ABB Relion 670 series, Technical Reference Manual

Page: 248 / 1232

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
117 , 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
117 .
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
LoadEnchMode 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.
6.4.2.3 Basic operation characteristics
Each impedance zone can be switched OnEnabled and OffDisabled by the setting
parameter Operation .
Each zone can also be set to Non-directional , Forward or Reverse by setting the
parameter DirModeSel .
The operation for phase-to-ground and phase-to-phase fault can be individually
switched
Enabled and Disabled by the setting parameter OpModePG and OpModePP.
For critical applications such as for lines with high SIRs as well as CVTs, it is possible
to improve the security by setting the parameter
ReachMode to Underreach. In this
mode the reach for faults close to the zone reach is reduced by 20% and the filtering is
also introduced to increase the accuracy in the measuring. If the
ReachMode is set to
Overreach no reduction of the reach is introduced and no extra filtering introduced.
The latter setting is recommended for overreaching pilot zone, zone 2 or zone 3
elements and reverse zone where overreaching on transients is not a major issue either
because of less likelihood of overreach with higher settings or the fact that these
elements do not initiate tripping unconditionally.
The offset Mho characteristic can be set in Non-directional , Forward or Reverse by the
setting parameter OffsetMhoDir . When Forward or Reverse is selected a directional
Section 6
1MRK505222-UUS C
Impedance protection
242
Technical reference manual