[50ShadesOfDirt]

Again, there are two problems that I believe we can pull out of an entire dirty power discussion and do something about ... THD and Surges.

Unfortunately, talking about THD only from the gen side is making it sound like all problems are cured by getting a low-THD gen. They aren't ... everything in your house is dumping THD back out on the line (smps devices are among the worst by their very design, UPS, etc). If you were to argue that THD is bad, then your electronics are getting eaten up by the THD that is already in your house wiring, and not from the gen (how often do you run your gen, after all?). My belief (IMHO) is that THD is the lesser evil.

Think of the house wiring as a pond, and everything in the house has dirtied up the water already, because almost every device is dumping THD into that pond. Your "clean power" gen is dumping clean water into the pond, which immediately turns dirty (clean + dirty = dirty). Your "dirty power" gen is dumping dirty water into the pond (dirty + dirty = dirty). In commercial/industrial sites, they can clean their entire lake of dirty water with specialized equipment, and end up w/ less dirty water. The grid requires this of them.

When you transfer back to the grid, clean water is getting dumped into your dirty pond by the grid, along with the occasional (and free) instant-death-dealing surge. Kiss that HVAC goodby, not slowly from THD, but instantly, from a single surge event.

What can you do about a THD problem? I'm no EE, but as best I can tell, little to nothing ... what commercial/industrial sites can do, with an army of EE's at their disposal, isn't what we can do at the home level. Can you stick a $200 UPS in front of something that is supposedly susceptible to high-THD? Sure, but now that same UPS is dumping more THD back out on the wiring.

The only clean pool of water I can find is the small puddle that is internal to your device that has an SMPS in front of it; this is a DC puddle. Behind it, that SMPS is dumping THD back out into the AC house pond. Our prevailing codes in the US say that this is a one-way street only ... clean up incoming power, but do nothing about outgoing power.

What can you do about the Surge problem? I'm no EE, but as best I can tell, you simply install an SPD device in your load center, and smaller SPD devices in front of susceptible devices. Have an HVAC that costs 10k? Stick an SPD in front of it. Same w/ little $$$ electronics. No THD being dumped back out on the line, from your SPD effort.

So, do you get a low-THD gen? Absolutely no problem, if you are willing to trade fuel choice, authorized dealer network only, complexity, low-watts, and other issues for the chance to dump clean water into your dirty pond water. Economics-wise, it's better for the economy when you buy a $5000 honda, than a $1000 westinghouse. Ignore those trade-offs (and issues, costs & headaches) behind the curtain ...

As I run my gen's for 1000 hrs / yr, out in rural nowhere, I too, am excited by "inverter technology" ... but I don't know where the marketplace will go with their designs. A "jet-engine complexity" Honda, and other similar designs, are the first iteration, and they don't fit rural applications easily. They are more like a JD tractor that only the dealer can repair, if you can get it there. I've seen only one open-frame gen w/ an inverter panel, but it had none of the features of a westinghouse non-inverter open-frame.

I took the $4000 in savings, bought $300 worth of SPD's, and called it good, as surges were infinitely more fixable than THD. 5 years later, it's still good. If my $300 TV does die a horrible death (and I can somehow trace it to THD, in some form of electronic autopsy), I still have $3700 left over to buy another $300 TV ...

YMMV, and watch out for those marketing folks ...

 

[50ShadesOfDirt]

The grid does indeed have low THD ... it puts out "clean water" (under high pressure), as the grid supplier works to keep it that way through constant monitoring at many points.

One problem the grid has is that commercial/industrial sites do throw THD back out on the grid, and cause trouble in the grid "ocean" for other customers downstream or nearby those sites. This is why the grid will punish these offending sites with chargeback fees, and is also why those sites have EE's & specialized equipment to counter the THD levels. If it is a thing on a grid, and in a commercial building's "lake", then it's a thing in our smaller "pond" (the scale changed, but not the problem).

The power comes into our pond, circulates (and some is consumed), and then returns to the source (the grid). The grid (or your gen) keeps the pressure high, so that anything consumed is replaced, but it still circulates.

