Author Topic: GTI design, unfolder, self switched vs controlled  (Read 421 times)

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Offline fourtytwo

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GTI design, unfolder, self switched vs controlled
« on: April 24, 2017, 05:09:57 AM »
Hi All, I am in the process of designing a GTI for my PV system. An unfolder is required to connect the uni-polar output of the smps to the bipolar grid. I am interested to see if anyone has experience of the self switched mosfet circuit enclosed ? I am presently using a controlled mosfet bridge along with opto-couplers and auxiliary power supplies controlling it from the micro. I occasionally get timing problems and have blown a mosfet as a result, this self switching version seems rather appealing as a way to reduce complexity and eliminate timing problems, but is it robust ?
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Offline tomw

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Re: GTI design, unfolder, self switched vs controlled
« Reply #1 on: April 24, 2017, 07:46:23 AM »
42;

GTI? Perhaps a Volkswagen GTI?

We encourage posters to either not use abbreviations or they define them before they use them.


Just saves the 20 questions to get the info.

They can mean different things to different fields / people.

Just saying..

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Offline fourtytwo

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Re: GTI design, unfolder, self switched vs controlled
« Reply #2 on: April 24, 2017, 10:40:15 AM »
GTI? Perhaps a Volkswagen GTI?

Very funny  ;D I thought perhaps in a forum entitled  "Automation, Controls, Inverters,  MPPT, etc" it would be fairly obvious and I am sure you know it's Grid Tied Inverter but whilst we are on the subject, what is MPPT ?

Offline DBCollen

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Re: GTI design, unfolder, self switched vs controlled
« Reply #3 on: April 24, 2017, 05:51:14 PM »
Maximum Power Point Tracking
Dustin.

Offline lighthunter

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Re: GTI design, unfolder, self switched vs controlled
« Reply #4 on: April 25, 2017, 05:35:36 PM »
Very interesting concept. Ive not seen it before. At first I misunderstood the term "unfolder"  This circuit is then only used for the timing synchronizer? Seems like it should work for that but the proof be in the actual performance. Be best to build it on breadboard and watch how it behaves with scope. 

If this is your complete circuit and not just a timing reference then i would say not a good idea. It only switches once per half cycle which would produce much distortion (any energy that falls outside or inside of the sine curve). Most grid tie inverters switch many times per half cycle dumping their power into an electrical flywheel (inductor) which outputs a true sine curve matching the original plus some current flow. Impossible to do this with only one switch per half cycle unless you used a class c amp with inductor and capacitor tank circuit tuned to 50/60hz, be tough to deliver any appreciable power though. From what Ive read H bridge is only way to go above a few hundred watts.  Anyway, have fun with it!

LH

Offline fourtytwo

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Re: GTI design, unfolder, self switched vs controlled
« Reply #5 on: April 27, 2017, 01:18:29 PM »
Very interesting concept. Ive not seen it before.
Hello LM thank you for your reply, me too :) I came across it some years ago drawn by a guy looking for help with a Chinese GTI that kept blowing Mosfets, at first I thought he had drawn it wrong but apart from changing a few diodes to zeners it seems to work perfectly except.... said problem of unreliability. His GTI was a simple 12/24V 300W plug in the wall type.  I just wondered if anybody had encountered a circuit like this in big multi-kw grid tie's.

I should mention that what the drawer labelled as +/-360V is actually the output from a high frequency pwm and is in fact a 100hZ unipolar sinewave. This circuit (H bridge) is sometimes called an inverse rectifier, sometimes an unfolder and I am sure has several other names too, I have also seen thyristor implementations.

At the moment my GTI unfolder is as I say a traditional hard switched H bridge using timing derived from the microprocessor that ultimately is derived from the grid zero cross however I have lost a mosfet in unexplained circumstances so if that happens to often I may try this one except I dont have a pcb for it at the moment and its hard to do high voltage stuff on breadboards.

Anybody out there torn down there GTI to know what the active circuit is immediately before the grid connection ?