ABB RELION REX640 Technical Manual page 1723

1MRS759142 C
REX640
Technical Manual
Requirements for measurement transformers
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
current transformer burden can be calculated on the basis of the rated accuracy limit
factor F at the rated burden, the rated burden S
n
burden S of the current transformer.
a
S S
+
in n
F F
= ⋅
a n
S S
+
in a
GUID-69D9C832-2B4C-4595-802D-FD7C6C4BB70A V1 EN
F The approximate value of the accuracy limit factor (ALF) corresponding to the actual CT burden
a
F The rated accuracy limit factor at the rated burden of the current transformer
n
S The rated burden of the current transformer
n
S The internal burden of the current transformer
in
S The actual burden of the current transformer
a
Example 1
The rated burden S of the current transformer 5P20 is 10 VA, the secondary rated
n
current is 5A, the internal resistance R
corresponding to the rated burden is 20 (5P20). Thus the internal burden of the current
transformer is S = (5A)
in
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
(0.113 + 0.020) Ω = 3.33 VA. Thus the accuracy limit factor F
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 operate times of the protection relay 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.
, the internal burden S
n
= 0.07 Ω and the accuracy limit factor F
in
2
* 0.07 Ω = 1.75 VA. The input impedance of the protection
Section 12
corresponding to the actual
a
and the actual
in
(Equation 383)
=(5A)
a
corresponding to the
a
n
2
*
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