Ask the experts: Understanding the impact of harmonics

Josh Kingsley:
All right, let's get right into the questions. Our first one is going to be coming from Jacob on LinkedIn. I'm going to phrase this the best way that I possibly can. "What are harmonics, and why are they bad?" Where do you start with harmonic analysis to know if you have to worry about harmonics if you're putting in a VFD? Dan, how would you answer that?

Dan Carnovale:
Josh, great way to start, and Jacob thanks for the question. Where do you start with harmonics? First of all, what do we know about harmonics? Again, distortion on the voltage and current waveform, where do they come from? We're going to actually take a look at where harmonics come from. So, let me just draw it out here, I love to have my little drawing tablet here. What happens is that people put in motors, and they decide to switch over and put in a variable frequency drive, or they put in other loads like fluorescent lighting, or LED lighting or whatever. Then, all of a sudden harmonics show up in their system.

So, where they come from? It is actually from the load. So, let's take a look at an example with the drive. We have a motor that may be 100 horsepower motor, and we have some harmonics that are generated by the drive. I'll come back to that in a minute. We're going say those harmonics come out of the drive, or out of the electronic load, and they flow back up through the system. As all currents do, currents want to flow the path of least impedance, so they flow back up through the system and through the transformer.

Now, what's really interesting about harmonics is it seems overly complicated, but it's really just Ohm's Law - voltage equals current times impedance. When we start without the drive, we have an equation that looks like this. Voltage distortion equals current distortion times harmonic impedance. Basically, for the first scenario where we don't have a drive, voltage distortion equals zero because current distortion equals zero. But, as we add more and more current distortion here, in other words, let's say we go from 10 amps, maybe with one drive to 100 amps with multiple drives of different frequencies, fifth harmonic, and again, we'll come back to a little bit of that, we add more and more voltage distortion.

So again, we start out with a relatively clean sine wave in terms of voltage, we draw a current that doesn't look like the voltage, we call that nonlinear load. That nonlinear load, that current, doesn't look like the voltage. That current pushing back up through the system, then actually causes voltage distortion. Voltage distortion comes from current distortion, the more current distortion we have, the more voltage distortion we get. Again, great question to start with, because harmonic currents come from the loads, harmonic voltage distortion ends up being part of the end result of that.

The next step is how do you figure out what the solutions are, how do you model the system to determine what's the best solution for it? And I'll tell you one little secret - just because you have a drive and you have some harmonics in the system, it doesn't always mean you have a problem. So, the problems result from current distortion causing heating and things through the system, which again, we'll address in other questions, maybe, but voltage distortion causing this operation. We look at this from a problem and solution standpoint, and that's really a good starting point. So Jake, thanks for the question.

Josh Kingsley:
And, also big shout out to Vien and Hamid, who were part of the chat that I referenced at the beginning, who pretty much asked the same questions to help define harmonics. 

Josh Kingsley:
Let's jump to our next one, which is coming from Alex on LinkedIn. What he's asking is fairly related. So, this should follow pretty well. "What are the key harmonic solutions for VFDs, and how do you decide which one to pick?"

Dan Carnovale:
Again, Alex, that's a great question. Because if you think about it, as I mentioned, and I'll go back to my habit to use the drawing tablet here, but if I put my drive on here, and I say that I have harmonics coming out of my drive. I'm simplifying this, obviously, it could be multiple drives, it could be multiple different harmonic loads, whatever they happen to be. But, as that current comes out of the drive, and it wants to go somewhere, we have to either find a new path for it to go through, that's a lower impedance, going back to that voltage distortion equals current distortion times harmonic impedance. Or. we can actually limit that current from coming out.

So, what are some of the solutions? I would start with DC chokes built into the drive or line reactor built into the drive, or an additional line reactor in front of the drive. And what that does, is basically constricts the current. I always think of it like water going through a hose. If I constrict that hose, you know we've all kind of pinched a hose together, get a higher pressure at other end, but we're really limiting the flow through there. By putting a drive here, with the line reactor or a DC choke built in, we take a current that looks something like this, and we end up having a current that looks more rounded off, and the harmonic pattern that is associated with that is much less. In other words, we have a lot less distortion.

If in the first case, we didn't have a reactor, we had 20 amps of 5th harmonic current, with the second case, we might have 10 amps of 5th harmonic current. That actually is probably the least expensive version of things. So, like our DG drives, for example, and our other variable frequency drives have DC chokes built right in, and 5% is the right number for that. As you look at other things, we talked about - If we try to take the current, make it go somewhere else, we could actually put a filter over here. So, a filter looks like a reactor and a capacitor in series.

At a particular frequency, let's say this 5th harmonic current, when this current gets to this point, current wants to divide and go according to impedance. If this was ideally zero ohms, guess what? Current, even if it's 100 amps times harmonic impedance, if this is zero ohms, then our voltage distortion goes back to zero. In reality, it's not going to be zero, but this is how harmonic current flows through the system. And most times, when you make a filter like this, half of the current or so goes this way, half goes this way. So, those are two things that you could do.

The other thing is to select, perhaps, the right drive. We talked about 18-pulse clean power type drives, or medium voltage drives, perhaps 24-pulse to keep that limit down to a reasonable level. Then you go all the way up through broadband filters in front of the drives, and active front ends, and so forth.  There's a lot of different solutions, and honestly, that's why we put the paper together, because it can be confusing, and it all comes back to a couple of things, really, the technical advantages that you have and the cost. And, we've put a bunch of frequently asked questions and made some videos as well. Those references are available for you as well.

