THD - higher harmonics from genny backfeeding into source - Page 1
Forum section: electronics / general

author: 50ShadesOfDirt
author_rank: Regular Contributor
timestamp: October 19, 2022, 06:06:24 pm
content: I'm not an EE ... still struggling to understand all aspects of THD.If a generator puts out "up to 23% THD", and it's feeding a high-end Inverter w/ internal Battery-charger on a dedicated line, can the higher harmonics backfeed back into the generator's AVR?The inverter happily eats the power from the generator, but I do need to tune the generator's frequency and voltage output, so they get closer to 60hz and 240v at my constant load. When I asked the generator manufacturer for said tuning procedure(s), they pushed back on my scenario above, and said the "backfeed" might burn up the AVR. Is this a reasonable restriction, in your collective opinion?In fact, the inverter can pass thru any power it isn't consuming for battery charging (which takes 2 to 4 hours, if solar isn't happening that day), and none of that power passing through to the house has ever given the rest of my appliances any grief that I've been able to spot and tie back to THD. This could also be where the extra harmonics are going, or, the rest of the house appliances are kicking THD back out?My understanding of the harmonics is that, given 23% THD, approximately 80% of the power is clean and is eaten by the inverter (or passed thru), and the other 20% is dirty (at higher harmonic levels) and is "consumed somewhere" (heat in inverter components, possible backfeed back into generator, just bouncing around on house wiring)?What say y'all?

author: 50ShadesOfDirt
author_rank: Regular Contributor
timestamp: October 20, 2022, 01:31:30 pm
content: The generator manufacturer clarified a bit, and said it was really the solar panels that was putting the generator AVR at risk. Don't understand this one at all ...I would think that the inverter, being high-end, would keep it all separate, but I'll have to call in to them and see what they say.With the potential for so many things to throw THD back out on the lines, maybe I'll just stock a few AVR spares?

author: 50ShadesOfDirt
author_rank: Regular Contributor
timestamp: October 21, 2022, 02:08:31 pm
content: Found a blurb about solar panels & THD:"Solar inverters generate harmonics, although they usually are limited to an acceptable level for the installation. Just like all other forms of electronic equipment, photovoltaic inverters inject harmonics into the electrical installation. And just as other sources of harmonics can lead to overheating and other electrical system problems, so can photovoltaic inverters." (Schneider Electric blog)However, only DC wires feed from solar panels (mine are not micro-inverter'd) into the mppt charge controller, which itself is tied into the high-end inverter (brains of the RE power system). I'm going to go out on a limb and say that the high-end inverter manufacturer does the right thing with all forms of incoming power (solar panels, battery-bank, backup generator), and spits out only pure sine wave power leading to house (still waiting a response from tech support) ... so, the power gets a cleaning before entering the house wiring.Unfortunately, the second that clean power hits the house and gets consumed, almost all appliances & devices start throwing their own THD back out onto the line, so there is still THD running amok. I believe this is the normal (?) state of affairs ...In the case of the generator manufacturer's statement of THD (from solar panels) hurting their genny AVR, I'm calling partial BS, because of the high-end inverter (ie: Victron, Magnum, Outback, etc.) in my setup. I think they just want a get-out-of-jail-free card, if my genny ever dies a horrible death ...If a genny sits in an arrangement of backup power with an inverter, solar panels, mppt charger, battery-bank, etc., *and* the inverter is *low-end* (cheesy, low cost), then the inverter might not keep things straight, and THD could spill over from one side to the other. Someone could also have PWM inverters, instead of higher-end pure sine wave models ... these could wreak havok; for the most part, these old-tech PWM inverters seem to be out of style, compared to pure sine wave.If a high-end inverter is in the system, then no backfeeding of THD to genny should happen. Unfortunately, no fluke test equipment laying around, so no testing of the limb that I'm out on ...I do note that I've been running backup generators for years in this system, and I get reasonable lifetimes out of each; the engine usually dies before anything else, because of high hours on the unit.Case closed (I think)

author: gf
author_rank: Super Contributor
timestamp: October 21, 2022, 02:59:20 pm
content: What kind of inverter/charger is it? Pure off-grid, or a hybrid which can also run grid-tied?A pure off-grid inverter cannot feed back at all, since the inverter output is never connected to the AC input. The load is connected either to the inverter output, or to the AC input (-> bypass), but not to both at the same time.A hybrid, OTOH, can augment utility power with battery or DC solar power, and can also feed back if the inverter produces more power than consumed by the load. You need to ensure explicitly that it does not (-> "zero feed-in").

author: boB
author_rank: Frequent Contributor
timestamp: October 21, 2022, 03:37:29 pm
content: Schneider battery based inverter/charger and DC coupled (to battery) PV/solar MPPT charger controller ? No problem at all.They have been doing this for years now. Schneider bought Trace Engineering and those guys are where OutBack and Magnum and MidNite Solar (inverters just coming onto the market) came from up in Washington state. Not Victron though, but they are high end also.Most low-end generators can make very high THD but the inverter/chargers, especially the ones you mentioned, are designed to work with almost all, including the worst generators.Not sure where you heard this generator can make 23% THD but I wouldn't doubt it.You don't want to back-feed a generator. That is correct. But a Schneider XW or Connext or whatever is designed so that it has an extremely low chance of doing that. All of the higher end inverter companies know this.BTW, there can be voltage THD distortion which you can see easily on an oscilloscope and there is current THD when power is being drawn from the source.In your case, the generator IS the source. You are correct that a lot of generators can get hurt if the inverter/charger tries to raise its source voltage.boB