Schweitzer Engineering Laboratories SEL-751 - Figure 4.57 A-Phase Second Harmonic Blocking

4.86
SEL-751 Relay Instruction Manual Date Code 20170927
Protection and Logic Functions
Group Settings (SET Command)
HIF Detection Event Reports and Histories
The SEL-751 stores HIF detection information as compressed events and as
event summaries, logs, and histories. See High-Impedance Fault Event Sum-
mary on page 10.30, High-Impedance Fault Compressed Event Report on
page 10.34, and Figure 7.20, Figure 5.15: MET H (HIF) Command Response,
Figure 7.28: LOG H (HIF) Command Response, and Figure 7.27: HSG Com-
mand Response for more information.
Second- and Fifth-
Harmonic Blocking
Logic
When a distribution feeder supplies many transformers, magnetizing inrush
currents may cause sensitive overcurrent elements to operate when the line is
energized. The second-harmonic blocking logic can prevent this by blocking
such elements until inrush currents have subsided. As shown in Figure 4.57
and Figure 4.58, this logic uses the ratio of the second-harmonic content of
each phase to the fundamental current of the same phase to calculate the per-
cent harmonic content. The fifth-harmonic blocking logic is analogous to the
second-harmonic logic.
When the SEL
OGIC torque-control equation HBL2TC evaluates to logical 1,
and if the second-harmonic content of a particular phase (e.g., IAHC2 for
A-phase in Figure 4.57) exceeds the adjustable pickup threshold HBL2P for
the pickup time delay HBL2PU, the blocking Relay Word bit for that phase
asserts. Once the output is asserted, if the second-harmonic content falls below
the threshold for the dropout time delay HBL2DO, the output deasserts. If any
of the phase outputs asserts, Relay Word bit HBL2T also asserts. The same
logic applies to fifth-harmonic blocking.
Figure 4.57 A-Phase Second Harmonic Blocking
Begin 24-Hour Initial HIF Tuning Process HIFITUNE, INI_HIF
Normal HIF Tuning in Progress NTUNE_A, NTUNE_B, NTUNE_C
Increase the HIF Tuning Threshold DUPA, DUPB, DUPC
Decrease the HIF Tuning Threshold DDNA, DDNB, DDNC
Load Reduction Detected LRA, LRB, LRC, LR3
Ta b l e 4 . 29 HI F R e lay Wo rd B it s (Sheet 2 of 2)
HIF Activity Relay Word Bits
Relay
Word
Bits
Settings
SELOGIC
Setting
|IA|
0.05 * INOM
HBL2TC
HBL2AT
HBL2PU
HBL2DO
|IAHC2|
|IA|
HBL2P
100