Author Topic: Micro inverters versus String Inverters  (Read 6721 times)

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MattM

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Micro inverters versus String Inverters
« on: April 10, 2014, 01:44:47 AM »
Am I understanding the main differences between the Micro inverters and String Inverters is whether I would want to be off grid or grid tied.  From how I comprehend the diagrams I either off set my normal grid tie electricity with Microinverters or sink a load into storage and go for String Inverters to become off grid.  The Microinverters seem to be paperweights when grid power goes out.  If I want uninterrupted electricity then opt for a larger String Inverter and use storage to power the house at night.

I was liking the idea of Microinverters until I noticed where the disconnect is attached.  Instant buzz kill.

mab

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Re: Micro inverters versus String Inverters
« Reply #1 on: April 10, 2014, 08:31:49 AM »
My understanding is that for grid-tie you can have micro or string; most of the grid tie I've seen have been string.

If you want an off-grid system with storage then you need an off-grid inverter who's input voltage relates to your storage battery (I don't think yu can have off-grid without a battery of some sort), and the solar connected to the battery either via an MPPT controller, PWM controller or direct connection plus a dump controller.

just to confuse the issue some off-grid inverters can be backfed a.c. (SMA sunny island for e.g. - I think)) so you could connect your solar via a grid-tie inverter to your private a.c. grid.

MattM

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Re: Micro inverters versus String Inverters
« Reply #2 on: April 10, 2014, 09:39:48 AM »
I do believe both the utility companies and the fire codes create most of the headaches, regardless if you grid tie onto a public or private grid.  Just from my initial research.

john8750

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Re: Micro inverters versus String Inverters
« Reply #3 on: April 10, 2014, 11:36:36 AM »
Micro-inverters could be used on an off-grid system.
But, how does the safety circuit work to disconnect main power in a black out, if connected to the city grid.
Does the micro-inverter need a battery?
Keep the fun in it. Give me sun light.
John Smith

DamonHD

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Re: Micro inverters versus String Inverters
« Reply #4 on: April 10, 2014, 12:44:09 PM »
And grid-tied inverter needs to watch the grid connection and if it behaves unexpectedly including apparently being disconnected then then inverter has to shut down.

Is that you mean?

Rgds

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OperaHouse

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Re: Micro inverters versus String Inverters
« Reply #5 on: April 10, 2014, 12:58:16 PM »
Are you looking at a particular micrinverter or is this a theoretical question?  I don't have any first hand
knowledge of current products but my understanding is as follows.  In any engineering application there is always
one way that has a cost benifit.  With common panels now in the 300W range, an inverter for each panel is now
practical.  Boosting to a higher voltage allows a common buss collector wires to be smaller and less expensive.
For an array that may get partial shading, microinverters will be far more efficient.  If they incorporate CAN
communications the health of each panel will be available. Having to match panels in a string will be a thing of
the past.   It all may seem like a big complication right now, but these systems will become more commonplace
because they are better. As in everything else the hobbiest is going to be pushed out by technology.   I will
be switching to a 36V array buss and I will be modifying some boost converters to power point so my odd 12V
panels can tie into the higher voltage buss much like a microinverter.

Mary B

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Re: Micro inverters versus String Inverters
« Reply #6 on: April 10, 2014, 05:29:23 PM »
The downside to micro-inverters is no off grid capability that I have seen. Some say back feed with a pure sine inverter to make it think the grid is there. That would likely violate electrical code and if you ever had a fire insurance would look at it and say sorry, house had code violations we are not paying. Plus I do not know anyone who has dome it, getting the extra inverter to like the output from other inverters is an unknown.

birdhouse

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Re: Micro inverters versus String Inverters
« Reply #7 on: April 10, 2014, 08:18:05 PM »
i don't know why you would ever want to try to use micro inverters for off grid?? 

especially with the current technology and price points that morningstar, outback, xantrex and midnite offer. 

