7.1.6.2
Impedance characteristic
The Mho distance function ZMHPDIS is present with four instances so that four
separate zones could be designed. Each instance can be selected to be either forward
or reverse with positive sequence polarized mho characteristic; alternatively self
polarized offset mho characteristics is also available. One example of the operating
characteristic is shown in Figure
A) where zone 5 is selected offset mho.
The directional mho characteristic of Figure
has a dynamic expansion due to the
source impedance. Instead of crossing the origin as for the mho to the left of Figure
, which is only valid where the source impedance is zero, the crossing point is
moved to the coordinates of the negative source impedance giving an expansion of the
circle of Figure
R
jx
Mho, zone4
Mho, zone3
Mho, zone2
Mho, zone1
X
Zs=0
R
Zs=Z1
Zs=2Z1
IEC11000223-2-en.vsd
C
D
IEC11000223 V2 EN
Figure 65:
Mho characteristic and the source impedance influence on the mho characteristic
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 than a plain cross polarized characteristic. 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
LoadEnchMode
to
On
. 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
1MRK502052-UEN B
Section 7
Impedance protection
181
Technical manual
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