Low-Cost Inductor Current Ripple Excitation Design for Rapid Converter-Based Electrical Impedance Spectroscopy on a Monocrystalline Solar Module

Electrical Impedance Spectroscopy has been thoroughly researched on distributed energy devices for fault diagnostics using DC-DC converters. Through DC-DC converters, EIS uses externally generated AC signals superimposed on a modulating duty ratio signal to create a current or voltage excitation signal. To achieve a high resolution in the excitation signal, the switching frequency of the DC-DC converter must be at least ten times higher than the impedance bandwidth of the device under test. For batteries and fuel cells, devices with lower impedance bandwidths (less than 10 kHz) do not significantly impact the design of the DC-DC converter; however, for solar PV modules that have higher impedance bandwidths (greater than 50 kHz), the design of the DC-DC converter becomes complex and costly, while also requiring significant processing power for high frequencies. This paper thus, assesses the feasibility of using a DC-DC converter's inductor current ripple as an excitation signal to help reduce the switching frequency requirements of traditional EIS. With this technique, the switching frequency of the converter would only be required to be as high as the impedance bandwidth of the DUT thus lowering costs related to the DC-DC converter and processing requirements. The design considers the power regulation requirements of the load to determine the optimal peak-peak inductor current ripple for both power regulation and the device EIS excitation. The experimental results show that, because of the frequency distribution of the PV module's Nyquist impedance, the optimal design for the inductor current ripple is best suited to characterize the impedance of PV modules under well-lit conditions. Under these conditions, more than 50% of the PV module impedance can be characterized with good accuracy when compared against results from a commercial Frequency Response Analyzer. Under low lit conditions the inductor current ripple proves ineffective in the excitation when compared to traditional EIS implementation, characterizing less than 10% of the curve. Thus, the information this approach offers is at the high-frequency end and offers insights into the module's series resistance as well as the majority carrier information at a low cost which surpasses what is currently available for high-frequency excitation.