Date Code 20110408 Instruction Manual SEL-551 Relay
Appendix F
Setting Negative-Sequence
Overcurrent Elements
Setting Negative-Sequence
Definite-Time Overcurrent Elements
Negative-sequence instantaneous overcurrent elements 50Q1 and 50Q2
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 having negative-sequence instantaneous overcurrent elements trip for
this transient condition, delay negative-sequence instantaneous overcurrent
elements by at least 1.5 cycles (transient condition lasts less than 1.5 cycles).
Use the SEL
OGIC
®
Variable timers described in SELOGIC Control Equation
Variables/Timers on page 3.38.
Effectively, negative-sequence instantaneous overcurrent elements 50Q1 and
50Q2 are turned into negative-sequence definite-time overcurrent elements by
running them through timers. Use the timer output for tripping.
Continue reading in Coordinating Negative-Sequence Overcurrent Elements
on page F.2 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 elements 51Q1T and 51Q2T should not
be set to trip directly when they are set with low time-dial settings 51Q1TD
and 51Q2TD, respectively, that result in curve times below 3 cycles (see
curves in Figure 4.1–Figure 4.10). This is because negative-sequence current
can transiently appear when a circuit breaker is closed and balanced load
current suddenly appears.
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