CT Thermal Rating?
CT Thermal Rating?
(OP)
All,
I am working on a project that has two 3750kVA 13.8-4.16kV XFMRs installed. They want to swap out these XFMRs to a 7500kVA XFMR. The 4.16kV current on the new XFMR is 1040A. The main breaker CTs are currently 800/5 for the old 3750kVA XFMRs. I was told that the thermal rating would allow the 800/5 CT to work for the new XFMRs if the CT had a CONTINUOUS THERMAL CURRENT RATING FACTOR of 1.33 or higher. I thought that a CT was only good for the rating I have never heard of a CONTINUOUS THERMAL
CURRENT RATING FACTOR that would allow the CT to operate in a range higher than the 800/5. I was told that a 800/5 CT with a 1.5 TR could operate at 1200/5. Is this true? Please advise. I am going to have the main breaker de-energized on Monday to take the CT nameplate to see if the CT can be used for a higher capacity.
I am working on a project that has two 3750kVA 13.8-4.16kV XFMRs installed. They want to swap out these XFMRs to a 7500kVA XFMR. The 4.16kV current on the new XFMR is 1040A. The main breaker CTs are currently 800/5 for the old 3750kVA XFMRs. I was told that the thermal rating would allow the 800/5 CT to work for the new XFMRs if the CT had a CONTINUOUS THERMAL CURRENT RATING FACTOR of 1.33 or higher. I thought that a CT was only good for the rating I have never heard of a CONTINUOUS THERMAL
CURRENT RATING FACTOR that would allow the CT to operate in a range higher than the 800/5. I was told that a 800/5 CT with a 1.5 TR could operate at 1200/5. Is this true? Please advise. I am going to have the main breaker de-energized on Monday to take the CT nameplate to see if the CT can be used for a higher capacity.






RE: CT Thermal Rating?
It is common to have a thermal factor on a CT above 1. In fact they can be as high as 2. But what happens is that say for an 800/5 ct operated at 1.5 times it's rating, the primary current will be 1200 amps, while the secondary current will be 7.5 amps.
Bottom line is you need to check the CT nameplate, and if you can't find one, assume the thermal rating is 1.
RE: CT Thermal Rating?
Also be sure to check what the max current on your relay/metering equipment is.
RE: CT Thermal Rating?
RE: CT Thermal Rating?
RE: CT Thermal Rating?
RE: CT Thermal Rating?
RE: CT Thermal Rating?
GE Catalog No. 750X10G6
Inst GEH-230
Type J9S-0
800/5 Ratio
25-400 cycles
Std Amb 55C
It does not state the RTF so do I assume a 1.0?
It is a massive CT; it looks like it would have a greater than 1 RTF, but The namplate does not state. I tried google the CAT NO and TYPE but come up short. Any suggestions? I am trying to convince the owner that they should replace the 800/5 and go with a CT above the 7500kVA XFMR 4160V FLA rating (1040A) in case they ever want to use this XFMR near its rating. Shouldn't the 800/5 be replaced by 1200/5? What are your thoughts? Thanks in advance.
RE: CT Thermal Rating?
Does the nameplate not state the accuracy either? Are you reading the nameplate on the actual CT or on the breaker? The nameplate on the CT itself should have the accuracy and the rating factor listed per the standard. If you keep digging, I'm confident you'll find that the RF is at least 1.5.
If you can't find the rating factor anywhere (including asking GE Somersworth), then, in my opinion, you need to replace it.
RE: CT Thermal Rating?
RE: CT Thermal Rating?
By "keep digging" I meant call GE...start with their Somersworth NH factory.