51 Relay Setting vs. CT Ratio

[Flapjack]

SKM and some other publications recommend selecting the CT ratio on the primary side of a transformer to be 200% of transformer FLA base rating and to set the 51 pickup at 110-140% of FLA. I have a pdf of IEEE Industry Applications Society that also recommends CT ratio to be 200% of transformer FLA and <300% of transformer self cooled rating or 150% of transformer max rating.
IEEE C37.110 says the maximum design load current should not exceed the CT rated primary current.

Ignore being under the transformer damage curve and to the right of the inrush. I am only inquiring about the relay setting compared to the CT ratio.

The IEEE Industry Applications Society pdf shows examples of time-current curves and the trip settings are beyond the CT ratio primary value. I have attached two examples from the pdf that show long time settings of 2.5 and 2.73 times the CT primary value.

My questions are: Is this common? Are there any issues to look out for when selecting a trip setting that is 2-3 times the CT rated primary current?

I would think if the CT is designed to handle 20 times rated current and still be within 10% error, it has no problem handling 2-3 times the CT rating for the 51 setting, but I'd like to hear from those who have more experience in this area than I. Maybe SKM is being overly conservative in their recommendation?

 

[Paul1955]

SKM and some other publications recommend selecting the CT ratio on the primary side of a transformer to be 200% of transformer FLA base rating and to set the 51 pickup at 110-140% of FLA. I have a pdf of IEEE Industry Applications Society that also recommends CT ratio to be 200% of transformer FLA and <300% of transformer self cooled rating or 150% of transformer max rating.
IEEE C37.110 says the maximum design load current should not exceed the CT rated primary current.

Ignore being under the transformer damage curve and to the right of the inrush. I am only inquiring about the relay setting compared to the CT ratio.

The IEEE Industry Applications Society pdf shows examples of time-current curves and the trip settings are beyond the CT ratio primary value. I have attached two examples from the pdf that show long time settings of 2.5 and 2.73 times the CT primary value.

My questions are: Is this common? Are there any issues to look out for when selecting a trip setting that is 2-3 times the CT rated primary current?

I would think if the CT is designed to handle 20 times rated current and still be within 10% error, it has no problem handling 2-3 times the CT rating for the 51 setting, but I'd like to hear from those who have more experience in this area than I. Maybe SKM is being overly conservative in their recommendation?

The instantaneous (50) setting is usally around 200% of the maximum contribution of the fault current on the transformer low side. The time overcurrent (51) is set above the maximum allowable load beyond the transformer top rating....sometimes as high as 200% FLA. Many utilities select the rating of the CT primary current close to the 125% of the rated current of the transformer primary. Often times this results in a non-standard rating for CT primary windings, so the next practical rating is used. My experience in industrial applications (steel plants, etc.) has been that the CT is close to 200% of transformer FLA rating. Appendix C in IEEE C37.91 (2008) has several examples of setting transformer protection relays that may be of interest. There are also several (free) videos on the EasyPower website (www.help.easypower.com) that are very beneficial too.....

Paul

 

[mivey]

SKM and some other publications recommend selecting the CT ratio on the primary side of a transformer to be 200% of transformer FLA base rating

Well you can swag it like that or consider the application. First, sizing the relay CT over FLA is for consideration where you might overload the transformer. If you won't have 2X FLA then forget the 200% and go with expected overload percentages.

Second, you do want to make sure you can handle the fault current without saturating the relay CT. Now that may require a higher ratio to keep within 20X of the CT rating. Even that is not always possible with very high fault currents so you may have to have a second set of high current CTs, or use a peaking scheme, or other mitigation techniques.

and to set the 51 pickup at 110-140% of FLA.

That will depend on the application. If no steady overload is expected, then you can get closer to 100%. On the other hand you might get 200%, like with cold load pick up on a utility feeder (so we might set to 200-225% of normal load). If protecting a transformer, the damage curve and adjacent protection curves will also help determine the pickup.

IEEE C37.110 says the maximum design load current should not exceed the CT rated primary current.

