| At present,the current differential protection is still used in the transmission line connected to photovoltaic plants.The fault characteristics of the photovoltaic plants are not fully considered,which leads to incorrect operations of protection for transmission line connected to photovoltaic plants,and even a large area of photovoltaic plants off grid chain accidents.The fault characteristics of photovoltaic are different from those of traditional synchronous generators because of the influence of control strategy of inverter.The current differential protection principle based on the fault characteristics of synchronous generators is challenged severely.Therefore,it is necessary to study the fault characteristics of transmission line connected to photovoltaic plants and the measures to improve the performance of current differential protection.Firstly,the fault characteristics of photovoltaic are analyzed based on the topology of grid-connected photovoltaic systems and the control strategy of photovoltaic inverter.Besides,the expression of short circuit current is deduced,which concludes that the short circuit current provided by photovoltaic plants is composed of fundamental frequency component and harmonic component.Then,the ideal current model with harmonic and attenuation DC component is built based on the fault characteristics of short circuit current at PV side and system side.It is concluded that the fuzzy operating zone of the sampled value current differential protection in the transmission line connected to photovoltaic plants increases significantly through theoretical analysis.The ideal current model and the actual grid-connected model are built separately,which verified that the sensitivity of the sampled value current differential protection in the transmission line connected to photovoltaic plants decreases.Finally,an improved scheme based on Spearman rank correlation coefficient is proposed to solve the problem that the sensitivity of sampled value differential protection decreases when the transmission line fails.The sampled value differential protection is still used as the main criterion in the scheme,and the auxiliary criterion is used only in the fuzzy operating zone of the sampled value differential protection.Among them,the auxiliary criterion is constructed by using the difference of Spearman rank correlation coefficient between internal and external faults.Theoretical analysis and digital simulation show that the improved scheme can identify the internal and external faults under various working conditions accurately and sensitively,and reduce the influence of fuzzy operating zone effectively. |
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