Research On DC Line Protection Method Of Multi-terminal Flexible HVDC System In Power Grid

Multi-terminal flexible high voltage direct current(MTDC)system has many advantages such as flexible power flow control,high reliability of power supply,and few investment for converter stations.As an effective way to achieve grid connection and transmission of largescale clean energy,it is the main development direction of flexible HVDC technology.However,due to its low impedance characteristics,when a DC line fault occurs,the fault current rises quickly to several times as the rated current within a few milliseconds,causing the converter station blocked and shut down.Moreover,multiple converter stations inject fault current into the fault point.All these bring great challenges to the analysis of the fault characteristics and the relay protection.This paper focuses on the theoretical research and simulation analysis of the fault current characteristics,fault voltage traveling wave(TW)characteristics and DC line protection methods of MTDC system.The main research work of this paper is as follows:A calculation method of fault current for DC line faults in MTDC system is studied.The fault equivalent circuit of MTDC system including multiple converter stations is established,the differential equations required for the calculation of the fault current are determined and the fault current is calculated.A simulation model of true bipolar four-terminal flexible HVDC system is built in PSCAD/EMTDC.The effectiveness of the fault current calculation method are verified by the simulation results,and the influence of fault resistance and system parameters on the fault current is analyzed.This research provides theoretical basis for threshold setting of DC line fault detection of MTDC system.The frequency-domain expressions of the first and the second traveling wave of the fault voltage TW are derived.Based on the analysis of the transient process during DC line fault in MTDC system,for typical faults,according to the fault location,the frequency-domain expression of the first and the second fault voltage TW is derived by the Bergeron model of the DC line with TW dispersion considered.Its effectiveness is verified by the PSCAD/EMTDC simulation results.For the great requirements of the speed,selectivity and reliability of relay protection for MTDC system,a DC line single-ended protection method with anti-lightning criterion is proposed.Based on the research in Chapter 2,the protection start criterion is built by the rate of change of fault current.According to the research in Chapter 3,wavelet singular value entropy is used as the fault characteristic,and the fault criterion is built to identify fault area.For the problem that TW protection is susceptible to disturbance from lightning signals,auxiliary criterion are built by the ratio of the first and second TW wavelet modulus maximum to prevent protection from malfunctioning.Many simulation results and comparative analysis with existing schemes show that this method can identify fault area and type quickly and correctly,without the influence of lightning disturbance,which has high reliability and meet the requirements of relay protection for MTDC systems.