How do I calculate total demand distortion (TDD)?

The IEEE 519 standard requires you to calculate the total demand distortion and compare it to the tables. So how do you calculate TDD? 

The question is if I'm calculating TDD, what's different between THD and TDD? So, let's take a look at a THD calculation first. Then we'll move on to TDD. THD – basically we calculate as the sum of the square root of the sum of the squares of all the harmonics divided by the fundamental times a hundred percent. If we take a look at this graphic, we take the fundamental as a hundred percent or a hundred amps, let's say, for example 20%, third harmonic and so forth. And, we add all those up and we end up with 20 squared, 12 squared, four squared, two squared divided by a hundred squared or a hundred. If we don't put that under the square root, we end up with 23.8%. That's how we calculate THD. 

Now, if we want to calculate TDD, the difference is just the denominator. So, if we're looking at TDD, we see that the denominator now is not one, which is the fundamental only, it's really the fundamental at the peak demand. So, the peak demand would be, for example, every month, your utility sends you a bill and they give you a demand charge. If you're a large consumer of power, that demand charge is a 15 minute window. What you would do is divide by that number. And, so if you think about it, that number's your worse case scenario as far as your maximum load. If your harmonics don't change over the month during the maximum load, you're dividing by a bigger number. That means your TDD is typically easier to hit than it is for total harmonic distortion. So the only difference would be that if you're looking at THD versus TDD, the number on the bottom, the denominator is basically one or the fundamental, and in the case of TDD it would be the maximum demand load for the month.