Schweitzer Engineering Laboratories SEL-351-5, Instruction Manual

Page: 191 / 706

4.47
Date Code 20070117
Instruction Manual SEL-351-5, -6, -7 Relay
Loss-of-Potential, Load Encroachment, and Directional Element Logic
Directional Control Settings
If setting ORDER = U (ungrounded or high-impedance grounded system; see
Figure 4.14 ), the following settings are made internally and hidden:
Z0F = -0.10
Ω
secondary
Z0R =
0.10
Ω
secondary
Z0F and Z0R (
Ω
secondary) are set in reference to the phase current channels
IA , IB , and IC , as are settings Z2F and Z2R. However, settings Z0F and Z0R are
applied to Figure 4.12 , and Figure 4.14 , where neutral current I
N
, from neutral
current channel IN, is also applied. Settings Z0F and Z0R are adjusted
internally (with CT ratio settings) to operate on this I
N
current base, when
needed (effectively, Z0F • CTRN/CTR and Z0R • CTRN/CTR). See Internal
Enables on page 4.30 .
Deriving Z0F and Z0R Settings
Figure 4.25 shows the voltage and current polarity for an SEL-351 in a zero-
sequence impedance network (the same approach can be instructive for
negative-sequence impedance analysis, too). For a forward fault, the SEL-351
effectively sees the sequence impedance behind it as:
Z
M
= V
0
/(–I
0
) = –(V
0
/I
0
)
V
0
/I
0
= –Z
M
(what the relay sees for a forward fault)
For a reverse fault, the SEL-351 effectively sees the sequence impedance in
front of it:
Z
N
= V
0
/I
0
V
0
/I
0
= Z
N
(what the relay sees for a reverse fault)
If the system in Figure 4.25 is a solidly-grounded system (mostly inductive;
presume uniform system angle), and the load is connected line-to-neutral, the
impedance plot (in the R + jX plane) would appear as in
Figure 4.26 a, with
resultant Z0F and Z0R settings as in Figure 4.26 b. The zero-sequence line
angle noted in Figure 4.26 a (
∠
Z0MTA) is the same angle found in
Figure 4.10 and Figure 4.12 (in the “equation box” with the “Enable” line).
The preceding method of automatically making settings Z0F and Z0R (where
both Z0F and Z0R are positive values; still Z0R > Z0F) usually suffices for
mostly inductive systems—
Figure 4.25 and Figure 4.26 just provide a
theoretic background.
1 A nominal current, 150 V voltage inputs 0.5
1 A nominal current, 300 V voltage inputs 1.0
Relay Configuration z (
Ω
secondary)