Transformer CT Calculations

Status
Not open for further replies.

adamscb

Senior Member
Location
USA
Occupation
EE
We have two transformers that we want to size some CT's on the secondaries. One is a 7500/10500 kVA, 34.5 kV step down to 4160. Also we want a CT on the system neutral (wye connection). The other transformer is 3750/5250 kVA, 34.5 kV step down to 480 (again, we want a CT on the neutral). The 480V transformer is dual-throat, the 4160V unit is single-throat.

Here are my thoughts:

1600:5 on the phase, 3000:5 on the neutral (4160V unit)
4000:5 on each phase on each throat, 3000:5 on the neutral (480V unit)

Thoughts?
 

paulengr

Senior Member
We have two transformers that we want to size some CT's on the secondaries. One is a 7500/10500 kVA, 34.5 kV step down to 4160. Also we want a CT on the system neutral (wye connection). The other transformer is 3750/5250 kVA, 34.5 kV step down to 480 (again, we want a CT on the neutral). The 480V transformer is dual-throat, the 4160V unit is single-throat.

Here are my thoughts:

1600:5 on the phase, 3000:5 on the neutral (4160V unit)
4000:5 on each phase on each throat, 3000:5 on the neutral (480V unit)

Thoughts?

First thought is the 480 transformer is stupidly huge. Over 6000 A is what I calculated and it’s going to be hard and very expensive on switchgear. Strongly suggest NOT doing this. A good practical limit on 480 is around 2500 kVA if you don’t care about arc flash. Otherwise 1500 kVA is a practical limit to stay under 40 cal.

On the 480 side need 8000:5 if this even exists. You will probably have to do it in two stages or use something like an optical C.T.

Also why make the neutral larger?? Typically it’s the other way around since with reasonably balanced currents it will be close to zero. I can’t think of a scenario where the neutral exceeds it.

Third what is the purpose? If it is for metering typically you need to be linear over the whole range. But if it’s for protection often smaller CTs suffice. And since both are rarely the same you need both sets.
 

adamscb

Senior Member
Location
USA
Occupation
EE
I know the 480 is really big, it’s just the way the plant was designed. Keep in mind that I said it’s dual throat. So 4000:5 on each throat should get there.

Your question on the neutral, these are for relay protection, and since ground current can be really high on a solidly grounded system I thought making them large was a good thing. I was taught at least the size of the phase CT’s, if not more. So actually the CT on the 480 neutral should be 4000:5 in my mind

third this is all for protection. Reducing saturation is the main goal here
 

paulengr

Senior Member
I know the 480 is really big, it’s just the way the plant was designed. Keep in mind that I said it’s dual throat. So 4000:5 on each throat should get there.

Your question on the neutral, these are for relay protection, and since ground current can be really high on a solidly grounded system I thought making them large was a good thing. I was taught at least the size of the phase CT’s, if not more. So actually the CT on the 480 neutral should be 4000:5 in my mind

third this is all for protection. Reducing saturation is the main goal here

Neutral currents can’t exceed phase currents. If we only load one phase IN =IX. If we load any other phase they vector add and IN will be less. The one possible but rare exception is triplen harmonics. These are not produced by drives, only general and mostly single phase loads.

The normal problem with neutral currents is high resistance grounds where the C.T. must be physically large (window) but with a very low ratio like preferably say 25:5. The two requirements are counter to each other because a big window makes core losses high and a low ratio demands low core losses.

In fact in your case the BYZ CAN be your issue. However I suggest that due to the fact that you are going to find that getting a C.T. window that big is completely impractical use either a calculated or wired neutral. If you wire all 3 phase CTs in wye you will get a neutral. But this is old school analog. On digital microprocessor relays they can calculate it. They just vector add the three phase CTs and call it “residual” instead of “neutral” ground fault protection. The accuracy will be equal to the sum of the three errors but since your C.T. accuracy is already under 1% error the total error is under 3% which is plenty for protection purposes. So I wouldn’t even try to use a neutral C.T. for practical reasons.

Finally if you insist on doing it, most relays have an option to use a high impedance neutral input like say 20 mA instead of 5 A. The reason for this is going back to the issues with small ratios like 25:5. It is far easier to work with a ratio like 25:0.020. It doesn’t matter in your case but you will need every trick if you attempt to make that big of a CT window. This is especially critical on the 4160. Squeezing a bunch of unshielded bus bars or cables into a C.T. window at medium voltage is just asking for voltage stress and tracking damage down the road. Start off on the right foot.

