What does oversizing my electrical equipment do for harmonics (k-rated transformer, generator, etc.)?

We often get a question: What if I just oversized my equipment? Will that take care of my harmonics? 

The real answer to the question of oversizing equipment and dealing with harmonics is - what's your goal? Think about harmonics as a two-part problem. One is overheating and the second part is MIS operation or voltage distortion issues. The first part is a current issue. The second part is a voltage issue. If you oversize a cable, for example, what's going to happen is that cable is able to take more current. That's clearly a current issue. So, if you have a smaller cable on a neutral and it overheats, you put a bigger cable, you're going to relieve some of the capacity issue. And, you're actually going to be able to live longer with that cable. A transformer on the other hand is very similar, but in some respects, it also can change the way things flow. So if you look at a K-rated transformer, for example, a 75 KVA standard transformer might be here, a K rated transformer might be here, might be bigger physically, but electrically, it does the same thing. But, the impedance to that system and how the impedance works really dictates some of the losses, the voltage distortion and the impedance of the system changes how that flows. 

Generally speaking, a K rated transformer is an oversized transformer. That's just, again, living with harmonics. And that really relates to current. Where it gets a little bit different is with voltage distortion on generators. We have a FAQ on why generators react differently to harmonics, but basically it comes down to impedance. 

So if I oversize my generator, what am I doing? Let me explain that with a piece of paper here. If I take a normal utility transformer and I feed my load and I say this is a thousand K and I have Z equals 5% or 5.75. And I put a certain load on here. Let's say it's a bunch of variable frequency drives. It's a lot of variable frequency drives and I'm producing harmonic current. Basically we've talked about it before, but voltage distortion at the common bus is current distortion times harmonic, impedance, and that's basically just Ohms law. So, the more harmonic current I put here, the more I distort my voltage and it's just proportional. 

The impedance comes into play as far as how much. If I put a little bit of load on there, my voltage is like this and my current maybe looks like this. But eventually I might start to distort my voltage with that impedance. So my voltage gets a little out of whack for that current. Now, what happens is if I put on a thousand KVA generator.

The impedance is going to be about 18 to 20%. Let's say that impedance is three or four times as big as what the normal utility source is. So if I turn off the utility source and I feed that current now through the generator what's going to happen is my impedance is three to four times. Then that means my voltage distortion gets worse by three to four times. Now my voltage distortion gets a lot worse over here. And we might have multiple zero crossings and all kinds of other things that we've talked about, some of the FAQs. So, really oversizing the generator doesn't in this case really just make the generator happy from a current standpoint. But in fact, what it does is if I make that generator 2000 KBA.

Then the difference in impedance from here to here is not five to 18%. It's basically a thousand over 2,000 in terms of putting it on a per unit basis, times 18%, which really gives us about 9%. So, now we're going from 5% to 9%, instead of going from three to four times as much voltage distortion like one and a half to two times. So, again, when we think about what we're doing by oversizing equipment, it really comes down to oversized cables, transformers to handle the current oversized generators, to make the impedance effectively smaller, to make the voltage distortion less. And that's really what making the equipment bigger is going to do for us.