CT Thermal Rating?

[JBUDA54]

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.

 

[cranky108]

An 800/5 CT I don't think is that common. Make sure this is the ratio, and not the setting of a multi-ratio CT.

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.

 

[bacon4life]

IEEE C57.13 lists the standard rating factors as thermal current rating factors shall be 1.0, 1.33, 1.5, 2.0, 3.0, or 4.0.

Also be sure to check what the max current on your relay/metering equipment is.

 

[scottf]

I'd be very surprised if the bushing CT inside a breaker didn't have a rating factor of 2.0, meaning can handle 200% rated current. Note that if you're in the ANSI/IEEE world, that rating factor is based on a 30C average ambient with the max ambient not exceeding 40C. IEEEC57.13 has a chart to show how to de-rate the RF for average ambient conditions above and below 30C.

 

[jghrist]

What is the breaker rating? It would be unusual for a breaker to have a CT rated lower than the continuous current rating of the breaker. There is no thermal overload rating on breakers.

 

[JBUDA54]

The breaker rating is 1200A

 

[scottf]

You probably have 1200:5A MR (multi-ratio) CTs in that breaker, with the connections made for 800:5A ratio. The CT most likely has a rating factor of 2.0 or 1.5, so either way you should be ok to leave the CT settings alone and run at 1200A continuous. Or you could change the CT connection to 1200:5A.

 

[JBUDA54]

I was able to read the CT nameplate of the 5A Main Breaker. And for once it matched the DWGs.

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.

 

[scottf]

You can't tell anything about the rating factor by the physical size of the CT.

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.

 

[JBUDA54]

I snapped a picture of the CT Nameplate. This was an extremely difficult task in that the CT nameplate faced the Breaker compartment Wall and there was little room to view the NP. I had to take a mechanics mirror and place it in front of the NP and Snapped a Photo of the nameplate. I looked at the Nameplate and I am confident it is for the CT do to the factor the top line Reads: General Electric Type J9S-0 Current Transformer, and the next line is a massive font 800/5 ratio. No accuracy was specified on the nameplate, just the CAT NO, INST, and the cycles.

 

[scottf]

JBUDA54-

By "keep digging" I meant call GE...start with their Somersworth NH factory.