Transcript
Introduction to Residential Interharmonics
Good afternoon, everyone, and welcome to today’s white paper webinar. Today, we’re talking about a very interesting case here of interharmonics in a residential situation.
Interharmonics are kind of a subspecialty of power quality that you don’t see interharmonic issues very often, and when you do, they’re typically industrial-type loads. Traditionally, interharmonics weren’t really present in residential loads, but that’s starting to change with the advent of high-power electronics showing up in even low-power consumer electronics.
And here we have an example of a space heater, and this is a plug-in space heater. This was the space heater that you see here. The claim to fame of the space heater was that it was energy efficient because it had a continuously variable heat output. So instead of a three-position switch that gave you 600 watts, 900 watts, 1200 watts, this heater could continuously vary its heat output. And so it was supposedly more energy efficient because of that. But the way it does this produced flicker and interharmonics, and the mechanism of where interharmonics causes trouble is traditionally with causing excessive flicker.
What Are Interharmonics?
Before I show you the waveforms of this heater and go into details about why it was doing this, let me back up a little bit and just make sure we all understand what interharmonics are.
Remember, harmonics and interharmonics are all frequency components that are sine waves, and a sine wave has an amplitude, a frequency, and a phase angle. And here the frequency is what we’re talking about. Harmonics are sine waves of voltage or current whose frequency is an exact multiple of 60. So the third harmonic, for example, is a sine wave at 180 hertz. The amplitude and phase angle could be anything but the frequency is an exact multiple of 60. That makes it a harmonic.
The reason why harmonics are so useful is because of the Fourier theorem, which says that any waveform distortion pattern can be represented as a combination of harmonics, assuming it is periodic, repeats itself.
Harmonic Decomposition Example
Here in the upper left we have one channel of voltage, voltage in red, current in blue. The red is a nice 60 hertz sine wave from the utility. The blue is the load. This is a single-phase electronic load. It only draws current near the peak of the voltage waveform. So this current is not a sine wave, but the distortion is periodic. It’s the same shape every cycle, so it repeats itself on a 60 hertz basis.
And because it has that property, the waveform is distorted but in the same pattern every cycle, that means you can represent it as a combination of harmonics. So here we have the harmonic breakdown. It’s got some 60 hertz first harmonic. It has 1.2 amps at the third harmonic. It has some fifth harmonic. And if you add these harmonics together point by point, you eventually re-create the original distortion.
So because this distortion is periodic, it’s the same every cycle, only harmonics are present. There are no frequencies that are not multiples of 60.
Non-Synchronous Distortion
Where we introduce interharmonics is when you have distortion that’s not the same every cycle. That distortion is different from cycle to cycle. That gives rise to interharmonics. So you have interharmonic currents that are drawn when you have loads whose current isn’t completely driven by the voltage timing.
An interharmonic is really any frequency that’s not an exact multiple of 60. You get this when you have waveform distortion that is sustained, but it’s not the same every cycle. It’s not completely periodic. Typically these are loads where the current variation doesn’t depend on the voltage. For example, arc welders, furnaces, bar furnaces. These are classic loads where the arc current varies quite a bit based on what’s actually happening with the welding or the arc, not the 60 hertz voltage.
So you have non-synchronous loads. The current change isn’t synchronized with the voltage waveform. Another way of thinking about this is the harmonics are the synchronous distortion. Interharmonics are the non-synchronous distortion. By synchronous we mean synchronized with this 60 hertz timing.
In some other loads that may be like this, distributed generation where it is constructing its own synthetic 60 hertz waveform—these are supposed to, of course, phase lock to 60 hertz, but any error in that gives rise to typically low frequency interharmonics. Loads where the current isn’t dependent on the voltage waveform completely or loads that have an AC to DC conversion have that possibility because once it’s DC, then the load variations don’t really follow the voltage variations anymore. So these are all, as you can see, traditionally large loads, large industrial loads, not typically residential loads.
IEEE 519 Interharmonic Measurement
Interharmonics are measured per IEEE 519 in five hertz wide bins. The standard defines five hertz wide spacings between DC zero hertz and three kilohertz, and the math behind how this computation works gives us a measurement every five hertz. So you have a measurement at, say, 240 hertz and then 245 hertz and 250 hertz.
There are various groupings of these interharmonics and harmonics. The ones that happen to fall on an exact multiple of 60 are the harmonics. The standard gets a little bit confusing in how it groups these with interharmonic and harmonic groups and subgroups.
