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Axpert MKS 5kVA solar panel configuration


cvzyl
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Hi guys

 

I have a question about the solar panel configuration for the above inverter. The Voc max for the inverter is spec'ed as 145V. The Voc for my panels is 37V so I had them wired into 4 parallel strings of 3 panels each in series. This gives max open circuit voltage of 111V. I thought going as high as possible with the voltage (max panels in series) is the best solution.

 

Once my installation was up and running I noticed that I typically only get 2.4-2.5kW out of the 3 kW installed. I understand that I would only rarely see output at the rated capacity but it bothered me that on a bright day there is still quite a gap between actual and potential output.

 

Scratching around in the manual I then found the following:

87Lgrtj.jpg

 

Clearly my voltage is way too high and strings of 2 panels in series would be the optimum. In fact, further down in the manual it gives the following suggestion:

9CIuZjS.jpg

 

So, do I need to have my panels re-wired? What configuration are you guys running? How much of a difference can I expect if I bring the Voc down into the optimum range?

 

Regards

Cobus

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Hi @cvzyl

I think your specs are for the KS model which has a PWM charge controller, the MPPT version's voltage range is from 60 - 115V DC.

The voltage @ Pmax of your panels are probably just over 30V = 3 in parallel +/- 90V,

I would think 3 strings of 4 in series.

(30 x 3) x (4 x 8.5) = 3060W

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It might have something to do with the efficiency of the MPPT controller. An MPPT controller is really just a microprocessor controlled adjustable buck converter, so all the issues you have with a buck converter you will also have with an MPPT controller. Now without getting into the details too much (because I don't get half the math for this yet!), it will be designed to work best within certain limits. I've been told by installers that what you want is approximately twice your battery absorption voltage. In the end it will depend on the design of the buck converter, because what it has to do is store a bunch of magnetic energy in an inductor, and then release that magnetic energy back into electric energy. So what you do is wind it up using a high voltage at low current, then wind it down at a lower voltage to get more current. There's a whole lot of info about when the converter runs in continuous mode, and when it runs in discontinuous mode, and so on and so forth... so I'd say you may have to experiment a bit.

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Hi @cvzyl

I think your specs are for the KS model which has a PWM charge controller, the MPPT version's voltage range is from 60 - 115V DC.

The voltage @ Pmax of your panels are probably just over 30V = 3 in parallel +/- 90V,

I would think 3 strings of 4 in series.

(30 x 3) x (4 x 8.5) = 3060W

Must have been half asleep, your pv panel connections are correct:

3 in series = (30 x 3) = 90V

4 parallel strings = (4 x 8.5) = 34A

90 x 34 = 3060W

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I found this useful calculator for designing a buck converter.

 

http://www.daycounter.com/Calculators/Switching-Converter-Calculator.phtml

 

Though probably Greek to most people... it might help give you an idea even if you have no idea what the numbers are. Fill in some values, then look how the inductance value (L) differs based on input voltage.

 

For some reference values, you can use 0.2V voltage drop for a really good Schottky diode and 0.7 for a good Mosfet (transistor) at high current. Switching frequency is usually between 20khz and 100khz (my Microcare uses 40khz, from the current ripple on my Victron Blue Solar I'm deducing somewhere above 100khz). Current ripple should be 10% max. Minimum output current is usually 1 amp or so (below that the converter just cannot operate), output current set to whatever the max is you want to push through there (say 10A).

 

Fill in these values, then play only with the input voltage. Notice how the optimum value of L differs based on the input voltage.

 

The L value of the inductor is fixed though, it is a physical magnetic component with a fixed amount of copper wire wound around it, so a lot of the time you're not operating at this optimum point. The idea is to get close to it.

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@cvanzyl

 

Did you play around with the wiring etc?

 

Out of interest I'm running the same inverters (2x of) - and I have 9x 300W renesola's - so 2700W of PV and on an ideal day i sit around 2.3 - 2.4kW as well - which according to the installers etc is about right.

 

My roof is also not the perfect pitch and faces NE - so also not ideal. I haven't heard of anybody here that gets the " full"  advertised rate :) - inefficiencies, wire loss etc etc - all adds to what you eventually end up getting.

Perhaps post in general around what most members get VS installed PV?

 

Regards

 

B

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Just remember that your solar panel specifications are based on Standard Test Conditions (STC) of 1000 w/m2 irradiance, 25 Degrees Celsius and Air Mass of 1.5.

I think on a clear day we will get a solar irradiance of between 600 - 1000 w/m2 but with scattered clouds around it drops to about 200 - 400 w/m2 and with an overcast sky it drops down to 50 - 150 w/m2.

Also the power output of the panel decreases as the temperature rises.

So don't expect to see your full rated, PV panel power available at the inverter.  

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