Study On Fast Fault Detection Method For MMC-HVDC Lines

The MMC-HVDC technology has gained increasingly interests in recent years.The DC line protection is one of key technical problems to be solved in the operation of MMC-HVDC.Compared with the traditional DC systems,the fault current of MMC-HVDC rises fast and the peak is large,meanwhile the fault transient process is complex.In order to quickly remove the fault and restore the normal operation of non-fault lines,fast fault detection methods are needed.Thus the study of fast fault detection methods is of great importance to the operation of MMC-HVDC.In this dissertation,the fast fault detection methods for DC lines are proposed.According to the transient characteristics of different fault types and the corresponding fault detection requirements,the corresponding solutions are proposed.The main research work and novel achievements are as follows:Aiming at the fast fault detection for monopolar grounding fault in MMC-HVDC,the band characteristic difference between the fault signal and the interference signal is revealed,and a novel fault detection approach using wavelet time entropy is proposed.The proposed method solves the problem that the detection method based on the change rate of voltage and current is susceptible to noise and other interference signals.Furthermore,the identification of the fault line is needed for MMC-MTDC system.It is demonstrated that the transient difference current can be used to distinguish the fault line.Then a short-term energy based approach for monopolar grounding line identification in MMC-MTDC system is proposed to reduce the influence of the transient process on the recognition result.The proposed approach can accurately identify the fault line while the fault current is very small and the transient process is complex.The proposed method solves the problem that it is difficult to distinguish the fault line by the change rate of voltage and current.A single terminal fault location method for MMC-HVDC transmission line using active pulse is proposed.Active pulse generation principle based on submodule control is proposed by analyzing the characteristic of MMC submodule.And then a single-terminal fault location method using active pulse is propose d.Dichotomizing recursion SVD is used to detect the arrival time of active pulse,by which the detection results on different decomposing layers do not shift,and the accurate measurement of the arrival time of the wave head can be realized.The proposed active pulse method does not require additional equipment and does not influenced by transition resistance.Fault location can be repeated since the pulse can be sent repeatedly,so the reliability and accuracy of fault location is improved.A bipolar fault detection method based on temporal characteristics for MMC-MTDC is proposed.By analyzing the bipolar fault traveling wave characteristic and the inverter characteristic,a bipolar fault detection method based on the difference of the ratio of increasing time to reducing time is used to distinguish the fault.The proposed method is not affected by fault resistance,can overcome the influence of high frequency and high amplitude noise.The proposed method can detect the fault within 1ms,so the fault current in the rising phase can be cut off and the risk of MMC-MTDC system outage can be reduced.A fast bipolar permanent fault identification approach for MMC-MTDC lines is proposed.By analyzing the characteristics of DC line voltage under different fault conditions(permanent fault,instantaneous fault and protection mal-operation),it is found that the difference between positive pole voltage and negative pole voltage of the line with permanent faults is zero,while that of the line with instantaneous fau lts is not zero.According to this feature the permanent fault can be identified.Furthermore the robust locally weighted regression approach is used to realize the fast permanent fault identification in transient process.The proposed approach can reduce the impact cause by instantaneous fault and protection mal-operation and enhance the stability and power supply reliability of MMC-MTDC system.