Replacement and enhancement of power electronics at the flywheel machine EZ2

The flywheel generator “Energiezentrale 2″ (EZ2), consisting of motor, flywheel, generator and auxiliary generator, aged 50 years, supplies ASDEX Upgrades copper Toroidal Field coils with DC current of up to 80 kA. Since there is no redundancy for this machine, a high availability has to be ensured. Therefore, during the 2023/24 experimental brake the inverter was replaced, that supplies the rotor of the 5.7 MVA doubly fed induction drive motor that accelerates the 223 t flywheel of EZ2. For starting the machine, a liquid rheostat is connected to the rotor. This resistor is also used for braking the machine. The inverter of 2 MVA is able to control the speed of the machine in the operating range between 1275 and 1650 rpm. In addition, the rectifier has been replaced that supplies the excitation of the high current pulse power synchronous generator. By this excitation the voltage at the stator clamps is controlled that ramps up the DC current in the toroidal field coils of ASDEX Upgrade by the help of diode rectifiers. The main objectives of the excitation controller are a fast rise and a constant DC current to get a useful toroidal field during a plasma pulse. Also auxiliary rectifiers have been replaced, as two 400 A rectifiers heating the slip rings of the synchronous machine in order to reduce high abrasion, a rectifier for compensation of the remanence field of the machine resulting in a high idling current and the excitation inverter feeding the auxiliary generator that supplies DC emergency oil pumps.