What is the difference between THD and TDD?

People ask this question a lot:  what's the difference between THD or total harmonic distortion and TDD, which is total demand distortion?

The main difference between THD and TDD is simply the denominator in this equation. If you take a look at total harmonic distortion, we're taking all the harmonics, summing the squares and taking the square root. Then the only difference is the fact that we divide by the fundamental in the first one, which is sort of the instantaneous value right now, or the 60 Hertz value right now. The TDD value is dividing by the maximum fundamental current or your demand over a window. For example, on a utility bill, there's a maximum demand. If it comes out in KVA or kilowatt, we take that and convert it to current. We use that value in terms of the maximum loan demand. So, in most cases, that number's going to be bigger, if that number's bigger, what that means is the denominator's going to be bigger and make the THD or the TDD value in this case, smaller. 

Let's take a look at an actual example. So, if we have an 18 pulse drive and we have a total demand load of 225 amps, that's the maximum kind of demand on that unit. Why do we buy a variable frequency drive? We put a drive in so we can run it at a lighter load or a slower speed typically. So we can save energy or maybe change the speed for process control. But, if we have THD calculation and we take a look at it, like in this case, the actual ASO harmonic or 8.2 amps are harmonic, but the fundamental in this case at 30 Hertz is 30.8 amps. So if we do the THD calculation, we're going to take the 8.2 divided by 30.8, and we're going to end up with 26.7%. If we do the total demand distortion, we're going to take 8.2 divided by 2 25, and we're going to get 3.6 percent distortion. 

TDD is a lot lower number. And, if you look at this graphic, you can see that the TDD climbs ever so slowly as we go from light load to heavier load, and the THD value goes down as we go to heavy load. What's important about that is not the value of percentage. What's important about that is that the amps of harmonic are slightly increasing, which really is what we care about in terms of amps of harmonics. Again, this is what's important. And, if we take a look at IEEE 519 current distortion, they want to look at TDD because the demand distortion is what's important. It's not the instantaneous thing. Remember harmonics affect you over time. Let's take a look at a quick summary of that on a piece of paper here for a minute. 

So if I look at my power system and I have a certain amount of harmonic load, let's say I have a bunch of drives or whatever VFDs here. And , I produce a certain amount of harmonic current. If I do THD calculation, you know, instantly right now with my load and I don't have any other loads on my power system. And I say, I have 10 amps of harmonics over a hundred amps of fundamental times, a hundred percent. Then my THD value is 10%. Well, maybe overnight, or when there are other loads on the system, I may have more 60 Hertz or less. But, in the case of total demand distortion, what if I had my worst case scenario for that was when I had all my other pumps and motors and cross the line things on here. And I still had 10 amps of harmonic current, but I had maybe let's make it exaggerated, a thousand amps of fundamental current. 

Well then, in that case, multiply that out. I get 1% distortion, but you can see in this case that whether or not I have 10 amps of harmonic current here or 10 amps of harmonic current there that is what's important. And, really if this transformer can handle a thousand amps, let's say it's a thousand KVA transformer and can handle 1200 amps. Well, that's not a problem for this transformer, but 10 amps on this transformer is the same as this 10 ASO harmonic current here, or this 10  of harmonic current here affects the voltage the same way on the primary and secondary. So my VT, H D VT, H D here is affected by the actual Amsel harmonic current. And ,what they're trying to do by using TDD is get you away from thinking about percentages and getting lost in the percentage values, because it can be a little misleading in terms of the actual effect of the harmonics in the power system. Essentially because the utility cares about you screwing up the neighbor's voltage distortion at the common bus. They really want to use that demand distortion and see how much harmonic current you're drawing compared to the stiffness of the system and the impedance of the system. And, that's why the IEEE 519 current table looks the way it does.