ABB 615 series Technical Manual page 583

1MAC059074-MB A
615 series ANSI
Technical Manual
rated burden, the rated burden S
current transformer.
S S
+
in n
F F
= ×
a n
S S
+
in a
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Example 1
The rated burden S of the current transformer 5P20 is 10 VA, the secondary rated current
n
5A, the internal resistance R
corresponding to the rated burden is 20 (5P20). The internal burden of the current
transformer is S = (5A)² × 0.07 Ω = 1.75 VA. The input impedance of the protection relay
in
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
3.33 VA. Thus, the accuracy limit factor F
The CT burden can grow considerably at the rated current 5A. The actual burden of the
current transformer decreases at the rated current of 1 A while the repeatability
simultaneously improves.
At faults occurring in the protected area, the fault currents can 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 an 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.
The accuracy limit factors corresponding to the actual burden of the phase current
transformer to be used in differential protection must fulfill the requirement:
(
F K Ik T
> × × × ω
a r max dc
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Ik The maximum through-going fault current (in I
max
T The primary DC time constant related to Ik
dc
ω The angular frequency, that is, 2 x π x f
T The time to saturate, that is, the duration of the saturation-free transformation
m
K The remanence factor 1/(1-r), where r is the maximum remanence flux in pu from the saturation flux
r
, the internal burden S
n
= 0.07 Ω and the rated accuracy limit factor F
in
corresponding to the actual burden is about 46.
a
T
−
m
T dc
( 1 ) 1 )
e
× − +
) at which the protection is not allowed to trip
R
max
n
Section 4
Protection functions
and the actual burden S
in
(Equation 113)
n
= (5A)² × (0.113 + 0.020) Ω =
a
(Equation 114)
of the
a
577