can anyone show me with an example how to calculate
the over dimensioning required for a CT in order to keep
it from saturating due to transient effects?
Suggestion: Reference:
1. Sutherland P. E. "Applying CTs with Digital Ground Relays," IEEE Industry Applications Magazine, Vol. 7, No. 2, pp 71-79, March/April 2001
Reference 1 indicates that the best solution is to design the system to prevent the line CTs from full saturation during external phase faults. So beware of various clandestine calculations!
See Chapter 3 of ANSI/IEEE Std. 242 (IEEE Buff Book). Basically, standard CTs for relaying (those in accordance with ANSI/IEEE C57.13) will be accurate within 10% at currents up to 20 times rated current at rated burden. If your maximum fault current is going to exceed 20 times the CT rated amperes, then you may have a saturation problem.
The CT can saturate for currents below 20x rated, due to transient DC component of the fault current waveform.
I have a copy of the 1976 edition of the Westinghouse Applied Protective Relaying book, which gives the following formula -
Transient saturation is avoided if :
Vk >= 6.28*I*R*T
Vk = CT kneepoint voltage
I= symmetrical secondary current, A RMS
R= total secondary resistance
T= dc time constant of the primary circuit in cycles
T=(Lp/Rp)*f
Lp = primary circuit inductance
Rp = primary circuit resistance
f = system frequency
This formula will give a reasonable approximation. More details are available from the folllowing reference: