Date Code 20080213 Instruction Manual SEL-351A Relay
Appendix F
Setting Negative-Sequence
Overcurrent Elements
Setting Negative-Sequence Definite-Time
Overcurrent Elements
Negative-sequence instantaneous overcurrent elements 50Q1–50Q6 and
67Q1–67Q4 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–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
Figure 3.12 and Figure 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 Figure 7.23 and Figure 7.24) and use the outputs of the
timers for tripping.
Continue reading in Coordinating Negative-Sequence Overcurrent Elements
on page F.3 for guidelines on coordinating negative-sequence definite-time
overcurrent elements and for a coordination example. The coordination
example uses time-overcurrent elements, but the same principles can be
applied to definite-time overcurrent elements.
Courtesy of NationalSwitchgear.com
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