Is a temporary measurement as good as a long term trend for harmonics?

A common question that comes up is – “If I'm concerned about harmonics, do I need to do permanent measurements or can I use temporary measurements to really get a good sense of what's going on? 

The real question is if you're going to do measurements, what are you going to use those measurements for? And, there are a few reasons why you would do harmonic measurements. One might be just a benchmark. Let's say you're building a system and you're adding in a lot of different harmonic producing loads. Your nonlinear loads. You might want to know where you're at today and what it might look like in the future. And in that case, you'd want to get a pretty good level of high and low levels of harmonics, meaning during the day, overnight, and weekends. So in that case, you might want to look at some trends. In other cases, you might just be looking to model the power system. And, in fact, when you're modeling a power system, a lot of times that's ahead of another design. So you can get a decent idea of what's there with a snapshot. 

And what you have to know about that is if you're going to put that measurement into a model, I usually use a high average level of measurement. So, not the worst case, not the best case – meaning the lowest case, but what you're going to use is something that's a high average so that when you go to monitor your system, you're not excessively modeling harmonics and making it worse than it actually would be in real life. In those cases, a lot of times I'll do a snapshot measurement and use that for my model and then maybe tweak it again over time to see where the high and low situations might be. 

We've also done some FAQs on why harmonics are higher overnight than during the day. In fact, it’s typical for the harmonic distortion percentage to be higher overnight than during the day. What you really want to know if you're modeling a filter — designing a filter is what are the actual amps that you're going to see through that filter? The actual amps are not necessarily coincidentally the same as the highest level of percent distortion. The other thing that you might want to think about is if you're doing power factor correction? This level of power factor correction requires this size harmonic filter and designing – a power factor correction bank. You could overcompensate if you make the filter too big and go leading power factor, or maybe raise the voltage. I want to draw one last thing here, as we think about power systems and where and how I'm doing my measurements. I typically might have a single utility transformer and then maybe I have multiple transformers down through my system. 

And I want to measure different locations now. Some people will say — do I need permanent meters at every single location? Measure what's going on again. If you're designing a harmonics filter that you're putting here, measuring this current is probably going to be good enough because it accumulates everything going on these downstream loads, including all the harmonic sources from down here. If you are designing individual harmonic filters, say for VFDs down here, then perhaps you want to know what's happening on those branches and knowing what's going on this branch. When there may or may not be other loads on the other branches is important from a current standpoint, but also from a voltage standpoint. So, let's say you measure down here and your VTD is 4%.

If you don't know what's going on the rest of the system, it's really hard to tell how this load is going to affect that situation because that voltage distortion may be coming from this load individually, or it may be coming from the other loads beside that. The other thing is if there's a capacitor on the system, you want to measure wherever there are capacitors on the system, ideally through the capacitor, but also through the power system behind it. So you can check for resident points. Again, I'll typically do snapshot measurements. And, if anything looks different than I would expect, I'll go back and do maybe some longer term measurements. But, the trending that you would do for long term measurements is what we say through one electrical cycle of load. So that means, again, nights and weekends, maybe one shift through a process to see the light and heavy loads of the situation, but that really dictates if you're going to do temporary or permanent measurements for harmonics.