In Situ Series Resistance Estimation of an Off-Grid PV String by Charge-Controller Actuation: A Feasibility Case Study Within a Digital Twin Framework

Abstract

Series resistance (Rs) is one of the most informative single parameters for the state of health of a photovoltaic (PV) string. Determining it without a curve tracer or interrupting production remains difficult, because maximum power point (MPP) tracking (MPPT) confines operation near the MPP. This study proposes a procedure in which the digital twin (DT) of an off-grid PV string with battery storage commands a scheduled, condition-gated current-voltage (I–V) sweep from the MPP toward open circuit through the current-limit register of the commercial charge controller. The reduced power continues to charge the battery. The single-diode model is fitted with the ideality factor and shunt resistance held fixed; the string-level Rs, inclusive of the DC cabling, is obtained by Levenberg–Marquardt iteration. In two measurement periods on the same four-module string, added series resistances of 0.33 to 1.18 Ω were recovered within 8% with a pooled regression slope of 0.97. Twelve clear-sky baseline sweeps over eight days agreed with a sample scatter of 3.4%. Feeding the in situ Rs back into the twin raised the operating power mean absolute percentage error (MAPE) from 7.1% to 12.9% when substituted alone and reduced it to 4.7% when the companion parameters were refit. A sweep takes about 144 s, costs about 5 Wh, roughly 0.1% of the daily yield. The results are limited to a single string at a single site and are presented as a feasibility study.