Voltage and Current Harmonic Distortion

If you simply measure the THD on both sides you will then know how much of the distortion is being absorbed by the core of the transformer and converted to heat.

If you want to calculate this, you will have to take into account at least a 20% tolerance band for differences from one transformer core to the next. That will provide no benefit to just measuring it. Potentially you could assume that the transformer passes all of the distortion right through, but in reality, some of it is lost as heat. It really depends on how severely loaded the transformer is from other circuits. And, some of those other circuits are going to absorb some of that distortion as well.

Current distortion is only significant if you are approaching the peak VA rating of the transformer at the specified voltage. Once you have hit the current ceiling, then voltage distortion starts becoming significant. If you are getting voltage distortion, it will affect other devices connected to the primary side of the transformer. But if the voltage source is "stiff enough" then the current variation does not really present a problem. As I've heard it said, "Voltage is King!"

I think you should MEASURE the THD, rather than calculate it. Distortion is caused by non-linear loads and sources whose reactance won't be available to you. Use a meter like:

http://www.fluke-direct.com/shop/itemDetail.do?itm_id=307818&itm_index=0&item=43B/003

I have that, and upgraded last year:

http://www.fluke-direct.com/shop/itemDetail.do?itm_id=321625&itm_index=0&item=434-II&manufacturerItemNumber=434-II

Very handy to have around.

cuba pete and woodward dl:

thanks for the responses.

I do have a power analyzer (fluke 435)

The reason i need to be able to calculate is because we are designing a plant where 90%+ of the loads will be drives AND the power source is a diesel generator. The harmonic calculators i have project a THDv of about 14% (above IEEE519 dedicated) and the THDi is really high, 31.2%

I am trying to see if a transformer that is fed with that type of power will reduce the THD at all on the load side so as to isolate the loads on this transformer (single phase loads like UPS online double conversion)

My guess is that the THD on the load side will be slightly less than on the line side so i would like to calculate by how much somehow.

Has anyone had experience with this that can shed some light?

Oh, well then you are not some student or worse!

Right now, you have a great opportunity to pick your transformers so that the harmonic distortion is completely absorbed by installing the right type of phase shifted windings. This is accomplished in a way that prevents the need for any other special equipment to deal with the harmonics.

For free expert help you want to look up the following company:

Eaton Electrical

(Phone) 412-893-3262

(Cell) 412-855-3452

www.eatonelectrical.com

I have visited their lab in Pennsylvania and learned more in a couple of hours than I learned in a full semester long course on the subject in college. You will get it right the first time and save money.

Funny you mentioned Eaton,

I have reviewed their web education materials (web casts) on harmonics. They are very instructive as you mentioned. It must be even better to visit their site.

I will revisit the information i have from them on this. If i remember correctly, the webcasts on harmonics mention phase shifting transformers to mitigate harmonic distortion from non linear sources that are rich in third harmonics (Fluorescent lights for example) However the frequency drives are not rich in third harmonics but in 5th harmonics.

Thank you very much for the contact information!

I will give them a call and see what solution they recommend.

"There is only 10 types of people. Those who understand binary and those who don't :-)"

Line side THD of Voltages in normal working conditions will always be < 5% for volts. as per IEEE norms on the licensees.

Current distortion in line side will be caused by the loads. But, for Delta wound transformers, this will be nullified!

So, for transformer rating selection, you practically need to know the distortions caused by the loads.

Best will be to practically measure for existing cases, use the same experience for new plant designs. Any deciations will be corrected by the filters to be incorporated. Type to decide after commissioning of the plant. This will be economical.

Refer K rating of the transformer that decides the transformer rating under harmonics conditions.

The next logical question is, once you've established your THD values, what will you do about them?

I ask as harmonic mitigation is something I do.

Maybe once you've have completed your first round of calculations we could get in contact via private email.

One thing you do it communicate with the manufacturer of devices that be taking that type of THDi and THDv and ask what is the level of tolerance of their equipment.

The next step once you have a handle on that the THD will be in the system you communicate with the manufacturers of the devices that will use this power and see if power quality as far THD (v and i) those levels are acceptable. If not then come up with a different setup that will provide acceptable power quality to the devices.

Chances will be very much reduced to have THD problems in the field.Also less costly if those are taken into account in the planning stage.