Analysis Of Commutation Failure Mechanism And Suppression Measures For HVDC Transmission Lines

At present,China has entered the era of large power grid Internet.UHVDC transmission is to achieve ’West to East,North-South mutual economic,national networking’ strategy in the most important part.Commutation failure occurs continually in HVDC.It will cause the DC current to increase,reduce the voltage,reduce the DC transmission power,shorten the life of the converter valve,and convert the DC bias and inverter AC system over-voltage and other consequences,and even lead to DC blocking,so that the entire system outage.In this thesis,the commutation process commutation process has been detailed analysised,the commutation failure is the essence of the inverter side of the extinguishing angle is too small and can not make the thyristor carrier Sub-compound.The quantitative relationship between the arctanging angle and the physical quantities is analyzed,and several reasons for the commutation failure are summarized,some methods to prevent commutation failure are put forward.As the asymmetric failure caused by three-phase physical quantity changes in different,after the failure of the commutation station commutation phase commutation failure probability will be different.In this thesis,the variation of the commutation failure probability of each converter valve is analyzed by calculating the change of the three-phase physical quantity after each asymmetrical fault,and the change of the amplitude and phase of the commutation voltage is integrated by the change of the arc extinguishing angle y.The relationship between the transition resistance and the failure probability of each valve is verified.All the conclusions are verified by PSCAD simulation platform.Series capacitors can effectively inhibit the occurrence of commutation failure.In this thesis,by analyzing the different running states of the series capacitor system and ordinary converter,the vector capacitor is used to analyze the causes series capacitor suppress commutation failure.These reasons includ increasing the amplitude of the commutation voltage and changing the commutation voltage phase lag;the DC current increases rapidly after the fault,which leads to the increase of the capacitor voltage,which in turn will suppress the increase of the DC current,and the inhibition capacity is proportional to the increase of the current;the series capacitor can cancel part of the commutation reactance,when the commutation of electromagnetic transient process to speed up,for the overlapping arc angle μ smaller.Finally,the PSCAD simulation platform is used to verify that the series capacitor converter system can effectively reduce the critical transition resistance of each fault.In order to solve the commutation failure problem,this thesis proposes a commutation failure suppression scheme,which consists of two parts:the commutation failure prediction and the series capacitor suppression commutation failure.The difference between the maximum commutation area and the critical commutation area is used as a measure of whether commutation failure occurs.If the difference between the maximum commutation area and the critical commutation area falls below the set safety value,it is considered that commutation failure will occur.This prediction method is clear and simple,fast and reliable.Detection of frequency-intensive.The simulation results show that the prediction method has high success rate.The prediction results show that commutation failure will occur immediately into the series capacitor to inhibit the occurrence of commutation failure.The simulation results show that the series capacitor can effectively avoid the commutation failure and can effectively reduce the critical transition resistance under various faults.