Operating-Point-Optimized Control Strategy for Modular Multilevel Converters in Low-Frequency AC Transmission Systems

For the internal control of MMC topologies, degrees of freedom (DOF) that can influence both the transient and steady-state performance are used. Several operation modes, each with a specific selection of DOF, can be used to reduce either the branch energy variation, as an indicator of the required module capacitance, or the branch current RMS value, as an indicator of the branch losses. In this article, an operating-point-optimized operation mode based on an analytical model is presented, which uses a weighting function to find a tradeoff between branch energy variations and branch losses for each operating point, depending on the characteristics of the converter and connected systems. This generalized approach is valid for M3C, M2C, and b2b-M2C. The operating-point-optimized operation mode is presented for application in a low-frequency ac transmission system connecting an offshore wind farm to the onshore grid, whereby M3C or b2b-M2C is used as an ac to ac converter station. Using the novel operation mode, the MMC topologies can be optimally dimensioned and compared with each other at each operating point. The analytical approach is validated with a simulation model of a $\mathbf {320}\,\mathbf {MW}$ low-frequency ac transmission system and additionally tested on a low-voltage test bench.