Research On Adaptive Reclosing Of AC Line In AC/DC Hybrid System

With the rapid development of DC transmission technology and new energy power generation technology,China has formed an AC/DC hybrid power grid with high proportion of power electronic equipment.One of the most important and serious faults in the LCC-HVDC AC/DC hybrid power grid is the commutation failure fault on the inverter side,and the commutation failure on the inverter side will occur when the AC line circuit breaker on the inverter side removes the fault or coincides with a permanent fault.The grid-connected tie line of photovoltaic power station often adopts three-phase reclosing,and its fault nature discrimination method needs to be further studied.Therefore,this paper studies the adaptive reclosing of AC lines in AC/DC hybrid system.The main research work includes:(1)Aiming at the problem that the single-phase tripping of the AC line will cause the commutation failure of the inverter side,a method for selecting the reclosing mode of the AC transmission line on the inverter side is proposed.The influence of the phase shift of the commutation voltage on the turn-off angle after the single-phase tripping of the AC system is analyzed.The equivalent impedance of DC system and AC system is calculated by using the current and voltage information on both sides of the line after single-phase grounding fault,and the phase forward angle of commutation voltage is calculated accordingly.By comparing the phase forward angle of commutation voltage with the critical angle,the judgment of commutation failure is realized.If commutation failure occurs,the three-phase reclosing mode is adopted,otherwise the single-phase reclosing mode is adopted.This method can avoid the blocking of DC transmission system and shorten the normal operation time of AC/DC hybrid system.(2)Due to the influence of the DC transmission control system,the line voltage and current contain non-integer harmonic components after the single-phase ground fault and the fault phase circuit breaker tripping of the AC line on the inverter side of the AC/DC hybrid power grid.Based on this,a single-phase fault property discrimination method suitable for the AC line on the inverter side is proposed.The electrical characteristics of the inverter side AC line fault single-phase circuit breaker after tripping are analyzed and discussed.The applicability of the commonly used fault property discrimination methods in the inverter side AC line is analyzed and studied.The results show that the commonly used methods may misjudge.A least squares fitting current model including the non-integer harmonic components generated by the DC transmission control system is established.The model is solved to obtain the power frequency component and the low-frequency free oscillation component current,and the single-phase fault property is distinguished by comparing the amplitude of the two.(3)The three-phase reclosing is usually used in the grid-connected tie line of photovoltaic power station.After the trip of the three-phase circuit breaker,the energy storage on the line decays rapidly,and it is difficult to accurately identify the fault nature.An active injection three-phase adaptive reclosing method for photovoltaic grid-connected tie-line is proposed.Cables and switches are added to the centralized grid-connected photovoltaic power station to connect from a photovoltaic power generation unit to the station side of the tie line.After the photovoltaic power station is out of operation,the three-phase fundamental frequency voltage signal is injected into the tie line by the photovoltaic power supply.Based on the short-circuit current energy on the tie line and the amplitude of the injected signal voltage,the fault nature is identified.PSCAD/EMTDC simulation verifies the correctness of the theoretical analysis and the effectiveness of the proposed reclosing method.