How do I select CT ratios?

[mbrooke]

38 kv vacuum breaker, 16ka worse case short circuit current

15.5kv vacuum breaker, 7ka worse case short circuit current

How do I select the correct CT ratio for each of these? What is considered the normal industry standard? I'd like a value that will work, yet can be "standardized"

 

[adamscb]

You select the CT ratio based on the calculated full-load current that would be flowing through the circuit breaker. The current flowing through should never be above the primary rating of the CT (under normal conditions, i.e. not counting fault situations). Sizing is not done based on calculated short-circuit, all of the CT ratios listed would be saturated under a short circuit condition.

For example if you're drawing between 0 and 600 amps, use the 600:5 CT, and so on and so forth. The closer you get to 600, the more you might want to bump up to the 1200:5 CT ratio because of saturation.

 

[mbrooke]

You select the CT ratio based on the calculated full-load current that would be flowing through the circuit breaker. The current flowing through should never be above the primary rating of the CT (under normal conditions, i.e. not counting fault situations). Sizing is not done based on calculated short-circuit, all of the CT ratios listed would be saturated under a short circuit condition.

For example if you're drawing between 0 and 600 amps, use the 600:5 CT, and so on and so forth. The closer you get to 600, the more you might want to bump up to the 1200:5 CT ratio because of saturation.

But doesn't the rating of the relay itself also play a role in terms of fault current or this is arbitrary?

 

[adamscb]

But doesn't the rating of the relay itself also play a role in terms of fault current or this is arbitrary?

Not to my knowledge. The C### refers to the accuracy of the CT (they're all accurate enough for general purpose), and the 1 or 2 sets is if you have more than one cable per phase, at least this is how I'm interpreting the Excel table.

 

[NewtonLaw]

But doesn't the rating of the relay itself also play a role in terms of fault current or this is arbitrary?

Almost without exception, the relay will be designed to operate at some value of current input between 0 to 5 amps for normal load current and will additional inverse tripping time at higher levels of current even through the CT will be in saturation. I.E. the relay LSIOC and HSIOC operates more rapidly at higher fault magnitudes but only incrementally so. The TOC will see the greatest increase in operating speed (shorter operating time) at fault magnitudes.

Hope this helps,

Newton Law

 

[mbrooke]

Not to my knowledge. The C### refers to the accuracy of the CT (they're all accurate enough for general purpose), and the 1 or 2 sets is if you have more than one cable per phase, at least this is how I'm interpreting the Excel table.

Over current and bus bar differential. One set is for the OV relay the other for differential.

 

[GoldDigger]

But doesn't the rating of the relay itself also play a role in terms of fault current or this is arbitrary?

The 600:5 and similar ratings assume that the relay or other device connected to the low current side of the CT is designed to operate with a 5A full span current.
That is why it says 600:5 and not just 120:1.
If your device wants to see a full scale current other than 5A, then you have to do a little more work in choosing the CT.
You can use a 600:5 CT with a device that wants to see 1A when the primary current is 120A. It will work just fine, but will be overbuilt for that task.
If you want to get a 10A output to your device, you could not use any of the CTs listed because they would be seriously saturated before that point.

None of these ratings directly address fault current. Using a CT instead of a suitable shunt for thermal trip and coil for magnetic trip is an entirely different design problem.

 

[iwire]

That is why it says 600:5 and not just 120:1.

Nice, I don't work with these kinds of CTs but see them often and wondered why the ratios were expressed that way.

 

[mbrooke]

The 600:5 and similar ratings assume that the relay or other device connected to the low current side of the CT is designed to operate with a 5A full span current.
That is why it says 600:5 and not just 120:1.
If your device wants to see a full scale current other than 5A, then you have to do a little more work in choosing the CT.

All the SEL relays can be ordered with either a 1 A or 5 A input rating, in this case 5A.

You can use a 600:5 CT with a device that wants to see 1A when the primary current is 120A. It will work just fine, but will be overbuilt for that task.
If you want to get a 10A output to your device, you could not use any of the CTs listed because they would be seriously saturated before that point.

