CT RATIO CHANGING

[appunni]

Sir,

We have a 200/1-1A CT in one substation. The classes of cores of CT are PS and PS respectively. We want to enhance the CT ratio from 200 to 400A and only one core is enough for application. There is no provision for enhancing the ratio both in primary and secondary of the CT. If we connect secondary cores in series, can the same CT be used as 400A/1A single core? What is the limitation for using 200A primary winding as 400A primary winding practically? If the cores are of 5P10 and PS classes, can the secondary cores be connected in series?
With regards,
appunni

 

[DanDel]

My first question is: Why do you want a 400:1A instead of a 200:1A CT?
You cannot make a 400:1A CT by connecting the secondaries of two 200:1A CTs in series. As a matter of fact, if you are planning on adding a 200:1A to the existing CT, why don't you simply install a 400:1A CT and remove the existing one?

 

[askit9197]

First , a 200/1-1A CT cannot be used as a 400 A one as the excess primary current causes heating in the cores. Second connecting the secondaries can never give you a different ratio. You can instead use a ICT (interposing CT) of ratio 1/0.5 to get a ratio of 400/1 from the same ct.Be sure to check the burden of this arrangement

 

[scottf]

What is the rating factor of the the 200:1A CT you have? If it is 2.0 or higher, then you can run it at 400A continuously. Otherwise, buy a new CT. As stated above, connecting the secondaries together will not give you desired results.

 

[appunni]

Sir,
Thanks for your replies. The reason for replacing 200/1A to 400/1A is to completely utilise the current carrying capacity (which will come around 360A) of the feeder completely.
My analysis is like this. 200/1A CT means one turn in primary and 200 turns in secondary. If two such cores are connected in series, we get 400 turns in secondary with one turn in primary. This is equivalent to 400/1A CT. What are the mistakes for analysis? Please explain. Advance thanks.
with regards,
appunni

 

[scottf]

The problem is that there are 2 sets of 200 turns around 2 different cores...meaning that the flux density in each is roughly the same...

Think about it this way...for your scheme, the secondary windings of each winding would be conncted in series, therefore, each secondary winding would have the same current through it. If 200 A is in the primary, then there would be 1A in each secondary....that means it's still a 200:1A ratio. Actually, for your scheme to work ratio wise, you would have to put the secondaries in parallel. Problem is that it still won't work correctly.

Bottom line....either your 200:1A current CT has a ration factor of 2.0 (which means you can run it at 400A) or you get a new 400:1A CT.