Fault Location Algorithm For UHVDC Transmission Lines Considering Wave Speed Changes

With the development of power electronics technology,UHVDC transmission has been widely used due to its economic,stable,and long-distance high-power transmission of electricity in the case of the mismatch in the regional distribution of power supply and demand in China.In view of the large proportion of DC line faults in UHVDC transmission system faults,and DC lines are too long,it is very difficult to patrol the lines,which brings a lot of inconvenience to remove the line faults.So,a fast and accurate fault location algorithm is of great significance for quickly clearing faults,restoring power,and ensuring the safety and stability of the power grid operation.The fault location methods of UHVDC transmission lines mainly include traveling-wave-based algorithm,fault-analysis-based algorithm and natural-frequency-based algorithm.The traveling-wave-based algorithm has high ranging accuracy but weak resistance to transition resistance,and the fault-analysis-based algorithm has high reliability but low ranging accuracy.The natural-frequency-based algorithm has poor anti-interference ability,and there is a problem of terminal dead zone.Up to now,the traveling-wave-based algorithm is still widely used in actual engineering,and the fault-analysis-based algorithm and natural-frequency-based algorithm have not been actually applied.Based on the problems in the fault location methods,the work of this paper is arranged as follows:1.A simulation model of UHVDC transmission system has been built,the operating characteristics of the model under normal and fault conditions has been verified.Based on the field engineering parameters of Lugu UHVDC transmission system,this paper builds a simulation model of ±800kV UHVDC transmission system in PSCAD.It is verified whether the system's key parameters such as voltage,current and power meet the standard under normal operating conditions.It is verified whether the system's changes in voltage,current and trigger delay angle can reflect the actual control and regulation characteristics of the UHVDC transmission project.2.This paper simulates the fault location method that does not consider the change of wave speed,this method uses the fault location algorithm that combines the principle of traveling wave and fault analysis,and analyzes the factors that affect the fault location result.The fault location method that does not consider the change of wave speed first calculates the distributed current along the line according to the principle of fault analysis,then obtains the fault traveling wave propagation curve according to the distributed current,and then finds the intersection point of the fault traveling wave propagation curve of the rectifier and inverter side to obtain the fault distance.The simulation result that the fault location has a great influence on the ranging error shows that the choice of traveling wave velocity is an important factor affecting the ranging accuracy.3.As the influence of wave speed on the ranging accuracy,a new fault location algorithm for UHVDC transmission lines considering wave speed changes is proposed,and the algorithm is verified by simulation analysis.When the UHVDC transmission line fails,first of all,use the VMD to decompose the signal measured at the rectifier and inverter side,perform mathematical morphological filtering on the decomposed single-component signal,and then SWT is used to obtain the instantaneous frequency and arrival time of the first traveling wave reaching the measurement point;according to the relationship between frequency and wave speed,the propagation speed of the traveling wave at this frequency is obtained;then the traveling wave velocity and the time for the traveling wave to reach the measuring point are brought into the fault location formula to calculate the fault distance.Finally,the accuracy and reliability of the fault location algorithm are verified by simulation examples.This paper proposes a fault location algorithm for UHVDC transmission lines considering wave speed changes.It uses VMD and SWT to identify the instantaneous frequency of fault voltage traveling waves.The calculation efficiency is high,there is no end effect and modal aliasing phenomenon,and the engineering application is strong.Due to the mathematical morphological filtering of the component signals,the frequency recognition accuracy is high,and the fault location method considering wave speed changes improves the location accuracy.