Voltage compensation circuits connect DC voltages accumulated in capacitors having respectively different charging voltages which are 2^K (where K=0, 1, 2, . . . ) times the minimum voltage into AC voltages. The voltage compensation circuits are connected in series to an electric power system in series. When the voltage dip occurs in an electric power system, a combination of voltage compensation circuits is selected from the voltage compensation circuits. The sum of the output voltage compensates the voltage dip. During normal operation, a high speed mechanical short circuit switch by-passes the voltage compensation circuits to reduce power loss.