ABB RELION REX640 Technical Manual page 740

Protection functions
at the rated burden, the rated burden S
burden S
F
=
a
Example 1
In the example, the rated burden S
rated current 5A, the internal resistance R
factor F
burden of the current transformer is S
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
accuracy limit factor F
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
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
I
kmax
T
dc
ω
T
m
K
r
The parameter
The value of the parameter
the construction of the CT. For example, the value
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
an accurate value is not available.
740
of the CT.
a
S S
+
in n
F
⋅
n
S S
+
in a
(ALF) corresponding to the rated burden is 20 (5P20). The internal
n
corresponding to the actual burden is 46.
a
Chapter 4.3.3.12 Technical data
Tm
-
Tdc
(T (1 - e
ω
⋅ ⋅ ⋅
r kmax dc
Maximum through-going fault current (in pu) at which the protection is not
allowed to operate
Primary DC time constant related to I
Angular frequency, that is, 2 · π · 50 Hz
Time to saturate, that is, the duration of the saturation-free transformation
Remanence factor, 1/(1-r)
r gives the maximum remanence flux density in the CT core.
r is needed. The value r = "0.4" is recommended to be used when
, the internal burden S
n
of the CTs 5P20 is 10 VA, the secondary
n
= 0.07 Ω and the accuracy limit
in
2
= (5 A) · 0.07 Ω = 1.75 VA. The
in
=(5 A)2 · (0.113 + 0.020) Ω = 3.33 VA. Thus the
a
.
) + 1)
kmax
r depends on the magnetic material used and on
1MRS759142 F
and the actual
in
(Equation 173)
(Equation 174)
r = "0.4" means that the
Technical Manual
REX640