ABB Relion 670 series, Technical Manual

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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 153: 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 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
GUID-E6CC3CA7-72BE-40FC-A557-7BDB62F7BC1E v2
In ZMFPDIS, each zone measurement loop characteristic can be set to mho characteristic or offset
mho characteristic by setting
OpModePEZx or OpModePPZx (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 and offset Mho characteristics with different mode settings are
indicated in figure 154 .
1MRK 502 066-UUS B Section 7
Impedance protection
309
Technical manual