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bergmans-trading-cc avatar image
bergmans-trading-cc asked

Semi Off Grid dilemma

Hi All

Skylla with Quattro setup

I am from South Africa, so some of you will know that except for the constant looming of loadshedding , our electrical grid is in a bad state.
When ever one wants to install a Grid Connected solar system, does not matter what brand or type, Grid Tied or Hybrid, when you connect the input of your inverter to ESKOM( our power utility) in the country side you need to hand in an application first with all plans and designs prior to installation. Even if you do not plan on feeding back to the grid. They are very strict regarding this. Which is understandable, but very frustrating and tedious and can take very long.

When connecting to municipality in a town they are somehow and somewhat less strict.


This leads me to my question, and please direct me to the relevant thread if this has been covered somewhere else. Also do not hesitate to shut my train of thought down if it is not possible or not legal. I am merely pondering with ideas around a problem lots of customers and installers face

Is it at all possible to have the following system :—Grid point transformer is a 50kVa point—

So any loads on the system cannot ex

3 X Quattro 15 KVA 3 phase setup (no AC input)

6 x 250/100 MPPT

40 kw PV array for MPPTs

Fimer or Fronius 50 kVa string inverter on AC output of Quattros

50kW array for string inverter

100

kWh lithium battery bank

3 x Skylla TG 48V in paralell to charge battery bank (specified hours at night or cloudy days)

Cerbo GX

All relevant fusing and isolation and earthing of course.

The important part is an 3 phase automatic on load change over switch.

Once the battery reaches a certain SOC a relay can be triggered to energise a contactor to automatically switch the load back to grid.

Center changeover

It is important that the hybrid and pv inverters are not connected to the grid what so ever.

The only grid connection is the Skylla chargers that feed the batteries if needed.

This way you are not completely off grid witch is fine because the main goal is minimal reliance on the grid without the time consuming and difficult long process.

Perhaps someone gan assist me or advise me if this train of thought is completely off the rails.

Thank you

MultiPlus Quattro Inverter ChargerLithium BatteryoffgridSkylla
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Alexandra avatar image Alexandra ♦ commented ·

@Bergmans Trading cc

Why not just keep the loads on the inverters and trigger the skyllas to charge or maintain the batteries? Unless you feel the bank is too small?

The issue with switching back and off the grid is you are relying on the grid to be there when you switch back. Would there be any equipment that gets messed up by the changing over of power?

I would still have the contactor or changeover there though in case the system goes down or needs maintenance and the grid is available to use as an interim.

Not sure the rules and regulations in SA for the setup so cannot answer on that.

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randyoo avatar image randyoo commented ·

I'm located in the USA, and may actually implement a somewhat similar solution to my utility's demands for bureaucratic design/schematics, permission, inspection, etc. The problem with a contactor (I think) is that running appliances can be damaged by a phase that could be up to 180º out. Of course, the battery chargers would be inefficient, compared to passing AC directly. Ideally, we could use the Multi/Quattro AC input, which would ensure synchronization of phases before engaging the relay/contactor for pass-through. Sadly, for us, it seems to be out of the question.


Most likely, I'll search for the most efficient 48V battery charger I can find, and just run off the inverters full-time, as @Alexandra suggests. If forced to switch over to public grid, though, I'd like to have a delay, to avoid damage to running appliances.

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completeelectrical avatar image completeelectrical commented ·
I have a somewhat similar requirement but less so because of the load shedding but more due to unstable and erratic quality of supply. For instance we get anywhere between 12-18hrs supply a day but its on and off throughout the day and the voltages vary from 186 to 240v.

What I want the Skylla to do is keep charging the batteries whilst we run a clean inverted supply constantly as if we go through the invertor, it just acts as passthrough so long as its above 186v. Yes I know I can set the lower threshold but then its switching the invertor on and off all through the day which I'm sure is worse than constantly running in invertor mode.

My only concern is the charging cycle of the batteries is constant so will possibly diminish the battery life quicker than normal


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Mike Dorsett avatar image
Mike Dorsett answered ·

Seems like this would be similar in regulation terms to adding a stand by generator, with manual change over switch. In terms of compressors running during change-over, you would want at least a 5 minute 'off' in order for the gas pressure to drop, so the compressor is not overloaded on restart. Some fridge compressors can take care of this themselves.

Best way is to dimension your solar system so as you are virtually self sufficient - usually about 120% to 150% of normal needs, then you only need the grid in bad weather - usually when it would be likely down!!

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