Current Transformer Questions 2

[111R]

I'm attempting to fully understand CT operation and I'm getting stuck on a few seemingly simple concepts despite reading for quite a while. I realize this list is exhausting, so a few answers will still be helpful.

The excitation curve is a plot of secondary RMS exciting current vs secondary RMS exciting voltage.

1) I look at the accuracy classes of CTs and they are rated at C100, C200, C400, and C800. I found out that this means that a C800 CT can supply 800V into the secondary at 20x rated current (usually 5A*20=100A) within 10% accuracy. So, B1.0, B2.0, etc mean the same thing, but max burden impedance is given rather than secondary voltage. I am looking at PAGE 5 of the following PDF and I don't understand a few things:

http://www.idc-online.com/technical..._engineering/current_transformer_concepts.pdf

Why is this a C400 CT? 486V is quoted as the secondary voltage at 10A. How is 486V within 10% accuracy? It's way beyond the knee point. And, why 10A...shouldn't it be at 100A (20*5)? I guess I don't really understand what the excitation curve indicates. The knee point voltage on this curve seems to be around 200V. What does the knee point tell us?

2) The knee point is where the core saturates, correct? So, the terms excitation curve and saturation curve are synonymous? What about magnetization curve?

3) Is the linear section before the knee point the desirable area to stay within for accurate current monitoring?

4) The IEEE30 and IEEE45 refer to the angles of a line tangent to the knee point. They indicate the knee point voltage of the CT. There is also IEC 10/50 which is the point where a 10% increase in exciting voltage causes a 50% increase in exciting current. Since I'm in the US, I doubt I will use IEC often. How do I select between IEEE30 and IEEE45? Why is there a need for two? IEEE30 will give a higher knee point value, but is there an application for one over the other?

Thanks for your time.

 

[randyman]

Hello 111R,

With regards to metering CTs don't always assume they too, will saturate at fault levels. That depends on the type of CT (wound-type or window-type) and the current ratio. Many window-type CTs have enough capacity to drive a low burden well over 20 times rated current without even entering saturation. The point here is do not assume anything just because the CT is rated for metering duty - they are not self limiting and do not automatically turn off beyond their rating factor. In the IEC world they do have specs which force a CT to be self-limiting, refered to as Instrument Security Factor and can be typically set to 5 or 10 times rated current.

With regards to whether or not a metering CT could drive a relay would depend on knowledge of the secondary loop impedance and the exciting characteristics of that CT. You cannot make a general statement regarding application of metering class CTs for use for protection purposes - generally they are not considered. But the same evaluations of calculating performance of relaying CTs can be applied to metering CTs provided you have the information to do so.

 

[111R]

Does anyone happen to have a cutaway picture of a typical bushing CT? I'm not finding anything on Google.

 

[111R]

Also, for PTs, I see ratios defined as 700/1200:1:1 or 350/600:1:1. What does the extra :1 ratio mean?

 

[mgtrp]

It probably means that there are two secondary windings.