Scale-Up Methodology of a Modular Multilevel Converter for HVdc Applications

Modular multilevel converters (MMCs) are a realistic alternative to the conventional voltage source converters for medium-voltage (MV) and high-voltage direct current (HVdc) applications. The number of submodules (SMs) per arm of the MMC can be as high as 512 to achieve desired high dc voltage levels required for HVdc with a very low total harmonic distortion (THD) (e.g., <;0.1%) of the MMC ac-side interface voltage. Although the low THD of the MMC output voltage with a high number of SMs is desirable, the MMC control implementation and complexity is also important to be considered for the high number of SMs. The MMC control complexity significantly increases as the number of SMs increases. Redesigning the number of SMs in MMCs also becomes quite difficult and may require significant control upgrade, which in turn also needs additional tests and validations. This paper presents an MMC scale-up control methodology applicable for MV and HVdc applications. The number of SMs can be conveniently increased or reduced without any significant control modifications. The proposed control method and capacitor voltage balancing algorithm are implemented in the real-time digital simulator and MMC support units based on field-programmable gate array boards. The performance of the proposed MMC control method is investigated for a point-to-point MMC-based HVdc system under various operating conditions.