Research On Commutation Failure Of AC/DC Hybrid Grid And Its Impact On AC Protections

Commutation failure is one of the most typical faults of HVDC transmission systems,and is mostly caused by faults on the inverter side AC system.As the coupling degree of AC-DC hybrid system in China continues to deepen and the scale and capacity of DC grids continue to increase,simple AC faults can easily induce one or more commutation failures.Continuous commutation failure will result in DC blocking,causing huge power flow transferring.It may cause cascading trips and develop into a serious systemic failure.In this regard,the key link in the cascading failure of AC-DC hybrid system--the boundary of commutation failures needs to be clear.Because the clear boundary can help us to realize the early warning of cascading failures,and on this basis,by studying the process of commutation failures,in AC-DC hybrid grid,the new fault characteristics and the relationships between control informations need to be analyzed,and the influences of traditional AC protections in the new scenario need to be analyzed to optimize the performance of protections.By using the new transient characteristics new protection principles for the AC hybrid system need to be proposed to shorten the fault removal time.It is of great significance to suppress the continuous continuous commutation failure and prevent the protection from being mis-moved or refused.It effectively blocks the cascading failure of the AC-DC hybrid grid and ensures the safe and stable operation of the large-scale power grid.This paper firstly comprehensively combs the existing research results on the factors that affecting commutation failure,detection methods,prevention and resistance methods,and the impacts of commutation failure on AC and DC systems.Secondly,the mechanism of commutation failure under different fault scenarios and system parameters is studied.The mechanism of the electrical parameters and control parameters of the system on the commutation process is clarified,and the interaction relationship between the factors is analyzed.A commutation failure boundary detection method based on deep recurrent neural network is proposed.By taking advantage of DRNN(Deep Recurrent Neural Network),a new method of fast commutation failure diagnosis is introduced.By using the converter bus voltages,DC current and the trigger angle commands of the inverter,this method can consider multiple factors effecting the commutation process,can trace the cause of commutation failure accurately and has certain predictive function.Digital simulation experiments show that the mathematical model constructed in this paper can solve the problem of commutation failure boundary detection.Thirdly,the simulation model is built by PSACD,and various fault feature extraction methods such as improved Fourier algorithm are used to analyze 50Hz fault current and the fault impedance measurement characteristics of the fault line fed to the grid side when the commutation fails.And its impact on traditional communication protection.Finally,the mechanism of aperiodic component current generation during the commutation failure of six-pulse inverter is deduced,and a aperiodic component-based differential is proposed by using the characteristics of fault and non-fault phase aperiodic component current during commutation failure.The new principle is protected and the feasibility of the protection principle is verified by PSACD simulation results.