Grid tie inverter - parallel AC sources 1

[aegcmac]

I need to grid-tie a bunch of about 50 small inverters that process DC input of say, up to 17 Vmp and 6.5 Imp and back feed this through a small sine wave inverter rated to +/- 120 watts into an existing 120 VAC / 60 Hz AC grid. The AC grid is not a public utility, rather the end-user's AC supply, so IEEE or U.L. standards for back feed do not apply. The key here is the functionality, not bureaucratic approval of the precise quality of the back feed.

It's easy to get DC to AC, and relatively easy to get the AC in a sine wave, but synchronizing to another AC sine wave source for a parallel connection seems complicated.

Is it that complicated? If you could live with harmonic distortion of up to 5% - 7% and efficiency of the conversion from DC to AC in the 85% to 90% range, is it really that difficult to process the DC and parallel these two AC sources?

Or, if we have the small sine wave inverter, we need to parallel it with the AC grid on site. The key here is being able parallel the two AC sources.

A company in Europe, for example,

http://www.mastervolt.com/sunmaster/soladin.asp

has a mini grid-tie inverter that processes the DC input from a solar panel, but not available in 120 VAC/ 60 Hz. They seem to be doing it almost exactly as I need, but in more/less a Radio Shack form of package, with a nominal 24 VDC input and the output is European 230 / 50Hz. I don't need the packaging and all the bureacratic approvals, just need the function in a 120 VAC / 60 Hz environment.

Can anybody help?

 

[jbartos]

Comment: Beside what the inverters can deliver and of which quality, there are other engineering and safety issues to be tackled, namely:
1. The paralleled inverters will cause an increased short circuit currents since the paralleled system impedances Zsys,i for i number of inverters will approximately become:
Zsys,total=(Zsys,i)/(i), which can become very small.
2. The harmonic interaction, if there are no input harmonic filters such as LINEATOR, will increase, causing voltage peaks due to harmonic superpositions.
3. The probability that an inverter going "plazma" posing danger to maintenance personnel, will increase with the number of paralleled inverters.
4. Etc.

 

[aegcmac]

I totally agree. I do believe there are issues with grid tied inverters just as there are issues with generators coming on line. It's just that when you look at the leading inverter MFRs and where you have in some cases several hundred 2500 watt inverters paralleled into a Main on a single site including remote monitoring, control and data acquisition, it starts to become like requiring Microsoft to get approval to build a Windows program or NASA to set off a small model rocket.

The other issue is that the end-user owns the facility including all wiring and systems within it, but the PU owns everything from the meter. So you have some rebellion towards the PU, which is a form of localized monopoly of the power supply, and whenever you attempt changes within a buracracy like the public utility it most always involves politics. So really, it's the politics that is the real barrier because the MFRs know how and are happy to build highly reliable, safe inverters for grid tie. The engineers within the PU typically have no problem with this inverter technology once they actually see it.

Thanks for the tip on the hamonic filters. I'll look into this further. I've posed the single question of adjusting phase angle in another forum also, Circuit Engineering, just to try to cover all bases and get any input possible for a relatively simple solution. On a small scale private project like mine, and where I have voltage and frequency sufficiently tight I'm thinking this phase angle issue is one of the last major pieces of the puzzle.

 

[stevenal]

Hold up on the sync check relay. These work on the theory that the two systems in question will lock together after they are closed together at the proper time. Two inverters with separate clock signals will not lock together. The solution is a common clock as you've suggested above. If a common clock is used, out of sync closure is not possible. The relay is not needed.

I disagree with your assertion that standards do not apply. Regardless of statutes, they are a good source of accepted engineering practice.

 

[aegcmac]

stevenal:

I regret if I gave the impression that standards should not apply, rather I do firmly believe in uniform standards. It's just that I hear of many cases where on public systems leading technology experts are having to answer to the bureaucrats, the politicians vs. simply satisfying the engineers who understand it, etc, and in privately owned/controlled systems the operators are usually more concerned about system integrity and safety than anybody since they tend to be proud of the system they build in the first place and their neck is 100% on the line. So I know we must have standards of acceptable engineering practices, etc, but I just think we could do with a little less politics.

A light bulb first went on when you mentioned a common clock. Thanks for the input. But now I'm having trouble seeing it with just one clock, if that's what you meant, where it might take two clocks to get the two AC sources synced at the point they both tick at the same time, etc. It may be I need to better understand the common clock, and I will pursue this, but could you elaborate on this a little please?

 

[buzzp]

You probably have tried this company but check trace engineering. They seem to have a handle on the market for alternate power. Although, they may not make something in your range.

