Siemens SIPROTEC 7SD610 Manual page 37

Current transformer 10P10; 30 VA → n = 10; P
Current transformer 10P20; 20 VA → n = 20; P
The operational accuracy limit factor n' is derived from these rated data and the actual secondary burden P':
With
n' =
n =
P =
N
P =
i
P' =
Usually, the internal burden of the current transformers is stated in the test report. If it is unknown, it can be
roughly calculated from the DC resistance R
2
P ≈ R · I
i i N
The ratio between operational accuracy limit factor and rated accuracy limit factor n'/n is set at address 251
K_ALF/K_ALF_N.
The CT error at rated current, plus a safety margin, is set at address 253 E% ALF/ALF_N. It is equal to the
„current measuring deviation for primary nominal current intensity F1" according to VDE 0414 / Part 1 or IEC
60044. It is
– 3 % for a 5P transformer,
– 5 % for a 10P transformer.
The CT error at rated accuracy limit factor, plus a safety margin, is set at address 254 E% K_ALF_N. It is
derived from the number preceding the P of the transformer data.
Table 2-1 illustrates some usual protective current transformer types with their characteristic data and the rec-
ommended settings.
Table 2-1 Recommended settings for current transformer data
CT class Standard
5P IEC 60044-1
10P
TPX IEC 60044-1
TPY
TPZ
PX IEC 60044-1
BS: Class X
C100 ANSI
to
C800
1)
If n'/n ≤ 1.50, setting = calculated ratio; if n'/n > 1.50, setting = 1.50
SIPROTEC, 7SD610, Manual
C53000-G1176-C145-6, Release date 02.2011
operational accuracy limit factor (effective overcurrent factor)
rated accuracy limit factor of the current transformers (distinctive number behind P)
rated burden of the current transformers [VA] at rated current
internal burden of the current transformers [VA] at rated current
actually connected burden (devices + secondary lines) [VA] at rated current
Error at rated current
Transforma- Angle
tion ratio
1,0 % ± 60 min
3,0 % —
0,5 % ± 30 min
1,0 % ± 30 min
1,0 % ± 180 min
± 18 min
= 30 VA
N
= 20 VA
N
of the secondary winding.
i
Error at rated
accuracy limit
Address 251 Address 253 Address 254
factor
≤ 5 % ≤ 1,50
≤ 10 % ≤ 1,50
ε ≤ 10 % ≤ 1,50
ε ≤ 10 % ≤ 1,50
ε ≤ 10 % ≤ 1,50
(only I∼)
≤ 1,50
≤ 1,50
Recommended settings
1)
3,0 % 10,0 %
1)
5,0 % 15,0 %
1)
1,0 % 15,0 %
1)
3,0 % 15,0 %
1)
6,0 % 20,0 %
1)
3,0 % 10,0 %
1)
5,0 % 15,0 %
Functions
2.1 General
37