Recommendations for current transformers
The more important the object to be protected, the more attention has to be paid
to the current transformers. It is not normally possible to dimension the current
transformer so that they repeat the currents with high DC components without
saturating when the residual flux of the current transformer is high. TR2PTDF
operates reliably even though the current transformers are partially saturated.
The accuracy class recommended for current transformers to be used with
TR2PTDF 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
=
×
+
+
(Equation 79)
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 operate times of the protection relay comply
with the retardation time.
1MRS757644 H
Protection functions
620 series
Technical Manual
545