For example, the harmonic subgroup is centered around a harmonic, and it is composed of the five hertz wide bin centered on the harmonic and the two on either side. So that gives us a 15 hertz wide band centered around the harmonic. So you’d have the fourth harmonic subgroup, you’d have the fifth harmonic subgroup here, and then the space between those two is the interharmonic subgroup. Any energy in this region is considered non-synchronous, and any in this 15 hertz wide band here is considered synchronous.
The reason why it gives that plus or minus five hertz on either side is because if there’s any slight variations from cycle to cycle, that’s still mostly synchronous. The IEEE 519 standard is about harmonic distortion, synchronous distortion. So it explicitly excludes interharmonics from those limits.
Here are the different groupings from the standard. We have the raw five hertz wide bins that can either fall on an exact multiple of 60 or off a multiple of 60, and the harmonic groups and subgroups, interharmonic groups and subgroups. Normally if you are dealing with traditional harmonic distortion, you’d really only be concerned about this harmonic subgroup because that’s what IEEE 519 puts thresholds on. The interharmonic subgroup here is the in between. This is the non-synchronous distortion.
Most of the time there aren’t interharmonics, and so really you’re only concerned about the harmonic subgroup. But if you do have that situation where you do have non-synchronous distortion and need to quantify that, these other groupings are available.
How a Space Heater Causes Interharmonics
Let’s jump back to the subject here in the white paper, and that’s this space heater. How could a space heater cause this kind of complex power quality issue? Well, it goes back to its claim to fame of being able to continuously vary the heat, and it does this by taking that heating element and switching it off and on rapidly, so the average on time, the duty cycle, gives it the heat it wants. It only has a 12 amp resistive load, and because 12 amps is the most it can draw from a 15 amp branch circuit. All it can do is switch that 12 amps off and on, but what it can do is control the timing so that the average on time is whatever amount of heat it wants.
Switching Methods
Now, it does this with probably the worst possible method. That’s a phase angle fired method. The most utilities friendly way of varying the duty cycle of that kind of resistive heating element is by switching it off and on in whole cycles. So if you wanted, say, 50% heat, you’d have it on for 10 cycles and off for 10 cycles, or on for four cycles, off for four cycles. You switch it in whole integer cycles.
Still kind of friendly is at least doing it at integer cycles, even at just one off, one on. Still somewhat friendly is at least only switching it off and on at a zero crossing. At this point, even the friendliest and good, it’s still synchronized. And even with this advanced single cycle, it’s still in whole half cycles. And that does produce less transient because we’re switching off at zero crossing. But here we get into the possibility of interharmonics or even some DC component to it, even harmonics.
And the worst possible way is phase angle fire, where every single switch on may be at a different point in the voltage waveform. Typically, these are switched with TRIACs or DIACs or SCRs, which inherently turn off at zero crossing, so the off portion of the current is always a zero crossing, but the on portion can vary. And this is also how a traditional light dimmer works.
Waveform Analysis
So if we look at the waveform from this space heater, we can see, here in the figure, that it is switching off and on at different points in the waveform. Here we’re zooming in on one of the graphs from the white paper. And you can see this little snippet, it’s on for 40 milliseconds, then off for 20 milliseconds.
In this on time where the current switches on, here the current’s in blue, there’s a different point in the waveform every time. So here it’s at the positive portion of the voltage waveform. Here at the next on time, it’s the negative portion at a different point in the waveform. So since the on times are not the same every cycle, it’s non-synchronous. The current looks different from cycle to cycle.
Even though it’s a resistive heating element and it’s just a resistor, which normally is a linear load, can’t produce any harmonics, the way it’s being switched turns this overall load into a non-linear, non-synchronous element because each cycle of current, if you look at one cycle at a time, looks different. And that gives rise to interharmonics because we have cycles of current that look different from cycle to cycle.
Spectrum of the Space Heater
So here’s the spectrum. If we take the waveform here in figure two, and if we use the IEEE method of taking 12 cycles of this waveform and doing a harmonic analysis or interharmonic analysis, here’s the spectrum. Here are the five hertz wide bins. So we have spacing at five hertz wide. There’s DC, five hertz, 10 hertz. The peak here is 60 hertz.
Now remember, this is a 12 amp steady state load. If this thing were switched on continuously, it would be drawing 12 amp steady state. But in the waveform we’ve chosen here from figure one, we have about five and a half amps at 60 hertz. And we have about two and a half amps at 45 hertz and 75 hertz. These are not multiples of 60. These are interharmonics. They’re not synchronized with 60 hertz.
In fact, these interharmonics are much higher than the actual harmonics. Here we have 120 hertz and 180 hertz are two of the larger harmonics, and they’re actually really small. We have much more interharmonic distortion than we do harmonic distortion.
