AC cable rating for a 1MW PV install

[Mark.W]

Hi Chaps

Just as a matter of interest really, can I use derating factors when designing the ac cable size?

eg PVWatts gives a derating factor of 0.77.

PVWATTS: Changing System Parameters

So for a 1kW install the actual ac power would only be 770W, could I then design my cable on 770W?

Regards
Mark

 

[SolarCity]

Not read the link but I don't see any reason why derating should be applicable. A 1MW install should be capable of producing 1MW for a sustained amount of time and the cable needs to be able to support that.

 

[Mark.W]

I agree that a 1MW install should be able to produce 1MW, but in reality systems do have losses. PVWatts lists some of those losses as inverter losses when converting DC to AC, losses from VD across blocking diodes, DC wiring, soiling etc. This derating factor according to PVWatts can be an overall derating factor 0.77, which would mean only 770KW would flow from a 1MW install. This difference could mean a smaller AC cable and in turn a greater profit. I mean we use derating factors like grouping, ambient temperature etc when designing our cable, why could we not use derating factors like inverter efficiencies etc when designing the output from a PV install??

 

[Gavin A]

I just lost a long post here... to cut a long story short. No, absolutely not. Think about what you're trying to calculate vs what PV watts is trying to calculate and why you should be erring on the side of caution the opposite way to PV watts.

But in essence you really only need to worry about the peak inverter output anyway - actual peak, not the nominal figure, as the inverter shouldn't be pushing more than that through it.

I shouldn't be having to have this discussion with someone who's involved in MW plants though should I?

eta or have you been dropped in the deep end or something?

 

[Mark.W]

No Gavin I am more than capable of designing MW installs, thanks for your concern this is just as a matter of interest!

 

[Worcester]

No Gavin I am more than capable of designing MW installs, thanks for your concern this is just as a matter of interest!

In which case then your question is REALLY weird.

As the ROI economics will outweigh all your considerations. the cost of the hook up to the grid will one of the most signifciant factors, and I doubt that they would want to install a 770kW system as opposed to a 1MW system if they are paying for a 1MW hook up..

 

[The Solar King]

Important to remember this NREL are a U.S. organisation and they do things a little different....... As Gavin said this could be a long post. The very first item could be eroneous if panels only have a positive tolerance. Just read through the de-rating factors and see which ones are relevant. There are means of calculating output in PVsol that take into account a number of these such as cabling if you can be bothered to set them.

More importantly, go enjoy the sun this weekend while it is shining. The weather is likely to de-rate next week!!

 

[Earthstore]

I suggest you install your undersized cable, then come back on here to ask why you have an overvoltage problem, that is if the DNO has not shut your system down first.

All your calcs are based on STC of panels, but all PV systems should exceed their STC rating at various times of the year, therefore you have the potential of exceeding your 1MW but this should be restricted by the inverter, if all is designed correctly of course.

 

[solardxb]

I tend to find PV Watts limited to the US region, and somehow kills your system yield.

Should not be used or taken seriously to find the output for a 1MW power plant

Most reliable inverter manufactures have great sizing tools, which are quite good.