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1MAC050144-MB C
Section 4
Protection functions
615 series ANSI
363
Technical Manual
(Equation 44)
F
a
The approximate value of the accuracy limit factor (ALF) corresponding to the actual CT burden
F
n
The rated accuracy limit factor at the rated burden of the current transformer
S
n
The rated burden of the current transformer
S
in
The internal burden of the current transformer
S
a
The actual burden of the current transformer
Example 1
The rated burden S
n
of the current transformer 5P20 is 10 VA, the secondary rated current
is 5A, the internal resistance R
in
= 0.07
Ω
and the accuracy limit factor F
n
corresponding
to the rated burden is 20 (5P20). Thus the internal burden of the current transformer is S
in
=
(5A)
2
* 0.07
Ω
= 1.75 VA. The input impedance of the IED at a rated current of 5A is <
20 m
Ω
. If the measurement conductors have a resistance of 0.113
Ω
, the actual burden of
the current transformer is S
a
=(5A)
2
* (0.113 + 0.020)
Ω
= 3.33 VA. Thus the accuracy
limit factor F
a
corresponding to the actual burden is approximately 46.
The CT burden can grow considerably at the rated current 5A. The actual burden of the
current transformer decreases at the rated current of 1A while the repeatability
simultaneously improves.
At faults occurring in the protected area, the currents may be very high compared to the
rated currents of the current transformers. Due to the instantaneous stage of the differential
function block, it is sufficient that the current transformers are capable of repeating the
current required for instantaneous tripping during the first cycle.
Thus the current transformers usually are able to reproduce the asymmetric fault current
without saturating within the next 10 ms after the occurrence of the fault to secure that the
trip times of the IED comply with the retardation time.
The accuracy limit factors corresponding to the actual burden of the phase current
transformer to be used in differential protection fulfill the requirement.
(Equation 45)
Ik
max
The maximum through-going fault current (in I
R
) at which the protection is not allowed to trip
T
dc
The primary DC time constant related to Ik
max
ω
The angular frequency, that is, 2*π*fn
T
m
The time-to-saturate, that is, the duration of the saturation free transformation
K
r
The remanence factor 1/(1-r), where r is the maximum remanence flux in p.u. from saturation flux
The accuracy limit factors corresponding to the actual burden of the phase current
transformer is used in differential protection.
F
F
S
S
S
S
a
n
in
n
in
a
=
×
+
+
Содержание 615 Series ANSI
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Страница 29: ...1MAC050144 MB C Section 615 series ANSI 22 Technical Manual ...
Страница 389: ...Section 4 1MAC050144 MB C Protection functions 382 615 series ANSI Technical Manual ...
Страница 485: ...Section 6 1MAC050144 MB C Supervision functions 478 615 series ANSI Technical Manual ...
Страница 635: ...Section 10 1MAC050144 MB C Other functions 628 615 series ANSI Technical Manual ...
Страница 707: ...Section 15 1MAC050144 MB C Applicable standards and regulations 700 615 series ANSI Technical Manual ...
Страница 711: ...Section 16 1MAC050144 MB C Glossary 704 615 series ANSI Technical Manual ...
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