Common DC bus for solar panels or microinverters? - Page 1
Forum section: electronics / projects

author: moffy
author_rank: Super Contributor
timestamp: February 18, 2023, 01:27:11 am
content: Microinverters for solar panels have a lot of positives because they will maximise the output of each panel. But the problem I see is they add a lot of complexity, because each microinverter has MPPT and sine wave inverter built in. A large number of relatively complex units leads to a relatively higher failure rate than a single unit inverter. What if the microinverters were replaced with an MPPT step up converter to say a common DC bus of 200V, and that bus was then fed into a single sine wave mains inverter? You would still be able to maximise the power of each panel, but the roof mounted MPPT step up converter would be much simpler than a microinverter and therefore hopefully more reliable. The trick, I guess, would be how to stabilise the DC link with all of the step up converters feeding in, in parallel. Having DC on the roof would be no different to series connecting the panels before feeding into the inverter.Seeing this is such an obvious idea it has probably already been done. Well what are your opinions?

author: Someone
author_rank: Super Contributor
timestamp: February 18, 2023, 01:48:57 am
content: Quote from: moffy on February 18, 2023, 01:27:11 amYes, per panel (or substring) optimisers.

author: TheMG
author_rank: Frequent Contributor
timestamp: February 18, 2023, 02:00:07 am
content: Yes, more inverters means that there is statistically a higher probability of something failing, however, it's important to remember that if a microinverter fails, you lose the output of just one panel. With a single string inverter, if it fails, you've got nothing.If you have an MPPT controller per panel, well, you might as well use microinverters as there really aren't that many extra parts!As mentioned, optimizers do exist for series strings, which can, for instance, limit the decrease in string output power if you have a shaded panel. But then you still have all the disadvantages of a string inverter.There's also a safety benefit to microinverters in that you're not dealing with high DC voltages, which can produce some pretty nasty very hot arcs if a bad connection develops under load (DC arcs do not readily extinguish like AC arcs since there is no zero-crossing).

author: moffy
author_rank: Super Contributor
timestamp: February 18, 2023, 02:18:09 am
content: Quote from: Someone on February 18, 2023, 01:48:57 amThanks, they do seem to work in series, up to 600V total voltage, rather than a common parallel DC bus.

author: moffy
author_rank: Super Contributor
timestamp: February 18, 2023, 02:23:16 am
content: Quote from: TheMG on February 18, 2023, 02:00:07 amI believe the microinverters are a lot more complex than a simple step up inverter, also being connected to mains they are subject to the faults and surges of mains voltage. The 240V AC is much better than high voltage DC, that's why I was trying to limit it to around 200V DC.

author: jonpaul
author_rank: Super Contributor
timestamp: February 18, 2023, 02:36:57 am
content: 5 kva /200V=25A min solar15 KVA / 200v0V = 75AetcAdd more,amps if,efficiency <100%take NEC, IEC code, determined wire guage for that ampacity and run legnthSoon ,apparant why higher V is advantageSome EV battery's have gone from 300..400 up to 600..800V.Insulation is cheaper than copper!Jon

author: moffy
author_rank: Super Contributor
timestamp: February 18, 2023, 02:44:22 am
content: Quote from: jonpaul on February 18, 2023, 02:36:57 amWell noted Jonpaul it is a trade off of safety vs efficiency, a compromise no doubt.

author: uer166
author_rank: Super Contributor
timestamp: February 18, 2023, 04:06:30 am
content: Quote from: moffy on February 18, 2023, 01:27:11 amEh, there is little point since:a) It is as expensive (or close to) as microinverters, yet without the benefit of carrying AC on roof only for safety, or redundancy since you need a central inverter.b) Modern global max search string inverters can handle partial shading no worse than microinverters or optimizers.A string inverter pretty much always wins for $/kW, so no reason to use microinverters unless you're a nerd, or youabsolutelyneed a N+ system. DC microgrids can be a neat thing though, just maybe not here.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 18, 2023, 04:36:33 am
content: Until recently, if you wanted daytime backup without batteries, the only choices available were string inverters - most notably the brands SMA and MPP. Enphase IQ8 changed that but as I see it, it's not a straightforward upgrade to add batteries later on, MPP did the best in that respect.

