ABB RELION RET670 Applications Manual page 461

1MRK 504 163-UUS A
Transformer protection RET670 2.2 ANSI
Application manual
is not known, the following approximations may be considered for lines with rated
voltage 400 kV:
• K = 0.9 for lines longer than 100 miles
L
• K = 0.85 for lines between 50 and 100 miles
L
• K = 0.8 for lines shorter than 50 miles
L
Multiply the required resistance for the same safety factor K
actual voltage and 400kV when the rated voltage of the line under consideration is
higher than 400kV. The outer boundary RLdOutFw obtains in this particular case its
value according to equation 377.
= ×
RLdOutFw K R
L L
EQUATION1343 V1 EN-US
It is a general recommendation to set the inner boundary RLdInFw of the oscillation
detection characteristic to 80% or less of its outer boundary. Exceptions are always
possible, but must be considered with special care especially when it comes to settings
of timers tP1 and tP2 included in oscillation detection logic. This requires the
maximum permitted setting values of factor kLdRFw = 0.8. Equation
corresponding maximum possible value of RLdInFw.
= ×
RLdInFw kLdRFw RLdOutFw
EQUATION1344 V3 EN-US
The load angles, which correspond to external δ
proposed oscillation detection characteristic in forward direction, are calculated with
sufficient accuracy according to equation
æ
Z
d
= × ç
2 arc tan
ç
Out ×
2 RLdOutFw
è
EQUATION1345 V1 EN-US
æ
Z
d
= × ç
2 arc tan
ç
In ×
2 RLdInFw
è
EQUATION1346 V1 EN-US
The required setting tP1 of the initial oscillation detection timer depends on the load
angle difference according to equation 381.
= × =
0.9 137.2 123.5
min
= W
98.8
Out
379 and
ö
æ
155.75
S
÷ = ×
2 arc tan
ç
÷
×
è 2 123.5
ø
ö
æ
155.75
S
÷ = ×
2 arc tan
ç
÷
2 98.8
è
ø
max
Impedance protection
with the ratio between
L
W
378
and internal δ boundary of
In
380 respectively.
ö
= °
64.5
÷
ø
ö
= °
76.5
÷
×
ø
Section 8
(Equation 377)
presents the
(Equation 378)
(Equation 379)
(Equation 380)
455