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 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
e
a
r
dc
T
m
Tdc
>
×
×
×
×
−
+
−
max
(
(
)
)
ω
1
1
(Equation 67)
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 x π x f
n
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 pu
from the saturation 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.
The required minimum time-to-saturate T
m
in MPDIF is half-fundamental cycle
period (10 ms when f
n
= 50 Hz).
Two typical cases are considered for the determination of the sufficient actual
accuracy limit factor F
a
:
Protection functions
1MRS757644 H
510
620 series
Technical Manual
