ABB Relion 670 series, Technical Manual

Page: 355 / 1432

R
X
Z3
Z2
Z1
Z4
Z5
ZRV
R
X
Z
S
=0
Z
S
=Z1
Z
S
=2Z1
IEC15000056-1-en.vsdx
IEC15000056 V1 EN-US
Figure 176: Mho, offset mho characteristics and the source impedance influence on the mho
characteristic
The mho characteristic has a dynamic expansion due to the source impedance. Instead of crossing
the origin, as for the mho to the left of figure 176 , which is only valid where the source impedance
(Zs) is zero, the crossing point is moved to the coordinates of the negative source impedance
given an expansion of the circle shown to the right of figure 176 . Z1 denotes the complex positive
sequence impedance.
The magnitude of the polarized voltage is determined completely by the positive sequence voltage
magnitude from before the fault. This will give a somewhat less dynamic expansion of the mho
circle during faults. However, if the source impedance is high, the dynamic expansion of the mho
circle might lower the security of the function too much with high loading and mild power swing
conditions.
Basic operation characteristics
GUID-B9D7C3D8-811A-419B-8373-712EC122EB08 v1
In ZMFPDIS, each zone measurement loop characteristic can be set to mho characteristic or offset
mho characteristic by setting zxOpModePE or zxOpModePP (where x is 1-5 depending on selected
zone).
ZMFPDIS fixes zone 1 and 2 in
Forward mode and zone RV in Reverse mode. Zone 3-5 can be set to
Non-directional, Forward or Reverse by setting the parameter DirModeZx (where x is 3-5
depending on selected zone).
If
DirModeZx (where x is 3-5 depending on selected zone) is selected as Non-directional, the
directional element will not have any effect on the measurement loop and operation of the
function. When
DirModeZx (where x is 3-5 depending on selected zone) is selected as Forward or
Reverse, directional lines are introduced. Information about the directional lines is given from the
directional element. Basic Mho characteristics with different mode settings are indicated in figure
177 .
1MRK 502 066-UUS B Section 7
Impedance protection
349
Technical manual