Comparative analysis of current control methods for modular multilevel converters.

摘要

Modular Multilevel Converters (MMCs) are power electronic converters comprised of a series connection of sub-modules. Their modular structure allows for the possibility to design high-voltage converters that are suitable for utility applications due to the modular fail-safe structure with reduced switching frequency requirements. Some areas of interesting research specific to the MMC topology include modulation techniques, control methods, capacitor voltage balancing strategies, and circulating current suppression control. This thesis presents the development of a predictive current control for MMCs that has the benefit of inherently reduced circulating currents within the converter's phase units. Two other typical MMC current control strategies are implemented for comparison with the predictive current control.;The operation and modeling, multi-loop control design, and digital simulation of a MMC are presented using MATLAB/Simulink software. An effective control scheme is implemented using a cascade control approach, with an outer power controller and an inner current controller. The outer loop is implemented with a conventional synchronous proportional-integral (PI) controller. The inner loop is then implemented with PI, proportional resonant (PR), and predictive controllers and the controller error signal dynamics for each method are observed. The predictive arm-current controller is shown to have significantly reduced circulating currents in the phase units, which reduces arm current distortion and submodule capacitor voltage ripple.