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Best Axpert string config 5kva


Arandoza
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Hello all,

I currently have a 5kva RCT Unit.

The current string configuration is 3 X 250w panels in series and with 3 strings currently, maximum 4 possible.

I was reading thru one of the other Axpert variant models and they seem to recommend keeping the voltage in the 60-70 volt range for optimum performance.

So they recommend 2 X 250w panels by 6 strings, also coming out at 12 panels effectively.

Was just wondering if any of you had tried either or both of these configs and which one you found to be best?

My panel specs are

Voltage at Pmax (Vmp) 31.02 Volts

Current at Pmax (Imp) 8.06 amps

Open circuit Voltage (Voc) 36.99 volts

Thanks

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I noticed, in an Axpert manual pulled off the net somewhere, that the optimum PV voltage is around 60V for the MPPT charger to be most efficient. My own unit's manual (the Mecer variant) did not state this, but I see it does state that the MPPT range is up to 110V. So it seems there was a hardware change at some point, which allowed proper use up to about 100V (which is a string of 3). Makes sense, as your cabling carries less current this way. Mine is running fine on strings of 3, although it switches the MPPT on at abouts 60 V already. For a string of 3 it means the panels are not generating enough voltage yet, but it is the ideal value for a string of 2. So I am also considering changing mine to strings of 2 instead, even if just to overcome the constant mode changing while the sun is coming up and while it sets.

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A lot depends on your battery voltage.

I was told not to go over 3 x battery voltage on your MPPT input side.

2 x battery voltage would be much better for the MPPT.

 

I won't stare myself blind at the battery voltage thing. I think the reason why somebody said that, was because of efficiency in the MPPT. If you're looking at a stand-alone charge controller only used for charging a 48V battery bank, yes, then I would recon the MPPT would be most effective with and input voltage of up to 150V (3 x battery voltage), but if you are however converting the PV input to supply the AC load, then you might want to have a higher input voltage on the MPPT for it to be more efficient. Maybe Plonkster can shed some light on this.

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Well, remember that with normal DC invertors, the PV voltage is first converted to battery voltage, then only used again from the batteries for the conversion back to AC.

Only with some Hybrids and GTi the PV voltage is converted strait to AC, but then the PV input voltage would be much higher than 230V DC.

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I won't stare myself blind at the battery voltage thing. I think the reason why somebody said that, was because of efficiency in the MPPT. If you're looking at a stand-alone charge controller only used for charging a 48V battery bank, yes, then I would recon the MPPT would be most effective with and input voltage of up to 150V (3 x battery voltage), but if you are however converting the PV input to supply the AC load, then you might want to have a higher input voltage on the MPPT for it to be more efficient. Maybe Plonkster can shed some light on this.

It all depends on the design of the MPPT. An MPPT is essentially a buck converter, and the input voltage is one of the parameters used in the design of the buck converter (That is, picking the size of the inductor and the capacitor that makes up the converter).

There isn't a one-size-fits-all solution, and so there will be an optimum voltage for the input. Going outside of that means you lose some efficiency, but still much less than if you weren't using MPPT at all. The manufacturer will usually be able to tell you what voltages they designed for.

 

Quick buck converter crash course. Analogy: When drawing a bath of water to exacting temperature requirements, there are two ways to do this. The one is to adjust the ratio of hot to cold water on the input. But lets say you have really simple controls and no such gradual adjustment is possible. Instead, you can only turn it full-on or full off. How would you control the temperature? Simple (right?), you turn on the hot water until it gets a little too hot... then you turn on the cold water until it gets a little too cold, and so you continue, using the thermal mass of the water.

 

Buck converter works the same. It has a big old coil, usually wire wound around a ferrite core (the inductor), with a switch on the input side. The switch can only be full-on or full-off (because electronics get hot if you turn them halfway on). When it turns on, the current flowing through the inductor generates a magnetic field. This magnetic field will eventually saturate the ferrite core. If you now turn the switch off, this magnetic field collapses and generates electricity again (in the reverse direction). A diode is installed in the circuit to catch this reverse EMF and channel it to the battery. Now, in the same manner as you controlled your water temperature, you can control the magnetic energy content in the ferrite core by switching the current on and off with the right mark-space ratio.

 

But quite clearly, your ability to do so fairly efficiently will depend on how large the inductor is, and what the input voltage is, just as with the water experiment where it's much easier if you're using a bigger basin rather than a coffee mug, or water at 60 degrees instead of boiling point.

 

So there is no rule that fits all, it really depends on the design. I was told to go for twice the absorption voltage of the batteries, which for a 24V system is around 60V, same as what superdiy says.

 

Agreed that if you're going directly to AC, then the higher the better :-)

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Thanks everyone.

Oomd and I read the same manual it seems.

And I also read a few other items that seemed to suggest that ideally one wants to keep the pv voltage around 60-70v.

After considering all your reason's, and looking at the data from the axpert - pv volts and amps, I seem to think that the 3 panel under 115v Max is probably the best config. As I noticed it seems to allow the voltage to rise to 90-115 volts and a few amps pv when load is light, and when the load is heavy it drops to around 60v and 25 amps pv, which makes me think that with a limited voltage max range of 60-70 with 2 panels per string, it may not be able to control the pv power supplies in such a subtle way?

Also running the higher voltage should mean less chance for failure of the pv cables to the inverter, and less resistance or loss.

For now I think I will stick with the 3x4 string config.

Thanks again for all the valuable input. :)

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Hi everybody. I also got the RCT 5KVA busy buying solar panels now. Has anyone brought Benq 330 w panels.

 

Specs 20.4 % efficient

Vmp = 54.7

Imp  = 6.09

Voc  = 64.8

 

 Going to connect 2 in series. 

 With 4 strings. (8x330w=2640watt)

 

Or Renesola 300Watt

 

Specs 15.5 % efficient

Vmp = 36.6

Imp = 8.20

Voc = 44.8

 

Going to connect 3 in series.

With 3 strings. (9x300w =2700watt)

 

There are a price difference but i think the Benq is much better.

Any advice? 

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Hi Vulgrim,

Things to consider are

1. Temperature effect in cold conditions and how this could push your limits of the Mppt.

2. How much space you have for the panels - unshaded of course, if you have the space go with 250w panels 3 X 4 strings, you will have the full 3000 watts :)

in my case I am able to get away with 9 panels and the last three may only help on really cloudy days, but even then the 2.25 kW (9 panels) of panels will generally generate at least 700 or so watts reliably, when overcast, with clear skies I have seen the inverter showing 2.02kw being generated. When the ironing is being done :) but most of the time the system happily carries a load varying from 400 - 1000watts,

One can get 3 X 250w panels for around R2770 per panel, and I installed mine 3 at a time as a set over a six month period, it was easier on the pocket and allowed me to better appreciate what the panels, inverter and batteries were doing. And after 9 panels I am happy with the result.

Enjoy

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