Interharmonics and Flicker
So why do we care about these interharmonics? Well, because they’re not synchronized at 60 hertz, that causes voltage drop that changes from cycle to cycle. So that gives rise to a continuously variable voltage drop, and that’s where we get into flicker, because flicker is about continuous voltage modulation.
If this were a highly distorted waveform, like a VFD, well, that’s bad for voltage distortion, but it’s not gonna cause light flicker because if the shape of the waveform is the same from cycle to cycle, the voltage drop from that load is also the same from cycle to cycle, so it’s not causing any voltage fluctuation, so it’s not gonna cause flicker. On the other hand, interharmonics are by definition different from cycle to cycle, so not only is the current changing from cycle to cycle, but so is the voltage. And that voltage fluctuation from cycle to cycle gives us the light flicker.
And indeed, that’s what this load does. It produces a tremendous amount of flicker. And in fact, it’s perhaps the worst possible situation for flicker.
Interharmonic Guidelines and Flicker Sensitivity
Here we have figure five, some interharmonic guidelines that are related to flicker. There aren’t really explicit limits on interharmonics in the 519 standard, but in the annex, it does talk about interharmonics as it relates to flicker. And these are suggested guidelines for limiting interharmonics. These limits are designed to keep the PST flicker under 1.0. And if you’re familiar with the IEEE 141 standard, that’s the threshold of irritability. If the flicker exceeds 1.0 for PST, people are gonna become irritated by that amount of light flicker.
The curve gets smaller and smaller. The amount of allowed interharmonic distortion is really small here in the 45 to 55 hertz range, and also from like 70 to 75 hertz. So keeping in mind those limits in where people are the most sensitive to flicker, that happens to be where most of the energy is. Remember this heater peaked at 45 hertz and 75 hertz. So that’s pretty much the worst possible places to have your interharmonic energy.
And again, that’s where the limits are the absolute smallest. So this small 12-amp load will cause a huge amount of flicker here. In figure six, we see we’re greatly exceeding the flicker limits on not only that branch circuit, but because this is the most sensitive spot, this is gonna cause a large amount of flicker on the other 120 volt leg on that transformer secondary, and even other customers on that same secondary.
Practical Implications
So you might expect something big like a 20 kilowatt tankless electric water heater to cause flicker, but here’s how something as simple as a plug-in the wall space heater can cause a large amount of flicker for everyone on that transformer secondary because of the interharmonic currents it draws at just the right frequencies where people are absolutely the most sensitive.
Now, this type of SCR switching, phase angle fired switching, can also appear in kitchen appliances like countertop heaters, hot plates, sauce cookers, things that have a variable amount of heat and use this same switching technology.
So if you have a customer that’s complaining about flicker, ask them, “Hey, do you have a space heater plugged in? Does it go away if you turn it off? Does it happen at mealtime when someone’s cooking dinner or cooking lunch?” And also consider that other customers on that transformer secondary may have one of these, and it’s as simple as a plug-in the wall device. These are not large tankless electric water heaters, not multi-kilowatt EV chargers. These are simple plug-in the wall devices that produce these interharmonics.
THD vs. Total Distortion Including Interharmonics
It’s really something that is tough to quantify with the existing waveform distortion standard. If we look at the harmonic content, this is much smaller than the interharmonic content. Here the 120 hertz, the even harmonic, the second harmonic is the highest at half an amp.
But overall, if you go by IEEE 519, the total harmonic distortion is only about 11% from this load because the THD metric in 519 only includes harmonic subgroups. It does not include interharmonics because 519 is about synchronous harmonic distortion. It explicitly removes interharmonics from these metrics. But if you do include them, if you include all the interharmonic components, now we’re at 143% distortion. So you get very different measurements when it comes to overall waveform distortion based on whether you include harmonics only or if you include interharmonics in addition.
So this kind of plug-in the wall device, it kind of falls through the cracks of the waveform distortion standards because it’s not really something that was that prevalent in the past. But it’s becoming more and more common as the electronics for this high power switching become less and less expensive.
Conclusion
The bottom line is interharmonic loads are becoming more common as the technology to produce this kind of current becomes more common and less expensive, even trickling down into residential loads. So be on the lookout for this type of load. One of the big markers is excessive flicker from customers who didn’t really expect it, where there’s no obvious hard start motors, there’s no heat pumps causing lots of voltage sags. These sort of high power, very fast switching devices are becoming more and more common.
If anyone has any questions on the white paper, feel free to call at any time at 1-800-296-4120 or send us an email at support@powermonitors.com. Thanks for attending everyone. Everyone, have a great weekend.