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.
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
(Equation 421)
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
ω The angular frequency, that is, 2*π*fn
T
m
The time-to-saturate, that is, the duration of the saturation free transforma-
tion
K
r
The remanence factor 1/(1-r), where r is the maximum remanence flux in p.u.
from saturation flux
The accuracy limit factors corresponding to the actual burden of the phase
current transformer is used in differential protection.
The parameter
r
is the maximum remanence flux density in the CT core in
p.u. from saturation flux density. The value of the parameter
r
depends on
the magnetic material used and on the construction of the CT. For instance,
if the value of
r
= 0.4, the remanence flux density can be 40 percent of the
saturation flux density. The manufacturer of the CT has to be contacted
when an accurate value for the parameter
r
is needed. The value
r
= 0.4 is
recommended to be used when an accurate value is not available.
The required minimum time-to-saturate T
m
in TR2PTDF and TR3PTDF is half
fundamental cycle period (10 ms when fn = 50Hz).
1MRS759142 F
Requirements for measurement
transformers
REX640
Technical Manual
1919
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