Study On Transient Characteristics Analysis And Fault Current Calculation Of LCC-HVDC Receiving Terminal During The Whole Process Of Fault

With the development of ultra-high voltage DC transmission technology in China and the need to build a new type of power system while focusing on the new energy,whether it is a large-scale power grid with high proportion of DC transmission,or a high-voltage DC transmission system with wind and fire bundling as the transmission terminal,the system will face many problems such as rapid and variable system fault transient processes and complex fault electrical quantities which is difficult to solve.Moreover,with the continuous increase of high-voltage DC transmission capacity and the increasing proportion of new energy generation,the short circuit current calculation methods for traditional power grids are no longer applicable.Therefore,this paper studies the fault transient processes of the LCC-HVDC system and the air-condition baling LCC-HVDC transmission system in the scenario of symmetrical short circuit faults on the AC side of the system receiver.The following achievements have been achieved in this paper:Due to the high nonlinearity and complexity of fault time response of the converter station in LCC-HVDC system,an accurate analytical solution of fault current is difficult to solve.A short circuit current calculation method based on the subsection response characteristics of the transfer function of the DC control system is proposed.First,analyze the conduction of the inverter valve block when a three-phase short circuit fault occurs on the AC side of the receiving terminal,and obtain the equivalent circuit of the inverter commutation failure.After that,by analyzing the transient response process of the DC control system,a transfer function model is established based on the response characteristics of the control system.Finally,by simplifying the transfer function and inverse transformation,an approximate analytical expression for the entire transient process of DC current is obtained,and the contribution of DC current is converted to the AC side to obtain a segmented analytical expression for AC short circuit current that takes into account the contribution of DC current.To solve the problem of complex fault traversal characteristics and difficulty in solving the short circuit current in the air fire baling system sent through LCC-HVDC during short circuit at the receiving end,a short circuit current calculation method based on the response characteristics of the grid side converter control system is proposed.First,by analyzing the characteristics of terminal voltage fault traversal during system faults,the entire electromagnetic transient process during terminal voltage faults is obtained.Second,by simplifying and decoupling the control system of the wind turbine grid side converter,the active and reactive current control system of the grid side converter is obtained.At last,a differential equation model is established based on the power transient response characteristics of the control system,and the approximate analytical expressions of the active and reactive currents of the grid side converter are obtained by solving the differential equation.Aiming at the problem of transient overvoltage caused by DC commutation failure on the sending side of the wind and fire binding system through LCC-HVDC,the overall analysis of the control structure and control strategy of the system is first carried out,and then the impact of different control strategies and parameters of the system on the transient electrical quantity of the machine terminal during the fault process is studied on the basis of considering the coupling relationship between the fan and DC,and parameter optimization strategies to improve the fault characteristics of the system are proposed.