Evaluation of the Suitability of Existing Earth‐Fault Detection Methods for Insulation Monitoring Devices in Photovoltaic Power Generation Systems

To ensure the safe operation of photovoltaic (PV) power generation systems, it is essential to detect and address promptly earth faults and degradation of the insulation resistance to earth. Safe operation requires the use of insulation monitoring devices for photovoltaic systems (PV‐IMDs) that can continuously and reliably monitor the insulation resistance to earth under both normal and earth‐fault conditions. In this study, we conducted theoretical evaluations of the suitability of various existing earth‐fault detection methods for use in PV‐IMDs based on two equivalent circuit models representing the impedance between photovoltaic‐module strings (PV strings) and earth. We also experimentally evaluated two equivalent circuit models and measured their performances under both conditions. We conclude that the equivalent circuit model consisting of a single insulation resistance and RC‐series paths in parallel is more suitable for the PV string‐to‐earth circuit than the popular model, which consists of a single insulation resistance and a single leakage capacitance in parallel. In conclusion, direct‐current‐based methods are more suitable than alternating‐current‐based methods for PV‐IMDs. © 2026 Institute of Electrical Engineers of Japan and Wiley Periodicals LLC.