ABB Relion 650 Series Technical Manual page 113

1MRK 506 326-UUS B
5.3.6.2 Impedance characteristic
The distance function consists of five zones. Each zone can be selected to be either forward or
reverse with positive sequence polarized mho characteristic alternatively self polarized offset mho
characteristics with reverse offset. The operating characteristic is in accordance to figure
where zone 5 is selected offset mho.
jx
IEC09000143 V1 EN-US
Figure 57: Mho, offset mho characteristic 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 57, 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 57. 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.
The mho distance element has a load encroachment function which cuts off a section of the
characteristic when enabled. The function is enabled by setting the setting parameter
LoadEnchModex (where x is 1-5 depending on selected zone) to Enabled . Enabling of the load
encroachment function increases the possibility to detect high resistive faults without interfering
with the load impedance. The algorithm for the load encroachment is located in the Faulty phase
identification with load encroachment for mho function FMPSPDIS (21), where also the relevant
settings can be found. Information about the load encroachment from FMPSPDIS (21) to the zone
measurement is given in binary format to the input signal LDCND.
Line distance protection REL650
Technical manual
Mho, zone4
Mho, zone3
Mho, zone2
Mho, zone1
R
Offset mho, zone5
© Copyright 2011 ABB Power Grids. All rights reserved
Section 5
Impedance protection
SEMOD154224-11 v4
57
X
Zs=Z1
Zs=2Z1
IEC09000143-1-en.vsd
Zs=0
R
107