Josh Kingsley:
So essentially, you're trying to diagnose what exactly the problem is, where it's coming from, what kind of budget you have to have for solutions, and then you're going to have a bevy of options?

Dan Carnovale:
Exactly, yes.

Josh Kingsley:
Perfect. Okay.

Dan Carnovale:
Josh, it's like being a power quality doctor. We use that analogy, because if you go to the doctor and you say, "Hey Doc, I have a problem." He doesn't automatically just throw a cast on your leg, he says, "Well, what are your symptoms?" And, if you look at the symptoms here, and I have a lot of heating and other things associated with that, I know I have to deal with the current. If I have a lot of voltage distortion, I know I have to deal with the voltage. We look at it as it is, applying the right solution according to what you need in your system, and sometimes doing nothing. The least expensive solution sometimes is the right solution. It just depends. Harmonics are the new normal that we have to deal with.

Josh Kingsley:
Got it. So, let's walk down that path a little bit. When a solution is correct to do nothing, paint a picture of how you determine that it's not as big of a problem as you think it is.

Dan Carnovale:
Yes. I'll start with another thought here like, so let's say somebody wants to convert their warehouse. They put a bunch of VFDs, originally running motors and a conveyor system, and all of a sudden, you put in one drive and two drives and three drives. You convert all these motors over to have VFDs. At what point do you have a problem? There's some rules of thumb that we go by, sometimes if you look at the kVA of this transformer, let's say 1000 kVA.

If you get to the point where about 20% of your load on that transformer is harmonic type load, then maybe you start to do some more formal analysis. It's kind of the back of the envelope thing. 20% of the full load of that, which is going to relate to both the voltage and current issues. If you look at this bus here, and you're looking at the voltage here, this is where harmonics are a self infecting problem. Right? So, the more you put down here, the more you distort the voltage on the bus. So, you go from a nice clean sine wave to something that's distorted.

And as you distort the bus and you determine that I'm now having problems with some of my loads in my system. Again, the one thing, Josh, about those harmonics is it's not as cut and dried as some other power issues. Some gray areas exist obviously, and for one situation, maybe an industrial plant, you can live with a lot of harmonics, maybe for a hospital with MRI machines, you can just have a little bit as an example.

Josh Kingsley:
So going back to that diagnosis, you need to know what your loads are doing and what the potential failure could be. A little bit of nuisance tripping every once in a while in an industrial facility might be dealt with, but an MRI ever tripping is probably a bad situation?

Dan Carnovale:
Yes, exactly.

Josh Kingsley:
Got it. You also mentioned that those were self-infecting problems where you can be harming your own power source that you have, is there such thing as a problem that doesn't infect yourself, but infect something else?

Dan Carnovale:
Well, that's actually where the IEEE standard comes in. I go back to a scenario where you have your transformer here, and you're feeding your loads, and then on the same system we have another utility customer, and you could literally be affecting those guys by your harmonics, and that's where the IEEE standard comes in at that point of common coupling, where they has to be mediators.

Couple of examples, Josh. I ran into a situation where there was a ski lift, and they had to drive on the ski lift. As they added more and more things to the top of this hill, more and more people were riding the ski lift, all of a sudden, the people in the really nice houses at the top of the hill with a ski lift, their lights were flickering. Tt was affecting them with the ski lift. So the answer is what? Turn off the ski lift and not have people go up and down the hill, or maybe put some filtering to absorb those harmonics or send them back to the load, because current always has to flow out of the load and then back to the load where voltage we can deal with in other ways.

Another scenario related to that was back when people were replacing fluorescent lights a lot. They replaced half of the fluorescent lights in a big commercial building, like 12 story building. And as they got about halfway through, those lights started to flicker. And. so now you're thinking, "Okay, I got halfway through, all of my lights that I just put in, even I'm saving money with them from an efficiency standpoint, they're starting to flicker. Now I have another problem." And that's where, again, you have to start to deal with it. So, it could be you affecting yourself, you affecting your neighbor, or in a big scale things, like that steel mill affecting the hospital on a big scale and utilities to deal with it.

Josh Kingsley:
And the utility is probably really interested in playing mediator in situations like a ski lift and people that are living on top of the mountain. 

Josh Kingsley:
Okay, let's get into our next question. We're going back to LinkedIn, and we're specifically going to go to the chat that we had, Brian Souter asked a question, I'm going to try and paraphrase this as much as I possibly can.

Josh Kingsley:
The first thing he's asking is, “What's the relationship between the existence of harmonics and a three-phase system and apparent neutral currents?” To throw a couple of the factors in that he's throwing out, he's got the equipment properly grounded and he's seeing high neutral currents of both the medium voltage switchgear and the downstream 480 VAC side. Even though the plant technically does not have a four-wire system, which would be the three phase plus the neutral, but he's seeing the high neutral currents coming in at his SEL-51 relays and Siemens wLs 480 volts switchgear.

And, the problem has been bad enough that the breakers are causing erroneous ground trip fault. So, Siemens hadn't put in EMI filters, I know there's a ton of stuff going on there, if you can help unpack what we can do to talk about this and maybe comment on the Siemens proposed solution and what your thoughts are on that.

