Reverse characteristic of a cell

Reverse characteristics of a cell (that is, the current behavior when a reverse voltage is applied) is involved in all situations where the currents are not well balanced in a module array. This is the case namely in "mismatch" situations (of cells in a module, or modules in an array), partial shadings, or heterogeneous arrays (with different orientations, i.e different irradiances).

Severe consequences of the Reverse Bias in arrays can result in so-called "hot spot" phenomena. These are the overheating of unbalanced (bad or shaded) cells, which can lead to their destruction. Bypass diodes mounted in the PV modules are intended to protect them against these dangers.

PVsyst offers a specific tool for visualizing and understanding these special array behaviors. But they are not taken into account in the simulation process of PVsyst, which doesn't calculate the electrical array behaviour in such detail at each time step. Therefore the reverse bias model exact determination is not crucial in PVsyst, as it is only used in the phenomenological array behaviour tools.

Empirically, the cell characteristic's behavior under reverse polarization is quadratic with applied voltage. This result comes from our own measurements and is confirmed in Roger et al.¹

\(IRev = Iph + aRev · (V + Rs·I)²\)     for     \(V < - Rs·I\)

This expression could be valid until the avalanche zone (Zener), located around V = –30 V. However, in reality, under irradiation (with high photocurrent Iph), the dissipation—which varies with the cube of reverse voltage—reaches a destructive limit well before this point. For example, cells in Arco M55 modules dissipate about 18 W at a reverse voltage of –18 V and 25 W at –20 V, corresponding to a temperature increase of about 100°C. This is even more dangerous because the temperature rise sharply increases the parameter Brev, and therefore the reverse current, creating an unstable situation.