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phasing inverter output to mains AC

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IamSmooth

Thu Apr 12 2007, 08:46PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

I have just installed a solar system in my house that uses inverters to convert the DC to AC. They analyze the MAINS AC, adjust the phase and then feed it back to the grid in sych. I am going to guess that it finds the zero-crossing of the ac signal.

I know nothing about this subject and would like to read up on it. Does anyone have any good reference books and/or links where I can read on this? I tried google but I got a lot of hits, many of which are sales ads for inverts, and I would rather take the advice of someone here that probably knows how to build one.

Avalanche

Thu Apr 12 2007, 09:07PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

I'd have to recommend you download Steve's PhD thesis from here

It came in very handy when I was building my inverter, and is probably exactly what you're looking for. The zero crossing thing is sort of right, in fact the inverter itself has to be slightly off-phase (but still in synch) in order for it to have a load. IIRC Steve's inverter uses a very slow 'PLL' to lock it to the mains frequency, but I never experimented with grid tie anyway.

If you haven't already found it, there's also the fieldlines message board which deals with remote living and power systems, etc.

IamSmooth

Thu Apr 12 2007, 10:04PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Thanks, Avalanche. I looked through it and it seems to be way more than I'm after. For example, here is a link that shows the design of a basic inverter, converting DC to AC.

I guess one could use the MAIN's ac signal through a comparator and use this square-wave output instead of the 555. Does this sound resonable? Does anyone knowledgeable with inverters know how the commercial devices work?

Steve Ward

Fri Apr 13 2007, 03:25AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Thanks, Avalanche. I looked through it and it seems to be way more than I'm after. For example, here is a link that shows the design of a basic inverter, converting DC to AC.

I guess one could use the MAIN's ac signal through a comparator and use this square-wave output instead of the 555. Does this sound resonable? Does anyone knowledgeable with inverters know how the commercial devices work?

If you think Conners inverter design is over your head, then you really dont want to see commercial designs. You need to face the facts, to do something that you are after WILL be complex. You are asking for real sine wave output (meaning a high switching frequency to re-construct a sine wave), and then you want it to phase properly with the mains. It aint gonna happen with a 555 and a few transistors.

Steve Conner

Fri Apr 13 2007, 11:03AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I agree. My inverter is probably about the simplest, most ghetto grid intertie device you can make. If you don't understand how it works, or my write-up on it, you don't understand how grid intertie works and you need to crack some books. Or go back and read my stuff more carefully. I had access to commercial grid-intertied inverters when I was doing that work, so my work contains the knowledge I found when I was poking around inside them.

Also, there is not too much information out there. Connecting a DIY inverter to the grid is illegal in most countries, including here, so not many people are prepared to admit that they did it by publishing their work. I published mine as a deliberate protest against regulations that I thought were stupid and regressive to the cause of alternative energy, like a geek version of chaining yourself to a tree. Having said that, my academic supervisors were cool with it.

The chapter of my thesis that deals with the inverter has a references section that contains links to as much information as I did find. (You did download just that chapter, right? You don't want to wade through all 250-odd pages.)

IamSmooth

Fri Apr 13 2007, 01:37PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Steve, I must have missed the right part. I was reading through about 100 pages and it was talking about cost-effectiveness of various systems. Can you tell me the pages of your thesis that deals just with the inverter design?

Steve Conner

Fri Apr 13 2007, 01:39PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

You can download the inverter chapter on its own here:

Or if you already have the full version, look at Appendix A, IIRC.

Avalanche

Fri Apr 13 2007, 01:48PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

I think with inverters and switchmode power supplies in general, the more you start to understand the principals on how they work, the more you start to accept why all the complexities are there. I can understand how at first glance that even a simple inverter will look over-engineered, but once you break everything down into blocks, for example feedback, overcurrent, low battery alert, etc the more it all makes sense as a system.

The schematic with the 555 is one of those things that would work on the bench with the right load connected, but there are numerous reasons why you wouldn't want to use it as an inverter. If you tried to grid-tie that, it would probably just burn out.

You might want to read up about switch mode power supplies in general, because the principals are the similar and there's generally more info available. Maybe also check out a few datasheets for SMPS controllers, such as the SG3525

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