chance of CT saturation depending on TURNS RATIO?

[RIYAZAHAMED]

I came across this question by a friend. He was questioned in an interview. 2 CT's one with 100/1 A another with 100/5 A turns ratio are there. Which will go into saturation soon? Is there any thing to do with TURNS RATIO?


Any answer will be highly appreciated!

 

[Skogsgurra]

If the burden resistor is the same and the primary current is the same, the 100/5 CT will saturate before the 100/1 CT.

But, who would use CTs that way? I find the question just moot.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[scottf]

The correct answer is: there is no way to tell from that information.

One would need to know the accuracy and burden rating of each CT and the applied burden. Or see an excitation curve and Rct data for each CT.

CT performance is based on ampere-turns and not just secondary turns.

The Rct of a 1A secondary rated CT is going to be much higher than a 5A rated CT, in most cases.

 

[waross]

The primary of each window type CT will consist of one turn. As the current in each the primary is increased, the magnetic flux in the core will increase. At some level, the flux will saturate the core.
A metering CT may saturate at a lower level than a protection CT.
The only way that the question is valid is if a particular plant or station uses one secondary rating for protection and another secondary ratio for metering.
For instance, if standard practice with that employer is to use 100:5 CTs for metering and 100:1 CTs for protection, then the 100:5 CTs may saturate first, not because of the ratio but because they are metering class CTs.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[oldfieldguy]

Too many variables, friend.

Let's make an assumption for the sake of illustration, and say that both CT's use the same configuration, wight and type of iron for the core. The one with the fewer turns will saturate at a lower voltage.

You can illustrate this by using a multi-ratio CT and doing a saturation check on the full winding versus any of the partial windings.

In your case, you have one CT that's 100:1 and one that's 20:1 (100:5). If the cores were identical, you would expect the 100:5 to saturate much lower. However, there's a likelihood that the cores are not the same. Fortunately, at these ratios the saturation test can be performed relatively easily.

old field guy

 

[scottf]

oldfieldguy-

This is obviously an exercise in theory, since the question, in an of itself, is a poor one.

However, I don't really agree with your sentiment, in that the question was presumably, which CT would saturate first...and I assume that means during a fault. If the core was the same and the applied burden was the same, one cannot assume that the 100:5A ratio will saturate first, because the internal resistance, Rct, will be much much lower, than a 100:1A ratio, assuming that normal wire used for a 5A secondary is used on the 100:5A ratio and normal wire used for a 1A secondary is used on a 100:1A ratio.

In fact, if we say the core is the same in both and the applied burden is the same in both, the 100:1A ratio CT will almost certainly saturate before the 100:5A CT during a transient in real life. In the end, the ampere-turns is the same in both units at a given primary current level.

 

[pwrtran]

Saturation is not determined by the turn ratio it is determined by the type of core steel and cross section area of the core that allows maximum flux density can be passed through before B becomes flat. For a certain built CT and same secondary burden, the lowest ratio will saturate first.

 

[scottf]

pwrtran...sorry, but you're confusing knee-point voltage and saturation as a function of primary current, applied burden, internal resistance, and system x/r.

Try to do a CT performance relay calculation with an excitation curve alone and you can't do it. You also need the Rct of the CT. While the turns ratio differs by a factor of 5, the Rct would likely differ by a factor of 15-20 (5 times the number of turns/length of wire at approx. 1/5th the wire diameter).

Vs = Is x (Rct + Rl + Rb)

In my experience, the increase in Rct normally outpaces the difference in Is (5 times) and the difference in the knee-point voltage.

 

[pwrtran]

scottf -

....??

Say 100:5 tap has 20 turns and 500:5 tap has 100 turns. Rct/turn is a constant value and the total R is linear, how come the turn ratio increase 5 time and Rct increase by 15-20 times? Doesn't make sense.

When a CT working in a linear portion your expression is okay. However because our interest is in the non-linear region, so when it passes the knee point (saturation point is about 20% higher than the knee point) your equation is no longer adequate as it does not include the excitation current which is Ie, and Ist=Ie+Is which Ist is the total secondary terminal current. In order to support the Vs, more excitation current Ie is required to produce more flux but the flux density becomes flat is due to the material. If the material is changed like air there will be no saturation.

 

[scottf]

pwrtran-

The difference is that CTs with a 1A rated secondary use smaller secondary conductor. We're comparing a 100:1A to a 100:5A. For example, a 100:5A CT would have 20 turns of approx. #12AWG (2mm) with approx. 1.5 ohm/1000ft and a 100:1A CT would have 100 turns of approx. #18AWG (1mm) with approx. 6.4 ohm/1000ft. Wire size would vary depending on the rating factor of the CT, but it's close for discussion's sake. So you have 5 times the number of turns (assume 5 times wire length) of a wire with approx. 4 times the resistance/length...that's where I get the figure of 20 times Rct.

 

[waross]

Are we considering saturation during testing by energizing the secondary or saturation by over current on a one turn primary? Will there be a difference between primary energization and secondary energization?

Bill
--------------------
"Why not the best?"
Jimmy Carter