R
X
Z3
Z2
Z1
Z4
Z5
ZRV
R
X
ZS=0
ZS=Z1
ZS=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
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