Research On Analysis Of Distributed Generation Fault Characteristics And Protection Schemes For Micro-grid

Micro grid can minish the impact from fluctuation of distributed generator(DG) by control and management system as a new type of power system. But microgrid also brings disadvantages on power attemperation, function and relay protection. Researching on it and taking corresponding step to eliminate the disadvantageous influence is the necessary task to apply microgrid technology from experimentation to practice.Firstly the inverter's electric model is built and theoretic analysis method is proposed. Inverter interfaced distributed generator(IIDG) shows evident non-linear characteristic for the limits on power and current from control system. Two typical control modes of inverters are investigated, then the outer electric characteristic of IIDG in different types of fault condition are described by the control target and restriction equation of inverters. Combined with the fault limit conditions, the theoretic calculation method for fault response of inverter is proposed when different type fault occurs in system. The technique and conclusion can supply reference to system analysis and fault calculation.The new protection principle and integrated protection scheme of microgrid are proposed. Based on characteristic of microgrid,an inverse-time overcurrent protection with low voltage acceleration is proposed which only uses local currents and voltages information without communications. The new protection method can adapt system operation modes and guarantee that line fault can be cleared quickly in grid-connected or islanding condition. For actualizing the selectivity of relay protection, energy directional element based on fault component of instantaneous currents and voltages is proposed to avoid wrong direction judgement of directional element which use phase components for control system of inverter during fault transient. Then after analyzing the earthing modes of low voltage distribution system and the impact on phase to ground protection,the phase to ground protection scheme of microgrid is proposed according to particularity of microgrid. Based on local currents and voltages information, integrated protection scheme of microgrid is proposed which satisfies speediness and selectivity by cooperating different kinds of protection principles and suitable setting. The integrated protection scheme also dispenses with setting switch and credibly trip in all operation modes and all fault type condition.Micro-grid protection and control management system is developed which contains industrial computers as management device and protection equipments as intelligent agent. Control information and messages are delivered through RS-485 communications. The system realizes the function of protection, islanding detection,and grid reconnection in microgrid. It can supervise the state of all devices in microgrid with favorable interface. Distributed generators model and AC microgrid simulation system are built on RTDS(Real Time Digital Simulator), then the protection and control integrative device with new protection scheme proposed are tested by the numbers. The test results show the validity of direction judge and inverse-time overcurrent protection with low voltage acceleration are correct.The investigations above contain fault analysis and protection schemes for microgrid which DGs are connected to. The methods and conclusions can supply the theory foundation and technical reference for design, experiment, operation control and relay protection in analysis and practice.