Modeling And Control Of Multi-Terminal Direct Current Transmission System Based On Modular Multilevel Converter

Multi-terminal direct current(DC)transmission system based on modular multilevel converter(MMC),with superior reliability,redundancy and flexibility over the conventional voltage source converter high votage DC transmission system(VSC-HVDC),have attracted a great deal of attention globally and widely used in electrical engineering now.Multi-terminal DC transmission system based on MMC with various operation modes can meet multi-objective complex equirements.The stable operation of multi-terminal DC transmission system based on MMC involves alternating current/direct current(AC/DC)power grid and coordinated control of multiple MMC devices,which needs high requirements of control system.This thesis conducts research on modeling and control of multi-terminal DC transmission system based on MMC.The main contents are as follows:Firstly,this paper introduces the configuration of multi-terminal DC transmission system based on MMC and analyzes the functions of MMC voltage controlled station and power controlled station.According to three phase half bridge sub module topology of MMC,MMC instantaneous value model(MMC-IVM)is deduced in detail by differential-mode(DM)component and common-mode(CM)component.MMC-IVM indicates that the AC side current can be controlled by phase leg voltage DM component and DC side current can be conyrolled by CM component The AC/DC side energy is coupled by phase leg sub module capacitor,which realizes AC/DC control variables decoupled.Based on MMC-IVM,MMC power controlled station controller and voltage controlled station controller with modulation strategy and sub module voltage sharing control strategy is designed completely.Secondly,the small signal stability analysis of multi-terminal DC transmission system based on MMC is carried out.The MMC average value model of power controlled station based on phase leg equivalent capacitance is deduced by law of energy conservation.The virtual resistance can increase damping and the reference of phase leg capacitance voltage is used to avoid measuring phase leg capacitance voltage directly.Then,a small signal stability analysis model of three-terminal flexible DC transmission system is established.It is composed of a single MMC voltage station and two MMC power stations.One is to employ the Routh criterion to analyze the stability of power controlled station and use Bode diagram analysis of resonant characteristics and root locus method to analyze the effect of power changes on system stability.Another is based on three-terminal DC transmission system,using Bode diagram analysis of resonant characteristics and root locus method to analyze the influence of virtual resistance,phase leg reactors,DC reactors and other parameters on the system stability.The results show that small signal model can be used for system stability analysis and guiding the design of main circuit and controllers parameters.At the same time,virtual resistance can increase the system damping and reduce the resonance peak of DC transmission system,and improve the system stability effectively.Finally,the electromagnetic transient simulation model of 26-level MMC is established by PSCAD.The simulation results show that on the basis of sub module voltage sharing,the power controlled station realizes the decoupling control of active power and reactive power on AC side and the voltage controlled station realizes the DC side voltage control and AC side reactive power control,which verifies the effectiveness of MMC-IVM and control strategies.Three-terminal flexible DC transmission system simulation model is established by MATLAB/SIMULINK.The simulation study of active power flow reverse and step with virtual resistance are carried out.The results reveal that three-terminal flexible DC transmission system can realize multidirectional complex power flow and the virtual impedance improves the transient process and system stability.In addition,the demonstration project of Tangjiawan three-terminal flexible DC transmission system in Zhuhai is introduced.The experiment study of two-terminal and three-terminal are carried out.The multi-terminal DC transmission system based on MMC can control multi-terminal AC/DC power grid energy exchange and improve the reliability of the system as well.