CT calculation 16

[144x]

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?

 

[jbartos]

Suggestion: Visit

http://www.selinc.com/techpprs/6107.pdf

http://www.selinc.com/flyers/311lflyr.pdf

http://www.geindustrial.com/pm/ppt/bus2000.pdf

for more info

 

[nitro]

Joan 271273 please forward me the CT info you have it would be greatly appricated.

 

[jbartos]

Suggestion: Visit

http://www.geindustrial.com/industrialsystems/meter/notes/GET-97D.pdf

for more info on a CT protection against surges, and errors due to harmonics under "Wave Form."

 

[jbartos]

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!

 

[DSF]

can anyone show me with an example how to calculate
the dimensioning required for a CT in order to keep
it from saturating due to transient effects?

 

[redtrumpet]

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.

 

[peterb]

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:

http://www.geindustrial.com/pm/notes/ger3973.pdf