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tony-pecora avatar image
tony-pecora asked

How to couple a large solar PV array (Fronius) to a smaller Quattro in stand alone application?

We want to couple a 100kWp solar pv array (Fronius) to 3 x Quattro 48/15000 units.

While under direct sunlight, the 100kWp solar PV array will supply power for daytime loads + battery charging, with the battery bank tied to the 3 x Quattro units providing a buffer during sporadic cloud cover.

Due to the 1:1 rule, the Fronius must be connected to the AC-in of the Quattro units, but as it is a stand alone system, without grid connection and without genset supply during extended sunlight periods, how can the Fronius remain operational while not sensing additional AC supply?


Basically, how can we couple a larger rated Fronius inverter to a smaller rated Quattro in an off-grid system without a generator?


AC PV Coupling
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ponzoa avatar image ponzoa commented ·
Questions to be able to offer solutions here;

1) what are the actual power demand requirements here. This will define the size of your Victron installation as that is the power it will need to be able to supply at night or with clouds etc.
2) have you confirmed or purchased anything yet or is it still in design/quote phase?

Ideally, you haven't closed anything with the client yet. If not and once Q1 is defined, you can build from there.
a) size your Victron onverter/charger system and batteries. (The batteries MUST be able to priovide the full capacity power)
b) define Fronius on AC Out to be same power rating or smaller.
c) define reast of solar array desired using Victron MMPTs and set to DC Bus (Lynx distributor or similar.
d) select GX device and how you're going to connect to monitor and provide support/updates.

The Fronius inverters require a grid to syncronise with or they will NOT produce/provide any power. In the case, connecting them on the AC out, Victron provides the (grid equivalant) reference.

Good luck.

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matt1309 avatar image
matt1309 answered ·

Hi @Tony Pecora

The easy answer is normally wire on ACIN side along with the grid however without that it's more challenging.

It's probably more of a question for you. Where do you want the excess power to go. If you've not got a grid and you battery/quattro's are maxed out where would you expect/want the power to go?


I would say you've got three options.

1. (Very carefully) Design a dump load system similar to what you see in hydro installations (however 100kwp it'll need to be a very large dump load). Dump load needs to be able to drop output power of PV to well below 1:1 rule, ideally 0 for safety as I wouldn't want to rely on frequency shifting given the large difference between quattro capacity and peak KW of PV. (I'm also not 100% sure if this voids warranty not sure if 1:1 rule is included in warranty.) D

2. Get a grid connection. Excess power goes there. (Essentially ESS system with AC coupled PV on ACIN, loads and everything else on ACOUT1).

3. Get a huge amount of batteries/quattro's to meet the 1:1 rule.




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Michelle Konzack avatar image
Michelle Konzack answered ·

What you are writing does not work:

We want to couple a 100kWp solar pv array (Fronius) to 3 x Quattro 48/15000 units.


You would need to spit the PV Array into e.g. 45kWp on ACout and 55kWp on ACin, but


Due to the 1:1 rule, the Fronius must be connected to the AC-in of the Quattro units, but as it is a stand alone system, without grid connection and without genset supply during extended sunlight periods, how can the Fronius remain operational while not sensing additional AC supply?

Stand Alone System mean Off-the-Grid, hence your ACin is useless and a Fronius can NOT run on ACin, because it need a Voltage to orient to.

If ACin is NOT connected to the Grid, the Fronius does not start.


Basically, how can we couple a larger rated Fronius inverter to a smaller rated Quattro in an off-grid system without a generator?


You cant!


Your only Option is to put 45kWp on ACout and use 5 Victron RS450/200 for the remaining 55kWp to charge the batteries direct.

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tony-pecora avatar image tony-pecora commented ·

There must be a way.

It seems absurd that we are not able to cater for incremental cloud cover for a large 'daytime' (100kWp+ solar array) off grid power system, without having to over-engineer the inverter chargers and battery bank.


Many facilities operate during daytime hours, and require large daytime loads, but minimal nightime loads. Does Victron not have a solution for large daytime stand alone applications?

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tony-pecora avatar image
tony-pecora answered ·

Hi Matt, thank you for the response, but grid is not an option, however, we do have big dump loads available.

The question mainly refers to the operation of the Fronius Inverter. Does the Fronius remain operational even when the genset is off?


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matt1309 avatar image matt1309 commented ·
As @Michelle Konzack has mentioned it does not.


