IEEE Power & Energy Magazine - May/June 2021 - 27

The main drawback is their dependence on the magnitude of the short circuit current. Therefore, sensitivity issues introduced by the large variation of short circuit current levels due to topology or operational mode
changes are not resolved. Also, the unconventional nature
and power factor of fault current contribution from PEinterfaced units can cause directional elements to fail.

Distance Relays
A common alternative to directional overcurrent relays is
distance relays, which can detect the direction of the fault
current and are not affected by its magnitude. Distance
relays are applied for transmission line protection, and their
operation is based on the impedance seen as calculated from
the measured voltage and current.
The measured impedance is compared with a locus in the
R-X diagram, which defines the reach of the protection zone.
The measured impedance is an indicator of the fault location
since the R and X values of the line increase in proportion
to the distance of the fault. If the measured value lies within
this locus, it is interpreted as an in-zone fault, and a trip signal is issued to the respective circuit breaker. Distance relays
operate with one instantaneous zone as primary protection
and additional time-delayed zones that serve as backup protection for forward or backward faults.
The settings of distance relays are dependent on the zones
that they are designed to protect. Typical values for the first
zone are 80-85% of the feeder length on which the relay
is placed. This 15-20% safety margin is usually considered
to provide reliable operation against measurement errors of
the current and voltage, deviations between the considered
and actual line parameters, and relay inaccuracies. A second
zone protects 100% of the feeder length plus 50% of the next
feeder. A third zone could cover both the protected and the
second line as well as 25% of the third line. Figure 6(b) presents the first two zones in the R-X plane of a distance relay,
which uses quadrilateral characteristics.
Infeed and outfeed currents of a DER in a microgrid can
lead to underestimation or overestimation of the fault location
by the distance relay. In the island microgrid of Figure 4(a),
illustrated here as Figure 6(a), a three-phase symmetrical
fault occurs at 30% of the line, i.e., line 2, connecting buses 5
and 6. It is considered that the relay controlling CB4 located
in the line, i.e., line 1, connecting buses 2 and 5, operates with
the characteristics of Figure 6(b), where zone 1 covers 80%
of line 1, and zone 2 covers 100% of line 1 and 50% of line 2.
The current magnitude I CB4 measured by the relay of CB4
can be affected by the presence of the PV and the power factor of the PV fault current.
To illustrate possible miscalculations of a distance relay, the
measured impedances for different values of fault resistance
and power factors of the PV during the fault are presented in
Figure 6(b). The magnitude of the PV fault current remains the
same in each case. When the PV operates at unity power factor,
the measured impedance is slightly affected, moving closer to
may/june 2021

the limit of zone 2. However, as the PV current becomes more
reactive (the power factor decreases), the estimated impedance
by the distance relay is severely affected, leading to relay overreach. A trip signal is issued to CB4, thus instantaneously disconnecting an unfaulty line and the PV.
The ambiguity in the operation of a PE-interfaced unit
during unbalanced fault conditions may adversely affect the
impedance calculation as well. Furthermore, distance relays
may be proven ineffective as a protection scheme for short
lines where the discrimination between in-zone and out-ofzone faults is difficult. Hence, distance relaying is not a practical solution for microgrid protection.

Differential Protection
Differential protection schemes have also been proposed
for microgrid protection. The scheme compares the current
flowing in and out of the protected equipment with a given
threshold to determine whether to isolate the protected element. The comparison of current is performed by two devices
located at the buses where the terminals of the protected element are connected. Each device compares its local current
measurement with the measurement sent by the device on
the other end. Measurements are transmitted through a communication network based on Ethernet, fiber optic, power
line carrier, or wireless communication technologies.
When applied for line protection, differential relays may
be located in remote locations. The installed communication
network will inevitably introduce communication delays
and package drop issues to the transmitted signals. This
can lead to unsynchronized data gathered by the differential relays, which can affect the protection performance. For
line protection, technologies like fiber optics are required
to ensure that high-bandwidth communication between the
devices is established. Conventional line differential protection, as applied in transmission systems, can be applied in
microgrids to protect distribution feeders with no loads connected along them (e.g., lines connecting power or storage
stations with a load center).
Other novel differential protection approaches are based
on the communication between conventional protection
devices, e.g., directional overcurrent relays. Such a microgrid
protection scheme based on the differential protection principle is illustrated in Figure 7(a), which is equivalent to a
zone selective interlocking scheme.
In Figure 7(a), the relays located at the line ending and
receiving ends provide the main protection of the feeders,
e.g., the relay controlling CB15 and CB17 for the line connecting buses 3 and 4. Directional overcurrent or simple
overcurrent relays can also be installed at DERs and load
connection points, respectively, for short circuit protection,
e.g., controlling CB14 and CB16 for wind turbine and load
disconnection. The main protection against faults along
the line is achieved through communication between the
relays on their ends. Directional overcurrent relays can be
employed at every circuit breaker on a feeder, transmitting a
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