One CT Phase Reading Significantly Lower
One CT Phase Reading Significantly Lower
(OP)
While reviewing the motor starting data for a 1750HP, 7.2KV motor, we noticed that B phase starting current was significantly lower (about 50%) than A and C phases. This is not something that has been known to happen in the past. The last 3 starts had the following voltages and currents:
Start 1: Ia - 958 Va - 6766 Ib - 450 Vb - 6797 Ic - 967 Vc - 6769
Start 2: Ia - 1032 Va - 6752 Ib - 583 Vb - 6777 Ic - 1086 Vc - 6760
Start 3: Ia - 1102 Va - 6748 Ib - 605 Vb - 6783 Ic - 1130 Vc - 6755
While running, all 3 phases are fairly balanced, Va - 6755 Ia - 98A, Vb - 6729 Ib - 93A, Vc - 6755 Ic 96A. We injected current from 30-130A into the CT and the indication was 27A @ 30A injected - 125A @ 132A injected, so low-range accuracy seems to be okay. I'm trying to think what would cause the B phase CT to read so much lower. The 5% unbalance while running seems to indicate the motor is OK. Is it possible that the CT is becoming saturated at higher currents? The CT is about 30 years old but I haven't heard of a transformer shifting its saturation point before...
Thank you.
Start 1: Ia - 958 Va - 6766 Ib - 450 Vb - 6797 Ic - 967 Vc - 6769
Start 2: Ia - 1032 Va - 6752 Ib - 583 Vb - 6777 Ic - 1086 Vc - 6760
Start 3: Ia - 1102 Va - 6748 Ib - 605 Vb - 6783 Ic - 1130 Vc - 6755
While running, all 3 phases are fairly balanced, Va - 6755 Ia - 98A, Vb - 6729 Ib - 93A, Vc - 6755 Ic 96A. We injected current from 30-130A into the CT and the indication was 27A @ 30A injected - 125A @ 132A injected, so low-range accuracy seems to be okay. I'm trying to think what would cause the B phase CT to read so much lower. The 5% unbalance while running seems to indicate the motor is OK. Is it possible that the CT is becoming saturated at higher currents? The CT is about 30 years old but I haven't heard of a transformer shifting its saturation point before...
Thank you.






RE: One CT Phase Reading Significantly Lower
After a few cycles, the currents should be basically equal.
David Castor
www.cvoes.com
RE: One CT Phase Reading Significantly Lower
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(2B)+(2B)' ?
RE: One CT Phase Reading Significantly Lower
If so, is it operating correctly? There may be some components that have failed or changed, which will give you an unbalance on starting, but once it is out of circuit in normal operation, you will see the balanced currents only from the motor.
ausphil
RE: One CT Phase Reading Significantly Lower
Unfortunately, we do not have the capability of capturing waveforms for this. That would help out a lot.
RE: One CT Phase Reading Significantly Lower
I would expect current imbalance due to DC offset on start to be randomly distributed between the phases; if it consistently appears on one phase, I would be worrying about a poor connection that is marginal at normal load, but becomes significant at start.
It might be possible to find under steady state conditions with an IR scan.
RE: One CT Phase Reading Significantly Lower
David Castor
www.cvoes.com
RE: One CT Phase Reading Significantly Lower
We're going to swap CT leads between B and A phases and see what happens.
The LRA of the motor is around 950A, so I do not think it's an issue of DC offset.
RE: One CT Phase Reading Significantly Lower
What instrument is measuring the currents you are quoting? If really needed set up independent smart monitoring and compare readings.
Rafiq Bulsara
http://www.srengineersct.com
RE: One CT Phase Reading Significantly Lower
We are told the breaker was replaced and the same results were obtained.
We are told that LRC is 950 and your device (whatever it may be... would really like to know) is telling you 450... as an "inrush".
I have suspicions about your recording device. I would look for some explanation other than random variation of dc component based on closing angle, because it does not appear to be random. I think you are on the right track swapping CT output circuits.
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(2B)+(2B)' ?
