IEEE Power & Energy Magazine - May/June 2021 - 56

the measured values to a processor or relay. It has also been
proposed to asynchronously connect microgrids to secondary networks using ac-dc-ac power electronic interfaces to
enable the control of power exports and fault current characteristics and that could have the added benefit of reducing
the arc flash risk. However, this would entail a considerable cost, and adding the ac-dc-ac converters to the power
path could have adverse reliability impacts. This would be
a critical consideration, given that the purpose of secondary
networks is to provide high-reliability service.
For the problem of low fault current availability from
IBRs, the solution most frequently applied at present is to
significantly increase the total inverter megavolt-ampere
capability in the IBR plants. For example, an IBR plant
designed to serve 10 MVA of a load may use 20-MVA, or
higher, inverters to provide more fault current. This has obvious cost ramifications, and, even then, it is generally a good
idea to conduct electromagnetic transient simulations with a
detailed, code-based, manufacturer-specific inverter model
to ascertain whether the protection system will respond as
intended under all conditions. In summary, there exist solutions to the key challenges, but they tend to be quite expensive and complex. There is a need for research that leads to
the development of new solutions that offer effective protection at a lower cost. Some of the potential pathways that
might lead to promising developments in advanced solutions
to these protection challenges are described in the following.

Technologies for Very Low-Cost
Differential Protection
As noted, various members of the differential protection family are widely viewed as the most viable solutions for the
protection of IBR-sourced microgrids. Preliminary results
suggest that, from a technical perspective, differential protection should work within certain conditions, and in the future,
this technique may be demonstrated within a wider variety of
system configurations with DERs. The primary obstacle to
the wider use of differential methods is cost. This could be
addressed with new, inexpensive relays and associated transducers/sensors for distribution use, and low-price, cybersecure, burst-tolerant, reconfigurable communications systems.

New Protection Techniques
and Associated Hardware
Alternative protection techniques may still prove effective in protecting IBR-energized microgrids in secondary
networks. Potential candidates are described next, but this
should not be taken as an exhaustive list, and innovation in
this area of protecting IBRs and microgrids in secondary
networks is key.
Traveling Waves

Traveling wave-based protection is already in commercial
use in transmission. Two main types have been described in
the literature, and they are discussed in the following:
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1) Time of arrival: This family of techniques relies on the
fact that when a fault occurs on a line, it creates electromagnetic waves that propagate in both d- irections
at essentially the speed of light and that reflect from
various boundaries in the faulted circuit. The arrival
times of these electromagnetic waves can be used to
provide a distance-relaying fault location function
on the circuit and trigger appropriate action. Timeof-arrival techniques were first developed to facilitate extremely fast protection of transmission circuits
against faults relatively far from the relay's measurement point, and they have been highly effective in that
role. However, the transmission circuits protected by
time-of-arrival techniques tend to be much longer than
distribution circuits (transmission line lengths commonly used in studies of time-of-arrival techniques are
≥100 km, whereas most distribution circuits are shorter
than 20 km). Also, transmission circuits protected by
time-of-arrival methods tend to be much simpler than
distribution circuits, with far fewer tap points and interfaces. Thus, in a distribution circuit, during a fault
one typically observes dozens of arrivals of primary
and reflected electromagnetic waves within the first
few microseconds, making the use of the time of arrival very difficult in distribution.
2) Waveshape: The waveshape-based traveling wave
techniques make use of the frequency dependency of
circuit impedances. When the electromagnetic waves
mentioned in the preceding propagate from a fault,
their waveshape is altered by the fact that the line acts
as a filter, changing the relative magnitudes of various
frequency components. The degree to which the waveshapes change can be used to estimate how far along
the line a wave has propagated and thus to provide a
distance-to-fault estimate. Several variants of this technique have been reported in the literature. The approach
has been shown to work on transmission circuits, but
like the time-of-arrival technique, it would be very difficult to apply in distribution. The two main reasons are
that 1) the frequency-dependent parameters of the distribution line would need to be known with precision,
and they typically are not, and 2) in distribution, the line
length across which electromagnetic waves travel is so
short that the frequency filtering effect is negligible.
Although there are difficult challenges to overcome,
traveling wave techniques remain an active area of research
because they have some highly desirable properties. The
most obvious one is that traveling wave techniques can
enable protection to be extremely fast, but other properties
make traveling wave solutions intriguing for IBR-sourced
systems and secondary networks. For example, traveling
wave solutions do not rely on high-magnitude fault currents; they detect a fault itself, not the system's response to a
fault. This means that traveling wave techniques could still
work in fault current-limited, IBR-sourced power systems.
may/june 2021

IEEE Power & Energy Magazine - May/June 2021

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