MPPT and Integrated Diagnostics for Multi-String PV System With Multi-Input ESC

Multi-string configuration is commonly seen in practical photovoltaic (PV) applications, to secure the Maximum Power Point (MPP) for each string, individual string is equipped with its own DC-DC converter for voltage regulation. This study presents an integrated framework that achieves dual purposes simultaneously: maximum power point tracking (MPPT) control and detection of the change in internal resistance. An multi-channel extremum seeking control (ESC) strategy is developed to maximize the power output of multi-string PV system by regulating the voltage input to the DC-DC converter, overall MPPT can be obtained by delivering the power from strings to the DC bus. Simulation results show that the ESC can achieve multi-string overall MPPT with periodic dither signals such as sinusoidal and square wave signals. As change in internal resistance is coupled with many degradation factors for the PV cell, e.g. increase in the series resistance and decrease in the shunt resistance, the capability of integrated detection of internal resistance can be used as a diagnostic probe for PV online diagnostics. Square Wave ESC dither signal is a useful tool to detect the change of internal resistance as the rise time or settling time on a square wave rising edge is more amplified with internal change of a PV system. Therefore, the voltage output transient under square-wave dithering can be changed remarkably with degradation induced change of internal resistance. Simulation results have also confirmed such correlation. The ESC with square wave dithering can thus provides the dual benefits of MPPT and internal resistance change simultaneously.