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Abstract

The ASDEX Upgrade (AUG) tokamak requires an electrical power up to a few hundred MVA for a time period of 10–20 s. The power and energy is provided by three separate networks based on flywheel generators. In 1999, during a routine check performed on generator EZ3, it was discovered that coupling bolts of the flywheel generator shaft were deformed. Given that the active load of the generator (~100 MW) in service is well below the design value of the shaft (~800 MW), the damages may only be explained by a torsional resonance of the shaft line, itself excited by active power transients from the converter loads. A value of 23.6 Hz was calculated for the first eigen-frequency of the shaft line. Frequencies between 10 and 30 Hz have been identified in the spectrum of the load curves. Since torsional shaft oscillations are characterised by very low damping, torsional resonance can become dangerous even for over-dimensioned generator shafts. Therefore, a novel `torque' measurement system was installed. The paper presents results from calculations and measurements showing that devices capable to measure the stress in the shaft line are essential for generator protection in feedback controlled fusion experiments.