Date Code 20001006 Setting Negative-Sequence Overcurrent Elements F-1
SEL-351 Instruction Manual
APPENDIX F: SETTING NEGATIVE-SEQUENCE
OVERCURRENT ELEMENTS
SETTING NEGATIVE-SEQUENCE DEFINITE-TIME OVERCURRENT ELEMENTS
Negative-sequence instantaneous overcurrent elements 50Q1 through 50Q6 and 67Q1
through 67Q4 should not be set to trip directly. This is because negative-sequence current can
transiently appear when a circuit breaker is closed and balanced load current suddenly appears.
To avoid tripping for this transient condition, use negative-sequence definite-time overcurrent
elements 67Q1T through 67Q4T with at least 1.5 cycles of time delay (transient condition lasts
less than 1.5 cycles). For example, make time delay setting:
67Q1D = 1.50
for negative-sequence definite-time overcurrent element 67Q1T. Refer to Figures 3.12 and 3.13
for more information on negative-sequence instantaneous and definite-time overcurrent elements.
Negative-sequence instantaneous overcurrent elements 50Q5 and 50Q6 do not have associated
timers (compare Figure 3.13 to Figure 3.12). If 50Q5 or 50Q6 need to be used for tripping, run
them though SELOGIC
®
Control Equation variable timers (see Figures 7.25 and 7.26) and use the
outputs of the timers for tripping.
Continue reading in Coordinating Negative-Sequence Overcurrent Elements in this appendix for
guidelines on coordinating negative-sequence definite-time overcurrent elements and a following
coordination example. The coordination example uses time-overcurrent elements, but the same
principles can be applied to definite-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
Figures 9.1 through 9.10 in Section 9: Setting the Relay). This is because negative-sequence
current can transiently appear when a circuit breaker is closed and balanced load current suddenly
appears. Refer to Figure 3.20 for more information on negative-sequence time-overcurrent element
51QT.
Courtesy of NationalSwitchgear.com
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