Capacitor Voltage Ripple Suppression in a Modular Multilevel Converter Using Frequency-Adaptive Spatial Repetitive-Based Circulating Current Controller

In this article, a new circulating current control architecture is proposed to suppress the submodule capacitor voltage ripple in a modular multilevel converter (MMC). The proposed cascaded control architecture uses the spatial repetitive controller (SRC) in both the outer voltage loop and the inner current loop, and can effectively suppress the second harmonic component of the submodule capacitor voltage ripple even under variable line frequency conditions. The dual SRC-based circulating current controller injects a second harmonic component into the circulating current to suppress the submodule capacitor voltage ripple. The SRC, which is a frequency-adaptive periodic digital controller, incorporates a phase sampling technique to achieve dynamic change in the sampling frequency, can adaptively tune its gain characteristics according to operating line frequency of the MMC. Unlike the existing techniques, the proposed controller uses the submodule capacitor voltage information for circulating current harmonic reference generation and, hence, does not require additional sensing of the output current. Detailed mathematical analysis and architecture of the proposed controller are presented in this article. A 400-V, 1-kW MMC laboratory prototype is developed and experimental results demonstrating the performance of the proposed circulating current control architecture for variable frequency and load conditions are also presented.