Schweitzer Engineering SEL-351P, Quick Start Manual

Page: 628 / 674

Date Code 990215 Setting Negative-Sequence Overcurrent Elements F-1
SEL-351P Manual Técnico
APPENDIX F: SETTING NEGATIVE-SEQUENCE
OVERCURRENT ELEMENTS
S
ETTING
N
EGATIVE
-S
EQUENCE
D
EFINITE
-T
IME
O
VERCURRENT
E
LEMENTS
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 SEL
OGIC
®
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.
S
ETTING
N
EGATIVE
-S
EQUENCE
T
IME
-O
VERCURRENT
E
LEMENTS
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 9.20 in Section 9: Setting the SEL-351P Recloser Control ). 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.