Research On Control Method And Protection Adaptability Of Low Frequency AC Transmission System

Low frequency AC transmission has the advantages of both high voltage AC and high voltage DC power transmission,which has become a very competitive large-capacity long-distance power transmission technology.modular multilevel matrix converterhas come into the view of experts and scholars because of its advantages of directly realizing AC-AC transformation,output arbitrary power factor and small harmonic content,and has become a frequency conversion device of low frequency transmission system.However,the current research on low-frequency transmission mainly focuses on economic evaluation and steady-state control,and rarely mentions the problem of fault crossing and protection adaptability.Therefore,the asymmetric fault crossing control and traditional protection adaptability of low-frequency transmission systems are studied in this paper.In this paper,the system topology diagram of M3C is introduced,and the internal electrical volume and working mode of M3C are described in detail.The active power expression of the bridge arm is calculated and different frequency components are analyzed in detail to show the frequency quantity of each part of the flow in the M3C bridge arm.At the same time,the relationship between the power of the bridge arm and the capacitor voltage is analyzed,the generation mechanism and influencing factors of M3C capacitor voltage ripple are introduced,and the expression of ripple is derived.Aiming at the control part of low frequency transmission system,this paper combines the M3C topology and obtains the 8-order mathematical model after M3C decoupling by double αβ0 transformation.The voltage fluctuations between subconverters and within subconverters are decoupled by double αβ0 conversion of the capacitor voltage of the bridge arm.Independent control is carried out based on the decoupled parts to realize the normal operation of M3C under the steady-state condition.The problems of asymmetric faults in low-frequency transmission systems and the reasons why steady-state control can not solve them are described.An asymmetric fault crossing control strategy for M3C based on active power DC component balancing is proposed.The expressions of fault current and fault voltage of M3C were analyzed,and the active power of each bridge arm during the asymmetric fault was calculated.The DC component of the power was extracted and converted into the form of fluctuation quantity by double αβ0 transformation.This part of fluctuation is suppressed to zero and the negative sequence current that needs to be injected into the system is determined.On the basis of M3C steady-state control,negative sequence current control is added to realize asymmetric fault crossing.The effectiveness and feasibility of the proposed control method are verified by simulation.Aiming at the problem of protection adaptability of low frequency transmission system,this paper analyzes the reason why traditional phasor differential protection is not applicable in low frequency transmission system.Based on waveform similarity theory,a fast protection principle suitable for low frequency lines is proposed by Jacard distance.The protection criterion and protection setting principle are introduced in detail.The feasibility and effectiveness of the proposed protection principle are verified by simulation.