ERL L-PRO 4000, User Manual

Page: 54 / 406

4 Protection Functions and Specifications
4-4 L-PRO 4000 User Manual D02706R02.30
Restrict the reach in the R region for the phase distance relays where load en-
croachment is an issue. The shaped Mho characteristic provides the best fit for
the application keeping the number of relay settings at a minimum and pro-
vides the benefits associated with the Mho characteristic.
The Mho characteristic used by the relay is developed by the classical ap-
proach using the measurement of the angle between 2 vectors.
These vectors are defined as:
where
V is the actual line voltage for ground distance relays or the actual line to
line voltage for the phase distance relay.
I is defined as above for ground distance relays or the line to line current for
phase distance relays.
Z
set
is the setting reach and V
ref
is a positive sequence memory voltage
stored within the relay.
V
ref
is the polarizing quantity for the Mho elements, and is more completely
described in “Relay Method of Memory Polarization” on page 4-8.
To make the reach of the ground distance relay relate to the line positive se-
quence impedance the classical
K
o
factor is used.
This factor is defined as
The relay includes a directional element to supervise the phase (21P) and
ground (21N) Mho elements, for all five Zones (21P1 to 21P5 and 21N1 to
21N5). The directional element improves security of the Mho elements for re-
verse faults such as: bus faults, phase-phase faults during high load conditions.
The directional element does not supervise Zone 3, Zone 4 and Zone 5 ele-
ments if these zones are set as offset characteristic. The directional element is
described in “Directional Element” on page 4-10.
(1)
(2)
(3)
(4)
A I Z
set

V – =
B V
ref
=
I
phase
K
o
3 I
0

+
K
0
Z
0
Z
1
–
3 Z
1
------------------ =