Performance and safety aspects of PV modules under partial shading: a simulation study

Abstract

The effect of partial cell shading on module and systems has been investigated by electronic circuit simulations. The shaded cell’s transition from generation to dissipation, with increasing shadow fraction has been analyzed in detail. High shunt resistance cells show a maximum dissipation point (MDP) at low shadow fractions (518%), while low shunt resistance cells show a maximum dissipation plateau at high shadow fractions (typically >50%). Many cell manufacturers reject cells that have Irev(-10V) > 2.5 A, with the argument that high dissipation and associated high temperatures (local hot spots) under reverse bias will not occur in this way. However, in this study we show that cells, under partial shading conditions, with reverse-bias currents as low as Irev(-10V)=0.1A yet can suffer from dissipations up to 86W. This result confirms the need for a revised hotspot test as suggested by Wohlgemuth et al. We show that for module-level optimizers dissipation occurs at higher shadow fractions and that the dissipation is lower than for system-level optimizers. Finally, we suggest that a new generation of inverters with global MPP tracking potentially pose a safety and long-term module reliability concern, since the maximum power can be accompanied with high dissipations in partially shaded cells. Inverters that operate at the high-V local MPP produces less power in situations of partial cell shading, but high dissipative powers in a shaded cell can be prevented.