VFD question

[tw/nci]

I understand that VFD's produce 5th and 7th Harmonics. Do VFD harmonics have any detrimental effect on other systems in the typical office building and what kind of things would you look for. Thanks for your help

 

[Besoeker]

I understand that VFD's produce 5th and 7th Harmonics. Do VFD harmonics have any detrimental effect on other systems in the typical office building and what kind of things would you look for. Thanks for your help

A VFD with a 3-phase input produces 5th, 7th, and 11th, 13th, 17th, 19th etc. (6n?1), n being the integer series 1, 2, 3, etc.
These harmonics result in supply voltage distortion. This causes increased losses in conductors, transformers, well most things. Capacitors are particularly susceptible.

But, in an office environment, I would have thought that single phase non-linear equipment (including single-phase fed VFDs) would be more of a problem. Such equipment produces 3n (triple n) harmonics which, in addition to the 6n?1, problems, also significantly increases neutral currents, sometimes to the point of damage.

Each single phase load might not be much on its own, but their sheer numbers can make them a problem.

 

[tw/nci]

What kind of corrective action should be taken or at least moniotored in a building with a vast amount of 3 phasse VFD's. Thanks

 

[jim dungar]

Look at your voltage distortion. Do not get caught up in looking at every source of harmonics, instead look at the 'points of common coupling' in your facility (which are usually transformer secondaries).

Harmonics, of and by, themselves are usually not a concern. Rather, it is how the harmonics affect the 'power grid' that causes problems.

 

[marti smith]

Has anyone installed a harmonic mitigating transformer for these issues? Do they produce the desired result?

 

[Besoeker]

What kind of corrective action should be taken or at least moniotored in a building with a vast amount of 3 phasse VFD's. Thanks

Jim is right, of course. Harmonics will have an adverse affect on the supply voltage. But just how much depends on the specific circumstances.
And you need to know how much before you can determine what, if any, corrective action is required.

What can be done if it is deemed to be a problem?
Passive harmonic filters.
Active harmonic filters.
Replacing some of the VSDs with active front end units.
Polygonal transformers giving a 30deg shift for some of the VSDs.

All would require some system analysis to work out the best option(s).

I guess the best option is to ensure it isn't a problem in the first place. The design work can, or should be, done at the outset. We design and install VSD systems and often we have to agree contractual commitments to maximum distortion limits that will be achieved post install. This is prior to award of contract so you depend on getting it right by design. Or pay for the consequences if you don't.

That's fine if it is a new installation. But you don't often start with a clean sheet of paper. You have to make the best of what you've got.

 

[Besoeker]

Has anyone installed a harmonic mitigating transformer for these issues? Do they produce the desired result?

I don't believe they reduce the harmonics for a single drive.
They could if you looked it from the supply where more than one drive is connected. That's basically how 12-pulse drives reduce their contribution to supply harmonics.

 

[robbietan]

newer VFds usually have either a line reactor built in or have a 12 pulse/18 pulse converter - both done to lessen the harmonics produced by the VFD

 

[Besoeker]

newer VFds usually have either a line reactor built in or have a 12 pulse/18 pulse converter - both done to lessen the harmonics produced by the VFD

Yes. For example 12 pulse converters have (12n?1).
The arrangement cancels out 5th, 7th etc. on the supply.
Line reactors help some by making the supply current a bit less lumpy but, in my experience, are not a hugely effective means of harmonic attenuation.

 

[PJS3]

GREEN NEWBIE jumping in on this discussion if I may...
I just got a comment from a client on my design that since the two AHU's (a single 5hp each - 480V,3phase) that we're putting in have integrated VFD's that somehow a TVSS is required... :-?I'm lost on that one. I can only assume that he is concerned about harmonics from the VFD's... But (at the risk of showing my experience level) I don't understand how a TVSS addresses that. Thoughts?

I thought this would fall in line with this discussion....

 

[robbietan]

GREEN NEWBIE jumping in on this discussion if I may...
I just got a comment from a client on my design that since the two AHU's (a single 5hp each - 480V,3phase) that we're putting in have integrated VFD's that somehow a TVSS is required... :-?I'm lost on that one. I can only assume that he is concerned about harmonics from the VFD's... But (at the risk of showing my experience level) I don't understand how a TVSS addresses that. Thoughts?

I thought this would fall in line with this discussion....

I hope no "TVSS solves harmonics" gadget is involved here.

Having a TVSS will definitely protect the VFD from transients, if there are any in the first place.

And here I am thinking that the VFD will lessen transients from motor starting due to its "softstarter" property.