Schweitzer Engineering Laboratories SEL-351-5, Instruction Manual

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4.4
SEL-351-5, -6, -7 Relay Instruction Manual
Date Code 20070117
Loss-of-Potential, Load Encroachment, and Directional Element Logic
Load-Encroachment Logic
Load-Encroachment Logic
The load-encroachment logic (see Figure 4.2 ) and settings are enabled/
disabled with setting ELOAD (= Y or N). (If group setting VNOM = OFF,
then ELOAD can only be set to “N.” See
Potential Transformer Ratios and PT
Nominal Secondary Voltage Settings on page 9.42 for more details on the
VNOM setting.)
The load-encroachment feature allows phase overcurrent elements to be set
independent of load levels. This is especially helpful in bus overcurrent
applications. A bus relay sees the cumulative currents of all the feeders but
still has to provide overcurrent backup protection for all these feeders. If the
phase elements in the bus relay are set to provide adequate backup, they often
are set close to maximum bus load current levels. This runs the risk of tripping
on bus load current. The load-encroachment feature prevents this from
happening as shown in the example that follows in this subsection.
q To Figure 4.21 .
Figure 4.2 Load-Encroachment Logic
(90˚)
(270˚) (—90˚)
(0˚) (180˚)
Example Settings
X
1
R
1
Positive-Sequence Threshold = (Phase Channels Nominal Rating) • (0.1)
Load In
ZLOUT
ZLOAD
Load Out
ZLIN
PLAR
PLAF
> ∠
Z
1
>
NLAF
NLAR
> ∠
Z
1
>
PLAR
Pos.-Seq.
Threshold
|Z
1
| (Magnitude)
∠
Z
1
(Angle)
V
1
I
1
V
1
I
1
Z
1
=
q
Relay
Word
Bits
ZLF
ZLR
Settings
ZLIN
(Load In
Region)
ZLOUT
(Load Out
Region)
ZLR
= 19
.00
Ω
sec
.
NLAR
= 217˚
PLAR = 162
˚
PLAF = 2
6˚
NLAF = —
18˚
ZLF =
11.
90
Ω
sec.
to q