Inverter/ MicroInverter Questions

adam1984

So heres the deal guys... Im having some confusion. So just when i thought i had it all figured out i realized i know absolutely nothing about solar. I understand the inverter converts DC to AC power. I understand that on an oscilloscope dc power is a flat line and AC is a sine wave. I don't get whats going on inside the inverter. I understand it takes steps and involves a square wave, but not sure how. Does it reconstruct the wave by pulsing more and more square waves until it looks a sine wave? PART 2-Microinverters? I was under the assumption that only charge controllers were MPPT, and now i read that microinverters (one per panel) track the maximum power. So the micro inverter is hooked up straight into the panel then the output goes to the charge controller? If its job is to convert the DC panel output to AC, then why would it convert to AC before it gets to the battery? Just really confused and frustrated. Any explanation on microinverters and how inverters work would help. you guys are great.

niel

Re: Inverter/ MicroInverter Questions

over simplifying this, in an mppt controller there is a conversion to ac and then back to dc, but is not inline with utility voltages or frequencies. the gt inverter holds this conversion to utility specs without going back down to dc. the gt inverter is more efficient as a result of less steps or conversions than the mppt controller would be, but the mppt controller reaps more available power than a straight pwm controller would.

BB.

Re: Inverter/ MicroInverter Questions

Look at switch mode power supplies.
And many use a buck mode type switching power supply (used to drop voltage).
Although, a Enphase Micro inverter probably uses some form of boost supply (used to increase voltage).
Here is some basic information about AC inverters.

So, look at blocks:

MPPT Solar Charge Controller:

• solar panel - buck mode - battery bank

GT Inverter (large, high voltage type):

• solar panel string - buck mode - sine wave inverter - AC line

GT Inverter (Enphase low voltage input)

• solar panel - boost mode - sine wave inverter - AC line

Hybrid GT inverter System (battery bank GT):

• Solar panel - buck mode MPPT charge controller - battery - boost mode - Inverter - AC line

I don't know if the above confuses you or helps you. Note that the control/feedback circuitry in these guys is very complex and behave differently depending on the input and output needs.

For example a Hybrid inverter in off-grid mode operates like an AC voltage-mode source. A hybrid inverter in Grid Tie mode operates like an AC current-mode source.

-Bill

icarus

Re: Inverter/ MicroInverter Questions

I think you have a bit of confusion.

One wouldn't use a micro inverter with a battery based system. (the enphase micro inverter that I think you are talking about is a grid tie inverter).

So in a battery based system you have PV-wiring-charge controller- battery. (all DC (except inside an MPPT controller).

Loads are powered like this,, battery (DC)- Inverter (converts to AC) and then AC loads.

A enphase inverter goes PV (DC)- enphase inverter (AC)- grid- loads.

Hope that clears that up.

Tony

adam1984

Re: Inverter/ MicroInverter Questions

The info helped me bb. And also, the last post was correct. I was not understanding why you would convert to AC to charge a battery, but since these are for grid tie systems, no batteries! this makes sense now. Not sure how i get so confused.... bb thank you for the links. Tony that cleared things up completely. Glad i have you guys. They should pay you for doing this. Really helps alot although i guess not so much on your end... but i really do appreciate all the help i get not only in this post, but all of them.

BB.

Re: Inverter/ MicroInverter Questions

You are welcome Adam...

Now, to blow your mind--Yes you can do what you said (convert to AC to charge a battery using a GT type inverter)--and here is how:

GT Inverter (large, high voltage type) to Off Grid Inverter:

• solar panel string - buck mode - sine wave inverter - AC line - AC output of Off Grid Inverter - Battery Bank

It turns out that some (most, all?) sine wave inverters are actually bi-directional power devices. Yes, you hook up a normal off-grid (stand alone inverter) to power your cabin--and everything works the way you expect it too...

Now, connect a Grid Tied inverter (either the high voltage solar array type or, presumably, the smaller Enphase mini-GT inverters will work too) to the running Off-Grid inverter. The only restriction being that the GT inverter+solar panels have to have a lower peak output than the Off-Grid inverter's output (like 3kW of GT inverter back-driving a 4 kW off-grid inverter).

The OG inverter setups up your 120/240 VAC 60 Hz "grid) power (Just like the utility). Then you have your GT + solar array connected to the same "Grid" power (off grid inverter's AC output).

If the AC load is greater than the GT inverter's output, the off-grid inverter will make up the difference. If the GT inverter's output is greater than the load, then any excess energy "flows" backwards through the off-grid inverter and actually charges the battery bank (several brands/models of inverters have been setup this way and found to work very nicely).

The draw back (there is always a catch)--1) the off grid inverters have never been tested/approved for such a use. And 2) there is no battery charge controller so it is very possible to overcharge your battery bank (and worse) unless you manually turn off the GT inverter when the batteries are "charged" or you setup a couple charge controllers to "deal with" the excess energy (there are several ways to do that too).

Anyway--the more you know, the more you realize you don't know very much.

-Bill

RCinFLA

Re: Inverter/ MicroInverter Questions

To your question on how a sinewave is created from D.C.....

The duty cycle of the squarewave is continously varied to match the instantaneous sinewave voltage. The result is filtered, resulting in very clean sinewave.

To show example. Take a 350 volt peak 50% duty cycle squarewave (equal time high and low) at 25 kHz and filter it. This results is output that is average of the input waveform, in this case 350 vdc times 0.5 equals 175v output.

Do the same at 10% duty cycle and you get 350v times 0.1 equals 35 v output.
At 90% duty cycle you get 350 v times 0.9 equals 315 v output.

I used 350 vdc in the example because that is about the minimum needed to create a 240 vac sinewave which has a peak of 339 vdc. There needs to be a little extra D.C. voltage to account for voltage drop in the switching MOSFET devices.

To get the neqative half of sinewave the D.C. polarity is flipped. This is all done with four switches in an H- bridge configuration.

A microGT inverter has a DC-DC switching boost converter to take the panel voltage up to greater then 350 vdc.