Today, stationary excitation systems with thyristor power converters are preferrably used for the voltage stabilization of slip-ring synchronous machines. The power densities of the corresponding power station generators supplied by ALSTOM are between 4MVA and 750MVA, the main focus being at approximately 200MVA. Over 400MVA, excitation systems from ALSTOM are used around the world (under the name SEMIPOL since 1965).