ABB RET670 - Page 188

RFPP 3 X1£ ×
IECEQUATION2306 V1 EN (Equation 89)
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
loa d min
Z
V
S
=
EQUATION1718 V1 EN (Equation 90)
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:
min
load
max
Z
V
3 I
=
×
EQUATION1719 V1 EN (Equation 91)
Minimum voltage V
min
and maximum current I
max
are related to the same operating
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.
Section 3 1MRK504116-UUS C
IED application
182
Application manual