ABB Relion 670 series - Page 318


AB
V
12
1
L L set
IZ
1
comp AB AB set
V V I Z  
pol
V
AB
I jX
AB
IR
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 set1 
and
Z RVset1
where
Z set1 
and
Z RVset1
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 RVset1
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