Impedance Differential Protection For Active Distribution Network

With a large number of distributed generation(DG)permeation,the traditional distribution networks have become a multi-source network with bi-directional power flow and short-circuit current,also known as Active Distribution Network.Then the sensitivity,selectivity and reliability of traditional over-current protection will face severe challenges.However,new protections for distribution network proposed at present are affected by the complex fault characteristics of DG and the existing synchronization conditions of distribution network,so it is difficult to guarantee the protection performance in active distribution network.In order to meet the relay protection demand of active distribution network and improve the sensitivity and reliability of protection,an impedance differential protection for active distribution network is proposed in this paper.Firstly,based on the definition of differential impedance and restraint impedance,the principle of impedance differential protection is proposed by analyzing the impedance characteristics of the fault during internal fault and external fault.The main criterion is constructed by the amplitude difference between the differential impedance and the restraint impedance during internal fault and external fault,while an auxiliary criterion is constructed by the amplitude characteristic of fault current to cope with the existing dead-zone fault.The protection performance of the proposed protection under the influence of transition resistance,CT saturation and data synchronization error is analyzed theoretically.The active distribution network model is built in PSCAD/EMTDC to verify the performance of the impedance differential protection principle.Secondly,an implementation scheme of impedance differential protection is proposed based on the impedance differential protection principle.Aiming at the weak feed characteristics of active distribution network,the corresponding start-up scheme and phase selection scheme are designed.At the same time,considering the synchronization condition of active distribution network,a new data synchronization method unaffected by channel delay and sampling delay is proposed.Moreover,an impedance calculation scheme that can automatically select the impedance calculation method according to the phase selection result is designed.Finally,the software and hardware platform of the impedance differential protection device and communication scheme are designed.According to the implementation scheme of impedance differential protection,the software flows of protection start,fault phase selection,data synchronization,impedance calculation and impedance differential protection module are designed.Furthermore,the communication network and communication protocol of impedance differential protection in active distribution network are designed.