IEEE PES T&D Conference & Exposition 2022 - 127

The current differential principle is used for line, generator,
busbar, transformer, and motor protection. For line applications,
current measurements are performed by the local relay and
transmitted to the remote relay. Thus, the differential calculation
is performed at both ends of the line. Figure 3(a) illustrates
a two-terminal line application where the line relays measure
their local currents and send the measurements to the corresponding
remote terminal through a communications channel.
The current signals need to be time-synchronized before performing
the differential current calculation.
The 87L scheme must not misoperate for the fictitious
differential currents created by CT saturation errors associated
with the nonlinear nature of their iron cores and the
charging currents due to the line or cable capacitance. Also,
87L algorithms need to accommodate errors due to asymmetrical
latencies in the communications channels. They
must thoroughly monitor messages for errors in the transmission
of the measurements. They should detect these measurement
transmission errors when they occur and disable
the differential scheme. 87L implementations solve these
challenges in different ways, providing the scheme with the
required security and sensitivity.
A scheme that has proven to be secure and sensitive
includes differential elements with the characteristic
depicted in Figure 3(b). The ratio of the remote to local
current (/ )II
RL is a complex number, which can be plotted
on a complex plane (also called the alpha plane). The
alpha plane differential element operates when the current
ratio plots in the operating region (outside the relay
restraining region).
Modern 87L schemes with elements having the alpha
plane characteristics are simple and easy to apply. The factory
configuration of these elements can be used with confidence
(no need for parameter determination like distance
relaying) for traditional two-terminal applications.
On industrial sites, it is common to run a cable out of
a transformer, as shown in Figure 4. In this application,
because of the breaker and CT location, the protection zone
includes the transformer and cable. The 87L should be able
to compensate for phase angle shifts associated with transformer
construction (like a transformer differential relay
does) and consider the significant differential current error
due to cable charging capacitive current and transformer
magnetizing inrush current. Modern 87L relays can accommodate
both requirements. Long transmission lines and
medium-length underground cables exhibit large capacitive
currents, which can be compensated for by the 87L relay.
There is no practical distance limit for 87L schemes since
they operate reliably in the presence of communications
channel asymmetry errors.
Communications Channels for
Current Differential Protection
Line current differential schemes rely heavily on a reliable
communications channel. The ideal channel is a direct link
April 2022 Show Issue
Relay 1
(most likely an optical fiber) between protective relays. This
arrangement is not practical for many utilities because optimization
of available bandwidth is important. The most
common method to take advantage of the fiber-optic bandwidth
is to multiplex different data flows. Today, three popular
options exist for communicating differential protection
signals between relays:
✔ Time-division multiplexing (TDM)
✔ Packet-switched communications
✔ Wavelength-division multiplexing (WDM)
Time-Division Multiplexing
TDM has been used for many years by utilities and is a highperformance
solution. Types of TDM are SONET, mainly
used in the United States, and SDH, used in other parts of
the world. Traditionally, utility communications relied on
TDM channels to send 87L data as well as other services.
Today, these networks are still used all over the world,
L
IL
IR
R
IL
IR
Relay 2
(a)
Im (IR/IL)
Operating
Region
Re (IR/IL)
(b)
figure 3. The differential element in each line terminal relay
uses local and remote currents to detect internal faults.
L
IL
Relay 1
IL
IR
IR
R
Relay 2
figure 4. Line differential scheme protecting a transformer
and a cable.
ieee power & energy magazine
127
Restraining
Region
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