Schweitzer Engineering Laboratories SEL-251 - Minimum Response Time for Time-Overcurrent Elements; Negative-Sequence Elements Protect for Phase Faults

Date Code 20000421 Applications 5-11
SEL-251, -2, -3 Instruction Manual
MINIMUM RESPONSE TIME FOR TIME-OVERCURRENT ELEMENTS
For time-overcurrent element coordination problems at higher fault current levels, you can
incorporate a minimum response time into the time-overcurrent element. Such a time-
overcurrent element cannot operate until a set minimum time passes. A ground/residual time-
overcurrent element example appears below.
To implement this scheme, set 51NT and 50NLT with the same magnitude pickup setting (51NP
= 50NL). Set the 50NLT setting with the desired minimum response time.
Using SELOGIC Control Equations:
A(12) = 51NT (ground/residual time-overcurrent element)
B(12) = 50NLT (ground residual definite-time overcurrent element)
V(56) = A*B
TR(1246) = V+... (TR is the programmable trip condition)
effectively, TR(1246) = 51NT*50NLT+...
close TRIP output contacts = TR + ...
Figure 5.7: Minimum Response Time Using Ground/Residual Elements
NEGATIVE-SEQUENCE ELEMENTS PROTECT FOR PHASE FAULTS
Table 5.1 shows different system conditions and the currents generated by each.