Abstract: A method and an apparatus for improving the voltage quality of a secondary power supply unit uses a compensation device with a pulse-controlled power converter which is connected serially to a transmission line through the use of a coupling transformer. Positive and negative phase-sequence system deviations are determined and a basic transmission ratio space-vector is generated from them. Control signals for a pulse-controlled power converter are generated from the basic transmission ratio space-vector through the use of the link voltage of the pulse-controlled power converter. As a result, a compensator voltage is connected serially into the transmission line. A power system voltage of an ideal power supply system is thus generated for a secondary power supply unit from a distorted power system voltage.