| As a new breed of voltage-sourced converter(VSC),Modular multilevel converter(MMC)has drawn attention from both industry and academia,since it combines the advantages of VSCs with the perfect output waveform,scalability,low switching frequency and low converter losses.Voltage-sourced converter high-voltage direct current(VSC-HVDC)systems with overhead lines have great application prospect and development potential.However,there still existed some critical issues which have not been solved,and there is an urgent need for detailed analysis of VSC-HVDC systems for overhead lines transmission.This thesis focuses on several important issues in steady-state characteristic and fault performance of overhead line based MMC-HVDC.The main works are organized as follows.The thesis is organized as follows:(1)Based on the continuous model,the ac side and dc side continuous model are presented.In weak system connection situation,the recovery responses of traditional current vector controller and power synchronize controller are compared,and the reson for the unstability with current vector controller is discussed.For the converter with the power synchronize controller,the dc power ratio limit of the receiving system is discussed based on the voltage response analysis.(2)A valve losses evaluation method based on piecewise analytical formulas for the modular multilevel converter based high-voltage direct current(MMC-HVDC)transmission systems is proposed.According to the generating mechanism,the total valve losses of the MMC are divided into three parts:conduction losses,essential switching losses,and additional switching losses.The conduction losses and the essential switching losses are represented by analytical formulas,and the additional switching losses are described by an analytical upper limit.(3)An improved shut-down control scheme for MMC-HVDC systems is proposed.To extend the application scope,the proposed method consists of two parts:the main control scheme and the back-up control scheme.The shut-down control scheme is divided into three parts:the energy feedback stage,the controllable energy dissipation stage and the uncontrollable energy dissipation stage.In the energy feedback stage,the SM capacitor voltage is reduced by increasing the modulation index m,activating the third harmonic injection,adjusting the converter transformer tap and inserting the redundant sub-modules.The capacitive energy in the SMs can be utilized at utmost.In the controllable energy dissipation stage,the SM capacitors are discharged by the dc line or the start-up resistors,which could get rid of the discharge resistor.In the uncontrollable energy dissipation stage,the SM capacitors are fully discharged through the SM resistors.(4)Analytical method for dc side harmonic currents calculation and dc loop resonance analysis of the LCC-MMC hybrid HVDC systems is proposed.Based on the linearization theory,the MMC is represented by an equivalent passive electric network.In the following calculation,the line commutated converter(LCC)at the rectifier side is replaced by the three-pulse harmonic voltage sources;the MMC at the inverter side is replaced by the equivalent passive electric network;the dc transmission line is represented by the coupled multi-phase line model.The proposed method is efficient and effective in evaluating the dc side harmonic currents and dc loop impedance,and shows great value in practical engineering application.(5)Based on the HVDC circuit breaker scheme and the MMC with dc fault blocking capability acheme,two aspects in the dc fault performance for multi-terminal MMC-HVDC are analyzed:i)a dc side short circuit current calculation method for evaluating the performance requirement of HVDC circuit breakers is proposed;ii)the impact on the transient stability of the connected ac system after dc side fault is analyzed according to different stages with the dc fault handling method,and the comparision the between these two schemes is performed based on simulation results. |
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