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The increased power and voltage ratings of semiconductors fosters the use in medium voltage components, such as ac–dc converters. With a bipolar medium frequent rectangular (MF RECT) voltage, the converter size can be reduced and the efficiency increased. Possible insulating materials for medium voltage transformers are oil and insulation paper. The breakdown voltage of insulation materials is depending on the frequency and the shape of the voltage curve. Until today, only a few investigations of the breakdown voltage of insulating materials at sinusoidal and RECT voltages up to a few tens of kilohertz have been published. To design and optimize MF transformers at RECT voltages, further investigations are necessary. Therefore, a test bench for measuring the breakdown voltage at RECT voltages in the frequency range between one and ten kilohertz up to several tens of kilovolts is developed. The test bench is inspired by a Marx generator, which is a high voltage source for impulse voltages. By substituting resistances and spark gaps by IGBTs and adding galvanically insulated power sources, a medium frequent bipolar voltage can be generated. The test bench is characterized with respect to slew rate, input power, frequency, and voltage amplitude. Its usability is shown in first breakdown voltage measurements of insulation papers, used for transformer insulation. [ABSTRACT FROM AUTHOR]