Research On Simulation And Protection Scheme Of Hvdc Transmission System

China’s energy and economic centers are distributed in reverse,and large power supply bases are far away from load centers.HVDC technology,which transmits largescale power across long distances across regions,has developed rapidly in recent years.HVDC system simulation is an important tool for analyzing the characteristics of AC and DC systems,and HVDC system relay protection is an important line of defense to ensure the safety of AC and DC systems.This paper studies the simulation and protection schemes of HVDC transmission system,the main research contents include:(1)The inductance/capacitance based switch of LCC-HVDC and its parameter setting method are proposed.The inductance/capacitance equivalent switch model has the characteristic that the equivalent admittance of the system remains unchanged when the switch state is changed,which greatly reduces the computing resources required for matrix inversion.Taking the six-pulse commutation system as an example,this paper uses the time domain modeling method to analyze the voltage and current state of the inductance/capacitance equivalent switch model in a switching cycle,comprehensively considering the transient response,steady state process,switching loss and simulation error.Aiming at minimizing the difference between the performance of the inductance/capacitance equivalent switch and the performance of ideal switch,the corresponding parameter setting method is proposed.Comparing the model with the simulation results of PSCAD the validity of the parameter setting and the accuracy of the simulation results are verified.(2)The integrated electromagnetic transient simulation model and solution algorithm of LCC-HVDC transmission system are proposed.The integrated model models the converter transformer and the converter valve as a whole.The converter transformer is based on a simplified model,and the converter valve is based on an ideal switch model.The solution algorithm takes the instantaneous value of the three-phase voltage of the AC system as the input,and the switching state of the converter valve is represented by a state matrix,and the electrical network structure of the current simulation step is determined according to the state matrix.Dividing it into the main network and the sub-network,and based on the topological analysis results,the electrical main network equation is expressed and solved in a unified form.Solve the secondary network according to the results of the main network,and finally update the current simulation step size of each node voltage and branch current.The model is compared with the PSCAD simulation results to verify its efficiency and accuracy.(3)A DC line protection scheme for MMC-based DC grids using AC/DC transient information is proposed.In the proposed scheme,the difference between the transient voltage root mean square ratios(DTVR)on both sides of the current-limiting inductor of DC lines is used to identify the DC line fault,and the instantaneous zero sequence voltage(IZSV)of the AC side of the converter station is used to discriminate the faulted poles.Also,the DC grid model under the fault state is analyzed,and a simplified mathematical model of fault analysis is proposed,which can be used to set the fault criterion of faulted pole discrimination.The test results show that the proposed protection scheme can fast and accurately identify faults,and has good tolerance to fault resistance,noise and abnormal sampling.(4)A DC line protection scheme for MMC-based DC grids using line mode fault impedance is proposed.In the proposed scheme,the calculated value of the line mode fault impedance is used to detect the DC system fault,and the instantaneous zerosequence voltage of the AC system of the converter station is used to determine the faulted pole.The fault voltage and current are converted to the modal domain for analysis,and the corresponding model domain fault network is obtained.The voltage and current at the fault point,the propagation process of the fault voltage and current under each fault condition,and the line mode fault voltage and current measured at the protection device are quantitatively analyzed in detail.The test results show that the proposed protection scheme can fast and accurately identify faults,and the algorithm complexity is low,which is beneficial to improve the speed of the protection scheme.