RE: One CT Phase Reading Significantly Lower
We swapped A and B CT's, but the low reading stayed on B phase (indicated, i.e. A phase actual)
Before swap:
Ia:1102 Va:6748 Ib:605 Vb:6783 Ic:1130 Vc:6755
After swapping A and B CT leads:
Ia:1103 Va:6710 Ib:627 Vb:6739 Ic:1030 Vc:6713
I think we have a loose connection between the CT and the relay.
RE: One CT Phase Reading Significantly Lower
Just to mention something:
1102/605=0.549
1103/627=0.568
958/450=0.470
1032/583=0.565
1102/605=0.549
average =0.540
That's nt too far from 1/sqrt(3) = 0.577
I don't think it means anything (CT wiring error would show up during low current test... have a hard time imagining actual error that would cause sqrt3 during start but not at run), but just pointing it out on the off-chance it might trigger an idea for someone else.
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(2B)+(2B)' ?
RE: One CT Phase Reading Significantly Lower
RE: One CT Phase Reading Significantly Lower
"shorted turns" can be actual physical damage or failure inside the CT on the winding itself, or they can be a completed path through the CT due to improper or shifting mounting hardware.
Finding out if this is a problem is very easy with a variable voltage AC source capable of outputting line frequency AC up to the expected saturation level. All three CT's should saturate at essentially identical voltages. If you have a problem, the "bad" CT will saturate at significantly lower voltage. The test can be conducted from the compartment where your motor protection relay is located.
old field guy
RE: One CT Phase Reading Significantly Lower
If that is correct, the problem is in the relay or the wiring from the TB where the swap was made to the relay.
If I am interpreting it wrong and the same primary phase showed low current (relay indicated B low on one reading then A low after swap) then the problem is in the CT or the primary circuit.
RE: One CT Phase Reading Significantly Lower
A - leave the CT's in place on the same phase and swapped wiring on the CT secondary
or
B actually able to move the entire CT from one phase to another?
If you did A, then you already ruled out the CT. If you did B, then I would do B next, in which case you will be able to divide the problem either into CT (if problem remains with CT) or wiring/relay (if problem swaps).
If you just want to test, broadly three areas:
1 - CT.
2 - Wiring
3 - Relay.
Excitation test is good suggestion. Off the top of my head, I am not positive shorted turn would cause the scenario (why wouldn't it show up at low currents), but it's easier to check it than to guess where that's it. Also it might reveal other CT problems. Here are some more checks:
1 - Lift the CT circuit ground and perform a 500vdc megger test to ground. This may also require removing the relay from the circuit to prevent damage first. A short to ground can act like a short but you wouldn't find it in an excitation test if you isolate the CT first and the circuit ground is at the relay panel.
2 - Do some resistance checks of this phase and compare to the other phases.
3 - Try swapping out relays
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(2B)+(2B)' ?
RE: One CT Phase Reading Significantly Lower
If you did A, then you already ruled out the CT. If you did B, then I would do A next, in which case...
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(2B)+(2B)' ?
RE: One CT Phase Reading Significantly Lower
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(2B)+(2B)' ?
RE: One CT Phase Reading Significantly Lower
electricpete - we did A because, as you mention, B is very difficult to do. We actually swapped leads on the back of the Multilin relay. The CT's are commonly grounded, along with the relay. I would hope it is not a bad relay, because we swapped out the relay with the same results. However, it is not impossible that we have 2 bad relays (the other relay went bad in another way).
Currently, we are checking wiring between B CT and the relay. We ohmed out the wiring, and it wasn't obviously high. Of note, the wiring for B CT is significantly longer than A and C because it goes to another building where it was previously used for ammeter indication (that was removed many years ago).
RE: One CT Phase Reading Significantly Lower
Doesn't that pretty much rule out the wiring and indicate it has to be the relay?
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(2B)+(2B)' ?
RE: One CT Phase Reading Significantly Lower
So, I guess it is a need to be methodical and recheck the facts that are presented and re-examine what was ruled out, because we seem to have ruled everything out. Perhaps argues to going back for more testing if that is easily done.
