ABB RELION REX640 Technical Manual page 717

1MRS759142 C
REX640
Technical Manual
Example 1
The rated burden S of the current transformer 5P20 is 10 VA, the secondary rated
n
current 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
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 about 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 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 the operating 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 1 (
> ⋅ ⋅ ⋅ ⋅ −
a r max dc
GUID-DFB88A81-EC60-40C7-8670-7FC2B03FA2B1 V1 EN
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
r
flux
The parameter r is the maximum remanence flux density in the CT core in pu from the
saturation flux density. The value of the parameter r depends on the magnetic material
used and also on the construction of the CT. For instance, if the value 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.
= 0.07 Ω and the rated accuracy limit factor F
in
T
−
m
T
e ) 1 )
dc
+
) at which the protection is not allowed to operate
R
max
n
Section 4
Protection functions
= (5A)² ×
a
corresponding to the
a
(Equation 162)
n
711