min
2 1 0
0.8 cos sin
2 1 0
load
R R
RFPG Z
X X
× +
£ × × ¶ - × ¶
× +
é ù
ê ú
ë û
EQUATION1721 V2 EN (Equation 331)
Where:
∂ is a maximum load-impedance angle, related to the maximum load power.
To avoid load encroachment for the phase-to-phase measuring elements, the set resistive
reach of any distance protection zone must be less than 160% of the minimum load
impedance.
EQUATION579 V3 EN (Equation 332)
Equation 147 is applicable only when the loop characteristic angle for the phase-to-phase
faults is more than three times as large as the maximum expected load-impedance angle.
For other cases a more accurate calculations are necessary according to equation 148.
load min
R1Zx
RFPPZx 1.6 Z cos sin
X1Zx
IECEQUATION2307 V2 EN (Equation 333)
All this is applicable for all measuring zones when no Power swing detection function
ZMRPSB (78) is activated in the IED. Use an additional safety margin of approximately
20% in cases when a ZMRPSB (78) function is activated in the IED, refer to the
description of Power swing detection function ZMRPSB (78).
8.12.3.8 Zone reach setting higher than minimum load impedance
The impedance zones are enabled as soon as the (symmetrical) load impedance crosses the
vertical boundaries defined by RLdFwd and RldRev or the lines defined by ArgLd. So, it
is necessary to consider some margin. It is recommended to set RLdFwd and RldRev to
90% of the per-phase resistance that corresponds to maximum load.
The absolute value of the margin to the closest LdAngle line should be of the same order,
that is, at least 0.1 • Z
load min
.
The load encroachment settings are related to a per-phase load impedance in a
symmetrical star-coupled representation. For symmetrical load or three-phase and phase-
to-phase faults, this corresponds to the per-phase, or positive-sequence, impedance. For a
1MRK 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 379
Application manual
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