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CT calculation 16
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jbartos
Electrical
- Jan 15, 2000
- 6,440
jbartos
Electrical
- Jan 15, 2000
- 6,440
Suggestion: Visit
for more info on a CT protection against surges, and errors due to harmonics under "Wave Form."
for more info on a CT protection against surges, and errors due to harmonics under "Wave Form."
jbartos
Electrical
- Jan 15, 2000
- 6,440
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!
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!
redtrumpet
Electrical
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.
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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:
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:
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