EArray < EOutInv

[laurahin]

Hi,

I printed out EArrMPP, EArray, EOutInv, and E_Grid from my recent PVsyst runs so I could track the losses through the simulations.  Based on the PVsyst help info, my understanding is that EArrMPP is what would come out of the array if MPP was always maintained and EArray is what the array actually delivers, accounting for power point offsets.  When no inverter or POI clipping was involved, everything made sense:  EArrMPP > EArray > EOutInv > E_Grid.  However, as soon as any limiting occurred, EArray decreased so that it was approximately equal to E_Grid, giving EArrMPP > EArray < EoutInv > E_Grid, which makes no sense.  Can you explain this?  (I fully admit that my understanding of EArrMPP and EArray might be incorrect.)  I’m concerned about this because we sometimes report available energy out of the array to clients wanting to employ BESS on the DC side and I believed that EArray would be the appropriate value to use.

Thanks.

[dtarin]

• EArrMPP = DC input into the inverter when max power point tracking
• EArray = EarrMPP - (IL_Pmax + EGridLm) 
  • It is the mppt DC energy minus the clipping losses, even clipping due to POI limit.
  • POI clipping will also move the inverter operating point in reality, so I suppose that it why it is included? 
  • The help says the following,  

  • Quote taking inverter operating point displacements into account

• EOutInv = EArrMPP - InvLoss
• EGrid = EOutInv - (EACOhmL + EMVOhmL + EMVTrfL + .... + EGrdLim)
  • This will change depending on if you have defined AC losses, whether you have accounted for unused energy from grid limit separately, etc.

In looking at the equations above, we can see that it is indeed possible for EArray to sometimes be close to EGrid. If you have not implemented a grid limit and display as separate loss, you wont have EGrdLim (it will = 0).

Regarding BESS, it depends on how your model is set up, so can't really say whether it is correct or not. This is DC coupled, correct? Are you sending EArray directly to grid (minus IL_Oper/Pmin/Vmin/Vmax/Imax) and IL_Pmax to batteries?

[laurahin]

2 hours ago, dtarin said:

 

Thanks for your reply, but I don't understand the answer.  As stated above, the help section indicates that EArray is like EArrMPP when inverter operating point displacements are taken into account.  To me, this means that any energy lost because the inverter is unable to maintain proper PP tracking is subtracted from EArrMPP to get EArray.  As far as I know, that's nothing like "the mppt DC energy minus the [inverter and POI] clipping losses," so either the documentation is unclear or I don't understand PP tracking (or both).  Perhaps the equations shown above could be included in the documentation, for clarity.  I don't see why EarrMPP - (IL_Pmax + EGridLm) would be of any interest and wouldn't have paid it any attention if I'd seen the equation.  

We are modeling the storage system outside of PVsyst, so the question is which value gives the full available DC energy out of the array, regardless of whether the battery is charging or the energy is being sent to the grid.  The answer appears to be EArrMPP.  Thanks!

[Michele Oliosi]

Hi @laurahin,

I second @dtarin's answer, and from a quick look the equations seem correct. Note that EOutInv = "Available Energy at Inverter Output". In this case it is to be interpreted as the maximum possible energy at inverter output, if the inverter operated at MPP. Hence EOutInv = EArrMPP - InvLoss

The grid limitation is always effectively interpreted in PVsyst by inverters changing their operating point -> this diminishes the EArray just as inverter clipping would. This is the reason EArray = EarrMPP - (IL_Pmax + EGridLm). EGridLm here is the grid limitation loss.
 

1 hour ago, laurahin said:

Perhaps the equations shown above could be included in the documentation, for clarity.  I don't see why EarrMPP - (IL_Pmax + EGridLm) would be of any interest and wouldn't have paid it any attention if I'd seen the equation.  

Yes ! We should have been more detailed in the documentation. Maybe some old FAQ post clarifies this, but I couldn't find it. Where did you get this info @dtarin

 

1 hour ago, laurahin said:

documentation is unclear

This is likely the case, we will do our best to address this

[dtarin]

9 hours ago, Michele Oliosi said:

Yes ! We should have been more detailed in the documentation. Maybe some old FAQ post clarifies this, but I couldn't find it. Where did you get this info @dtarin

@Michele Oliosi With 8760 data. I have not seen it elaborated on the forums prior. 

[laurahin]

Two more points of follow-up:

1) In the PVsyst report, a variable "EArray" is shown in the "Balances and main results" table, but "Array nominal energy" and "Array virtual energy at MPP" appear in the waterfall diagram. It would seem confusing if the report really shows array output with different PP regulation assumptions in these two locations, but maybe nobody looks that closely.

2) Going back to what DC power would be available before the inverter:   I now think inverter power point tracking is irrelevant to this situation.  If we shunt all of the energy to the BESS before it reaches the inverter, the battery charge controller will do the tracking.  Since we aren't asked to model the BESS, we can give the client EArrMPP and let them figure it out.  

I think one source of confusion is that power point tracking, applied at the inverter or charge controller, interacts with components up- and downstream in the system.  Comments?

Thanks!

[dtarin]

7 minutes ago, laurahin said:

1) In the PVsyst report, a variable "EArray" is shown in the "Balances and main results" table, but "Array nominal energy" and "Array virtual energy at MPP" appear in the waterfall diagram. It would seem confusing if the report really shows array output with different PP regulation assumptions in these two locations, but maybe nobody looks that closely.

 

I agree, PVsyst only allows the last two variables in the table to be selected by the user, but all variables in this table should be able to be edited and selected by the user. If there is a way to edit all variables, I haven't found it yet.