Yes for the relay CT. FWIW, metering CTs have rating factors and you can get 3-4X through those, depending on the ambient.

Ignore being under the transformer damage curve and to the right of the inrush. I am only inquiring about the relay setting compared to the CT ratio.

OK, from here forward but I'll not start over.

The IEEE Industry Applications Society pdf shows examples of time-current curves and the trip settings are beyond the CT ratio primary value. I have attached two examples from the pdf that show long time settings of 2.5 and 2.73 times the CT primary value.

My questions are: Is this common? Are there any issues to look out for when selecting a trip setting that is 2-3 times the CT rated primary current?

Not if they have a 3X rating factor for the given ambient or something else is checking for overloads so we won't have long-term overloads that burn up our CTs.

I would think if the CT is designed to handle 20 times rated current and still be within 10% error, it has no problem handling 2-3 times the CT rating for the 51 setting

For short fault duration that is true but not long time settings

Also, the 20X must consider the burden (Zb) , X/R ratio (for asymmetry), and remanence (residual flux component). The typical formula is:

{ [I_fault * Zb * (X/R +1)] / [ 1 - per unit remanence] } <= 20

 

[mivey]

The instantaneous (50) setting is usally around 200% of the maximum contribution of the fault current on the transformer low side.

For utility feeders there is not much point in that unless you don't really want an instantaneous or just are tripping for faults from elsewhere, like primary contact or something.

The normal instantaneous looks down about 80% of the line or it looks to just above the next device to keep the protection zones separated.

Some others: You might have a hot-line tag instantaneous that looks at something between 100-200% of normal LOAD current. You may also have a high-current instantaneous to limit reclosing on high level faults or to protect certain gear.

There are many schemes but setting way above available fault essentially disables the instantaneous.

 

[Phil Corso]

MB&E... 2 comments!

1) Why doesn't the 13.8kV OCR, have an instantaneous-element?

2) Does the 50/5 CT downstream of the 13.8kV CB represent a zero-seq or core-type, for Ground-fault detection?

Regards, Phil Corso

 

[mivey]

I have attached two examples from the pdf that show long time settings of 2.5 and 2.73 times the CT primary value.

My questions are: Is this common? Are there any issues to look out for when selecting a trip setting that is 2-3 times the CT rated primary current?

Sorry I forgot to look at the pics. Those are not set to more than the primary rating. Those are the pickups.

400/5 = 80:1 ratio. Then 2.5 * 80 = 200 amps.

200/5 = 40:1 ratio. Then 40 * 2.73 = 109.2 amps.

 

[mivey]

MB&E... 2 comments!

1) Why doesn't the 13.8kV OCR, have an instantaneous-element?

It does. It is set to 15X on one example and 10X on the other example.

2) Does the 50/5 CT downstream of the 13.8kV CB represent a zero-seq or core-type, for Ground-fault detection?

But of course.

 

[Flapjack]

Thanks for the replies.

So unless the CTs have a rating factor for ambient, the 51 pickup should always be set under the relay CT primary value? Let's say a 400:5 CT had a thermal rating of 2x. Does this mean the 51 pickup could be set at 800 A?

I wonder why the TCC curves I attached previously show 51 pickup values beyond the CT primary value and nowhere do they even mention anything about the trip exceeding the CT rating and possible issues...

 

[Flapjack]

Sorry I forgot to look at the pics. Those are not set to more than the primary rating. Those are the pickups.

400/5 = 80:1 ratio. Then 2.5 * 80 = 200 amps.

200/5 = 40:1 ratio. Then 40 * 2.73 = 109.2 amps.

No, I thought the first one shows a 51 setting at 1000 amps. The scale at the bottom has 13.8 kV reference. It is 2.5 x CT sec. The CT would see 12.5 amps, or 12.5 *400/5 = 1000.
The other TCC curve has a .48 kV reference and needs to be converted to 13.8 kV, but you can see the long time setting around 16 kA @ 480 V, which is ~556.6 A @ 13.8 kV. 2.73 * 5 = 13.65, so the 13.65 * 200/5 = 546 A.

