Research On Control And Protection Technology Of Hybrid High Voltage Direct Current Transmission System

The hybrid high voltage direct current(hybrid-HVDC)transmission system is composed of line commutated converter(LCC)and voltage source converter(VSC)through different kinds of connections and topologies.Combining the advantages of conventional direct current transmission and flexible direct current transmission technology,hybrid-HVDC plays a significant role in the scenario of large-scale new energy grid-connection,optimizaed utilization of wide-area resources,and the improvement of stability of HVDC systems.Due to the difference in the commutation mechanism between LCC and VSC,there are essential differences in fault characteristics.As a result,the propagation of faults in hybrid-HVDC systems will further exacerbate the complexity of the interaction effects of faults.Therefore,it is of great significance to study the fault characteristics and propose corresponding fault protection strategies for the development of DC transmission systems.This paper focuses on the control and protection technology of hybrid-HVDC transmission systems.Firstly,the research background is expounded,and research actuality of the topology and control protection strategy of hybrid-HVDC transmission system is summarized and analysed.Secondly,based on the characteristic of single direction of power flow of China's West-to-East Power Transmission Project,this paper takes LCC-MMC hybrid-HVDC transmission system as the main research object.The basic principles of hybrid-HVDC is studied,including commutation units and connection modes.On the basis of mathematical model analysis,the research of basic control strategy is carried out,including converter coordinated control strategy,MMC modulation strategy and bridge arm circulation suppression strategy.In terms of fault control and protection technology,the fault types studied in this paper include the AC faults at the sending end,AC faults at the receiving end and the DC side faults.Aiming at the problem of drop and even interruption of the transmission power due to the AC faults at the sending end,and the problem of double frequency fluctuation of the DC side resulted from AC asymmetrical fault at the sending end,an integrated control strategy combining power transmission control,third harmonic injection method and DC voltage fluctuation suppression strategy is proposed to improve the operation characteristics of the hybrid-HVDC transmission system.For the problem of uneven power transmission on both sides and abnormal increase in DC voltage caused by the AC faults at the receiving end,and the problem of current distortion,active power and reactive power double frequency fluctuation owing to AC asymmetrical fault at the receiving end,the power balance control based on the active regulation of LCC and the positive and negative double loop current control strategy are proposed to reduce the impact of AC faults at the receiving end on the hybrid-HVDC transmission system.Aiming at the shortcomings of the traditional half-bridge sub-module structure which cannot achieve the DC fault self-clearing,a current-transferring sub-module based on reverse blocking IGBT and parallel thyristor is proposed,which could effectively ensure the safety of the switching devices,as well as the self-clearing of DC faults.For the current-transferring submodule,the operating principle and additional investment cost are analysed.For the MMC composed of current-transferring sub-module,the corresponding fault control and protection strategy and the restart control strategy are put forward.