Value of Voltage when using Degradation of panel

[LoicA]

Hi

I am studying the impact of panel degradation on the voltage and thus the voltage threshold loss (VMin 60°C).

Where to find the value of the array voltage at 60°C the 20th year ?

In the System window, the value shown does not take into account the degradation of panel power thus the panel voltage.

In the Simulation -> Results > "Predefined Graph", it's possible to draw the "Distribution of array voltage" but it does not answer to my question and it's not easy to use.

By using the hourly data I saw some values of voltage (see image enclosed) but two questions emerged :

1/ Why the voltage is constant over the day ? Both the T° and irradiation are varying. The value of 596V is the VminInverter, is there a coincidence ?

2/ Why some values of EGrid or PR are negative ? (not visible in the image enclosed).

Thanks

Loic

[André Mermoud]

The long-term degradation is applied to the STC values for the PV module model (50% in current, 50% in voltage), and therefore slightly affects the operating voltage.

However in the case of your table, the Array voltage attains the Inverter VmppMin, and is therefore clipped to this value. This is the array operating voltage, not the MPP voltage.

The impact of the degraded voltage in the simulation is simply the possible increase of the loss due to voltage threshold.

NB: The degraded module is defined during the simulation process. There is no tool for analyzing this degraded module explicitely.

You can create this degraded module by redefining the STC values Isc, Voc, Imp and Vmp.

NB: I don't know any publication about the impact of the relative degradation on the current and the voltage. I have put an equivalent decrease for I and V, but without experimental justification.

You may have defined night losses in your system, like Auxiliary consumptions or transformer iron loss.

[LoicA]

Dear Mr Mermoud, thank you for your answer.

Ok for the 50%/50% ratio on the I and V impact.

Anyway if we look at the T° between 12°C and 22°C according to the screen on the first post of this topic, the MPP voltage should be respectively 616.5V and 598.5V. So it's over the VmppMin 596V and with a not negligeable variation (616.5-598.5 = 18V). So why the voltage array is fixed at 596V? For your information the VMin 60°C is 531V.

It could be interesting to show the V_MPP and I_MPP (I am talking about the MPP not the Array Voltage) somewhere in PVsyst in case of panel degradation.

Is the Isc and Voc dispersion in the degradation part used in the calculation process ? I mean, are all the panel become differents when dispersion is applied, (V and I), or all the panel are the same and at the end (as shown on the diagram losses) a "Mismatch for degradation dispersion" is applied according to the Monte Carlo % results ?

In the first case it's not easy to calculate the degraded V_MPP and I_MPP, in the second case it is and I can do it myself.

Ok for the explanation of the negative Egrid and negative PR that makes sense and you are right we have defined auxiliaries losses and there are night losses.

Edited October 12, 2016 by LoicA

[LoicA]

Hi,

I have one another question,

It's about a different project, but still around IL_Vmin interrogations,

Can someone explain me the IL_VMIN losses showed in the image below ? UArray > Vmin_Inverter (596V), so where does the losses come from ?

 

Thank you in advance

[André Mermoud]

When the "Minimum voltage for PNom" is specified in the inverter's parameters, this corresponds indeed to an input current limitation.

This current limit corresponds to the value I limit = PNom / VmppPNom

Now when the Impp of the array is higher than this current Limit, the loss with respect to the available Pmpp is accounted in the IL_VMin loss.

[LoicA]

Hello Mr Mermoud

This is clear thank you.

I also found some information here :

Navigation PVsyst Help : Physical models used > Grid inverter > Inverter Model: Input and Output

Best regards, Loic