Modeling of Inverter power limitation based on input and output voltage. Also temperature derating for multiple input voltages

[Pranav Maheshwari]

There are 3 inverter characteristics which I want to model in PVSyst:

1. Temperature derating for multiple MPP voltage. Following is an example of Sungrow RS series inverters temperature derating profile at multiple MPP voltages: 
   There is considerable difference in temperature derating for different MPP voltage, but we get to model only a single behavior.
   
    
2. Power limitation as per input voltage:
   
   In the OND file, the MPP voltage range mentioned is 40-560V, but it can be seen from the above graph that full power is only attainable b/w 235-480V
   
   If we put the Min. Voltage for Pnom=235, then it automatically sets the Max current per MPPT at 11A (but actual mentioned in datasheet is 16A)
    
3. Power limitation as per grid voltage:
   
   Although PVSyst assumes a constant grid voltage, so this behavior may not be essential to model. But if in future there is some variable grid voltage, specially for standalone systems based on loads, this will also become critical.

Hope my problem statement is clear!

Regards,

Pranav

[André Mermoud]

1. - Temperature derating

In PVsyst, there is indeed one only value for the temperature derating Nominal power.

However the effect on the simulation will be extremely low because:

   - The inverter temperature is usually not known/defined with such an accuracy of 2-3 °C at each time step.

   - There are usually very few hours in the year with such an inverter  temperature  (this is the environmental temperature around the inverter)

   - When such temperatures are attained, the error will be effective only when the PV array Power  is higher than the PNom of the inverter (overload conditions).

2. - Input voltage conditions

This is indeed not implemented in PVsyst. But the behaviour of the simulation is rather close to these limitations.

   - On the left (low voltages), this corresponds to the input current limitation, defined in PVsyst by the  "VmppMin for PMax"  parameter  (see the help 

 " Component Database > Grid inverters > Grid inverters - Main interface > Grid inverters, main parameters")

  - On the right (high voltages), this curve corresponds to the increase  of voltage when the inverter is in overload conditions (displacement of the operating point).  Again, an error in the simulation will only occur in overload conditions, when the falling edge of the I/V curve is close (i.e. crosses) this PNom curve of the inverter. 

3. - Grid voltage condition

This is not applicable as the Grid voltage is not known during the simulation (it is not an input parameter).

Again, this may only affect overload losses, in very particular Grid voltage situations.

[Pranav Maheshwari]

Yes, for the 2nd point, I did define the "VmppMin for PMax" as 235V and VmppMax as 480V. Automatically the max current per MPPT was set to 11A, which resulted in around 0.2-0.4% clipping loss associated with current.

So although the cause of clipping is because of input voltage limit, but in simulation its reflected as current clipping because of how we've modelled. 

So this can be an correct approach right?

[André Mermoud]

Yes, this is normal.

NB: In your voltage definitions, a current of 11A * Voltage of 235V means that you have an inverter with PNom = 11*235 = 2.58 kW  (per MPPT input).

This doesn't correspond to you previous posts where you had  a voltage limitation of 235V for an inverter of 5 kW, i.e. a current of 21.28 A

[Pranav Maheshwari]

@André Mermoud

This is the same case, inverter contains 2 MPPTs, so thats why per MPPT Pmax would be 2.5 kW only

[gespe]

Hello, 

I have a similar question, but I don't see the answer above. How do I configure the upper end of this Power vs DC voltage curve? 

 

 

[André Mermoud]

This PNom deratig according to the input voltage is not yet implemented in PVsyst. PVsyst puts a fixed voltage limit  (here 1250 V) for the MPP tracking.

However the PVsyst results should be rather correct. They will only be affected if you define high string lenghts (Vmpp close to these maximum operating voltages).

As the falling slope of the P/V curve is close to the falling edge of this derating, the overload losses of PVsyst (by displacing the operating point on the I/V curve) are correctly evaluated when the MPP point is over PNom. 

The conditions for "stopping" the inverter will be similar  (see the FAQ  just below).

https://forum.pvsyst.com/topic/194-why-sometimes-the-overload-losses-increase-significantly-without-reason/#comment-369

NB: This is on our roadmap. We will work on this topic during the next months.