Got it, and makes sense.

None of these ratings directly address fault current. Using a CT instead of a suitable shunt for thermal trip and coil for magnetic trip is an entirely different design problem.

Any idea what typical relays such as this:

https://selinc.com/products/551/

And this:

https://selinc.com/products/351/

Can handle in terms of short circuit secondary current? My understanding is that a higher CT ratio such as 2000/5 will put far more current through the relay before saturation than say a 600/5.

 

[mbrooke]

The 600:5 and similar ratings assume that the relay or other device connected to the low current side of the CT is designed to operate with a 5A full span current.
That is why it says 600:5 and not just 120:1.
If your device wants to see a full scale current other than 5A, then you have to do a little more work in choosing the CT.
You can use a 600:5 CT with a device that wants to see 1A when the primary current is 120A. It will work just fine, but will be overbuilt for that task.
If you want to get a 10A output to your device, you could not use any of the CTs listed because they would be seriously saturated before that point.

None of these ratings directly address fault current. Using a CT instead of a suitable shunt for thermal trip and coil for magnetic trip is an entirely different design problem.

Also, these make of relays ask for amps secondary...

So if I want my 51 to pick up at 600amps on a 1200/5 CT, I would enter 2.5 amps?

1200/5=240

Desired amps / 240= amps secondary in the settings sheet?

 

[adamscb]

Any idea what typical relays such as this:

https://selinc.com/products/551/

And this:

https://selinc.com/products/351/

Can handle in terms of short circuit secondary current? My understanding is that a higher CT ratio such as 2000/5 will put far more current through the relay before saturation than say a 600/5.

The 551 can handle on the secondary of the CT: "15 A continuous, 500 A for 1 s,linear to 100 A symmetrical" (found on Page 20 of the Data sheet).

The 351 can handle on the secondary of the CT: "15 A continuous, 500 A for 1 s,linear to 100 A symmetrical,1250 A for 1 cycle" (found on Page 28 of the Data sheet).

I didn't do the math, but I think you're fine with using either one, from a short-circuit perspective. These things are designed with short circuit ratings in mind. On Page 31 of the Datasheet on the 351, on Instantaneous it can trip somewhere between 0 and 1/4th of a cycle, and on the secondary it can withstand 1250 A for up to one cycle, so you should be good. I couldn't find how fast the 551 could trip on instantaneous in the Datasheet.

Also, these make of relays ask for amps secondary...

So if I want my 51 to pick up at 600amps on a 1200/5 CT, I would enter 2.5 amps?

1200/5=240

Desired amps / 240= amps secondary in the settings sheet?

Correct

 

[Ingenieur]

Look at pg 8
https://cdn.selinc.com/assets/Liter...ers/TP6038_19920817_Web.pdf?v=20150812-085648

Rule of thumb
i fault/ct primary rating < 20
C rating 2 times excitation voltage at i fault

assume
nominal load 200 A
i fault 3000

3000/20 = 150
so any rating over 150:5 should be fine
but since load is 200 need larger
300:5 puts load at 67% or 3.33 A out
400:5 at 50% or 2.5 A
either should be fine
3000/300 = 10 < 20
3000/400 = 7.5 < 20

Use 300:5
assume burden of 0.5 Ohm
Sec fault i x burden = 3000/60 x 0.5 = 25 v
So a min C of 50 is required

 

[mbrooke]

The 551 can handle on the secondary of the CT: "15 A continuous, 500 A for 1 s,linear to 100 A symmetrical" (found on Page 20 of the Data sheet).

The 351 can handle on the secondary of the CT: "15 A continuous, 500 A for 1 s,linear to 100 A symmetrical,1250 A for 1 cycle" (found on Page 28 of the Data sheet).

I didn't do the math, but I think you're fine with using either one, from a short-circuit perspective. These things are designed with short circuit ratings in mind. On Page 31 of the Datasheet on the 351, on Instantaneous it can trip somewhere between 0 and 1/4th of a cycle, and on the secondary it can withstand 1250 A for up to one cycle, so you should be good. I couldn't find how fast the 551 could trip on instantaneous in the Datasheet.



