Schweitzer Engineering Laboratories SEL-351A - Figure 4.25 Zero-Sequence Impedance Network and Relay Polarity; Figure 4.26 Zero-Sequence Impedance Plot for Solidly-Grounded, Mostly Inductive System

4.47
Date Code 20080213 Instruction Manual SEL-351A Relay
Loss-of-Potential, Load Encroachment, and Directional Element Logic
Directional Control Settings (Not in SEL-351A-1)
If the system in Figure 4.25 is a solidly-grounded system (mostly inductive;
presume uniform system angle) and load connected line-to-neutral. The
impedance plot (in the R + jX plane) would appear as in Figure 4.26a, with
resultant Z0F and Z0R settings as in Figure 4.26b. The zero-sequence line
angle noted in Figure 4.26a (∠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.
Figure 4.25 Zero-Sequence Impedance Network and Relay Polarity
Figure 4.26 Zero-Sequence Impedance Plot for Solidly-Grounded, Mostly
Inductive System
Z0MTA—Zero-Sequence Maximum Torque Angle
Setting Range:
–90 to –5.00 degrees
5.00 to 90.00 degrees
If enable setting E32 = Y and ORDER contains a V or S, setting Z0MTA
should be set. Z0MTA must be set to compensate for the neutral-ground
resistor and is used in the Best Choice Ground Directional Element logic to
make proper forward and reverse fault determination (see Figure 9.12).
If enable setting E32 = AUTO, then Z0MTA is set equal to Z0ANG and
Z0MTA is hidden.
I
0
V
0
Z
N
Z
M
Forward
Reverse
Zero-Sequence Reference Bus
SEL-351
(a) Impedance Plot (b) Z0F and Z0R Settings
Z
N
= R
N
+ jX
N
(Reverse)
–Z
M
= —R
M
— jX
M
(Forward)
R
jX
Reverse
Forward
Z0F
Z
0
Z0R
Z
N
— Z
M
Z
N
> Z0R
Z0F > — Z
M
Z0R > Z0F
Zero-Sequence
Line Angle
(Setting Z0ANG)
Z0MTA
Courtesy of NationalSwitchgear.com