5A CT secondary

[BrkfldEE]

My system has a 1200:5A MR CT that is being used on the 800:5A tap. If the transformer was fully loaded at it's 2 stage forced cooling rating, the total CT secondary Amps would be 6. Is it ok to use a CT with a nominal current above the 5A rating?

 

[davidbeach]

It depends. What's the thermal rating factor of the CT? What's the continuous current rating of the devices in the CT circuit?

 

[BrkfldEE]

The CT is a C800 and the relay device appears to be rated at 5A. It is a Beckwith M-3310 transformer protection relay. We are replacing a GE BDD transformer differential relay. I'm not sure what the GE BDD relay device's continuous current rating is.

 

[JG2828]

The M-3310 is rated to withstand 2x continuous current, so 10 amps. The BDD is rated for 2x the tap value setting.

 

[BrkfldEE]

Ok, so that takes care of the relay devices connected to the CT, but what about the CT itself? Will it eventually overheat or something if constantly used at a current greater than 5A?

 

[stevenal]

Take another look at Davidbeach's post.

 

[JG2828]

I think I just found what you are looking for.
Take a look at page 6 of the attached pdf.

3.2.1.3 For a multi-ratio internal bushing current transformer with a maximum rating equal to the nameplate rating of the equipment in which it is installed, but which is operating on a reduced tap, the calculated equipment capability should
be reduced by the factor:

SQRT(It/In)

Where It is the reduced tap current rating, and In is the maximum current rating of the current transformer.
If information is not readily available on the continuous thermal rating factor of a bushing current transformer, the manufacturer should be consulted.

 

[JG2828]

Basically you want to use 960 amps primary on an "800/5" CT. You need to find out the TRF or continuous thermal rating factor of the CT. It will be a number between 1 and 4. Multiply that number by 800 and if the result is greater than 960 you should be all set.

 

[mmt019]

What is the manufacturer and Cat # of the CT?

 

[BrkfldEE]

I don't know the manufacturer or cat # of the CT. I also don't know the thermal rating factor. It is in the bushing of an existing ASEA transformer. I plan on taking a trip to the substation where the CT is in a couple weeks. I will see if I can get the info then. But my question is pretty much answered, I just need to find out what the thermal rating factor of the CT is.

 

[jghrist]

If it's an energized transformer, chances are you won't be able to determine the CT rating factor safely. It won't be on the transformer nameplate. You could get the information from ASEA with the transformer serial number. Or, you could assume that the rating factor is at least 1.2 or ASEA wouldn't put the CT on the transformer.

 

[scottf]

I can't understand where the formula in 3.2.1.3 comes from. It should be It/In, i.e. no sqrt, in my opinion.