Influence On The Relay Protection And Phase-selector Of The Cross-voltage Short-circuit Fault In Mixed-Voltage Transmission Lines System

With the expansion of power grid scale,in order to solve the problem of land resource shortage,the transmission mode of multi-circuit lines on the same tower has been established.All the existing protection devices which rely on single-terminal electrical quantities to constitute operation criterion will be unpredictably impacted.For the mixed-voltage transmission lines system,which means setting up two or more transmission lines of different voltage levels on the same tower,the fault feature is more complex.To solve this problem,the paper targets on the distance protection and phase selector commonly used in medium and high voltage transmission systems,analyzing its influence in cross-voltage short-circuit fault.Main work is as follows:(1)For mixed-voltage four-loop transmission lines system on the same tower with weak electric connection,the paper has obtained the calculation method of its fault current by constructing the complicated sequence network connecting two voltage level systems.Then,the superposition principle is used to decompose the system into weak electric connection system and equivalent current source system.fault current of these two part is calculated respectively,and finally fault current of mixed-voltage four-loop transmission lines system on the same tower with strong electric connection is got by the two part of current superposed.(2)Based on the fault current calculation method,the expression of the measured impedance is obtained.In comparison with the set value or the actual fault impedance,the influence of cross-voltage factors on the measured impedance is studied.Also,relationship is discussed when measured impedance enters or exits the action area,or the adaptability of protection relays is affected.(3)By analyzing the magnitude and phase angle of fault sequence currents in the protection installation site,the adaptability or influence of the phase selector based on principle of the phase-to-phase current suddenly-change and principle of the zero-negative sequence current angle are studied.The results show that the phase selection results are related to the phase angle difference of the power between different voltage level systems.In the single-phase fault,the phase selector detecting suddenly-changing quantity and the phase selector detecting steady state quantity still meet theoretically correct phase selection conditions,but its adaptability will be affected.In the two-phase fault,these two phase selection principle will mistake the fault for single-phase grounding.Aiming at the case that the two-phase fault is mistakenly selected as single-phase fault,the relationship between the phase angle of the power between different voltage level systems and the system parameters is proposed.The theoretical analysis of the paper is verified by the simulation of the fault model of the four-circuit lines on the same tower in PSCAD.Through the influence analysis and relationship calculation,it is possible to correctly judge the action of distance protection and phase selection device.