As can be seen in the fig 8.5, CCVT equivalent circuit is a R-L-C circuit. If transformer is considered ideal, it can be described by integro differential equation of the type, . The corresponding differential equation is given by .
For a solid 3 - phase fault say near the CCVT bus at t=t0. v(t) = 0 for . Thus, during fault the governing differential equation is given by .
This equation is expressed in standard form as follows: where is natural frequency in radians per second and is the damping constant. Thus, and . Because of the property of tuning reactor, = 50 or 60 Hz. We know from the background in network analysis that response of such a circuit to step excitation, depends upon .
Because R is quite small, . If ; then we expect underdamped response. The response depends upon the damping and and point on the voltage waveform where the fault strikes. Such transients are known as subsistence transients. Fig 8.7 shows subsistence transients of CCVT. It can be seen that subsistence transients can reduce the accuracy of distance relays.
. The corresponding differential equation is given by 
. 
. Thus, during fault the governing differential equation is given by 
. 
where 
is natural frequency in radians per second and 
is the damping constant. Thus, 
and 
. Because of the property of tuning reactor, 
= 50 or 60 Hz. We know from the background in network analysis that response of such a circuit to step excitation, depends upon 
. If 