80A at 150+ VDC through some of these.  i'd think micro inverters would cost a fortune to handle that amount of panels in a off grid scenario, and it still seems it would be hard to install them in a legit way. 

i believe oztules was using a BIG grid tied inverter with battery charging to control a large PV souce.  i think he had it set where the inverter thought the grid was always down, so the amps went to his battery bank.  then he somehow tricked it to thinking the grid was active again when he wanted to draw from the solar.  he could then pull his 240v output straight from the panels without discharging his battery bank.   

i might be missing something there.  it was a while back when he was explaining the system. 

adam

oztules

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Re: Micro inverters versus String Inverters
« Reply #8 on: April 11, 2014, 02:01:12 AM »
Some misconceptions here.
Yes I did/do use large grid tie inverters to charge the battery bank... it is only 800ah@48v or about 39kwh in size.

However, I have had over 4kw from the grid ties charging the batteries, and running the house at the same time.

First you need a decent pure sine wave inverter/charger.
Only the SMA island one works in the commercial world as far as I know.
I built a 8000w pure sine wave unit for less than $400, and included  18uh choke in the drive arm of the torroid transformer I made.

This choke is the key somehow. I had started out to kill off the 20khz switching signal out of the iron cores way, but instead found that a tiny choke made the idle come down to 25 watts or so. It seems to help the fets to switch on fully, before the current starts to flow, as it does two things..... lowers the idle current from 200 watts to only about 25 watts...... and the other benefit, is you can hook up as many kilowatts of grid tie as you please, and the transfomer and fets run very cool.

The grid tie units are not tampered with in any way, and will switch out  from + - half a hertz, and any voltage variations within the AS3777 ( I think.. code).... very strict.

So it is the inverter that is critical in it's design.

Mine looks like this.





The box is from a gutted grid tie unit, and the transformer is made up from 2 torroids out of 3kw grid ties.


................oztules
Flinders Island Australia

oztules

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Re: Micro inverters versus String Inverters
« Reply #9 on: April 11, 2014, 05:19:00 AM »
Oh, and that torroid weighs near on 60 lbs. If I had used EI laminations, you would need near 120 lbs, and probably twice the magnetising current.

The grid ties are standard. The inverter provides the 50hz "grid". The inverter then sends any excess power that the house does not use to the batteries backwards through the mosfets while they are switched on in their normal inverter function. Any grid tie will do, but the inverter is the key.

It should also be noted, that you can't regulate the grid ties in this setup. You need a very big dump load, or in my case, they are automatically interrupted ( AC relay) when the batt voltage is high. They then spend a few minutes as per the AS standards..... checking for power purity, before switching on again.... then get turned off again by the comparator ... and so it goes....... kind of a bang bang controller with 3 minutes hysteresis I guess..... no pretty, but works well.

The grid ties are soft start because of the mppt functions built into them.

Good setup if your panels are a way a way from the battery shed. I transmit the power at 350vdc to the grid ties. They are next to the main inverter at the battery bank.

As an aside.... I do get tired of hearing how home made electrons can burn a house down that the grid can't. My inverter will drive 6kw for lengthy periods, 24000 watts for very short spells..... the grid can do lots more than that. I fail to see the argument at all.



................oztules


Flinders Island Australia

Rob Beckers

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Re: Micro inverters versus String Inverters
« Reply #10 on: April 11, 2014, 07:25:56 AM »
Actually you can use grid-tie inverters (micro- or string-inverters) off-grid. It's been done for years, and is called "AC coupling". Xantrex wrote a nice white-paper about it. It still requires a regular off-grid battery-based inverter; the grid-tie inverter(s) see the local grid, synchronize to it, and feed the local loads. Any excess energy (beyond what the local loads need) will funnel back over the battery-based inverter to charge the batteries. Works like a charm with all the major battery-based inverters! In fact, it should work with any H-bridge based inverter since that topology works "both ways" when it comes for moving energy.

A word of warning: Do not use the SMA Sunny Island as your battery-based inverter for AC-coupling. They change their frequency to signal to the grid-tie inverters that the batteries are full. That works with SMA's products (SunnyBoy and WindyBoy) but will kick any regular grid-tie inverter off-line when the batteries are at just 70% or so.

I'm simplifying things a little in the above, but not by much. There's the need for charge control, and current handling of the battery-based inverter. Read up on AC-coupling and you'll get the idea. It does work (know several that use this and have for many years).

