Research On Mechanism And Control Strategy Of Continuous Commutation Failures In HVDC System

With the commissioning of a large number of HVDC projects,China has formed a cross-regional,long-distance and large-scale AC-DC hybrid grid operation pattern.Inverter-side AC system faults can easily cause HVDC commutation failure.Serious continuous commutation failure may even cause the HVDC system to be blocked,interrupting power transmission,and causing a huge impact on the safe operation of the power grid.The first commutation failure occurred extremely quickly,which is very difficult to suppress.The development of continuous commutation failure takes a certain amount of time.If the continuous commutation failure is successfully avoided,it can effectively prevent the propagation and development of AC faults and reduce the probability of subsequent chain accidents.Therefore,the paper studies the influencing factors,occurrence mechanism and suppression methods of continuous commutation failure of HVDC system,which is of great significance for the development of HVDC technology and the safe operation of the power grid.Firstly,the paper deduces the method of judging commutation failure based on the turn-off angle,and summarizes the variables that directly affect the value of the turn-off angle.The grey correlation analysis method is used to quantitatively analyze the correlation between different variables and the change of the turn-off angle,and the results show that the leading firing angle is a variable that plays a decisive role in the value of the turn-off angle,which should be the focus of research on the mechanism of continuous commutation failure.Then,by analyzing the input and output characteristics of the pole control level,the paper concludes that the leading firing angle command has the same harmonic characteristics as the pole control input,and contains a large number of 2nd harmonic components during the AC fault.Besides,the commutation voltage is calculated using symmetrical component method during different types of faults in the AC system,and the results show that a fluctuation component with a period of 6 commutations is introduced to the leading firing angle,owing to equidistant firing control system.Under the common regulation of the pole control level and the valve control level,the leading firing angle has a periodic fluctuation characteristic,which is easy to cause continuous commutation failure in the HVDC recovery process.The paper further analyzes the operation principle of the commutation failure prevention module,and points out that it has poor coordination with PI controller at the pole control level and weak suppression effect on commutation failure when the two-phase fault occurs.To this end,the paper adds negative sequence component detection to the module and redesigns the feedforward link.The simulation results show that the improved commutation failure prevention module has better suppression effect on continuous commutation failure.Finally,the paper proposes to reduce the amplitude of the leading firing angle fluctuation in order to suppress the continuous commutation failure,and designs a leading firing angle inner loop controller based on fuzzy control theory.The simulation results show that the proposed controller can significantly improve the tracking accuracy of the leading firing angle and reduce the amplitude of the periodic fluctuation during the fault,which can effectively improve the ability of HVDC to resist continuous commutation failure.