Research On The Protection Schemes Of Distribution Network With Microgrid

Microgrid is one of the main methods of make use of distributed renewable energy and will be the key technology for smart grid construction in the future.The topology will changed when microgrid connected to distribution network.A radial distribution network supplied through a source is replaced by a network with multi-source supplied.The power flow presented in steady state of pre-fault network and fault current level,direction presented in the fault transient state network and the fault steady state network changed.These changes lead to the misoperation or refuse to operation of protection devices.Traditional protection schemes and algorithms cannot satisfy the requirements of protection relay after microgrid connected.In order to maintaining the security,stability of operation and the reliability of power supply,protection schemes for distribution network including mirogrid are studied and analyzed in this paper.In this paper,a microgrid equivalent to a controlled parallel unit that including a DG and a load,then,this paper simulates and analyses the impact on fault current and reclosing device operation in distribution network,fault situations cover different fault location and different microgrid output power.Also analyses the impact on overcurrent protection performance of feeders,such as selectivity,sensitivity,and reliability.Based on the analysis of various impact on the distribution network with microgrid,an adaptive distance protection scheme,directional relaying scheme based on phase-angle difference and differential protection scheme based on fault component are presented to protect distribution network.(1)Microgrid simulation model designed and a model of distribution network establitished.Microgrid as a controlled unit can be equivalent to a controlled current source or constant current source in parallel with load.the output current of microgrid can be controlled by two input selector in the simulation.The impact of distribution network protection with microgrid is simulated.(2)Adaptive distance protection:the paper analyses the calculation of setting impedance under the conditions of symmetrical and asymmetrical short-circuit,summarizes the calculation methods of branch coefficient under different faults.During a asymmetricalshort-circuit,positive,negative and zero sequence component of voltage and current.Calculated with symmetrical component method,and then superimposes voltage and current at fault location.Then,calculates branch coefficient.At last,Simulation results have demonstrated the feasibility of the scheme.(3)Directional relaying scheme based on phase-angle difference: for earth faults,according to phase-angle difference between voltage and current through the protection relay to discriminate the fault direction.For unearth faults,using phase-angle difference between phase-to-phase voltage difference and current difference through the protection relay to discriminate the fault direction.This paper illustrates the princeple and designes flow chart of proposed scheme.(4)Differential protection scheme based on fault component:for the shortcomings and disadvantages of conventional differential protection,this paper uses fault component as a criterion so as to active distribution network isolates fault quickly and correctly.Theoretical analyses discriminant methodes when fault occurred at operating zone or not at.Aiming at the effects of distribution network when microgrid connected,a few solution proposed based on detailed Theoretical analysis.And simulation showes schemes feasibility.