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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.
7.12.3.7
Zone reach setting lower than minimum load impedance
GUID-68C336F4-5285-4167-B3F8-B0963BD85439 v4
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 the equation.
Z
loadmin
U
2
S
-------
=
EQUATION571 V1 EN-US
(Equation 348)
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 349)
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, both phase-to-phase and
phase-to-earth fault operating characteristics should be considered.
To avoid load encroachment for the phase-to-earth measuring elements, the set resistive
reach of any distance protection zone must be less than 80% of the minimum load impedance.
RFPE 0.8 Z
load
×
£
EQUATION792 V1 EN-US
(Equation 350)
This equation is applicable only when the loop characteristic angle for the single phase-to-
earth faults is more than three times as large as the maximum expected load-impedance
angle. For the case when the loop characteristic angle is less than three times the load-
impedance angle, more accurate calculations are necessary according to equation
[2] RLdRv=RLdRvFactor*RLdFw
[3] RLdRv=RLdRvFactor*RLdFw
1MRK 505 343-UEN B
Section 7
Impedance protection
319
Application manual
Содержание Relion 670 series
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