
all of the information is available to the central controller, which
can optimally select the adequate setting groups of the relays. Its
main drawback is its lower reliability because the central controller is a single point of failure. Thus, any malfunction of the
central controller affects the operation of the adaptive protection
scheme. Another disadvantage is the requirement of a complex
wide area network for the communication of the central controller with every device in the microgrid.
Distributed approaches have also been proposed for the
online operation of adaptive protection. Local controllers
installed at different microgrid locations update the settings of their respective relays based on the information they
receive from local devices and their interaction with neighboring controllers. A distributed architecture for adaptive
protection is presented in Figure 8(b). Its main advantage is
the increased reliability since there is no single failure point.
Also, the communication infrastructure is simpler since the
local controllers use a wide area network to communicate
only with their neighbor controllers and a local network to
update their setting groups. The main drawback is that a
relay might select a suboptimal setting since the state of all
microgrid components is unknown.

Test Systems for Protection Scheme
Testing and Validation
The performance of an adaptive protection system must be
evaluated before deployment to avoid design gaps that could
jeopardize personnel safety or power asset integrity. The adaptive protection system should be tested for both islanded and
grid-connected operational modes, changes in microgrid topology with the connection and disconnection of DERs, different
behavior of PE-interfaced generation units during faults, multiple load levels, and various types of faults in diverse locations.
An evaluation based on software simulations offers the
flexibility to examine various scenarios, but it does not consider, or cannot model precisely, the conditions encountered
in the real application. For example, it is not known how the
commercial relay implements the protective functions, how
much time it requires to conduct calculations, and what time
delay is introduced between the tripping command of a relay
and the opening of the associated circuit breaker. Therefore,
pure software-based evaluation lacks important information
to evaluate the performance of an adaptive protection system
under real conditions.
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IEEE Power & Energy Magazine - May/June 2021

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