IEEE Electrification Magazine - March 2014 - 68

more complex fault current paths and affect protection
strategies in microgrids.
in this article, protection system challenges in
microgrids are discussed and protection practices in the
context of a functional microgrid system at the illinois
institute of technology (iit) are presented. We address the
structure and design of the iit microgrid and analyze
technical approaches for protecting the iit microgrid in
grid-connected and island modes.

Challenges in Microgrid Protection Systems
conventional protection schemes for radial distribution
networks cannot be applied to microgrids without major
modifications. such modifications would require addressing the impact of der integration, microgrid topology, and
fault current levels in grid-connected and island modes. in
this section, the challenges and possible solutions to
microgrid protection are discussed.

Challenges
the integration of der units would impose major challenges
in microgrid protection schemes. the conventional distribution system protection is designed for radial distribution networks that include feeders at one end with high fault
currents. in radial distribution networks, fault currents
always flow downstream, i.e., from utility feeders toward
fault locations. As a single source of power generation, utility
feeders provide high fault currents, which trigger Pds along
feeder paths. However, traditional protection schemes face
the following fundamental challenges in microgrids.

Fault Current-Level Modifications
fault currents depend on microgrid operation modes. in
grid-connected mode, utilities contribute to microgrid fault
currents while, in islanded mode, microgrids' potential
fault currents are lower. the fault current injection capability of ders with power electronic interfaces is limited to
twice their rated currents, and lower fault currents would
not trip overcurrent (oc) relays. in traditional distribution

Utility
Grid
Ig

Missing
Operation

Utility
Grid
Ig

PD1
IDER

Sympathetic
Tripping
PD1
IDER

PD2
PD3

PD2
DER

DER

Feeder 1
(a)

Feeder 1 Feeder 2
(b)

Figure 1. The impact of DER units on relay operation. (a) The
malfunction of PDs due to downstream faults and (b) the sympathetic
tripping of PD.

I E E E E l e c t r i f i c ati o n M agaz ine / MARCH 2014

networks, fault currents decrease as feeder impedances
increase when fault points shift downward along feeder
paths; however, microgrid ders contribute to fault currents along feeder paths. der units, especially generators
with rotating prime movers, such as synchronous or
induction generators, would have higher contributions to
fault currents when compared with ders with power electronic interfaces. As fault current paths could be different
in grid-connected and island modes, the two operation
modes would require different relay settings. As a result,
fixed settings for microgrid relays may become impractical
as the dynamic behavior of ders may also affect the coordinated settings of relays.

Impact of DERs on the Operation of Protective Devices
der units have plug-and-play characteristics in microgrids,
which may require modifications to traditional relay settings. in such cases, der locations and fault currents would
determine precise relay settings. the impact of der
integration on Pd operations is summarized in the following two categories:
xx
Malfunction of PDs due to downstream faults: in a
downstream fault, shown in figure 1(a), utility grid
and der unit currents (I g and I DER, respectively) contribute to the total fault current. if I DER is large enough,
I g will be reduced because of a higher voltage contributed by I DER at Pcc. thus, Pd1 may not trip because of
a lower fault current even though feeder 1 experiences a higher fault current.
xx
Sympathetic tripping: in figure 1(b), Pd3 should trip to
clear the fault. However, if the der unit contribution
to the fault current is large, Pd2 may trip in response
to high current I DER, which would disconnect feeder 2
from the utility grid.

Impact of Microgrid Topology on
the Coordination of Protective Devices
the looped or meshed networks in microgrids will affect
fault current magnitudes and directions. for example, the
fault current in a loop is divided between two parallel
paths. Hence, Pds on the upstream feeder may have currents that are twice as large as that in each fault path
within a loop. Accordingly, looped or meshed structures in
microgrids could impact the Pd coordination.

Possible Solutions for Relay Coordination
Balancing DER Technologies
for OC Protection
A possible solution to overcome low contributions of der
units to fault currents in microgrids is to balance der unit
contributions with those of other generation technologies
by introducing generating units with higher fault currents
to increase total fault currents in microgrids to proper levels that could be detected by oc protection systems. synchronous generators, including permanent magnet

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