ABB REL650 series Technical Manual page 115

1MRK 506 335-UUS A
jx
IEC09000143 V2 EN
Figure 39: Mho, offset mho characteristic and the source impedance influence on the mho characteristic
Technical manual
Mho, zone4
Mho, zone3
Mho, zone2
Mho, zone1
R
Offset mho, zone5
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 39, which is only valid where the
source impedance 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 39. Z1
denotes the complex positive sequence impedance.
The polarization quantities used for the mho circle are 100% memorized positive
sequence voltages. 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
In ZQMPDIS (21), each zone measurement loop characteristic can be set to mho
characteristic by setting CharPPZx or CharPGZx (where x is 1-5 depending on selected
zone). These mho characteristics can be classified into - Offset or Directional. The
directional mho characteristics can be set to Non-directional, Forward or Reverse by the
setting parameter DirModeZx (where x is 1-5 depending on selected zone). The offset mho
characteristic can be set to Forward or Reverse by the setting parameter MhoCharZx
(where x is 1-5 depending on selected zone).
Section 6
Impedance protection
X
Zs=Z1
Zs=2Z1
IEC09000143_2_en.vsd
Zs=0
R
109