Modeling modules with different powers in a single MPPT

[Gustavo Pianovski]

I'm working on a project with 3 different modules (705, 710 and 715 Wp) connected to a single MPPT inverter:

 

Ideal Condition (No Losses)
	Module	705 Wp	710 Wp	715 Wp
	Config.	A	B	C	Total
Module	Imp (A)	17.55	17.59	17.63
	Vmp (V)	40.2	40.4	40.6
	P (Wp)	705	710	715
Electrical Configuration	String/Inv	8	36	37	81
	Mod/STR	30	30	30
Arrangement	Vmp String (V)	1205.1	1210.9	1216.7
	Imp String (A)	140.4	633.2	652.3
	P config. (kWp)	169.2	766.8	793.7	1729.7

 

An alternative modeling approach is to change the .OND to 3 MPPTs and use the MULTI-MPPT tool. This will allow me to model all three module types in the same inverter.

Will making this change to .OND cause other problems?

 

I did the simulations below to test:

 

• Simulation 1: 3 inverters with original .OND (1 MPPT per inverter). Each inverter is configured with a different module, totaling 5,176 kWp.
  • E_grid = 11261 MWh
     
• Simulation 2: 3 inverters with modified .OND (3 MPPTs per inverter), only to allow modeling in PVsyst. Each inverter is configured with 27 strings of the three types of modules, totaling 5,176 kWp. The PVsyst MULTI-MPPT tool is used to group the 3 MPPTs on the same inverter.
  • E_grid = 11261 MWh

The energy result (E_Grid) is the same in both cases. I understand that PVsyst doesn't consider any differences between the models. Therefore, we must calculate a loss due to this electrical configuration.

 

I understand that this electrical configuration will result in a greater mismatch. One way I found to calculate the electrical losses of this configuration is to consider that "the array voltage is always limited by the lowest voltage string." In this case, the voltage of the 710Wp and 715Wp strings will be equal to the voltage of the 705Wp string. I calculated the power of each configuration (P config. (Wp) = Vmp String (V) X Imp config (A)):

 

Edited 18 hours ago by Gustavo Pianovski

[Gustavo Pianovski]

Continuation...

 

Actual Condition (COM Losses: string voltage limited by the smallest)
	Module	705 Wp	710 Wp	715 Wp
Module	Config.	A	B	C	Total
	Imp (A)	17.55	17.59	17.63
	Vmp (V)	40.2	40.2	40.2
Electrical Configuration	P (Wp)	705	706.6	708.2
	String/Inv	8	36	37	81
	Mod/STR	30	30	30
Arrangement	Vmp String (V)	1205.1	1205.1	1205.1
	Imp config (A)	140.4	633.2	652.3
	P config. (kWp)	169.2	763.1	786.1	1718.5

 

In the first table (ideal condition), the total power is 1729.7kWp. In the second table, the total power is 1718.5kWp. The difference between these two power outputs is 0.65%. I'm considering applying this loss to the mismatch simulation. Is this analysis correct?

 

I tested a second methodology: using the Detailed Study tool in the Mismatch Losses tab in PVsyst. In this analysis, the Voltage Difference between the 715Wp and 705Wp strings is 11.5V (1216.7V – 1205.1V (Table 1). Applying a voltage difference to the string of 11.5V, PVsyst returns a mismatch of only 0.05% (Figure below). Why is the loss calculated by PVsyst so low?

 

What is the best way to model this project and a loss to be applied due to the power difference of the modules?