ABB RELION REX640 Technical Manual page 658

Section 4
Protection functions
652
Example 1
In the example, the rated burden S
current 5A, the internal resistance R
corresponding to the rated burden is 20 (5P20). The internal burden of the current
transformer is S = (5 A)
in
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
= 3.33 VA. Thus the 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
section.
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
ω
⋅ ⋅ ⋅
r kmax dc
GUID-721C9BED-DF96-47AF-B6BD-362478ED0DF6 V1 EN
I Maximum through-going fault current (in pu) at which the protection is not allowed to operate
kmax
T Primary DC time constant related to I
dc
ω Angular frequency, that is, 2 ⋅ π ⋅ 50 Hz
T Time to saturate, that is, the duration of the saturation-free transformation
m
K Remanence factor, 1/(1-r)
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.
The minimum time to saturate (T
of the CTs 5P20 is 10 VA, the secondary rated
n
= 0.07 Ω and the accuracy limit factor F
in
⋅ 0.07 Ω = 1.75 VA. The input impedance of the relay at a
2
corresponding to the actual burden is 46.
a
Tm
-
Tdc
) + 1)
kmax
) in TR3PTDF is 10 ms.
m
1MRS759142 C
(ALF)
n
⋅ (0.113 + 0.020) Ω
2
=(5 A)
a
Technical data
(Equation 121)
REX640
Technical Manual