About CT and relay performance

[lume7006]

Hello guys,

A circuit has a small load of 75 kVA at 4.16 kV, and a very high shortcircuit level, around 50 kA.
If we choose the CT in order to "see" the load current it will be a very low ratio (let's say 150/5) but, it will not held 100 A secondary neither saturation criteria.
However, taking into account saturation and less than 100 A sec, the ratio needs to be so high that relay will not see any load, I mean, the secondary current has a magnitude so small that the sensibility of the relay does not see it and then, it will consider as the circuit were open.

What options should we consider to solve this?
Any idea is welcome!

Best Regards

 

[davidbeach]

Will the saturation cause the relay to fail to operate? Doesn't matter how saturated if the relay still does its job.

Why do you need to see load with your protection CT? Why does the relay care if the circuit is open? Since maximum fault current is about 4800 times load current you won't ever do a good job of measuring load and responding to faults with a single iron core CT. Perhaps this is an application for an optical CT.

 

[lume7006]

Hello,

My concern is because the relay manufacturer states in the relay manual that the minimum current to be detected is 0.1*In, this means 0.05 A sec, otherwise, the relay "understands" the circuit breaker is open.

According to our calculations, the CT may be saturated and as the relay will be receiving more than 100 A sec, then, there could be potential problems with this circuit, of course only in case of a fault!

 

[alehman]

lume,
Do you know why the relay needs to know if the breaker is open or closed? If it really is necessary, perhaps you could use an auxiliary contact on the breaker to signal the relay instead.

Also, what David said. If it's just an overcurrent trip, so long as the secondary current is high enough on the time-current curve or above the instantaneous pickup, saturation may not matter.



Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[lz5pl]

Recently I had a similar situation - load is power transformer 10 MVA - about 52 A, 3-phase SC - 24 kA (110 kV feeder). Problem was not the protection core, but commercial metering. I used CT with secondary reconnection of ratio. So metering core was connected on 200/1, but protections - on 800/1 A, cl. 5P30 core.

------------------------
It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[marks1080]

You should use a pallet from the breaker to let the relay know if it is open or closed. I do not know what you mean by "understands".

 

[lume7006]

Hello,

What I meant with "relay understands" was taken from relay manual that states:
If the current flowing through the relay is less than 0.05 Asec then, the relay recognizes the breaker is open.
So, I am not sending any other signal to the relay, the relay itself is making a decision based on the flow of current below a design threshold.

Coming back to my initial question:
May I choose a lower CT ratio despite having more than 100 A sec and have calculated a voltage that it is into the saturated region of the CT or select a higher ratio?

Thank you in advance

 

[slavag]

Hi.
Sorry, but I don't see problem.
You can install 150/5A CT w/o any problem, but isn't "good" for you, your load about 10A. You need 75/5A or 50/5A CT, Saturation isn't problem, relay will work, if CT will saturated too.
You need only check if this CT is "standed" with 50kA SC current, you can use, for rxample, ring type CT.
relay current input in the newer relay have a thermal withstand capability about 500A for 1s, 1250 for half-wave.

Good Luck.
Slava

 

[marks1080]

I still think it best to use a breaker pallet for breaker position. If the circuit faults open (not short circuit) the current on the secondary of the CT could likely be less than you describe... It's not good enough for your relay to think the breaker is open in this situation. The relay should understand the true position of the breaker and then make the decision to open it.

EG: Breaker pallet indicated breaker is closed. Secondary current is approx. 0. Relay should be programmed to know there is likely an open fault somewhere on the circuit and trip the breaker.

That is a very basic example, and in practice more elements would likely be involved. This is also more of a general comment, as I have no looked at your diagrams. Maybe it applies to you, maybe it doesn't.

Mark

 

[racobb]

marks1080

What do you mean by a breaker "pallet". Just curious as I have not heard the term before here in the US.

Aside, as you say the breaker needs to know and with the advanced relays today, we normally input a breaker a and b contact so it will see the transition.

