ABB Relion 620 Series - Page 546

The accuracy limit factors corresponding to the actual burden of the
phase current transformer to be used in differential protection fulfill the
requirement.
F K Ik T e
a r dc
T T
m dc
> × × × × − +
−
max
/
( ( ) ) ω 1 1
(Equation 80)
Ik
max
The maximum through-going fault current (in I
r
) at which the protection is not
allowed to operate
T
dc
The primary DC time constant related to Ik
max
ω The angular frequency, that is, 2*π*fn
T
m
The time-to-saturate, that is, the duration of the saturation free transforma-
tion
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
):
Protection functions
1MRS757644 H
546 620 series
Technical Manual