Dan Carnovale:
Okay. Yes, there's a lot in there. Brian, I'll reach out to you after this and give you more specifics, but I'm going to address your question the best I can, just to give you a little bit of an overview. If we look at a three-phase system, and we have three phases going out, and a neutral. What your referencing was, if I have loads on each phase connected to neutral, you're talking about zero sequence. If we have, let's say, phase A, 10 amps, 10 amps on phase B, 10 amps on phase C, if that's 60 hertz current, because it's all displaced by 120 degrees, we have 10 amps, plus 10 amps, plus 10 amps. In the neutral here, we get zero.

What's interesting is you mentioned zero sequence. If I have, let's say, five amps, or 10 amps, or whatever of 3rd harmonic, or zero sequence, that current is additive on the neutral. When it gets back to this point, it's 1, 2, 3, and it adds up to be 15 amps in this case. That's why people historically have doubled neutrals. Now, when you look at how things go, what you're referencing is, you're talking about a medium voltage system and a 480 volt system that doesn't have a neutral. In this case, we're not talking about those type of low frequency. In this case, 3rd harmonic, for example, zero sequence harmonics, what you're talking about, I think, is some of the higher frequency harmonics.

Dan Carnovale:
And those can come from sometimes the outputs of VFDs. The pulse-width modulated output, we usually don't think about harmonics from coming from the output of the drive, the drive is a rectifier, DC bus with a capacitor and the inverter, and the pulse-width modulation on this output, that high frequency stuff, sometimes I've seen, if the input current looks like that double-hump current that I drew earlier, the output current can have some 39th harmonic, like really high order 3rd harmonics.

So again, I'll get back to you on some of the details of that, but where I've seen this in larger industrials is the influence of those currents coming through, not the conductors themselves, because there are no conductors near situation but the capacitance to ground, they can follow that capacitive path, especially the higher frequencies. And that's why the EMI filter was recommended likely. If it took care of the problem, that would make sense because you're actually putting a conductor in parallel with that capacitance to ground. So, very complicated situation, but I'll definitely reach out to you and talk through the details of the answers to that.

Dan Carnovale:
By the way, before I forget, sorry, Josh, one other thing that does affect 480 volt system is this high resistance grounded systems, you can see that on the voltage measurement, on HRG system. If you measure current, a lot of times you can filter that out or not see that and just look at the 60 hertz part. But, if you're looking at voltage to ground, sometimes you will see that pick up.

Josh Kingsley:
Got it. Thank you, Dan, and hopefully that gets us down the path. Brian, do us all a favor and check your email. Our Eaton electrical services and systems team is going to be reaching out to you to discuss this further. They'll be able to set up a time. I mean, this is the Ask the Experts session, and we truly have an expert on here, Dan. But we also have experts outside that can help, and that's what they're there. So be looking for a note from them.

All right, the questions are coming in fast and furious, but the producers are in my ear as normal reminding me to tell you that this episode of Ask the Expert is brought to you by the Power XL EGP 18-pulse VFDs. Visit Eaton.com/EGP to learn more about the product. 

Josh Kingsley:
Dan, let's jump back into the questions. It looks like this one's coming in, we're going to jump over to Facebook, give some love to them. We've touched on this a little bit, but can we get a little more specific into, "Is there a way to select a VFD so you can avoid harmonic problems?"

Dan Carnovale:
Yeah, that's a really good question. Whether it's a drive or really any electronic load, let's say, again, LED lights, we say that those electronic loads have characteristic harmonics. So, I've been drawing this current that looks like this, and I'm going to back up a little bit and explain that a little bit more. This current has a 60 hertz component, and it has other frequencies that added together kind of a superposition add up to be that waveform. It might have a 5th harmonic and it might have a 7th harmonic. And you say, "Well, how do you know which frequencies those are? So, 5th harmonic is five times 60,300 hertz.

How do you know what that waveform would have? There's a nice easy calculation here. It's NP plus or minus one. When we have 6-pulse drive, that's six, N is just an integer 1, 2, 3, 4,5. One times six plus or minus one, which would give us 5th and 7th, and then two times six plus minus one, which gives us 11th, and 13th, and three times, so 17th, and 19th. So, if I were to take that same equation and now I have an 18-pulse drive. What would be my predominant harmonic? It would almost look like I have a nice clean sine wave, and I would have 18 plus or minus, 17th and 19th.

Then two times 18 plus minus one, so 35th, you're stretching my math here, Josh. 35th to 37th. But anyway, when I do all that, and I look at the frequency spectrum, so if this is magnitude, and we'll call the fundamental 100%. The fundamental being 60 hertz, I would have 5th and 7th and 11th, and 13th, and 17th and 19th. What you notice is, those frequencies as they go up in frequency, the magnitude of the harmonic goes down. So this is 5th, 7th, 11th, 13th, 17th, 19th, and so forth.

Now, when I draw the spectrum for just the 18-pulse drive, I still have my fundamental. By the way, what's the fundamental doing? That's what's doing the work. The harmonics are extra currents that are part of the process of converting from AC to DC, and we have to live with them. So, if you have less harmonics to start with, 17th and 19th, all these other ones go away. That's why we look at certain applications, and we say, at some point, I might want to go with 18-pulse drive, because even though my cost's a little bit more out of the gate, I might not have to deal with the harmonics after the fact.

So, we look at, where can I take advantage of higher pulse numbers and so forth? And, I can do that within the individual drive itself, and we're using phase shifting transformers as an example.

