Why sometimes the Overload losses increase significantly without reason ?

[André Mermoud]

When the PV power exceeds the DC power Pnom(dc) corresponding to the inverter Pnom(ac) value, the inverter has to displace the operating point along the P/V curve of the array, in order to just draw the necessary power.

This displacement is usually towards higher voltages. The power loss is (Pmpp - Pnom(dc)) for this hour.

 

Overload, usual conditions

 

Now if the voltage corresponding to this Pnom(dc) is over the VmppMax of the inverter, there is no possible operating point with both conditions [Pnom(dc)] and [Vnom(dc) < VmppMax] : the inverter has to stop !

When you come to this situation, the solution is to diminish the number of PV modules in series.

 

Overload with high voltage operating point

 

NB: this behavior arises when the hypothesis of PVsyst are met, i.e. the inverter really stops working above the VmppMax value.

This is not necessarily the case in the reality. You should ask the manufacturer for identifying the exact behavior of his inverter.

In some cases the real VmppMax is higher than the value specified on the datasheet (we have a case where the specified VmppMax is 800V, and the cut happens at 904V).