| Under the pressure of increasing shortage of traditional energy and increasing environmental pollution,the application of distributed power generation technology with new energy has attracted unprecedented attention and development.However,it can’t be ignored that the probability of faults of collector lines in wind farms is higher.Occasionally,large-scale wind farm units are disconnected.The State Grid Corporation proposes to change the grounding mode of the main transformer of wind farm from non-grounding to grounding through small resistance,and allocate corresponding protection.However,when grounding fault occurs in small resistance grounded wind farm,its zero sequence protection often malfunctions and refuses to operate,which seriously endangers the safe operation of power grid.At present,the conventional zero sequence protection setting method of collector line is difficult to remove faults reliably and quickly,which may affect the safe operation of wind farm,damage equipment and even directly affect the stable operation of power system.It is necessary to deeply study its unique fault characteristics and propose a protection method that can adapt to the complex fault characteristics of wind farms.In this paper,the mathematical model of DFIG wind turbine is established,and the simulation model of DFIG wind farm is established.Considering the output characteristics of wind farm collector when grounding fault occurs,this paper establishes a DFIG wind farm system grounded by small resistance using PSCAD software platform,which lays a foundation for the later analysis of fault characteristics of wind farm collector line and the drawbacks of zero-sequence protection for grounding fault of conventional wind farm.According to the established wind farm model,this paper briefly introduces the current wind farm wiring forms and wind farm protection configuration,and analyses the advantages and disadvantages of various grounding modes.This paper adopts a more suitable small resistance grounding mode for wind farms,which is conducive to the rapid and accurate removal of faults in wind farms to avoid large-area wind turbine disconnection.In view of the frequent occurrence of zero sequence main protection rejection,misoperation and backup protection overstepping in existing wind farms,the main reasons for the poor quality of protection are analyzed.The setting value of conventional zero sequence current protection is a certain value,which can’t actively adapt to the change of fault transition resistance.It is difficult to ensure the reliability and sensitivity of protection because of the influence of transition resistance.At the same time,this paper proves that there is a strong correlation between zero-sequence current and zero-sequence voltage when single-phase grounding fault occurs in wind farm,which is proved by simulation.Then,an inverse time-limited collector line protection method which can adapt to complex fault conditions of wind farms is proposed.In view of the similarity between zerosequence voltage and zero-sequence current,zero-sequence voltage can be used to simulate zero-sequence current on any collector line.In this paper,zero-sequence voltage is used to simulate zero-sequence current,and the correlation between simulated zero-sequence current and measured zero-sequence current is used to discriminate the fault collector line.In this scheme,the data are decomposed by wavelet transform beforehand,and the characteristic frequency band is extracted according to the maximized energy.The interference can be eliminated effectively,the fault features can be amplified,and the communication requirements are not high.The proposed correlation is not affected by the fault transition resistance.It can effectively deal with the maloperation and rejection of zero sequence protection in small resistance grounded wind farms.Finally,summarizing this paper,according to the problems found in the research process and the development needs of wind farm protection,the future research work is prospected. |
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