Saturation generally hits at around 20x on a C.T. specifically designed for protection. We eschew accuracy for range. So instead of say 0.1% accuracy and saturation at maybe 10 A at the output we allow up to 50-100
A before saturation. This used to be a problem back in the bad old days of induction disc relays. Today the WDM algorithm automatically accounts for saturation and estimates currents well beyond saturation. The digital inputs are typically clamped at around 20x anyways so it doesn’t matter if you saturate or not...the relay input gave out long before the C.T. does. As long as the LT setting is within the C.T. range you are good to go. If it’s an industrial plant generally speaking set the C.T. at or as close to the transformer size as practical or a little below if you have very little diversity like one transformer and one or two motors (extra capacity for starting). On electric utilities count on running the transformer up to 25% over name plate at some point. Try to do this because even some microprocessor relays have a built in hard limit on the settings of about 50-200% of the C.T. ratio for the tap for LT. It is aggravating to have to “fix” this. Nobody ever attempts to accurately track a dead short except the test stands.
 

adamscb

Senior Member
Location
USA
Occupation
EE
I guess we're still talking about two different scenarios. You're talking about unbalanced loads on the neutral, but I'm talking about a single phase to ground fault on a solidly-grounded system. But the more I think of it the phase CT's will see that current anyway and trip the relay out. Any thoughts on that?

Also we're using microprocessor-based relays, so from what I gather you're saying as long as the CT rating is above the FLA of the transformers, I shouldn't be worried?
 

paulengr

Senior Member
I guess we're still talking about two different scenarios. You're talking about unbalanced loads on the neutral, but I'm talking about a single phase to ground fault on a solidly-grounded system. But the more I think of it the phase CT's will see that current anyway and trip the relay out. Any thoughts on that?

Also we're using microprocessor-based relays, so from what I gather you're saying as long as the CT rating is above the FLA of the transformers, I shouldn't be worried?

I1+I2+I3=IN. By definition you will see a phase overcurrent but not necessarily enough to trip.

If you have a phase to phase fault it will be worst case kVA x 1000 / (480 x 1.732) / %Z assuming infinite bus on primary and dead short at the terminals plus inductive backfeeding. For a phase to neutral fault it will be 58% of the phase-phase fault (277 vs 480). But ground faults occur across the grounded conductors which are generally higher impedance. Hence NEC mandates ground fault tripping because at higher currents you can’t realistically expect phase over current protection to consistently trip on ground faults.

So no, no reason to go bigger.

It’s preferable to exceed “FLA” moderately but don’t go crazy with it, do not double because some relays won’t adjust below a tap if 0.5 (2.5 A minimum trip). I have gotten burned by this issue more than once. It doesn’t hurt to be moderately under, say no more than 70%. CTs have ratios, not kVAs. The kVA is expressed by burden. With microprocessor relays burden is meaningless because it is so low. We aren’t powering coils for induction discs.

If you look in the relay manual and get deeply into the ADC side of things you will realize.a couple things. The relay is over sampling at 16-32x, and it has a peak current limit of around 10x the rates input or 5 A in North America. This is obviously woefully short of handling say a mild 10 kA fault on say a 100:5 CT with a maximum linear input of 1 kA. The CT starts to clip in contrast at 100 A (20 x 5 A) and fully saturates quite a bit higher but the relay gave out long before the C.T. did. But we can look at how much it is flat topped (crest factor) and then estimate currents much higher albeit with reduced accuracy. SEL has a couple white papers describing how this works. Suffice to say there is a lot of digital magic going on in the background that you just don’t have to worry about.

CT books just don’t do any justice to this because they were written when relays were all induction disc style. They didn’t have much dynamic range anyways, they are slow, and have large burdens.

Also I think you missed residual or digital phase C.T. ground fault vs true neutrals. You just add I1+I2+I3 as vectors to calculate IN. It sees the vector sum (geometric mean) of the errors in each.of the three phases. In a high resistance ground where 10 A is reasonable for the resistor and you set the trip typically at half that or 5 A, a few Amos of errors will nuisance trip so a neutral CT is required (with a 0.020 A output). But with solidly grounded systems with a tap of say 10-25% or thousands of amps in your case the error in the vector sum is probably less than the ADC conversion noise.

You will be using some very large bushing CTs with big windows, thin cores, and 1-3% accuracy. Should be fairly inexpensive. Check CT vendors or larger transformer shops to see what’s available. Ritz might be a good starting point.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
We have two transformers that we want to size some CT's on the secondaries. One is a 7500/10500 kVA, 34.5 kV step down to 4160. Also we want a CT on the system neutral (wye connection). The other transformer is 3750/5250 kVA, 34.5 kV step down to 480 (again, we want a CT on the neutral). The 480V transformer is dual-throat, the 4160V unit is single-throat.

Here are my thoughts:

1600:5 on the phase, 3000:5 on the neutral (4160V unit)
4000:5 on each phase on each throat, 3000:5 on the neutral (480V unit)

Thoughts?
Didn’t this XF come with internal CTs already??
7.5 MVa is a pretty big unit.
Planning on 87 protection I assume with them on both sides?

 

paulengr

Senior Member
Didn’t this XF come with internal CTs already??
7.5 MVa is a pretty big unit.
Planning on 87 protection I assume with them on both sides?


I’ve never ordered nor seen internal CTs.

