Ratio correction or interposing CTs 3

We are stuck with a situation where protection CTs (class 10P10, 10VA, 500/5) are used for metering - is this a problem?

The 500/5 amp ratio is too large for our current readings, we want use interposing/ratio correction CTs such that our metering is at 200/5 amp scale. What considerations do I have to follow to specify this ratio correction CT?

 

[jnims]

One thing you will have to watch is the burden you are placing on the 500:5 CTs once you have added the auxilary or interposing CTs. The impedance reflected to the 5A side of the 500:5 CTs will be (12.5/5)^2 times the impedance of your meter. If your wiring is short and you are using an electronic meter (usually a low burden), it is probably not a problem.

 

[ChrisN]

Thanx jnims for your reply, but I don't understand how you derived the impedance factor that you quoted on your reply.

 

[jnims]

To achieve a 200:5 ratio, you will need a 5:12.5 auxilary CT (500:5 * 5:12.5 = 200:5). Suppose you had a 1 ohm impedance connected to the 12.5A side of the auxillary and 12.5A was being passed through the impedance, the voltage would be 12.5V. On the 5A side of the auxiliary CT (assuming an idea transformer), you would have 5A and 31.25V since VA (or Amp-turns)are equal on each side. The impedance seen by the 500:5 loop is 6.25 ohms(31.25V/5A)or (12.5/5)^2 times the actual impedance. Before electronic meters and relays, it was necessary to check the burden to avoid saturation. As a result, it was highly unusual to connect auxiliary this way.

Hope this helps

 

[SooryaShrestha]

I think the CT with designation of 10P10 should not be used for measurement unless the big errors will not be a problem. I suggest that the CT be replaced with good accuracy CT. We are using high accuracy electronic meters these days at reasonable prices to get good readings. Earlier we find most of the time the readings do not match and sometimes the readings have no meaning because of low accuracy. Using 10P10 CT means at rated current the accuracy is +/- 3%. In your case since you are going to use around 40% of the primary current, the accuracy will be still lower. Thus with such CT, we will have readings which in proper sense not usable for any serious study.

 

[jack6238]

For jnims- I just want you know that was an excellent response. You get a star for me! great job.

 

[ChrisN]

Thanx for your help guyz. This was my first thread and I am touched by your responses, their certainly gave me direction. I really appreciate it.

 

[jbartos]

Suggestion: When applying those various CTs, it is necessary to remember that they have their rated VAs. Else, they may actually burn just like any other transformers. See Reference:
ANSI/IEEE Std C57.13-1978 An American National Standard IEEE Standard Requirements for Instrument Transformers

 

[peterb]

Regarding the accuracy of the CTs, some standard CTs have both a protection and a metering accuracy rating - the rating stated depends on the application for which the CT is supplied. I suggest that you contact the CT manufacturer or supplier to verify whether this is the case with yours.
If you are in fact using programmable electronic metering, the CT ratio should not be a problem, as you would simply program the actual ratio. Perhaps you should consider replacing the metering rather than adding the interposing CTs, which will add complexity and further measuring errors to your circuit. If you are stuck with using analog metering, consider replacing the 5A full scale deflection meters (by the way, are we talking ammeters here or energy metering?) with meters scaled for 2A FSD.