PI passivity-based control of modular multilevel converters for multi-terminal HVDC systems

In this work, a decentralized PI passivity-based controller (PI-PBC) is applied to the Modular Multilevel Converters (MMCs) to ensure global asymptotic stability of a multiterminal MT-HVDC system. Since continuous MMC state-space models naturally have time-periodic steady-state solutions, a first step towards the derivation of the controller is the formulation of an equivalent model characterized by constant steady-state solutions, obtained via a multi-frequency orthogonal coordinates transformation. For the design of the controller, the overall system is represented in an appropriate port-Hamiltonian formulation, which allows the derivation of the stabilizing control law using passivity-based arguments. The results are validated on a three-terminal simulation benchmark.