Research On Commutation Failure And Identification Method Of Weak Receiving End DC Transmission System

With the rapid development of China’s direct current transmission technology,China’s power grid has formed a pattern of AC-DC hybrid.High-voltage direct current transmission has the advantages of high voltage level,long transmission distance and large transmission capacity,which solves the uneven distribution of load and energy in China.The problem is that Yongfu DC is the first intra-provincial DC transmission system in China.The inverter system(Guangxi side)AC system is slightly weaker than the Yunnan side.Therefore,when the inverter side AC system fails,it is easy to cause DC transmission.The commutation failure of the system fails.As a common fault of the DC transmission system,the commutation failure will cause a sudden change of DC current and DC voltage,which poses a serious threat to the safe and stable operation of the DC system.Therefore,the commutation is effectively recognized.Failure and taking the corresponding measures to suppress the failure of commutation have great significance for the DC transmission system.The main research contents of this paper are as follows:(1)The simulation model of Yongfu HVDC transmission system is built in PSCAD/EMTDC simulation software.The single-phase ground fault occurs on the inverter side AC system bus,two-phase short-circuit fault,two-phase short-circuit ground fault,three-phase connection The ground fault and the change of the electrical quantity when the DC transmission line fails,the simulation results show that the DC transmission system can respond to the fault state reliably and reasonably under various faults.(2)theoretically analyzed the process of commutation failure,and the factors that cause the commutation failure,summed up the method of suppressing the commutation failure.Through a large number of simulations,the single-phase grounding of the inverter-side AC system is generated,and the two-phase short circuit When the threephase ground fault occurs,the critical resistance of the commutation failure and the continuous commutation failure is caused,and the critical ratio of the commutation failure when the converter transformer ratio is increased is obtained.(3)The fault diagnosis method of commutation failure based on empirical mode decomposition(EMD)and Elman neural network is studied.The EMD decomposition method is used to decompose the DC current signal into a relatively stable eigen from high frequency to low frequency.Modal function components and residuals,using approximate entropy and sample entropy to calculate component complexity and feature reconstruction,using it as feature vector,using neural network for commutation failure fault identification,the simulation results show that the fault recognition rate is used by this method.Not good.(4)A method of fault recognition for commutation failure based on fast ensemble empirical mode decomposition(FEEMD)and Elman neural network is proposed.The DC current signal is decomposed into several IMF components and one by FEEMD method.The quantity R is used to calculate the approximate entropy and sample entropy of the IMF component.The Elman neural network is used for fault identification.The simulation shows that the method of combining sample entropy with Elman neural network can judge the commutation failure of the DC transmission system.Status and line failure.