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Harmonic distortion

Hi jas42,

It is not a simple problem, the Generac XG series has a total harmonic distortion from 4.7 t0 25% , the XP series, less than 5%. But what is THD?.

The AC voltage supplied by the grid has a sinusoidal form (as shown in the next post) , very near to the ideal smooth curve represented by the math function sine. The main characteristic of this ideal curve is that it has not sharp changes in its trajectory. When this curve is converted in voltage, and is fed to a resistive device, such as an incandescent lamp, it impulses a current with the same wave form, a smooth variable current waveform.

Any deviation from this ideal sine waveform is called distortion. In the AC supplied by the grid, there are many big generators carefully designed to produce the best waveform and quality electrical power. In a 60 cycles per second (60 Hz) system, this frequency is named the fundamental frequency.

Due to design difficulties and costs, many portable generators produce not only a non sinusoidal waveform but more than the funamental frequency; along with the 60Hz, there are multiples of this freq., double, triple etc and they are called harmonics.

If the fundamental frequency is 60 cycles per second, at to say 120 volts, the other unwanted frequencies, especially the third harmonic (180 Hz), are present in a variable amount, to say 5volts, 12 volts etc. the fifth may be 3 volts in example. The sum up of the total amount of unwanted voltages, generated at frequencies different to 60 Hz, expressed as a percentage of the the fundamental 60 Hz 120 volts, is called Total Harmonic Distortion.

This THD, is fed mixed with the 60 Hz, deforming the smooth curve, and depending of the load, may or may not be too harmful. An incandescet lamp or a portable drill are not affected, but a sofisticated electronic control may be alterated in its functions seriously, induction motors may overheat, as power transformers.

To filter this harmonics, there are special harmonic filters. Most common filters or economic line conditioners, merely filter out spikes or noise, regulators mantain line voltage inside safe margins, but to avoid harmonics out of your delicate electronics, you will need dedicated harmonic filters, that will control spikes too.

An UPS (uninterruptible power supply), may or not may solve the problem.
Be aware of simple UPS that will provide an horrible "square" wave full of power spikes and deformations ( see the photo) that will harm anything but an incandescent lamp. There are other that provide a "modified sine wave", similar to the produced by the common inverters, better than square . The pure sine wave inverters or UPS produce a clean, distortless voltage wave appropiate for all kind of equipment.

But, UPSs and inverters (domestic) run on low voltage DC current. UPSs have an internal battery, used only when normal AC supply is cut. Most of them (the small ones ) have a by pass relay to feed directly the protected equipment with the AC mains. The true online UPSs convert the AC supply to DC and then generates an AC sinusoidal, clean voltage wave, independent of the incomming supply. This is a solution, costly but solution.

The other solution would be exchange your generator for an XP10000.

In few words:

Harmonic filter, true online UPS, XP10000 genset, or take the risk to damage your equipment.

Best regards.
My Online UPS keeps the voltage at a rock solid 240v when connected to my non-inverter gen as the volts jumps around all over the place especally with a light load,, but I havent seen the gen ever go over 250v, as it stays within 215-245v and its quite steady at 230v with or over a load of about 20%
 

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Or get a Sinewave UPS that acts as a "shield" for your sensitive items(both AVR and PFC), on utility power and/or generator power
But remember you need a generator friendly UPS for non inverter gens, as most UPS will remain on battery mode as the the power from the generator wont be steady enough for a normal UPS to get out of battery mode... I have no idea how my UPS that is a generator friendly UPS can smooth out the voltage without going to battery mode. The UPS does click over to battery mode though, when theres a sudden demand for power from the generator and the voltage drops for a second.
 

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I have been told that our grid runs up to about 3-4% THD worst case, but I have no way to measure it. I have looked at it with my scope and I can see some distortion in the waveform. My WEN GN625i inverter has a much better looking waveform than my grid power.

On grid power I worry more about surges and spikes than THD.
Thanks for the info. Much appreciated. I agree. Surges / spikes are cause for concern.
I have found a lot of info on THD. Some seems reliable, but most is anecdotal. My goal is to confirm its level of risk (probability & impact). The only ways I can think of doing this is to either measure it directly (best way in my opinion) or get confirmation from the group at my local power distributor. If I can eliminate THD as a low probablility / low impact risk, my home backup solution will change.
Cheers
 

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If I can eliminate THD as a low probablility / low impact risk, my home backup solution will change.
If you go with a gen that is <5% THD you will likely be fine now and into the future. I had to change out my old gen (20%+ THD) for a cleaner waveform because of my new furnace. But, I wanted more power (old gen was only 3KW) anyway, so it worked out fine for me.
 

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Two words: Inverter Generator.

If I may be so blunt, you seem to be overthinking it. If nothing has been damaged in the last two plus decades, I wouldn't worry about the quality of the AC your utility company provides.

But if power quality is paramount, get a dual-conversion UPS to power your sensitive electronics. For longer outages, an inverter generator would be your best bet. It will provide even better AC power quality compared to utility power.

Out of curiosity, what sort of "sensitive electronics" do you have?
I agree about not worrying about my grid supply. If I can replicate it, via my backup generator, I'm golden.
If I need an inverter generator, that's what I'll get. I'm just trying to confirm as many facts as I can before I go shopping for a $1.00/Watt generator vs a $0.10/Watt generator.
 
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