How to design a single phase , 50Hz, 240VAC inverter (grid tied)

[waveboy]

Hi,
Would you agree, that this is a simple job? One simply has to set up sinusoidal PWM into a H bridge, and have output current limitation.
You make sure your DC bus is say , 400V, so that its always above mains peak...then you simply sinusoidally PWM it.....the sinusoidal PWM is made by a ref sine into a comparator with a sawtooth into the comparators other input...and you monitor the current drawn...and adjust the PWM pulse widths until your average output current is correct. Would you agree with this? Its almost an "open loop" operation?

 

[itsmoked]

Would you agree, that this is a simple job?

No, not at all.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[IRstuff]

Its almost an "open loop" operation?

you monitor the current drawn...and adjust the PWM pulse widths until your average output current is correct.

This is almost the textbook definition of NOT open loop

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[waross]

The term; "Re-invent the wheel." comes to mind.
But we may be underestimating you. Let us know how it works out.
By the way, shifting the zero point of the PWM signal to control or limit the current may be non-trivial.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waveboy]

Thanks, reference GTI's......they have no current control...they cant, because the current drawn from them depends on the loading of the phase........they have no voltage control...the voltage at their output is determined by the mains.
The GTI's only have current and voltage limiting.

The attached LTspice sim shows an open loop GTI doing a pretty good job with no control whatsoever......i admit that a little bit of extra control is needed to get that current sinusoid and in phase with the voltage...but as you can see.....the jobs almost done here, and thats open loop!

This is why i say the GTI is nowhere near as difficult as its made out.

 

[waveboy]

sorry, i am going to have to learn how to attach more than one file here...back soon
..for some reason i cant upload the ltspice sim?

 

[LionelHutz]

That's OK. You didn't ask any actual questions about the simulation, so no-one needs to see it.

 

[waveboy]

Do you think the attached would be OK to take forward as a Grid Tied inverter?
It puts a sinusoidal current into the mains. Is mostly analog...would just need some software to generate the sinusoidal reference and do all the protections needed for a GTI.
(LTspice of grid ited inverter attached)

 

[waveboy]

Also, here is the schem of the GTI attached (i dont think you can have more than one attache per post?)

 

[jfpe]

waveboy,
What are you planning for a power source feeding your inverter? Where does your isolated 400VDC come from?

-JFPE

 

[BrianPetersen]

Why are you designing something like this, rather than buying one? What do you want to achieve, that a good purchased unit doesn't already do?

Why re-invent the wheel?

 

[waveboy]

Yes, we could buy an excellent one from China.
However, we need it to last >10 years, and so we will need to work with the manufacturer over the years to put right any failure points as they (possibly) show up.
The Chinese are great people, they are genuinely decent and superb. China has become "first world" without invading other countries. The Chinese are most honorable.
However, within the next 10 years, the Chinese will (possibly rightly) take over the entire south china sea, and all of its wonderful oil and mineral reserves. The "nine dash line" will belong to China.

When this happens, we believe the West will ban all further trade with China, and so we wont be able to get the support we need for this inverter.
We have to think of such things before we jump in with wallets flashing.

The Chinese are truly decent people though....and we really want to trade with them.

(We are already heavily invested in China, and are really worried that we may get banned from continuing this relationship with China. Especially with all this "climate" talk.
Is this for real?...this villification of oil?.....is it something to do with the fact that the west is running out of oil....therefore they want to villify it?
...its this kind of rhetoric that worries us that soon there may be a ban on trade with china...but we sincerely hope not....we are not pulling our current investments out of china though)

 

[waross]

Buy the design rather than the inverter.
That is; Arrange to build your own to a Chinese design under license.
You will be able to replace failed components and with the source code you will be able to tweak performance.
Costs a little more but worth it.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!