Schweitzer Engineering Laboratories SEL-311C - Setting Negative-Sequence Time-Overcurrent Elements; Figure F.1 Minimum Response Time Added to a

F.2
SEL-311C Relay Instruction Manual Date Code 20060320
Setting Negative-Sequence Overcurrent Elements
Setting Negative-Sequence Time-Overcurrent Elements
Setting Negative-Sequence
Time-Overcurrent Elements
Negative-sequence time-overcurrent element 51QT should not be set to
trip directly when it is set with a low time-dial setting 51QTD, that results
in curve times below 3 cycles (see curves in Figure 9.1 through Figure 9.10).
This is because negative-sequence current can transiently appear when a
circuit breaker is closed and balanced load current suddenly appears. Refer to
Figure 3.26 for more information on negative-sequence time-overcurrent
element 51QT.
To avoid having negative-sequence time-overcurrent element 51QT with such
low time dial settings trip for this transient negative-sequence current
condition, make settings similar to the following:
SV6PU = 1.50 cycles (minimum response time; transient condition lasts
less than 1.5 cycles)
SV6 = 51Q (run pickup of negative-sequence time-overcurrent element
51QT through SEL
OGIC
®
control equation variable timer SV6)
TR = …+ 51QT * SV6T +… (trip conditions; SV6T is the output of the
SEL
OGIC control equation variable timer SV6)
Figure F.1 Minimum Response Time Added to a
Negative-Sequence Time-Overcurrent Element 51QT
51QT * SV6T
I (3I
2
)51Q
SV6PU
t