IEEE Power & Energy Magazine - May/June 2021 - 44

the distance protection will underreach or overreach different branches.
Another challenge with distance protection is that distributed load along the length of the feeder causes the error
to be even larger. With any sort of load or generation inside
the protective zone, the distance to the fault will be miscalculated because distance protection assumes that impedance
to the fault can be calculated using its voltage and current,
which does not account for other current flows into the fault
or protection zone.
Moreover, a highly unbalanced system can cause issues
with load encroachment into the distance protection zone,
challenges for the directional elements with unbalance, and
errors in the impedance calculations performed in distance
relays. For example, for a phase A to phase B line-to-line
fault, the impedance calculation would calculate the difference in voltage between phases A and B divided by the
difference in current between A and B to calculate the
impedance to the fault. This type of calculation does not
work if there is an unbalanced load that creates a nonzero
difference in the current between A and B.
Differential Protection

Figure 7 shows the currents measured at two locations in a
microgrid used for differential protection by subtracting the

currents at the sending and receiving ends of the line. Note
that the current is unbalanced before the fault due to unbalanced loads in the microgrid. Also, there are some loads
in between the two sensors, so the difference is not exactly
zero before the fault. When a line-to-line fault (phase A
to phase B) is applied inside the protection zone at 0.9 s, the
low-fault-current IBR-based microgrid does not increase currents significantly at the receiving or sending end, but the fault
appears in the difference.
There is excellent current discrimination provided by
the differential principle, despite the distributed load. This
method is found to be the most reliable in both detecting
the fault and locating the faulted section. Since it works for
low fault currents from both ends, it would obviously work
in the grid-connected mode or with synchronous sources in
the system since these would provide much higher fault currents. Depending on the size and amount of load inside the
differential protection zone, the differential can still have
challenges in islanded inverter-based systems, where the
fault current may be only a few times higher than the load
inside the zone.
The example shown in Figure 7 is a true differential
scheme in which the current at one end of the zone is subtracted from the current at the other. While effective, this
scheme requires high-speed data communications between

Current (A)

500

Phase A
Phase B
Phase C

250
0
-250
-500
0.85

0.9

0.95

1

Time (s)
(a)

1.05

1.1

1.15

Current (A)

500

Phase A
Phase B
Phase C

250
0
-250
-500
0.85

Current (A)

1.2

1,000
750
500
250
0
-250
-500
-750
-1,000
0.85

0.9

0.95

1

Time (s)
(b)

1.05

1.1

1.15

1.2

Phase A
Phase B
Phase C

0.9

0.95

1

1.05

1.1

1.15

1.2

Time (s)
(c)

figure 7. The currents for a phase A to phase B line-to-line fault inside a differential protection zone when an IBR-based
microgrid is in island mode: the (a) sending end, (b) receiving end, and (c) differential currents.
44	

ieee power & energy magazine	

may/june 2021
IEEE Power & Energy Magazine - May/June 2021

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - May/June 2021
