Research On HVDC Control System And Characteristics Of Commutation Failure Under AC System Fault

With the development of High Voltage Direct Current(HVDC)technology,more and more HVDC system connected into alternating current(AC)power grid,the transmission power are dramatically increased as well.Because the converter is made from thyristor,its half controlled character leads commutation failure problem in HVDC system.Therefore the mechanism of commutation failure in HVDC system should be deeply analyzed.Many experts and scholars have been studying this subject for many years.There are three major methods to predict commutation failure,which is minimum extinction angle method,the minimum voltage drop method,and the minimum commutation area identification method.But the research also indicates that the commutation failure of the DC system often influenced by AC fault time.The same fault impedance may have different result with different fault time.Many researches indicate that the phase angle and the amplitude of AC voltage and the control system influence the commutation process a lot.But the tradition method often ignores these problems which makes error in judgment.In my research,the type of transformer connection group is deeply considered,which also influence commutation process.In order to thoroughly discusses and analyses the cause of commutation failure.Main work and study of this article are as followsThe first: the mechanism of commutation failure is analyzed in this paper,which considers the influence of different groups of converter transformer.Then an improved criterion of commutation failure by analyzing the actual trigger process of each valve is proposed in this paper.This theory is reference for the further study.The Second: the problem of commutation failure are sensitive to the fault time,different time have infect the critical impedance a lot.Based on commutation area,this problem is deeply analyzed in this paper.The Third: the adjustment of DC control is considered.The main control can be dividing into two types of technology,constant extinction angle control and constant voltage control.Both type of control stagey are widely used in DC project,and both of them have good ability to resist commutation failure.The progress are of DC control are analyzed,and different control strategy are considered in this paper.