-RoB-

camillitech

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Re: Micro inverters versus String Inverters
« Reply #11 on: April 14, 2014, 02:40:07 PM »
Actually you can use grid-tie inverters (micro- or string-inverters) off-grid. It's been done for years, and is called "AC coupling". Xantrex wrote a nice white-paper about it. It still requires a regular off-grid battery-based inverter; the grid-tie inverter(s) see the local grid, synchronize to it, and feed the local loads. Any excess energy (beyond what the local loads need) will funnel back over the battery-based inverter to charge the batteries. Works like a charm with all the major battery-based inverters! In fact, it should work with any H-bridge based inverter since that topology works "both ways" when it comes for moving energy.

A word of warning: Do not use the SMA Sunny Island as your battery-based inverter for AC-coupling. They change their frequency to signal to the grid-tie inverters that the batteries are full. That works with SMA's products (SunnyBoy and WindyBoy) but will kick any regular grid-tie inverter off-line when the batteries are at just 70% or so.

I'm simplifying things a little in the above, but not by much. There's the need for charge control, and current handling of the battery-based inverter. Read up on AC-coupling and you'll get the idea. It does work (know several that use this and have for many years).

-RoB-

Yup,

I've been doing it for years with a Trace SW4548e and SMA GTI's works a treat, has many advantages and I still have regular DC inputs to my battery bank from wind and solar. You just have to make sure you have plenty of reliable dump loads. Funny enough I've just been considering the micro inverter scenario for just one 200w panel. I power up a wireless relay antenna over a kilometer away in exchange for free broadband, one 200w panel mounted on the frame with a micro inverter on the back would supply the 85w needed for the aerials and feed any excess down the armoured cable to my house. When the sun don't shine then it would just be fed as normal from my own 'mini grid'.

http://lifeattheendoftheroad.wordpress.com/2012/05/15/commissioning-the-powerspout/

There's some links on there explaining the theory, sure it's not the answer to everything but if your usage is matched with production then it's far more efficient. It also has the advantage that GTI's are far cheaper and can work at much higher voltage than regular 'off grid' MPPT's.

Cheers, Paul 

lifer

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Re: Micro inverters versus String Inverters
« Reply #12 on: April 14, 2014, 06:56:25 PM »
I built a 8000w pure sine wave unit for less than $400, and included  18uh choke in the drive arm of the torroid transformer I made.

This choke is the key somehow. I had started out to kill off the 20khz switching signal out of the iron cores way, but instead found that a tiny choke made the idle come down to 25 watts or so. It seems to help the fets to switch on fully, before the current starts to flow, as it does two things..... lowers the idle current from 200 watts to only about 25 watts...... and the other benefit, is you can hook up as many kilowatts of grid tie as you please, and the transfomer and fets run very cool.

Sorry for being kind of off-topic.. I'm very interested in @oztules inverter's design (I even tried to PM him for details but he might didn't get my message anyway).

Does anyone have any complete schematics of such an inverter (3-4kW)? I guess this is the simplest (and more feasible) way to build a diy sine inverter. You only need a full bridge stage driven by a SPWM signal and a regular low frecquency transformer. And, of course, that mighty choke (and an output capacitor).

Thanks in advance for any information!

oztules

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Re: Micro inverters versus String Inverters
« Reply #13 on: April 18, 2014, 06:34:57 PM »
I would not entertain a high frequency unit.... the commercial ones are substandard for house use, and the DIY would be no better.
Their surge ratings are hopeless.

The simplest way to make a fine 8000w inverter is to just buy the power jack LF control and power boards.... yes I know power jack are universally shunned for good reason,  but they seem to have cottoned onto a manufacturer of LF pwm control and power boards that are used in a lot of chinese lf inverters. They are tough and very reliable.

Buy the boards, and just add a 18uh choke and a very large high grade transformer ( from  very big welder or cheap grid tie LF units). If you want 6000w continuous, you need a big transformer, if you want 3kw continuous, then a smaller one.. it will surge to >20000watts for short periods ( minutes). It will start most anything you can find to run.

A fan is required for the heat sinks.... not because they get hot in normal use, but the computer requires to see it there.

and thats it.

All the information you need is in this topic:
http://forums.energymatters.com.au/post40869.html#p40869

.............oztules
Flinders Island Australia

phil b

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Re: Micro inverters versus String Inverters
« Reply #14 on: April 22, 2014, 01:36:54 AM »
That was a good read Paul. Thanks!
Oz- that was genius! I couldn't duplicate that one easily without more info... Thanks!
Phil