Not quite convinced

[ptonsparky]

A while back I started a thread on failed Power Factor Correction capacitors and VFDs. I would refer to it if I could find it.

This is a seasonal operation so full scale testing for harmonics at this time could result in a $10,000 utility bill for a non productive month. Total connected HP is 1425. VFD hp is 69 HP. Less then 5% of the total. Of that 69 HP, only 3 3hp motors do not have line and load reactors. From my reading I believe 10% to 15% of total HP was the magic number where concern may/should begin.

We ran a short test with the 60 HP of VFDs and a 100 HP fan with PFC. We used a Fluke 43B and several Fluke true RMS meters. As expected the THD with just the drives running was up to 57%. THD dropped to 3.4% after the 100 HP was started with only the 5th harmonic showing on graph. Only .2 amp increase in capacitor amperage was noted. I expected more.

The plant does not suffer from low voltage even at max load. 3-300 KVA xfmrs with less than 25000 sca according to utility. Considerably less than I expected. 2.4% impedance 480/277 with no neutral load.

The plant PFC is manually controlled via fused disconnect at the start/end of day. Could I use an educated guess that maybe it was cycled on-off-on before it could discharge competely or am I still in denial?

 

[boater bill]

I am having trouble seeing the entire picture Tom. I guess I missed the original thread. Did the PF caps fail?
The line input reactors should reduce the line harmonics, but may not totally eliminate them. The only time I have seen PF caps fail is either a short on the lines with improper protection or the oil leaked out.

If you find your original thread please post a link.

 

[jim dungar]

The lighter the loading the more THD each drive represents. As the system loading increases the effective system THD naturally decreases. Other sources of high THD at light load are "idling" motors and transformers because the percentage of magnetizing current is very high compared to the actual work they are doing.

 

[ptonsparky]

2-3 years ago we installed a 100 KVAR plant PFC capacitor. One year ago in Jan we installed the 2- 20 hp & 4 - 5hp drives. The other 3 hp dirves were existing. In Dec of 06 we decided to meg the motors & do general PM. We discovered the plant PFC failed, and also one each for a 200 hp & 125 hp. Tracing the billing back to see when they got hit with poor PF charges led us to Jan. Some time after startup of the new system & control we lost the Plant cap, and possible the other two but not sure. The PFC components that failed were physical the smallest. Other larger sized components of other PFCs were not bothered.

Many suggested a harmonic problem & I was inclined to agree until I researched it some more. The drives make up such a small portion of the entire load and are started after almost all other motors and PFC is engaged, I am starting to wonder if something else could have been the cause.

Some time last summer one of the utility transformers burned and subsequent repair literally blew the top off of another.

The original PFC paid for itself within months. New Harmonic PFC will take one heck of a lot longer.

 

[jcormack]

I would suspect voltage transients before harmonics.....not to say that harmonic resonance won't blow caps. I always spec capacitors of a voltage rating higher than what they will be used on. For example I use 600 volt devices on 480 apps, 4800 on 4160, etc. This reduces the equivalent KVAR of the device as the KVAR is related to the Square of the voltage - a 100 KVAR 600 volt unit used on 480 volt would be approx 64 KVAR. I NEVER blow up capacitors due to overvoltage now & I have lots of surge "Capacity" to help soak-up transients. When you begin to exceed the voltage ratings of a cap the current rises exponentially. Are these caps switched with the motor starter? On-line all the time? Often if switched in banks they can develop resonance between banks. I had three banks of 75 KVAR each in a unit substation, there was not imp. between them. We had lost 2-3 units there & also the main wire that fed all three had "burnt off" so I went to investigate. With one unit on the current flow was normal, with 2 on still basically normal -when the 3rd was added the PF became leading and the current flow from each was suddenly 1.75 x FLA. System dynamics play a large role on "ringing" and harmonic problems with caps......I can say large banks of caps close to automatic tap-changers is generally a "bad" thing........

 

[ghostbuster]

I would suspect voltage transients before harmonics.....not to say that harmonic resonance won't blow caps. I always spec capacitors of a voltage rating higher than what they will be used on. For example I use 600 volt devices on 480 apps, 4800 on 4160, etc. This reduces the equivalent KVAR of the device as the KVAR is related to the Square of the voltage - a 100 KVAR 600 volt unit used on 480 volt would be approx 64 KVAR. I NEVER blow up capacitors due to overvoltage now & I have lots of surge "Capacity" to help soak-up transients. When you begin to exceed the voltage ratings of a cap the current rises exponentially. Are these caps switched with the motor starter? On-line all the time? Often if switched in banks they can develop resonance between banks. I had three banks of 75 KVAR each in a unit substation, there was not imp. between them. We had lost 2-3 units there & also the main wire that fed all three had "burnt off" so I went to investigate. With one unit on the current flow was normal, with 2 on still basically normal -when the 3rd was added the PF became leading and the current flow from each was suddenly 1.75 x FLA. System dynamics play a large role on "ringing" and harmonic problems with caps......I can say large banks of caps close to automatic tap-changers is generally a "bad" thing........

I would have to agree with you 110% w.r.t. voltage transients causing caps to blow.Also caps will tend to "attract voltage transients" from utility switching and cause operational problems within the plant.We have one customer that will actually turn off their cap bank to ensure critical parts can be produced and shipped(note: they are willing to pay for the bad p.f. to make sure the parts are produced on time)

 

[ptonsparky]

With the exception of the 100 KVAR, all the caps are switched on with the motors. The 100 is suppposed to be on at start of production until shutdown, be it one month or one day. I think we are getting awful close to unity, .96 to .98 when the 100 is on. Would using a 100kvar 600v help any? Salesman said "bigger is better" at the time. And, yes I should know better.

 

[rattus]

How are PFCs rated?

How are PFCs rated?

Are these caps rated in terms of RMS voltage or peak voltage?

Also, I don't understand how PFCs would attract transients. They will tend to absorb transients which ought to be a good thing.

 

[bob]

I think we are getting awful close to unity, .96 to .98 when the 100 is on. Would using a 100kvar 600v help any?

You say that as if its a bad thing to be at unity PF. IT not. Nothing is going to happen if you happen to get to unity except lower the bill from the utility.
If you go past unity the utility probably will not penalize the plant.

 

[robbietan]

Also, I don't understand how PFCs would attract transients. They will tend to absorb transients which ought to be a good thing.

transients have high frequencies (some around 20 kHz!) and they go right to the capacitors whose Xc is lowered considerably due to this high frequency.

 

[robbietan]

We ran a short test with the 60 HP of VFDs and a 100 HP fan with PFC. We used a Fluke 43B and several Fluke true RMS meters. As expected the THD with just the drives running was up to 57%. THD dropped to 3.4% after the 100 HP was started with only the 5th harmonic showing on graph. Only .2 amp increase in capacitor amperage was noted. I expected more.


I assume this is current harmonics. It would have been better if you used TDD, here you can see the difference in the amount of harmonic current versus the total load. even a high THD current would not be a factor if the amount is (for example) 5 amps and total load current is 500 apms.

however, them capacitors act as a trap for harmonics due to their low resistance. heating would result and this shortens their useful life.

 

[rattus]

Also, I don't understand how PFCs would attract transients. They will tend to absorb transients which ought to be a good thing.

transients have high frequencies (some around 20 kHz!) and they go right to the capacitors whose Xc is lowered considerably due to this high frequency.

Robbie, I know how a cap works. I am criticizing the use of the verb "attract". The implication to me at least is that the caps somehow make the transients worse, and I cannot believe that they do.

They will however convert voltage spikes into current spikes which may create objectionable magnetic fields.