5
Table 2 SEL-351 Phase, Negative-Sequence, Residual-Ground, and Neutral Overcurrent Elements (Sheet 2 of 2)
Overcurrent Element
Number of Elements Directional Control Torque Control Definite-Time Delay
Operating Quantity
Negative-sequence current (3I2) 1 inverse-time (51Q) Yes Yes NA
6 instantaneous (50Q1–50Q6) Yes, on first 4 Yes, on first 4 Yes, on first 4
Neutral current (IN) 1 inverse-time (51N) Yes Yes NA
6 instantaneous (50N1–50N6) Yes, on first 4 Yes, on first 4 Yes, on first 4
Inverse-time overcurrent element settings include a wide percentage of disc travel, and the amount of current. Set-
and continuous pickup current range, continuous time- ting EM Reset Delay = N resets the elements immedi-
dial setting range, and time-current curve choices from ately if current drops below pickup for at least one cycle.
both US (IEEE) and IEC standard curves shown in
Table 3.
Table 3 Inverse Time-Overcurrent Curves
Overcurrent Elements for Phase
Fault Detection
IEEE IEC
The SEL-351 Protection System provides the tools nec-
Moderately Inverse (U1) Standard Inverse (C1)
essary to provide sensitive fault protection, yet accom-
Inverse (U2) Very Inverse (C2) modate heavily loaded circuits. Where heavy loading
Very Inverse (U3) Extremely Inverse (C3) prevents the phase overcurrent elements from being set
sufficiently sensitive to detect lower magnitude phase-to-
Extremely Inverse (U4) Long-Time Inverse (C4)
ground faults, residual-ground overcurrent elements are
Short-Time Inverse (U5) Short-Time Inverse (C5) available to provide sensitive ground fault protection
without tripping under balanced heavy load conditions.
Use multiple inverse curves to coordinate with down- Likewise, when heavy loading prevents the phase over-
stream reclose fast and delay curves. Sequence coordina- current elements from being set sufficiently sensitive to
tion logic is also included to provide coordination detect lower magnitude phase-to-phase faults, negative-
between fast and delayed curves on the SEL-351 and sequence overcurrent elements are available to provide
downstream reclosers. Figure 2 represents an SEL-351 more sensitive phase-to-phase fault detection without
coordinated to a downstream SEL-351R Recloser Con- tripping under balanced heavy load conditions. Phase
trol. Inverse-time relay curve settings include a wide and overcurrent element pickup can be set high to accommo-
continuous pickup current and time-dial (vertical multi- date heavy load, yet remain sensitive to higher magni-
plier) range. tude three-phase faults. Block any element during
transformer inrush with programmable second-harmonic
blocking.
52 Recloser On extremely heavily loaded feeders, when phase over-
current elements cannot be set to provide adequate three-
SEL-651R
phase fault sensitivity and also accommodate load, the
SEL-351 SEL-351 load-encroachment logic adds security. This
t Delay logic allows you to set the phase overcurrent elements
Curves below peak load current to see end-of-line phase faults in
Retrofit older heavily loaded feeder applications. This load-encroach-
Fast existing reclosers with ment logic uses positive-sequence load-in and load-out
Curves SEL-651R Recloser Control elements to discriminate between load and fault condi-
tions based on the magnitude and angle of the positive-
I sequence impedance (Figure 3). When the measured
positive-sequence load impedance (Z1) resides in a
Figure 2 Coordinate Overcurrent Protective Devices
region defined by the load-encroachment settings, load-
The SEL-351 Protection System inverse-time overcur- encroachment logic blocks the phase overcurrent ele-
rent relay curve settings offer two reset characteristic ments. As Figure 3 shows, when a phase fault occurs, Z1
choices for each element. Setting EM Reset Delay = Y moves from a load region to the line angle and allows the
emulates electromechanical induction disc elements, phase overcurrent elements to operate.
where the reset time depends on the time-dial setting, the
Schweitzer Engineering Laboratories, Inc. SEL-351-5, -6, -7 Data Sheet