Is the grid affected by our homes? A very good question, and I don't know how the grid is protected from each of us in terms of THD ... one googled explanation is:

"The proliferation of nonlinear loads and the increasing penetration of distributed energy resources in medium-voltage (MV) and low-voltage (LV) distribution grids make it more difficult to maintain the power quality levels in residential electrical grids, especially in the case of weak grids. Most household appliances contain a conventional power factor corrector (PFC) rectifier, which maximizes the load power factor (PF) but does not contribute to the regulation of the voltage total harmonic distortion (THD V ) in residential electrical grids."

I'd take this to mean the grid is good/bad in your area ... warrants more research & reading.

The other problem the grid has is "surges", and there isn't anything the grid folks can do about those ... your clean water from the grid comes with the occasional free and big surge. Inside your house, more free micro-surges are happening, to the tune of dozens, hundreds, or more. While we can block surges, I don't see how we can block THD, when almost every device in your house is contributing to it.

Linear or non-linear loads? I'll let google answer that:

"with linear loads, the voltage and the current sine waves are going to look the same. Current that is drawn from a regular motor or a light can look like the clean voltage, even if it is not in phase or a little bit leading or lagging—it depends on the power factor of the load."
"with nonlinear loads, the current is not going to look like the voltage. The more drives or the more distorted current we add—the more voltage distortion we have on the system."

And finally ...

"Times have changed. Harmonic problems are now common in not only industrial applications but in commercial buildings as well. This is due primarily to new power conversion technologies, such as the Switch-mode Power Supply (SMPS), which can be found in virtually every power electronic device (computers, servers, monitors, printers, photocopiers, telecom systems, broadcasting equipment, banking machines, etc.). The SMPS is an excellent power supply, but it is also a highly non-linear load. Their proliferation has made them a substantial portion of the total load in most commercial buildings."

So there's the kicker ... the (non-linear load) SMPS is both good and bad! It's good for the device behind it ... it's bad for any other device in the pond, unless they also have an SMPS in front of them. I have dozens, if not hundreds of SMPS "wall-warts" in operation, just walking around my house and doing a quick count. Each one is throwing THD back out on the wiring, by definition of how it works. If I had just one, I'd agree ... nothing to see, move along. But, a dozen ... a hundred? I can't do anything about these devices nor the THD, so I just let them duke it out. After 5 years, I'm still waiting for things to blow up, because THD should've been handled differently.

So, my dirty gen adds dirty water to my already dirty pond, and each SMPS is eating that dirty water, cleaning it up, and presenting clean water to the device behind it, and throwing dirty water back out. Maybe it's possible that I've lost a wall-wart or three, over the 5 years I've operated dirty gens, but I haven't lost a TV or other "sensitive" device behind those wall-warts, or that has built-in power protection. Each wall-wart is ... $10 or so. I might be one of the few that puts 1000/hrs/yr of (in my case, dirty) water into the pond from a gen.

Whereas, almost anyone can point to a single surge event (lightning, grid) and show several devices that fried right after that surge. Nobody can point to a 1000 micro-surges, which happen daily. Yet, a few SPD's later, I've protected against surges from without, and micro-surges from within. I just can't do that with THD, given all the devices in my pond, and given that there are no equivalent THD SPD's (TPD's?). And a $5k low-THD gen? If our use-case is home backup power, that's clean water for 10 hours per year (insert your own number). If the use-case is rock-band sound/audio mobile power generation, at 50 events per year, 8 hours each ... yeah, I'd throw a Honda at it, and hopefully the business income would pay for it. Food truck? Another great Honda use-case ...

I'm not sure ANYONE can point at a fried/dead device and definitively say THD did that. Maybe device manufacturers will one day add autopsy circuits to these things. Tilt this way for surge, and that way for THD.

I couldn't come up with a better metaphor than ponds/lakes/oceans, and perhaps someone will ...

You could argue the total number of electrons with me ... I can't grasp the numbers, where trillions of the little devils are running around in the system. I just see plenty of research that says that our house is a system (in a chain of small to big systems), and things in our house (like an SMPS) actually causes THD themselves (not just a gen causing/not-causing THD), and I extrapolate from there. Then I look for something practical that I can do, right now; SPD's were very practical.