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Hypothetical Solar Proposal - Possable or not???


Wetkit
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Right, so currently I do have a 3kw Infini Plus invertor,

235Ah battery bank,

18 x 235w PV panels configured in 2 separate strings of 9 each.

 

Problem I have is that I have more than enough PV, but the inverter is prone to overloads.

I have been looking at adding a normal grid tie invertor into the mix and running that unit off 1 string.

Due to me having pre-paid power, I would have to get a grid tie unit that offer a limiting device.

Currently only 2 companies does this.

Microcare has their own limiter device working only on their inverters.

ExSolar uses a dedicated meter with a Fronius invertor. Good implementation, but $$$.

Thing is, both options is very expensive, say R25k and over.

 

Now I have this idea to run 2 x Infini's in series.....

I know you cannot parallel them, but I have not seen anything in the documentation that say you cannot put 2 in series, or put a Grid-tie inverter onto the output of one???

 

So here is what I was thinking off...

This will cost less than any of the above options, and I will have a backup unit if one fails.

 

post-23-0-76352000-1445511391_thumb.jpg

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I'm not an expert but I don't think you can push grid-tied energy into a micro-grid without some kind of control system. When you have the grid to push back into... that can sink a fair amount of power. But when you don't have that, you need a battery bank and a good control circuit.

 

The usual way to do this, is to have something like a Multiplus with the GTI connected to its output. Then you configure it to adjust the output frequency between 50hz and 53hz. As long as the GTI supports GFPR (Grid frequency-dependent power reduction), your microgrid controls the power fed into it by adjusting the frequency.

 

So looking at your image, I'm worried about the Infini on the left that has nowhere to go with excess power... if that inverter can mess with the frequency of your micro-grid, and the other one adjusts to it... it will work. Otherwise... I'm not an expert, but I think it's not going to work.

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Assuming one can install them in series, at some stage the first inverter may be overloaded as it will have to supply the total load of 3500Kw.

I agree with @SilverNodashi - 2 separate systems, 2Kw each would be enough.

 

My garage doors and driveway gate have backup batteries, I haven't bothered connecting them via the inverter.

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Re gate motors. I found out that my gate motor's charger isn't capable of carrying the full current required when it's actually moving the gate. It relies heavily on the battery for this. Now as solar people we know batteries well... they don't like deep discharges. You want to get it full as quickly as possible to avoid sulphation. This applies even more to those cheap-*ss SLA batteries used in automatic gates. So I have my gate motor on the inverter. It only needs around a peak of 40W anyway (for a few minutes right after the gate was operated).

It's a Centurion DS5, in case that's of value to anyone with a similar device :-)

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Re gate motors. I found out that my gate motor's charger isn't capable of carrying the full current required when it's actually moving the gate. It relies heavily on the battery for this. Now as solar people we know batteries well... they don't like deep discharges. You want to get it full as quickly as possible to avoid sulphation. This applies even more to those cheap-*ss SLA batteries used in automatic gates. So I have my gate motor on the inverter. It only needs around a peak of 40W anyway (for a few minutes right after the gate was operated).

It's a Centurion DS5, in case that's of value to anyone with a similar device :-)

Both my gate and garage motors are connected to the inverter as well. It's cheaper to replace those small 7Ah batteries every 2-3 years than replacing the expensive solar batteries ;)
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Victron does something similar with a normal and grid tie inverter combination, where they use frequency shifting to limit the load.

Yup, in fact their system can operate with pretty much any GTI as long as that GTI reacts to a frequency change. In the "assistant" where you configure this, you type in the frequencies, and it simply does a linear calculation between the two frequencies. If I recall correctly, you can also put a current transformer onto the GTI line and what it will do then is adjust the frequency upwards until the signal from the current transformer drops low enough. But all those toys do cost a bit of moolah.

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