Siemens SIPROTEC Manual page 52

Functions
2.1 General
According to the above table, address 251 is to be set to 1.5 if the calculated value is higher than 1.5. This
results in the following setting values:
Address 251 K_ALF/K_ALF_N = 1.50
Address 253 E% ALF/ALF_N = 3.0
Address 254 E% K_ALF_N = 10.0
The presettings correspond to current transformers 10P with rated burden.
Of course, only those settings are reasonable where address 253 E% ALF/ALF_N is set lower than address
254 E% K_ALF_N.
Transformer with voltage control
If a power transformer with voltage control is located in the protected zone, a differential current may occur even
during normal operation under steady-state conditions. This differential current depends on the current intensity
as well as on the position of the tap changer. Since this is a current-proportional error, the best way is to treat
it like an additional current transformer error. Calculate the maximum fault current at the limits of the control
range and add it (referred to the mean current of the control range) to the current transformer errors for the
addresses 253 and 254). Perform this correction only at the end facing the regulated winding of the power
transformer.
Calculation example:
Transformer
This results in the following:
Rated current at rated voltage
Rated current at U
Rated current at U
The maximum deviation from this current is
This maximum deviation δ
and 254 E% K_ALF_N as determined above.
Please consider that this deviation through voltage control is referred to the mean current at rated apparent
power and not to the rated current at rated voltage. This requires an adequate correction of the setting values
as discussed in Section 2.1.4 under „Topological Data for Transformers (optional)".
52
YNd5
35 MV
110 kV / 25 kV
Y-winding with tap changer ±10 %
= 184 A
I
N
+ 10 % I = 167 A
N min
– 10 % I = 202 A
N max
[in %] has to be added to the maximum transformer errors 253 E% ALF/ALF_N
max
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011