Study On The Inrush Current Of High-voltage Built-in Transformer And The Countermeasures For The Misoperation Of Zero-sequence Current Protection

With the increasing capacity of China's power grid and the reduction of system impedance,the short circuit current of the system has increased correspondingly,which has increased the risk of the safe operation of the power grid.High-impedance transformers are used in areas where the short-circuit current is large because of their ability to compensate for system impedance and limit short-circuit current.The high-voltage built-in transformer,which is one of the high-impedance methods,has built-in high-voltage windings,which has the advantages of economical design and low operation loss.Therefore,it is widely used by domestic counterparts.However,due to its special structure,this kind of transformer makes the inrush current problem more prominent.In recent years,there have been a number of incidents in which a high-voltage built-in transformer air-drop generates an inrush current and causes the zero-sequence current protection to malfunction.The emergence of new issues urgently requires new research idea and solution,and the author has carried out comprehensive exploration.Firstly,the mechanism and characteristics of zero sequence inrush are analyzed.Due to the difference of closing angle,remanence and iron core,the saturation degree of iron core is different.The zero sequence inrush current produced by three phase inrush synthesis is not 0,which is no longer biased toward the side of the time axis.The zero-sequence inrush current is mainly dominated by the second and third harmonics,and appear to be larger than the fundamental wave.On this basis,the inrush current characteristics of the high-voltage built-in transformer are studied emphatically.Starting from its special structure,simulations and analytical calculation methods are used to obtain that the high-voltage built-in transformer inrush current has a zero-sequence component with high content,large amplitude,and attenuation.Slowly,the impact of various factors on the zero-sequence inrush current was studied by building a simulation model in PSCAD/EMTDC.When the inrush current of the above characteristics flows through the zero-sequence network path,it may cause mis-operation of the configured zero-sequence current protection.Combined with the specific protection configuration,this paper briefly analyzes it.In order to effectively solve the above problems,this paper analyzes and summarizes the existing preventive measures,and determines the solution from the perspective of enhancing the ability to protect themselves.By using the second harmonic blocking in differential protection,the second harmonic of the zero-sequence current is introduced into the blocking criterion,and the zero-sequence current protection is modified accordingly so that it is not affected by the excitation inrush current.In this paper,the adaptability of this scheme is analyzed.It is found that it may be affected by CT saturation,which fails to identify inrush currents and CT saturation of asymmetrical ground faults.To solve this problem,a new zero-sequence current protection scheme is proposed based on the combination of the zero-sequence current second harmonic content and the rate of change.Finally,the effectiveness of the new scheme is verified by building a model in PSCAD/EMTDC.Because the inrush current may lead to misoperation of the zero-sequence current protection configured in the power grid,this paper proposes to establish a risk assessment system for the zero-sequence current protection of the network covering the system from the perspective of the risk of early warning.Based on the big data platform,by collecting operation data and historical data,we calculate the misoperation probability of zero sequence current protection by computer simulation system inversion.In the calculation of the protection malfunction probability,considering the parameter data produced by the influence inrush can not be completely obtained,different misaction probability models are specially designed for the different completeness of the parameters,and the corresponding risk assessment indexes are given.In addition,this dissertation also studies the data relevance for the parameters that affect the protection misoperation,introduces the Apriori algorithm used in the field of mobile communication,and mines the correlations of the parameters that affect the misoperation probability,and shows through specific examples.The above studies can provide predictive guidance for the operation of existing or newly-built substations.