Correct

Its ok and thank you As long as those will handle the short circuit currents for a few cycles I can live with that. #10 CU will be good from the relay to the CT?

FWIW I know of their more costly versions being used in transmission applications with fault currents in the 63,000 and 80,000 range- so its nice to know these can handle similar. Though not to similar, 16ka is about the max here.

 

[mbrooke]

Look at pg 8
https://cdn.selinc.com/assets/Liter...ers/TP6038_19920817_Web.pdf?v=20150812-085648

Rule of thumb
i fault/ct primary rating < 20
C rating 2 times excitation voltage at i fault

This is what I had in mind. So it looks like I also have to take the fault current into account making sure. Is this for the sake of the relay or the CT?

assume
nominal load 200 A
i fault 3000

3000/20 = 150
so any rating over 150:5 should be fine
but since load is 200 need larger
300:5 puts load at 67% or 3.33 A out
400:5 at 50% or 2.5 A
either should be fine
3000/300 = 10 < 20
3000/400 = 7.5 < 20

Use 300:5
assume burden of 0.5 Ohm
Sec fault i x burden = 3000/60 x 0.5 = 25 v
So a min C of 50 is required

Thanks! If I have 800amps max continuous, 1200/5 is what you would select?



...................................................


BTW, I am also taking into account bus differential with these CTs. At least one of them. A SEL587Z would put me at well over 25v. Unless I am told compelling reason to go with a 487B.

 

[Ingenieur]

This is what I had in mind. So it looks like I also have to take the fault current into account making sure. Is this for the sake of the relay or the CT?







Thanks! If I have 800amps max continuous, 1200/5 is what you would select?



...................................................


BTW, I am also taking into account bus differential with these CTs.

depends on fault current and relay Z or burden

the ct can handle up to 20 x rating in almost all cases
In this case 24000 A or an i fault = 3000% x i load

and tripping is usually at 800-1200% give or take

but it usually is not an issue if sized for mid i load at mid range

 

[mbrooke]

depends on fault current and relay Z or burden

the ct can handle up to 20 x rating in almost all cases
In this case 24000 A or an if of 3000% of i load

But it will saturate before that value is reached or saturate after 24,000? Assuming typical burden.

and tripping is usually at 800-1200% give or take

800%? :blink: Time over current will supplement instantaneous values.

 

[SG-1]

All the SEL relays can be ordered with either a 1 A or 5 A input rating, in this case 5A.




Got it, and makes sense.




Any idea what typical relays such as this:

https://selinc.com/products/551/

And this:

https://selinc.com/products/351/

Can handle in terms of short circuit secondary current? My understanding is that a higher CT ratio such as 2000/5 will put far more current through the relay before saturation than say a 600/5.

Page 20 of the Data Sheet for the 551 says:

General AC Input Currents
5 A nominal: 15 A continuous, 500 A for 1 s, linear to 100 A symmetrical.
Limiting Dynamic Value:
1250 A for 1 cycle (sinusoidal waveform)

Check out this GE publication for selecting CTs:
https://www.idc-online.com/technica...ring/Selecting_current_transformers_part1.pdf

 

[Ingenieur]

But it will saturate before that value is reached or saturate after 24,000? Assuming typical burden.

800%? :blink: Time over current will supplement instantaneous values.

Should be linear up to that point, not saturated
note the relay spec others have posted: 100 A linear or 20 x 5 A
and should trip well before that in a short time 6 cycles or less
if the ct is ranged at 2/3 it will trip at 3000% of load i
in my experience oc trip is set for 6 cycles at far less

I don't know what that means?
there may be point where the curves overlap depending on settings

 

[GoldDigger]

Once you hit core saturation, the output current will still continue to rise with increasing input, just not as fast since you now have only the coupling of an air core inductor.

 

[Ingenieur]

This is pretty interesting
http://store.gedigitalenergy.com/faq/Documents/750_760/CTsaturation.pdf

you need to get some pretty high fault/rating ratios before you have issues