Research On Control Strategy Based On New MMC Tap Station In HVDC Transmission Syatem

With the development and application of flexible HVDC transmission technology,some areas has the potential requirements to send or receive power alongside its long transmission corridor,which could fully utilize the existing power transmission apparatus.This thesis proposed a series-connected AH-MMC(SCAH-MMC)tap,which is suitable for long distance overhead line transmission system and can be built alongside the existing HVDC transmission c orridor to provide power to remote areas.Compared to parallel-connected MMC,SCAHMMC reduces the total investment requirement for HVDC tap station and possesses the dc fault ride-through capability.In this thesis,the three-phase series asymmetric MMC controller is designed based on the analysis of the topology and working principle of three-phase series asymmetric MMC.At the same time,the thesis emphasize the usefulness of algebraic formulas in gaining analytical insights which give birth to two feedforward control methods for SCAH-MMC to suppress third harmonic current.The simulation of the PSCAD/EMTDC platform verifies the dc fault ride-through capability,the rationality of controller design and the third harmonic suppressor.In order to ensure the safe and stable operation of the new tap station in the system,a control strategy solving the issue of dc voltage balance under the unbalanced grid condition is proposed;At the same time,proposing a method based on single-phase control to eliminate the second harmonic voltage at dc side according to single-phase SCAH-MMC DC side equivalent circuit.The simulation verifies the rationality of the AC side asymmetric fault control strategy and the design of the second harmonic voltage suppressor.This thesis also studies the overall control strategy of the new tap station after accessing the point-to-point HVDC transmission system.The master-slave control strategy is applied to the whole system.At the same time,the DC-side fault control strategy of the system is formulated.The final simulation verifies the feasibility of the master-slave control strategy and the DC-side fault control strategy of the system.