Analysis of a Stator Earth Fault Protection System of a Medium Voltage Converter-Fed Synchronous Motor

For grinding applications cycloconverter-fed gearless mill drives are being applied with increasing powers and new risks emerge with the systems scaling-up. Traditional parasitic second order effects may be not neglected, like thermal expansions, electromagnetic effects and EMC emissions. In this frame, harmonics and partial discharges of windings in medium voltage converter-fed machines build additional leakage current background that masks eventual phase insulation failures or produce nuisance trips in the protective system. Despite these perturbations, for reliable operation, the electrical protection system must detect properly a phase-to-ground fault of the motor in the 100% length of stator winding with the trade-off between sensitivity ad insensitivity. This work presents a study and discussion based on modeling and simulation for assessing the limitations and range of application of a 100% earth fault protection configuration, considering the operation of this protection acting also as earth leakage protection.