A CT Connected to another CT

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D!NNy

Senior Member
Location
San Luis Obispo
hi friends can any one help me on this..


I have seen a CT of 200:5 on a resistance grounding neutral, Secondary of 5amp CT is connected to another CT of 200:5 amp type.
I am trying to understand the purpose.

Is this some thing like to see Sensitive Earth Fault Current?
 

hurk27

Senior Member
Welcome to the forum, you only need to start one thread as all forums are open to everyone and those interested in answering your question will post soon, so I closed the others with a link to this one.
As for the reason of having two CT's, I not very familiar with that kind of configuration so maybe someone will chime in on it.

Most time we just install the manufactures designed system, but I have heard of a buck boost design to change the ratio thus changing the sensitivity of the GFP reversing the polarity of one CT in series with another will reduce the output voltage but why not just get the correct CT? but like I said this is kind of over my head.

Again Welcome
 

richxtlc

Senior Member
Location
Tampa Florida
hi friends can any one help me on this..


I have seen a CT of 200:5 on a resistance grounding neutral, Secondary of 5amp CT is connected to another CT of 200:5 amp type.
I am trying to understand the purpose.

Is this some thing like to see Sensitive Earth Fault Current?

The second ct is an aux ct and is used to change the ratio of the primary. With the 2 cts at 200:5 connected as described, the ratio is 1600:1 or 8000:5.
 

rcwilson

Senior Member
Location
Redmond, WA
Three ways of wiring two CT's:

1. Primary in series with secondaries paralleled. CT ratio is cut in half. (200 A primary: 5 A + 5 A = 10 A secondary). 200/10 = 100/5 CT.

2. Primary in series with secondaries in series. CT ratio is the same but twice the output voltage before saturation.
(200A primary, each CT helps push the same 5 amps through the circuit.)

3. Series connected with one CT's secondary feeding the primary of the second CT. Ratio = product of ratios.
(200 A primary #1 = 5 A secondary #1 = 5A primary #2 = 0.125 A secondary #2) 8000/5 effective ratio (600:1)

Which way is your connected?


A 200:5 CT donut has very little voltage output. Its ratio is 40:1, one primary turn = 40 secondary turns. The single primary turn is the conductor going through the middle of the donut. With only 40 turns of secondary wire on the core, each turn has to generate 1/40 of the voltage needed to push the required current. Unless we build a CT with a lot of magnetic steel in the core it is difficult to get enough voltage output to have good accuracy under all conditions.

Stacking two CT donuts on the same conductor and putting the secondaries in series doubles the voltage output.

Putting the secondaries in parallel gives the same voltage output but twice the current so the ratio is 1/2.

A problem with using the two CT's to get an effective 8000:5 (1600:1) ratio is the high burden on the CT. A 0.1 ohm relay circuit on the secondary looks like a 160 ohm load on the primary of the CT (40:1x 40:1 x 0.1 ohm = 160 ohm). At 200 Amps primary, that first CT will have to put out 5A x 160 ohms = 800V. That is well beyond the capability of most CT's in that ratio. The CT's will saturate and not deliver accurate signals.
 

rcwilson

Senior Member
Location
Redmond, WA
Correction on Impedance of CT's in series.

Correction on Impedance of CT's in series.

My math was incorrect above. Ganging two 200:5 CT's together gives an 8000:5 CT, since 200:5 = 40:1 and 40x40 x 5 = 8,000. (had that right).

Ganging the CT's reduces the sensitivity of the relay. A 1.0 A relay pickup setting will = 40 amps primary pickup on a single 200:5 CT and 1600 Amps on the ganged CT's.

My math on the impedance burden was wrong. The 200:5 CT will reflect the impedance to the primary based on the inverse of the CT ratio squared. A 0.1 ohm relay circuit will present 01./(40 x 40) = 0.0000625 ohms on the primary of a 200:5 CT. Nothing to worry about as long as the 8000:5 ratio is what we wanted.

The CT burden problem occurs when we try to increase sensitivity by using the CT to step up the current. If our 40 amp sensitivity is too high, why not boost the current to the relay by using the second CT backwards as a 5:200 CT? The new pickup appears to be 1.0 amps primary. But that's when the 0.1 ohm relay circuit looks like 160 ohms to the first CT, increasing the voltage output needed and possibly saturating the CT resulting in no current output. (Tried it. Didn't work. Tripped utility instead.)
 

AdrianWint

Senior Member
Location
Midlands, UK
Is this some thing like to see Sensitive Earth Fault Current?

It might be or might, as others have already suggested, be to 'adjust' the ratio of the CT.

You'd need to tell us a little more...... are the CT's wired in series , ie. the SECONDARY of the first CT is connected to the PRIMARY of the second CT

OR

are both the SECONDARIES connected together & if so are they in phase ie. S1 to S1 & S2 to S2 or are they in anti-phase ie S1 to S2 & S2 to S1?

This information should help us help you....
 

D!NNy

Senior Member
Location
San Luis Obispo
Welcome to the forum, you only need to start one thread as all forums are open to everyone and those interested in answering your question will post soon, so I closed the others with a link to this one.
As for the reason of having two CT's, I not very familiar with that kind of configuration so maybe someone will chime in on it.

Most time we just install the manufactures designed system, but I have heard of a buck boost design to change the ratio thus changing the sensitivity of the GFP reversing the polarity of one CT in series with another will reduce the output voltage but why not just get the correct CT? but like I said this is kind of over my head.

Again Welcome

Thank you Sir
 

D!NNy

Senior Member
Location
San Luis Obispo
My math was incorrect above. Ganging two 200:5 CT's together gives an 8000:5 CT, since 200:5 = 40:1 and 40x40 x 5 = 8,000. (had that right).

Ganging the CT's reduces the sensitivity of the relay. A 1.0 A relay pickup setting will = 40 amps primary pickup on a single 200:5 CT and 1600 Amps on the ganged CT's.

My math on the impedance burden was wrong. The 200:5 CT will reflect the impedance to the primary based on the inverse of the CT ratio squared. A 0.1 ohm relay circuit will present 01./(40 x 40) = 0.0000625 ohms on the primary of a 200:5 CT. Nothing to worry about as long as the 8000:5 ratio is what we wanted.

The CT burden problem occurs when we try to increase sensitivity by using the CT to step up the current. If our 40 amp sensitivity is too high, why not boost the current to the relay by using the second CT backwards as a 5:200 CT? The new pickup appears to be 1.0 amps primary. But that's when the 0.1 ohm relay circuit looks like 160 ohms to the first CT, increasing the voltage output needed and possibly saturating the CT resulting in no current output. (Tried it. Didn't work. Tripped utility instead.)

Thanks for your response. I was busy last two days. i need to do some research on CT's based on the information you gave me here. i will get back to you once i have any more questions.
 

D!NNy

Senior Member
Location
San Luis Obispo
It might be or might, as others have already suggested, be to 'adjust' the ratio of the CT.

You'd need to tell us a little more...... are the CT's wired in series , ie. the SECONDARY of the first CT is connected to the PRIMARY of the second CT

OR

are both the SECONDARIES connected together & if so are they in phase ie. S1 to S1 & S2 to S2 or are they in anti-phase ie S1 to S2 & S2 to S1?

This information should help us help you....


Hi Friend

A) 200:5 ---> B) 200:5
CT A : 200 amps primary to 5 amp Secondary this is connected on a neutral grounding resister.
CT B : 5 amp secondary of CT A is connected to 200amp primary of CT B and 5amp Secondary of CT B is Connected to Ground fault relay.

Thanks
 
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