Josh Kingsley:
Got it. But Dan, I want to dig into something a little bit more. If you pick the right drive, and equipment and gear right out of the gate, you can use less corrective equipment later. We're really digging into the question of, is this a retrofit, or is this a new build? Can you expand a little bit more about if you would diagnose the situation differently one way or the other, and what your approach for each of those would be?

Dan Carnovale:
Josh, that is a good follow up. Because what I find is, a lot of times designers come to me and say, "Am I going to have a problem with this? This goes back to those 20% more of my transformer capacity, am I going to have problem?" I mean, there's no guarantees with harmonics in terms of magnitudes and certain systems. You really have to look at it from a system level. But, when you start to think about the solutions, if you pick the right drive out of the gate, you could end up having less filtering and so forth later.

So, one of the things we addressed in the paper was around the individual drive approach versus system approach. That really does go into those kinds of things. Again, it's a technical and economic choice. I'll show you an example here. I'm running this as a simplified case, but if I have a bunch of drives here, and I'm creating some harmonic distortion, and I start to mess up my voltage distortion here, and maybe I want to transfer over to a generator, what we've seen in the past is, you have a generator here, and when you're running on the normal source, this utility source transformer is 5.75%, or something like that, generator impedance coming back looking into that is 18%.

So, what am I going to do to take care of the harmonic issue? Well, maybe when I'm running on my normal source, it looks like that, but when I'm running on my generator source, my voltage distortion really gets crazy. And, the reason is, because, remember going back to Ohms Law, voltage distortion equals current distortion times harmonic impedance. If the harmonic impedance here is three times on the generator as it is on the transformer, guess what? If I put a single solution in here, then all the harmonic current goes here, no matter which source I'm on.

So sometimes the answer is corrected on the system level. Again, this is drawn as one drive, but this could very well be 10 MCC full drives. Looking at it from that standpoint, the other thing is, let's say, I didn't want to have to deal with this after the fact, if I make this 18-pulse, 90% of the time, I don't have to think about it anymore. Sometimes we do passive filters, sometimes we do active filters. Active filters look at the current going through the system, and an active filter is, like your cell phone has noise cancellation. Basically, it injects equal and opposite frequencies.

So if the harmonics coming out of the driver going this way, the active filter sends them that way. And again, since current flows in a loop that goes out and back, and as far as the bus goes, the voltage goes back to normal. You have lots of different things to think about related to that, but it can be an individual drive solution, or a system solution. Let me just add one more thing to that. Let's say this is a water treatment plant, let's say you had 10 drives across here. I'm going to start over here. Let's say you have your source, and you have 10 drives, but one of the things you could do is you could put a filter on every single drive. I won't draw them all.

But if you end up with a filter on every drive, so you have a drive plus a harmonic filter, that adds a little bit of cost to every drive, but it might be the right way to do it. Now, what if you had five drives that are running all the time and five redundant ones? Well, you basically have five filters that are sitting there with nothing to do half the time. Now that might be where a system level like an active filter. Even if an active filter costs more to begin with, you might put it on a system level and still be able to deal with the problems on many drives instead of one drive. I don't know, Josh, if that helps to clarify a little bit better.

Josh Kingsley:
The real question is, do you want to put the active filter in, put the extra cost up front, or do you want to do it in service calls, and you're adding an active filter to somewhere between five and 10 drives if you've got redundant in that particular situation?

Dan Carnovale:
Yes.

Josh Kingsley:
Let me ask one more quick follow up to you. You mentioned having a main power source and a generator power source. Let's think of a scenario where you decide sometime down the road after the building was built originally, that you need to have a backup generator, what are the things that you're thinking about? It seems like you mentioned that the impedance could be different based on the main power source versus the generator. Talk about how you would diagnose that up front as you're doing the generator retrofit.

Dan Carnovale:
What I run into a lot Josh is, if you do have your generator connected in here, and again, this would be some sort of transfer scheme. I'll just draw it as if they're both connected in parallel, but you'd have a generator here again. Let's say this is simplified 6%, 18%. And then you have transfer scheme with breakers or contactors or whatever. And, then you feed your loads down here. If you had no generator, but you added one later, and you had a whole bunch of harmonic producing loads, and again, we're going to call harmonic producing loads the new normal, anything electronic, but specific to today, we're talking about drives.

But you have all these VFDs, or other things that are adding harmonics to the system, and they want to flow back to the path of least impedance. If the generator is offline, that current's going to flow back up to the source. And, by the way, it's going to flow out, say, for example, on A and then back on B and C. A lot of people think harmonics flow to ground, but they don't, they flow out on the phase conductors, and back on the other phases, or the neutral. But, anyway, as we send the currents out, and then we switch over to the generator, so we take this out, we run over here to the generator, sometimes we'll have problems, and like I said, maybe we go from a voltage distortion that looks somewhat bad to something that looks horrible.

And when I say horrible, I mean, it could get really ugly, pretty fast, because again, you got three times the impedance. And what's that going to cause? You might have what we call multiple zero crossings. I'll explain a couple of scenarios with that. But, you could have a bad voltage distortion. By cleaning up the harmonics here, this goes back to normal. And what's interesting, Josh, you kind of made me think of something. What do people usually test their generator with? They'll bring in a resistive load bank, and they'll test it with a resistor. Well, of course, if you have a linear voltage, and you have a linear current, you have no distortion, it's going to look fine.

