at the rated burden, the rated burden S
n
, the internal burden S
in
and the actual
burden S
a
of the CT.
F F
S S
S S
a n
in n
in a
= ⋅
+
+
(Equation 173)
Example 1
In the example, the rated burden S
n
of the CTs 5P20 is 10 VA, the secondary
rated current 5A, the internal resistance R
in
= 0.07 Ω and the accuracy limit
factor F
n
(ALF) corresponding to the rated burden is 20 (5P20). The internal
burden of the current transformer is S
in
= (5 A)
2
· 0.07 Ω = 1.75 VA. The
input impedance of the relay at a rated current of 5 A is < 20 mΩ. If the
measurement conductors have a resistance of 0.113 Ω, the actual burden of
the current transformer is S
a
=(5 A)2 · (0.113 + 0.020) Ω = 3.33 VA. Thus the
accuracy limit factor F
a
corresponding to the actual burden is 46.
The CT burden can grow considerably at the rated current of 5 A. At the
rated current of 1 A, the actual burden of the current transformer decreases,
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 CTs. Due to the instantaneous stage
of the differential function block, it is enough that during the first cycle, the
CTs can repeat the current required for instantaneous tripping.
The CTs should be able to reproduce the asymmetric fault current without
saturating within the next 10 ms after the occurrence of the fault, to secure
that the operating times of the protection relay comply with the times
stated in
Chapter 4.3.3.12 Technical data
.
The accuracy limit factors corresponding to the actual burden of the
phase current transformer to be used in differential protection fulfill the
requirement of the equation.
Fa > K I (T (1 - e ) + 1)
r kmax dc
-
Tm
⋅ ⋅ ⋅
ω
Tdc
(Equation 174)
I
kmax
Maximum through-going fault current (in pu) at which the protection is not
allowed to operate
T
dc
Primary DC time constant related to I
kmax
ω Angular frequency, that is, 2 · π · 50 Hz
T
m
Time to saturate, that is, the duration of the saturation-free transformation
K
r
Remanence factor, 1/(1-r)
The parameter
r
gives the maximum remanence flux density in the CT core.
The value of the parameter
r
depends on the magnetic material used and on
the construction of the CT. For example, the value
r
= "0.4" means that the
remanence flux density may be 40 percent of the saturation flux density. The
manufacturer of the CT should 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.
Protection functions
1MRS759142 F
740 REX640
Technical Manual
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