Reverse characteristic of a cell

Reverse Characteristics of a cell (i.e current behaviour when a reverse voltage is applied on it) 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 visualising and understanding these special array behaviours. But they are not implied in the simulation process of PVsyst, which doesn't calculate the electrical array behaviour in detail at each hour. 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 behaviour of the cell's characteristic under reverse polarisation is quadratic with the applied voltage. This result comes from our own measurements, and is confirmed in Roger and al¹

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

This expression could be valid till the avalanche zone (Zener), situated around V= -30V. But in reality, under irradiation (high photocurrent Iph), the dissipation, which varies with the cube of the reverse voltage, reaches a destructive limit well before this elbow. For example, the cells constituting the Arco M55 modules, dissipate about 18 W at a reverse voltage of -18V, and 25W at -20V, corresponding to a rise in temperature of the order of 100°C. This is even more dangerous as the temperature's rise sharply increases the parameter bRev , and therefore the reverse current, leading to an unstable situation.