SEL-411L-2 Data Sheet Schweitzer Engineering Laboratories, Inc.
8
The principle of operation for ac saturation is based on
the observation that all CTs of the differential zone per-
form adequately for a short time into the fault. If so, the
differential current does not develop during the external
faults, but the restraint current increases. This external
fault pattern differs from the internal fault pattern in that
both the differential and restraint currents develop simul-
taneously. The algorithm monitors the difference by respond-
ing to changes in the instantaneous differential current
and the instantaneous restraint currents that the relay
measures during one power cycle. The algorithm declares
an external fault if it detects sufficient increase in the
restraint current, no accompanying increase occurs in the
differential current, and the situation persists for a prede-
termined portion of a power cycle. When both currents
develop simultaneously, the EFDAC logic does not assert.
The principle of operation for dc saturation checks if the
dc component in any of the local 87L zone currents is rela-
tively high as compared with the CT nominal and the ac
component at that time. If the dc component is high and
the differential current is low compared with the restraint
current, EDFDC asserts in anticipation of possible CT satu-
ration, resulting from overfluxing by the dc component.
The SEL-411L-2 combines the output from both logics
to drive an external fault-detected (EFD) Relay Word bit.
The relay uses the OR combination of the ac path and the
dc path, not only to drive the local external fault detector,
but also to transmit information about the external fault
to all remote terminals.
The EFD Relay Word in Figure 7 is an OR combination
of the local and remote external fault detectors. This allows
all terminals to receive an alert about an external fault
even if one of the terminals has minimal current contri-
bution to the fault. Upon assertion of the EFD Relay Word
bit, all 87L elements switch to high security mode. No
user settings are necessary for the EFD logic.
In-Line Transformers
The 87L function performs in-line power transformer
vector group, ratio, and zero-sequence compensation. The
function also provides logic for blocking during overex-
citation conditions and offers both harmonic restraint and
blocking to accommodate transformer inrush. Proper com-
pensation of the measured current occurs at the local relay
prior to remote terminal transmission of current data. Once
the local relay receives data from the remote terminals, it
can consume these data by using the same signal processing
and algorithms as in the plain line application (see Figure 8).
Time-Overcurrent Differential
Protection
The SEL-411L-2 allows protection of lines with tapped
loads without the current measurement at the tap. You can
make such partial line current differential applications
selective, and these may be acceptable if you connect
tapped and unmeasured load through a step-down power
transformer. The transformer impedance reduces the level
of line differential currents for network faults fed from
the low side of the transformer, providing better coordina-
tion margins.
This application allows you to protect lines having multi-
ple load taps without the need to invest in high-grade
communications and install the SEL-411L-2 relays at
every tap of the line.
Overall, in the partial line current differential applications of
the SEL-411L-2, we suggest following this approach:
➤The 87L elements are applied as instantaneous but
are intentionally desensitized to prevent operation
for faults in the tapped load.
➤The differential time overcurrent elements provide
sensitive, but time-coordinated protection for the
low-current line faults, some internal faults in the
tapped transformer, and remote back-up for short-
circuit protection in the tapped load network.
Use the selectable time-overcurrent elements to config-
ure the differential time-overcurrent protection while
coordinating with the phase-, negative-, or zero-sequence
short-circuit protection of the tapped load network.
Security With Respect to
Communication Events
Noise in a communications channel can corrupt data. The
SEL-411L-2 uses a 32-bit BCH code to protect data
integrity. Any data integrity protection has a non-zero
probability of defeat. To reduce the probability that a
standing noise condition could result in corrupted data
Figure 7 Combined External Fault Detector
EFDAC
EFDDC EFD
EFD1
EFD2
. . .
Local Terminal
To Outgoing Packets
Remote Terminals
(Incoming Packets)
Figure 8 Compensation for In-Line Transformers at the
Local Relay Allows the Algorithms to Remain Unchanged
87
L+T
Relay 1
CT 1 CT 2
Relay 2
iCT1 iCT2
1
TAP1
T1T21
TAP2
