A Review of Broadband Frequency Techniques for Insulation Monitoring and Diagnosis in Rotating Electrical Machines

Rotating electrical machines greatly suffer from insulation degradation and failure. The adoption of wide band-gap semiconductors in Variable-Frequency Drives (VFDs) hinder the utilization of traditional techniques such as on-line Partial Discharges (PD) to monitor and diagnose the insulation status and failure. Alternative techniques exploiting the broadband frequency behaviour of machine windings, and the increased high-frequency content of VFD excitation arise as a valid alternative. The present work provides a comprehensive analysis and review of cutting-edge broadband frequency techniques aimed to diagnose winding insulation degradation and faults. First, the broadband nature of machine windings is illustrated by explaining the per-turn equivalent circuit and the physical interpretation of its parameters. Then, the different insulation monitoring and diagnosis methods are thoroughly reviewed. The analysed methods include the Frequency Response Analysis (FRA) applied to rotating electrical machines, the analysis of Common Mode (CM) broadband impedance and parasitic capacitances, and methodologies based on the analysis of transient high-frequency current ringing. Specific focus is placed on the instrumentation, which often represents the main applicability limitations.