Research on novel railway uninterruptible flexible connector with series-connected transformers and back-to-back converter

To reduce negative sequence current penetration into power system caused by single-phase electrified locomotives, phase sequence commutating method among traction feeders has been adopted by electrified railways in China, which results in a neutral section without power supply between two feeders in traction substation. Loss and recovery of power supply to locomotives when passing neutral section will cause over voltage and over current in locomotives, which greatly threatens the safety of railway and passengers. A novel railway uninterruptible flexible connector is proposed in this paper, which can eliminate power supply dead zone when locomotive passing neutral section. It has the series-connected structure of back-to-back converter and transformers. Back-to-back converter is used to transfer active power needed by locomotives from one traction feeder to neutral section and transformers are adopted to reduce the capacity of converters and the complexity of control system. Instantaneous power theory is used to detect sag of neutral section voltage to start compensation and current increase to end compensation. Double close-loop control strategy is used for back-to-back converter and control system parameters are designed in accordance with typical second-order system design methodology. PSCAD/EMTDC simulations are carried out to prove the effectiveness of proposed uninterruptible flexible connector and its control system.