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One CT Phase Reading Significantly Lower

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.

RE: One CT Phase Reading Significantly Lower

Is this the peak inrush current or the symmetrical locked rotor current?  Due to the dc offset current, the short-lived inrush current will not be same in all three phases since it is a function (partly) of the voltage phase angle on energization.

After a few cycles, the currents should be basically equal.  

    

David Castor
www.cvoes.com

RE: One CT Phase Reading Significantly Lower

A waveform might gives some clues if you've got one.

=====================================
(2B)+(2B)'  ?

RE: One CT Phase Reading Significantly Lower

Do you have a soft start arrangement?

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

(OP)
The motor is an across-the-line start, and the breaker has been swapped out with the same results.  The currents are peak inrush.
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 think I'd check the cable connections between the breaker and the motor, and also the connections from the bus to the breaker, and the breaker stabs.  

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

If these are peak inrush values, there may not be any problem at all.  The three phases will never be equal.

 

David Castor
www.cvoes.com

RE: One CT Phase Reading Significantly Lower

(OP)
Unfortunately, IR scans would be impossible on the back of the breaker due to sheet metal blocking the view.
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

With the given info, I tend to agree with dpc. Do you have any readings from "good" times? What is rated FLA of the motor?

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 given 3 readings where A is all significantly lower (more than a factor of 2 lower in some cases).

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.  

=====================================
(2B)+(2B)'  ?

RE: One CT Phase Reading Significantly Lower

(OP)
The relay we are using is a Multilin 469.
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

As you said in your first post, CT saturating for some reason seems to be most likely culprit.

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.
 

=====================================
(2B)+(2B)'  ?

RE: One CT Phase Reading Significantly Lower

You may have a bad relay. Inject current in the relay to test it.  I have had a couple of the 469 input circuits go bad.

RE: One CT Phase Reading Significantly Lower

Could be a shorted turn on the CT.  This significantly lowers saturation voltage and the CT "gives up" during high loads.

"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

I interpret your readings as showing that the relay indicated B phase was low both before and after swapping phase A & C.

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

When you "swapped the CT's", did you
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

=====================================
(2B)+(2B)'  ?

RE: One CT Phase Reading Significantly Lower

Correction in bold:
If you did A, then you already ruled out the CT. If you did B, then I would do A next, in which case...

=====================================
(2B)+(2B)'  ?

RE: One CT Phase Reading Significantly Lower

I would suspect it's much more likely you did A, B would be very tough to do.

=====================================
(2B)+(2B)'  ?

RE: One CT Phase Reading Significantly Lower

(OP)
rcwilson - you interpreted correctly, the relay indicated low on phase B both with B CT going in and with A CT going into the B phase input.
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

Quote:

We actually swapped leads on the back of the Multilin relay
You swapped leads right at the relay and the problem remained on the same indicated phase (different actual phase)?
Doesn't that pretty much rule out the wiring and indicate it has to be the relay?

=====================================
(2B)+(2B)'  ?

RE: One CT Phase Reading Significantly Lower

I guess if you come down to it, there is nothing that satisfies everything you told us. Seems highly doubtful that a relay problem would only show up during start and not run (unless for some reason there is a setting that controls how that inrush parameter is calculated on an individual phase basis), and also you told us relays were swapped and stayed with same phases.

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.

=====================================
(2B)+(2B)'  ?

RE: One CT Phase Reading Significantly Lower

It is normal for transformer inrush current to be high on two phases and low on the third.  Is this also the case with motors?
 

RE: One CT Phase Reading Significantly Lower

Not for motors (other than random variation in dc offset which can create 2 high and one low).

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.

=====================================
(2B)+(2B)'  ?

RE: One CT Phase Reading Significantly Lower

It's good to distinguish between motor inrush and locked rotor current during motor starting - they are not quite the same thing  Inrush includes the locked rotor current PLUS the brief period of dc offset current which will vary depending on the phase angle of the voltage at the instant the contacts close to energize the winding.  This can be asymmetrical in one or more phases.  The locked rotor current is the symmetrical value of current after the initial inrush has decayed.

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

http://www.gedigitalenergy.com/products/manuals/469/469man-dh.pdf

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?

=====================================
(2B)+(2B)'  ?

RE: One CT Phase Reading Significantly Lower

Actually that sqrt(3) error is sqrt(3) high, can't arrange the vectors to create sqrt(3) low.

* 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?

=====================================
(2B)+(2B)'  ?

RE: One CT Phase Reading Significantly Lower

(OP)
I believe the starting currents we are reading are peak inrush.  The manual I have doesn't really describe it.  It is the current in the Event History.  We are definitely not reading the "learned starting current" because that's in the Learned History.

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

So, when you swapped A and B input leads, did you swap one lead per channel or 2 leads per channel?

=====================================
(2B)+(2B)'  ?

RE: One CT Phase Reading Significantly Lower

(OP)
We swapped one lead per channel.  On the Multilin, CT's are on G6 (A-phase), G7 (B-phase), and G8 (C-phase), with common ground on H6, H7, H8.  We swapped leads on G6 and G7, but left the leads on H6 and H7 since it should have been commonly grounded.

RE: One CT Phase Reading Significantly Lower

The neutral wiring is not checked if you just swap one lead per channel and in theory a high resistance on the neutral could cause everything seen.   I said it was a longshot, because I assume the neutral connections are something like short jumpers between H6/H7/H8, with extra lead landed on one that connects to the long lead that provide interconnection  to the ct neutral. But sometimes the real world connections and daisy chains are not quite what we expect/assume. So it would be useful to do as thorough a check on the wiring as possible. Ideally you'd like to watch with your own eyes as both leads per channel are swapped, and examine closely whether any other leads are landed on these two pairs of terminals and see if they all make sense.

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?

=====================================
(2B)+(2B)'  ?

RE: One CT Phase Reading Significantly Lower

Quote:

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.
By common ground I think you mean wye point? I'm not clear exactly what transducer you're referring to. But if there is a difference on the neutral side, it would show up in swapping both leads per channel (vs only one lead per channel).

=====================================
(2B)+(2B)'  ?

RE: One CT Phase Reading Significantly Lower

At any rate, if you are suspecting or investigating that the neutral side has some wiring differences between A phase and B phase, then we are on the same page.

=====================================
(2B)+(2B)'  ?

RE: One CT Phase Reading Significantly Lower

for mesutphen:

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

mesutphen:

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


 

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