Voltage drop in series diode

[RAMVEA]

default Voltage drop across series diode in PVsyst version 6.39 is 0v where as same in updated version default is given 0.7v.

1.Why there is incerase in loss.

2.Does it refers to bypass diode or the reverse-blocking diodes.

3.As most of the newer modules dont have reverse blocking diode, why have we set the default as 0.7v rather than 0v which may make more sense.

[André Mermoud]

When creating a new calculation version, the voltage drop in the series diode is initialized at a null value. The default value of 0.7 V only appears if you ask for this (click the checkbox).

In old installations people used to include a blocking diode in series with each string, for avoiding reverse current from neighbour strings in case of mismatch. However this is useless as even when a string is almost completely shaded, its voltage remains close to Voc. Moreover, these diodes were subject to failures, which were not always detected.

To my knowledge, nobody uses such diodes anymore nowadays.

These diodes have nothing to do with the by-pass diodes, which are quite necessary for the protection of modules against Hot-spots. All crystalline modules do have such diodes. These diodes are completely inactive in absence of serious mismatch conditions (partial shadings).

[aaronrodg]

There is no way to insure the even sharing of the current between the three parallel diode strings, as there will be slight variation in junction voltage drops.

[RAMVEA]

Thanks for the valuable information.

We have a case where there is 2 strings 21 in each string with different module makes , so the voltage flowing through the strings will be some where 800 & 815 respectively. so will there be any flow in reverse current in the 800 V string.

if there is any reverse current flow, what value of current will be flowing.

Until what voltage difference, the reverse current may flow.