VFD-Induced Torsional Stresses in Pumped Hydropower Storage Applications

Despite consistent maintenance and monitoring equipment installed in pumped storage hydropower (PSH) facilities, many shafts and electrical component failures are reported, possibly resulting from undetected sources. These sources include undetectable vibrations or, in certain conditions, high-frequency mechanical or electrical harmonics. They may induce premature material fatigue and aging, leading to accelerated wear and premature system failure. This paper presents a direct method for plotting Campbell diagrams of large motors supplied by variable frequency drives for torsional analysis purposes. The method is applied to two-level, three-level neutral-point clamped, and seven-level cascaded H-bridge multilevel inverters, commonly used industrially available voltage source inverter (VSI) topologies for pumped PSH plants. These diagrams display the locations where torsional stress components induced by VFDs can interfere with shaft resonance modes. These locations are where threatening torsional stresses can build up. The method simplifies the determination of the magnitude of stimulus forces in the motor airgap that may threaten the shaft. An analytical assessment of the cascaded H-bridge VFD-induced torsional stresses under unbalanced operation is also proposed. The accuracy of the theoretical developments is supported by selected simulations results at different operating points and different fault conditions. Hybrid experimental-numerical VFD-induced harmonic stress analysis is also performed to demonstrate the relevance of the proposed study. Results are crucial for a robust design of the system integration to avoid catastrophic failures.