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Functions
2.1 General
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
51
Table 2-1 Recommended settings for current transformer data
1)
If n'/n
≤
1.50, setting = calculated ratio; if n'/n > 1.50, setting = 1.50
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
2
Cu
Length 20 m
Device 7SD5 , I
N
= 5 A
Burden at 5 A, 0.3 VA
The resistance of the secondary lines is (with the resistivity for copper
ρ
Cu
= 0.0175
Ω
mm
2
/m)
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
N
= 5 A is calculated
P
l
= 0.175
Ω
· (5 A)
2
= 4.375 VA
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
CT class Standard Error at rated current Error at rated
accuracy limit
factor
Recommended settings
Transforma-
tion ratio
Angle Address 251 Address 253 Address 254
5P IEC 60044-1 1,0 %
±
60 min
≤
5 %
≤
1,50
1)
3,0 % 10,0 %
10P 3,0 % —
≤
10 %
≤
1,50
1)
5,0 % 15,0 %
TPX IEC 60044-1 0,5 %
±
30 min
ε ≤
10 %
≤
1,50
1)
1,0 % 15,0 %
TPY 1,0 %
±
30 min
ε ≤
10 %
≤
1,50
1)
3,0 % 15,0 %
TPZ 1,0 %
±
180 min
±
18 min
ε ≤
10 %
(only
I ∼
)
≤
1,50
1)
6,0 % 20,0 %
PX IEC 60044-1
BS: Class X
≤
1,50
1)
3,0 % 10,0 %
C100
to
C800
ANSI
≤
1,50
1)
5,0 % 15,0 %