Current Transformer Stability

[lyntondave]

Hiya,

I'm new here so forgive me if I've not posted in the correct area etc...

My Question/Scenario:

If I have a 4 CT system providing REF protection of a Generating Set, zoned protection looking back at the alternator zone. CT's are 1000/5 ratio, magnetization curves show that the Vkp of these CT's is around 7-7.5 volts. Resistance of the CT's are 0.5-0.6 ohms, maximum theoretical short circuit current is 2900A three phase and 7685A single phase fault,(ok so the generator would probably stall before supplying this fault current, but it is the theory I am interested in) So then a fault of this magnitude would produce a CT secondary of around 40A x 0.6 ohm CT resistance (not taking into account any cable/lead resistance) I believe this would require a maximum stable voltage of 24V.

When looking at suitability of a CT from a stability viewpoint is it just as simple as what I have done above? ie Ohms law to determine the Vkp requirement.

Many Thanks,

Dave

 

[mgtrp]

A Vkp of 7V sounds very low for a generator CT, are you sure that you've calculated/measured that correctly?

 

[lyntondave]

Agreed, but yes all around the 7v mark. I plotted a mag curve and one saturates a little closer to 8v, I think these are 5p5 CT's

 

[lyntondave]

I also feel this is low, too low. I just want to ensure the theory I have used is correct. I feel that in a short circuit scenario the CT would saturate before the PD could clear the fault.

 

[LeifEricson]

The voltage capability of the CT has to be high enough to overcome the voltage drop across its own winding plus the voltage drop across the connected burden (pilot wiring, relay, etc). Your 5P5 CTs should have a rated burden associated with them, normally expressed in VA (5, 10, 20, 30 VA are common ratings) which should be at least as high as the actual connected burden. Also, the amount of dc current present in the fault has to be accounted for due to its propensity for causing the CT to saturate. For more information on this you can refer to IEEE C37.110, "IEEE Guide for the Application of Current Transformers Used for Protective Relaying Purposes". On first glance, your 5P5 is undersized if the expected maximum fault current is greater than 5 times the rated current...should probably be at least a 5P10.