Research On Protection Strategy Of Traditional Multi-Terminal HVDC System

Due to the unbalanced development of energy and economic,the advantages of high-capacity and long-distance power transmission of HVDC have played an important role in the strategy of "sending electricity from west to east and connecting the whole country".However,the traditional two-terminal HVDC can only achieve point-to-point HVDC power transmission,with the economic development and construction of the power grid,the power grid must be able to achieve multiple power supply and multi-drop power,so the multi-terminal HVDC system has received more and more attention based on the two ends of the HVDC transmission system.Compared with the traditional two-terminal HVDC transmission system,the multi-terminal HVDC system can provide multi-power supply and more power take-off,providing a more flexible and faster transmission mode.However,as the number of terminals connected to the HVDC system increases,the operation mode of the system will be more diversified,which imposes higher requirements on the control and protection of system.Due to the complexity of the control system,the protection system of multi-terminal HVDC transmission is more complicated,which mainly reflecting the accurate judgment of the type of the fault and the position of the fault and the coordination of the protection actions after the fault,especially in the case of the fault of the HVDC line.In this thesis,the current research status and existing problems of the multi-terminal HVDC transmission system protection strategy are described.Firstly,the main transmission modes and operation characteristics of the multi-terminal HVDC transmission system are described in detail.The existing problems of the traveling wave protection strategy are pointed out.Secondly,the key technologies of HVDC transmission such as main wiring,converter equipment technology and the main control and coordination strategies are analyzed in depth.Then,based on the structure of three-terminal HVDC transmission system,this paper proposes a protection strategy configuration principle based on typical coordinated control strategy,the three-terminal system is divided into protection zones,including the converter station protection and DC line protection,and detailed protection configuration strategy for each area is given,fault location and circuit area fault location method is given.Complete protection of the configuration case can improve the protection of export logic strategy.Finally,a three-terminal HVDC system model is built in the PSCAD/EMTDC simulation software to realize the basic coordination control strategy,clear the fault point and set the fault exit logic,respectively,for sending and receiving end simulation converter station grounding fault,valve group Short-circuit fault and transformer fault,verify the correctness of the logic of export protection.The simulation results show that the proposed export protection strategy can ensure the non-fault part can work normally when the three-terminal system take any part of the failure in the system,and the power flowing of the converter station of the fault part will not reverse because of choosing different way of blocking the emergence situation when one part is normal,the other part is not,which avoiding the consequences of greater impact on the AC system of the fault converter station.