I find it easier to think of like this: It's not the genset keeping the fronius on, it's the quattro's. The quattros make a micro grid. The fronius sees that as the grid and turns on. I find it easier to think of it more like genset is keeping quattro batteries topped up and quattro is generating the microgrid. ie just having a generator hooked up to a fronius would as MIchelle mentioned end badly.


The dump load setup definitely natively supported. You'd have to have make custom infrastructure (that is very carefully managed and dump loads would need to be massive) to ensure fronius output was kept inline with 1:1 rule. 100kwp dump loads is like 33 residential immersion heaters, which would need a lot of water....


This definitely isn't supported though. You'd have ensure the dump load infrastructure can operate under unusual frequency for frequency shifting to remain unaffected. You'd also have to be very confident your dump load is able to maintain 100kw of output otherwise you could end up with a very dangerous setup.

In a hydro setup you'd also have a mechanical shut off in the event the dump loads failed/didnt respond fast enough. ie commonly you'd have a blocking plate that falls in front of water stream controlled by an electromagnet to turn off hydro. This is the mechanical fail over element. You'll likely need a PV alternative to this. This is normally designed to require manual reset as it's an indication of an issue with the system. I'm not sure what an appropriate PV equivalent would be (relay is the first thing that comes to mind but that doesnt feel sufficiently redundant like the hydro equivalent).

I would imagine the cost of hiring experienced firms to design such custom infrastructure would be very expensive.


The "proper" way to do it i would say is what Michelle's suggested. Of splitting the solar over AC coupling and DC coupling.

That way you've got the AC coupled PV that can't overload the victron inverters but you dont lose any solar production as the DC coupled MPPT's can ramp up/ramp down accordingly.


Is there a reason why you want the 100kwp all as AC coupled. DC coupled is more efficient when you're not using the power straight away.

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tony-pecora avatar image tony-pecora matt1309 commented ·
Thankyou for the response.

The facility has large daytime loads, and wants to utilize solar PV under direct sunlight to power those loads, while using a battery bank for sporadic periods of cloud cover, ie. battery bank to act as a buffer for 1 hour of cloud.

Any more than 1 hour of cloud cover, then the generators will kick in.





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matt1309 avatar image matt1309 tony-pecora commented ·

@Tony Pecora

For the sake of cost/ease of install. I would go with what Michelle suggests.

DC Couple 55kw and then AC Couple the rest to achieve 1:1 factor of 1 rule. It's fully supported installation. No weird custom setups needed.

Any other route i can think of requires some sort of quirk/additional planning/thought and additional safety measures. The efficiency losses from going down to low voltage DC with MPPT's and back up via inverter would in theory be minimal.


Or if project isn't urgent and efficiency is preferred take a look at the RS series inverters. I believe they cannot be paralleled yet but are expected to be at some point.

I believe these will have the efficiency closer to AC coupling if power is used directly (transformer-less inverter).


The dump load with fall back safety mechanisms idea might work but I'd constantly be worried about them failing and frying the whole system.


It might be worth a maths calculation, cost of efficiency losses going from PV to battery voltage and back up vs PV to 240Vac. Over say 5 years. Then compare that with quotes from electrical engineers for installing the custom failover/dump load mechanisms.

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Michelle Konzack avatar image Michelle Konzack commented ·
No, but you can not connect a Fronius to a GenSet, because they will kill each other.


For your MEGA project you definitive need 9 Quattros 15kVA.


The Fronius are not designed for Stand-Alone-Use.


This has NOTHING to do with Victron!

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Tim Larsson avatar image
Tim Larsson answered ·

Could you not use node red to control some contactors to 55kw of the fronius setup such that when batteries reach say 90% you turn off the ac connection to these? Thus sort of conforming to the 1:1 rule on ac output?

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Tim Larsson avatar image Tim Larsson commented ·

If you measure the power need of the facility and rotate in part of the extra 55kW of fronius inverters as you need them with some logic in node red. Could this not work?

Obviously you would also need some backup in form of a dump load. As I am quite new to this I would assume that there are more saftey considerations to be had which I am not aware of.

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matt1309 avatar image matt1309 Tim Larsson commented ·

I'd agree with you Tim, you'd almost want some non electrical form of safety. In hydro/wind when you have dump loads you tend to have a mechanical override/safety/brake mechanism. Like you said the risk is a contactor and or dump load could fail. And you're stuck hoping that the frequency shifting can activate quickly enough to not damage the system.


I imagine the solar equivalent is a giant umbrella....

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