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Date Code 20011112
Line Current Differential, Distance, Out-of-Step, Overcurrent,
3-3
Voltage, Synchronism Check, and Frequency Elements
SEL-311L Instruction Manual
outside the restraint region, and the difference current is above a settable threshold. The relay
restrains when the Alpha plane ratio remains inside the restraint region, or when there is
insufficient difference current.
M311L075
Trip Region
Restraint Region
Re
I
R
I
L
( (
Im
87LANG
2
87LANG
2
1
87LR
1
∠
180˚
I
R
I
L
( (
87LR
Figure 3.3: SEL-311L Relay Restraint Region Surrounds External Faults
The shape of the restraint region is described by two settings, as shown in Figure 3.3. Setting
87LANG determines the angular extent of the restraint region. Setting 87LR determines the outer
radius of the restraint region. The inner radius is the reciprocal of 87LR. All three types of
elements (phase, negative-sequence, and zero-sequence) further qualify trips with a differential
pickup setting. For example, setting 87LPP qualifies trips generated by the phase current
differential elements 87LA, 87LB, and 87LC. If the A-phase current ratio travels outside the
restraint characteristic, and the A-phase difference current exceeds setting 87LPP, then element
87LA asserts, indicating an internal fault. Differential pickup settings 87LGP and 87L2P provide
similar supervision for the ground current differential element 87LG and the negative-sequence
current differential element 87L2, respectively.
Traditional line current differential relays, phase comparison relays, and charge comparison
relays can also be represented as a restraint region on the Alpha plane. See technical paper "The
Effect of Multiprinciple Line Protection on Dependability and Security" by Jeff Roberts,
Demetrios Tziouvaras, et al. to see how other Alpha plane restraint regions compare to the
restraint region of the SEL-311L Relay. In every case, the SEL-311L Relay gives significant
improvement in security, sensitivity, speed, dependability, or all four.
Setting the Restraint Region and Supervision Elements
This section discusses setting the restraint region and differential overcurrent supervision
elements to protect a two-terminal line. Three-terminal settings are similar and are discussed
later. Set the phase differential elements 87LA, 87LB, and 87LC to reliably detect internal
three-phase faults. Set the negative-sequence differential element 87L2 to reliably detect internal
unbalanced faults (87L2 is restrained when all three of the phase currents from any line terminal
exceed 15 A in a 5 A nominal relay or 3 A in a 1 A nominal relay. 87LG is restrained when two
or more phase currents from any line terminal exceed 3 • Inom).
Summary of Contents for SEL-311L
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