ABB RET670 Applications Manual page 188

Section 3
IED application
182
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RFPP 3 X1
IECEQUATION2306 V1 EN
Load impedance limitation, without load encroachment function
The following instructions are valid when Phase selection with load encroachment,
quadrilateral characteristic function FDPSPDIS (21) is not activated. To deactivate the
function, the setting of the load resistance RLdFwd and RldRev in FDPSPDIS (21)
must be set to max value (3000). If FDPSPDIS (21) is to be used for all or some of the
measuring zones, the load limitation for those zones according to this chapter can be
omitted. Check the maximum permissible resistive reach for any zone to ensure that
there is a sufficient setting margin between the boundary and the minimum load
impedance. The minimum load impedance (Ω/phase) is calculated as:
2
V
=
Z
loa d min
S
EQUATION1718 V1 EN
Where:
V is the minimum phase-to-phase voltage in kV
S is the maximum apparent power in MVA.
The load impedance [Ω/phase] is a function of the minimum operation voltage and the
maximum load current:
V
=
min
Z
loa d
×
3 I
ma x
EQUATION1719 V1 EN
Minimum voltage V and maximum current I
min
conditions. Minimum load impedance occurs normally under emergency conditions.
As a safety margin is required to avoid load encroachment under three-
phase conditions and to guarantee correct healthy phase IED operation
under combined heavy three-phase load and ground faults, consider
both: phase-to-phase and phase-to-ground fault operating characteristics.
To avoid load encroachment for the phase-to-ground measuring elements, the set
resistive reach of any distance protection zone must be less than 80% of the minimum
load impedance.
1MRK504116-UUS C
are related to the same operating
max
(Equation 89)
(Equation 90)
(Equation 91)
Application manual