High current modules - Overload

[Douglas_L]

Hi,

 

I'm simulating a setup of Trina DEG21C.20 650W and according to the datasheet we can have 28 panels in one string with a temp diff of -55 degrees and a DC voltage of 1500. This gives us 18.2 kW/string with a 19.63 A Isc/string.

 

The 330KTL has a MPPT config with 4/5/5/4/5/5 inputs, and I’m planning to use 24 strings/Inv (132% max_overload, 146 % Pnom DC/AC), è 4 strings/MPPT. We are slightly above the Max current/MPPT (4*19,63 = 78,52 A) , but I assume this would only result in some losses when peak production, and we are underneath 115 A for the max short circuit current/MPPT.  If we look at the NOCT the the Isc is of 14,79A and at 4 inputs/MPPT this would lead to 59.16 A. 

 

I've received information that the PVsyst yield report does not take into consideration of the high current and only the overload factor (power) to determined the losses. I've checked how many hours the weather data is at 800 W/m2 (current being at 14.79A) or above and its only 200 h a year (stands for about 4-5% of all sunny hours at my location, South Sweden). So in my conclusion the inverter should be clipping the current during these 200 h, but I don't think this will affect the yield that much, but how can I check this in the yield report/hourly values for this system?

[André Mermoud]

First of all: the one-diode model for the low-temperatures is correct down to about -10°C.

With most modern modules, the one-diode model fails at very low temperatures, the voltage thermal coefficient is changing sign, meaning that the voltage is decreasing instead of increasing when the temperature diminishes. This may lead to an error in the Voc(TMin) evaluation for the max. nimber of modules.  Please check with a linear model (for example by explicitly setting the muVoc value in the PV module definition (page  "Additional data"), and specifying that the program should use this value in the "Project's settings").

Now the limitation of such a configuration is mostly dominated by the Power overload. When Power overload occurs, the inverter will move the operating point on the I/V curve towards higher voltages, therefore lower currents. This explains that the current limitation is almost never reached in usual configurations, it is always "preceded" by a power overload. Attaining the  current limit is only possible when you define very short strings, at the limit oif the minimum voltage of the inverter. 

The PVsyst simulation completely manages all these situations. This is explained in the Help  " Physical models used > Grid inverter > Inverter Operating Limits"