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This paper proposes a new three-phase transformerless STATic synchronous COMpensator (STATCOM) for shunt reactive power compensation in medium/high-voltage applications. The proposed STATCOM is based on a hybrid Boost Modular Multilevel Converter (hybrid BMMC) with a reduced number of Full-Bridge SubModules (FB-SMs). Operating with almost zero displacement factor at the ac terminals of the converter guarantees sufficient charging and discharging time intervals for the Half-Bridge SMs (HB-SMs) which ensures well-balanced capacitor voltages. Conventionally, each arm in a hybrid BMMC contains FB-SMs and HB-SMs with a ratio of 2:1 respectively. In this work, a new hybrid BMMC with a ratio of 1:2 is employed, which enables operating with a reduced number of FB-SMs with the same boosting capability, therefore, enabling transformerless operation. Nonetheless, the tradeoff is the loss of the dc fault blocking capability compared to the 2:1 configuration. This may be acceptable in STATCOM applications, as the possibility of having a dc side fault in such application is limited, compared to, for instance, MMC in high voltage dc network applications. In this work, detailed illustration of the proposed STATCOM configuration along with its controllers are presented. Finally, simulation case studies are considered to demonstrate the effectiveness of the proposed approach. The simulation results show promising and satisfactory performance. [ABSTRACT FROM AUTHOR]