Minimum voltage V
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 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
Z
load
≤
⋅
0 8
.
ANSI13000273 V1 EN-US
(Equation 150)
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
.
RFFwPG
Z
R
R
X
X
load
≤
⋅
⋅
∂ −
⋅
+
⋅
+
⋅
∂
0 8
2
1
0
2
1
0
.
cos
sin
min
ANSI13000274 V1 EN-US
(Equation 151)
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
Z
load
≤
⋅
1 6
.
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
.
1MRK 504 163-UUS A
Section 8
Impedance protection
Transformer protection RET670 2.2 ANSI
281
Application manual
Summary of Contents for RELION RET670
Page 1: ...RELION 670 SERIES Transformer protection RET670 Version 2 2 ANSI Application manual ...
Page 2: ......
Page 48: ...42 ...
Page 64: ...58 ...
Page 74: ...68 ...
Page 104: ...98 ...
Page 194: ...188 ...
Page 518: ...512 ...
Page 618: ...612 ...
Page 648: ...642 ...
Page 666: ...660 ...
Page 672: ...666 ...
Page 682: ...676 ...
Page 844: ...838 ...
Page 868: ...862 ...
Page 956: ...950 ...
Page 964: ...958 ...
Page 1004: ...998 ...
Page 1014: ...1008 ...
Page 1015: ...1009 ...