Characteristics Of UHV DC Fault And Its Influence On Relay Protection Of Received

With the rapid development of China’s economy, China’s power development pace continues to accelerate,the power system operating voltage level continues to improve, the electrical size of the network structure is constantly expanding. China’s construction of HVDC transmission project will reach about 50 by 2020,a number of these are UHVDC projects,so the development of long-range, high-capacity ultra-high voltage AC and DC transmission system has become an inevitable trend.When the abnormal situation of AC and DC systems occurs, losses are huge in Such a large project.This thesis mainly discusses the UHVDC system failure characteristics and the impact of protection issues made by AC-DC hybrid system to the grid by the end from a starting point of UHVDC system.Firstly,this thesis made a brief presentation of the fault type and protection of UHVDC system,then gave a detailed analysis of the process of UHVDC system commutation failures,made a detailed description of the control system after thatBased on the above theory the PSCAD simulation model with the actual parameters of Yunguang±800 kV UHVDC system was built.Then made simulation analysis of the different kinds of transient characteristics of various types of DC system fault and the various digrees of commutation failure characteristics caused by AC system failures.Through a lot of simulation,the results that the certain critical conditions which the system can withstand and the relations between different degrees of commutation failures and AC system failures wre found out.From the interaction approach of AC and DC systems, the rapid and continuous changes transient process characteristics of DC power were analyzed, and on this basis, in the failure process of AC-DC hybrid system, the current system was made as a controllable power, and the AC and DC systems analysis meter which contains DC control system was used to establish interaction mathematical model of AC and DC hybrid power grid.In order to study the impact of AC-DC Hybrid system to AC protection, based on method of combining theoretical calculation and simulation, The impact of AC-DC Hybrid system failure to AC protection was studied. This thesis analyzed the impact made by different types of AC fault to current differential protection in two situations which divided as single circuit transmission lines and double circuit transmission line. Through the research of zero sequence components and negative sequence component in AC system in the condition of line failures without grounded and unbalanced fault, the results were found to analyze the impact of zero-sequence and negative-sequence protection of AC system.