ABB RELION RET670, Applications Manual

Page: 461 / 1016

is not known, the following approximations may be considered for lines with rated
voltage 400 kV:
• K
L
= 0.9 for lines longer than 100 miles
• K
L
= 0.85 for lines between 50 and 100 miles
• K
L
= 0.8 for lines shorter than 50 miles
Multiply the required resistance for the same safety factor K
L
with the ratio between
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 .
min
0.9 137.2 123.5
L L
RLdOutFw K R
= × = × = W
EQUATION1343 V1 EN-US
(Equation 377)
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 378 presents the
corresponding maximum possible value of RLdInFw .
98.8
= × = W
RLdInFw kLdRFw RLdOutFw
EQUATION1344 V3 EN-US
(Equation 378)
The load angles, which correspond to external δ
Out
and internal δ
In
boundary of
proposed oscillation detection characteristic in forward direction, are calculated with
sufficient accuracy according to equation
379 and 380 respectively.
155.75
2 arc tan 2 arc tan 64.5
2 2 123.5
d
æ ö
æ ö
ç ÷ = × = × = °
ç ÷
ç ÷
× ×
è ø
è ø
S
Out
Z
RLdOutFw
EQUATION1345 V1 EN-US
(Equation 379)
max
155.75
2 arc tan 2 arc tan 76.5
2 2 98.8
d
æ ö
æ ö
ç ÷ = × = × = °
ç ÷
ç ÷
× ×
è ø
è ø
S
In
Z
RLdInFw
EQUATION1346 V1 EN-US
(Equation 380)
The required setting tP1 of the initial oscillation detection timer depends on the load
angle difference according to equation 381 .
1MRK 504 163-UUS A
Section 8
Impedance protection
Transformer protection RET670 2.2 ANSI 455
Application manual