Not sure how to connect these modules to the inverter

[danyof]

For sure this is an easy one, but I am not totaly sure how to connect these modules with the inverters (how many per inverter I can connect)
In bold there are the ones used in the calculations below:

INVERTER

[B]Max. DC input 33,800 W[/B]
Max. input voltage 1000 V
Max. input current per MPPT 23 A
Max. short circuit current per MPPT 32 A
Operating voltage range 200 V - 980 V
MPP voltage range at full loading 480 V - 800 V
[B]Rated input voltage 620 V[/B]
Max. number of inputs 6
Number of MPP trackers 3

MODULES

[B]STC ELECTRICAL CHARACTERISTICS[/B]
Rated Maximum Power Pmax (W)
345
Open-Circuit Voltage Voc (V)
46.6
Short-Circuit Current Isc (A)
9.48
Module Efficiency (%)
17.7
[B]Maximum Power Point Voltage Vmpp (V)
38.1[/B]
[B]Maximum Power Point Current Impp (A)
9.06 [/B]

Using the maximum power point data:

Should I connect in series 16 modules in order to get 38.1 x 16=609,6V (with 9.06A)
And then connect those groups in parallel with a total of 2 to make 9.06A x 2=18,12A

And all that connected it to 1 inverter (having space for 2 more groups of connection)?

That way we would get 609,6V x 18,12A=11046W per one
11,046W x 3=33,138W in total per inverter (which is still under the Max. DC input of 33,800 W)



Is that right? Should I use other data for the module?
Thanks a lot!

[sensij]

It is hard to believe you are installing such a large system and have to ask this. Anyway, how many panels total do you plan to install?

What you've written is basically correct, but more information is needed to say if 16 panel strings are best.

In any case, you should also calculate the temperature corrected string Voc and make sure it is under 1000 V.

[danyof]

Thanks for your reply. I am not personally installing this, but it is rather for an assignment I have to solve. It is just a preliminary layout to get exactly 100Kw installed.

For calculating what you said, taking the Voc in standar conditions would be 46.5V x 16 = 744 V which is less than 1000 V so it would be ok.
What other information should be taken into account to decide whether 16 panel strings is the best or not? Thanks!



P.s: I have been almost 2 years since the last subject I had in the faculty related with electricity, so I wanted to make sure I was on the right path.

[sensij]

You should find the minimum design temperature for that location, and adjust the Voc for that temp and verify it. In colder locations, the Voc could be much higher. I agree that it looks likely 16 panels should be OK, but it is an important calculation to perform. Minimum design temps can be found here.

You'll want to make sure that all of the panels in a string can be mounted in the same orientation.

[danyof]

Thanks for your usefull tips!
For whoever finds this topic interesting, I leave here some guidelines I found to calculate the Maximum Voc for the minimum temperature:

http://www.smainverted.com/2015/01/22/calculating-max-pv-voltage-is-not-scary/

However, I still have one doubt. Making all the calculations gives me a power of around 97-98 Kw (With 3 Inverters in Total)

I've been told to get exactly 100Kw, so I guess to use a 4th inverter to just supply 2 Kw is not economic.

Could I therefore increase the string size (to 18) and overpass the Rated input voltage of 620 V of the inverter (taking into account the Operating voltage range is 200 V - 980 V). Is it okay to work in that condition? It would be 685,8 V.

Thanks!

[sensij]

As long as the max Voc (at min temp) is still within the 1000 V limit, you can increase the string length. The "rated" voltage is probably just the operating voltage used for efficiency testing, but as long as you are in the mppt range, the efficiency should be good enough.

[danyof]

Just one last question:

Checking the datasheet you can find:
Temperature Coefficient of Pmax: -–0.44 %/C

This means, that the power will increase when the temperature decreases. This will happen in the coldest months, but that will also decrease the solar iradiation (due to solar position on the sky) which compensates the effect of the increase of the power. So my question is: How can i valorate this effect to see if it exceeds the Maximum Power Input of the Inverter? Or should I just ignore it?

Thanks!

[inetdog]

Just one last question:

Checking the datasheet you can find:
Temperature Coefficient of Pmax: -–0.44 %/C

This means, that the power will increase when the temperature decreases. This will happen in the coldest months, but that will also decrease the solar iradiation (due to solar position on the sky) which compensates the effect of the increase of the power. So my question is: How can i valorate this effect to see if it exceeds the Maximum Power Input of the Inverter? Or should I just ignore it?

Thanks!

A real life inverter will have a maximum output power specification but will be perfectly safe with a higher DC input power available and will just operate away from the Maximum Power Point to avoid damage (this is called clipping). Some inverters will also put a limit on just how much "extra" input power capacity can be handled.
Since this is an assigned problem you may not have been given that information and are expected to assume that the maximum input power is an absolute limit.
If that is the case you need to design for an occasional panel power output of up to 125% of the nominal panel output because of "edge of cloud" effects as well as low temperatures.

[ktran1]

do google search for solar string inverter calculation, then you can size number of panels correctly. Some inverters just take in MAX DC 600V