It can be seen that severity of dc offset component in fault current is maximum when from equation (1)
a)
and
b)
For example, if angle of transmission line is
, then with
and = sec = 5msec, the severity of dc offset current would equal
, which is also the peak value of symmetrical ac component of the current. This leads us to an important conclusion. viz.,
Peak value of dc offset current can be as high as the symmetrical ac peak.
Polarity wise the dc offset current can be positive or negative.
Dc offset current may be totally absent eg. If
, .
While, in above analysis, we have considered a single phase current, a 3 phase fault on a 3 phase transmission line
would always induce dc offset current in at least two phases. DC offset has adverse impact on the CT performance.
In the remaining lecture, we analyze effect of the dc offset current on CT performance.
, then with
and 
= 
sec = 5msec, the severity of dc offset current would equal
, which is also the peak value of symmetrical ac component of the current. This leads us to an important conclusion. viz., 
, 
.
