Risk Assessment And Suppression Strategy Of HVDC Commutation Failure Research Based On Harmonics Analysis Under AC Fault

Commutation failure is a typical fault of HVDC.Multiple continuous commutation failure will even lead to DC blocking,which will seriously threaten the safe and stable operation of the power grid.When AC system faults,due to the asymmetry of commutation voltage,the saturation of converter transformer and the coupling effect of converter,make the various harmonics generated in the system are closely related to the process of commutation failure.However,most of the research on commutation failure is based on the analysis of the drop in voltage amplitude,which is still lack of considering the influence of harmonic.Therefore,it is of great significance to study the mechanism of commutation failure considering the influence of harmonics,propose an effective risk assessment method and suppression strategy for commutation failure,so as to improve the stability of AC-DC hybrid transmission system.Based on the method of commutation area,this thesis analyzes the influence mechanism of the distortion of commutation voltage waveform caused by harmonics on commutation failure.According to the analytical formula of harmonic commutation area,defines the harmonic commutation area coefficient to evaluate the influence of harmonics on the commutation area.Meanwhile,analyzes the generation mechanism of each non-characteristic harmonic in AC faults.In order to solve the problem that is only considering the amplitude drop of the commutation fundamental voltage as a single criterion is difficult to reflect the complex transient process,the commutation failure risk index is proposed,which considers the influence of fundamental voltage reduction and harmonic on commutation failure.Meanwhile,in order to further distinguish the influence of these two factors,the commutation failure main factor index is proposed.These two indexes are easy to obtain in practical engineering,and have better engineering practicability.On this basis,a risk assessment method of commutation failure caused by AC system fault considering harmonic effect is proposed.The simulation analysis is carried out for different AC systems faults with PSCAD/EMTDC shows that the voltage distortion caused by low-order non-characteristic is the main reason to increase the risk of commutation failure;and when the fault is strong,the drop of fundamental voltage is the main cause of commutation failure.In order to solve the effect of low order harmonics on commutation failure when AC system faults,this thesis proposes two suppression strategies to reduce the risk of commutation failure from the perspective of voltage distortion.They are the external strategy of adding a double tuned filter and the internal strategy of adding a harmonic voltage loop on the inverter side.The simulation analysis is carried out for different AC systems faults with PSCAD/EMTDC shows that the double tuned filter strategy can significantly reduce the 2nd and 3rd harmonics which have a great influence on the commutation process on the bus voltage when the parameters are properly configured and the impedance of the system changes little;The harmonic voltage loop strategy can dynamically adjust the DC current command value according to the harmonic content when the bus voltage distortion reaches a certain threshold,it can reduce the large fluctuation of DC current,the distortion of converter transformer magnetizing current and commutation voltage.This strategy can restrain the subsequent commutation failure of the system and improve the fault recovery characteristics,Moreover,this strategy is low in cost and easy to practice,which has certain reference and application value for practical engineering.