IEEE Power & Energy Magazine - May/June 2021 - 35

PV On

Time to Trip (s)

100

PV Off
Setting Group 1
Setting Group 2

t = 2.239 s
I = 0.932 kA
t = 0.555 s
I = 2.12 kA

1
t = 0.912 s
I = 0.932 kA
0.1
500

1,000

2,000

5,000

10,000

Fault Current (A)

figure 11. The effect of topology changes on different setting groups of the relay controlling CB4.
table 2. A comparison between the theoretical and
experimental results.
Calculated
R3 Tripping
Time (s)

HIL R3
Tripping
Time (s)

PV off, adaptive protection
system inactive

0.555

0.580

PV on, adaptive protection
system inactive

2.239

2.255

PV on, adaptive protection
system active

0.912

0.938

Case

can lead to severe security and reliability issues. State-ofthe-art relays can address certain challenges, e.g., directional overcurrent relays can avoid sympathetic tripping, but
they are not able to deal with different fault current levels.
Distance relays can address different fault current levels,
but their performance is degraded when a downstream DER
contributes to the fault. Differential protection schemes can
address microgrid protection issues, but communication network latency makes them less robust, and higher investment
costs are required, e.g., the provision of high-bandwidth
communication infrastructure. Adaptive protection appears
as a promising technique for the secure and reliable operation of microgrids. An adaptive protection scheme calculates
optimal setting groups for each microgrid operation mode
offline. Then, in real-time operation, the setting group for
each relay can be selected based on the current operation
mode of the microgrid.
Alternatively, the proper setting groups are calculated
online for the current microgrid topology by executing fault
calculations at different locations. Finally, the importance of
advanced HIL testing for the acceptance of microgrid protection solutions cannot be overstated. They provide a realistic environment for a detailed evaluation of the performance
and robustness of novel protection schemes under real operating conditions.

For Further Reading

and the wind turbine. Relay settings, namely, pickup current,
time dial, inverse-time characteristic, and current transformer
ratio, are also illustrated in Figure 10 for the involved intelligent
electronic devices.
The performance of an adaptive protection system was
also evaluated. In this case, the central controller identifies
the current microgrid status and modifies the setting groups
of the relays. In Figure 11, the change in the setting group
of the relay controlling CB4 is observed to avoid protection
blinding. Under the new setting group, the relay can identify
the fault significantly faster. The fault clearing time for each
case is presented in Table 2. As expected, the actual fault
clearing times in the simulation are higher than the calculated ones from the time-overcurrent characteristics due to
delays introduced in the setup.

N. Hatziargyriou, Ed., Microgrids: Architectures and Control. Hoboken, NJ: Wiley, 2014. doi: 10.1002/9781118720677.
fmatter.
V. A. Papaspiliotopoulos, G. N. Korres, V. A. Kleftakis,
and N. D. Hatziargyriou, " Hardware-in-the-loop design and
optimal setting of adaptive protection schemes for distribution systems with distributed generation, " IEEE Trans.
Power Del., vol. 32, no. 1, pp. 393-400, Feb. 2017. doi:
10.1109/TPWRD.2015.2509784.
A. Hooshyar and R. Iravani, " Microgrid protection, "
Proc. IEEE, vol. 105, no. 7, pp. 1332-1353, July 2017. doi:
10.1109/JPROC.2017.2669342.
S. Beheshtaein, R. Cuzner, M. Savaghebi, and J. M.
Guerrero, " Review on microgrids protection, " IET Gener.
Transm. Distrib., vol. 13, no. 6, pp. 743-759, Mar. 26, 2019.
doi: 10.1049/iet-gtd.2018.5212.

Final Note

Biographies

The majority of microgrids are dominated by PE-interfaced
generators and can be operated either interconnected or
islanded from the main grid. The changes in the operational
mode (grid connected or islanded) and DER connections
(switch on/off), the low-fault current contribution, and the
unconventional behavior of PE-based generation adversely
affect the performance of conventional protection schemes
applied in distribution networks. Moreover, in islanded
microgrids, the improper coordination between protection
systems and the fault ride-through requirements of the DER

Dimitris Lagos is with the National Technical University of
Athens, Athens, 15780, Greece.
Vasileios Papaspiliotopoulos is with the National Technical University of Athens, Athens, 15780, Greece.
George Korres is with the National Technical University
of Athens, Athens, 15780, Greece.
Nikos Hatziargyriou is with the National Technical University of Athens, Athens, 15780, Greece.

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