Research On Three-phase Adaptive Reclosing Technology For Half-wavelength Transmission Line

Half-wavelength AC transmission technology holds great promise as a form of transmission that enables the transmission of power over extremely long distances and large capacities.But new challenges also arise,with more serious consequences in the event of a fault.The use of adaptive reclosing on EHVAC transmission lines not only avoids secondary shocks to the system when reclosing on permanent faults,but also effectively prevents the occurrence of reclosing overvoltages,which is of great significance and relies on the technical principle of line fault survivability.This paper investigates the problem of adaptive reclosing on half-wavelength transmission lines,with the aim of finding an adaptive reclosing solution that is suitable for the characteristics of this system.This paper first establishes its equivalent model based on uniform transmission line theory and half-wavelength transmission system parameters,analyzes and derives the mathematical expressions for primary and secondary arcs,and gives the steps to suppress the latent supply current using HSGS.Then,by analysing the overvoltage problem caused by a single-phase earth fault occurring on the line,it is derived that for half-wavelength transmission lines,if single-phase reclosing is used,the overvoltage caused by the process will exceed our operating standards,and the idea of using the scheme of tripping the threephase circuit breaker to isolate the fault is proposed.At the same time,from the time domain waveform analysis to get the residual voltage after the circuit breaker tripping offset characteristics: according to the theory of distributed parameter circuit,half-wavelength transmission line three phase tripping if the fault still exists,due to the transmission line and the earth between the short-circuit current path and the earth zero potential clamping physical role,the residual voltage waveform is bound to decay and oscillate around the zero potential axis;if the fault disappears,the line and the earth between the current path and earth zero potential clamping role no longer exists,the residual voltage waveform will be shifted to the side of the zero potential axis.Analysis of the residual voltage decay oscillation characteristics,the frequency difference value directly reflects the location of the source of transient voltage oscillation.If the line fault disappears,the residual voltage frequency difference value response line full length;if the line fault exists,the frequency difference value response line internal fault location.And from the perspective of dynamic components stored energy,simulation calculations to obtain the line after the circuit breaker tripped the stored energy value and its decay time length,indicating that it is sufficient to complete the fault survivability discrimination.Based on the theoretical analysis,a detailed method for identifying the nature of faults on half-wavelength transmission lines corresponding to time-domain and frequency-domain characteristics is given.Among them,the residual voltage time domain detection methods are: crossing zero axis detection,offset accumulation slope detection,envelope mean value detection,and short time window integration detection.In the frequency domain perspective,a method for identifying the nature of the fault using the line residual voltage frequency difference Δf is proposed,which is not affected by the system boundary conditions,while a double-ended criterion is given for the single-ended dead zone problem,increasing the reliability of the method.Finally,the reclosing time limit is adjusted taking into account the influencing factors such as arc extinction at the fault point and recovery of the dielectric insulation strength of the arc channel.Based on the method of residual voltage feature extraction in the time and frequency domains,a three-phase adaptive reclosing scheme for half-wavelength lines is presented,including its implementation steps and the results of a typical case test analysis of the influencing factors and applicability of the criteria,which verifies the correctness and reliability of the proposed method.