Then, they have a real scenario where the power goes out and they're running on the generator and they have a problem. So, this is something that I have run into a lot and people need to deal with. What I mentioned over here with multiple zero crossings is I had an example where somebody was using a welding line in an auto plant. Stamping presses were causing enough voltage distortion on the system to cause the welder instead of being 20 cycles long, the welder was eight cycles, they were counting. A nice way to keep track of time, by the way, is if you're looking at a sine wave, every time it crosses zero here, and then back here again, that happens to be 16.67 milliseconds.

Well, if it crossed there, and here, and maybe over here again, now every time it crosses zero, it thinks that 16.67 milliseconds, so the welding line that was supposed to be 20 times that was only getting eight times that. So again, harmonic problems are a little bit difficult to track down after you have this issue, because it's not always just something overheats and burns up. It's something over time heats and burns up or dismiss operation thing is a little bit intermittent, sometimes it does it, sometimes it doesn't. So again, the things with the generators, and especially with voltage distortion, it gets pretty bad, can be difficult to track down. It, again, follows that simple rule of electricity, which is Ohms Law.

Josh Kingsley:
And, your welds situation that you bring up, there's a strong possibility that you don't find out you had a bad weld until a little ways down the path when whatever you were welding was actually installed, and the customer is coming back to you saying, "What are you doing?"

Dan Carnovale:
Well, there's actually a recall on an actual part on a car, I won't tell you specifics. That's one of those things, you're exactly right. You wouldn't know it, we call that product quality as a result of power quality. Right? That can happen in a lot of different scenarios.

Josh Kingsley:
Got it.

Josh Kingsley:
Okay, so I'm going to jump into another Facebook question here. This one's from Preston, and it says, "We have a facility with drives on almost everything. The incoming transformer is a 2500 kVA ... It says YYY, I'm thinking they're meaning Y delta. Lots of harmonics on utility, but they disappear when transferred to the generator.” So, I mean, it sounds like pretty similar scenario to what we were talking about using generators. Let me know if you caught everything that I said.

Dan Carnovale:
So, 22.47, 2500 kVA, lots of harmonics, everything on here is on drives. They're running everything, and he says they disappear. The harmonics disappear when they go in generator. I don't know if he means specifically, maybe you get to have a follow up question. So, the current, again, has to flow somewhere. The current's going to typically flow out and back on the transformer out to all the different drives. If we transfer to the generator, we would have the same thing. If the generator's running in parallel, likely a lot of the harmonics will still go back to the utility as that's a lower impedance in terms of current.

Now, as far as voltage goes, what he may be saying is, it runs a lot better on the generator, the opposite could be true in terms of impedance, remember, we said this z is 5.75. If for some reason he had a very large generator here, it's a comparable thing. If this was, let's just exaggerate it, say 10MVA, 10MVA would be four times bigger than that. Even though the impedance is bigger in terms of 18%, the effective impedance would be 2500 divided by 10,000 times 18. You end up with something that's going to be less than the original impedance. I'll follow up with you. Did you say Preston? Josh, I think I can follow up with a specific around that question, or if he wants to follow up with an answer. But generally, harmonic currents have to flow, that's just a natural thing of the system.

Now, the other thing that could happen is let's say this generator is very small. This is actually an interesting phenomenon that happens, the current from the system could look like this when you look at the harmonics on the drives, and then when it's run on the generator, it has really, really high impedance, it actually could look like this. He may actually be squashing out his harmonics by having such a high impedance here, that this looks like a line reactor in front of all the drives. Lots of things, it's a very dynamic situation can go on but that would be a couple of guesses and then I can follow back up with Preston.

Josh Kingsley:
Yes, diagnose, diagnose, diagnose, and don't forget the Eaton electrical services and systems team is also available to help talk through some of this, and keep in mind we have the white paper coming as well.

Dan Carnovale:
Yeah, that's a really good point. It sounds like he's been taking some measurements. So, measurements are really the key. Diagnosing that with measurements is really important.

Josh Kingsley:
Got it.

Dan Carnovale:
It goes back to the question you asked me earlier, which is how do you know new system versus old system. I think I forgot to answer that part of it, which is, on a new system, you have to model, on an old system you can measure. It's a lot easier on an existing system to figure out what's going on versus on a new system. But you can do it on both.

Josh Kingsley:
Got it. Did have a follow up it is YY. It looks like he answered most of those questions. If he does come back with anything else, you can pop that up to the top as well. 

Josh Kingsley:
Next one I'm going to go to is LinkedIn and this is Ebrahim on LinkedIn. His question is, “What is the impact of the other harmonic into the system. An example is the first, second and third harmonic, you were talking about being tested on the mass.” So, let's go back to that again.

Dan Carnovale:
Okay. Let's do this, Ebrahim.  If we look at the fundamental 60 hertz, then we look at, let's say, the 2nd harmonic, which is going to be twice as fast. And then we look at the 3rd harmonic, all those. I don't know if anybody's done the power systems thing, but there's the thing called sequence, positive negative zero sequence. Positive sequence is what we think about 460 hertz. Positive sequence is, A followed by B followed by C. They are 120 degrees apart; they rotate this way. Phase A is there, phase B is here, and phase C is here.

Those three phases added up, again, we say would add up to zero, but that's a three-phase system, positive sequence. There's a neat kind of thing that happens when you go 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, will just go there, it goes positive, negative, zero, positive, negative, zero, positive, negative, zero, positive, et cetera. Positive sequence harmonics, which this one, this one, this one, this one, they act like the 60 hertz stuff.

