Research On Fault Location Of Pole-to-Ground Fault In Medium Voltage DC Distribution Line

The large-scale application of new energy is inseparable from the development of smart grid.As a part of smart grid and the main medium of high proportion of new energy access to power system in the future,the safe and stable operation of DC distribution network is becoming more and more important.In actual operation,various faults will inevitably occur in the DC distribution network,and the probability of poleto-ground fault is the largest.When a pole-to-ground fault occurs in one of DC lines,the voltage of the non-fault pole line rises sharply.If not handled in time,the long-term overvoltage will break through the insulation of the line,and the pole-to-ground fault will evolve into an inter-pole short-circuit fault,resulting in the unstable operation or even collapse of the DC distribution network.In addition,the DC distribution lines are mostly underground cables and the DC distribution network topology is flexible.The fault location method needs high accuracy and good adaptability.However,the existing technology of fault location is difficult to meet these requirements.Therefore,it is particularly important to study a more universal and more accurate fault location method of pole-to-ground fault for DC distribution lines,to quickly and effectively isolate the fault section and accurately locate the fault point.(1)The difference of fault characteristics when pole-to-ground fault occurs in DC lines under different grounding modes is revealed.Different from the AC distribution network,the topology of the DC distribution network is flexible,and the grounding modes of the system under different converters are different.Therefore,when the poleto-ground fault occurs in the system,the fault characteristics are also quite different.In this thesis,the fault characteristics of pole-to-ground fault under all effective grounding modes of VSC and MMC converters are analyzed in detail from the aspects of fault current flow path,composition,duration and voltage change in DC distribution system under symmetrical single-pole connection mode,which provides a theoretical basis for the subsequent establishment of zero-mode equivalent model of converter.(2)A fault section location method based on steady-state zero-mode current direction is proposed.By establishing a unified zero-mode equivalent model of the converter under different grounding modes,the direction difference of the zero-mode current in the fault section and the healthy section in the zero-mode network of the single-phase grounding fault is fully excavated,and the section location criterion based on the steady-state component direction of the zero-mode current is proposed to realize the effective identification of the pole-to-ground fault section.The section location method proposed in this thesis can adapt to all effective small current grounding methods,and has low requirements for sampling frequency and data processing,and has strong resistance to transition resistance.(3)An accurate fault location method for pole-to-ground fault based on offline injection modules is proposed.On the basis of successfully isolating the fault line,the fault circuit is constructed by using the offline injection modules at both ends of the line.The parameters of the fault location module are adjusted to change the damping state of the fault circuit.The fault location equation is simplified by the inductance extremum characteristic.A fault location algorithm based on RLC second-order dynamic circuit is proposed.The off-line ranging method proposed in this thesis is not affected by the fault distance.It has strong resistance to transition resistance,no synchronization requirements for information on both sides of the line,and has good universality and positioning accuracy.Based on the electromagnetic transient simulation software PSCAD/EMTDC,an experimental model of pole-to-ground fault location for DC distribution lines is built.A large number of simulation experiments are carried out for different fault conditions.Through the analysis of the simulation results,the feasibility of the pole-to-ground fault section location and accurate ranging method proposed in this thesis are verified.The thesis contains 54 figures,9 tables,and 70 references.