IEEE PES T&D Conference & Exposition 2022 - 126

L
R
Wind
IL
IR
Photovoltaic
Relay 1
Battery Energy
Storage System
figure 1. Line current differential protection in applications with nonconventional sources.
currents, and the sequence component content of the current
(all positive-sequence quantities, for example) do not interfere
with the principle that the current differential protection
uses. Figure 1 illustrates a line current differential (87L)
scheme for a power line connected to a substation with wind,
photovoltaic, and battery energy sources. The 87L scheme
compares the currents going into the line to the ones coming
out. The limitations of the power electronic-based sources
do not affect the operation of the current differential elements
of the protective scheme.
A transmission line fault, internal to the protection zone
of the 87L scheme, creates a differential current, which is the
sum of the currents measured at the line terminals. Because
the differential current is near zero when there is no internal
fault, 87L schemes can be set more sensitively than other
line protection schemes. The limited fault current contribution
of power electronic-based sources does not significantly
affect the detection of the fault.
Introduction to Line Current
Differential Protection
Today, the most common line protection scheme in applications
involving wind farms, photovoltaic fields, and battery
storage facilities is the 87L scheme. This is because
87L schemes are highly effective at detecting faults when
abnormal current signatures are present in networks with
I2
all combinations of IBR and SG energy sources. Other
traditional
line protection methods, such as time-overcurrent,
distance protection (which is based on impedance
measurements), and directional-comparison (which
determines the direction to the fault at the line terminals
and exchanges this information using a communications
channel), are not always effective in inverter-based generation-dominated
grids.
Line current differential protection has become a popular
method for underground cable protection common in
large industrial facilities. This popularity is because 87L
is applicable for cables and short lines, whereas time-overcurrent
and distance protection are difficult, if not impossible,
to coordinate in these applications. Operating times
of 87L are also faster than the previously mentioned methods.
Furthermore, the engineering labor and skill required
to set the 87L elements is much less than with other protection
methods.
Line current differential schemes require a digital communications
channel. Ideally, a direct fiber-optic link can be
used. However, the 87L data can be multiplexed with other
services using communications technologies, such as timedivision
multiplexing (TDM), multiprotocol label switching,
and wavelength-division multiplexing (WDM). The use of
operational technology (OT) edge node equipment makes it
possible to use packet-based networks for 87L applications.
I1
I1 + I2 + ... + IN ≈ 0
IN
figure 2. Current differential principle: the sum of currents
is close to zero when there are no internal faults.
126
ieee power & energy magazine
Line Current Differential
Protection Basics
Line differential protection schemes use Kirchhoff's Current
Law to detect power system faults within assigned protection
zones. While there are no fault conditions, the sum of
the currents going into the protection zone of the protected
equipment, such as a power line, is close to zero, as Figure 2
illustrates. When a fault occurs within the protection zone,
the currents do not add up to zero and their sum (the differential
current) exceeds a minimum operating threshold,
indicating there is an internal fault condition.
April 2022 Show Issue
IL
IR
Relay 2

IEEE PES T&D Conference & Exposition 2022

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