ABB RELION 670 Series Technical Manual page 318

Section 7
Impedance protection
ANSI15000060 V1 EN-US
Figure 155: Simplified mho characteristic and vector diagram for phase A-to-B fault
Offset Mho
The characteristic for offset mho is a circle where two points on the circle are given by the two
vectors
reactance settings in forward and reverse directions.
The condition for operation at phase-to-phase fault is that the angle β between the two
compensated voltages is greater than or equal to 90° (figure 156). The angle will be 90° for fault
location on the boundary of the circle.
The angle β for A to B fault can be defined according to equation below.
arg
β =
ANSIEQUATION15008 V1 EN-US
where
1
Z
RVset
312

I jX
AB

I Z 1
L L 1 2 set
and where
1 1
Z Z
set RVset

1
V I Z
− ⋅
AB AB set

( )
V I Z 1

− − ⋅
AB AB RVset

is the positive sequence impedance setting for phase-to-phase fault opposite to zone
direction and is defined as
 
V V I
comp AB

V
AB
V
pol
and are settable through the resistance and
1 1
Z Z
set RVset





Z 1
AB set

I R
AB
ANSI15000060-1-en.vsdx
GUID-3E13E6D5-0832-4386-9677-9A40BFF42F8F v1
1MRK 502 066-UUS B
(Equation 87)
Technical manual