Arm voltage estimation method for compensated modulation of modular multilevel converters

The Modular Multilevel Converter (MMC), being a complex system, requires a number of controllers to function properly. These range from low level switching pattern generation to high level control loops. Among these controllers are modulation techniques that calculate the insertion indices, control inputs that decide the number of sub-modules inserted in a given arm. One such technique is compensated modulation which divides the reference voltages, generated by high level controllers, by the respective arm voltages (sum of capacitor voltages) when calculating the insertion indices. This prevents the arm voltage ripple from affecting the generated output voltage. For this purpose, the arm voltages can be measured (closed-loop approach) or estimated (open-loop approach). This paper presents an arm voltage estimation technique for compensated modulation of MMCs. The proposed technique combines the benefits of the open-loop approach with that of the closed loop one.