ω The angular frequency, that is, 2*π*fn
T
m
The time-to-saturate, that is, the duration of the saturation free transformation
K
r
The remanence factor 1/(1-r), where r is the maximum remanence flux in p.u. from saturation flux
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
m
in TR2PTDF is half fundamental cycle
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
max
is typically 10 I
r
for a
substation main transformer. At a short circuit fault close to the supply
transformer, the DC time constant (T
dc
) of the fault current is almost the same
as that of the transformer, the typical value being 100 ms.
Ik
max
10 I
r
T
dc
100 ms
ω 100π Hz
T
m
10 ms
K
r
1
When the values are substituted in Equation 54, the result is:
F
a
> × × × × − + ≈
−
K Ik T e
r dc
T T
m dc
max
/
( ( ) )
ω
1 1 40
GUID-7F1019C5-C819-440B-871B-5CFD1AF88956 V1 EN
2. Re-energizing against a fault occurring further down in the network:
1YHT530004D05 D Section 4
Protection functions
615 series 411
Technical Manual
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