 

[mivey]

Thanks for the replies.

So unless the CTs have a rating factor for ambient, the 51 pickup should always be set under the relay CT primary value? Let's say a 400:5 CT had a thermal rating of 2x. Does this mean the 51 pickup could be set at 800 A?

Yes

I wonder why the TCC curves I attached previously show 51 pickup values beyond the CT primary value and nowhere do they even mention anything about the trip exceeding the CT rating and possible issues...

I took the pickups from the text to be amps.

 

[mivey]

It is 2.5 x CT sec.

I thought the text read you could set the pickup to between 0.5X and 20X the secondary or you could set the pickup amps to between 0.05*5 and 20*5 or 0.25-100 secondary amps which, in the 400:5 case, would give a 20-8000 amp primary range.

PS: I will have to look to see what they actually mean.

 

[mivey]

No, I thought the first one shows a 51 setting at 1000 amps. The scale at the bottom has 13.8 kV reference. It is 2.5 x CT sec. The CT would see 12.5 amps, or 12.5 *400/5 = 1000.
The other TCC curve has a .48 kV reference and needs to be converted to 13.8 kV, but you can see the long time setting around 16 kA @ 480 V, which is ~556.6 A @ 13.8 kV. 2.73 * 5 = 13.65, so the 13.65 * 200/5 = 546 A.

In the first example I see the primary is indeed picking up at 2.5*5*80 = 12.5*80 = 1000 amps (the curve plot starts 10% above that).

It is the secondary that is protecting the primary for long-term overloads: 1*5*240 = 5*240 = 1200 amps secondary or 362 amps primary (the curve starts at 10% above that).

 

[mivey]

Looking at the secondary, the Cutler Hammer is also protecting the primary CT at 111.3 amps primary long-term pickup.

 

[mivey]

Looking at the secondary, the Cutler Hammer is also protecting the primary CT at 111.3 amps primary long-term pickup.

Looking at the second example that is.

 

[Flapjack]

In the first example I see the primary is indeed picking up at 2.5*5*80 = 12.5*80 = 1000 amps (the curve plot starts 10% above that).

It is the secondary that is protecting the primary for long-term overloads: 1*5*240 = 5*240 = 1200 amps secondary or 362 amps primary (the curve starts at 10% above that).

Ah, I did not take that into consideration. So am I correct to say that the 51 pickup on the primary doesn't matter if the secondary provides protection for any overloads which translate to an amperage less than the CT rated primary value of the primary relay? It would be okay for the 51 primary setting to exceed the CT value, regardless of the thermal rating factor, because it is essentially only there for overcurrent protection and not overload protection?

 

[mivey]

Ah, I did not take that into consideration. So am I correct to say that the 51 pickup on the primary doesn't matter if the secondary provides protection for any overloads which translate to an amperage less than the CT rated primary value of the primary relay?

I wouldn't say it doesn't matter, but yes. I would try to avoid it if possible.

It would be okay for the 51 primary setting to exceed the CT value, regardless of the thermal rating factor, because it is essentially only there for overcurrent protection and not overload protection?

Yes, if you can't avoid it but just make sure you have protection from somewhere else.The secondary protection can protect the primary CT from overloads the same as it can protect the main transformer.

 

[Bugman1400]

For utility feeders there is not much point in that unless you don't really want an instantaneous or just are tripping for faults from elsewhere, like primary contact or something.

The normal instantaneous looks down about 80% of the line or it looks to just above the next device to keep the protection zones separated.

Some others: You might have a hot-line tag instantaneous that looks at something between 100-200% of normal LOAD current. You may also have a high-current instantaneous to limit reclosing on high level faults or to protect certain gear.

There are many schemes but setting way above available fault essentially disables the instantaneous.

I think he was referring to the highside faults. The 150-200% of a lowside fault as reflected to the highside is a typical pickup for the 50 element. This will trip fast for faults in the highside breaker and into the 13.8kV wdgs of the xfmr.