Negative sequence harmonics are kind of like A, B, C, we talk about, let's say, for a motor, we have a motor that spins, and the 60 hertz is making it spin this way. The negative sequence harmonics make the motor want to spin the opposite way. Let's say you have a system, and I have enough voltage distortion on my bus here, even across the line motor might see a voltage that looks like that, which means it's going to draw current into it. That might be, say, for example, negative harmonic, which is the 5th harmonic.

So I'm creating maybe with some VFDs over here, harmonic currents that are going out, and I'm screwing up my bus voltage, and then that negative sequence, harmonic current comes into this motor, makes it want to spin backwards. It's like dragging your foot all the way the floor and the gas pedal, and tapping your brakes a little bit, it's very inefficient. The forward motion doesn't allow the motor to spin as fast as it would want, and it makes it less efficient, heats it up, you can have premature damage and failure of motors. Negative sequence harmonics work like that.

Zero sequence harmonics, we addressed earlier, and they add up in the neutrals. Now you can have overheating in the neutrals to the point where you can have double the current the neutral as you do on the phases. Going back to our earlier example, phase A, B, C neutral, I could have 10 amps of say 60 hertz, 10 amps, we'll go with 10 amps of each in this case. 10 amps, 10 amps, 10 amps, 10 amps, 60 and 10 amps of, let's say, 3rd harmonic, or any of the zero sequence, and usually we talk about the odd multiples of the 3rd. So 3rd, 9th, 6th isn't usually very common. But anyway, what we would have is on the neutral out here, we would have zero amps of 60 hertz, as we mentioned before, 60 hertz, A, B, C, adds up to zero.

But the 3rd harmonics, A, B, C. 10 plus 10 plus 10, is going to add up to 30 amps on the neutral. Now, if you look at what this phase wire is going to be rated here, it's going to have 10 amps plus 10 amps, but you do that in a RMS fashion. It's 10 squared plus 10 squared, and that's going to be 14.1 amps. That's square root by the way. 10 squared plus 10 squared, it's 14 amps on phase A, that could be a 20 amp wire, 12 gauge wire. If I use 12 gauge wire on my neutral, it's only rated for 20 amps, I have 30 amps, I'm going to burn up my neutral, that's why they double the neutrals.

Zero sequence harmonics add up on the neutrals, and we talked about it earlier with the question about the medium voltage and 480 voltage systems where maybe high frequency ones, we mentioned 39th harmonics. If you keep expanding this list out, you get to 39th, it's going to be zero sequence. Positive sequence harmonics just add extra current to the system, it's like adding extra load on your wires and it can overheat things. Negative sequence tends to make things want to go backwards. Zero sequence tends to add up again. By the way, these also rotate as vectors or phasers, but they rotate together. When you add them up on the neutral, all three of them add up exactly in phase with each other, if I could draw correctly. What you end up with is three times the current of the neutral.

That whole discussion really goes to the fact that, each of the harmonics or characteristic harmonics have different patterns to them. And, if you look at VFDs, we say 6-pulse, we already said 5th, 7th, 11th, 13th. 12-pulse, we could figure that out. 18-pulse, we could figure that out. But, like fluorescent lights or LEDs, or maybe a three-phase rectifier for vehicle charging, for example, that might have 5th, 7th, whatever. This might have 3rd, 5th, 7th. LEDs might have 3rd, 5th, 7th. But, they might be a different spectrum, a different pattern. The nice thing about harmonics is they're pretty predictable, in terms of the characteristic harmonics, meeting the signature associated with each type harmonic. I hope that helps answer the question a little bit better.

Josh Kingsley:
Got it. 

Josh Kingsley:
Next question I'm going to jump to, this is on LinkedIn, and this one comes from Ray. I know that you mentioned the regulation earlier, but let's talk, IEEE-519. So, maybe just get into what is it? How do you use it? And why does it matter?

Dan Carnovale:
Did you say Ray, Josh?

Josh Kingsley:
Correct, Ray.

Dan Carnovale:
Ray, that's a great question. 519 is an important topic around harmonics. When we think about the voltage and current distortion, either our loads creator or neighbors loads creator, the current distortion is going to cause heating and other things in our system. We have to think about that, and how we're going to affect ourselves. If that current goes back out onto the power system, it may affect our neighbors a little bit through the transformers that we use in common with utility.

But, if we screw up the voltage at the utility level, now we have to deal with that, it's going to be more complicated. Again, the utility is the mediator, will call them, and we say that that point where everything comes together is the point of common coupling, the point where one customer shares the source for utility with another customer. And, the 519 standard, actually is interesting, I have it, you can look it up. But, it's about a 20 page document. When it first came out in 1982, roughly, they added a whole bunch of information into it. For about 10 years, they worked on reviving and redesigning a little bit.

In 1992, they redid it again, and it's one of those things that you think should be updated pretty frequently. But, it wasn't updated for a long time. Until 2014 was the last revision, and that's the one we're using today. But they went from about 100 page document down to about 20 pages. And, they try to be very prescriptive in terms of what they're doing with that. So, if you have, again, a single utility source, and let's say multiple customers or multiple transformers that are fed off the same source, sometimes you end up with basically what would be the harmonics coming from one customer affecting, let's say in a VFD scenario, or something like that.

Josh Kingsley:
Real quick, can we pull up his drawing on the screen as well? Yes, sorry. Go on.

