Study On Control Methods Of VSC-MTDC Transmission System

VSC-HVDC transmission technology has been applied in the fields of new energy access,improving power supply reliability,alleviating the operating pressure of power grids in load-intensive areas.With the increase of the number of receiving end systems,the development of multi-terminal flexible DC transmission technology is of great significance in adjusting energy structure,including interconnection,power transmission and wind power access.In this context,the VSC-MTDC voltage source multi-terminal HVDC system based on MMC of modular multi-level converter is studied.Firstly,the structure and topology of multi-terminal HVDC system are analyzed.The principle of MMC is analyzed,its mathematical model is deduced,the method of restraining circulation,the circulation inhibition;A modulation strategy based on fast sorting algorithm is designed to solve the problem of capacitor voltage equalization of sub-modules.The effectiveness of modulation strategy and circulation suppression is verified by PSCAD/EMTDC simulation.This paper analyses the converter station-level control mode of voltage-source multi-terminal HVDC transmission system,deduces the mathematical model of converter station,designs the inner-loop controller taking into account the leakage reactance of connecting transformer,and analyses the five outer-loop control modes,and designs the controller separately.Three main control modes at system level are analyzed.Aiming at the problem of over-voltage in constant active power control,the constant active power control is improved,and the constant active power control with voltage limitation function is designed.The four-terminal VSC-MTDC model is built by PSCAD/EMTDC to verify the effectiveness of the control function.Aiming at the unbalanced distribution of active power among traditional drooping control converter stations in VSC-MTDC,considering economic operation,a method of transforming the peak value of active load rate and balancing parameters of drooping converter stations into constraints by constructing the optimal power flow model of the system,aiming at minimizing network losses,and updating the drooping coefficient with the optimized results is proposed.This method can make the converter station with droop control in the system adjust the droop coefficient according to the different active reference instructions of the sender converter station,and participate in the power regulation flexibly.Five-terminal VSC-MTDC system simulation model is built in PSCAD/EMTDC to verify the effectiveness of the proposed method.The simulation results show that the proposed method can reduce the loss of DC network and realize the active power balanced distribution among drooping converter stations.