Novel Protection And Control Scheme And Implementation Technology For Microgrid

A novel power supply system consisting of distributed generations,storage devices,smart consumers and controllable load,control and protection devices,namely'microgrid',has become an important method of solving problems raised by the high penetration of distributed generations into distribution network.However,the flexibility of operation modes of microgrid with inverter-based distributed generations presents a severe challenge to traditional distribution network protection principles.Besides,the complex structure of microgrid,the reasonable cooradintion of main-grid feeder protection,grid-connection point protection and internal component protection of microgrid becomes one of the problems related to microgrid protection.This paper focuses on grid-connected point protection and internal feeder protection of microgrid.Firstly,based on the fault characteristic analysis of inverter-based distributed generation,the feasibilities of widely-used traditional power direction elment and positive sequence fault component direction element are studied respectively to proposetheoretical basis to grid-connected point protection and internal feeder protection of microgrid.According to requirements of fault isolation strategy for operation behaviors of grid-connected point switch,logical coordination of main-grid feeder preotction,internal feeder protection and paralleling opearation standard of microgrid is achieved by different time-delay segments in various fault cases.The variations of positive sequence power direction and positive sequence current amplitude at grid-connected point are utilized to construct a novel fault direction identification method.To solve its dead zone problem in three-phase short circuit fault case,over-current protection and low-voltage protection are designed as auxilliary criterions.Using current differential protection for reference,the amplitudes of the defined positive sequence differential impedance for the internal and external fault are analyzed,and a novel positive sequence differential impedance protection principle is proposed based on the impedance amplitude difference.To improve the performance,an operation criterion with braking characteristic is constructed to overcome the adverse effects of load branch.An auxiliary criterion based on the amplitude of positive sequence impedance and current is constructed for internal three-phase short circuit faults without transition resistance.Compared with traditional current differential protection,the proposed protection strategy has no need for synchronization measurement.Using the intelligent measurement and control terminals as a platform,protection software process is designed and a set of positive sequence impedance differential proection device faced to engineering application is developed.The feasibility of protection scheme is verified by simulation and device test results.On the basic of the various characteristics of energy function of each phase for different fault types at protection location of mircogrid feeder,amodified pilot protection method based on leading phase energy directionis proposed.An amplitude supplementary criterion to overcome effects of load switching is constructed and the adaptability of the proposed method in the presentence of inverter-based distributed generations is analysed.Simulation results show that the proposed scheme is able to identify fault directions and its performance is not only immune to the load change but resistant to transition resistance to some extent.