Rules of thumb for sizing CT's in high fault current applications
Rules of thumb for sizing CT's in high fault current applications
(OP)
I was just reading an article in the latest IEEE IAS magazine that discussed CT Saturation Calculations and the applications of CT's on circuit with high avaliable fault currents. The rule of thumb gien in the article was that the saturation current of the CT must be higher than 1.5 times the maximum current vaule or avalaible fault current. So for example a P15 rated CT would be suitable for a circuit whose avaliable fault current is 10x the CT primary rating because the saturation point of this P15 CT (accurate and will not saturate below 15x CT primary) would be 1.5x larger than the avalaible fault current. The article then goes on to an accuracy limit factor Kn that accounts for a lower than rated burden on the CT thus allowing it to be applied for higher currents.
I did some follow up reading in the IEEE buff book and found there that for applying CT's they actually carried out a calculation based on the CT's secondary current at a given fault value and the relay burdent the secondary current was being driven into to see how much secondary voltage needed to be developed in the CT to push the current through the burden. After the voltage was determined it used a CT excitation plot to find out what excitation current was assocated with this voltage for a given CT size. In order for the CT to be considered adequate the excitation current had to be below 10% of the secondary current that would be in the CT for a fault current.
Another article that I read talked about a rule of thumb where it stated that for a given CT the knee-point on the CT excitation curve should be at least twice that of the voltage developed at the secondary of the CT based upon the secondary current at max fault and the burden this curernt was driven through.
So after looking at this information I was curious to hear weather most people used some sort of rule of thumb when applying CT's or if they actually carried out calculations with currents and burdens to determine correct application of CT ratio?
I did some follow up reading in the IEEE buff book and found there that for applying CT's they actually carried out a calculation based on the CT's secondary current at a given fault value and the relay burdent the secondary current was being driven into to see how much secondary voltage needed to be developed in the CT to push the current through the burden. After the voltage was determined it used a CT excitation plot to find out what excitation current was assocated with this voltage for a given CT size. In order for the CT to be considered adequate the excitation current had to be below 10% of the secondary current that would be in the CT for a fault current.
Another article that I read talked about a rule of thumb where it stated that for a given CT the knee-point on the CT excitation curve should be at least twice that of the voltage developed at the secondary of the CT based upon the secondary current at max fault and the burden this curernt was driven through.
So after looking at this information I was curious to hear weather most people used some sort of rule of thumb when applying CT's or if they actually carried out calculations with currents and burdens to determine correct application of CT ratio?






RE: Rules of thumb for sizing CT's in high fault current applications
For nearly all of the HV breakers / transformers (bushing type protection CTs), in the US, it seems that most utilities select a 2000:5, C800 and are done with it. No love stories, once in a while a request for secondary lead resistance on bus diff (Standard tests requested).
Having said that, seeing how things are done in the IEC world makes me wonder why so much more time and effort is spent in studying this?
Is it that the 2000/5 C800 is so versatile / well designed that no further work is required? I have seen much larger CT's on large generators, but again what I see most on HV equipment in the US.
Good question and I hope I didn't hijack your thread. I wonder this myself often.