Research On The Influence Of Commutation Failure Of AC-DC Hybrid Grid On Relay Protection

China has the world’s highest voltage level and largest UHV AC-DC hybrid grid.The large-scale and large-capacity DC transmission connecting to the grid has led to more complex and changeable fault characteristics of the new power system.In particular.the existence of DC commutation failure greatly affects the performance of traditional power frequency-based relay protection and incorrect action of the protection may lead to systemic accidents such as DC blocking,large-scale power transfer.and chain tripping,seriously endangering the safe and stable operation of power grid.Therefore,it is particularly necessary to analyse the fault characteristics of the hybrid grid with DC feed-in,clarify the influence of commutation failure on relay protection,deeply dig the correlation of multi fault information,and finally propose new principles of relay protection with stronger adaptability,so as to avoid system cascading failures caused by a single fault.This article has achieved the following results.(1)A time-domain segmented fault transient analysis method is proposed,which takes into account the DC control strategy and the change of the converter topology.This paper deeply analyzes the working mode of the DC control system,clarified the switching mode of the control strategy when commutation failure occurs.On the basis of considering the DC control strategy and the topological structure of the converter.the fault transient process of the converter is analyzed in different stages.This paper further analyzes the expressions of the voltage and current of the converter during the commutation failure and clarifies the short-circuit current contributed by the DC line during AC fault,laying a theoretical foundation for the study of fault transient of AC/DC hybrid power grid.(2)An optimization scheme of AC differential protection based on the auxiliary criterion of current amplitude is proposed.This paper analyzes the DC short-circuit current characteristics feeding into AC system under different fault scenarios through a large number of simulations.Starting from the principle of differential protection,the impact mechanism of DC commutation failure on differential protection is revealed,and the refusal of differential protection is clarified.The root cause is that the DC feed causes the current phase difference on both sides of the AC line to change.This paper further proposes a differential protection optimization scheme to eliminate the influence of the DC component.At the same time,the difference of the current amplitude on both sides of the line is used as an auxiliary criterion,which effectively solves the problem of refusal in internal fault.(3)A transient protection scheme based on the spectral variation characteristics of fault currents on both sides of the line is proposed.This paper analyzes the traveling wave propagation process of high-voltage transmission lines and the spectral characteristics of fault current,and clarifies the fluctuation characteristics of the ratio of spectral currents on both sides of the line during faults.The characteristics of the spectral fluctuation variance of the ratio of fault currents on both sides are significantly different,and a fault identification scheme based on double-terminal transients is constructed.The proposed scheme can realize rapid identification and removal of faults,and at the same time has a strong ability to withstand transition resistance(4)A circuit breaker failure/dead zone protection scheme based on station domain information is proposed.Starting from the existing failure protection principle,this paper proposes a protection improvement scheme based on the optimal differential calculation method of trailing current.From the perspective of wide-area protection,the construction mode of station-area centralized-inter-station distributed wide-area protection is constructed,the fusion and interaction mechanism of station-area and inter-station information is clarified.By collecting station domain information and distributing interactive information between stations realizes quick action of circuit breaker failure and dead zone protection,and solves the problem of long delay in stepby-step cooperative protection action.