ABB Relion 670 series - Zone Reach Setting Lower than Minimum Load Impedance

Setting of the resistive reach for the underreaching zone 1 should follow the condition to
minimize the risk for overreaching:
RFPG 4.5 X1£ ×
ANSIEQUATION2305 V1 EN (Equation 326)
The fault resistance for phase-to-phase faults is normally quite low compared to the fault
resistance for phase-to-ground faults. To minimize the risk for overreaching, limit the
setting of the zone1 reach in the resistive direction for phase-to-phase loop measurement
based on equation
142.
61
R
FPPZx X Zx


IECEQUATION2306 V3 EN (Equation 327)
The setting XLd is primarily there to define the border between what is considered a fault
and what is just normal operation. See figure 183 In this context, the main examples of
normal operation are reactive load from reactive power compensation equipment or the
capacitive charging of a long high-voltage power line. XLd needs 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 RLdFwd and RldRev, 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
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 RLdFwd,
RldRev and LdAngle 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
143.
1MRK 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 377
Application manual