author: moffy
author_rank: Super Contributor
timestamp: February 18, 2023, 05:20:06 am
content: Quote from: uer166 on February 18, 2023, 04:06:30 amThanks for the reference to "global max search string inverters", looks like the need for the microinverter might be past. I will have to read up about it but if they solve the partial shading problem then the need for an alternative solution disappears."A string inverter pretty much always wins for $/kW, so no reason to use microinverters unless you're a nerd", doubtless it is true but dependent on the current cost of electricity you are paying.

author: moffy
author_rank: Super Contributor
timestamp: February 18, 2023, 05:22:17 am
content: Quote from: NiHaoMike on February 18, 2023, 04:36:33 amIf you want batteries a common DC bus might make sense.

author: Someone
author_rank: Super Contributor
timestamp: February 18, 2023, 05:22:53 am
content: Quote from: moffy on February 18, 2023, 02:18:09 amBoth exist but the few benefits of a parallel arrangement are dwarfed by the benefits of series operation. Even then, panels are so cheap optimisers rarely make sense.

author: moffy
author_rank: Super Contributor
timestamp: February 18, 2023, 05:46:43 am
content: Quote from: Someone on February 18, 2023, 05:22:53 amSeems to be the beginning of a consensus.

author: boB
author_rank: Frequent Contributor
timestamp: February 18, 2023, 05:57:39 am
content: A roof with hot and cold extremes isn't the best place for electronics to be located but they make it work OK

author: moffy
author_rank: Super Contributor
timestamp: February 18, 2023, 07:21:43 am
content: Quote from: boB on February 18, 2023, 05:57:39 amNor is a car engine bay, but it mostly works with good design.

author: fourtytwo42
author_rank: Super Contributor
timestamp: February 18, 2023, 09:50:55 am
content: Quote from: moffy on February 18, 2023, 05:20:06 amI had not heard of the expression "global max search string inverters" before but I guess that's what I designed & built myself, the screen shot shows the voltage scanning periodically (~10mins here) that often has no effect because mppt is already on the correct maxima, but later as a deep shadow moves across the array you can see the mppt voltage being corrected to the real maxima by the periodic voltage scan. The problem is the voltage scan frequency is limited by the loss of power during the scan the speed of which is limited by the input capacitance of the inverter required to reduce grid ripple loss of efficiency so it's all tradeoffs as usual.P.S. you can also see mppt has a nasty habit of drifting back to the old incorrect maxima after a correction, always more improvements to do

author: moffy
author_rank: Super Contributor
timestamp: February 18, 2023, 10:21:43 am
content: Quote from: fourtytwo42 on February 18, 2023, 09:50:55 amWow, the shading in the afternoon looks to have a big impact. Did you build your own inverter, or a "max search string" tracker for the DC input to the inverter?

author: fourtytwo42
author_rank: Super Contributor
timestamp: February 18, 2023, 10:45:03 am
content: Quote from: moffy on February 18, 2023, 10:21:43 amThe whole shebang although I only recently added what I call the shading scanner as a move of house meant the shading was much more pronounced than it ever was before. Actually it was surprisingly little additional code because the rest of the code provides all the infrastructure so to speak. I simply scan in bins of 5V or so down from 150V to ~30V recording the highest power and it's associated voltage along the way then returning to that point at the end of the scan. No need to store all the individual bins saving memory! All written in assembler on a PIC24xxxx running around 40Mips as I recall. I have now seen a few research papers on this by searching the phrase here but did not turn up any actual products.I should mention shading is particularly bad this time of year (winter) as the sun is low and this is a ground mount array.

author: tszaboo
author_rank: Super Contributor
timestamp: February 18, 2023, 01:22:07 pm
content: Quote from: moffy on February 18, 2023, 01:27:11 amNot exactly this. With a DC link setup, you have two wire going to and going from each panel. If you put them in series, then it's only two panels.The Solaredge inverters work this way. Individual MMPT for each panel, same current on all DC optimizers, and one central box to convert this to AC. It seems to have enough system variability to be more cost effective than per panel microinverters.Though multiple panel inverters are interesting. Like the Hoymilles that Dave took apart recently. These could be more cost effective in some cases. But in any case, the Enphase inverters are just bad value.Quote from: Someone on February 18, 2023, 05:22:53 amThey make sense if you run out of roof space, have shading on the panels, want to have better resale value, ease of mind. In my case, I spent 7% of the total cost on the optimizers, and the central inverter is cheaper (plus, no DC disconnect), so the total cost of having them is even less.

