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The accuracy class recommended for current transformers to be used with 87T is 5P, in
which the limit of the current error at the rated primary current is 1 percent and the limit
of the phase displacement is 60 minutes. The limit of the composite error at the rated
accuracy limit primary current is 5 percent.
The approximate value of the accuracy limit factor F
a
corresponding to the actual current
transformer burden can be calculated on the basis of the rated accuracy limit factor F
n
at
the rated burden, the rated burden S
n
, the internal burden S
in
and the actual burden S
a
of the
current transformer.
F
F
S
S
S
S
a
n
in
n
in
a
=
×
+
+
GUID-26DEE538-9E1A-49A2-9C97-F69BD44591C9 V2 EN
(Equation 200)
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 protection relay at a rated
current of 5A is < 20 mΩ. 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 protection relay comply with the retardation time.
1MAC059074-MB A
Section 12
Requirements for measurement transformers
615 series ANSI
1135
Technical Manual
Содержание Relion 615 series
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