IEEE Power & Energy Magazine - May/June 2021 - 57

Also, the distance-relaying function provided by travelingwave techniques could be used to enable a relay to determine whether a reverse current through an NP is feeding
a primary side fault (trip) or is simply normal IBR-sourced
current back-feeding the grid (do not trip). Both families
of traveling-wave techniques for distribution would benefit
from hardware development in the areas of extremely highsampling-rate meters and optical sensors that have faster
responses than electromagnetic transducers do and that do
not suffer from magnetic saturation.
Undervoltage Relaying

When a local fault occurs within a microgrid in either onor off-grid mode, the voltage on the affected circuit usually
falls to a low level, commonly lower than 0.2 pu (noting that
high-impedance faults are a separate, challenging issue).
Thus, undervoltage relaying should give a reliable indication
of the existence of a fault, but because the voltage gradients
along the circuit will be small, undervoltage relaying has
difficulty indicating the location of the fault. Still, preliminary results suggest that this technique might have a role to
play in the protection of microgrids, particularly those powered by distributed IBRs, and work in this area is ongoing.
Machine Learning of System Conditions

It has been demonstrated that machine learning techniques
can be used in distribution system protection. On radial distribution circuits, when appropriate training sets are available,
several classifications and clustering techniques are effective
in enabling a relay to distinguish between in-zone and out-ofzone faults by using only local measurements. These classifiers
" learn " the circuit through training data sets for each potential
system configuration and several different event types. These
data sets are typically derived from batches of simulations on
the circuit. The need for high-quality data sets is a challenge
to the use of machine learning in protection, particularly for
nonradial systems and for structures that have variable boundaries, such as networked and ad hoc microgrids.
Techniques Assisted by the Inverters Used in DERs

As is discussed in the Part 1 article, a dependence on IBRs
creates certain challenges for protection system designers.
However, the unique capabilities of IBRs could also provide
opportunities for new, holistic protection system designs in
which the source IBRs actively participate in the protection scheme. In particular, IBRs have a unique ability to
change their output current in predictable ways in response
to system conditions, for example, by changing the negative
sequence component and by adding specific low-order harmonics. These changes in the output current could provide
a crude means of communicating to other system elements,
such as relays, the conditions detected by the IBR, and the
information passed in this way could be harnessed to create a cooperative system control and protection system. To
realize such a holistic design, inverter response standardizamay/june 2021

tion would be required, and participation by IBR and relay
manufacturers would be essential.

Conclusions
This article discussed power system protection in the presence
of two technologies designed to improve power system reliability: secondary networks and microgrids. It was shown
that each technology is effective on its own in improving
reliability. Also, because each technology provides a unique
reliability benefit (secondary networks via redundancy in
the power path at distribution points and microgrids via the
use of local sources), there is some incentive to use the two
technologies together. Unfortunately, there are compatibility challenges, particularly in the area of protection. These
difficulties are magnified if a microgrid uses IBRs and if
its sources are distributed instead of centralized. The article discussed some potential pathways toward protection
technologies that overcome these challenges and enable
DERs and microgrids to be successfully deployed on secondary networks.

For Further Reading
M. Behnke et al., Secondary network distribution systems
background and issues related to the interconnection of distributed resources, National Renewable Energy Lab., Golden, CO, Tech. Rep. NREL/TP-560-38079, 2005.
E. Greenwald, Electrical Hazards and Accidents: Their
Cause and Prevention. New York: Wiley, 1991.
J. Domin and T. Blackburn, Protective Relaying Principles and Applications, 4th ed., Boca Raton, FL: CRC Press,
2014.
E. Sortomme, S. S. Venkata, and J. Mitra, " Microgrid
Protection using communication-assisted digital relays, "
IEEE Trans. Power Del., vol. 25, no. 4, pp. 2,789-2,796,
2010. doi: 10.1109/TPWRD.2009.2035810.
R. Salcedo et al., " Benefits of a nonsynchronous microgrid on dense-load LV secondary networks, " IEEE
Trans. Power Del., vol. 31, no. 3, pp. 1,076-1,084, 2016. doi:
10.1109/TPWRD.2015.2420594.
M. J. Reno, S. Brahma, A. Bidram, and M. Ropp, " Influence of inverter-based resources on microgrid protection:
Part 1, " IEEE Power Energy Mag., vol. 19, no. 3, pp. 36-46,
2021.
M. Ropp, M. Reno, W. Bower, J. Reilly, and S. S. Venkata, " Secondary networks and protection: Implications for
DER and microgrid interconnection, " Sandia National Laboratories, Albuquerque, NM, SAND2020-11209, November
2020.

Biographies
Michael E. Ropp is with Sandia National Laboratories, Albuquerque, New Mexico, 87185, USA.
Matthew J. Reno is with Sandia National Laboratories,
Albuquerque, New Mexico, 87185, USA.
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