Fast-dynamic Control Strategy for MMC-based Traction Substation in MVDC Railway Power Supply System

For modular multilevel converter (MMC)-based traction substation (TSS) in 24kV medium voltage direct current (MVDC) railway power supply system (RPSS), fast dynamic response is core requirement for its DC-link voltage control. In this paper, a fast-dynamic control strategy by a combination of dynamic reference generation (DRG) concept and model predictive control (MPC) is proposed to achieve fast dynamic response of MMC-based TSS’s DC-link voltage control. In outer DC-link voltage control loop, an improved DRG strategy based on extended state observer (ESO) (remarked as ESO-based IDGR strategy) is proposed to regulate DC-link voltage, in which an ESO is utilized to estimate system total disturbance (sum of TSS’s output power and power loss), and ac side current references are generated based on discrete-time model of TSS’s DC-link voltage. In inner current control loop, a model predictive current control (MPCC) strategy based on continuous-control-set MPC (CCS-MPC) principle is developed to simultaneously regulate MMC’s ac side current and circulating current with fast dynamic response. With the proposed control scheme, good disturbance rejection and fast voltage reference tracking abilities, as well as satisfactory steady-state performance can be achieved for MMCbased TSS. Finally, several simulation results are provided to demonstrate effectiveness and feasibility of the proposed control strategy.