Interaction Fault Analysis And Coordinated Protection Of AC/DC Side Faults In The Converter Station In Photovoltaic DC Grid-Connected System

Compared with the traditional PV system that is an AC-connected system,there are serveral characteristics that the PV DC-coinnected system has,such as simple control modes,small system loss and a high PV utilization rate.In order to realize the coordination between AC and DC protections in the DC-connected system,a simulation model has been established based on the PV DC-connected system,which is a demonstration project.Then,the DC-side electrical characteristics after AC faults and the AC-side characteristics after DC faults have been analysised based on this model.And a pervasive method,which is uesd to analysis the influence of AC?DC?faults on the DC?AC?side,i.e.,the interaction influnce between AC and DC sides after faults,is proposed.Finally,considering the interaction influnce,the coordination scheme of AC and DC protections has been designed.And the main contents are as follows.Firstly,the research purpose of this paper,as well as the research status of the research at home and abroad is introduced.Mainly includeing the introduction of the PV system,the introduction of modular multilevel converter,the status of fault analyses in AC/DC hybrid systems and related researchs on the coordination of AC/DC protections.Then,the simulation model of the PV DC-connected system,which is different from topologies of traditional PV connected systems,is established.Where,the DC/DC converter in a PV station is composed of several Boost full-bridge isolated converters.And the control strategy is designed based on the characteristics of the topology.The grid-connected converter station is mainly composed of a MMC,in which submodels comprise half bridge submodules and full bridge submodules.Thirdly,the interaction influnce between AC and DC sides after faults is analyzed.The mapping relation between AC and DC electrical quantities is analyzed firstly.Based on the relation,the sequence component network,which is used for ayalysing the characteristics of AC-side electric quantity after DC short faults.And existing conditions of the sequence component network is specifed.Where,the network during the normal system operation equals the positive sequence network that indicates the mapping relation between the traditional AC-side positive sequence component and the DC-side DC component.And the‘negative sequence network'demonstrates the mapping relation between the traditional AC-side negative sequence component and the DC-side 2nd harmonic.Based on the mapping relation between the traditional AC-side zero sequence component and the DC-side power-frequency component,the‘zero sequence network'is established,as well.After that,the AC component of the voltage is deduced,according to Kirchhoff's law and superposition principle.Finally,according to previous analyses,the coordination scheme of AC and DC protections in the converter station are proposed.Since coordination often lacks between AC and DC protections in the converter station,with certain problems such as the DC protection may act in error after an AC fault.Therefore,a protection criterion,which could identify fault areas and distinguish AC faults from DC faults,for a MMC is proposed,according to the 2^nd harmonic current in DC side and the negative sequence current in AC side before and after the converter blocking.Besides,based on this criterion and the DC-side low voltage protection,an AC/DC coordinated protection is proposed.