Accurately Extracting the Shunt Resistance of Photovoltaic Cells in Installed Module Strings V. d’Alessandro, P. Guerriero, S. Daliento, and M. Gargiulo Department of Biomedical, Electronics, and Telecommunications Engineering, University of Naples Federico II, via Claudio 21, 80125 Naples, Italy. Email: vindales@unina.it Abstract — A straightforward non-invasive method is proposed to accurately evaluate the shunt resistance of an elementary cell of a photovoltaic module connected in an installed string without the need of preliminary knowledge of the intrinsic diode parameters. The approach relies on the measurement of the current-voltage characteristic of the whole string after intentionally shading the selected cell. Calibrated PSPICE simulations are employed to both illustrate and test the method. As a case study, the shunt resistances of several cells belonging to a series array of 10 commercial modules are determined. Index Terms — Photovoltaic (PV) cell, PV array, series resistance, shunt resistance. I. INTRODUCTION The shunt resistance R sh in a photovoltaic (PV) cell is a parasitic parameter that can be reviewed as an indicator of the cell quality, since it describes the existence of an alternative (shunt) path for the current flow through the inherent cell diode or along the cell edges due to non- uniformly distributed manufacturing defects as e.g., lattice imperfections or impurities in/near the depletion region [1]. If the R sh of the cells is low, i.e., the shunt paths exhibit a high conductance, large leakage currents are undesirably derived. This in turn entails a performance degradation of the PV field due to the reduction in the power produced, especially at low irradiation levels (e.g., during cloudy days and/or far from noon) [2]-[4]. The shunt currents might also affect the open-circuit voltage V oc and short-circuit current I sc in cells characterized by intolerably low quality (i.e., with R sh <0.5 Ω). Moreover, low-R sh cells are particularly susceptible to hot-spot formation when shaded [4]. It is commonly recognized that an accurate extraction of the shunt resistance (not usually provided by the module manufacturer), as well as of the other key cell parameters, is of utmost importance for the design optimization of PV systems due to the increased reliability of the models implemented in simulation tools, as well as for quality control and performance estimation. Most conventional approaches are based on the measurement of the I–V characteristics of the PV cell under (different levels of) illumination or/and in the dark. In particular, R sh is usually determined from the slope of the I–V curve in the short-circuit current point [5]-[8] or in the reverse region [9]-[11]. However, this approach is unviable when the slope is influenced also by the soft-breakdown phenomenon often arising at low reverse voltages in silicon cells [3], [12]. An alternative technique has been also proposed, which makes use of the V oc and I sc values measured under very low irradiance conditions [13]. Another important issue concerns the measurement of the shunt resistance of individual cells embedded in a commercial module, which can be in principle carried out only after a critical cell de-encapsulation, or adopting a sophisticated two-terminal procedure that relies on the simultaneous application of a DC voltage source, an AC signal generator, and an operational amplifier connected to a phase-sensitive lock-in amplifier [2]. As an alternative, one could perform a gross (i.e., low- granularity) quality testing by determining the R sh corresponding to the whole panel. However, this approach is particularly prone to errors due to the enhanced flattening of the I–V curve, combined with the unavoidable data noise and the limited current resolution of available measurement systems; besides, it does not allow the identification of an uneven cell quality distribution within the module, or the detection of a cell failure. In this work, we propose a simple non-intrusive procedure to accurately quantify the shunt resistance of a selected cell belonging to a PV module connected in an installed string. The method relies on the measurement of the I–V curve of the whole string by keeping the chosen cell under ideally dark conditions. The approach is applied to a string comprising 10 commercial silicon modules. II. EXPERIMENTAL The experimental investigation was conducted on an array composed by 10 encapsulated mono-crystalline silicon 50Wp modules, each partitioned into two sub- panels provided with a bypass diode (located in the junction box mounted on the rear) and comprising 20 68 cm 2 elementary cells in series. As shown in Fig. 1, the string was mounted on the rooftop of the Department for experimental purposes. The PV panels were individually characterized through a self-powered monitoring circuit recently developed in house for diagnostic services [14]: the open-circuit voltage V oc of the individual modules was found to range between 20 and 25 V, while the short-circuit current I sc was detected to span from 2 (winter) to 3 A (summer) at sunny midday. A custom version of the H&H ZS3060 electronic DC load [15] rated for 3 kW 800 V was employed to measure the I–V characteristics of the overall string.