Inverter Clipping by MPPT

[Dan Nicksy]

I am having a simulation issue that I hope you can help me resolve. My simulation yields unrealistically high clipping losses as a result of MPPTs with uneven numbers of strings.

I have the following stringing on a Huawei SUN2000-25KTL:

• • MPPT1: (2) strings of 330W = 11,880W; DC/AC overbuild = 1.43; 40% of DC capacity
  
  • MPPT2: (2) strings of 335W = 12,060 W; DC/AC overbuild = 1.45; 40% of DC capacity
  
  • MPPT3: (1) string of 330W = 5,940 W; DC/AC overbuild = 0.72; 20% of DC capacity
  
  • Total: 29,880W; 1.1952 DC/AC overbuild

(I believe) PVsyst treats the three MPPTs on the SUN2000-25KTL as three independent inverters of 8.3 kWAC each, and clips each one individually. I have confirmed with the inverter manufacturer that the inverter does not limit power on an MPPT basis, but limits the power output of the inverter as a whole to 25 kWAC.

My intention is to modify the OND file to use unbalanced MPPTs. I will define 40%+20%=60% of the capacity on the Primary MPPT (for 3 strings of 330W modules), and 40% of the capacity on the Secondary MPPT (for 2 strings of 335W modules.) In that way the DC/AC overbuild of the Primary and Secondary simulation-MPPTs will be the same as the DC/AC overbuild of the real-world inverter, and PVsyst will simulate clipping only when appropriate for the entire inverter.

Can you confirm that this is an acceptable solution for PVsyst simulation, and would accurately represent the inverter as a whole? Is there any reason this would not provide accurate results? Note that for this simulation I am using the No Shadings option.

Thanks in advance for your help,

-Dan