A novel comprehensive control scheme of low frequency of the modular multilevel converter

This paper analyzes the active power in modular multilevel converters (MMCs), and presents a novel comprehensive control scheme of low frequency operation for the arm capacitor voltages and high frequency circulating current. The inner loop of this scheme employs the arm current direct control which can control the ac side currents, dc-link current and circulating currents simultaneously; an improved feed forward current control scheme is presented, reducing the capacitor voltage dynamic fluctuations as well as avoiding feed-forward currents into the ac and dc sides. The outer-loop capacitor voltage control of each arm is almost independent of each other and can be easily implemented by distributed control, which introduces Proportional and Resonant (PR) regulator to suppress different frequency capacitor voltages. Meanwhile, the capacitor voltages balancing control between upper- and lower-arms of three phase are added to the outer loop to achieve circulation constraints control. Lastly the effectiveness of the scheme is verified under low frequency operation of three-phase load and an AC machine via the OPAL-Rtlab real-time simulation system.