Fullbridge MMC control for hybrid HVDC systems

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Abstract
The Fullbridge based Modular Multilevel Converter (FBMMC) has been widely considered as one of the best solutions for High Voltage Direct Current (HVDC) systems, especially due to its capability of limiting dc short-circuit currents. Generally, the use of FBMMC on hybrid HVDC configurations with Line Commutated Converters (LCC) is considered for systems where there is no need for power reversion. However, whenever fast power reversion is necessary, the FBMMC is also one of the solutions because some features of its control strategies are relevant for this kind of HVDC application. This paper proposes a reduced switching frequency voltage balancing algorithm for FBMMC that is robust during the transition from positive to negative (and vice-versa) arm voltage levels. Such transition is mandatory for power reversion in hybrid systems with LCC. It also proposes a capacitor voltage control for zero dc voltage operation, which is necessary during power reversion of the hybrid configuration previously mentioned. The control methods proposed in this article have been implemented in time domain simulations of a hybrid HVDC system where a strategy to quickly reverse the power flow is performed. The results showed the effectiveness of the proposed control method.
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