Office Harmonics

[Besoeker]

Do you really model an entire facility before bid time?

Yes. Particularly where the drive content is significant.
The invitation to tender (ITT) often makes it mandatory in such cases.
I'm sure it is also used as a means of filtering out those bidders not qualified to undertake the work. It sorts out the men from the boys.

Do you really know the behavior of all the other harmonic sources prior to them actually being purchased much less installed?

Yes, generally to a sufficient degree of accuracy. The ITT usually has a single line diagram back to the PCC. What you don't usually get is the level of background harmonics at that PCC.
And, as experience has shown me, that can throw in a curved ball. You live and learn.

When I bid the engineering portion of a study I know I will not have any control over the conductors, raceways, and lengths that will be finally installed. One of the last projects I was involved in was switched from aluminum to copper conductors after the original approval process had been completed.

As I said above, we normally have a single line diagram so we are likely to know conductor size and length. And is almost always multi-core armoured conductors Values for Z, X, and R are given in the electrician's bible - BS7671 aka "The Regs". Neither conduit nor aluminium conductors are altogether common in UK. But I take your point. Modelling the harmonic behaviour of conductors in a conduit might be more of a challenge.

Yes the input is a simple rectifier just like most VFDs have. The harmonics result from the 'pulses' of current drawn based on the load requirements Because not all supplies will be drawing equal amounts of currents at all times there is a probability of cancelation of the harmonics. .

Well....
The rectifier bridges all take current at the same time so the harmonics won't actually cancel each other.
It's differences in timing, not relative magnitude, that results in cancellation.

 

[jim dungar]

It's differences in timing, not relative magnitude, that results in cancellation.

Absolutely, it is the timing. But, with 1-phase loads, connected in wye (star), magnitude is also a factor.

This thread began with a question of K-rated transformers in office environments. Modeling offices with unknown loading is not as feasible as modeling your 3-phase VFD circuits where the only major variable appears to be circuit length.

 

[nakulak]

I'd love to see a post of one of your recent models with the one line (if you are so inclined).

 

[Besoeker]

Absolutely, it is the timing. But, with 1-phase loads, connected in wye (star), magnitude is also a factor.

Yes, magnitude matters but differences won't result in cancellation. For all of the SMPSs (with a plain rectifier input) on the same phase, their current wavevorms will coincide and just add. Not cancel.
Have them spread around the three pases and the triple-n harmonics will add arithmetically in the neutral. So, still no cancellation.

This thread began with a question of K-rated transformers in office environments. Modeling offices with unknown loading is not as feasible

I should't have thought modelling anything with unknown loading would be altogether feasible.....:grin:

as modeling your 3-phase VFD circuits where the only major variable appears to be circuit length.

If only.
But it is usually just a tad more complex......
The type of variable speed drive*, the input circuit configuration, supply transformer characteristics, system fault level, other connected equipment, harmonic filters, potential supply resonance.....
The supply cabling from the supply transformer to the drive isn't usually a major part of that. At least for the projects we mostly undertake.

 

[Besoeker]

I'd love to see a post of one of your recent models with the one line (if you are so inclined).

Not so easy to post in it's entirety - it's several sheets each with 200 or so lines lines and the size might not please the admin.
So here is a summary of one of them. I don't know how well it will come out:

And here is a chart of admittance/impedance. I generally do this where there are capacitors or passive harmonic filters to ensure that resonances are avoided.

 

[nakulak]

wow, and you do all this work before you even get the job ? I'm impressed.

 

[Besoeker]

wow, and you do all this work before you even get the job ? I'm impressed.

Impresses the potential customers too....:grin:
You do what is needed to get the business.
Then you have to perform. We have to commit to performance figures on harmonics, efficiencies, delivery dates on documentation, hardware, installation, the whole nine yards..... Get any part of it wrong or late and you are into stiff financial penalty clauses, or even rejection of the entire kit and caboodle. And potentially could get hit with the costs of it being supplied by others. That maybe explains why a lot of work goes into preparing tenders.
You have to talk the talk. And walk the walk.

Stress doesn't have enough syllables to explain how it feels at times. Maybe that's just how I take it. I'm an engineer, not a salesman. It's both a blessing and a curse.

And, thank you for your kind comment.
Very much appreciated.

 

[Dnkldorf]

While we're on the subject of office harmonics, I have a question I can't figure out.

When I replaced 3 transformers (208Y) from typical dry to zig zag, the power factor went up, and the building stopped getting hit the PF factor penalty from the POCO.

I have yet to understand how this happened. Nothing else changed, just the replacement of the trans to get rid of the harmonic heat.

Why would this affect the PF?

Any ideas?

 

[Mayimbe]

Not so easy to post in it's entirety - it's several sheets each with 200 or so lines lines and the size might not please the admin.
So here is a summary of one of them. I don't know how well it will come out:
...

Do you use any computer program to help you with these designs?
Or have you make yourself an algorythm to help you make this design given the input variables?
If you have, Ill be the first to buy your program.

...the power factor went up...

I believe it has nothing to do with the zig zag transformer.

 

[StephenSDH]

Just for my own knowledge?? Is it the peaks of admittance or impedance that are the system resonance frequencies? (I'm guessing it is admittance) You want to make sure a harmonic you expect to be in the system (like 7th) isn't at a resonance frequency.

Thanks, Steve