Recently had a breaker that would not auto reclose because the input contacts were not adjusted properly and the relay complained beacuse of fail to trip.

Electromechanical over current relays did not know, but the reclosing relay did. Now they are all in the same relay!

Alan

 

[alehman]

May I choose a lower CT ratio despite having more than 100 A sec and have calculated a voltage that it is into the saturated region of the CT or select a higher ratio?

This depends entirely on the relay functions and pickup points. As I said, if it is just overcurrent, quite possibly it will be fine.

lz5pl - using more than one tap on a CT will result in inaccurate performance for both devices.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[lz5pl]

using more than one tap on a CT will result in inaccurate performance for both devices.
======
Alan, could you explain this a bit more detailed. I used separate cores for measuring and protection, so I didn't expected to have some problems with accuracy or saturation.

------------------------
It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[slavag]

Hi.
1. Mark, you are right, is always better connect aux. contacts for CB indication (52a, 52b). But today possible use for it low current supervision for the CB position indication too. It's include in the logic of newer relays, for SOTF functionality for example.

2. Alan, as lz5pl saied, is multi core CT, not multitap.
For multitap CT, you are right, unpossible use more than one tap.

Have a nice weekend.
Slava

 

[JJHorak]

You might check into into the short time rating of the wiring and relays. Don't count on CT saturation to limit the secondary current too much. CTs will gladly put out many hudreds of amps.

Example: C50, 100:5 CT. Some such CTs can have an internal resistance as low as 0.03ohms. Assume external wiring raises total resistance to 0.10 ohms. You could reproduce, without saturation, somewhere around 10,000A primary, giving 500A secondary. You could get maybe 2x that much secondary current, depending on the actual CT knee point and secondary burden.

 

[apowerengr]

size the CT and ratio to handle the short circuit issue for protection. meter the transformer secondary for load data.

 

[lume7006]

Hello again,

I thank everybody for having given feedback.
Now, taking againg the first question I would like to emphasize:

1) If I only considered ANSI/IEEE requirement that CT ratio must give less than 100 secondary Amperes.
What happen if this is not hold?
I have seen that some relay manufacturer state that relay may support much more than this value, but ANSI standard recommends to choose CT Ratio based on getting less than 100 secondary Amperes.
Is it really important to get less than 100 A or not?
I mean, is it any effect on the CT because of this high current? (and this effect is something different from saturation!)

2) I was reading IEC standard and there is not a recommendation of having less than 100 secondary A like ANSI.

Does anybody have a good reference or example to size CT's based exclusively on IEC standard?
ANSI standard gives at least two examples that can be followed and get a clear idea, however, IEC does not.

Best Regards

 

[davidbeach]

If you get the desired performance and nothing is damaged, what would be the issue. 100A secondary (more correctly 20x rated secondary) is the point at which you have 10% error into a standard burden. Less burden can mean more current for the same knee point voltage. I'd hate to ever see a CT applied so that an IEC CT had 100A on the secondary as that would generally be 100x rated secondary.

 

[cranky108]

Good point about 5 amp and 1 amp CT's, however we are seeing more and more 1 amp CT's here in the US. Fortunitually these are in distribution applications and in high ratio types that keep the current low.

 

[marks1080]

Alan: If you have a CT ratio that works, forget about the ANSI standard.

Remember that as long as your CT can provide the secondary current necessary to active your relay to trip than it doesn't matter if you go into saturation (again - another general comment, but usually holds true).

I often find myself very confused when I see discussions about different standards in this forum. The utility I work for governs itself and we are not subject to ANSI, IEEE, NEC, etc... We have developed our own standards that I've been told either meet or exceed other industry standards. Often i find myself at a disadvantage because of this.

 

[slavag]

I don't understand what is standard 100A.. more or less.
What David and Mark saied, if CT provide a needed secondary current, all well.
Protective relay have a 40In setting. Only check thar relay and CT will no dameged, it's all.
Lot of application for MV have a "small" CT ( 50-100A) with SC aboyt 25-40kA.

Best Regards.
Slava.

Mark, btw, we have also our own standard.