Dan Carnovale:
Yes, so as these harmonics come out of this system here, again. There are always transformers back to the system. If this current is affecting this transformer, it might actually cause damage to that, and this customer could see it. This is customer one and customer two. There's tables in the IEEE 519 standard, that basically define the current distortion that you're allowed to push up through your system and the voltage distortion that you're allowed to create at the common point, at the point of common coupling. Sometimes it's on the primary of the transformer, sometimes it's on the secondary, it depends.

But when you think about it, it's all related more to voltage than current. And people say, "Well, why would you say that?" It really becomes this. We use a term called ITHD, what's ITHD? It’s total harmonic distortion. That's the sum of all the harmonics, second squared plus third squared plus four squared, whatever, divided by the fundamental, the 60 hertz, that's THD, total harmonic distortion. In any given time, you can calculate that. With the IEEE-519 standard, they change that to it ITDD, total demand distortion, what that means is, everything on the top is relatively the same, except you're using worst case scenario, demand. Then this is the demand situation when you're at your peak.

So you might have a really light distortion, or let's say, in terms of amps, overnight, but the THD could be very high. But, your TDD is what's important. So how badly are you screwing up the current on the system, according to the size of this system? And, remember the 20% reference I gave earlier? Let's say your load on your transformer or this transformer is 50% of its capacity. They're going to let you produce less. The ITTD limit is based on your short circuit kVA. The short circuit amps, I guess. It's based on your load, and the ratio of those two determines how much of a level will allow you to, and most of the time, it's between 5% and 20% distortion.

When you look at TDD values in the IEEE standard, that really says, I can't screw it up more than five to 20% in that range. Now, if you look at an 18-pulse drive, most of the time, you're going to be below that 5%. That's where going back, Josh, to the original questions that we talked about, you don't even have to deal with it at that point, because it already "complies" with that. The way that the standard is written is you only comply at the point of common coupling. So, people sometimes use it down in that system, and we can talk about that more, if you want.

But, voltage distortion levels, the part that is really referring to more importantly, is VTHD here. Usually, that goes by voltage level. If I'm at 480, for example, that's the one change that was made from 1992 to 2014, and it went from 5% THD to 8% THD. What they found was, you didn't have to be quite as clean on most power systems at 480, so you could allow a little bit more voltage distortion.

Again, there's no magic number, so as soon as you get to 8.2% distortion, everything is going to start failing. But, those are guidelines, and the key word there is recommended practice. I don't see the utilities going out, jumping on people and say, "Hey, you have to do this proactively." But when there is a problem like the ski lift, they can be a little bit more proactive about it, trying to fix those problems.

Josh Kingsley:
Yes, I think that definitely gets into the IEEE thing, what I noticed more than anything is I always had the impression that regulations like that changed on a very frequent basis. So, it's nice to know that there's been a level of consistency, and they've also tried to bring clarity by making the actual publication a little bit shorter. 

Josh Kingsley:
So, I'm going to get into two questions here to make sure that we hit as many as possible. So Ashish from LinkedIn asks, "What's an active filter, and how can we address these harmonics in real time?" And then Vinay on LinkedIn also asked, "What are the losses that occur in a filter?" I think we're talking filtration.

Dan Carnovale:
Okay. I'll start over here with my drawings. What I'll do is, let's talk about an active filter again first. Let's say you have, and I'm just going to draw it now as simplified form, just a constant current source. No matter what your harmonic source is out here, we're going to say there's a certain amount of harmonics coming out of there. And, that could be a combination of things. It could be, let's say, it's a 6-pulse drive. So maybe I have 100 amps of 5th harmonic current, I could have 30 amps of 7th harmonic current, and I could have 10 amps of 11th harmonic current, and I can have five amps as an example of 13th harmonic current.

Current, as we mentioned, is going to flow out and flow back, and one of the things that I think is very important to understand is current has to flow in a loop, so no matter what, it's going to flow in a loop, so if current is created, meaning we allow that drive or that load to create the current, it's going to flow in the loop and come back. Where does current want to flow? It wants to flow the path of least impedance, and in this case, it only has one path. So it flows out on phase A back on B and C for example.

Dan Carnovale:
If I put a standard harmonic filter, and I mentioned this earlier, and it glossed over a little bit. But, basically, what you do is you tune this filter. If you look at a frequency spectrum of that, you look at basically something that looks like this. If this is impedance, and this is frequency, you have a point where that filter looks almost like very, very close to zero. Now, it's not really zero because there's a little bit of resistance in here. Let's just address that in terms of losses. If I push current through that path, first of all, only the current that's around the frequency here wants to flow through that path, because as you look at the impedance, impedance quickly is high here and high here. So the 5th harmonic filter is here, 7th is here, 11th is here, you're going to get a decent amount of 5th through it.

So maybe instead of 100 amps going there, maybe a 50 amps here, and 50 amps there. Then, you would determine the losses and that which is going to be minimal, because that resistance is fairly small. But, you're not going to get rid of much of the 7th, much of the 11th or 13th. They're going to want to still flow back in the system. The benefit, though, is the 5th is the biggest one. We can get rid of that in a fairly simple way. But, this takes a little bit more design, and you have to make sure that it's not going to be overloaded in the future.

And honestly, the main benefit of harmonic filter, believe it or not, is not harmonic filtering, power factor correction. You size it usually for power factor, and then you get the benefit of harmonics. We could talk about that more, but this is really a key. Now, if I don't have this kind of filter, let's just take this out for a second, I put an active harmonic filter in here, active filter. Okay? Now, what does an active filter do? An active filter senses what's the current coming through the system, and a lot of times it will sense it here with CTs on that main incoming line.

