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Minimum CT ratio for 5P20 CT for small MV drives

When select the CT ratio of 50/1 A for 6.6 kV drives where the SC rating of system is 40 kA, 50 protection may not operate because of CT saturation. Again, if the CT is not saturated, the relay terminal may not withstand the current >100 A (for 1 A sec). In case of 50/1 CT, for a fault current of 40 kA, the CT secondary will be having a current of 800A.

Is there any specific guideline to select the minimum CT ratio for a 40 kA system which will fulfill the followings:
1. Relay will be picked up for 50 function
2. Relay will not be burned because of excessive secondary current during high current fault

This has always been a concern with some mechanical relays because of the relatively burden when 50/51 relays are set to a minimum tap. I have not had much of this become a problem with electronic relays because of the fixed ct loading resistor used to convert amps to millivolts. Specifically, mechanical relays that are a problem are Westinghouse IT and CO that are set to a min tap. Also in this group are GE relays such as IAC.

I have always used the fault current divided ct ratio as a quick test. Result must be less than 100 or I would go thru detailed ct saturation calcs. I am not aware of a specific standard that might apply, other than it is necessary to assure that ct saturation must be avoided.

Now, if you are anticipating 40kA ground fault current, I assume this is not resistance grounded. Hence, I must ask why you want to use a 50/5 ct in the residual circuit. Here, the neutral connection of the phase ct's should detect ground faults and the phase ct's provide the ratio. In addition, residual ground detection will use 51 and 50. You also mention that this is applied as input protection to a small drive. Do the relays trip a mv motor contractor? Does this contractor have a fuse to handle the interrupting at max fault level?

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