ABB RELION 670 Series Technical Manual page 315

1MRK 502 066-UUS B
IEC15000056 V1 EN-US
Figure 153: Mho, offset mho characteristics and the source impedance influence on the mho
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 153, 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 153. Z1 denotes the complex positive
sequence impedance.
The magnitude of the polarizing 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
In ZMFPDIS, each zone measurement loop characteristic can be set to mho characteristic or offset
mho characteristic by setting
zone).
ZMFPDIS fixes zone 1 and 2 in
Non-directional , Forward or Reverse by setting the parameter DirModeZx (where x is 3-5
depending on selected zone).
DirModeZx (where x is 3-5 depending on selected zone) is selected as Non-directional , the
If
directional element will not have any effect on the measurement loop and operation of the
function. When
Reverse , directional lines are introduced. Information about the directional lines is given from the
directional element. Basic Mho and offset Mho characteristics with different mode settings are
indicated in figure 154.
Technical manual
X
Z4
Z3
Z2
Z1
Z5
R
ZRV
characteristic
OpModePEZx or OpModePPZx (where x is 1-5 depending on selected
Forward mode and zone RV in Reverse mode. Zone 3-5 can be set to
DirModeZx (where x is 3-5 depending on selected zone) is selected as Forward or
X
Z =0
S
R
Z =Z1
S
Z =2Z1
S
IEC15000056-1-en.vsdx
GUID-E6CC3CA7-72BE-40FC-A557-7BDB62F7BC1E v2
Section 7
Impedance protection
309