Load Impedance Limitation, Without Load Encroachment Function - ABB RELION RET670 Applications Manual

1MRK 504 163-UUS A
8.13.4.8
Transformer protection RET670 2.2 ANSI
Application manual

Load impedance limitation, without load encroachment function

The following instructions are valid when setting the resistive reach of the distance
zone itself with a sufficient margin towards the maximum load, that is, without the
common load encroachment characteristic (set by RLdFwd, RldRev and ArgLd).
Observe that even though the zones themselves are set with a margin, RLdFwd and
RldRev still have to be set according to maximum load for the phase selection to
achieve the expected performance.
Check the maximum permissible resistive reach for any zone 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 as:
2
V
=
Z
loa d min
S
EQUATION1718 V1 EN-US
Where:
V is the minimum phase-to-phase voltage in kV
S is the maximum apparent power in MVA.
The load impedance [Ω/phase] is a function of the minimum operation voltage and the
maximum load current:
V
=
min
Z
loa d
×
3 I
ma x
EQUATION1719 V1 EN-US
Minimum voltage V and maximum current I
min
conditions. Minimum load impedance occurs normally under emergency conditions.
To avoid load encroachment for the phase-to-ground measuring elements, the set
resistive reach of any distance protection zone must be less than 80% of the minimum
load impedance.
£ ×
RFPG 0.8 Z
load
GUID-AD2F1BE2-8442-4ED1-A558-21D092CEE170 V1 EN-US
This equation is applicable only when the loop characteristic angle for the single
phase-to-ground 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
Impedance protection
GUID-16C2EB30-7FEA-42E4-98C8-52CCC36644C6 v5
are related to the same operating
max
Section 8
(Equation 363)
(Equation 364)
(Equation 365)
443