Industrial System Harmonics Application 1

[ThePunisher]

HI all, I have been a very good reader of a lot of post here and would like to extend my appreciation and thanks.

I have been using IEEE 519 limits on the true PCC and we have always been compliant with it fortunately.

However, I have also been trying my best to understand and been looking for reasonable bases for applying IEEE 519 limits or figuring out specific limits we can use on buses way downstream of the TRUE PCC. There are several cases were the IEEE 519 limits are impractical when we are looking into downtream MCCs with VFDs that are significantly away from the PCC. In most cases, we apply IEEE 519 limits, spend huge cost on filters and call it a day.

However, it gives me interest to see if there are experienced folks here who can guide or help me understand if there are any withstand limits of cables, motors; and acceptable % harmonics we can establish to avoid nuisance tripping or mal-operation of relays or breakers that are less stringent than IEEE 519 limits when applied to significantly downstream of the TRUE PCC. The following are my dumb examples on what I am looking for:

- if harmonic currents flowing into the cables are 7%, this will be 107% total current (60 HZ + non-linear currents) and the cable sized to maintain 125% with de-rating should be able to withstand it.
- If harmonic voltage (THD) at the bus was 8%, this will be like 108% overvoltage assuming 100% 60 Hz voltage. If the insulation rating was greater than 108%, then the insulation would be ok

But yet, there are harmonic-generated losses which results to overheating, decrease in system efficiency and reduced design life of equipment....these are hard to quantify.

I read latest IEEE 519 and it did not address this, though it was partially discussed in IEEE 519-1992 (and later the paragraphs were deleted in the latest revision). I am assuming the standards committee were cautious of folks misapplying them if they were specified.

Bottom line, I am inquiring and asking for advice to establish REASONABLE voltage and current harmonic limits applied way downstream of the PCC where the harmonics are greater due to larger connected impedances seen at the harmonic sources.

 

[Coco_HueHueHue]

Hello,

I believe IEEE 519 is the way to go.
However, nothing prevents you from correcting the harmonics content "nearer" to your source of harmonics.

Take the example of a MV PCC (25kV) with a MV substation transformer 25/4.16kV and a LV substation transformer 4.16/0.6kV.
If your source of harmonic contents is on the 600V bus, correcting harmonics following IEEE 519 guidelines right there would obviously be cheaper than correcting it into your 25kV substation right at the PCC. I would expect your harmonics content to be acceptable at the PCC if they are corrected at the source.

If you are producing harmonics at every voltage levels of your substation... then... only a study can be conducted to see if its cheaper and/or easier to correct it at every voltage levels OR to put a big harmonic filter at the PCC bus.

Sizing equipment to "survive" the harmonics (oversizing copper and/or insulation as per your example) might work, but I would fear the utility supplying you might refuse to connect you or send you salty bills at the end of the month. Locally, here, the utility has their own interconnection standards that are mandatory to follow if you want to be connected at the PCC. I mostly use those instead of IEEE as they are often more stringent than IEEE.

I hope I understood your question correctly.
Coco

 

[Gr8blu]

@OP
Most of the (large) machine manufacturers - which means anything that fall into the electro-magnetic category - is expecting harmonic content equivalent to that allowed on a utility. That means - at worst - the IEEE 519 values. This is INDPENDENT of where the equipment is located on the local distribution side.

Why? Because historically, everything ran on systems that were either "true DC" or "true AC" (i.e. not a result of rectification or pulse width modulation).

Can machines and equipment be designed to handle higher harmonic content? Sure. But it effectively means that if you specify the machine needs to handle 10% current harmonics, it will be a 1250 hp design with a 1000 hp nameplate (to specifically handle the extra thermal stress from the higher harmonic content).

Converting energy to motion for more than half a century

 

[mparenteau]

Right or wrong...I used the following:

<5% VTHD
<33% ITHD

Anything above those #'s, I would look into filtering if operational/equipment issues were noticed, etc. I hadn't experienced any operational down-sides to those #'s, but your mileage may vary depending on your installation.

Mike

 

[jraef]

I like to look at IEEE-519 as being the Tablet brought down from the mountain that says only one commandment:
"Thou Shalt Not Infect Thy Neighbor's Facility with Your Harmonics!".

So then you have to look at harmonics like it's a "population of a country", which is made up of individual cities in your facility, with a lot of people (harmonics) that want to emigrate to an adjacent country. You can control emigration (harmonics export) at the border (the PCC), or you can control the emigration of the population at the individual cities (loads), which has the same net effect overall at the PCC. Since technically that's all that IEEE-519 is concerned with, either method is valid. But by controlling it closer to the sources, you also gain the benefits of having it not affect the equipment WITHIN your own facility. Ergo, by using the IEEE-5129 guidelines closer to local load sources of current harmonics, you end up with BOTH benefits.




" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Gr8blu]

Just noticed something else from the OP's post.
... if harmonic currents flowing into the cables are 7%, this will be 107% total current (60 HZ + non-linear currents) and the cable sized to maintain 125% with de-rating should be able to withstand it...

Yes - and no. It depends on the frequency of the harmonic content. Enough high frequency content will create the equivalent of skin effect - and now the cable sizing is insufficient to handle the thermal stress, since all the load is being carried by a relatively thin "skin" of conductor, instead of the entire cross-section (which is how it was originally intended to operate).

Converting energy to motion for more than half a century