Analysis and Control of Modular Multilevel Converter With Split Energy Storage for Railway Traction Power Conditioner

In this paper, a railway power conditioner (RPC) based on a modular multilevel converter (MMC) with a split supercapacitor energy storage system (SCESS) is studied. In this case, the MMC-SCESS-based RPC could not only provide normal negative-sequence current compensation, but also reduce the impact of power fluctuations caused by the locomotive braking or startup on the electric grid. First, this paper analyzes the power flow patterns and deduces the reference circulating current under different operation modes of the MMC-SCESS-based RPC. Then, the control objectives of the MMC-SCESS-based RPC are divided into two categories-the balance control and the current tracking control. The balance control methods are developed for the sub-module capacitor voltages and the state of charge of the supercapacitor, which are associated with the operation modes. To ensure the current tracking performance, a model-predictive direct current control method is presented for the MMC and the bidirectional energy converter. Finally, the effectiveness of the proposed control methods is verified by the experimental results of a downscaled prototype.