ABB Relion 670 series, Technical Manual

Page: 318 / 1432


AB
V
1 2
1
L L set
I Z

1
comp AB AB set
V V I Z
  
pol
V
AB
I jX

AB
I R

ANSI15000060-1-en.vsdx
ANSI15000060 V1 EN-US
Figure 155: Simplified mho characteristic and vector diagram for phase A-to-B fault
Offset Mho
GUID-3E13E6D5-0832-4386-9677-9A40BFF42F8F v1
The characteristic for offset mho is a circle where two points on the circle are given by the two
vectors
Z
set
1
and
Z
RVset
1
where
Z
set
1
and
Z
RVset
1
are settable through the resistance and
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 =
− ⋅
− − ⋅
( )








V I Z
V I Z
AB AB set
AB AB RVset
1
1
ANSIEQUATION15008 V1 EN-US
(Equation 87)
where
Z
RVset
1
is the positive sequence impedance setting for phase-to-phase fault opposite to zone
direction and is defined as
Section 7 1MRK 502 066-UUS B
Impedance protection
312
Technical manual