harmonics

[bhawks19]

I was told that if you supply power from a harmonic limitating transformer (zig zag or phase shifting) to a device that has an internal harmonics filter, such as a large refrigeration machine with a variable frequency drive then the two filters will fight, for lack of a better word, each other and not accomplish the task at hand, mitigating harmonics.

In fact, i am told, that this type of installation can promote component (load side filter) faliure

Is this true, and if so what exactly is happening to cause this?

thanks

 

[robbietan]

the zig zag transformer reduces the triplen harmonics that flow thru the neutral wire by redirecting to the phase wires. the harmonics there would cancel the harmonics that comes from the zigzag transformer.

a 3 phase VFD normally produces 5th and 7th harmonics, and the filter installed would probably be tuned to those frequencies. which means less 5th and 7th coming from the VFD to the electrical system.

I do not see any conflict here, and certainly no reason why the load side filter would fail. the load side filter just protects the load from the harmonics produced by the VFD itself.

 

[bhawks19]

harmonics

We have two cases where when we disabled the load side filter the issues went away,

Is there any known reference sites that may helpful in explaing any electrcial phenomon that may be occuring whne two harmonmic filters are installed on the same power lines?

thanks

 

[bhawks19]

We had two cases in which 2 seperate harmonic filtering transformers supplying loads with internal harmonic filters had issues. When the loads' internal harmonic filters were disabled the problems subsided....

I am told the problem is similair to the issues you have when you install two UPS's in series, though getting that explanation is also elusive..

Any reference sites or book suggetions on these areas of power quality.

thanks

 

[robbietan]

there are a lot of sites out there and one of them is electrotek.com which is in the business of power quality. mte corp is another good site.

one of my collegues pointed out that you could be having resonance problems since you added the zigzag transformer. why not take measurements of the harmonics involved, just to make sure.

the two UPS in series thing is tricky, depending on the UPS involved. depends on the type of waveform the UPS can accept. some UPS dont like having a square wave (or non sinusoidal wave) as their input.

 

[bhawks19]

i will check out these sites. thank you very much

 

[NeilWood]

Getting in here late, but, you said "We have two cases where when we disabled the load side filter the issues went away, ..." But what issues?
If the refrigeration unit drive is the only thing connected to the zig-zag and there is only one passive filter on the drive, then there should be no problem.
More info about the specific details of the problem (noise, overheating, nuisance tripping, etc) would be helpful. Normally, the zig-zag treats triplens and the the refer drive should be 3 phase and will have no triplens.

 

[bhawks19]

The one issue was we had a VFD passive filter burn up - York or Johnsion Controls blames the step up zig zag transformer

then the other issue was a nusiance trip, different machine, and again Johnson blames the zig zag transformer