Detection of Internal Resistance Change for Photovoltaic Arrays Using Extremum-Seeking Control MPPT Signals

Abstract

Maximum power point tracking (MPPT) and fault diagnosis are two important aspects of reducing the cost of energy for photovoltaic (PV) systems. Many mechanisms of PV degradation are associated with change in the internal resistance, especially the shunt resistance. This brief proposes a cost-effective scheme for detecting the change of PV internal resistance using the signals available in extremum-seeking control (ESC)-based MPPT. With the square-wave dither inputs, the steady-state trajectories of PV output signals effectively consist of a train of small-amplitude step responses. The transient characteristics of such step responses are affected by the PV internal resistance. A small-signal transfer function analysis is performed for the PV buck system, which justifies the impact of the internal resistance on the transient characteristics. A simulation study is first conducted for single-string PV systems, and the simulation results show that ESC can achieve satisfactory MPPT under a square-wave dither input. The transient characteristics, such as integrated absolute error, show a strong correlation with the internal resistance. The applicability of this scheme to multistring PV is demonstrated with a multi-input ESC based on the total power feedback, with the internal resistance of individual modules detectable from one output signal.