Blown fuses on power factor correction capacitors switched with aerato 1

[TEI66]

We have a waste water treatment plant aeration lagoon with 25 and 50HP 480volt induction motor driven aerators. PF capacitors are switched with the individual motors. These are 5KVAR and 15KVAR respectively. The caps are mounted integral to the motor control center immediately above the associated starter. They are connected between the contactor contacts and the OL relay. The motors are started FV across the line. They are sequenced at startup with a PLC.

There are two MCC. Each is rated 800 amps and are fed from a 2000 amp 480/277 volt 3 Phase WYE connected pad mounted service transformer.

There are a few other loads on the service but well over 90% is motors. The only VFD's are on fractional HP or less than 5HP peripheral equipment.

There are capacitors on the utility company OH primary line within 100 or so lineal feet from the pad mounted service transformer.

We have experienced random cap fuse blowing. Not always the same phase or even the same motor. Normally all aerators are running 24 x 7.

Anyone experienced anything similar? What did you determine was the problem.

 

[redtrumpet]

What size (amperage) and type of fuses?

 

[busbar]

Assuming here that the concern is with the low-voltge capacitor fuses and not those upstream on the padmount transformer.

You have no doubt checked for and ruled out overvoltage and excess harmonic conditions. Have you been able to determine the operating state of the motors when fuses have opened? Id est, during starting, running or stopping? Are both fuse sets operating simultaneously?

Having ruled out the obvious stuff, you may be stuck with having to babysit some monitoring equipment for awhile.

 

[busbar]

Are the LV capacitors connected in the usual ungrounded delta?

Might the utility be willing to switch out their MV capacitors for a few weeks to rule out some sort of interaction??

 

[peebee]

I'd suspect resonance. It's been a long time since I did a resonance analysis, so don't put any blind faith in my math.

Well, here goes. I assume 1hp = 1kVA and 80% PF motor. All analysis is single phase. 60 Hz = 377 rad/sec.

Motor Inductance:
50kVA motor @ 80% PF = 30 kVAR --> 10 kVAR single phase
10 kVAR at 277v = 7.67 ohms.
7.67 ohms = 20 mH at 60 Hz = Lm

Capacitor Capacitance:
15kVAR cap --> 5 kVAR single phase
5 kVAR at 277v = 15.34 ohms
15.34 ohms = 172.9 uF at 60Hz = Cc

Resonance at w = sqrt(1/LC)
w = sqrt(1/(20mH * 172.9 uF)) = 537 rad/sec = 85 Hz

85Hz is close enough to 60 that you may well be resonating between each 50hp motor and it's associated capacitor. I'd take the following steps:
1) Double check my analysis -- see disclaimer above.
2) Perform a more thorough analysis on the entire system.
2) Investigate the addition of chokes in series with your capacitors in order to detune the LC resonant circuit(s).

Something does seem strange to me here, usually resonances are found to be much higher than at fundemental, usually up around the 3rd or 6th harmonic.

 

[peebee]

I just double checked my bullet points and noticed that I count 1-2-2 instead of the more traditional 1-2-3. Told you not to trust me.

 

[jghrist]

When you check for resonance, generally you check the capacitance against the source impedance, not the load. Harmonic currents flow toward the source and if there is a parallel resonance between the capacitor and the source, there is a very high effective impedance to harmonic current flow. In this situation, there will be high harmonic voltage and high harmonic currents in both the capacitor and the source.

You also generally use locked rotor impedance for motors in harmonic analyses. You would expect resonance with the load near the fundamental frequency using the full load impedance. If the power factor is exactly compensated to 100%, it means that the capacitive reactance equals the load inductive reactance at the fundamental frequency. That is, it is in resonance.

 

[peebee]

Thanks jghrist. Sure seemed like something was wrong. Now that I look at my calc again, I only proved that you need Zc = Zl for power factor correction. Oops.

I'd still suspect resonance.

A quick look at the IEEE Red Book offers a couple helpful equations, including the following:
h ~= sqrt(kVAsc/kVARc)
where
h = order of harmonic
kVAsc = system short-circuit duty
kVARc = capacitor rating

Taking some guesses at the specifics of your installation, that would seem to put your resonance up into the 15th or 20th harmonic, where it would be unlikely that you'd have a problem.

Unfortunately, the Red Book also goes on to say the following:

"For more complex arrangements, which utilize capacitors in a number of scattered locations, the analytical determination of harmonic distribution is much more difficult and recourse to a computer study of the harmonic current flow may be necessary. Added capacitor locations multiply the combinations by which resonances may occur. . . . Somtimes a wait-and-see attitude is adopted in such cases. If trouble becomes apparent, such as blown capacitor fuses or cell failures, then corrective steps should be taken."

They go on to mention chokes as a corrective action, as I described above, and possibly adding or removing caps.

Also, as busbar alluded to, they state that caps should be ungrounded for two reasons: two eliminate zero-sequence harmonic currents and to avoid screwing up ground-relaying.

Hope this helps.

 

[jbartos]

Suggestion to the original posting: If those capacitor fuses blow randomly, the malfunction may be more related to some random phenomena in the power distribution systems, e.g. transients, surges, voltage changes, etc. Also, a faulty capacitor should not be ruled out.

 

[jghrist]

Can you determine if the fuse blowings are related to switching of the utility capacitor? Maybe the fuses only blow when the utility capacitors are on or only if they are off. If so, there may be a complex resonance situation as described by peebee.

 

[pwrman]

Have you checked the Capacitors with a tester lately, you my have some of them going bad. This not uncommon with this application. A simple handheld capacitor checker can determine this, compare the readings to the nameplate uf. Understand that older capacitors contain PCB's so if replacing a capacitor use the proper PPE, as PCB's are a Known cancer causeing agent. Other capacitors may contain Wesco oil which can burn the skin and eye's. So Be careful.
Electrical Power Solutions Ltd.

http://www.e-p-solutions.com

 

[Shortstub]

Responding to TEI66's query:

Are the preponderance of blowing cap fuses seem to be associated with the smaller units?

 

[SidiropoulosM]

Any investigation of resonance should include the transformer reactance as well as the power system reactance. I don't suspect resonance here. It's more likely that the cause of the problem is the transient voltage rise from capacitor switching. Are the capacitors switched simultaneously with the motors? Have you tried to switch the caps with a small time delay? Also, are all caps needed? You may be overcompensating. It will be difficult to find an analytical solution without accurate modelling of the motor's parameters and you'll have to do some experimenting. Michael Sidiropoulos

 

[Andiri]

Reply to original post. Look at your sequence of events. How long has this been a problem? Have you added or removed equipment from your system around the time the problem was first noticed? Has your utility company made any changes to the supply, or added other high inductance consumers to the same line that feeds your plant? Just a few basic Q's that may target the cause.

 

[Shortstub]

You mentioned randomness between MCC's. Is there a discernable pattern:

a) During normal load operation?
b) During low load operation?
c) During start (or stop) of larger motor?
d) During start (or stop) of smaller motor?
e) Any other "commonality?"

 

[Shortstub]

Further to my earlier post about a discernable pattern:

How is the "blown cap" detected? Cap fuses? Tripping of the motor's service breaker? Operation of the motor's fused disconnect? Visual inspection? Odor detection?

 

[peebee]

Smoke & char & black goo?

 

[Shortstub]

Thanks peebee,

Yep, what you said, AND:

What were effects on electrical protective equipment? Remember one man's "everything's OK" is another's "oh ....!"