author: jonpaul
author_rank: Super Contributor
timestamp: February 18, 2023, 02:23:07 pm
content: So, a 200..800V DC distribution bus requires different cables, connectors, breakers, inspection, approval than an AC 240..480V bus.One is wiser to be skeptical of any solar power claims, inverters, panel's installers, ....The claimed efficiency, power production, payback time etc. are very optimistic or outright déception by the mfg, installérs, gov.Before one decides on a solar power, inverters, topology0/ Panel mfg, inverters mfg, installers informationFinance, legal, track records, probabilities of existing in future years?1/ reliability record, warranty, support record for panel, inverters, installer?2/ Actual power production per panel, per year track record at your location?3/ operation, control and performance in case ofa. Utility mains failure, brown out or blackout?b. Internet connection failurec. inverters manufacturer or installer website, international connection or firm failure?d. roof repair cost to remove and replace solar panels?e. utility, local permits, safety, insurance required for install ( eg service panel, meter upgrades required)All these factors should be considered before choosing a solar power configuration, panel, inverters etc.Have a great dayJon

author: moffy
author_rank: Super Contributor
timestamp: February 18, 2023, 09:25:54 pm
content: Quote from: tszaboo on February 18, 2023, 01:22:07 pmYou make some very good points for the series MPPT.

author: moffy
author_rank: Super Contributor
timestamp: February 18, 2023, 09:33:23 pm
content: Quote from: jonpaul on February 18, 2023, 02:23:07 pmI agree Jonpaul that the claimed efficiencies and payback times are often optimistic at best. There is some good data from home owners and their production, but for relevance you need someone at a similar location to your own. Maintenance and failures is something that is often overlooked in costing. My poor neighbour after a few years, just out of warranty, had to replace his inverter, all savings gone. Same thing happened to another friend.

author: jonpaul
author_rank: Super Contributor
timestamp: February 18, 2023, 10:41:04 pm
content: an old friend is a distributor and importer of solar panels, mounts and,inverters. A,few points...10% failure in 2..5 yrs of microinverters is normal, thus a 10 KW installation of 20 panels will have,a,number failed in just 1..2,yearsThe actual energy yield depends on temperature, types of Mount, panel's quality, age ( decrease per year due to solarization), shading, dirt, film dust accumulated, and latatudeAll panel rated power is noon, clear day on equator, ambience température 20,dec,C, on Jones 21.practice reduces the rated,watts to 25..40%'rated,average over a,year.Jon

author: bdunham7
author_rank: Super Contributor
timestamp: February 18, 2023, 10:59:57 pm
content: Quote from: jonpaul on February 18, 2023, 10:41:04 pmAccurate information and quality products will improve your results. In my case with a 30-panel system in a very hot climate I have had 1 microinverter failure in 10 years and the system has delivered 100% of what it was estimated to provide over its lifetime so far. It was at 103%, but a bug in their software appears to lose the historical production from the inverter that was replaced.Of course, YMMV. My inverters have a 25-year warranty and a 10% failure rate in 2 to 5 years would be pretty expensive for the company. A previous line from the same company did have a high failure rate (but not that high) and they had to roll out a program to 'upgrade' them all to the model I have. I think those only had a 15-year warranty, but that's still pretty long. At the time I installed mine, it was string inverters that had a reputation for failure--so much so that solar companies were including a mid-life replacement in their cost savings analysis that they presented to customers.

author: moffy
author_rank: Super Contributor
timestamp: February 25, 2023, 06:36:08 am
content: Just a short update, ran across this video:where the fellow ran some comparative tests between a Fronius string inverter and Enphase microinverters, with shading etc. to compare the power produced, quite entertaining when he takes a cricket bat to a panel to simulate micro fractures. The result:- they are within a few percent of one another, maybe up to 5% sometimes, with the single string inverter just behind the multiple Enphase microinverters.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 25, 2023, 11:45:21 am
content: Yes, that is an outright lie and was never based on anything else except malicious marketing. Textbook example of FUD. It is surprising that companies like Enphase and SolarEdge got away with such aggressive and illegal practices.Now as I'm in a position of making recommendations, I do my best to remind people not to buy from such companies. I usually do not bother with ethics that much, but there is a limit of how much incorrect FUD one can tolerate. If you can't sell your product without blatantly false claims about your competitors, you should not be doing it.