Cost and reliability optimization of modular multilevel converter with hybrid submodule for offshore DC wind turbine

Abstract

The hybrid modular multilevel converter (HMMC) consisting of half- and full-bridge submodules (SMs), with DC fault ride-through capability, have become a promising candidate for offshore wind energy conversion system, especially all-DC output wind turbines. The topology of HMMC needs to be optimized for lower cost and higher reliability with active redundancy. This paper presents an optimization method for HMMC topology with cost and reliability objectives. The reliability is defined to be not only reliable under normal operation, but also able to ride through DC fault even with faulted SMs, which is different from the former model. Firstly, reliability and cost models with power loss, considering active redundancy configuration, are derived. Then, cost and reliability, with different numbers of original and redundant SMs, are quantified by taking a case study of 10-MW and 10-kV HMMC for offshore DC wind turbines. Finally, a multi-objective optimization approach that regards maximum reliability and minimum cost is proposed, and the optimal solution from Pareto-optimal set is selected according to fuzzy membership function, which determines the optimal topology for HMMC. The results show that reliability can be improve with redundant full-bridge SMs (FBSMs) but weakened with excessive half-bridge SMs (HBSMs). Furthermore, the incremental cost with a redundant FBSM is approximately 3 times that of HBSM. The optimal topology, 10 original SMs per arm (5 HBSMs and 5 FBSMs) with 1 redundant HBSM and 4 FBSMs, is validated by comparing with the results of former reliability model and single-objective optimization.