Siemens SIPROTEC 7SD610 Manual page 38

Functions
2.1 General
With this data the device establishes an approximate CT error characteristic and calculates the restraint quan-
tity (see also Section 2.3).
Calculation example:
Current transformer 5P10; 20 VA
Transformation 600 A / 5 A
Internal burden 2 VA
Secondary lines 4 mm
Length 20 m
Device 7SD610 , I
Burden at 5 A, 0.3 VA
The resistance of the secondary lines is (with the resistivity for copper ρ
Here, the most unfavourable case is assumed, i.e. the current (as is the case with single-phase faults) flows
back and forth via the secondary lines (factor 2). From that the power for nominal current I
P = 0.175 Ω · (5 A)
l
The entire connected burden consists of the burden of the incoming lines and the burden of the device:
P' = 4.375 VA + 0.3 VA = 4.675 VA
Thus the ratio of the accuracy limit factors is as follows
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.
38
2
Cu
= 5 A
N
2
= 4.375 VA
2
= 0.0175 Ωmm /m)
Cu
= 5 A is calculated
N
SIPROTEC, 7SD610, Manual
C53000-G1176-C145-6, Release date 02.2011