Schweitzer Engineering Laboratories SEL-311A, Instruction Manual, Page: 45 / 476

Date Code 20011205 Distance and Overcurrent Elements 3-5
SEL-311A Instruction Manual
V
B
V
AB
- Z
• I
AB
Unfaulted
V
A
V
C
Test Angle
sin (
θ
< 180
°
) > 0
V
C
mem
-jV
AB
- 0
.25
• V
C m
em
V
AB
- Z
• I
AB
-jV
AB
- 0
.25
• V
C m
em
Test Angle
sin (
θ
< 180
°
) > 0
V
A
V
B
V
C
V
AB
- Z
• I
AB
-jV
AB
- 0
.25
• V
C m
em
Test Angle
sin (
θ
> 180
°
) < 0
V
A
V
B
V
C
V
C
mem
External Fault
Internal Fault
Note: V
A
, V
B
, and V
C
are internal element voltages, not system voltages.
V
C
mem
M311B036a
Figure 3.3: Compensator-Distance Three-Phase Element Operation
Positive-sequence polarized and compensator distance mho elements each have different
operating advantages in different protection environments, but work equally well in the majority
of transmission line applications. Consider using compensator distance elements when:
·
A different phase-distance operating principle is desired for backup relaying.
·
Protecting a transmission line through a delta-wye transformer. The compensator
distance element reaches through a delta-wye transformer bank for phase-to-phase,
phase-to-phase-to-ground, and three-phase faults. Calculate the total primary
impedance as the sum of the per-unit transformer and line impedances, then convert
from per-unit to actual primary impedance at the protected bus voltage. The