Study On Sufficient Differential Protection For Distribution Network With Distributed Generators

Distribution network protection is faced with great challenges because of the distributed generation(DG).The traditional three section current protection loses its selectivity because of big differences of DGs'type,control mode,capacity and location.Therefore,distribution network usually isolates all DGs when any fault has been detected,which greatly limits the development of DGs.The protection methods for distribution network with DGs should not only solve the selection problem caused by DG,but also break the limit of the poor communication condition and low equipment configuration.At present,no ideal solution has been proposed.A novel sufficient protection principle which realizes the balance between the protection performance and the protection cost is put forward in this thesis.Under the framework of "sufficient",a new differential protection method for distribution network with DGs is proposed.First of all,the definition,connotation and characteristics of sufficient protection principle are discussed.The main idea of sufficient protection is to expand the traditional protection,which means sufficient protection no longer needs to detect all the faults,instead,fully utilizes available information to maximize the ability of fault recognition.The faults which have not been detected will be removed by DG isolation.Sufficient protection ensures practicality and gains selectivity with the sacrifice of a small part of speed,which meets the need of protection methods for distribution network with DGs.The characteristics and the matching method of the criterions of sufficient protection are analyzed,which provides a theoretical basis for sufficient differential protection criterion.What's more,the basic principles,advantages,characteristics of sufficient differential protection are discussed and the effectiveness and feasibility of the protection at the present stage are demonstrated.Secondly,a sufficient differential protection criterion based on the current phase difference on both sides of the protected line is raised and the calculation method of phase difference is proposed at the same time.In this thesis,phase difference calculation method is based on the calculation of current phase variation instead of the synchronization of both sides.The method takes the characteristics of the current on both sides as a bridge and calculates the current phase difference indirectly.The calculation method can be realized under the poor communication condition and its high calculation precision can meet the need of protection at the same time.Finally,omnibus protection criterions based on current amplitude,current phase and voltage phase are put forward.Based on the cooperation of omnibus protection criterions,DG isolation technology and the traditional three section current protection,a complete set of adaptive differential protection scheme is raised further.A new amplitude-phase protection criterion is put forward based on the fault characteristics of both current amplitude and phase,which has both advantages of amplitude criterion and phase criterion and improves the sensitivity and braking capacity at the same time.Based on the characteristic that voltage phases on different sides change in opposite direction after fault occurs,a voltage longitudinal protection is proposed.The protection only depends on the available voltage information in distribution network and can be used as an alternative scheme when current information disappears,improving the ability of fault recognition further.A new adaptive area allocation method and a new adaptive coordination method of the adaptive differential protection scheme are proposed.A distribution network with DGs model is established by Matlab/Simulink and the effectiveness of the proposed scheme is verified.