ABB REL670 2.2 IEC - Zone Reach Setting Lower than Minimum Load Impedance

to be set with some margin towards normal apparent reactance; not more than 90%
of the said reactance or just as much as is needed from a zone reach point of view.
As with the settings RLdFw and RLdRv
[2]
, XLd is representing a per-phase load
impedance of a symmetrical star-coupled representation. For a symmetrical load or
three-phase and phase-to-phase faults, this means per-phase, or positive-sequence,
impedance. During a phase-to-earth fault, it means the per-loop impedance,
including the earth return impedance.
8.12.3.7 Zone reach setting lower than minimum load impedance
GUID-68C336F4-5285-4167-B3F8-B0963BD85439 v5
Even if the resistive reach of all protection zones is set lower than the lowest
expected load impedance and there is no risk for load encroachment, it is still
necessary to set RLdFw, RLdRv
[3]
and ArgLd according to the expected load
situation, since these settings are used internally in the function as reference points
to improve the performance of the phase selection.
The maximum permissible resistive reach for any zone must be checked 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 with
equation
325.
Z
loadmin
U
2
S
-------=
EQUATION571 V1 EN-US (Equation 325)
Where:
U the minimum phase-to-phase voltage in kV
S the maximum apparent power in MVA.
The load impedance [Ω/phase] is a function of the minimum operation voltage and
the maximum load current:
Z
load
U
min
3 I
max
×
----------------------
=
EQUATION574 V1 EN-US
(Equation 326)
Minimum voltage U
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, it is required to avoid load encroachment under
three-phase conditions. To guarantee correct, healthy phase IED
operation under combined heavy three-phase load and earth faults,
[2] RLdRv=RLdRvFactor*RLdFw
[3] RLdRv=RLdRvFactor*RLdFw
Section 8 1MRK 506 369-UEN B
Impedance protection
352 Line distance protection REL670 2.2 IEC
Application manual