ABB RELION REX640 Technical Manual page 1724

Section 12
Requirements for measurement transformers
1718
F K Ik ( T
ω
> × × × × −
a r max dc
GUID-DA861DAD-C40E-4A82-8973-BBAFD15279C0 V1 EN
Ik The maximum through-going fault current (in I
max
T The primary DC time constant related to Ik
dc
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
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
main transformer. At a short circuit fault close to the supply transformer, the DC
time constant (T ) of the fault current is almost the same as that of the
dc
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
T T /
−
m dc
( 1 e ) 1 )
+
) at which the protection is not allowed to operate
r
max
in TR2PTDF and TR3PTDF is half
m
max
Equation 384, the result is:
1MRS759142 C
(Equation 384)
is typically 10 I for a substation
r
Technical Manual
REX640