Eaton vs AB

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ptonsparky said:
We have a customer that needs PFC. Yes, they are billed for poor power factor.

AB suggests keep PFC about 250 ft of conductor length separation. Thinking 3% line reactor plus the branch circuit lengths.

PFC mfg says they can be right next to each other. Now what?
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Reactors??? Are these for drives?
 
Poor PF is uaually corrected by installing capacitors. However, you need to take care when the load is VFD's.
They don't play well with capacitors.
 
bob said:
Poor PF is uaually corrected by installing capacitors. However, you need to take care when the load is VFD's.
They don't play well with capacitors.
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Nor are they necessary. VFDs correct the displacement PF already, they will present a .95PF (or greater) to the line side. No capacitors necessary, in fact not recommended at all. They can interact with the bus capacitors in unpredictable ways.

If someone at A-B offered that suggestion, they were likely referring to BULK PFC capacitors or Automatic PF correction systems, when used in installations that have VFDs but for correcting PF on OTHER loads. When you read something fom a PFC cap supplier, the reactors they are likely referring to would be in-line reactors used to help prevent spikes caused by the reactors from damaging other equipment when they come on and off line.
 
ptonsparky said:
The VFD driven motors will not, will not have PFC correction. They are a very small portion of the total motor load.

The accross the line motors will have PFC.
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So are you going to put caps at each motor? Or are you installing an automatic PF correction system?

It's just my opinion, but in my mind the best way is to install caps at each motor, connected down stream of the starter so they are ONLY on-line when the motor is on-line.
 
ptonsparky said:
I was more worried about the caps.
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It's a valid concern in my opinion.

If the PFC is for fixed speed motors on a per motor basis, load side of the starter you said, automatic correction should not be required. You are correcting for the motor reactive component of the current drawn. And that doesn't change much.
But, if you have variable speed drives or other non-linear loads, the supply voltage will be distorted to some extent. It may even be distorted on the supply side without anything on your plant running. That distortion is harmonics. Multiples of the supply frequency. Higher frequencies mean higher currents in capacitors.

To what extent it increases the current in the capacitors depends on the degree of distortion but I've come across a number of cases it has resulted in failures. Some spectacularly so. In one paper mill there was bulk PFC with automatic control. Every single bank had failed - I mean blown the guts out of it sort of failed. I've seen similar sorts of things in a few places. And sometimes trivial but inconvenient like the capacitors in every single fluorescent light fitting failed.

I can't say it will be like that for you. My background is power electronics so I suppose it's somewhat inevitable that I come across situations where the level of harmonics may be higher that average. That said, they are often not very good at utility level. And you can't do a lot about that.

We are not all that often involved in projects where PFC is required but when we are I always fit de-tuning reactors. These are calculated to block frequencies above 5th harmonic, 250Hz in our case.
But that may not be a practical proposition for you, not least for cost considerations.

So what to do? A power quality analysis would help to define the extent of the potential problem. Or absence of it.
 
The troublesome harmonic number may be figured out by using standard formula (based on capacitor bank size and system fault level) mentioned in power system Engineering manuals. A power quality analysis may be carried out to look for the same. If such a harmonic is indeed present, harmonic mitigation measures are to be taken immediately.
 
It has been many years ago, and I was still kind of green in this field, but we were installing capacitors in a plant at load side of each motor starter. We mounted quite a few and ran raceway/cabling but did not connect to the starter until there was a down day. Come back the next day and nearly every capacitor that had been in the circuit had blown fuse indicators all lit up:?

I did not fully understand what was going on at the time, and neither did my boss, but we eventually found out it was a harmonics issue and a large VFD in the plant was the biggest contributor to the problem.

We ultimately ended up putting two large capacitor banks with line reactors inline with them near the POCO transformer. Chances are there were times when it was over correcting but that is what was done. I think they were supposed to manually switch them off when they had down days or at least one of them depending on what processes were running. They typically ran 24/7 except for an occasional clean up day so most of the time the load was pretty consistent.
 
Sahib said:
The troublesome harmonic number may be figured out by using standard formula (based on capacitor bank size and system fault level) mentioned in power system Engineering manuals. A power quality analysis may be carried out to look for the same. If such a harmonic is indeed present, harmonic mitigation measures are to be taken immediately.
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If only it were so simple..........
 
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