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(2B)+(2B)' ?
RE: One CT Phase Reading Significantly Lower
RE: One CT Phase Reading Significantly Lower
For a transformer, the middle phase sees a different reluctance than the other two phases. If you look at the transformer core (3 leg or 5 leg) you can see the asymmetry. For a motor, the core is round... and all three stator phases look identical to each other, just shifted from each other in position by 2/3 of a pole pitch around the stator.
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(2B)+(2B)' ?
RE: One CT Phase Reading Significantly Lower
I have no idea what the OP's actual problem is, but inrush is a significant issue when setting motor short circuit protection and is quite real. Inrush is generally higher on newer high efficiency motors.
David Castor
www.cvoes.com
RE: One CT Phase Reading Significantly Lower
Page A-1 (pdf page 229/248) describes means to use 2 CT's to sense 3-phase current. There is also mention that polarity error can cause sqrt(3) error. Do you use something like this by any chance?
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(2B)+(2B)' ?
RE: One CT Phase Reading Significantly Lower
* There are 2 connections per CT channel... did you swap both of them (including the wye/neutral side connection)? If not, could perhaps have high resistance somewhere in that path on the neutral side (doesn't seem likely, but it would seem about the only thing that hasn't been ruled out).
Looking at your A and C phase currents, they might be a little high to be LRC given that voltage is below nameplate, but they also seem way too low to be peak instantaneous current (in fact peak instantaneous is always at least sqrt2 times LRC even if you don't have any dc, right?). I see in the manual they have some kind of "learned starting current" which is measured 200 msec after start, with the intention that the dc component has died about by then. Is that what you're reporting?
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(2B)+(2B)' ?
RE: One CT Phase Reading Significantly Lower
There was a portion of the B-phase wiring going to a separate building to drive a current transducer. We shorted that wiring and starting currents are now within 10A, and running currents are within 1-2A (used to be within 5-7A). Apparently our problem is CT burdening.
Considering we swapped the A and B input leads into the relay, I am guessing the transducer is between the relay output and the common ground. This is the only thing that explains to me why the problem did not move from B-indicated to A-indicated when we swapped leads.
RE: One CT Phase Reading Significantly Lower
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(2B)+(2B)' ?
RE: One CT Phase Reading Significantly Lower
RE: One CT Phase Reading Significantly Lower
The known longer run of wire on B phase seems to explain everything EXCEPT the results of the lead swap on the relay input... the problem should've shown on A phase indicated (B phase actual) if that long run were the culprit, wouldn't it?
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(2B)+(2B)' ?
RE: One CT Phase Reading Significantly Lower
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(2B)+(2B)' ?
RE: One CT Phase Reading Significantly Lower
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(2B)+(2B)' ?
RE: One CT Phase Reading Significantly Lower
If it were really connected like the dwg that you posted everything should be ok. Now maybe time to get in and rewire exactly to that dwg, rather than endlessly swapping connections. That means unbinding cable ties and tearing out the existing possibly. There is probably some sneak circuit or unwanted ground that cannot be seen otherwise. The remote circuit on phase B that you mention is not even shown there.
On the Multilin. Is that low reading possibly a stored value?
Are there any actual ammeters in the panel that show also a low reading on phase B?
BTW, don't assume anything. Check it yourself personally.
Just some advice from field experience.
rasevskii
RE: One CT Phase Reading Significantly Lower
On further thought you may have a bad CT on phase B. If the above checks/rewiring find no fault, then a mag curve on each of the 3 CTs may be done. Inject on the secondary (primary and secondary circuits lifted) an AC voltage and read the mA magnetizing current at about 10% steps until the CT begins to saturate (knee-point)(when a 10% V increase gives a 20% mA increase, that is the beginning of the saturation or knee-point) You will need a variac and step-up transformer for this, up to around 250...350V will be needed (or more).
On all 3 CTs the curve should be the same. If phase B CT is bad, it will show quite higher current than the others, or at least quite different.
On such a large and expensive motor, maybe ordering a complete new set of CTs would be the best answer. Otherwise how can you trust the protections?
rasevskii