1. the link is an inverter for a fuel cell, not a grid tie inverter.

2. I don't get your math, where did the x10 come from?

3. He'd need approval from the power company to connect his inverter to the grid and a improperly designed unit could kill a lineman or cause problems with the grid.

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A standalone inverter is a constant voltage source. A grid-tie inverter is a constant current source. Just being in sync with the mains is not good enough, if the inverter is configured as a constant voltage source it will either overheat or shutdown.

I don't think your power company would like that.

By the way cutting off the inverter when the mains fails isn't too hard to do. If the inverter is configured as a constant current source the output voltage will track the mains. If there's a power cut the inverter will try to power the whole grid so its output voltage will drop to near zero. All you need to do is cut-off the inverter when the mains voltage drops below a certain threshold.

True it needs to be approved which is beyond the scope of a hobbyist.

It's a shame because designing a grid tie inverter sounds would be a fun and challenging project. You could legally do it yourself if you powered your house from a diesel generator and a renewable power source and not be connected to the grid.

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10Kw unit, so it $0.1 x 10 x 24 x 365 = $8760

 

My mistake, I stand corrected.

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So is modifying a stand-alone inverter to connect to the grid the way to go or start completely from scratch?
I'm thinking that the SMPS and protection circuitry is already built in to a stand-alone unit, but is it too different or will need extensive modification to make it work?

I've seen a small grid-tie inverter (the Mastervolt Soladin 120) but they don't make it anymore and I can't get one anywhere. I'm thinking that if it can be made that small, a 150W inverter can do the same thing?

 

Plase read this thread from the beginning: the grid-tie inverter needs to be approved by the power compnay before you can use it.

I imagine that you could in theory use an existing inverter to scavenge parts from to build a gruid tie but it isn't possible to simply convert it to a grid tie inverter.

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thank you very muth

 

I am thinking of designing and building a grid-tie inverter for my final year project at uni. I've done some research into grid-tie inverters and stand-alone inverters (dismantled some stand-alone ones) and found the system seems quite simple but needs quite a lot of feedback / control because it could go horribly wrong pretty quickly. As far as i can see, the circuit consists of a step-up flyback circuit to turn 12v DC into 350v DC, then this is fed into a FET H-bridge which is controlled from a slightly higher than 50Hz oscillator. As the frequency is slightly more than the grid, the circuit will always try to lead the grid, but as the grid is essentially infinite current , this circuit is unable to change the frequency.
The main problems I am thinking is; failure modes (grid disconnect, anti-islanding, voltage or current spikes and large frequency changes) and the waveform I am introducing to the grid (modi-sine).

Is this still undoable as I attempted this project at home, then last year at college but didn't get a working prototype - lots of smoke and big flashes.

As Hero999 mentioned, the circuit would perform as a constant current source instead of a constant voltage source. What exactly would this entail (what's the difference between the two).

 

I'm sure people here would be very pleased if you post your completed design when you finish it

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You sounded very knowledgeable until this point.

A constant current source tries to keep the current the same regardless of the load and a constant voltage source tries to keep the voltage the same regardless of the load.

A grid tie inverter will inject a constant current into the grid.

A standalone inverter will supply the load with a constant voltage regardless of the current.

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Thanks Hero999, that clarifies something. I wiki'd constant current source after writing that, which obviously explains it more! After a little more investigation I have determined there are two main ways to step up the voltage (I already sorta knew); high frequency flyback circuit like PC SMPS and most high efficiency power supplies nowadays and low voltage into modified sine, harmonic removal etc feeds into power IGBT bridge or similar, which then drives a large low frequency transformer. The cheap (and nasty) in-car inverters are high frequency, small units while most UPS's seem to be big, bulky transformer style. It seems to me that the latter is the way to go (low voltage but higher amp switching) because I am less likely to get electricuted, it seems more simple (not dealing with saturated core ferrite transformers) and also, I believe that UPS's synchronise to the grid's phase so when the power goes out, it keeps feeding without any phase shifting, then eases back into synchronising when the power comes back up. The UPS circuitry is a lot more complicated but I think I can decypher the pointless bits and strip most of the redundant stuff out...

So on the constant current route - I have a switching circuit I've used before but its constant voltage (like most power supplies). Can anyone see a problem with the way i have turned the constant voltage circuit to a current circuit attached?

Attached Thumbnails

 

 

From my initial investigations into existing stand-alone inverters, I have found most of them use a TL494 and some PC PSUs I've dismantled use the same chip too. I've found the datasheet for this chip and some limited design notes. Does anyone have and suggestions for this chip or a better alternative?

I think the way to grid-tie is generate 350vdc from 12v, modulate the high voltage to modified sine at just over 50Hz, filter to produce purer sine and add to the grid. It's obviously more complicated than this so I've broken it down into smaller parts.

Does that sound right or am I missing something?

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That's modified sinewave inverter plus filter. It will work but the filter will need to be pretty bulky to work effectively. If you use high frequency PWM then a smaller filter can be used to remove the harmonics.

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I have experimented wit the above approach and it works. However, as noted it was inefficient. In the whereabouts of 60% if my mind serves me correctly. Of course we were using a very small motor set, 1/6 Hp. Electric motors become vastly more efficient with larger sizes. But at the time that's all we had and we ran out of time and money, thus the whole experiment was just that, an experiment.

The key here is to run the AC motor at reverse slip equal to its plate slip. For instance if you have a 4 pole motor at 60 hz, its synchronous speed is 1800 RPM. But if the plate lists its rated speed of 1750 RPM, then to operate as a generator, you would have to drive it at 1850 RPM. Accurate measurement and control of speed is a must, generator output drops quite rapidly above the slip speed.

 

Hi! I got interested as I'm thinking of building one myself, otherwise I won't graduate next semester. LoL. My idea is to build an H-bridge with the high side switches connecting at 60 Hz (it's 60 Hz here) and the low side ones running at high freq SPWM as Hero suggested.

The current harmonics will be filtered out by a low value sense inductor in series with the line. The focus of my project is proper timing of the control switches.

The problems I've encountered with the design so far are
1. getting proper line voltage feedback (I believe an unknown value lag would come from feeding back using a transformer)
2. I'm thinking of using FET switches with snubbers. With a DC current source feeding current into the H-bridge, overlap times would be necessary otherwise, there would be no path for the current, right? Following this argument, how will the SPWM scheme (requires off times) work with a current source?

As far as I've heard, most applications nowadays use MC. I would want to make one using only analog components though.

Also, this is my first time making the circuit, so any help/criticism of the work will do. Thanks.

PS. First time joining a forum. How do I post my schematics? LoL