Metering & protection CT construction

[power2020]

Hi guys,
Please explain me about construction of protection & metering CT'S.
i know Metering CT'S saturate earlier than protection CT's as their acuracy limit factor is more than metering CT's.
i would like to know the following.
1) what is the construction ( meterial etc) which made them early or late saturation.
2) which one will have more impedance & why??

3) any other differences in respective of construction only ?

thanks for your comments / answers

 

[scottf]

Power2020-

These are tough questions to answer specifically, but in general:

1) Normally the difference between protection CTs and metering CTs is the core construction and specifically, the size (cross sectional area) used. Protection cores are generally larger in cross sectional area than metering cores. The larger cross-section gives a lower nominal operating fluz density, meaning that the protection cores can handle a higher current level before saturating.

Sometimes metering cores are made from nickel versus the normal silitsium steel. The nickel has a higher magnetic permeability and lower losses, and thus delivers better accuracy. However, the B-H curve is different and Ni core saturate at much lower flux densities than Si core, thus making them only suitable from metering applications.

2) In general, protection cores have a higher indcutance and secondary resistance values than metering cores of the same ratio, but that varies quite a bit with the burden rating. The reason for the higher inductance is simply the larger cross-section cores. The reason for higher winding resistances is a longer overall secondary winding as a result of the larger core.


Note that the above is very generic.

 

[cuky2000]

Short background: CTs are designed such that the excitation impedances are as high as possible. On the other hand, the main source of CT errors is the magnetizing current. As higher is the grade core magnet to obtain better accuracy (small error).

1) What is the construction (material, etc) which made them early or late saturation. Core are made of silicon-iron or high-alloy nickel-iron. Typical alloy value are:
Measuring CT: 75% Ni
Protection CT: 50% Ni

2) Which one will have more impedance & why? The protection CT’s have more burden impedance than the metering counterpart. Per ANSI standard the value of the burden relays are 1, 2, 4 and 8 while the relays CT values are 0.1, 0.2, 0.5, 0.9 and 1.8. This is perhaps because the CT rating capacity in VA = ZI^2 requires bigger core volume to provide higher accuracy.

3) Any other differences in respective of construction only? Some designs are made with air gap to improve the core performance during transient events.
Typically, protection CT’s are made of mill-patterned cold rolled sheet metal Si-Ni
Here is an example of B-H curve for protection & metering CT’s published in the web site:

 

[scottf]

Cuky2000-

I'm not sure where your figures come from but, based on the materials that we use (instrument transformer manufacturer) and what I've seen other manufacturers use, there is virtually 0 nickel content in the core materials normally used for protection applications. What most manufacturer call Si-steel material is M4 steel, which is normally <0.3% Nickel...of course there's not much more Si either, so I've never understood why it's called Si-Steel.

In the IEEE market, there is no security-factor for metering CTs, so M4 steel is often used for them as well. In the IEC markets, and others with a security factor, nickel is almost always used. Ni-steel is normally about 80% nickel. That's on the main reasons that Ni-steel is about 10 times more expensive than Si-steel.

 

[cuky2000]

Scottf

You are right, most CTs cores for metering purposes are made of nickel alloy and protection CTs are usually made of high-grade oriented steel strip because of cost. However, I understand that some manufacturers are obtaining low price for steel-nickel imported from China and other development countries that also help them to reduce the number of different material in stock.

 

[scottf]

cuky2000-

Due to the excitation characteristics, nickel-steel would never be suitable for protection purposes, as the saturation point is quite low. In general, nickel is only suitable for metering applications.