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furnace

Mon Nov 17 2014, 05:44AM

Registered Member #4992 Joined: Wed May 23 2012, 03:57PM
Location:
Posts: 108

Hallo everyone!
I want to build a grid tie inverter but can't find to much info of the topic. My main concern was as to how I would pulse my half bridge mosfets so that it would synch to the grid. If someone can please shed some light on this it would be great.

Ash Small

Mon Nov 17 2014, 06:04AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Use a mains step-down transformer to trigger the bridge driver. When it goes positive it triggers the positive half cycle, and vice versa, but Steve Connor knows a lot more about these things than I do.

I think you may need some form of 'chopper' circuit as well, I read something about it once. Something like they use in 'sine wave inverters'.

Shrad

Mon Nov 17 2014, 09:46AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
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Posts: 780

disclaimer: this is subject to regulations and most grid management offices will strictly forbid (and most probably sue) any user messing with the direction of the current in their lines, as it will totally mess the load balancing between phases

Steve Conner

Mon Nov 17 2014, 10:00AM

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

I built a small one as part of my PhD research

Ash Small

Mon Nov 17 2014, 11:26AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Shrad wrote ...

disclaimer: this is subject to regulations and most grid management offices will strictly forbid (and most probably sue) any user messing with the direction of the current in their lines, as it will totally mess the load balancing between phases

Yep, there are regulations governing it in both UK and US, as far as I'm aware, but plenty of people sell excess energy to 'the grid'.

furnace

Mon Nov 17 2014, 11:31AM

Registered Member #4992 Joined: Wed May 23 2012, 03:57PM
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Posts: 108

Steve I can't seem get anything open your site, it's only the pictures that I can see , if you have a shchematic on it I would love to see it.

Ash small! I'm not sure if you can use a step down transformer because I don't know if the mosfets respon well to a sine wave, but if it does it would make things a lot easier, I was thinking of a tl494 or a 555 with npn and pnp transistors to chop my half bridge to 50 hz, but I'm not sure if it wil be synchronized with the grid?

Shrad

Mon Nov 17 2014, 11:50AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
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Posts: 780

Ash Small wrote ...

Yep, there are regulations governing it in both UK and US, as far as I'm aware, but plenty of people sell excess energy to 'the grid'.

Yep, but they sell it via certified equipment

The challenge there is to certify your equipment, and as far as it goes it involves making a demand, introducing it, meeting technical specifications and regulations, and pass a battery of tests so that you have your certification agreement

I have already seen the hassle of people trying to pass certifications for their laser projectors at photonlexicon or laserfreak (to IEC as well as FDA/FCC) and that involved issues of radiation exposure and laser classes (potential harm)

This kind of devices involves much more, counting potential harm, and it costs some money...

I guess making your own grid tie inverter is for economic purposes to resell some power for your own profit, and then this is of a greater benefit to buy an all made grid tie device, or you are trying to develop your own design and plan to get the funding commercialize it one day

I think that you also have the possibility to just have a synchronization device which is the interface between your inverter and the grid, and which behaves as a synchronization and shutdown master, but I have no clue

Anyway, this is an interesting subject so thank you for developing these questions!

Steve Conner

Mon Nov 17 2014, 12:24PM

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

Read this, a report on the inverter with complete schematics and firmware listing.

The trick Ash suggests is an ancient and skanky one, but it will work to a certain extent. You just drive the bases/gates of your switching devices from a little step-down transformer connected to the mains. Et voila you have a sort of rectifier that works backwards too. It won't be too efficient and the power quality will be dreadful.

Shrad

Mon Nov 17 2014, 02:11PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

why not simply make use of a class D amplifier design which uses the output of a (very) small AC transformer as the input?

always in sync, and easy to build and use (in my brain at least)

Ash Small

Mon Nov 17 2014, 05:10PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

furnace wrote ...

Ash small! I'm not sure if you can use a step down transformer because I don't know if the mosfets respon well to a sine wave, but if it does it would make things a lot easier, I was thinking of a tl494 or a 555 with npn and pnp transistors to chop my half bridge to 50 hz, but I'm not sure if it wil be synchronized with the grid?

I did say 'use it to trigger the bridge driver'. You only need to sense when it changes polarity, but you still need to chop the output to approximate to a 50 or 60 Hz sine wave that conforms to the exact frequency of the mains, which varies a bit. This is how the 'sine weave inverters' you can buy in the shops do it, some better than others. Some cheaper inverters just output a square wave. Steve's link does go into a fair amount of detail, though.

Obviously you still need a sufficiently large amplitude to be able to measure the crossing points, etc, but as Shrad pointed out, if you amplify the output from the step down transformer, you get a signal that corresponds to the mains frequency in your area.

This link gives the UK frequency over the last hour

there is/was one that gives 'real time' frequency somewhere, but I can't find it at the moment.

It did answer the OP's question regarding how to sense the change from +ve to -ve, though, I even threw in the 'chopper' trick for free

And dropped your name in

Here's one 'real time' link.

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