Damping of Torsional Resonances in Generator Shafts Using a Feedback Controlled Buffer Storage of Magnetic Energy

The power and energy for ASDEX Upgrade, Germany’s largest experiment for nuclear fusion research, is provided by three separate networks based on flywheel generators. Damage at couplings of the shafts of the flywheel generators EZ3 (144 MVA / 500 MWs) and EZ4 (220 MVA / 600 MWs) were discovered during a routine check. They can only be explained by subsynchronous resonances (SSR) which are excited by active power transients from the converter loads. Torque sensors were installed for generator protection. They cause an early termination of plasma experiments if a predefined torque level is exceeded. Since the low natural damping involved with torsional resonances was identified as a major cause of the SSR phenomena observed, two feedback controlled DC circuits were developed providing electromagnetic damping for the generator shafts in case of SSR excitation. Since April 2003, the damping circuits have been routinely operated during all plasma experiments. They provide sufficient active damping power to avoid a trip signal from the torque sensors. The paper summarises results from analysing, designing, commissioning and operating these damping circuits which might be of general interest since they are an effective and low cost solution for damping SSR in electric power systems.