Modulators are used to generate the pulsed power for the klystrons of the superconducting linear accelerator TESLA. They produce rectangular high voltage pulses of up to 120 kV. The electrical power during the pulse is typically 15 MW and can maximally be 16.8 MW. The pulse length is 1.6 ms with a repetition rate of 5 Hz, for app. 10 % of the modulators it is 10 Hz. This leads to a needed pulsed power of 8.9 GW. It is obvious that this energy can not be taken from the mains directly. Therefore it is stored in capacitor banks to be released during the pulse. Power supplies are needed to recharge the capacitor banks and to decouple the low repetition rate from the mains. The electrical supply companies have very strict rules of the amount of distortions that are allowed to be produced by a customer especially in the frequency range below 25 Hz (flicker frequencies). To meet these rules the power supplies have to operate in constant power mode. Different types of power supplies have been investigated to check a possible use for TESLA