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 Originally Posted by dom17 A 4.3kwp photovoltaic array system is to be installed on a roof with a pitch of 30o and facing due south. The Voc measured on the day of commissioning was 464 .4 volts at a temperature of 45oc and a irradiation of 800watts. Pmax 215 w Vpm 42 v Ipm 5.13 a Voc 51.6 v Isc 5.61 a (i)Calculate the number of modules required and method of connection.

A single panel has an open circuit (oc) voltage of 51.6 V, and the installed system reads 464.4 V, that is, exactly 9 times a single panel. So we are sure of one first thing: the PV arrays consists of at least one string of 9 panels connected in series (which add their voltage, and share the same current).

How much power a string of 9 modules would yield at STC (Standard Testing Conditions)? Nine times a single panel, so 9 x 215 W = 1935 W, but this would be at STC (irradiation 1000 W/sq.m., instead of the measured 800 W/sq.m.). Lacking any graphics or tables for the modules to check, one can assume the same panel string is going to generate about 80% of the energy under the measured conditions, so to get the desing power of 4.3 KWp we have to take that into account. So the nine panel string would produce, under 800 W/sq.m., only 1548 W, that compared to the desing power, is just about a third.

So, I would say that the system must consist of three parallel strings of nine panels in series in each strings. Under STC, each string will output at most Isc amps, and will generate at most 9 x Voc. The power generated would be, without taking into account losses, 27 x 215 x 0.80 = 4.6 Kw. Peak power at STC would be 5.8 Kwp

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 (ii)
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 Typical value for Isc under measured conditions.
All strings add their current to the input of the charge controller, so to get the Isc you would have to check the graphics for Isc according to irradiance, and multiply the Isc of a single panel by three (current in a single string is the lowest of the currents of the panels in the string).

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 (iii)
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 KW of inverter required if under sized by 0.81%
So I understand there is no storage in batteries and energy goes directly to the inverter. I don't really understand the "if undersized", because if the inverted gets on its input more power than designed to handle it can stop working or just burn. But power entering the inverter is the one generated in the panel array minus losses in the cabling, that I'm not sure they have been specified in the problem.

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 (iv)
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 DC module string cable rating, Array isolator rating
This is country dependant, every electrical legislation says a different thing. Just check the voltage and current cables are going to withstand at maximum, apply all the safety factors legislation asks for (i.e. to take into account unusual sunny and bright, but cold summer noons), take into account the max amount of losses in the cabling, and check the appropiate tables for cable diameters and shielding.

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 (v)
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 String/array isolator voltage rating.
I think this is a electrical circuit breaker for DC currents (so hardware is very different to, a much more expensive than that for AC) used when maintenance to the installation is needed, and you want to decouple the panel array from the rest of the installation. You should check the portfolios of this kind of hardware manufactures after checking legislation to see if there are any safety factors to apply to the array DC voltage.

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 (vi)
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 Size of area in M2 covered by PV array.
Just simple mathematics. An array of 3 by 9 panels, you may have to decide on the panel orientation (portrait or landscape) depending on the roof space available.

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 (vii)
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 Does the system pass or fail commissioning from the above information, giving reasons for your answer.
No idea what to say :-)

But, please remember I'm just a PV afficionado, not a certified professional installer, expert or anything like that, so take my advice with great care and don't trust my calculations very much.