Effects of the Operating Point of Photovoltaic Strings on Sizing of DC- and AC-Connected Energy Storage Systems for Power Smoothing

Abstract

Photovoltaic (PV) power generators are constantly exposed to operating condition variations. While the electrical characteristic of a PV generator has exactly one maximum power point (MPP) during homogeneous operating conditions, several MPPs may exist during nonuniform operating conditions. Since highly varying global MPP voltage causes large fluctuations in the inverter reference voltage, it would be beneficial to keep the operating point of the inverter all the time close to the nominal MPP voltage to secure more predictable and straightforward operation of the PV system. This article presents a study of the effects of the operating point on the operation of PV strings equipped with energy storage systems (ESS). A scenario in which the MPP closest to the nominal MPP voltage is used as the operating point is compared to operation in the global MPP. The effects of inverter sizing on the selection of the operating point are also studied. Moreover, DC and AC connections of the ESSs are compared. The study is based on measured current–voltage curves of a 23 module PV string. The results show that, in practice, there are no major differences in sizing of DC-connected ESSs between the two MPPs.