Study on AC fault ride-through of a MMC back-to-back HVDC system

The Back-to-Back HVDC transmission system (BtB) based on modular multilevel converter (MMC), can realize the asynchronous (frequency or phase is not synchronized) interconnection of two regional power grids., and it can effectively solve the power grid security and stability problems caused by power transfer. However, the rectifier and inverter equipment of BtB are directly connected by smoothing reactors, making the electrical connection between devices closer. Therefore, fault interaction would be transmitted between the AC systems and the DC side of BtB. The paper first introduces the mathematical model and conventional control strategies of the system. Next, the transient characteristics of the AC and DC system under the asymmetric AC fault are analysed emphatically. Finally, the negative-sequence voltage feedforward compensator and DC voltage limiter are designed to improve the AC fault ride-through capability of the system. Finally, the effectiveness of proposed fault ride-through strategies is verified through PSCAD/EMTDC.