IEEE recommends 87 transformer relaying at 10 MVA or larger along with sudden pressure rise protection not just pressure relief.

Just thinking about this though do you simply have two X1s, two X2s, two X3s, or is this wired with two secondary coils? If it’s two coils you can just treat them as independent. If you just have a wired bus you can’t realistically protect it unless you wire both CTs to provide the sum of both outputs and individuals for feeder protection. There are some crazy transformer or 87 relays from SEL that handle multiple sets of CTs that are commonly used in power plants that have some crazy bus schemes.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
I’ve never ordered nor seen internal CTs.

IEEE recommends 87 transformer relaying at 10 MVA or larger along with sudden pressure rise protection not just pressure relief.

Just thinking about this though do you simply have two X1s, two X2s, two X3s, or is this wired with two secondary coils? If it’s two coils you can just treat them as independent. If you just have a wired bus you can’t realistically protect it unless you wire both CTs to provide the sum of both outputs and individuals for feeder protection. There are some crazy transformer or 87 relays from SEL that handle multiple sets of CTs that are commonly used in power plants that have some crazy bus schemes.

wrote integrated, spell check changed it to internal...
But still, the larger transformers come with multi tap CT that are internal.
we have many of them that are internal mounted to the inside. When we change a bushing the CT stays, bushing comes out..

For transformers that size slip over CTs with mounts are generally specified with the transformer if it’s ordered. If it’s existing then he can order them with the mounts.
not sure why the sizes are that high, especially the neutral but you covered that, didn’t see a need to mention it again.
CTs are on the H side and the X side for 87 with opposing polarity. You just have to compensate the relay 30 degrees if it’s a nameplate setup. When you look at the raw phasors there will be a 150 degree difference in the currents, so that’s the need for the 30 degree compensation.
With a Dy1, the assumption is ABC on H1, H2, H3 respectively with an ABC rotation. You can “make” it a Dy11 by connecting it A on H3, etc with an ABC rotation. There the compensation will need to be 330 degrees.
 

paulengr

Senior Member
Normally also you do short circuit protection on the primary and overcurrent and short circuit on the secondary. I’m assuming you are doing “virtual main” so a fault on either side trips a main breaker. I like using Tavridas for that. Elgin Power makes them with a disconnect and grounding switch, very compact. But with two secondaries you might be able to exceed primary capacity. If that is the case then there is a trick to doing primary side protection. Open and close or at least sense the breaker via aux contacts. Energize with the usual trip curve to avoid magnetizing inrush. Then as soon as it subsides, switch to an overcurrent curve that is based on maximum current instead of avoiding inrush. A simple timer does the trick. This also inherently offers significant backup protection on the secondary side as well rather than the usual barely short circuit backup protection. It’s simple to do with an SEL 751A or 587, or a Basler. Not so simple with ABB.
 

paulengr

Senior Member
wrote integrated, spell check changed it to internal...
But still, the larger transformers come with multi tap CT that are internal.
we have many of them that are internal mounted to the inside. When we change a bushing the CT stays, bushing comes out..

For transformers that size slip over CTs with mounts are generally specified with the transformer if it’s ordered. If it’s existing then he can order them with the mounts.
not sure why the sizes are that high, especially the neutral but you covered that, didn’t see a need to mention it again.
CTs are on the H side and the X side for 87 with opposing polarity. You just have to compensate the relay 30 degrees if it’s a nameplate setup. When you look at the raw phasors there will be a 150 degree difference in the currents, so that’s the need for the 30 degree compensation.
With a Dy1, the assumption is ABC on H1, H2, H3 respectively with an ABC rotation. You can “make” it a Dy11 by connecting it A on H3, etc with an ABC rotation. There the compensation will need to be 330 degrees.

Even though the price is very high if you do true 87 relaying I like the digital relays because it’s easier to troubleshoot anfd you get 50/51 for free. You can plot the phasers for commissioning and troubleshooting and software “fix” polarity issues as needed.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
Normally also you do short circuit protection on the primary and overcurrent and short circuit on the secondary. I’m assuming you are doing “virtual main” so a fault on either side trips a main breaker. I like using Tavridas for that. Elgin Power makes them with a disconnect and grounding switch, very compact. But with two secondaries you might be able to exceed primary capacity. If that is the case then there is a trick to doing primary side protection. Open and close or at least sense the breaker via aux contacts. Energize with the usual trip curve to avoid magnetizing inrush. Then as soon as it subsides, switch to an overcurrent curve that is based on maximum current instead of avoiding inrush. A simple timer does the trick. This also inherently offers significant backup protection on the secondary side as well rather than the usual barely short circuit backup protection. It’s simple to do with an SEL 751A or 587, or a Basler. Not so simple with ABB.
651R has a “j” & “k” setting for a dual element 50/51
We use the 651R in combination with the Tavridas as a matched set with the compensation values for the voltage divider on the 651R door
 
Status
Not open for further replies.
Top