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Arandoza

DIY PV String Combiner Boxes

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Hello All,

I would like to learn how to correctly build a PV Array combiner box.  Obviously they differ dependant on the array configuration and amp / voltages of the array.

We use Axpert 5kw inverters and would like to have 6 strings per combiner box, each string with 3 x 275w panels. (50% Oversizing for bad weather, and to compensate for winter.)

 

I understand that the combiner box plays the following roles.

1.) Protection : Fuses for both Positive and Negative feeds from each string.

2.) Protection : Lightning and or Surge protection, which would go to a separate ground/earth spike.

3.) Disconnect : So that one can disable a individual string if needed, I assume MCBs are used.

4.) Output to inverter would use 6mm or better cable, to reduce losses and endure the combined currents onwards to the inverter.

 

What I would like to find out is how one should go about specifying the Fuses and Disconnects (MCBs) as well as the Lightning / Surge suppression and any other considerations.

 

The 5kva Axpert operates on 60-145v(Max Rating) per array, but typically is 65-110v, and the 275w Panel values are as below

Open Circuit Voltage (Voc)   38.38v

Max Power Voltage (Vmp)   31.34v

Short Circuit Current (Isc)    9.29a

Max Power Current (Imp)    8.77a

 

How does one then calculate the correct components for the combiner box?

 

I am sure many folks are interested in this topic and I expect it would be great if we had a Tutorial  / Guide on how to design and spec combiner boxes :) If there already is such a guide please point me in its direction.

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Hi Arandoza

My system runs much higher VOC voltages, and as a result, I paid special attention to the ratings of the switch gear I used. AC switch gear should not be used for DC applications.

There are technical specification that differ. Any good electrical wholesaler, should have a DC range of equipment. 

As for SURGE PROTECTION, the region your are in will also play a part here? As lightning is more prevalent in some areas. The surge protection is rated in TYPE 1 and TYPE 2 for AC applications, and a separate PV class.

Even fuses and fuse holders, specially suited to PV applications are available.

ISOLATORS too, required a different rating as well as wiring method.

I know your question was specifically related to STRING BOXES, but I hope this helps.

 

 

IMG_5583b.jpg

IMG_5584b.jpg

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The inverter I am installing, has a DC INPUT voltage of 250-900Vdc. So my installed array is 14 x 335w Canadian Solar. Intend to install additional panels, and a second inverter. For this reason, I made provision for the additional switch gear from the start.

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18 minutes ago, Arandoza said:

50% Oversizing for

I think that is too much. I generally don't advise more than 20% oversizing. @Coulomb has since come up with a more specific number for Axperts. While an MPPT can handle more power and simply throttle off the extra, there are two things you must be careful of.

The first is not to exceed the current rating of the switching MOSFET. As an example, a 150/60 controller from Victron has a maximum short circuit current specification (on the PV side) of 50A and a battery side rating of 60A. Given that your PV modules will make on the other side of 120V that means around 200% oversizing is possible before you exceed the 50A rating on the input side. But then consider a 250/60 which has only a 35A input. The only way you can oversize on that one is if you make sure your voltages are high enough. I do not know the rating of the Axpert's MPPT.

The second is overcurrent events. When solar conditions change suddenly an MPPT that was operating at the maximum point might suddenly find itself exceeding the max current. It now has to change its PWM ratio to get away from this condition, but until it does, it will operate outside its design for a couple hundred milliseconds. Solar chargers can handle this... but if you overdo it you will shorten their life.

As for making combiner boxes... I pretty much see it as fuses first, breakers second, and then it all comes together, and at that point you put the surge arrestor. Eg see this, but with fuses and breakers in both lines. And use panel-mount MC4 connetors.

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4 hours ago, Arandoza said:

3 x 275w panels. (50% Oversizing

I calculate that as 18 x 275 = 4950 W, just under 25% oversizing. That's still a lot, but you may well get away with it. The only sensible numbers I've seen for 4000 W rated Axpert MPPTs is 4500 W. It sounds like those are 60-cell panels, which are good for 3S into 145 V max Axperts. Perhaps you meant that 4950 W is about 50% more than you theoretically would need ignoring shading issues, Winter, etc.

With regard to fuses and breakers, generally it is desirable to combine some strings on the roof, so that you don't need to bring down 6 pairs of cables to the combining box. So you would have a fuse per string on the roof, combine usually pairs of strings with Y cables on the roof, then use 3 DC rated circuit breakers in the combining box. Noark make suitable dual-pole breakers, but I don't know about availability in South Africa.

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