Study On Active Injection Based Fault Identification And Location Method For PV DC Integration System

The large-scale photovoltaic plants integrating with the flexible direct current(DC)system can avoid the frequency and power angle stability problems caused by AC integration,and the the cables and reactive power facilities are saved.The economy of the system is improved.and the PV DC integration system has draw great attention.The DC/DC and DC/AC converters in PV DC integration system have the ability of fast fault current clearing.Short-circuit fault will lead to high speed action of converter's self-protection and then cause the power loss of the whole system.Therefore.it is very important to study the reliable fault identification and location methods to help avoid the long-term power loss and improve the reliability of the system.The effective way to avoid the long-term outage of the system is to identify and isolate the incipient fault,reducing the probability of short-circuit.Once the short-circuit is inevitable,the fault should be quickly located and isolated after blocking the converters to realize fast restoration of health areas.Because the weak characteristics of incipient and short circuit faults are diffcult to be extracted,the reliability of fault identifying and locating is challenged.In order to improve the reliability of location and make full use of the controllability of power electronic devices,an active injection based fault identification and location method is proposed.The specific research work and innovation results are as follows:(1)An incipient fault location method based on disturbance injection is proposed The incipient fault characteristics is revealed,and the disturbance injection control method based on a full and half-bridges hybrid modular multilevel converter(MMC)is proposed,and then the transfer resistance index based disturbance control fault identification and location method is proposed.In order to avoid the impact on the system,the disturbance injection criterion is established by using empirical mode decomposition and fuzzy logic membership function,to reduce interference to system operation.This method provides accurate identification and location method of incipient faults under weak fault characteristics,reducing the risk of serious faults which cause long-term power outage in the system.(2)An adaptive short-circuit fault identification method based on fuzzy logic is proposed.In order to solve the effect of noise and uncertainty of photovoltaic output on short-circuit fault identification.a short-circuit fault identification criterion based on fuzzy logic adaptive tracking photovoltaic output is proposed.By establishing the criterion of fuzzy membership function,the problem of conflicting system sensitivity requirements under different working conditions is solved,and the transient value in the extremely short data window length can be used to accurately identify the fault.(3)A fault location method based on DC/DC inverter fault injection control is proposed.Based on the dynamic characteristics of photovoltaic under faults condition,an active injection control method of DC/DC converter is proposed.On the basis of the designed injection signal,the corresponding fault section location method based on direction recognition is proposed.The method has reliable performance with transient resistance,strong noise,high reliability,and solves the problem of reliability degradation of transient value location method.(4)An incipient fault location method based on MMC fault injection control and shot circuit fault location method based on low-frequency signal are proposed.On the basis of analysis of converters transfer function,the injectionc ontrol method under fault condition of low-frequency signal based on a hybrid of full and half-bridges submodular MMC is proposed,and the frequency of the injected signal are designed.Based on the designed injection signal,the corresponding short-circuit fault location methods are proposed.The problem of reliable location under weak characteristic without additional equipment is solved.The method can be used for a full period of time without being affected by photovoltaics,and has higher reliability than the transient value localization method.