Zone Reach Setting Lower Than Minimum Load Impedance - ABB RED650 Applications Manual

1MRK 505 393-UEN B
7.1.3.7
[2] RLdRv=RLdRvFactor*RLdFw
[3] RLdRv=RLdRvFactor*RLdFw
Line differential protection RED650 2.2 IEC
Application manual
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
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.

Zone reach setting lower than minimum load impedance

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
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 67.
2
U
Z
= ------ -
loadmin
S
EQUATION571 V1 EN-US
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:
U
min
--------------------- -
Z =
load
×
3 I
max
EQUATION574 V1 EN-US
Minimum voltage U
min
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]
, XLd is representing a per-phase load
[3]
and ArgLd according to the expected load
and maximum current I
Section 7
Impedance protection
GUID-68C336F4-5285-4167-B3F8-B0963BD85439 v5
are related to the same
max
(Equation 67)
(Equation 68)
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