Phase Differential Protection Of Fault Component Of Positive Sequence Voltage For Half-Wavelength AC Transmission Line

Worldwide energy transmission planning needs to develop ultra-long-distance and large capacity transmission mode.Ultra-high voltage(UHV)half wavelength AC transmission provides an alternative technical way with significant technical and economic advantages.However,the advantage of half-wavelength AC transmission technology is based on safe operation,so it is important to study the relevant protection principles.In this paper,the advantages and demands of half-wavelength AC transmission technology are introduced.Combined with the research status of UHV transmission line protection and half-wavelength AC transmission technology,further related research is carried out.The uniform transmission line equation of transmission line is deduced by using the distributed parameter model.The voltage characteristics and influencing factors of lossless half-wavelength line and lossy half-wavelength line are analyzed.The symmetrical component method is used to analyze various types of faults,and the mathematical expressions of voltage and current at two sides of the line are obtained.The extraction method of fault component of positive sequence voltage is studied,and a new phase differential protection principle based on the fault component of positive sequence voltage of two sides of the line is proposed,which can be used as backup protection for half wavelength AC transmission line.The simulation results show that the principle can correctly distinguish internal fault and external fault,and has clear selectivity.A new principle of fault location for half wavelength AC transmission line based on the phase difference of fault component of positive sequence voltage of two sides of the line is also proposed.A practical method of estimating the equivalent system impedance on-line is studied.Six equations are constructed by using three sets of voltage and current measurements at any three time before the fault in this method.The equivalent system impedance can be estimated in real time with self-adaptability.