As far as the 2 clocks go, they are needed to make sure each runs off the same timing as the other. Otherwise, over time they will tend to drift apart and soon will be out of sync. So both inverters will have to be ran off the same clock or synchronized with another. The sync check relay is needed to tie in with the grid whether one or two inverters will be used. I have not designed with PLL (phase lock loops) but I believe this is the function of such a device- to hold two clocks in sync with another. Good luck.

 

[stevenal]

If there's only one clock there's only one tick involved.

I imagine a system something like this: The 5000 watt inverter would have an internal clock so it can output 60 hertz with no other source present. The smaller inverter would be of the grid tie variety, getting its signal from the existing AC line. The startup procedure: Turn on big inverter then turn on small inverter.

If you're trying to build the grid tie inverter circuitry yourself, I can't offer much help.

The main thing I'm trying to get across is this: Two inverters have separate internal clocks, one running at 59.999 Hz, the other at 60.001 Hz. Using a light bulb or sync check relay, a switch is closed when waveforms are in phase to connect the two systems together. The two inverters are not synched, and will continue to operate at their own individual frequencies.

 

[GOTWW]

http://www.homepower.com/files/GS_1.pdf

I have not read it yet, but a quick scan looks as it may be helpful.

 

[GOTWW]

BTW more, Notice the "no longer available or supported", Did the establishment shut them down???

http://www.homepower.com/files/GS_1.pdf

http://www.renewableenergyworks.com/pv/home_systems/acpv/microsine.html

 

[aegcmac]

Thanks everyone for the follow up input. It's amazing how once you walk through the door of a bridge rectifier or transformer with DC how difficult it is to return to a simple parallel connection as with DC. I really appreciate the input.

I have worked quite a bit with Trace equipment on some pretty large grid interactive systems where we take from the gird as we need it and backfeed into the grid any excess power generation. But they don't offer anything near the scale I'm needing. Interestingly, the web link GOTWW just provided is a mini grid tie inverter introduced into the American market several years ago by Trace that if still manufactured would be at least a significant help, but it's no longer made, there's only a few floating around, and the actual MFR in Europe has no interest in bringing it back to life. In fact it went from being called the Microsine when Trace introduced it to OK4U before it basically died in North America. I actually have a few of these units on my work bench from other mini projects a few years ago, but they're 100% epoxy or polymer filled so you can't learn a thing about them.

I'm starting to see why maybe Tom Edison fought for DC as the prime source over AC when they brought the first systems on line when you consider all the controls, monitoring and synchronours features needed to sync AC sources together. He lost the fight and AC became the prime source for efficiency reasons back then, but I better understand why he probably fought for DC.

I'm now thinking more seriously that to get two AC sources to parallel the voltage and frequency isn't as much the issue if you have these relatively stable and real close to each other. Voltage amplitude could be an issue, but again, if these are real close I'm not sure this would be an issue either. Given that voltage, frequency and even amplitude are within say, 2% of each other the real main issue is the phase alignment. I have an experiment planned this weekend to parallel two different AC sources where we concentrate solely on the phase alignment. I may smoke a few small inverters, but I believe that if I can get the phases aligned within +/- 5 degrees the sources will parallel. I'm going to try a couple of different methods including random, a simple clock tick pattern etc. If I can get them to lock together, they may not be in perfect sync, and like STEVENAL stated they may operate at their own frequency, and I know it may not be at the highest efficiency overall, but I'm thinking maybe sufficiently paralleled to be useful on a private site.

If you walk out your door and see a cloud of smoke in the distance it is probably me.

 

[GOTWW]

Just gonna have to tell Mr Gates, that he will have to wait until the New Technology is ready for prime time. But wait, that never stopped him before.

 

[GOTWW]

PS. They are not called trace anymore, wake up and realize they name have been changed to X-something. Maybe this is more appealing to the X0generation (ie. the neutral connection, that sucks up all the residual energy of the system).

 

[stevenal]

http://www.solarempowered.com/shop/item_OK4U.htm

 

[aegcmac]

Thanks STEVENAL, but that's the inverter I mentioned above. The OK4U was originally called Microsine 100, but they're no longer in manufacture and there's only a few floating around. In fact the site you mentioned states on their home page at

http://www.solarempowered.com

that they are on back order...which actually means these people apparently don't realize they'll be on back order forever unless they can put their hands on a few of them. I doubt there are more than 10 floating around the U.S. and once they're bought they're gone.

I would need 50 units of "something", so you could see why this won't be an option. However, it does show us that it can be done on a small scale.