IEEE Power & Energy Magazine - May/June 2021 - 21

coordinated control of microgrid resources increases energy efficiency, minimizes the overall energy consumption, and reduces the environmental impacts of energy production. At the same time, the ability of
microgrids to seamlessly transition to islanded operation when upstream network faults occur increases
the reliability and resilience of the customer supply. Furthermore, microgrids have been adopted as prominent and viable solutions for rural electrification in developing countries, isolated areas, or areas with
weak power transmission infrastructures.
Microgrid implementations are faced with challenges in their control, operation, and protection. Next to
control issues, protection has drawn the attention of researchers and industry since microgrid advantages
can be jeopardized if protection against faults proves deficient. The main challenges arise from the presence of multiple short circuit current sources due to the installation of distributed sources in traditionally
passive distribution networks, various fault current levels in different operational modes (islanded and grid
connected) of the microgrid, and the limited short circuit current contribution by power electronic (PE)interfaced energy sources. The coordination of fault ride-through curves and protection schemes is also an
issue that can affect the overall microgrid stability in islanded operation.
DC microgrids share similar concerns with ac microgrid protection. The different operational characteristics of dc circuit breakers and the requirement of faster fault clearance times compared to ac systems need
to be carefully considered. This article focuses on the challenges of ac microgrid protection, provides an
overview of the different protection methods, and presents the adaptive protection methodology as well as a
novel laboratory testing procedure for microgrid protection scheme evaluation.

Challenges in Microgrid Protection
In addition to the common characteristics of standard protection schemes, namely sensitivity, selectivity,
and speed, microgrid protection should also adapt to changing operating conditions. The most significant
concerns are the maloperation of protective devices, like protection blinding and sympathetic tripping, low
short circuit currents, the uncertain behavior of PE-interfaced units, and the lack of grounding.
The first challenge is protection blinding caused by the integration of DERs. Microgrids are part of
the distribution level of the power system where the protection schemes rely on overcurrent protective
devices, such as fuses and relays. This long-established protection technique might be inadequate for
microgrid protection since it depends only on the short circuit current magnitude for the unidirectional
behavior of the short circuit currents, i.e., from the upstream network to the fault. This might not be the
case in a microgrid. Figure 1 illustrates a microgrid that can operate in two modes: grid connected or
islanded. The circuit breakers are controlled by their respective protective relays. The presence of DERs
in different locations of the feeders provides additional fault current sources, altering the magnitude and
direction of fault currents coming from the upstream network. For example, as depicted in Figure 1, in
the event of a fault between buses 5 and 6, the short circuit current (i.e., I CB6) flowing from the photovoltaic (PV) through circuit breaker CB6 to bus 5 may lead to a reduction of the short circuit current
from the upstream network (i.e., I CB4). Therefore, the speed, or even the sensitivity, of the respective
upstream protective device, i.e., the relay controlling CB4, may fail to recognize the fault, the effect of
which is known as protection blinding.
The second challenge is the sympathetic tripping that impairs the selectivity of microgrid protection
schemes. Sympathetic tripping refers to the operation of a protective relay to a fault beyond its protection
zone. The unintentional operation is triggered by the DER backfeed current contribution to the fault, causing an outage of a healthy part of the system. The basic principle of sympathetic tripping is explained in
Figure 1 for the same fault between buses 5 and 6. During the short circuit, the wind turbine feeds a short
circuit current I CB14 to I CB4 in the direction of I CB12 . In the case of a large wind turbine current, the relay
controlling CB12 might command the circuit breaker to trip before the relays controlling CB7 or CB4 take
action. Thus, opening CB12 causes the disconnection of a healthy feeder.
The third challenge, especially for islanded microgrids dominated by the PE-interfaced generation,
is the drastically reduced short circuit current level. PE-interfaced units can contribute only about

By Dimitris Lagos, Vasileios Papaspiliotopoulos,
George Korres, and Nikos Hatziargyriou
may/june 2021

ieee power & energy magazine

IEEE Power & Energy Magazine - May/June 2021

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