ABB RELION RET670 Applications Manual page 287

1MRK 504 163-UUS A
Transformer protection RET670 2.2 ANSI
Application manual
Minimum voltage V and maximum current I
min
conditions. Minimum load impedance occurs normally under emergency conditions.
As a safety margin is required to avoid load encroachment under three-
phase conditions and to guarantee correct healthy phase IED operation
under combined heavy three-phase load and ground faults, consider
both: phase-to-phase and phase-to-ground fault operating
characteristics.
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.
RFFwPG 0 8 . Z
≤ ⋅
load
ANSI13000273 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 151.
RFFwPG 0 8 . Z
≤ ⋅ ⋅
load min
ANSI13000274 V1 EN-US
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.
RFFwPG 1 6 . Z
≤ ⋅
load
ANSI13000275 V1 EN-US
Equation 152 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. More accurate calculations are necessary according to equation 153.
max
2 R 1 R 0
⋅ +
 
cos sin
∂ − ⋅ ∂
 
2 1 0
X X
⋅ +
 
Section 8
Impedance protection
are related to the same operating
(Equation 150)
(Equation 151)
(Equation 152)
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