ABB RELION RET670 Applications Manual page 396

Section 8
Impedance protection
390
2
V
=
Z
loa d min
S
EQUATION1718 V1 EN-US
Where:
V 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:
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.
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 ground faults
both phase-to-phase and phase-to-ground fault operating characteristics
should be considered.
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.
£ ×
RFP G 0.8 Z
loa d
EQUATION1720 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
times the load-impedance angle, more accurate calculations are necessary according to
equation 300.
1MRK 504 163-UUS A
are related to the same operating
max
Transformer protection RET670 2.2 ANSI
(Equation 297)
(Equation 298)
(Equation 299)
Application manual