Analysis of the operation of PV strings at the MPP closest to the nominal MPP voltage instead of the global MPP based on measured current–voltage curves

Abstract

Under non-uniform operating conditions, photovoltaic (PV) generators may have several maximum power points (MPP) and voltage of the global MPP (GMPP) may vary quickly over a wide voltage range which may cause problems for tracking of the GMPP. Since highly varying GMPP voltage causes fluctuation of the inverter reference voltage, it would be beneficial to operate the PV system in a more predictable and straightforward manner by keeping the operating point of the inverter all the time close to the nominal MPP voltage. This article presents an experimental study of a scenario in which the MPP closest to the nominal MPP voltage (CMPP) is always the operating point instead of the GMPP. The analysis was based on 1,296,000 measured current–voltage curves of three different PV strings located at Tampere, Finland. 12 days of full-time measurements were analysed for each of the studied strings consisting of 6, 17 and 23 series-connected NAPS NP190GK PV modules. Furthermore, the effects of inverter sizing on the operating point behaviour of the strings were studied. The results show that the wide operating voltage range of the GMPP can be significantly reduced by operating at the CMPP at a cost of negligible energy losses. Energy losses due to power curtailment were much larger than energy losses due to operation at the CMPP instead of the GMPP.