New Circulating Current Suppression Control for MMC Without Arm Current Sensors

The occurrence of grid-side faults causes imbalance and distortion of ac side voltage. Thus, the operation of modular multilevel converter (MMC) system under grid imbalance is becoming important to improve the reliability of power system. Under unbalanced grid conditions, the use of
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stationary reference frame-based controller can simplify the control structure because it can avoid decomposing the positive- and negative-sequence
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components of current and voltage. On the other hand, several arm current sensorless control methods have been recently developed to enhance the reliability of MMC, while reducing the sensors and sampling signals. This study newly proposes the arm current sensorless circulating current suppression control (CCSC) method for the MMC based on
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stationary reference frame. Unlike the existing methods requiring the circulating current reference, the proposed CCSC method uses the only common mode component, which is obtained from instantaneous phase inductor voltage. Therefore, it removes arm current sensors, while satisfying all required performances of MMC and its reliability by the effective controls for circulating current and average value of SM capacitor voltage. The new algorithm for its implementation is also presented. The practical effectiveness of the proposed CCSC method is verified by both simulation and experimental results.