ABB RED615 Technical Manual page 417

1YHT530004D05 D
615 series
Technical Manual
ω The angular frequency, that is, 2*π*fn
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 p.u. from saturation flux
r
The accuracy limit factors corresponding to the actual burden of the phase current
transformer is used in differential protection.
The parameter r is the maximum remanence flux density in the CT core in p.u.
from saturation flux density. The value of the parameter r depends on the magnetic
material used and on the construction of the CT. For instance, if the value of 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
period (10 ms when fn = 50Hz).
Two typical cases are considered for the determination of the sufficient accuracy
limit factor (F ):
a
1. A fault occurring at the substation bus:
The protection must be stable at a fault arising during a normal operating
situation. Re-energizing the transformer against a bus fault leads to very high
fault currents and thermal stress and therefore re-energizing is not preferred in
this case. Thus, the remanence can be neglected.
The maximum through-going fault current Ik
substation main transformer. At a short circuit fault close to the supply
transformer, the DC time constant (T
as that of the transformer, the typical value being 100 ms.
Ik 10 I
max r
T 100 ms
dc
ω 100π Hz
T 10 ms
m
K 1
r
When the values are substituted in
> × × (
F K Ik T
a r max dc
GUID-7F1019C5-C819-440B-871B-5CFD1AF88956 V1 EN
2. Re-energizing against a fault occurring further down in the network:
in TR2PTDF is half fundamental cycle
m
) of the fault current is almost the same
dc
Equation
− T / T
× × − ( 1 m dc ) + 1 ) ≈ 40
e
ω
Section 4
Protection functions
is typically 10 I for a
max r
54, the result is:
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