And, then what it'll do is it'll inject equal and opposite currents this way. So, if current is flowing this way, in this way, say out on phase A back on B, and C, out on phase A back on B and C, where these two meet, again. It's like noise cancellation in my earphones here, or in your cell phone. It basically cancels it out. This 5th harmonic here, if it's 100 amps going this way, it literally could put 100 amps going this way, and cancel it out. What you see back here, then, is zero amps. The really nice thing about that is, you could do that at multiple different frequencies. Essentially, you could get it for all these frequencies.

Again, if you think about the benefit of that, it's huge. But, as we talked about the paper, this is also the most expensive solution. You think about, if I had never gone back to the example, we had 10 drives, and five of them were running all the time and five of them weren't, you don't know which five we're going to run. Maybe an active filter is still cheaper than putting a single filter or multiple filters on the system. That gives an overview of how active filters work. But, the other part of that addressing losses are what does this thing do? It injects current, the way it injects current is to a power converter, and this thing has losses.

Let's say you measure, not current up here, but let's say you measure power kilowatts. And, let's say your load was 150 kilowatts, and you have all your harmonic current on there, and you turn off your active filter at 150 kilowatts, and then you turn on your active filter, your kilowatts actually will go up a little bit. The reason they go up a little bit is because the current that you're taking out of here is not really real power like kilowatts, it's similar to reactive power in the sense that it's distortion power, and what it turns out to be is as that currents going through the system and getting corrected by this one, this actually might have, let's say, two to five kilowatts of losses.

Dan Carnovale:
My power that I measure on my system with that active filter on is 150, say 155 kilowatts with that on. So, your current originally looked like this in terms of very distorted, and what you're going to end up with is a really nice clean sine wave and power factor correct and everything, but you're going to pay for with a little bit of losses. Get rid of harmonics, add a little bit of losses, but the benefit is there. It's a system level thing, which is one of the things we address in the paper.

Josh Kingsley:
Got it. 

Josh Kingsley:
Okay, next question that I want to ask, this one comes from Bob on LinkedIn. And, the question is regarding high order harmonics. Do you see any specific high order harmonics that tend to be more damaging than others?

Dan Carnovale: 
I mean, one of those things is system specific, I think, going back to the other question that we talked about, because of capacitors. So all power systems have inductance and capacitance. When you do have higher order harmonics dependent on those capacitive paths, think about the impedance of the capacitor. When I say capacitive paths, I'm talking through air, power conductors and capacitance to ground or capacitance to other cables. When you think about a capacitor, it's one over two pi times the frequency times the capacitance. If you have a fixed capacitance that says the frequency goes up, the impedance of that path goes down.

So, what's going to happen is, high frequency currents can follow those paths. Now, if it's zero sequence, those are the ones that tend to find those ground paths that we talked about earlier. 39th or 45th, or whatever the odd multiples of the third way up there. But, as you look at like, let's say 17th and 19th harmonic, usually those magnitudes are so small, that they're not really as effective causing damage or problems with equipment on the system as nearly as much as the lower harmonics like 5th, 7th and 11th and 13th, or 3rd harmonic line, single phase system.

Josh Kingsley:
All right, it looks like we have one more question. Someone asked one that I think is going to be fairly quick. This is a question from LinkedIn: "What's the difference between THD and TDD?"

Dan Carnovale:
I'll draw this out on a different sheet here. Again, the TDD really gets addressed in the 519 standard. THD is going to be like all of your harmonic squared divided by the fundamentals. Again, you could do this for voltage and current. For TDD, we're only talking about current. So, let's say we're going to use an example where we have fundamental 100 amps, 3rd harmonic, 30 amps, 5th harmonic, say, is 12 amps, 7th harmonic will be, say, four amps and 11th harmonic will be two amps or whatever. This is fundamental, 3rd, 5th, 7th, say 11th. Okay.

The fundamental here is 100 amps. If I take 30 squared, so I get 900, and I take plus 12 squared, I get 144. I take four squared, 16, and I take two squared, I get four. What you'll notice right away is, when you square these amps, the biggest ones really become the predominant one. I don't have my calculator, but if I calculate this out, I would guess that you're going to get somewhere in a 30. It's not really that relevant for the exact numbers here. But, I'm going to say, once you take this, add it together, and you take the square root, so you get 33. Sorry, 33 divided by 100. So we multiply that by 100%, and we get 33% THD. Okay?

Now, remember, earlier I mentioned, if you were running overnight, and you had like a light load condition, you might have a higher percent THD. What can happen is, let's say the fundamental overnight goes to 50 amps. But, mostly the rest of it stays the same. So THD, you could have 33 divided by 50, and now guess what my THD is, 66%. So, you would say, is this number worse than this number? And, this is where THD can get you in trouble. It's really the same, the same amount of harmonic current is flowing through my power system, and heating up my equipment and whatever in actual amps. And actually, amps are the most important part. Okay?

What TDD tries to attempt is to try to make sure that you're using the maximum demand number so that you're not artificially giving a THD that's way high. Most of the time, let's say your maximum demand was 200 amps. Now you're actual THD would be 33 divided by 200. And that could be roughly 16.5% TDD. Going through all that, what I mentioned earlier was the 519 standard says I could never go above 20%, this number would be within the 20%, and we would have to carefully look at that, see if it makes sense. This number is above, this number is above, but honestly, they're all the same amount of harmonic current.