Date Code 20011112
Line Current Differential, Distance, Out-of-Step, Overcurrent,
3-1
Voltage, Synchronism Check, and Frequency Elements
SEL-311L Instruction Manual
SECTION 3: LINE CURRENT DIFFERENTIAL, DISTANCE,
OUT-OF-STEP, OVERCURRENT, VOLTAGE,
SYNCHRONISM CHECK, AND FREQUENCY
ELEMENTS
The SEL-311L Line Current Differential Protection and Automation System includes both
current and voltage based protection elements. Loss or absence of potential does not affect
current-based elements such as line current differential, overcurrent and tapped-load protection.
This section describes operational theory and settings guidelines for key elements.
C
URRENT
D
IFFERENTIAL
E
LEMENTS
The SEL-311L Relay contains five line current differential elements: one for each phase, and one
each for negative-sequence and ground current. The phase elements provide high-speed
protection for high current faults. Negative-sequence and ground elements provide sensitive
protection for unbalanced faults without compromising security. This section familiarizes you
with the operating principles of the line current differential elements in the SEL-311L Relay and
introduces a setting philosophy that gives secure, fast, sensitive, and dependable operation. For
most two-terminal and three-terminal applications, the 87L settings need not be changed from the
factory defaults.
SEL-311L
87LA
87LB
87LC
87LG
87L2
SEL-311L
Ia, Ib, Ic
M311L074
87LC
87LB
87LA
87LG
87L2
Figure 3.1: SEL-311L Relay Line Current Differential Elements
The SEL-311L Relay exchanges time-synchronized Ia, Ib, and Ic samples between two or three
line terminals. Each relay calculates 3I2 and 3I0 for all line terminals. Current differential
elements 87LA, 87LB, 87LC, 87L2, and 87LG in each relay compare Ia, Ib, Ic, 3I2, and 3I0 (IG)
from each line terminal. All relays perform identical line current differential calculations in a
peer-to-peer architecture to avoid transfer trip delays.
This section describes settings considerations for the differential elements. Phase differential
elements 87LA, 87LB, and 87LC reliably detect three-phase faults. The negative-sequence
element 87L2 detects internal unbalanced faults and is restrained when all three of the phase
currents from any terminal exceed 3 • Inom.
Settings Example: 230 kV Transmission Line
With Tapped Load
in
Section 9: Setting the Relay
shows how to use the zero-sequence element
for high-speed tripping with delta-wye grounded tapped loads.
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