Research On Analyses And Countermeasures Of Power Quality Problems In Cabled Distribution System

At present,transmission and distribution lines include overhead lines and power cables.Because overhead lines are easily affected by climate,environment and other conditions,it brings certain hidden dangers to the safe and stable operation of the distribution system and affects people’s life and industrial production.Because cables do not occupy the ground space,they are conducive to the overall beauty of the city appearance and high reliability of power supply,which makes the distribution system cabled at the same time,the parameter characteristics of cable lines will bring power quality problems to the distribution system,and even cause equipment damage.Therefore,it is of great significance to analyze the mechanism of power quality problems in cabled distribution system and study the countermeasures.Firstly,the structure and basic parameter calculation of single core and three core cables commonly used in 35 k V system are introduced,and the equivalent models of centralized and distributed parameters of cables are given.The parameters of 35 k V cables and overhead lines with the same cross-sectional area are investigated.The results show that the inductive reactance parameters of cables with the same voltage level and cross-sectional area are smaller than those of overhead lines,while the resistance parameters are similar,but the cable capacitance is smaller The parameters are much larger than those of overhead lines.Secondly,according to the high-frequency resonance engineering case of cabled distribution system,starting from the measured data,the harmonic emission characteristics of distribution system are analyzed,and the harmonic voltage and current are evaluated.It is found that the high-frequency harmonic voltage exceeds the standard for many times.The impedance model of distribution system is established,the resonance mechanism is analyzed,and the correctness of the mechanism derivation is verified by the measured data.According to the case of high-voltage fuse fusing of electromagnetic voltage transformer(PT)caused by single-phase grounding fault,the fault recording data on site are analyzed,and the over-current phenomenon on PT side is found,and the generation mechanism is analyzed.Then,the high frequency resonance simulation model of the distribution system is built.The 24 pulse rectifier with asymmetric parameters is used to simulate the load harmonic source.The cable adopts the lumped parameter π type equivalent model.The simulation results are consistent with the measured data.The effects of system short-circuit capacity,cable load state and cable length on the high frequency resonance are studied.According to the simulation model of low-frequency nonlinear oscillation,the correctness of the mechanism analysis of PT primary side over-current is verified.When the cable capacitance to ground increases,the current flowing through the primary side of PT increases and decays faster.Finally,a frequency shift method based on impedance reconstruction is proposed to solve the problem of high frequency resonance in cabled distribution system.The impedance reconstruction device is incorporated into the power distribution system to change the inherent impedance of the system and make the resonance frequency shift.From the root locus,it can be seen that the stability of the system is increased.The design steps of the impedance reconstruction device are introduced,and the influence of the parameters of the impedance reconstruction device on the effect of high frequency resonance control is simulated.Then,the neutral point of PT primary side is grounded through line resistance to control the high voltage fuse fusing phenomenon caused by low-frequency nonlinear oscillation.Simulation and practical application cases verify the effectiveness of the method.At the end of the paper,a measurement method and mechanism analysis of ground capacitance of distribution system based on signal injection method are introduced,which provides a new idea for power quality control of cabled distribution system.Based on the background of practical engineering cases,this paper reveals the power quality problems of cabled distribution system,and studies the corresponding countermeasures,which is of great significance to the safe and stable operation of distribution system.