Is harmonics blowing up my capacitors??
Is harmonics blowing up my capacitors??
(OP)
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!






RE: Is harmonics blowing up my capacitors??
Just disconnect and put them upstream of softstarter.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Is harmonics blowing up my capacitors??
" 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."
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(2B)+(2B)' ?
RE: Is harmonics blowing up my capacitors??
2nd motor reached full speed and cap just switched in?
Motor running in steady state?
Motor stopping ?
=====================================
(2B)+(2B)' ?
RE: Is harmonics blowing up my capacitors??
RE: Is harmonics blowing up my capacitors??
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
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Is harmonics blowing up my capacitors??
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?
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RE: Is harmonics blowing up my capacitors??
I suspect a voltage issue. Wrong voltage or wrong connections.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Is harmonics blowing up my capacitors??
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.
RE: Is harmonics blowing up my capacitors??
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:
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Is harmonics blowing up my capacitors??
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?
RE: Is harmonics blowing up my capacitors??
"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: Eng-Tips.com Forum Policies
RE: Is harmonics blowing up my capacitors??
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.
RE: Is harmonics blowing up my capacitors??
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
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Is harmonics blowing up my capacitors??
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.
RE: Is harmonics blowing up my capacitors??
RE: Is harmonics blowing up my capacitors??
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Is harmonics blowing up my capacitors??
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.
RE: Is harmonics blowing up my capacitors??
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Is harmonics blowing up my capacitors??
RE: Is harmonics blowing up my capacitors??
The frequency is 35 - 40 times fundamental and doesn't show in the report on harmonics because the highest harmonic registered is the 13th.
You have a sharp resonance there. What kVA rating and % uk does the transformer have? It looks like the leakage reactance and the capacitors form a high-Q resonance circuit.
I really cannot understand why the utility didn't tell you about this. It is quite obvious.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Is harmonics blowing up my capacitors??
RE: Is harmonics blowing up my capacitors??
If that is the case, from the report, there is a number of high frequency voltage 900 Hz to 1200Hz. If the capacitor is not tuned properly, this capacitor bank will act as a sink that draws this high frequency current. And that can be a huge amount as the reactance is very small. That explains the blown fuses, and funny humming noise.
RE: Is harmonics blowing up my capacitors??
I would say you need someone who knows how to run his power metering equipment to do some measurements at different points on the system.