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 53)
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 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 operate 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.
F K Ik T e
a r dc
T T
m dc
> × × × × − +
−
max
/
( ( ) )
ω
1 1
GUID-DA861DAD-C40E-4A82-8973-BBAFD15279C0 V1 EN
(Equation 54)
Ik
max
The maximum through-going fault current (in I
r
) at which the protection is not allowed to operate
T
dc
The primary DC time constant related to Ik
max
Section 4 1YHT530004D05 D
Protection functions
410 615 series
Technical Manual
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