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The modular multi-level converter (MMC) is a recently the most popular converter for high and medium voltage and power applications especially for Facts and HVDC transmission systems. But there are some technical problems in controlling the MMC in which one is balancing the voltages across the capacitor in each submodule without excessive switching of the power electronic devices. By using a reduced switching frequency technique (RSF) in voltage balancing in MMC legs, the average device switching frequency is reduced. This requires knowledge about an instantaneous capacitor voltage, so a Kalman observer is introduced for measuring instantaneous voltages across capacitors in each submodule. The proposed method needs only one current sensor per arm means two sensors for one leg. By using this method number of voltage sensors requisite will be reduced compared with the traditional voltage sensors method. So the sensor noise, system complexity, and cost will be minimized, consequently increasing the efficiency of the overall converter system. The Kalman observer estimates the capacitor voltages of each submodule by using arm currents and switching patterns. In addition to sensor cost problems, the paper also examines the effect of flowing current on the voltages of arm capacitors. This paper also deals with a controller that will control the circulating current in MMC. The efficiency of the proposed method is proved by simulating a single phase MMC using MAT LAB/ SIMULINK.