PF Factor Correction and Harmonics

[W@ttson]

Hello,

I have a client that has two utility feeds that feed into an ATS. One utility feed is from 2 miles away and sends it to the ATS via their own medium voltage distribution. The other is nearby to the ATS. The utility grids are separate.

The system has automatic power factor correction equipment connected to the 480V distribution bus. The PF correction only goes online when only the near utility feed is feeding the distribution. When the ATS is switched to the far away utility, the PF correction is not online at all.

My thought is that the nearby utility has a lot of parasitic harmonics from neighbors. The nearby utility feeds many large facilities that use hundreds of drives. Can it be possible that the harmonics from the nearby neighbors effects the 480V bus and therefore kicks on the power factor correction unit? Then when connected to the far away utility, the neighbors that are at nearby to that 2 mile away location generally do not have harmonic producing loads so the harmonic load is not an issue and therefore the PF Correction unit does not kick on?


Thanks

 

[David Castor]

If they are taking service at medium voltage, it's unlikely that harmonics from other utility customers are causing any issues at 480 V at this location.

 

[W@ttson]

why is that?

If they are taking service at medium voltage, it's unlikely that harmonics from other utility customers are causing any issues at 480 V at this location.

Why is that? I guess you are saying that the nearby neighbors are getting the service at 480V directly from the utility and therefore when it gets to the medium voltage part of the utility infrastructure that the harmonics would have been filtered out?

I have no other explanation then if that is the case.

 

[Elect117]

There can be numerous reasons at play, but...

The longer the line, the worse the power factor becomes because of the way impedance works. R+JX

The size of the wire / # of runs could also impact it.

Harmonic distortion is a usually taken as a percent of. So if the loads are heavily distorted at 480V, and the transformer is sized quite a bit larger than needed, then the total harmonic distortion seen on the primary side is lower. Harmonics typically show up as heat at the transformer anyway.

I would inquire what the power factor of the utility's line is at your service rather than harmonics. They might be using your capacitors as kind of like a switched bank to help with their own reactive power needs.

Utility's have their own capacitors and the feeder circuit from utility 1 might have a better power factor than the one from utility 2.

But all together I would also inquire as to how the capacitor bank is being switched / set up. It might only be sensing on the utility 2 side.

 

[W@ttson]

The longer the line, the worse the power factor becomes because of the way impedance works. R+JX

The utility feed that is far away (long line) is the one that provides the good service that does not require the PF correction caps.

 

[W@ttson]

Utility's have their own capacitors and the feeder circuit from utility 1 might have a better power factor than the one from utility 2.

That is interesting. Thank you

 

[David Castor]

why is that?

Why is that? I guess you are saying that the nearby neighbors are getting the service at 480V directly from the utility and therefore when it gets to the medium voltage part of the utility infrastructure that the harmonics would have been filtered out?

I have no other explanation then if that is the case.

The step-down transformers at each service provide a significant buffering impedance that reduces any harmonics in either direction. When power quality problems exist, the cause is usually local.

 

[Jraef]

Is the PFC system a capacitor bank, or are you referring to an Active Harmonic Filter system?

PFC caps are not typically tied to harmonics in that in typical Automatic PFC systems, they measure the DISPLACEMENT power factor to decide when to cycle in the capacitors and how much to add. VFDs create DISTORTION power factor, which takes place at much higher frequencies and are usually not seen by AFPC sensing systems.

My thought is that there is a significant change in the system impedance when changing the power sources, so much so that the APFC system is no longer seeing the same distortion PF, so it doesn't react. Either that, or the ATS used to be tied to a generator, and someone DELIBERATELY tied an aux. of the ATS to the APFC unit to disable it.

 

[W@ttson]

Is the PFC system a capacitor bank

Yes, its a bank of capacitors that are dynamically switched in or out in stages depending on what the power factor is measured as.

 

[W@ttson]

My thought is that there is a significant change in the system impedance when changing the power sources

There is 100% a huge difference in impedance. The one service is VERY stiff. The one that tends to call on the PF Correction caps. The other utility is pretty weak. Depending on the loads called on and the sequencing with the weaker, but less call on the PF Correction caps service would yield a huge voltage drop.