One CT Phase Reading Significantly Lower 1

[mesutphen]

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.

 

[dpc]

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

http://www.cvoes.com

 

[electricpete]

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

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

 

[ausphil]

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

 

[mesutphen]

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.

 

[potteryshard]

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.

 

[dpc]

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



David Castor

http://www.cvoes.com

 

[mesutphen]

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.

 

[rbulsara]

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

 

[electricpete]

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)' ?

 

[mesutphen]

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.

 

[electricpete]

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)' ?

 

[rcw retired EE]

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.

 

[oldfieldguy]

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

 

[rcw retired EE]

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.

 

[electricpete]

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)' ?

 

[electricpete]

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)' ?

 

[electricpete]

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

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

 

[mesutphen]

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).

 

[electricpete]

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)' ?