Study On A New Current Protection Strategy With Sufficient Criterion For Distribution Networks With Penetration Of DGs

In recent years,with the development of the renewable energy sources technology,more and more distributed generators(DG)have been integrated into distribution network.Therefore,distribution network has become a multi-sources network in which the traditional three-sect current protection can not be used.Besides,distributed generators have characteristics of diversity and randomness,getable datas are limited because of insufficient current intelligent level of distribution network,so there are many difficulties when researching new protection strategy of distribution network with DGs.In this situation,this thesis proposes a sufficient criterion based on the maximal current differences between adjacent transmission lines under the guidance of the sufficient theory.On this basis,this thesis prososes a sufficient current protection strategy to solve the incorrect operation problems of the three-sect current protection.Firstly,the sufficient criterion and its advantages on current protection application are discussed in details.Combined with DC automatic isolation technology,the cooperation of several sufficient criteria can improve the sensitivity and quickness of fault criterion.Thus,the complete protection strategies can be constructed.In this thesis,the short-circuit current characters of different types of DGs are analyzed,and its influence on the selectivity and sensitivity of three-sect current protection is also presented.The application of sufficient criterion on current protection is carefully demonstrated.To overcome the false operation and incoordination of three-sect current protection in distribution network with DGs,the active and reactive power variations of the DGs with inverters under different control strategies in short circuit states are analyzed.With the short-circuit currents magnitudes of adjacent transmission lines,a maximal current magnitude difference sufficient criterion is proposed.With the criteria,the suppress influence of DG on the current of downstream transmission line current can be processed.The proposed criteria has low requirement for communication which is suitable to distribution network with DGs.On the basis of the sufficient criterion based on the maximal current differences between adjacent transmission lines and the action information of the three-sect current protection,the sufficient current protection strategy against misoperation and the sufficient current quick break protection strategy are proposed to prevent false operation and failure to actuate.The strategies fully use the action information of the current protection before they actuate,which breaks through the traditional process of current protection innovatively and practicably.Besides,this thesis also proposes feeder partition strategy in order to configure protection strategy selectively.At last,this thesis builds models of distribution network with DGs in Matlab/Simulink platform to simulate and verify the effectivenss of the protection strategy.