F.2
SEL-351-5, -6, -7 Relay
Instruction Manual
Date Code 20070117
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
–
). This is
because negative-sequence current can transiently appear when a circuit
breaker is closed and balanced load current suddenly appears. Refer to
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
Continue reading in
Coordinating Negative-Sequence Overcurrent Elements
for guidelines on coordinating negative-sequence time-
overcurrent elements and a following coordination example.
51QT * SV6T
I (3I
2
)
51Q
SV6PU
t
Summary of Contents for SEL-351-5
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