Is harmonics blowing up my capacitors??

[imisspla07]

I have a power factor correction capacitor bank rated 600kVAR. It is in parallel with three 1250hp motors on soft start systems used for gas compression. The bank has a contactor that closes only when 2 or three motors are running and is unlatched when the soft start is ramping up the motors. So, basically, it is only latched in when 2 or 3 motors are running full speed. The first time we used the cap bank it blew the fuses upstream of the contactor (connected to the bus). We changed out the fuses to different fuses and the second time it ran it blew the cap bank. It was suggested we have harmonics issue, but the utility did a power quality study and showed .05% harmonics. Seems low to me. Also, it was mentioned it might be resonance. But when I went out there with the utility we only latched in 300kVAR; half of the full amount. This should have gotten us away from any resonance issue. Now, the last thing to note is that when we did latch in the 300kVAR, it made a very eerie sound, like someone was waving a light saber in front of you (wa, wa, wa, wa). Definitely didn't sound right, so we latched it back out. I have no idea what's going on with this thing. It's been noted maybe it's from the utility. Someone mentioned perhaps an open NGR at the utility. Any ideas would be GREATLY appreciated!

 

[Skogsgurra]

You cannot have capacitors parallel to a motor started with a soft starter.

Just disconnect and put them upstream of softstarter.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[electricpete]

Gunnar - it sounds like the caps are not in the circuit when the starter is ramping up:
" The bank has a contactor that closes only when 2 or three motors are running and is unlatched when the soft start is ramping up the motors."


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

 

[electricpete]

What was going on when the caps or their fuses "blew"...
2nd motor reached full speed and cap just switched in?
Motor running in steady state?
Motor stopping ?

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

 

[potteryshard]

You may want to check fundamentals first... Are the capacitors rated for the applied line voltage? Connected delta with a wye voltage rating?

 

[Skogsgurra]

OK. Not connected when soft starter running? Then I don't know what to say.

But if the utility says you have 0.05 % harmonics, I think they are very wrong and you may very well have a lot more. 0.05 % doesn't exist IRL.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jraef]

I would still suspect something in the capacitor contactor / soft starter interlocking circuit design. For example:

1) you say that the caps are only on-line when 2 of 3 motors are at full speed, but does that mean they are still on line when #3 turns on? If you think not, are you sure?

2) You do not provide any information on the soft starters. Most will include a bypass contactor, but do yours? Are you sure it is coming on? For example I have seen a few people attempt to use the "Energy Saver" feature on even an MV soft starter, which means the SCRs MUST be in the circuit and they will phase back based on loading. If that's the case, even after you have initially reached full speed, you may end up with high harmonics. If you have a byapss contactor this is a moot issue, but worth mentioning.

3) Any chance you are using the "Decel" function of the soft starter? If it's a centrifugal compressor they might. If they are, did someone remember to open the capacitor control contactor on Decel as well, not just Accel?

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

It sounds as if the capacitors are connected upstream of the soft starts. (Contactor closes when 2 motors are running).
I suspect a voltage issue. Wrong voltage or wrong connections.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[imisspla07]

Thanks for all the posts. To answer a few questions, we have a contactor upstream of the capacitors to latch them out when any motors are 'ramping up' to speed or when only one motor is running. This is controlled via PLC and it was verified after we blew the capacitors. They blew shortly after being latched in when all three motors were running. I am skeptical about the .05% harmonic answer given by the utility; not sure I buy that either. The voltage rating is correct; 4160V rated bank. I am unsure of how our technicians connected the capacitor bank, however. There is an "in line bypass contactor" in the soft start design, but I'm unsure how that works. I'm unsure about the decel as well, but the caps blew before the motors had a chance to decel.
So would the consensus be a connection problem, then, and not harmonics? I am very interested as to why the strange loud-soft-loud-soft, etc humming sound. Sure didn't sound right.

 

[Skogsgurra]

Did you hear that from the capacitors? You can hear a similar sound if you expose a small PF capacitor - like the ones used for flourescent tube fixtures - to an overvoltage.

What you hear is internal discharges that self-repair for a while and then, when the capacitor is "enough damaged" usually results in a blown fuse.

I did an experiment with that some time ago:

http://www.gke.org/pub/files/Metal-paper capacitor on overvoltage.pdf

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[imisspla07]

Skogsgurra,

Definitely came from the capacitor cabinet. We didn't hear the sound until the caps were latched in. Thanks for the paper, although I admit, I am not sure I really understand it. If this is the issue, however, what would be the recommended solution?

 

[jraef]

"Inline bypass contactor" is an oxymoron; you have an Inline contactor, a bypass contactor or two contractors, Inline and bypass. I suspect the two, that is fairly standard. High harmonics has a similar effect of high voltage on the capacitors, it over heats them and they start self healing as gunnar mentioned. But if you are confident that the soft starters are working properly, the high voltage issue may be the only remaining track. You didn't mention, but are these induction motors or are they maybe synchronous? I've seen a situation with multiple large MV compressors on a common header where when multiples are on line, the downstream units end up creating some sort of a venturi effect and pulled the first one into over speed, which then regenerated and became a load on the other motors. Not sure how that would raise the bus voltage though unless maybe they were synchronous. In our situation it just ended up as a load share problem.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[imisspla07]

Jraef,

You are correct; I mis-read my print. There is an in-line contactor and a separate bypass contactor. The motors are Mitsubishi / Toshiba Induction motors.

 

[Skogsgurra]

@ imisspla07

The issue in my example was overvoltage. That cascade thing is not important here. The fact that overvoltage kills capacitors is.

Check if they are D or Y connected and if the rated voltage is in accord with the 4160 if D or 4160/sqrt(3) if in Y.

Also, if they are in Y, there is - at least theoretically - a possibility that unsymmetry can cause some overvoltage. But not much, unless one of the capacitors is shorted. In that case, the remaining capacitors will be connected to 4160 V. Not good.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[veritas]

imisspla07

What is the fusing arrangement of the bank? Internally, externally or fuseless? What is the bank configuration? Any commissioning tests to check that the bank was balanced and no capacitor unit fuses blown? Did you manage to get any voltage readings just prior to the fuses blowing?

You may thinkl of inserting a good protection relay in circuit when energising again to meausre rms current and voltage.

Regards.

 

[imisspla07]

The bank is fused externally, two sets of 300kVAR. I don't know if any tests were done or not, but the manufacturer did replace the blown caps under warranty, but the strange sound still exists with the new bank.

 

[Skogsgurra]

But they do not blow up?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[imisspla07]

Skogsgurra,

Most recently when we latched them in and latched in 300kVAR they did not blow up or blow any fuses. They were only latched in for only a minute or so.

 

[Skogsgurra]

Intriguing. Did you by any chance put a clamp and a recorder on the bank? It would be really interesting to see what the current looks like.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[imisspla07]

Here's the report I received from the utility. I'm not sure I fully understand it.